Optimized multiphoton excitation coil for MRI

Abstract

An apparatus for multiphoton parallel transmission for magnetic resonance imaging (MRI) of a subject, includes a radio frequency (RF) transmit coil, a single-channel universal excitation coil positioned outside of the RF transmit coil, and a processor device coupled to the RF transmit coil and the single-channel universal excitation coil. The RF transmit coil is configured to perform at least one RF excitation pulse of a multiphoton parallel transmit excitation comprising a multiphoton excitation pulse configured to correct spatial inhomogeneities and the single-channel universal excitation coil is configured to perform a low-frequency excitation pulse of the multiphoton excitation pulse. The processor device is configured to control current waveforms provided to the RF transmit coil to perform the at least one RF excitation pulse and to control current waveforms provided to the single-channel universal coil to perform the low frequency excitation pulse of the multiphoton excitation pulse.

Description

MGH 2024-119-02 Q&B 125141.04925OPTIMIZED MULTIPHOTON EXCITATION COIL FOR MRICROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based on, claims priority to, and incorporates herein by reference in its entirety U.S. Serial No. 63 / 722,548 filed November 19, 2024, and entitled "Optimized Multiphoton Excitation Coil for MRI."STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] This invention was made with government support under award numbers 1R56EB034902-01, F30MH129062 and 5U01EB0267996-05 (agreements 2022A013744 and 2017A053881) awarded by the National Institutes of Health. The government has certain rights in the invention.FIELD

[0003] The present disclosure relates generally to magnetic resonance imaging and, more particularly, to systems and methods for multiphoton parallel transmission using one or a small number of single-channel universal excitation coils in the acquisition of MR images.BACKGROUND

[0004] Magnetic resonance imaging (MRI) is a well-known tomographic imaging modality which has already substantially impacted medical practice. MRI has become a staple of anatomic, physiologic, and functional imaging, and is routinely used in clinical medical practice. Typical clinical MRI scanners operate with a main external magnetic field strength, Bo, of 1.5T or 3T. Over the past 15 years, there has been a push towards higher MRI field strengths, as the signal -to-noise ratio increases approximately in proportion to the static field strength, Bo. Pushing the magnetic field to higher levels (up to 7T for clinical scanners) has improved sensitivity and spatial resolution but has also generated challenges arising from the wavelength of the radiofrequency (RF) fields needed for excitation. For high-field MRI RF excitation, the image intensity and contrast are modulated across the body in complex ways (the so-called “flip angle inhomogeneity” problem). In other words, contrast in high-field MRI can be obscured by the spatially non-uniform excitation flip angle profile of- 1 -QB\99565311.1MGH 2024-119-02 Q&B 125141.04925 conventional birdcage transmit coils. Parallel transmit methods which employ an array of transmit antennas were introduced to address this problem. In parallel transmit methods, multiple, individually driven waveforms are sent to the transmit coils. However, parallel transmit methods add significantly to the scanner cost and complexity and introduce a range of concerns about local tissue heating (the so-called “local-SAR” problem).

[0005] One solution that was developed to address the problems (e.g., flip angle inhomogeneity and local tissue heating) of conventional excitation and conventional parallel transmit excitation is the use of multiphoton parallel transmission (MP-pTx) to mitigate flip angle inhomogeneities or otherwise spatially shape the excitation in high-field MRI. MP-pTx is an excitation technique that utilizes a single, conventional birdcage coil supplemented with low-frequency (kHz) z-directed fields from a multichannel shim array and / or gradient channels. The individual amplitudes and phases of the z-directed fields can be chosen to create uniform excitations. The RF, shim array, and gradient waveform’s amplitudes and phases can be optimized by nonconvex optimization techniques, e.g., using a genetic algorithm followed by sequential quadratic programming (SQP). Analysis of the deposited energy, known as Specific Absorption Rate (SAR) is simplified compared to many high-field excitation methods such as parallel transmission because the method’s SAR is limited to that of a conventional birdcage coil (only the birdcage coil produces significant SAR). While the ability of MP-pTx employing a full shim coil array demonstrates the potential to create markedly improved excitation homogeneity in the presence of severe Bi+inhomogeneity at 7T, practical implementation remains constrained by the requirement for a fully controlled shim array, which is rarely incorporated into standard MRI hardware due to its expense and technical demands.

[0006] It would be desirable to provide systems, apparatus, and methods for MP-pTx that addresses the above described challenges of implementing multiphoton pulses to improve MRI excitation.SUMMARY

[0007] In accordance with an embodiment, an apparatus for multiphoton parallel transmission for magnetic resonance imaging (MRI) of a subject includes a radio frequency (RF) transmit coil configured to perform at least one RF excitation pulse of a multiphoton parallel transmit excitation comprising a multiphoton excitation pulse configured to correct spatial inhomogeneities, a single-channel universal excitation coil- 2 -QB\99565311.1MGH 2024-119-02 Q&B 125141.04925 positioned outside of the RF transmit coil and configured to perform a low frequency excitation pulse of the multiphoton excitation pulse, and a processor device coupled to the RF transmit coil and the single-channel universal excitation coil and configured to control current waveforms provided to the RF transmit coil to perform the at least one RF excitation pulse and to control current waveforms provided to the single-channel universal coil to perform the low frequency excitation pulse of the multiphoton excitation pulse.

[0008] In accordance with another embodiment, a magnetic resonance imaging (MRI) system includes a magnet system configured to generate a polarizing magnetic field about at least a portion of a subject, a gradient system including a plurality of gradient coils configured to apply at least one magnetic gradient field to the polarizing magnetic field, a radio frequency (RF) system including at least one RF transmit coil configured to apply RF excitation fields to the subject, a single-channel universal excitation coil configured to apply excitation pulses to the subject, and a computer system. The computer system is programmed to receive a set of parameters for a multiphoton parallel transmit excitation comprising a multiphoton excitation pulse configured to correct spatial inhomogeneities, and direct the plurality of magnetic gradient coils, the RF transmit coil, and the singlechannel universal excitation coil to perform a pulse sequence comprising the multiphoton parallel transmit excitation to acquire magnetic resonance (MR) data from the subject, wherein the multiphoton excitation pulse of the multiphoton parallel transmit excitation is performed using the RF transmit coil and the single-channel universal excitation coil.

[0009] In accordance with another embodiment, a method for generating a magnetic resonance image of a subject using a magnetic resonance imaging (MRI) system includes receiving, using the MRI system, a set of parameters for a multiphoton parallel transmit excitation comprising a multiphoton excitation pulse configured to correct spatial inhomogeneities, performing, using the MRI system, a pulse sequence comprising the multiphoton parallel transmit excitation to acquire magnetic resonance (MR) data from the subject, wherein the multiphoton excitation pulse of the multiphoton parallel transmit excitation is performed using a radio frequency (RF) transmit coil and a single-channel universal excitation coil of the MRI system, and generating, using a processor, an image of the subject using the acquired MR data.QB\99565311.1MGH 2024-119-02 Q&B 125141.04925BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The present disclosure will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements.

[0011] FIG. l is a block diagram of an example magnetic resonance imaging (MRI) system in accordance with an embodiment;

[0012] FIG. 2A is a block diagram of an example magnet assembly including a universal coil for multiphoton parallel transmit (MP-pTx) excitation in accordance with an embodiment;

[0013] FIG. 2B is a block diagram of an example magnet assembly with a local coil including a universal coil for MP-pTx excitation in accordance with an embodiment;

[0014] FIG. 3 illustrates an example universal coil for MP-pTx excitation in accordance with an embodiment;

[0015] FIG. 4 illustrates an example method for design of a universal coil for MP-pTX excitation in accordance with an embodiment;

[0016] FIG. 5 illustrates an example MP-pTx excitation using a universal coil in accordance with an embodiment;

[0017] FIG. 6 illustrates a method for generating a magnetic resonance image of a subject using an MP-pTx excitation and a universal coil in accordance with an embodiment;

[0018] FIG. 7 is a block diagram of an example computer system in accordance with an embodiment;

[0019] FIG. 8A is a block diagram of an example magnet assembly including a universal coil and a set of shim coils for MP-pTx excitation in accordance with an embodiment; and

[0020] FIG. 8B is a block diagram of an example magnet assembly with a local coil including a universal coil and a set of shim coils for MP-pTx excitation in accordance with an embodiment.DETAILED DESCRIPTION

[0021] FIG. 1 shows an example of an MRI system 100 that may incorporate the universal coil (or universal excitation coil) described herein and may be used perform the methods described herein. The MRI system 100 includes an operator workstation 102, which may include a display 104, one or more input devices 106 (e.g., a keyboard and mouse), and a processor 108. The processor 108 may include a commercially available programmable machine running a commercially available operating system. The operator workstation 102 provides the operator interface that facilitates entering scan parameters (e.g., a scan- 4 -QB\99565311.1MGH 2024-119-02 Q&B 125141.04925 prescription) into the MRI system 100. The operator workstation 102 may be coupled to different servers, including, for example, a pulse sequence server 110, a data acquisition server 112, a data processing server 114, and a data store server 116. The operator workstation 102 and the servers 110, 112, 114, and 116 may be connected via a communication system 140, which may include any suitable network connection, whether wired, wireless, or a combination of both.

[0022] The pulse sequence server 110 functions in response to instructions provided by the operator workstation 102 to operate a gradient system 118 and a radiofrequency (“RF”) system 120. Gradient waveforms for performing a prescribed scan are produced and applied to the gradient system 118, which excites gradient coils in an assembly 122 to produce the magnetic field gradients Gx, Gy, and Gzthat are used for spatially encoding magnetic resonance signals. The gradient coil assembly 122 forms part of a magnet assembly 124 that includes a polarizing magnet 126 and one or more RF coils, for example, a whole-body RF coil 128 and / or a local coil (e.g., a head coil 129). While a head coil 129 is illustrated in FIG. 2, it should be understood that a local coil can be configured for imaging other regions or anatomy of a subject. The one or more RF coils can include an RF transmit coil (e.g., RF coil 128 or incorporated in a structure of a local coil) and an RF receive coil (e.g., part of RF coil 128 or incorporated in a structure of a local coil). In some embodiments, the one or more RF coils can be driven independently or with a fixed amplitude / phase relationship. In some embodiments, the whole-body RF coil 128 and / or local coil (e.g., head coil 129) may be a birdcage coil. In some embodiments, the magnet assembly 124 may also include one or more shim coils (not shown), for example, a shim coil array. In some embodiments, the shim coil(s) may be used to, for example, compensate for or remove inhomogeneities from the main magnetic field, Bo, generated by the polarizing magnet 126. In some embodiments, the shim coil(s) (e.g., a shim coil array) may be located, for example, inside the gradient coil assembly 122 or at other locations in the magnet assembly 124. In some embodiments, the shim coil(s) may be incorporated in the structure of a local coil, for example, head coil 129.

[0023] RF waveforms are applied by the RF system 120 to the RF coil 128, or a separate and possibly distinct local coil (e.g., the head coil 129), to perform the prescribed magnetic resonance pulse sequence. Responsive magnetic resonance signals detected by the RF coil 128, or a separate and possibly distinct local coil (e.g., the head coil 129), are received by the RF system 120. The responsive magnetic resonance signals may be amplified, demodulated, filtered, and digitized under direction of commands produced by the pulse sequence server- 5 -QB\99565311.1MGH 2024-119-02 Q&B 125141.04925110. The RF system 120 includes an RF transmitter for producing a wide variety of RF pulses used in MRI pulse sequences. The RF transmitter is responsive to the prescribed scan and direction from the pulse sequence server 110 to produce RF pulses of the desired frequency, phase, and pulse amplitude waveform. The generated RF pulses may be applied to the wholebody RF coil 128 or to one or more local coils or coil arrays, such as, for example, the head coil 129.

[0024] The RF system 120 also includes one or more RF receiver channels. Each RF receiver channel includes an RF preamplifier that amplifies the magnetic resonance signal received by the coil(s) 128,129 to which it is connected, and a detector that detects and digitizes the I and Q quadrature components of the received magnetic resonance signal. The magnitude of the received magnetic resonance signal may, therefore, be determined at any sampled point by the square root of the sum of the squares of the I and Q components:M = 7 / 2+ Q2(1) and the phase of the received magnetic resonance signal may also be determined according to the following relationship:(p = tan’1) (2)

[0025] The pulse sequence server 110 may receive patient data from a physiological acquisition controller 130. By way of example, the physiological acquisition controller 130 may receive signals from a number of different sensors connected to the patient, such as electrocardiograph (“ECG”) signals from electrodes, or respiratory signals from a respiratory bellows or other respiratory monitoring device. Such signals are typically used by the pulse sequence server 110 to synchronize, or “gate,” the performance of the scan with the subject’s heartbeat or respiration.

[0026] The pulse sequence server 110 may also connect to a scan room interface circuit 132 that receives signals from various sensors associated with the condition of the patient and the magnet system. Through the scan room interface circuit 132, a patient positioning system 134 can receive commands to move the patient to desired positions during the scan.

[0027] The digitized magnetic resonance signal samples produced by the RF system 120 are received by the data acquisition server 112. The data acquisition server 112 operates in response to instructions downloaded from the operator workstation 102 to receive the realtime magnetic resonance data and provide buffer storage, such that no data is lost by data overrun. In some scans, the data acquisition server 112 passes the acquired magnetic resonance data to the data processing server 114. In scans that require information derived- 6 -QB\99565311.1MGH 2024-119-02 Q&B 125141.04925 from acquired magnetic resonance data to control the further performance of the scan, the data acquisition server 112 may be programmed to produce such information and convey it to the pulse sequence server 110. For example, during pre-scans, magnetic resonance data may be acquired and used to calibrate the pulse sequence performed by the pulse sequence server 110. As another example, navigator signals may be acquired and used to adjust the operating parameters of the RF system 120 or the gradient system 118, or to control the view order in which k-space is sampled. In still another example, the data acquisition server 112 may also process magnetic resonance signals used to detect the arrival of a contrast agent in a magnetic resonance angiography (“MRA”) scan. For example, the data acquisition server 112 may acquire magnetic resonance data and process it in real-time to produce information that is used to control the scan.

[0028] The data processing server 114 receives magnetic resonance data from the data acquisition server 112 and processes it in accordance with instructions downloaded from the operator workstation 102. Such processing may include, for example, reconstructing two- dimensional or three-dimensional images by performing a Fourier transformation of raw k- space data, performing other image reconstruction algorithms (e.g., iterative or back- projection reconstruction algorithms), applying filters to raw k-space data or to reconstructed images, generating functional magnetic resonance images, or calculating motion or flow images.

[0029] Images reconstructed by the data processing server 114 are conveyed back to the operator workstation 102 for storage. Real-time images may be stored in a database memory cache (not shown in FIG. 1), from which they may be output to operator display 104 or a display 136. Batch mode images or selected real time images may be stored in a host database on disc storage 138. When such images have been reconstructed and transferred to storage, the data processing server 114 notifies the data store server 116 on the operator workstation 102. The operator workstation 102 may be used by an operator to archive the images, produce films, or send the images via a network to other facilities.

[0030] The MRI system 100 may also include one or more networked workstations 142. By way of example, a networked workstation 142 may include a display 144, one or more input devices 146 (e.g., a keyboard and mouse), and a processor 148. The networked workstation 142 may be located within the same facility as the operator workstation 102, or in a different facility, such as a different healthcare institution or clinic.- 7 -QB\99565311.1MGH 2024-119-02 Q&B 125141.04925

[0031] The networked workstation 142 may gain remote access to the data processing server 114 or data store server 116 via the communication system 140. Accordingly, multiple networked workstations 142 may have access to the data processing server 114 and the data store server 116. In this manner, magnetic resonance data, reconstructed images, or other data may be exchanged between the data processing server 114 or the data store server 116 and the networked workstations 142, such that the data or images may be remotely processed by a networked workstation 142. This data may be exchanged in any suitable format, such as in accordance with the transmission control protocol (TCP), the internet protocol (IP), or other known or suitable protocols.

[0032] As mentioned, an existing MP-pTx technique for excitation (e.g., at high fields) can utilize a single, conventional birdcage coil supplemented with low-frequency (kHz) z- directed irradiation from a multichannel shim array as described in pending U.S. Patent Application Serial No. 19 / 120,196, filed April 10, 2025 and titled “System and Method for Multiphoton Parallel Transmit Excitation for MRI,” herein incorporated by reference in its entirety. However, the requirement for a fully controlled shim array can present a barrier to practical implementations of multiphoton excitation pulses. The present disclosure describes a single-channel universal coil (otherwise referred to herein as a universal excitation coil or universal Bizcoil) configured to generate a low frequency z-directed field (Biz) for a multiphoton excitation pulse as part of a MP-pTx excitation. The single-channel universal coil or a small number of single-channel universal coils (e.g., <5) can replace the multichannel shim array and be used to supplement a conventional RF transmit coil (e.g., a birdcage coil) for multiphoton excitation for an MP-pTx technique. Accordingly, with the disclosed universal coil, multiphoton excitation can be utilized with only a single or small number of additional coils. Advantageously, the single-channel universal coil can provide a simplified hardware configuration and lower the barrier to commercial implementation of MP-pTx, while maintaining reduced local SAT concerns as compared to conventional parallel transmission. .

[0033] The single-channel universal coil (or universal excitation coil or universal Bizcoil) can be designed to be optimized for an average subject (e.g., patient) and, therefore, can be used for other, previously unseen subjects. The universal coil (or a small number of universal coils) can be configured to produce the necessary field pattern for a spatially uniform multiphoton excitation. The present disclosure also describes various methods to generate a- 8 -QB\99565311.1MGH 2024-119-02 Q&B 125141.04925 universal coil design for MP-pTx pulses that mitigates flip angle inhomogeneity during nonselective excitations.

[0034] The present disclosure describes systems and methods for performing multiphoton parallel transmit (MP-pTx) excitation for MRI using a single-channel universal coil(s). The disclosed MP-pTx excitation may be used in a pulse sequence performed by an MRI system to acquire magnetic resonance (MR) data from a subject and the acquired MR data may be used, for example, to generate an image of the subject. An MP-pTx excitation can be used to control the spatial profile of excitation for MRI. In some embodiments, the MP-pTx excitation can include an on-resonance RF excitation pulse followed by a multiphoton excitation pulse. In some embodiments, the on-resonance RF excitation pulse may be generated or performed using an RF coil such as, for example, a birdcage coil. In some embodiments, the multiphoton excitation pulse may be applied before an on-resonance RF excitation pulse, the multiphoton pulse may be used alone, or a combination of on-resonance RF excitation pulses and multiphoton excitation pulses may be used. In some embodiments, an off-resonance RF excitation pulse may be used before or after the multiphoton excitation pulse. In some embodiments, the RF excitation pulse may have both on-resonant and off- resonant frequency components simultaneously.

[0035] The multiphoton excitation pulse can be configured to utilize the multiphoton excitation phenomenon and may be used to, for example, correct spatial inhomogeneities of the on-resonance RF excitation pulse. In some embodiments, the multiphoton excitation pulse includes an off-resonance RF excitation pulse and one or a small number (e.g., <5) of low- frequency excitation pulses applied simultaneously via optimized single-channel universal coil(s) with the off-resonance RF excitation pulse. Accordingly, the off-resonance RF excitation pulse may be supplemented with the low frequency excitation pulse(s). In some embodiments, the off-resonance RF excitation pulse may be generated or performed using an RF coil such as, for example, a birdcage coil. Advantageously, in some embodiments, the one or small number of low frequency excitation pulses may be generated or performed using one or a small number of single-channel universal coils. In some embodiments, the amplitudes and phase of the various pulses generated by the RF coil and the universal coil(s) may be modulated through time.

[0036] Advantageously, the disclosed systems and method for MP-pTx excitation using a single-channel universal coil provide improvements over conventional excitation, conventional parallel transmit excitation, and convention MP-pTx techniques in cost,- 9 -QB\99565311.1MGH 2024-119-02 Q&B 125141.04925 simplicity and in that it has reduced energy deposition (SAR) constraints as compared to conventional parallel transmit techniques. In some embodiments, the disclosed single-channel universal coil and MP-pTx excitation methods can be used to solve the flip angle inhomogeneity problem with a vastly cheaper hardware configuration and SAR concerns of a conventional birdcage coil. The universal coil (or small number of universal coils) can provide a low-cost way to apply an additional, low-frequency oscillatory field (or a small number of oscillatory fields) with a number of degrees of freedom (e.g., the amplitude of the current through the universal coil(s)).

[0037] The single-channel universal coil (or a small number of single-channel universal coils) can be incorporated into an MRI system (e.g., MRI system 100 shown in FIG. 1), and in particular, in a high field MRI system. FIG. 2A is a block diagram of an example magnet assembly including a universal coil for multiphoton parallel transmit (MP-pTx) excitation in accordance with an embodiment. Magnet assembly 224 (e.g., magnet assembly 124 of MRI system 100 shown in FIG. 1) can include a polarizing magnet 226, a gradient coil assembly 222, an RF transmit coil 250 (e.g., (e.g., a whole body RF coil 128 shown in FIG. 1) and an RF receive coil 252 (e.g., RF coil 128 or a local coil). In some embodiments, the RF transmit coil 250 can be, for example, a birdcage coil. As mentioned above, in some embodiments, the RF transmit coil 250 can be driven independently or with a fixed amplitude / phase relationship with one or more additional transmit (or excitation) coils (e.g., a transmit coil array). Various other elements of a magnet assembly are omitted from FIG. 2A for clarity. In some embodiments, a single-channel universal coil (or a small number of single-channel universal coils) 260 for multiphoton excitation can be positioned at a location within a magnet assembly 224 as shown in FIG. 2 A. For example, the universal coil 260 may be located in a volume or space between inner and outer gradient coils (not shown) in the gradient coil assembly 222. In some embodiments, as discussed further below with respect to FIGs. 3 and 4, the universal coil 260 may be on a cylindrical former outside the RF elements. In some embodiments, the cylindrical former may be of similar length as the gradient coil assembly 222. While the universal coil 260 is shown within the gradient coil assembly 222 in FIG. 2A, it should be understood that in other embodiments, the universal coil 260 may be positioned at other locations in the magnet assembly 224. In some embodiments, the universal coil 260 can be positioned at a location outside of an outer diameter of the RF transmit coil 250. In some embodiments, the universal coil 260 may be mounted to another component of the magnet assembly 224, for example, the RF transmit coil 250.- 10 -QB\99565311.1MGH 2024-119-02 Q&B 125141.04925

[0038] A current can be passed through the universal coil (or each of a small number of universal coils) 260 to generate a magnetic field, in particular, a Bizfield. Universal coil 260 may be powered by an amplifier 262 and waveforms generated by amplifier 262 may be controlled by a computer system 264 (e.g., operator workstation 102 or pulse sequence server 110 shown in FIG. 1). In some embodiments, the computer system 264 and amplifier 262 are configured to control the current supplied to the universal coil(s) 260. In particular, during a scan operation, the universal coil(s) 260 can be energized to provide one or a small number of z-directed, low-frequency fields for multiphoton parallel transmit excitation, as described further below with respect to FIGs. 5 and 6. In some embodiments, an optimized coil winding can be designed for the universal coil 260 and be used to provide a spatially targeted low- frequency field as discussed further below with respect to FIGs. 3-6.

[0039] FIG. 2B is a block diagram of an example magnet assembly with a local coil including a universal coil for MP-pTx excitation in accordance with an embodiment. In FIG. 2B, magnet assembly 224 (e.g., magnet assembly 124 of MRI system 100 shown in FIG. 1) can include a polarizing magnet 226, and a gradient coil assembly 222. A local coil assembly 270 can be positioned within the bore of the magnet assembly 224 and surrounded by the gradient coil assembly 222. The local coil assembly 270 can be configured for imaging of a specific region or anatomy of a subject, (e.g., the head, abdomen, etc.). The local coil assembly 270 can include an RF transmit coil 250 and an RF receive coil 252. In some embodiments, the RF transmit coil can be, for example, a birdcage coil. As mentioned above, in some embodiments, the RF transmit coil 250 can be driven independently or with a fixed amplitude / phase relationship with one or more additional transmit (or excitation) coils (e.g., a transmit coil array). Various other elements of a magnet assembly are omitted from FIG. 2B for clarity. In some embodiments, a single-channel universal coil (or a small number of single-channel universal coils) 260 can be incorporated in the structure of the local coil assembly 270 (e.g., local coil 129 shown in FIG. 1). In FIG. 2B, the universal coil 260 is illustrated as being positioned outside of the RF transit coil 252 in the local coil assembly 270. In some embodiments, the universal coil 260 may be mounted to another component of the local coil assembly 270, for example, the RF transmit coil 250.

[0040] As mentioned, a current can be passed through the single-channel universal coil (or each of a small number of single-channel universal coils) 260 to generate a magnetic field, in particular, a Bizfield. Universal coil 260 may be powered by an amplifier 262 and waveforms generated by amplifier 262 may be controlled by a computer system 264 (e.g.,- 11 -QB\99565311.1MGH 2024-119-02 Q&B 125141.04925 operator workstation 102 or pulse sequence server 110 shown in FIG. 1). In some embodiments, the computer system 264 and amplifier 262 are configured to control the current supplied to the universal coil(s) 260. In particular, during a scan operation, the universal coil(s) 260 can be energized to provide one or a small number of z-directed, low- frequency fields for multiphoton parallel transmit excitation, as described further below with respect to FIGs. 5 and 6. In some embodiments, an optimized coil winding can be designed for the universal coil 260 and be used to provide a spatially targeted low-frequency field as discussed further below with respect to FIGs. 3-6.

[0041] FIG. 3 illustrates an example universal coil for MP-pTx excitation in accordance with an embodiment. A single-channel universal coil (or universal excitation coil or universal Bizcoil) 300 can include a plurality of windings 304 or a winding pattern that is optimized during a coil design process as discussed further below with respect to FIG. 4. In some embodiments, the coil windings 304 can be positioned on a cylinder 302 (or cylindrical former) as illustrated in the example in FIG. 3. In some embodiments, the coil windings 304 can be designed for and positioned on other shapes of formers (e.g., a dome, etc.). The coil windings 304 and coil winding pattern shown in FIG. 3 is an example and it should be understood that various other patterns can be utilized based on the design and optimization process. As mentioned, the universal coil 300 can be designed and optimized for an average subject (e.g., a patient) so that the universal coil can advantageously be used for imaging of any subject.

[0042] FIG. 4 illustrates an example method for design of a universal coil for MP-pTX excitation in accordance with an embodiment. Although the blocks of the process in FIG. 4 are illustrated in a particular order, in some embodiments, one or more blocks may be executed in a different order than illustrated in FIG. 4, or may be bypassed. The method may be implemented by a processing system (e.g., workstation 102 and processor 108 shown in FIG. 1 or the computer system 700 shown in FIG. 7) including at least one electronic processor, where the at least one electronic processor may be a processor (e.g., including one or more individual processor devices) as described further below with respect to FIG. 7.

[0043] As mentioned, a single-channel universal coil for multiphoton excitation can be designed and optimized for an average subject (e.g., a patient) so that the universal coil can advantageously be used for imaging of new subjects, for example, in some embodiments new subjects with similar characteristics relating to the subjects used for optimization.Accordingly, in operation the universal coil needs to be configured to produce spatially- 12 -QB\99565311.1MGH 2024-119-02 Q&B 125141.04925 uniform excitations across a variety of subject-specific RF excitation Bi+maps and magnetic field inhomogeneity (ABo) maps. Accordingly, in some embodiments, the coil winding pattern of a single-channel universal coil can be optimized over a database of a plurality of subjects and the Bi+maps and ABo maps associated with each subject.

[0044] At block 402, RF excitation Bi+maps and magnetic field inhomogeneity (ABo) maps for a plurality of subjects can be retrieved, for example, from a database. In some embodiments, the Bi+maps and ABo maps for the plurality of subjects can be retrieved from data storage, for example, disc storage 138 of MRI system 100 shown in FIG. 1 or a storage device 716 of a computer system 700 shown in FIG. 7. The Bi+maps and ABo maps for the plurality of subjects can be obtained from, for example, measurements (e.g., using an MRI system 100 to acquire data from a subject) or calculation in digital models. In one example, Bi+maps can be generated from MR data acquired using a known pulse sequence such as, for example, a turbo-FLASH pulse sequence. In another example, ABo maps can be acquired using known pulse sequences such as, for example, a double-echo gradient echo (GRE) pulse sequence. In some embodiments, for the coil design and optimization process a first subset of the plurality of subjects (and their associated Bi+maps and ABo maps) can be selected for a training dataset and a second subset of the plurality of subjects (and their associated Bi+maps and ABo maps) can be selected for a test dataset. For example, if the plurality of subjects includes 15 subjects, the training dataset can include 12 subjects and the test dataset can include 3 subjects.

[0045] At block 404, in some embodiments, a Biztarget field can optionally be determined using at least the Bi+maps and ABo maps for the plurality of subjects. As discussed further below, in some embodiments, the universal coil for MP-pTx excitation can be designed using a optimization method that does not require explicitly determining a Biztarget field. For embodiments that utilize an optimization method that utilize a Biztarget field, for example, a stream function method that can be used to design a coil that best represents a given field, a Biz target field can be determined at block 404.

[0046] The B iz target field can be designed to create a uniform excitation using a multiphoton RF pulse where the first photon is a traditional RF excitation (typically off- resonant) using a conventional RF transmit coil, such as a birdcage excitation coil, and the second photon is derived from the Biztarget field oscillating at the much lower frequency needed to complete the excitation resonance condition. In one example, the Biztarget field needed by the MP-pTx pulse to achieve a uniform 90° excitation over the training dataset- 13 -QB\99565311.1MGH 2024-119-02 Q&B 125141.04925 during universal pulse optimization can be determined. In some embodiments, to determine the B iz target field, first an optimized MP-pTx pulse for the conventional MP-pTx technique utilizing a conventional multichannel shim coil array and a conventional RF transmit coil (e.g., a birdcage coil) can be determined. The optimized MP-pTx pulse can include, for example, an on-resonant RF excitation pulse applied using the RF transmit coil, a blip period, and a multiphoton pulse including an off-resonant RF excitation pulse applied using the RF transmit coil and sinusoidal waveforms applied using the shim coil array. In some embodiments, the relative phases of a Biot-Savart-simulated shim array can be fixed because the single-channel universal coil does not support local phase shifts in the current. Additionally, the relative magnitudes of the simulated shim array elements can be held constant between the blip period and the multiphoton pulse. As an example, for the multiphoton pulse, the frequency of the waveforms applied during the multiphoton pulse can be set to AcOxy=o)z= 2K(5 kHz) to obtain two-photon resonance. An optimization framework, for example, known optimization methods, may be used to optimize one or more parameters (e.g., amplitude, phase and frequency of each shim coil current, duration of the low frequency excitation pulse, etc.) of the multiphoton excitation pulse (e.g., of the MP-pTx pulse) to create a uniform transverse magnetization pattern at the end of the excitation. In one example, the MP-pTx pulse can be optimized using a genetic algorithm followed by a gradient descent using a Sequential Quadratic Programming (SQP) algorithm.

[0047] In some embodiments, a number of optimizations can be performed to design a plurality of optimized MP-pTx pulses and a subset of the plurality of optimized MP-pTx pulses can then be selected as candidates for creating or determining the Biztarget field. In one example, the three best performing optimized MP-pTx pulses over the training dataset can be selected as candidates for creating the Biztarget field for the universal coil. In some embodiments, the best performing pulse of the candidate pulses can be selected to creating the B iz target field for the universal coil.

[0048] Once the optimized MP-pTx pulse has been determined, the Biztarget field can be derived from the optimized MP-pTx pulse, for example, the Biztarget field can be the field that is created by or emanates from a simulated multichannel shim coil array using the optimized MP-pTx pulse. In some embodiments, the Biztarget field can be smoothed with a known method such as, for example, nine-pixel Gaussian kernel. In some embodiments, the Biz target field can be stored in data storage (e.g., disc storage 138 of MRI system 100 shown in FIG. 1 or data storage 716 of computer system 700 shown in FIG. 7).- 14 -QB\99565311.1MGH 2024-119-02 Q&B 125141.04925

[0049] At block 406, a design of a single-channel universal coil or a small number of singlechannel universal coils can be generated. Various optimization methods can be used to generate the design (e.g., a coil winding pattern) of the single-channel universal coil(s) for MP-pTx excitation. In one embodiment, the determined Biztarget field at block 404 can be used to generate a design for the universal coil. For example, a stream function boundary element method (SF-BEM or “stream function method”) can be used to optimize coil windings (e.g., coil windings 304 shown in FIG. 4) for a universal coil or a small number of universal coils (as discussed further below). The stream function method can be used to design a universal coil that best represents or approximates the Biztarget field. The singlechannel universal coil can be designed on, for example, a cylinder configured to be positioned outside of an RF transmit coil (e.g., a birdcage coil). It should be understood that, in some embodiments, the universal coil can be designed on other surface shapes, for example, a bullet, a dome, etc. The surface (e.g., the surface of the cylinder) can be meshed with a predetermined amount of triangular elements with specified basis functions at the internal vertices. A vector of basis function weights (e.g., node weights) indicating the relative weights of the basis functions on the meshed surface can be determined and then used to determine magnetic flux density map, inductance, force balance, and torque. A convex optimization problem can be used to determine the weight vector which best generates the Biztarget field, while satisfying torque, force balance, and wire density constraints. The optimal wire distribution (e.g., a winding pattern) can be determined by contouring the node (stream function) weights. Finally, in this embodiment, the generated universal coil design can be validated to confirm the universal coil can be used for an MP- pTx pulse (e.g., a universal MP-pTx pulse) and correct excitation inhomogeneity. To validate the optimized universal coil design, universal Bi coil MP-pTx pulses can be generated over the training dataset using the optimized universal coil design, for example, a universal coil having the optimized coil winding pattern and other parameters of the optimized coil design, except with a specified current constraint. In some embodiments, the generated design of the optimized universal coil can be stored in data storage (e.g., disc storage 138 of MRI system 100 shown in FIG. 1 or data storage 716 of computer system 700 shown in FIG. 7) and the optimized universal coil design can be used to fabricate a universal coil to be incorporated in an MRI system for MP-pTx.

[0050] As mentioned, in some embodiments, a design for a small number of single-channel universal coils (e.g., <5) can be generated. In some embodiments, multiple universal coils can- 15 -QB\99565311.1MGH 2024-119-02 Q&B 125141.04925 be designed using a singular-value decomposition (SVD) of target field maps (e.g., Biztarget field maps) for a plurality of subjects, for example, each subject in the database of Bi+field maps and ABo maps. In some embodiments, the Bi+maps and ABo maps for the plurality of subjects can be stored in data storage, for example, disc storage 138 of MRI system 100 shown in FIG. 1 or a storage device 716 of a computer system 700 shown in FIG. 7. An MP- pTx pulse can be designed for each set of Bi+and ABo field maps (and associated subject) of the database, which, by driving the driving channels in phase, allows the definition of a target field map (e.g., a Biztarget field map) for each subject. The target field maps for the subjects the database can then be analyzed and compressed using SVD into a set of field maps that naturally span the space of map solutions helpful to achieve a target flip angle. In some embodiments, other compression approaches can be used in this process. The number of field maps kept during compression can dictate the number of single-channel universal coils (i.e., the number of channels). Accordingly, each universal coil can approximate an average coil across the database of subjects and associated field maps.

[0051] As mentioned, in some embodiments, the universal coil for MP-pTx excitation can be designed using an optimization method that does not require explicitly determining a Biztarget field. In some embodiments, an optimization technique can be used that simultaneously optimizes the stream function (node) densities of a current loop basis set on a mechanical former (such as a cylinder) while attempting to generate a homogeneous excitation pattern. In these embodiments, a conventional MP-pTx pulse can be designed and optimized using the basis functions on the meshed surface itself as “virtual shim coils.” Accordingly, the universal coil can be designed directly from the relative weights of the basis functions used during optimization. In this embodiment all nodes have associated magnitudes for pulse optimization, while torque, force balance, wire density, and inductance constraints can be placed on the stream function nodal values. In this embodiment, the pulse and the stream function pattern can be simultaneously obtained during the optimization, and as such, there is no explicit Biztarget field calculation. Instead, the Biztarget field can be implicitly calculated when the pulse optimization is performed. Many iterations of this nonconvex optimization can be run and the best performing coil design can be implemented. In some embodiments, the generated design of the optimized universal coil can be stored in data storage (e.g., disc storage 138 of MRI system 100 shown in FIG. 1 or data storage 716 of computer system 700 shown in FIG. 7) and the optimized universal coil design can be used to fabricate a universal coil to be incorporated in an MRI system for MP-pTx.- 16 -QB\99565311.1MGH 2024-119-02 Q&B 125141.04925

[0052] At block 408, an optimized universal multiphoton excitation pulse can be determined based on the design of a single-channel universal coil or a small number of single-channel universal coils generated at block 406. In some embodiments, a universal MP-pTx pulse can be determined for an average subject (e.g., patient), for example, using Bi+maps and ABo maps for a plurality of subjects (e.g.., the Bi+maps and ABo maps and associated subject from block 402). For example, the universal MP-pTx pulse can be generated using the training dataset. The universal MP-pTx pulse can be utilized on average for most similar subjects. In some embodiments, the optimized universal multiphoton excitation pulse can be stored in data storage (e.g., disc storage 138 of MRI system 100 shown in FIG. 1 or data storage 716 of computer system 700 shown in FIG. 7) and the universal multiphoton excitation pulse can be used in an MRI system for MP-pTx. In some embodiments, at the time of scanning a subject (e.g., as discussed below with respect to FIG. 6), the MP-pTx pulse can be designed and optimized for the specific subject, for example, for Bi+maps and ABo maps measured for the specific subject being scanned.

[0053] FIG. 5 illustrates an example MP-pTx excitation using a universal coil in accordance with an embodiment. In FIG. 5, the multiphoton parallel transmit (MP-pTx) excitation 500 includes an on-resonance RF excitation pulses 502 followed by a multiphoton excitation pulse (or multiphoton pulse) 504. In some embodiments, the MP-pTx excitation 500 may also include an optional blip period 518, as discussed further below. In some embodiments, the MP-pTx excitation 500 may be used in a pulse sequence employed by an MRI system (e.g., MRI system 100 shown in FIG. 1) to acquire MR data for various applications such as, for example, body imaging or head imaging. The on-resonance RF excitation pulse 502 may be applied using an RF transmit coil (e.g., RF transmit coil 250, such as, a body or local coil) of an MRI system. In some embodiments, the on-resonance RF excitation pulse 502 may be circularly polarized and applied at the Larmor frequency (m0) using a single-channel, high frequency RF transmit coil such as, for example, a birdcage coil. The on-resonance RF excitation pulse 502 can generate an efficient, but spatially non-uniform excitation (e.g., transverse magnetic field Blxy).

[0054] The on-resonance RF excitation pulse 502 may then be followed by a multiphoton excitation pulse (or multiphoton pulse) 504. Multiphoton excitation pulse 504 can include an off-resonance RF excitation pulse 506 and a low frequency excitation pulse 510. The off- resonance RF excitation pulse 506 may be applied using an RF transmit coil (e.g., RF transmit coil 250) of an MRI system. In some embodiments, the off-resonance RF excitation- 17 -QB\99565311.1MGH 2024-119-02 Q&B 125141.04925 pulse 506 may be circularly polarized and applied at a frequency (m0— Amxy) off resonance from the Larmor frequency by an offset frequency (Amxy) using a single-channel, high frequency RF transmit coil such as, for example, a birdcage coil. The off-resonance RF excitation pulse 506 can generate a spatially-dependent transverse magnetization, Blxy. The low frequency excitation pulse 510 can be applied or performed simultaneously with the off- resonance excitation pulse 406. The low frequency excitation pulse 510 can advantageously be applied using a single-channel universal coil (e.g., universal coil 260 shown in FIGs. 2A and 2B). In some embodiments, the low frequency excitation pulse 510 can be applied at the offset frequency, Amxy. In some embodiments, the low frequency excitation pulse 510 may operate at a frequency in the tens of kilohertz, where minimal energy is absorbed by the body. The low frequency excitation pulse 510 can generate a z-directed oscillating field, B1Zp. While one low frequency excitation pulse is shown in FIG. 5, it should be understood that in embodiments having a small number (e.g., <5) of single-channel universal coils, the multiphoton excitation pulse 504 can include a small number of low frequency excitation pulses 510, where each low frequency excitation pulse 510 can be applied using a different single-channel universal coil in the small number of universal coils.

[0055] In the multiphoton excitation pulse 504, the field Blzgenerated by the low frequency excitation pulse 510 or, in some embodiments, the sum of the individual fields, B1Zp, generated by a small number of low frequency excitation pulses 510 can supply the small amount of additional energy needed to complete energy conservation in the spin transition, i.e., convert z-axis magnetization into magnetization in the xy-plane. In some embodiments with one universal coil, various parameters of the low frequency excitation pulse (or field) 510, for example, the amplitudepulse duration, waveform, etc., may be selected to, for example, optimize the uniformity of the overall excitation or to perform other imagingrelevant, spatially localized tasks. For example, in some embodiments, the multiphoton excitation pulse 504 can be used to correct the inhomogeneity problems of the on-resonance RF excitation pulse 502. In some embodiments, an optimization framework, for example, known optimization methods, may be used to optimize one or more parameters (e.g., amplitude, and frequency of the universal coil current, duration of the low frequency excitation pulse, etc.) of the multiphoton excitation pulse 504 to create a uniform transverse magnetization pattern at the end of the excitation 500. In some embodiments, gradient field 512 (Gx), 514(Gy), and 516 (Gz) may also be applied during the multiphoton excitation pulse- 18 -QB\99565311.1MGH 2024-119-02 Q&B 125141.04925504. Gradient fields 512, 514, 516 may be applied using a gradient coils of the MRI system, for example, gradient coil assembly 122 shown in FIG. 1.

[0056] While the MP-pTx excitation scheme 500 illustrated in FIG. 5 shows the multiphoton excitation pulse 504 performed after the on-resonance RF excitation pulse 502, in other embodiments, the multiphoton excitation pulse 504 may be applied before an on-resonance excitation pulse 502 or the multiphoton excitation pulse 504 may be used alone. In addition, in some embodiments, multiple combinations of on-resonance RF excitation pulses 502 and multiphoton excitation pulses 504 may be used. If the multiphoton excitation pulse 504 is applied prior to an on-resonance RF excitation pulse 502, the multiphoton excitation pulse 504 can excite spins to preemptively counteract the inhomogeneities that the on-resonance RF excitation pulse 502 may induce. If the multiphoton excitation pulse 504 is applied following the on-resonance RF excitation pulse 502 (as illustrated in FIG. 5), the multiphoton excitation pulse 504 may serve as a “correction” pulse, whereby the multiphoton excitation pulse 504 would attempt to correct, for example, the flip angle inhomogeneities that may be present following the on-resonance RF excitation pulse 502.

[0057] In some embodiments, the MP-pTx excitation 500 may include an optional blip period 518. The optional blip period 518 may be applied in a period between the on- resonance RF excitation pulse 502 and before the multiphoton excitation pulse 504. In some embodiments, the blip period 518 can consist of current 520 applied to the universal (or Blz) coil to impose a spatially-dependent phase on the transverse magnetization. The blip period 518 may also include currents 522, 524, 526 applied to the gradient coils. In some embodiments including a blip period 518, the amplitudes of the current applied to the universal coil and gradient coils during the blip period 518 may be selected (e.g., optimized and modulated) to create a uniform transverse magnetization pattern at the end of the excitation 500. In some embodiments, if the MP-pTx excitation includes a combination of multiple on-resonance RF excitation pulses 502 and multiple multiphoton excitation pulses 504, a blip period 518 may be applied between each individual pulse.

[0058] In some embodiments, in the MP-pTx excitation 500, the on-resonance RF excitation pulse 502 may be employed to efficiently complete most of the desired excitation, and then may be followed by the multiphoton excitation pulse 504 which may be configured to utilize the single-channel universal coil or a small number of single-channel universal coils) to “fix” the spatial inhomogeneities, resulting in a more uniform excitation achieved with a single conventional high-frequency excitation and without the specific absorption rate (SAR)- 19 -QB\99565311.1MGH 2024-119-02 Q&B 125141.04925 concerns of conventional parallel transmit techniques. The disclosed multiphoton excitation pulse 504 can advantageously utilize the multiphoton excitation phenomena for excitation uniformity mitigation in MRI. In addition, the disclosed multiphoton excitation pulse 504 may also be used to achieve other target (e.g., as desired by a user or operator of the MRI system) spatial excitation profiles or patterns.

[0059] In some embodiments, to address the spatial flip angle inhomogeneity problem, the disclosed system and method for MP-pTx excitation can use a conventional birdcage transmit coil (a single high-frequency channel) to apply an off-resonance Bixy field (e.g., via an off- resonance RF excitation pulse 502) such as, for example, a transverse a Bi+RF field, and can use a low-frequency z-directed single-channel universal coil to apply a low frequency z- directed field (Blz) which supplement the off-resonance Blxyfield. Using the low-frequency universal coil (or a small number of universal coils) to apply the low frequency z-directed Blzfield (e.g., via a low frequency excitation pulse 510) can help lower cost and significantly simplifies SAR management, since SAR is negligible at low frequencies, independent of how the universal coil is energized. In some embodiments, the disclosed MP-pTx excitation 500 can create a more homogeneous excitation at high field strengths. The disclosed system and methods for MP-pTx excitation can address the excitation (flip angle) inhomogeneity issue without the expense of added high frequency power channels or concerns about increasing local SAR above conventional single-channel excitations.

[0060] FIG. 6 illustrates a method for generating a magnetic resonance image of a subject using an MP-pTx excitation and a universal coil in accordance with an embodiment. Although the blocks of the process in FIG. 6 are illustrated in a particular order, in some embodiments, one or more blocks may be executed in a different order than illustrated in in FIIG. 5, or may be bypassed.

[0061] At block 602, one or more parameters for a multiphoton parallel transmit (MP-pTx) excitation (e.g., MP-pTx excitation 500 shown in FIG. 5) can be received or retrieved. The MP-pTx excitation can include a multiphoton excitation pulse (e.g., multiphoton excitation pulse 504 in FIG. 5) that, in some embodiments, may be applied before or after an on- resonance RF excitation pulse. The multiphoton excitation pulse can include an off-resonance RF excitation pulse (e.g., off-resonance excitation pulse 506 shown in FIG. 4) and one or a small number (e.g., <5) of low frequency excitation pulses (e.g., low frequency excitation pulse(s) 510 shown in FIG. 5). In some embodiments, the one or more parameters provided at block 602 can include parameters of the multiphoton excitation pulse including parameters of- 20 -QB\99565311.1MGH 2024-119-02 Q&B 125141.04925 the low frequency excitation pulse(s) (or field), for example, the amplitude, phase, frequency, pulse duration, waveform, etc. of the low frequency excitation pulse(s). As mentioned above, the one or more parameters may be selected to, for example, optimize the uniformity of the overall excitation or to perform other imaging relevant, spatially localized tasks. In some embodiments, an optimization framework, for example, known optimization methods, may be used to optimize one or more parameters (e.g., amplitude and frequency of each universal coil current, duration of the low frequency excitation pulse, etc.) of the multiphoton excitation pulse to create a uniform transverse magnetization pattern at the end of the MP- pTx excitation. For example, in some embodiments, the optimization of the parameters of the multiphoton excitation pulse may be performed using a conventional pulse optimization approach. In some embodiments, the one or more parameters may be provided by a user (or operator), for example, using a user interface or input devices of an MRI system (e.g., MRI system 100 shown in FIG. 1). In some embodiments, the one or more parameters may be retrieved from data storage of an MRI system (e.g., MRI system 100 shown in FIG. 1) or data storage of other computer systems. For example, parameters determined using an optimization method may be stored in data storage and retrieved from data storage for performing the MP-pTx excitation as part of an MR scan of a subject. In some embodiments, the MP-pTx excitation can be an optimized universal MP-pTx excitation, for example, as discussed above with respect to block 408 of FIG. 4, based on an optimized universal coil and can be utilized on average for new subjects. In some embodiments, at the time of scanning a subject (e.g., as discussed below with respect to FIG. 6), , the MP-pTx pulse can be designed and optimized for the specific subject, for example, for Bi+maps and ABo maps measured for the specific subject being scanned as discussed above with respect to block 408 of FIG. 4.

[0062] At block 604, an MRI system (e.g., MRI system 100 shown in FIG. 1) may be used to perform a pulse sequence with a multiphoton parallel transmit (MP-pTx) excitation (e.g., either a universal MP-pTx excitation or a subject-specific MP-pTx excitation) to acquire MR data from a subject. The MP-pTx excitation may be utilized during the excitation phase of known pulse sequences for acquired MR data from a subject (e.g., three-dimensional gradient echo (GRE), inversion recovery, spin echo, etc.). As discussed above, the disclosed MP-pTx excitation advantageously includes a multiphoton excitation pulse that, in some embodiments, may be applied before or after an on-resonance RF excitation pulse. The multiphoton excitation pulse can be used to correct spatial inhomogeneities, for example, spatial inhomogeneities of the on-resonance RF excitation pulse, to create a uniform- 21 -QB\99565311.1MGH 2024-119-02 Q&B 125141.04925 transverse magnetization pattern at the end of the MP-pTx excitation. In some embodiments, the on-resonance RF excitation pulse may be applied using an RF transmit coil (e.g., RF transmit coil 250) of an MRI system. In some embodiments, the multiphoton excitation pulse can include an off-resonance RF excitation pulse 506 and a low frequency excitation pulse 510. The off-resonance RF excitation pulse 506 may be applied using an RF transmit coil (e.g., RF transmit coil 250) of an MRI system. The low frequency excitation pulse can advantageously be applied using a single-channel universal coil (e.g., the single-channel universal coil shown in FIGs. 2A and 2B).

[0063] At block 606, an image of the subject may be generated using the NMR data acquired at block 504. The image of the subject may be reconstructed using known reconstruction methods. At block 610, the generated image of the subject may be stored or displayed. The generated image of the subject may be stored in, for example, data storage of an MRI system (e.g., MRI system 100 shown in FIG. 1) or data storage of other computer systems (e.g., computer system 700 shown in FIG. 7. The generated image of the subject may be displayed on a display, for example, a display of an MRI system (e.g., displays 104, 136 and / or 144 of MRI system 100 shown in FIG. 1) or a display of other computer systems (e.g., computer system 700 shown in FIG. 7).

[0064] FIG. 7 is a block diagram of an example computer system in accordance with an embodiment. Computer system 700 may be used to implement the systems and methods described herein. In some embodiments, the computer system 700 may be a workstation, a notebook computer, a tablet device, a mobile device, a multimedia device, a network server, a mainframe, one or more controllers, one or more microcontrollers, or any other general- purpose or application-specific computing device. The computer system 700 may operate autonomously or semi-autonomously, or may read executable software instructions from the memory or storage device 716 or a computer-readable medium (e.g., a hard drive, a CD- ROM, flash memory), or may receive instructions via the input device 720 from a user, or any other source logically connected to a computer or device, such as another networked computer or server. Thus, in some embodiments, the computer system 700 can also include any suitable device for reading computer-readable storage media.

[0065] Data, such as data acquired with an imaging system (e.g., a CT imaging system) may be provided to the computer system 700 from a data storage device 716, and these data are received in a processing unit 702. In some embodiment, the processing unit 702 includes one or more processors. For example, the processing unit 702 may include one or more of a- 22 -QB\99565311.1MGH 2024-119-02 Q&B 125141.04925 digital signal processor (DSP) 704, a microprocessor unit (MPU) 706, and a graphics processing unit (GPU) 708. The processing unit 702 also includes a data acquisition unit 710 that is configured to electronically receive data to be processed. The DSP 704, MPU 706, GPU 708, and data acquisition unit 710 are all coupled to a communication bus 712. The communication bus 712 may be, for example, a group of wires, or a hardware used for switching data between the peripherals or between any component in the processing unit 702.

[0066] The processing unit 702 may also include a communication port 714 in electronic communication with other devices, which may include a storage device 716, a display 718, and one or more input devices 720. Examples of an input device 720 include, but are not limited to, a keyboard, a mouse, and a touch screen through which a user can provide an input. The storage device 716 may be configured to store data, which may include data such as, for example, Bi+and ABo maps, Biztarget fields, single-channel universal coil designs, parameters of MP-pTx excitations, MP-pTx pulses, etc., whether these data are provided to, or processed by, the processing unit 702. The display 718 may be used to display images and other information, such as CT images, patient health data, and so on.

[0067] The processing unit 702 can also be in electronic communication with a network 722 to transmit and receive data and other information. The communication port 714 can also be coupled to the processing unit 702 through a switched central resource, for example the communication bus 712. The processing unit can also include temporary storage 724 and a display controller 726. The temporary storage 724 is configured to store temporary information. For example, the temporary storage 724 can be a random access memory.

[0068] In some embodiments, the disclosed single-channel universal coil can be used in combination with a set of shim coils (e.g., a multichannel shim coil array) for multiphoton parallel transmission. As mentioned, in prior techniques MP-pTx can be performed using a conventional birdcage coil supplemented with low-frequency (kHz) z-directed irradiation from a multichannel shim array as described in pending U.S. Patent Application Serial No. 19 / 120,196, filed April 10, 2025 and titled “System and Method for Multiphoton Parallel Transmit Excitation for MRI,” herein incorporated by reference in its entirety. To increase the number of low-frequency pulses for MP-pTx, a multichannel shim array can be used in addition to the single-channel universal coil. Accordingly, the off-resonance RF excitation pulse of the multiphoton excitation pulse can be supplemented by a plurality of low frequency excitation pulses.- 23 -QB\99565311.1MGH 2024-119-02 Q&B 125141.04925

[0069] The single-channel universal coil (or a small number of single-channel universal coils) and one or more shim coils can be incorporated into an MRI system (e.g., MRI system 100 shown in FIG. 1), and in particular, in a high field MRI system. FIG. 8A is a block diagram of an example magnet assembly including a universal coil and a set of shim coils for MP-pTx excitation in accordance with an embodiment. Magnet assembly 824 (e.g., magnet assembly 124 of MRI system 100 shown in FIG. 1) can include a polarizing magnet 826, a gradient coil assembly 822, an RF transmit coil 850 (e.g., (e.g., a whole body RF coil 128 shown in FIG. 1) and an RF receive coil 852 (e.g., RF coil 128 or a local coil). In some embodiments, the RF transmit coil 850 can be, for example, a birdcage coil. As mentioned above, in some embodiments, the RF transmit coil 850 can be driven independently or with a fixed amplitude / phase relationship with one or more additional transmit (or excitation) coils (e.g., a transmit coil array). Various other elements of a magnet assembly are omitted from FIG. 8A for clarity. Magnet assembly 824 can also include a set of one or more shim coils 880 (e.g., a single shim coil, a shim coil array, etc.). In some embodiments, shim coil(s) 880 may be resistive shim coils. In the embodiment shown in FIG. 8A, shin coil(s) (e.g., a single shim coil, shim coil array, etc.) 880 can be located inside the gradient coil assembly 822. For example, the shim coil(s) 880 may be located in a volume or space between inner and outer gradient coils (not shown) in the gradient coil assembly 822. In some embodiments, the shim coil(s) 880 are full-size shim coils and, accordingly, may be on a cylindrical former of similar length as the gradient coil assembly 822. While the shim coil(s) 880 are shown within the gradient coil assembly 822 in FIG. 8A, it should be understood that in other embodiments, the shim coil(s) 880 may be positioned at other locations in the magnet assembly 824. In some embodiments, the shim coil(s) 250 may be mounted to another component of the magnet assembly 224, for example, the RF transmit coil 850.

[0070] In some embodiments, a single-channel universal coil (or a small number of singlechannel universal coils) 860 for multiphoton excitation can also be positioned at a location within a magnet assembly 824 as shown in FIG. 8A. For example, the universal coil 860 may be located in a volume or space between inner and outer gradient coils (not shown) in the gradient coil assembly 822. While the universal coil 860 is shown within the gradient coil assembly 822 in FIG. 8A, it should be understood that in other embodiments, the universal coil 860 may be positioned at other locations in the magnet assembly 824. In some embodiments, the universal coil 860 can be positioned at a location outside of an outer diameter of the RF transmit coil 850. In some embodiments, the universal coil 860 may be- 24 -QB\99565311.1MGH 2024-119-02 Q&B 125141.04925 mounted to another component of the magnet assembly 824, for example, the RF transmit coil 850.

[0071] A current can be passed through the shim coils 880 (e.g., a shim coil array) to generate magnetic fields, in particular, Bizfields. Shim coils 880 may be powered by an amplifier 882 and waveforms generated by amplifier 882 may be controlled by a computer system 864 (e.g., operator workstation 102 or pulse sequence server 110 shown in FIG. 1). In some embodiments, the computer system 864 and amplifier 882 are configured to control the current supplied to the shim coils 880. In particular, during a scan operation, the shim coils 880 can be energized to provide a plurality of z-directed, low-frequency fields in addition to the z-directed, low-frequency field(s) provided by the single-channel universal coil 860 for MP-pTx, as described above with respect to FIGs. 5 and 6.

[0072] A current can be passed through the universal coil (or each of a small number of universal coils) 860 to generate a magnetic field, in particular, a Bizfield. Universal coil 860 may be powered by an amplifier 862 and waveforms generated by amplifier 862 may be controlled by a computer system 864 (e.g., operator workstation 102 or pulse sequence server 110 shown in FIG. 1). In some embodiments, the computer system 864 and amplifier 862 are configured to control the current supplied to the universal coil(s) 860. In particular, during a scan operation, the universal coil(s) 860 can be energized to provide one or a small number of z-directed, low-frequency fields for multiphoton parallel transmit excitation, as described above with respect to FIGs. 5 and 6. In some embodiments, an optimized coil winding can be designed for the universal coil 860 and be used to provide a spatially targeted low-frequency field as discussed above with respect to FIGs. 3-6.

[0073] In some embodiments, the multichannel shim array can be incorporated in a local coil. FIG. 8B is a block diagram of an example magnet assembly with a local coil including a universal coil and a set of shim coils for MP-pTx excitation in accordance with an embodiment. In FIG. 8B, magnet assembly 824 (e.g., magnet assembly 124 of MRI system 100 shown in FIG. 1) can include a polarizing magnet 826, and a gradient coil assembly 822. A local coil assembly 870 can be positioned within the bore of the magnet assembly 824 and surrounded by the gradient coil assembly 822. The local coil assembly 870 can be configured for imaging of a specific region or anatomy of a subject, (e.g., the head, abdomen, etc.). The local coil assembly 870 can include an RF transmit coil 850 and an RF receive coil 852. In some embodiments, the RF transmit coil can be, for example, a birdcage coil. As mentioned above, in some embodiments, the RF transmit coil 850 can be driven independently or with a- 25 -QB\99565311.1MGH 2024-119-02 Q&B 125141.04925 fixed amplitude / phase relationship with one or more additional transmit (or excitation) coils (e.g., a transmit coil array). Various other elements of a magnet assembly are omitted from FIG. 8B for clarity. In some embodiments, a single-channel universal coil (or a small number of single-channel universal coils) 860 can be incorporated in the structure of the local coil assembly 870 (e.g., local coil 129 shown in FIG. 1). In some embodiments, shin coil(s) (e.g., a single shim coil, shim coil array, etc.) 880 can also be incorporated in the structure of the local coil assembly 870. In FIG. 8B, the universal coil 860 and the shim coils 880 are illustrated as being positioned outside of the RF transit coil 852 in the local coil assembly 870. In some embodiments, the universal coil 860 or the shim coils 880 may be mounted to another component of the local coil assembly 870, for example, the RF transmit coil 850.

[0074] As mentioned, a current can be passed through the single-channel universal coil (or each of a small number of single-channel universal coils) 860 to generate a magnetic field, in particular, a Bizfield. Universal coil 860 may be powered by an amplifier 862 and waveforms generated by amplifier 862 may be controlled by a computer system 864 (e.g., operator workstation 102 or pulse sequence server 110 shown in FIG. 1). In some embodiments, the computer system 864 and amplifier 862 are configured to control the current supplied to the universal coil(s) 860. In particular, during a scan operation, the universal coil(s) 860 can be energized to provide one or a small number of z-directed, low- frequency fields for multiphoton parallel transmit excitation, as described above with respect to FIGs. 5 and 6. In some embodiments, an optimized coil winding can be designed for the universal coil 860 and be used to provide a spatially targeted low-frequency field as discussed above with respect to FIGs. 3-6. In addition, a current can be passed through the shim coils 880 (e.g., a shim coil array) to generate magnetic fields, in particular, Bizfields. Shim coils 880 may be powered by an amplifier 882 and waveforms generated by amplifier 882 may be controlled by a computer system 864 (e.g., operator workstation 102 or pulse sequence server 110 shown in FIG. 1). In some embodiments, the computer system 864 and amplifier 882 are configured to control the current supplied to the shim coils 880. In particular, during a scan operation, the shim coils 880 can be energized to provide a plurality of z-directed, low- frequency fields in addition to the z-directed, low-frequency field(s) provided by the singlechannel universal coil 860, as described above with respect to FIGs. 5 and 6.

[0075] Referring to FIG. 5, in an embodiment where a multichannel shim array is used in combination with the single-channel universal coil, the multiphoton excitation pulse 504 can include a plurality of low frequency excitation pulses 510 applied or performed by the single-- 26 -QB\99565311.1MGH 2024-119-02 Q&B 125141.04925 channel universal coil and a set of one or more shim coils. The plurality of low frequency excitation pulses 510 can include P total pulses and can be applied or performed simultaneously with the off-resonance excitation pulse 506. The sum of the individual fields, Blzgenerated by the low frequency excitation pulses 510 can supply the small amount of additional energy needed to complete energy conservation in the spin transition, i.e., convert z-axis magnetization into magnetization in the xy-plane. In some embodiments, for the low frequency excitation pulses generated by the set of shim coils, various parameters of each low frequency excitation pulse (or field), for example, the amplitudephase (<pp), pulse duration, waveform, etc., may be selected to, for example, optimize the uniformity of the overall excitation or to perform other imaging-relevant, spatially localized tasks.

[0076] Computer-executable instructions for multiphoton parallel transmit (MP-pTx) using a single-channel universal coil for magnetic resonance imaging according to the abovedescribed methods may be stored on a form of computer readable media. Computer readable media includes volatile and nonvolatile, removable, and non-removable media implemented in any method or technology for storage of information such as computer readable instructions, data structures, program modules or other data. Computer readable media includes, but is not limited to, random access memory (RAM), read-only memory (ROM), electrically erasable programmable ROM (EEPROM), flash memory or other memory technology, compact disk ROM (CD-ROM), digital volatile disks (DVD) or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to store the desired instructions and which may be accessed by a system (e.g., a computer), including by internet or other computer network form of access

[0077] The present disclosure has been described in terms of one or more preferred embodiments, and it should be appreciated that many equivalents, alternatives, variations, and modifications, aside from those expressly stated, are possible and within the scope of the invention.- 27 -QB\99565311.1

Claims

MGH 2024-119-02 Q&B 125141.04925CLAIMS1. An apparatus for multiphoton parallel transmission for magnetic resonance imaging (MRI) of a subject, the apparatus comprising: a radio frequency (RF) transmit coil configured to perform at least one RF excitation pulse of a multiphoton parallel transmit excitation comprising a multiphoton excitation pulse configured to correct spatial inhomogeneities; a single-channel universal excitation coil positioned outside of the RF transmit coil and configured to perform a low frequency excitation pulse of the multiphoton excitation pulse; and a processor device coupled to the RF transmit coil and the single-channel universal excitation coil and configured to control current waveforms provided to the RF transmit coil to perform the at least one RF excitation pulse and to control current waveforms provided to the single-channel universal coil to perform the low frequency excitation pulse of the multiphoton excitation pulse.

2. The apparatus according to claim 1, wherein the single-channel universal excitation coil is optimized for an average subject.

3. The apparatus according to claim 1, wherein the at least one RF excitation pulse comprises an on-resonance RF excitation pulse.

4. The apparatus according to claim 1, wherein the multiphoton excitation pulse comprises an off-resonance RF excitation pulse performed by the RF transmit coil simultaneously with the low frequency excitation pulse performed at least by the singlechannel universal excitation coil.

5. The apparatus according to claim 1, further comprising a set of gradient coils coupled to the processor and wherein the multiphoton excitation pulse further comprises one or more gradient pulses and the processor is further configured to control current waveforms provided to the set of gradient coils to perform the one or more gradient pulses.

6. The apparatus according to claim 1, further comprising a plurality of single-channel universal excitation coils, wherein the multiphoton excitation pulse comprises a plurality- 28 -QB\99565311.1MGH 2024-119-02 Q&B 125141.04925 of low frequency excitation pulses, wherein each of the plurality of plurality low frequency excitation pulses is performed by one of the plurality of single-channel universal excitation coils,7. The apparatus according to claim 1, further comprising: a first amplifier couped to the RF transmit coil and the processor; and a second amplifier coupled the single-channel universal excitation coil and the processor.

8. A magnetic resonance imaging (MRI) system comprising: a magnet system configured to generate a polarizing magnetic field about at least a portion of a subject; a gradient system including a plurality of gradient coils configured to apply at least one magnetic gradient field to the polarizing magnetic field; a radio frequency (RF) system including at least one RF transmit coil configured to apply RF excitation fields to the subject; a single-channel universal excitation coil configured to apply excitation pulses to the subject; and a computer system programmed to: receive a set of parameters for a multiphoton parallel transmit excitation comprising a multiphoton excitation pulse configured to correct spatial inhomogeneities; and direct the plurality of magnetic gradient coils, the RF transmit coil, and the single-channel universal excitation coil to perform a pulse sequence comprising the multiphoton parallel transmit excitation to acquire magnetic resonance (MR) data from the subject, wherein the multiphoton excitation pulse of the multiphoton parallel transmit excitation is performed using the RF transmit coil and the single-channel universal excitation coil.

9. The MRI system according to claim 8, wherein the single-channel universal excitation coil is optimized for an average subject.- 29 -QB\99565311.1MGH 2024-119-02 Q&B 125141.0492510. The MRI system according to claim 8, wherein the computer system is further programmed to generate an image of the subject using the acquired MR data.

11. The MRI system according to claim 8, wherein the multiphoton excitation pulse comprises: an off-resonance RF excitation pulse performed using the RF transmit coil; and a low-frequency excitation pulse performed using the single-channel universal excitation coil, wherein the low-frequency excitation pulse is performed simultaneously with the off-resonance RF excitation pulse.

12. The MRI system according to claim 11, wherein the multiphoton excitation pulse further comprises one or more gradient pulses performed using the plurality of gradient coils, wherein the one or more gradient pules are performed simultaneously with the off- resonance RF excitation pulse and the low frequency excitation pulse.

13. The MRI system according to claim 11, wherein the multiphoton parallel transmit excitation further comprises an on-resonance RF excitation pulse performed by the RF transmit coil.

14. The MRI system according to claim 8, wherein the RF transmit coil is a birdcage coil.

15. The MRI system according to claim 8, further comprising a local coil, wherein the RF transmit coil and the single-channel universal excitation coil are positioned in the local coil.

16. A method for generating a magnetic resonance image of a subject using a magnetic resonance imaging (MRI) system, the method comprising: receiving, using the MRI system, a set of parameters for a multiphoton parallel transmit excitation comprising a multiphoton excitation pulse configured to correct spatial inhomogeneities; performing, using the MRI system, a pulse sequence comprising the multiphoton parallel transmit excitation to acquire magnetic resonance (MR) data from the subject,- 30 -QB\99565311.1MGH 2024-119-02 Q&B 125141.04925 wherein the multiphoton excitation pulse of the multiphoton parallel transmit excitation is performed using a radio frequency (RF) transmit coil and a single-channel universal excitation coil of the MRI system; and generating, using a processor, an image of the subject using the acquired MR data.

17. The method according to claim 16, wherein the multiphoton excitation pulse comprises: an off-resonance RF excitation pulse performed using the RF transmit coil; and a low-frequency excitation pulse performed using the single-channel universal excitation coil, wherein the low-frequency excitation pulse is performed simultaneously with the off-resonance RF excitation pulse18. The method according to claim 16, wherein the single-channel universal excitation coil is optimized for an average subject.

19. The method according to claim 17, wherein the low frequency excitation pulse is configured to generate a z-directed oscillating magnetic field at an offset frequency.

20. The method according to claim 16, wherein the multiphoton parallel transmit excitation further comprises an on-resonance RF excitation pulse performed by the RF transmit coil.- 31 -QB\99565311.1

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