MRI using an off-resonant RF pulse to cause a phase shift or spoiling

Abstract

The present disclosure describes systems and methods of magnetic resonance imaging (MRI) that provide B1 field mapping and B1 field spoiling without a plurality of B0 gradient fields. An off-resonant pulse may be applied, or delivered, to the object of interest following an excitation pulse to induce a phase shift, which may be measured to determine the B1 field map. Also, a spoiling off-resonant RF pulse may be applied, or delivered, to the object of interest to remove unwanted signal coherence pathways in the region of the object.

Description

MRG 0694.000002W001Bi FIELD MAPPING AND Bi SPOILING

[0001] This application claims the benefit of U.S. Provisional Application No. 63 / 684,526 filed August 19, 2024, and entitled “Bi Field Mapping and Bi Spoiling,” which is incorporated by reference in its entirety

[0002] This invention was made with government support under a Small Business Innovation Research Grant Phase I (Award No: 2323231) awarded by the National Science Foundation. The government has certain rights in the invention.

[0003] Magnetic resonance imaging (“MRI”) permits robust, high-resolution imaging with tunable image contrast that is free of ionizing radiation, thereby making it an indispensable tool in clinical medicine and biomedical research. However, the financial burden of purchasing and maintaining such a technology is a considerable draw back that limits MRI access to mostly wealthy institutions in developed countries. In recent years, researchers have focused on developing lower cost and / or portable MRI systems to address these challenges. Most of these investigations have focused on redesigning various components of the MRI system. Other approaches have targeted the stringent requirements of the static Bo field gradient systems. Conventional linear Bo-gradient coils consume precious space in the magnet bore, require significant electrical power and water cooling to operate, require maintenance, and produce loud acoustic noise, which is a common complaint of patients and potentially damaging to their hearing. Elimination of the Bo-gradient system including the Bo-gradient coils would significantly reduce the infrastructure needs and financial burden of MRI, while simultaneously permitting silent MRI.

[0004] Additionally, approaches to reduce cost and provide portability in MRI systems have primarily addressed one system component such as, e.g., the magnetic field strength, as opposed to holistically reevaluating the conventional MRI system design. Hence, such reduced cost portable MRI systems often provide little clinical value due to, e.g., the images not being of sufficient quality to satisfy radiologists reading the images due to reduced magnetic field strength.

[0005] When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field Bo) during MRI, the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a transverse excitation RF transverse field, Bi, which is in the x-y plane and which is near the Larmor frequency, the signal emitted by excited spins after the excitation signal Bi may be received and processed to form an image.

[0006] There are a variety of techniques used to determine if the Bi field produced by a magnetic resonance coil or array is homogeneous or to what degree the field is inhomogeneous. Such techniques are often referred to as Bi mapping. In general, Bi mapping techniques may either implement spatially or non-spatially resolved Bi measurements. Bi measurements are spatially resolved if one or more spatial-encoding gradients are applied during acquisition and, in contrast, Bi measurements are non-spatially resolved when spatial-encoding gradients are not utilized during Bi measurements. Among other things, Bi maps can be used to adjust transmit gain to produce a radio frequency (RF) pulse at a specific flip angle, to design multi -transmit channel RF pulses, and / or to aide in the implementation of chemical shift imaging. Bi mapping can also serve as an aide in Ti mapping and / or other quantitative magnetic resonance imaging techniques. Some Bi mapping techniques are Ti dependent. That is, the signal utilized for Bi is often weighted as a function of Ti relaxation. Other Bi mapping techniques are Bo or chemical shift dependent. Still other techniques are inaccurate over certain ranges of Bi field, and / or are dependent on large RF power depositions.

[0007] Of the Bi mapping techniques, a sub-class of such techniques contains techniques that may be referred to as magnitude-based Bi mapping techniques. For example, one such magnitude-based Bi mapping technique using the signal ratio from a a-2a flip angle sequence to determine Bi. Although such a technique is more accurate than others over a larger range of flip angles, such a technique is Bo dependent and often relies on a relatively long repetition time (TR) requirement and total imaging time making its clinical transition challenging.

[0008] Of the Bi mapping techniques, a sub-class of such techniques contains techniques that may be referred to as phase-based Bi mapping techniques. For example, one such phase-based Bi mapping technique utilizes a Bi-dependent phase produced by an adiabatic hyperbolic secanthalf-passage and / or full-passage pulses. However, adiabatic pulses are typically longer in their pulse duration than purely amplitude modulated pulses, and as a result, the specific absorption rate (SAR) associated with such techniques can limit the clinical application of such techniques at a high magnetic field.SUMMARY

[0009] The present disclosure describes illustrative MRI systems and methods that utilize frequency-modulated Rabi encoded echoes (FREE), which do not utilize or need conventional Bo gradient coils for imaging. Additionally, the illustrative MRI systems and methods may utilize pulse sequences that are tolerant to large field inhomogeneity, enabling inexpensive, compact, permanent magnets for imaging. Moreover, the illustrative MRI systems and methods do no utilize cryogens, operate silently, and do not need high power electronics or cooling systems for Bo-gradient coils.

[0010] The present disclosure relates to mapping the transverse radio frequency (RF) field, Bi, produced by a single or multi-channel RF coil (e.g., the multi-channel RF coil can be operating in an arbitrary drive mode) without using an active or passive Bo gradient magnetic field applied to an object of interest. The illustrative Bi mapping as described further herein may enable improved model-based image reconstruction by incorporating the Bi maps as prior knowledge and / or permit image-space geometric distortion correction and the fine-tuning of transmit fields’ spatial configuration for RF transmission. The single or multi-channel RF coil may be described as a RF coil assembly configured to transmit, or generate, RF field gradients in at least two directions that are not parallel to each other and are orthogonal to the magnetic field, Bo, of the polarizing magnet. In one embodiment, the RF coil assembly includes a first RF coil and a second RF coil that is configured to transmit, or generate, a RF field gradient that is not parallel to the RF field gradient transmitted, or generated, by the first RF coil.

[0011] The illustrative systems and methods may utilize frequency-modulated Rabi-encoded echoes (FREE). Various MRI techniques and processes including FREE may be described in U.S. Pat. No. 12,372,595 entitled “Multi-Echo Radio Frequency (RF) Based Spatial Encoding in Magnetic Resonance Imaging Using Frequency-Modulated RF Pulses” and issued on July 29,2025, U.S. Pat. App. Pub. No. 2024 / 0219496A1 entitled “Systems and Methods for Radio Frequency (RF) Based Spatial Encoding in Magnetic Resonance Imaging Using Frequency- Modulated RF Pulses” and published on July 4, 2024, and PCT Pat. App. Ser. No. PCT / US2024 / 030266 entitled “Magnetic Resonance Imaging Using Frequency-Modulated Radio Frequency Pulses for Frequency Encoding of Magnetic Resonance Signals” and filed on May 20, 2024, each of which are incorporated by reference in their entireties.

[0012] One illustrative method of magnetic resonance imaging (MRI) an object includes providing a magnetic field, B0, along the object, applying a radio frequency (RF) pulse sequence to induce transverse signals within a region of the object where at least one pulse of the RF pulse sequence is spatially varying in the region of the object and includes an off-resonant RF pulse to induce a spatially varying phase in the region of the object. The illustrative method further includes acquiring the transverse signals induced by the RF pulse sequence and the off-resonant RF pulse, determining a phase shift in the transverse signals caused by the off-resonant RF pulse, determining a Bi field map based on the phase shift, and reconstructing an image from the transverse signals and determined Bi field map.

[0013] One illustrative magnetic resonance imaging (MRI) system to image an object includes a polarizing magnet assembly configured to provide a magnetic field, Bo, along the object, a radio frequency (RF) coil assembly comprising at least one RF coil to transmit, or generate, a RF field orthogonal to the magnetic field of the polarizing magnet, and a computing apparatus operably coupled to the polarizing magnet assembly and the RF coil assembly and comprising one or more processors. The computing apparatus is configured to apply a RF pulse sequence using the RF coil assembly to induce transverse signals within a region of the object. The at least one pulse of the RF pulse sequence is spatially varying in the region of the object and includes an off-resonant RF pulse to induce a spatially varying phase in the region of the object. The computing apparatus is further configured to acquire the transverse signals induced by the RF pulse sequence and the off-resonant RF pulse, determine a phase shift in the transverse signals caused by the off-resonant RF pulse, determine a Bi field map based on the phase shift, and reconstruct an image from the transverse signals and determined Bi field map.

[0014] One illustrative method of magnetic resonance imaging (MRI) an object may include providing a magnetic field, Bo, along the object and applying a radio frequency (RF) pulsesequence to induce transverse signals within a region of the object. The RF pulse sequence may include a spoiling off-resonant RF pulse to remove unwanted signal coherence pathways in the region of the object. The method may further include acquiring the transverse signals induced by the RF pulse sequence and reconstructing an image from the transverse signals.

[0015] One illustrative magnetic resonance imaging (MRI) system to image an object may include a polarizing magnet assembly configured to provide a magnetic field, Bo, along the object, radio frequency (RF) coil assembly comprising at least one RF coil to transmit a RF field orthogonal to the magnetic field of the polarizing magnet; and a computing apparatus operably coupled to the polarizing magnet assembly and the RF coil assembly and comprising one or more processors. The computing apparatus may be configured to apply a RF pulse sequence using the RF coil assembly to induce transverse signals within a region of the object. The RF pulse sequence may include at least one spoiling off-resonant RF pulse to remove unwanted signal coherence pathways in the region of the object. The computing apparatus may be further configured to acquire the transverse signals induced by the RF pulse sequence and reconstruct an image from the transverse signals.

[0016] The details of one or more aspects of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the techniques described in this disclosure will be apparent from the description and drawings, and from the claims.BRIEF DESCRIPTION OF DRAWINGS

[0017] FIG. 1 is a block diagram of an illustrative method of magnetic resonance imaging (MRI) using Bi mapping.

[0018] FIG. 2A is an illustrative RF pulse sequence.

[0019] FIG. 2B is a graphical depiction of an actual Bi field versus a mapped Bi field.

[0020] FIG. 2C is another illustrative RF pulse sequence for Bi mapping without using an off-resonant pulse but using a FREE sequence only.

[0021] FIGS. 3 A — 3C are illustrative RF pulse sequences including off-resonant, or Bloch- Siegert, pulses.

[0022] FIG. 4 is a block diagram of an illustrative method of image reconstruction of the method of FIG 1.

[0023] FIGS. 5A-5D are illustrative RF pulse sequences providing Bloch-Siegert crushing.

[0024] FIG. 6 is a block diagram of an illustrative magnetic resonance imaging (“MRI”) system that may be used to perform, or execute, the methods, processes, and techniques described in some embodiments of the present disclosure.

[0025] FIG. 7 is a block diagram of an illustrative model-based image reconstruction system that may be used to perform, or execute, the methods, processes, and techniques described in some embodiments of the present disclosure.

[0026] FIG. 8 is a block diagram of illustrative components that can implement the systems of FIGS . 6 & 7.DETAILED DESCRIPTION

[0027] In the following detailed description of illustrative embodiments, reference is made to the accompanying figures of the drawing which form a part thereof, and in which are shown, by way of illustration, specific embodiments which may be practiced. It is to be understood that other embodiments may be utilized, and structural changes may be made without departing from (e.g., still falling within) the scope of the disclosure presented hereby.

[0028] Illustrative systems and methods shall be described with reference to Figures 1-8. It will be apparent to one skilled in the art that elements or processes from one embodiment may be used in combination with elements or processes of the other embodiments, and that the possible embodiments of such systems and methods using combinations of features set forth herein is not limited to the specific embodiments shown in the Figures and / or described herein. Further, it will be recognized that the embodiments described herein may include many elements that are not necessarily shown to scale. Still further, it will be recognized that timing of the processes and the size and shape of various elements herein may be modified but still fall within the scope of thepresent disclosure, although certain timings, one or more shapes and / or sizes, or types of elements, may be advantageous over others.

[0029] The Bloch-Siegert shift is an apparent change in the resonant frequency of a spin due to the presence of a far off-resonance Bi field. “Far off-resonance” may be defined as a relative measure of the off-resonance frequency and the Bi field magnitude asW.RF(1)>

[0030] which may be much less than 1 but can be arbitrarily close to 1. The additional frequency offset induced by the off-resonant, or Bloch-Siegert, pulse is

[0031] It is to be understood that Bi as well as COBS may be spatially varying or a function of spatial position. The additional phase accrued by a spin due to the BS pulse (duration denoted by T) is the time integral of C BS, which may be written as

[0032] Bi mapping is conventionally achieved in conjunction with Bo gradients to encode information regarding the Bi field into the magnitude or phase of the resultant image. Further, it is to be understood that any RF pulse can be used to affect the Bloch-Siegert shift, but some are more advantageous than others.

[0033] Phase-based Bi mapping may be described as being advantageous over magnitudebased mapping methods in that the result is not dependent on the relaxation, diffusion, or other magnetic properties of the imaged object, except for those influencing the Bo field, whose effect may be removed by reference scan or using a spin-echo sequence. Using the Bloch-Siegert effect, the Bi field can be ascertained by applying an off-resonant, or Bloch-Siegert, pulse between spin excitation and signal reception, with flexible placement of a Bi image encoding module or other pulse sequences, which may encode in up to 3 spatial dimensions. In one embodiment, the Bi image encoding module or pulse sequence may utilize FREE. In other embodiments, the Bi image encoding module or pulse sequence may be any Bi -based image encoding module or pulse sequence capable of providing spatial-encoding.

[0034] An illustrative method 200 of magnetic resonance imaging (MRI) an object is depicted in FIG. 1. As shown, the method 200 includes applying a radio frequency (RF) pulse sequence 202 to induce transverse signals (e.g., rotary echoes, spin echoes, etc.) within a region of the object. The RF pulse sequence may be any pulse sequence capable of spatially-encoding a gradient across a transverse direction (x ory) or both transverse directions (x and y). It is to be understood that the method 200 of FIG. 1 may not utilize or need conventional Bo gradient coils nor an active or passive Bo field gradient applied to the object. In one embodiment, the method 200 may utilize the longitudinal encoding, Mz, produced by the rotary signals (e.g., rotary echoes) inducted by the RF pulse sequence. In one embodiment, the method 200 may utilize a passive Bo field gradient. In one embodiment, the method 200 may utilize the spin or stimulated echoes produced by the RF pulse sequence, with or without active or passive Bo field gradient(s). In one embodiment, the method 200 may utilize an active or pulsed Bo field gradient. In one embodiment, the method 200 may utilize a single Bo field gradient (as opposed to two or more or a plurality of Bo field gradients). The RF pulse sequence may be applied, or delivered, using one or more RF coils in a RF coil assembly positioned about the object of interest.

[0035] Generally, the RF pulse sequence may first include a RF excitation pulse or magnetization preparation module to excite spins in the region of the object (e.g., a plurality of nuclei). The RF excitation pulse may be described as a RF pulse tuned to a resonant frequency (e.g., near the Larmor frequency) of a plurality of nuclei subjected to a magnetic field. As such,the application of the RF excitation pulse places the nuclei in an excited state. The parameters of the RF excitation pulse may be arbitrarily chosen by the user.

[0036] The RF pulse sequence may utilize frequency-modulated Rabi encoded echoes (FREE). For example, and more specifically, the RF pulse sequence may further include a first frequency-modulated RF pulse to phase the spins excited by the RF excitation pulse and a second frequency-modulated RF pulse to rephase the spins dephased by the first frequency- modulated RF pulse. The first frequency-modulated RF pulse may be spatially varying in the region of the object so as to provide spatial encoding. In one or more embodiments, the RF pulse sequence comprises at least one adiabatic full passage (AFP) RF pulse.

[0037] The RF pulse sequence may further include an off-resonant pulse, which may be referred to as a Bloch-Siegert pulse, for use providing Bi field mapping. After application of the first resonating RF pulse, a first off-resonant RF pulse is applied to the plurality of excited nuclei. An off-resonant RF pulse is an RF pulse tuned such that that application of the off- resonant RF pulse to a plurality of nuclei does not place the plurality of nuclei in an excited state. For example, an off-resonant RF pulse is an RF pulse having a particular shape or frequency such that the application thereof to a plurality of nuclei subjected to a magnetic field will not be excited or will be excited to a minimal extent. The application of this off-resonant RF pulse occurs while the plurality of nuclei are already in an excited state, e.g., due to the excitation RF pulse. The application of the off-resonant RF pulse causes the resonance frequency of the plurality of excited nuclei to shift. Such a shift is often referred to as a Bloch-Siegert shift. The magnitude of such a shift is dependent on the Bi field applied to the plurality of excited nuclei.

[0038] Moreover, it may be described that the off-resonant, or Bloch-Siegert, RF pulses are highly off-resonant (a>RF> a)^ and add a spatial phase proportional to the square of the RF transmit field without significantly exciting spins. The off-resonant, or Bloch-Siegert, RF pulses produce equivalent transverse spoiling to Bo crushing and that Bloch-Siegert Bi mapping smoothly integrates with FREE. The Bloch-Siegert Bi mapping optimizes sequences, contrasts, and reconstructions by enabling users to map the Bi transmit field and crush unwanted, incoherent signals.

[0039] To completely remove Bo gradient coils, Bi imaging is used map its own transmission fields. By leveraging FREE with Bloch-Siegert Bi mapping as described herein, an RF field maybe mapped without Bo gradient coils. In the pulse sequence 300 shown in FIG. 2A, the Bi field of the x-channel is mapped. After excitation 301, the x-channel immediately transmits an off- resonant, or Bloch-Siegert (e.g., Fermi), RF pulse 302 departing a spatial phase proportional to the squared Bi field before spatial encoding for imaging. Two-dimensional phase-encoded multishot FREE 305, 306 may be used to perform the imaging such as described in, e.g., U.S. Pat. No. 12,372,595 entitled “Multi-Echo Radio Frequency (RF) Based Spatial Encoding in Magnetic Resonance Imaging Using Frequency-Modulated RF Pulses” and issued on July 29, 2025, which is incorporated herein by reference in its entirety. The FREE and Bloch-Siegert processes map the Bi field without using Bo gradient coils. The resultant echoes 308, which are measured, can be used to reconstruct an image with a Bi phase that determines the Bi map.

[0040] A potential application of the illustrative pulse sequence 300 of Figure 2A is two orthogonal RF coils with spatially varying fields to transmit the two sections of the pulse sequence. It may be described that the goal of the pulse sequence 300 of Figure 2A is to image a 2D object without Bo gradient coils and depart a phase in the x RF coil that can be used to determine a Bi field map from that coil. The first RF coil transmits the RF pulses 305 with the superscript “y.” This RF pulse spatially encodes the y-dimension using FREE. The second RF coil transmits the RF pulses with the superscript “x,” this includes a Fermi pulse 302, which will depart a Bi phase that enables a Bi field map, and a FREE pulse module 306 that will spatially encode the x-direction. The pulse lengths (TP), in this version of FREE, may be systematically varied according to an integer value m and n to sweep the pulse length in accordance with a desired &-space trajectory. The pulse sequence 300 of Figure 2A demonstrates one iteration of merging Bloch-Siegert RF pulses with FREE to determine both a Bi field map and image. As opposed to sweeping the pulse durations of the RF pulse, any version of FREE may be used with the illustrative Bl field mapping described herein.

[0041] A simulation where the Bi field was mapped by the FREE and Bloch-Siegert processes of the present disclosure is depicted in FIG. 2B. To map the -channel, the sequence would be repeated, but the off-resonant, or Bloch-Siegert, Fermi RF pulse would be transmitted from the y-channel immediately after excitation.

[0042] The pulse sequence 400 shown in FIG. 2C may be described as leveraging FREE's ability to produce a phase shift that is linearly varying with the Bi field. The illustrative pulsesequence 400 of FIG. 2B. uses only the FREE sequence having one excitation pulse 401 and two adiabatic refocus pulses 402, 403 with different time bandwidth products. The first adiabatic pulse 402 may define a time, Tp, and a positive FM polarity, and the second adiabatic pulse 403 may define a time, Tp + mTp, and a positive FM polarity to produce a phase shift. Such produced phase shift may be subtracted with the phase shift produced by another FREE sequence but with adiabatic pulses 402, 403 either switched in time, bandwidth, or FM polarity as shown in the reference scan 405. This subtracted phase shift may then provide a phase that is not affected by Bo effects and is linearly proportional to the Bi field. The Bi map may thereby be calculated using a Bloch simulation of the pulse sequence described that produces a polynomial fit required to go from phase to Bi field.

[0043] Illustrative RF pulse sequences 450, 460, 470 including off-resonant, or Bloch- Siegert, pulses 455 are depicted in FIGS. 3A-3C. As shown in each pulse sequence 450, 460, 470, an excitation RF pulse 451 may be first delivered, or applied, to the object of interest at a resonant frequency (e.g., near the Larmor frequency) of a plurality of nuclei subjected to a magnetic field. One or more imaging encoding modules 456 of the pulse sequence may then be delivered to the object of interest, and the off-resonant, or Bloch-Siegert, RF pulse 455 may be delivered, or applied, to the object of interest before the one or more imaging encoding modules 456 as shown in FIG. 3 A, after the one or more imaging encoding modules 456 as shown in FIG. 3B, or between at least two imaging encoding modules 456 as shown in FIG. 3C. Following the delivery of the pulse sequence, the signal 458 from the object of interest may be measured as designated as “ADC” in the FIGS. 3A-3C.

[0044] The method 200 further includes acquiring the transverse signals 204 induced by the RF pulse sequence delivered in process 202. Acquisition or measurement of the transverse signals 204 may be performed by one or more RF coils that are part of the RF coil assembly or a different set of receiver RF coils positioned with respect to the object of interest. As described herein, the transverse signals may be spatially-encoded by the RF pulse sequence, and knowledge of the spatial-encoding may be utilized to reconstruct an image of the object 206 based on the transverse signals. In other words, an image may be reconstructed 206 from the transverse signals using, e.g., the mathematical framework described herein. For example, an iterative, regularized least square approach may be used to infer the image during imagereconstructions. Further, it is to be understood that the transverse signals that are, acquired or measured, may be induced by the RF pulse sequence with or without the off-resonant pulse used for Bi mapping as described herein.

[0045] A more detailed, illustrative method of image reconstruction 206 of FIG 1 is depicted in FIG. 4. Although the processes or steps as shown are depicted in a particular order, it is to be understood that some processes or steps may be performed or executed in a different order or simultaneously with other processes or steps. As shown, an image may be generated 209 using the received transverse signals and knowledge of the spatial encoding of the transverse signals. For example, generation of the image 209 may include simulating or generating a model of the object being analyzed (e.g., using Bloch equations, (e.g., a set of three coupled differential equations), using Cayley-Klein parameters, etc.) and solving the inverse problem using the model and received transverse signals. In some cases, an analytical expression, derived from the Bloch equations, may be provided for the model, and in such case, the model can be directly calculated.

[0046] Further, image reconstruction 206 may include determining a phase shift in the transverse signals 210 caused by the off-resonant RF pulse. In other words, the transverses signals may be analyzed to determine the off-resonant RF pulse induced phase shift. The determined phase shift may then be used to determine a Bi field map 212. Once the Bi field map is determined, the image may be adjusted 214 based on the determined Bi field map. Additionally, it is to be understood that such processes may be performed iteratively or looped, e.g., until a maximum number of iterations or some convergence criteria is met, as depicted by the arrow extending from process 214 to process 210. In other words, the image may be generated 209, a phase shift may be determined 210, Bi field map may be generated 212, the Bi field map may be used to adjust, or correct, the image 214, and then such process may be repeated or looped.

[0047] In an ideal case, a 180-degree radio frequency (RF) pulse will only invert spins. But RF pulse imperfections and bandwidth limitations can cause all RF pulses (e.g., not just 180- degree pulses) to produce unwanted transverse magnetization that could interfere with signals of interest. Additionally, these signals can build into unwanted coherences that persist from cycle to cycle in any MRI pulse sequence. Spoiling refers to the disruption of these unwanted transversecoherences. In traditional MRI systems, spoiling may be executed either by modulating the initial phase of the RF pulse in an algorithmic approach (referred to RF spoiling) or by rapidly switching Bo gradients (referred to as Bo crushing). In most pulse sequences, Bo crushing is placed about RF inversion pulses.

[0048] Bi-encoded systems, such as described herein, primarily encodes with Bi gradients, rather than Bo gradients and often lack active Bo gradient coils, and thus, and such Bi-encoded systems do not and cannot crush unwanted coherence signals using traditional Bo crushing. Without a crushing-equivalent, Bi-encoded systems may not provide the highest image quality due to unwanted coherence pathways potentially interfering with signals of interest.

[0049] The illustrative systems and methods described herein may be configured to remove unwanted transverse coherences by using a Bi gradient and the Bloch-Siegert shift, which may be referred to as Bloch-Siegert crushing. The Bloch-Siegert crushing may be described as enabling Bi encoded systems to crush unwanted transverse magnetization and improve overall image quality.

[0050] The following equation defines the frequency modulation of an RF pulse:(4).

[0051] where coo is the Larmor frequency of interest, CORF is the variable pulse frequency, and z' is a unit vector indicating a field in the z-direction. The amplitude modulation function of an RF pulse is simply defined as coi(t) and is set to the envelop of the RF amplitude waveform.

[0052] The Bloch-Siegert shift occurs under the scenario in which:

[0053] During this case, the resonance frequency of the nucleus shifts and the off-resonant pulse acts as an additional contribution to the static Bo field. An analytical equation for the Bloch-Siegert shift frequency is as follows:

[0054] The Bloch-Siegert shift may utilize only an RF coil and yet induce a field change in space similar to a change in the magnetic field of the system. This unique feature enables the illustrative systems to mimic Bo gradient coils and their actions during a Bo crushing event. To mimic Bo crushing, an RF field gradient, such as the field produced by a surface coil, may be utilized and an RF pulse may be transmitted across space with a Bloch-Siegert shift. As described herein, an off-resonant, or Bloch-Siegert, pulse induces a frequency shift of the spins across space, so that during the RF pulse. The spatial frequencies of the object are:

[0055] The off-resonant, or Bloch-Siegert, pulses may include, but are not limited to, hyperbolic secant pulses, Fermi pulses, and hard pulses with a constant resonance offset. The spatial variation of frequencies the Bloch-Siegert and RF Field gradient create a spatial phase that is proportional to the square of the Bloch-Siegert shift across space. Bloch-Siegert crushing may be described as conceptually equivalent to modulating the Bo field through traditional Bo gradients. This spatial phase has previously been used for Bi mapping and Bi image encoding. In this use case for Bi crushing, the spatial phase is used to mimic Bo crushing.

[0056] Further, off-resonant, or Bloch-Siegert, pulses may be used to induce large amounts of spatial phase across space before and after inversion pulses to perform Bloch-Siegert crushing. This rapid inducing of a spatial phase rapidly dephases transverse signals produced by imperfect RF pulses thereby crushing unwanted coherence pathways.

[0057] Examples of Bi spoiling or crushing during Bi-encoded pulse sequences 500, 501, 502, 503 are depicted in FIGS. 5A-5D. In the example in Fig. 5A, off-resonant, or Bloch- Siegert, pulses, 510 which, in this case, are Fermi pulses, are applied or delivered about (e.g., before and after) the adiabatic full passage pulses 512 used for encoding and spin echo refocusing. The Fermi pulses 510 used for Bloch-Siegert crushing are applied at a resonance offset that satisfies Equation 5. The off-resonant, or Bloch-Siegert, pulses 510 may be describedas serving the same function as Bo crushers. Further, the off-resonant, or Bloch- Seigert, amplitude modulation function can be a Fermi pulse as shown in FIG. 5A or as a hard pulse, Hyperbolic secant, or many others.

[0058] The Fermi pulses 510 introduce a large amount of spatial phase that is proportional to the spatially-varying RF field that is transmitting the RF pulse and rapidly de-phases the transverse magnetization and removes imperfections from the adiabatic full passage pulse 512. It is to be understood that any RF pulse can be used in place of the adiabatic full passage pulse. The illustrative sequences 501, 502, 503 of Figures 5B, 5C, and 5D show three other potential placements of the Bloch Siegert Bi spoiling pulse 510. It is to be understood that one, two or more, or all of the illustrative pulse sequences 500, 501, 502, 503, or portions thereof, shown in Figures 5A-5D may be utilize during the same imaging sequences (e.g., at the same time) in efforts of removing the unwanted transverse magnetization during an MRI pulse sequence with or without Bo gradient coils. Further, it is to be understood that, while only one RF pulse is shown in Figures 5A-5D, any number of arbitrary RF pulses 512 may be used in an RF pulse sequence with or without Bo gradient coils. Additionally, as shown in FIG. 5D, a dead time period 514 may be utilized between the arbitrary RF pulse 512 and the Fermi pulse 510. The dead time period 514 may be described as a period of time sufficient to allow return to thermal equilibrium and allow the decay of the spins in the longitudinal and transverse magnetization directions. The dead time period 514 may be between about 0.5 seconds and about 10 seconds. In one embodiment, the dead time period 514 is 3 seconds. In one or more embodiments, the dead time period 514 is greater than or equal to 0.5 seconds, greater than or equal to 1 second, greater than or equal to 2 seconds, greater than or equal to 3 seconds, or greater than or equal to 5 seconds, and / or less than or equal to 10 seconds, less than or equal to 8 seconds, less than or equal to 6 seconds, or less than or equal to 4 seconds.

[0059] Thus, the illustrative systems and methods may use a Bi-encoded pulse sequence that utilizes RF pulses and RF field gradients to spoil or crush unwanted transverse coherences by using off-resonant, or Bloch Siegert, pulses to induce a spatial phase across space that, among other things, may enable Bi -encoded systems to share in the benefits of Bo crushing.

[0060] An illustrative magnetic resonance imaging (MRI) system 700 that can implement the processes and methods described herein is illustrated in FIG. 6. The MRI system 700 includes anoperator workstation702 that may include a display 704, one or more input devices 706 (e.g., a keyboard, a mouse), and a processor 708. The processor 708 may include a commercially available programmable machine running a commercially available operating system. The operator workstation 702 provides an operator interface that facilitates entering scan parameters into the MRI system 700. The operator workstation 702 may be coupled to different servers, including, for example, a pulse sequence server 710, a data acquisition server 712, a data processing server 714, and a data store server 716. The operator workstation 702 and the servers 710, 712, 714, and 716 may be connected via a communication system 740, which may include wired or wireless network connections.

[0061] The pulse sequence server 710 functions in response to instructions provided by the operator workstation 702 to operate a radiofrequency (RF) system 720, and optionally a gradient system 718. In some configurations as described herein, the MRI system 700 may not include a gradient system 718, and instead, spatial encoding can be provided by using frequency encoded and / or phased encoded FREE techniques described in the present disclosure to generate Bl gradients with the RF system 720. In other words, despite the gradient system 718 shown in FIG. 6, the gradient system 718 as well as the B0 gradient coils in assembly 722 associated therewith may not be utilized using the illustrative methods, processes, and techniques describe herein.

[0062] When the gradient system 718 is present and implemented for partial spatial encoding (e g., slice encoding), gradient waveforms for performing a prescribed scan are produced and applied to the gradient system 718, which then excites Bo gradient coils in an assembly 722 to produce the magnetic field gradients that are used for spatially encoding magnetic resonance signals along spatial dimensions otherwise not encoded using the methods, processes, and techniques described in the present disclosure. In these configurations, the gradient coil assembly 722 can form part of a magnet assembly 724 that includes a polarizing Bo magnet 726 and a whole-body RF coil 728.

[0063] RF waveforms are applied by the RF system 720 to the RF coil 728, or a separate local coil to perform the prescribed magnetic resonance pulse sequence. Responsive magnetic resonance signals detected by the RF coil 728, or a separate local coil, are received by the RF system 720. The responsive magnetic resonance signals may be amplified, demodulated, filtered, and digitized under direction of commands produced by the pulse sequence server 710. The RFsystem 720 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 710 to produce RF pulses of the desired frequency, phase, and pulse amplitude waveform. The generated RF pulses may be applied to the whole-body RF coil 728 or to one or more local coils or coil arrays.

[0064] The RF system 720 also includes one or more RF receiver channels. A RF receiver channel includes an RF preamplifier that amplifies the magnetic resonance signal received by the coil 728 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 a sampled point by the square root of the sum of the squares of the I and Q components:

[0065] and the phase of the received magnetic resonance signal may also be determined according to the following relationship:

[0066] The pulse sequence server 710 may receive patient data from a physiological acquisition controller 730. By way of example, the physiological acquisition controller 730 may receive signals from a number of different sensors connected to the patient, including electrocardiogram (ECG) signals from electrodes, or respiratory signals from a respiratory bellows or other respiratory monitoring devices. These signals may be used by the pulse sequence server 710 to synchronize, or “gate,” the performance of the scan with the subject's heart beat or respiration.

[0067] The pulse sequence server 710 may also connect to a scan room interface circuit 732 that receives signals from various sensors associated with the condition of the patient and themagnet system. Through the scan room interface circuit 732, a patient positioning system 734 can receive commands to move the patient to desired positions during the scan.

[0068] The digitized magnetic resonance signal samples produced by the RF system 720 are received by the data acquisition server 712. The data acquisition server 712 operates in response to instructions downloaded from the operator workstation 702 to receive the real-time magnetic resonance data and provide buffer storage, so that data is not lost by data overrun. In some scans, the data acquisition server 712 passes the acquired magnetic resonance data to the data processor server 714. In scans that require information derived from acquired magnetic resonance data to control the further performance of the scan, the data acquisition server 712 may be programmed to produce such information and convey it to the pulse sequence server 710.

[0069] The data processing server 714 receives magnetic resonance data from the data acquisition server 712 and processes the magnetic resonance data in accordance with instructions provided by the operator workstation702. 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.

[0070] Images reconstructed by the data processing server 714 are conveyed back to the operator workstation 702 for storage. Real-time images may be stored in a data base memory cache, from which they may be output to operator display 704 or a display 736. Batch mode images or selected real time images may be stored in a host database on disc storage 738. When such images have been reconstructed and transferred to storage, the data processing server 714 may notify the data store server 716 on the operator workstation 702. The operator workstation 702 may be used by an operator to archive the images, produce films, or send the images via a network to other facilities.

[0071] The MRI system 700 may also include one or more networked workstations 742. For example, a networked workstation 742 may include a display 744, one or more input devices 746 (e.g., a keyboard, a mouse), and a processor 748. The networked workstation 742 may be locatedwithin the same facility as the operator workstation 702, or in a different facility, such as a different healthcare institution or clinic.

[0072] The networked workstation 742 may gain remote access to the data processing server 714 or data store server 716 via the communication system 740. Accordingly, multiple networked workstations 742 may have access to the data processing server 714 and the data store server 716. In this manner, magnetic resonance data, reconstructed images, or other data may be exchanged between the data processing server 714 or the data store server 716 and the networked workstations 742, such that the data or images may be remotely processed by a networked workstation742.

[0073] Referring now to FIG. 7, an example of a system 800 for reconstructing images from data acquired using the illustrative methods, process, and techniques described herein, which in some instances may be a model-based image reconstruction framework, is shown. As shown in FIG. 7, a computing device 850 can receive one or more types of data (e.g., k-space data, receiver coil sensitivity data) from data source 802. In some embodiments, computing device 850 can execute at least a portion of an image reconstruction system 804 to reconstruct images from magnetic resonance data (e.g., k-space data) acquired using a FREE technique. In some embodiments, the image reconstruction system 804 can implement a model-based image reconstruction, as described above.

[0074] Additionally or alternatively, in some embodiments, the computing device 850 can communicate information about data received from the data source 802 to a server 852 over a communication network 854, which can execute at least a portion of the image reconstruction system 804. In such embodiments, the server 852 can return information to the computing device 850 (and / or any other suitable computing device) indicative of an output of the image reconstruction system 804.

[0075] In some embodiments, computing device 850 and / or server 852 can be any suitable computing device or combination of devices, such as a desktop computer, a laptop computer, a smartphone, a tablet computer, a wearable computer, a server computer, a virtual machine being executed by a physical computing device, etc. The computing device 850 and / or server 852 can also reconstruct images from the data.

[0076] In some embodiments, data source 802 can be any suitable source of data (e.g., measurement data, images reconstructed from measurement data, processed image data), such as an MRI system, another computing device (e.g., a server storing measurement data, images reconstructed from measurement data, processed image data), etc. In some embodiments, data source 802 can be local to computing device 850. For example, data source 802 can be incorporated with computing device 850 (e.g., computing device 850 can be configured as part of a device for measuring, recording, estimating, acquiring, or otherwise collecting or storing data). As another example, data source 802 can be connected to computing device 850 by a cable, a direct wireless link, etc. Additionally or alternatively, in some embodiments, data source 802 can be located locally and / or remotely from computing device 850 and can communicate data to computing device 850 (and / or server 852) via a communication network (e.g., communication network 854).

[0077] In some embodiments, communication network 854 can be any suitable communication network or combination of communication networks. For example, communication network 854 can include a Wi-Fi network (which can include one or more wireless routers, one or more switches, etc.), a peer-to-peer network (e.g., a Bluetooth network), a cellular network (e.g., a 3G network, a 4G network, etc., complying with any suitable standard, such as CDMA, GSM, LTE, LTE Advanced, WiMAX, etc.), other types of wireless network, a wired network, etc. In some embodiments, communication network 854 can be a local area network, a wide area network, a public network (e.g., the Internet), a private or semi-private network (e.g., a corporate or university intranet), any other suitable type of network, or any suitable combination of networks. Communications links shown in FIG. 7 can each be any suitable communications link or combination of communications links, such as wired links, fiber optic links, Wi-Fi links, Bluetooth links, cellular links, etc.

[0078] Referring now to FIG. 8, an example of hardware 900 that can be used to implement data source 802, computing device 850, and server 852 in accordance with some embodiments of the systems and methods described in the present disclosure is shown.

[0079] As shown in FIG. 8, in some embodiments, computing device 850 can include a processor 902, a display 904, one or more inputs 906, one or more communication systems 908, and / or memory 910. In some embodiments, processor 902 can be any suitable hardwareprocessor or combination of processors, such as a central processing unit (CPU), a graphics processing unit (GPU), etc. In some embodiments, display 904 can include any suitable display devices, such as a liquid crystal display (LCD) screen, a light-emitting diode (LED) display, an organic LED (OLED) display, an electrophoretic display (e.g., an “e-ink” display), a computer monitor, a touchscreen, a television, etc. In some embodiments, inputs 906 can include any suitable input devices and / or sensors that can be used to receive user input, such as a keyboard, a mouse, a touchscreen, a microphone, etc.

[0080] In some embodiments, communications systems 908 can include any suitable hardware, firmware, and / or software for communicating information over communication network 854 and / or any other suitable communication networks. For example, communications systems 908 can include one or more transceivers, one or more communication chips and / or chip sets, etc. In a more particular example, communications systems 908 can include hardware, firmware, and / or software that can be used to establish a Wi-Fi connection, a Bluetooth connection, a cellular connection, an Ethernet connection, etc.

[0081] In some embodiments, memory 910 can include any suitable storage device or devices that can be used to store instructions, values, data, or the like, that can be used, for example, by processor 902 to present content using display 904, to communicate with server 852 via communications system(s) 908, etc. Memory 910 can include any suitable volatile memory, non-volatile memory, storage, or any suitable combination thereof. For example, memory 910 can include random-access memory (RAM), read-only memory (ROM), electrically programmable ROM (EPROM), electrically erasable ROM (EEPROM), other forms of volatile memory, other forms of non-volatile memory, one or more forms of semi-volatile memory, one or more flash drives, one or more hard disks, one or more solid state drives, one or more optical drives, etc. In some embodiments, memory 910 can have encoded thereon, or otherwise stored therein, a computer program for controlling operation of computing device 850. In such embodiments, processor 902 can execute at least a portion of the computer program to present content (e.g., images, user interfaces, graphics, tables), receive content from server 852, transmit information to server 852, etc. For example, the processor 902 and the memory 910 can be configured to perform the illustrative methods, process, and techniques described herein.

[0082] In some embodiments, server 852 can include a processor 912, a display 914, one or more inputs 916, one or more communications systems 918, and / or memory 920. In some embodiments, processor 912 can be any suitable hardware processor or combination of processors, such as a CPU, a GPU, etc. In some embodiments, display 914 can include any suitable display devices, such as an LCD screen, LED display, OLED display, electrophoretic display, a computer monitor, a touchscreen, a television, etc. In some embodiments, inputs 916 can include any suitable input devices and / or sensors that can be used to receive user input, such as a keyboard, a mouse, a touchscreen, a microphone, etc.

[0083] In some embodiments, communications systems 918 can include any suitable hardware, firmware, and / or software for communicating information over communication network 854 and / or any other suitable communication networks. For example, communications systems 918can include one or more transceivers, one or more communication chips and / or chip sets, etc. In a more particular example, communications systems 918 can include hardware, firmware, and / or software that can be used to establish a Wi-Fi connection, a Bluetooth connection, a cellular connection, an Ethernet connection, etc.

[0084] In some embodiments, memory 920 can include any suitable storage device or devices that can be used to store instructions, values, data, or the like, that can be used, for example, by processor 912 to present content using display 914, to communicate with one or more computing devices 850, etc. Memory 920 can include any suitable volatile memory, nonvolatile memory, storage, or any suitable combination thereof. For example, memory 920 can include RAM, ROM, EPROM, EEPROM, other types of volatile memory, other types of nonvolatile memory, one or more types of semi-volatile memory, one or more flash drives, one or more hard disks, one or more solid state drives, one or more optical drives, etc. In some embodiments, memory 920 can have encoded thereon a server program for controlling operation of server 852. In such embodiments, processor 912 can execute at least a portion of the server program to transmit information and / or content (e.g., data, images, a user interface) to one or more computing devices 850, receive information and / or content from one or more computing devices 850, receive instructions from one or more devices (e.g., a personal computer, a laptop computer, a tablet computer, a smartphone), etc.

[0085] In some embodiments, the server 852 is configured to perform the methods, processes, and techniques described in the present disclosure. For example, the processor 912 and memory 920 can be configured to perform the illustrative methods, processes, and techniques described herein.

[0086] In some embodiments, data source 802 can include a processor 922, one or more data acquisition systems 924, one or more communications systems 926, and / or memory 928. In some embodiments, processor 922 can be any suitable hardware processor or combination of processors, such as a CPU, a GPU, etc. In some embodiments, the one or more data acquisition systems 924 are generally configured to acquire data, images, or both, and can include an MRI system. Additionally or alternatively, in some embodiments, the one or more data acquisition systems 924 can include any suitable hardware, firmware, and / or software for coupling to and / or controlling operations of an MRI system. In some embodiments, one or more portions of the data acquisition system(s) 924 can be removable and / or replaceable.

[0087] Note that, although not shown, data source 802 can include any suitable inputs and / or outputs. For example, data source 802 can include input devices and / or sensors that can be used to receive user input, such as a keyboard, a mouse, a touchscreen, a microphone, a trackpad, a trackball, etc. As another example, data source 802 can include any suitable display devices, such as an LCD screen, an LED display, an OLED display, an electrophoretic display, a computer monitor, a touchscreen, a television, etc., one or more speakers, etc.

[0088] In some embodiments, communications systems 926 can include any suitable hardware, firmware, and / or software for communicating information to computing device 850 (and, in some embodiments, over communication network 854 and / or any other suitable communication networks). For example, communications systems 926 can include one or more transceivers, one or more communication chips and / or chip sets, etc. In a more particular example, communications systems 926 can include hardware, firmware, and / or software that can be used to establish a wired connection using any suitable port and / or communication standard (e.g., VGA, DVI video, USB, RS-232, etc.), Wi-Fi connection, a Bluetooth connection, a cellular connection, an Ethernet connection, etc.

[0089] In some embodiments, memory 928 can include any suitable storage device or devices that can be used to store instructions, values, data, or the like, that can be used, forexample, by processor 922 to control the one or more data acquisition systems 924, and / or receive data from the one or more data acquisition systems 924 to generate images from data, to present content (e.g., data, images, a user interface) using a display, to communicate with one or more computing devices 850, etc. Memory 928 can include any suitable volatile memory, nonvolatile memory, storage, or any suitable combination thereof. For example, memory 928 can include RAM, ROM, EPROM, EEPROM, other types of volatile memory, other types of nonvolatile memory, one or more types of semi-volatile memory, one or more flash drives, one or more hard disks, one or more solid state drives, one or more optical drives, etc. In some embodiments, memory 928 can have encoded thereon, or otherwise stored therein, a program for controlling operation of medical image data source 802. In such embodiments, processor 922 can execute at least a portion of the program to generate images, transmit information and / or content (e g., data, images, a user interface) to one or more computing devices 850, receive information and / or content from one or more computing devices 850, receive instructions from one or more devices (e.g., a personal computer, a laptop computer, a tablet computer, a smartphone, etc.), etc.

[0090] In some embodiments, any suitable computer-readable media can be used for storing instructions for performing the functions and / or processes described herein. For example, in some embodiments, computer-readable media can be transitory or non-transitory. For example, non-transitory computer-readable media can include media such as magnetic media (e.g., hard disks, floppy disks), optical media (e.g., compact discs, digital video discs, Blu-ray discs), semiconductor media (e.g., RAM, flash memory, EPROM, EEPROM), any suitable media that is not fleeting or devoid of any semblance of permanence during transmission, and / or any suitable tangible media. As another example, transitory computer-readable media can include signals on networks, in wires, conductors, optical fibers, circuits, or any suitable media that is fleeting and devoid of any semblance of permanence during transmission, and / or any suitable intangible media.

[0091] As used herein in the context of computer implementation, unless otherwise specified or limited, the terms “component,” “system,” “module,” “framework,” and the like are intended to encompass part or all of computer-related systems that include hardware, software, a combination of hardware and software, or software in execution. For example, a component may be, but is not limited to being, a processor device, a process being executed (or executable) by aprocessor device, an object, an executable, a thread of execution, a computer program, or a computer. By way of illustration, both an application running on a computer and the computer can be a component. One or more components (or system, module, etc.) may reside within a process or thread of execution, may be localized on one computer, may be distributed between two or more computers or other processor devices, or may be included within another component (or system, module, etc.).

[0092] In some implementations, devices or systems disclosed herein can be utilized or installed using methods embodying aspects of the disclosure. Correspondingly, description herein of particular features, capabilities, or intended purposes of a device or system is generally intended to inherently include disclosure of a method of using such features for the intended purposes, a method of implementing such capabilities, and a method of installing disclosed (or otherwise known) components to support these purposes or capabilities. Similarly, unless otherwise indicated or limited, discussion herein of any method of manufacturing or using a particular device or system, including installing the device or system, is intended to inherently include disclosure, as embodiments of the disclosure, of the utilized features and implemented capabilities of such device or system.EXAMPLES

[0093] Example Exl : A method of magnetic resonance imaging (MRI) an object comprising: providing a magnetic field, Bo, along the object; applying a radio frequency (RF) pulse sequence to induce transverse signals within a region of the object, wherein the RF pulse sequence comprises an off-resonant RF pulse that is spatially varying in the region of the object and induces a spatially varying phase in the region of the object; acquiring the transverse signals induced by the RF pulse sequence; determining a phase shift in the transverse signals caused by the off-resonant RF pulse; determining a Bi field map based on the phase shift; and reconstructing an image from the transverse signals and determined Bi field map.

[0094] Example Ex2: A magnetic resonance imaging (MRI) system to image an object comprising:a polarizing magnet assembly configured to provide a magnetic field, Bo, along the object; radio frequency (RF) coil assembly comprising at least one RF coil to transmit a RF field orthogonal to the magnetic field of the polarizing magnet; and a computing apparatus comprising one or more processors and configured to: apply a RF pulse sequence using the RF coil assembly to induce transverse signals within a region of the object, wherein at least one pulse of the RF pulse sequence is spatially varying in the region of the object and comprises an off-resonant RF pulse to induce a spatially varying phase in the region of the object; acquire the transverse signals induced by the RF pulse sequence and the off- resonant RF pulse; determine a phase shift in the transverse signals caused by the off-resonant RF pulse; and determine a Bi field map based on the phase shift; and reconstruct an image from the transverse signals and determined Bi field map.

[0095] Example Ex3 : The method as in Example Exl or the system as in Example Ex2, wherein reconstructing the image from the transverse signals and determined Bi field map comprises reconstructing the image from the transverse signals and determined Bi field map without applying a Bo gradient magnetic field to the object.

[0096] Example Ex4: The method as in Example Exl or the system as in Example Ex2, wherein the reconstructing the image from the transverse signals and determined Bi field map comprises reconstructing the image from the transverse signals and determined Bi field map without applying a plurality of Bo gradient magnetic fields to the object.

[0097] Example Ex5: The method or system as in any one of Example Exl-Ex 4, wherein the RF pulse sequence further comprises: a RF excitation pulse to excite spins in the region of the object; and at least one frequency-modulated RF pulse to dephase or rephase the spins excited by the RF excitation pulse that spatially vary in the region of the object.

[0098] Example Ex6: The method or system as in Example Ex5, wherein the off-resonant RF pulse is applied prior to the at least one frequency-modulated RF pulse.

[0099] Example Ex7: The method or system as in Example Ex5, wherein the off-resonant RF pulse is applied after the at least one frequency-modulated RF pulse.

[0100] Example Ex8: The method or system as in Example Ex5, wherein the off-resonant RF pulse is applied after a first frequency-modulated RF pulse of the at least one frequency- modulated RF pulse and before a second frequency-modulated RF pulse of the at least one frequency-modulated RF pulse.

[0101] Example Ex9: The method as recited in Example Exl or the system as in Example Ex2, wherein reconstructing the image from the transverse signals comprises removing phase induced by the Bo field using a reference scan or a spin-echo sequence.

[0102] Example ExlO: The method or system as in any one of Example Exl-Ex9, wherein the RF pulse sequence further comprises a spoiling off-resonant RF pulse to remove unwanted signal coherence pathways in the region of the object.

[0103] Example Exl 1. A method of magnetic resonance imaging (MRI) an object comprising: providing a magnetic field, Bo, along the object; applying a radio frequency (RF) pulse sequence to induce transverse signals within a region of the object, wherein the RF pulse sequence comprises a spoiling off-resonant RF pulse to remove unwanted signal coherence pathways in the region of the object; acquiring the transverse signals induced by the RF pulse sequence; and reconstructing an image from the transverse signals.

[0104] Example Ex 12. A magnetic resonance imaging (MRI) system to image an object comprising: a polarizing magnet assembly configured to provide a magnetic field, Bo, along the object; radio frequency (RF) coil assembly comprising at least one RF coil to transmit a RF field orthogonal to the magnetic field of the polarizing magnet; and a computing apparatus comprising one or more processors and configured to: apply a RF pulse sequence using the RF coil assembly to induce transverse signals within a region of the object, wherein the RF pulse sequence comprises at least one spoiling off-resonant RF pulse to remove unwanted signal coherence pathways in the region of the object; acquire the transverse signals induced by the RF pulse sequence; and reconstruct an image from the transverse signals.

[0105] Example Exl3: The method as in Example Ex 1 1 or the system as in Example Ex 12, wherein reconstructing the image from the transverse signals and determined Bi field map comprises reconstructing the image from the transverse signals without applying a Bo gradient magnetic field to the object.

[0106] Example Exl4: The method or system as in any one of Example Exl 1-Exl3, wherein the RF pulse sequence further comprises: a RE excitation pulse to excite spins in the region of the object; and at least one frequency-modulated RF pulse to dephase or rephase the spins excited by the RF excitation pulse that spatially vary in the region of the object.

[0107] Example Exl 5: The method or system as in Example Ex 14, wherein the at least one spoiling off-resonant RF pulse comprises: a first spoiling off-resonant RF pulse applied prior to the at least one frequency- modulated RF pulse, and a second spoiling off-resonant RF pulse applied following to the at least one frequency- modulated RF pulse.

[0108] Example Exl6: The method or system as in Example Exl4, wherein the at least one spoiling off-resonant RF pulse is applied after the at least one frequency-modulated RF pulse.

[0109] All references and publications cited herein are expressly incorporated herein by reference in their entirety for all purposes, except to the extent any aspect directly contradicts this disclosure.

[0110] Unless otherwise indicated, all numbers expressing feature sizes, amounts, and physical properties used in the specification and claims may be understood as being modified either by the term “exactly” or “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the foregoing specification and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by those skilled in the art utilizing the teachings disclosed herein or, for example, within typical ranges of experimental error.

[0111] As used herein, the term “configured to” may be used interchangeably with the terms “adapted to” or “structured to” unless the content of this disclosure clearly dictates otherwise.

[0112] The singular forms “a,” “an,” and “the” encompass embodiments having plural referents unless its context clearly dictates otherwise.

[0113] As used herein, “have,” “having,” “include,” “including,” “comprise,” “comprising” or the like are used in their open-ended sense, and generally mean “including, but not limited to.” It will be understood that “consisting essentially of,” “consisting of,” and the like are subsumed in “comprising,” and the like.

[0114] Reference to “one embodiment,” “an embodiment,” “certain embodiments,” or “some embodiments,” etc., means that a particular feature, configuration, composition, or characteristic described in connection with the embodiment is included in at least one embodiment of the disclosure. Thus, the appearances of such phrases in various places throughout are not necessarily referring to the same embodiment of the disclosure. Furthermore, the particular features, configurations, compositions, or characteristics may be combined in any suitable manner in one or more embodiments.

[0115] The words “preferred” and “preferably” refer to embodiments of the disclosure that may afford certain benefits, under certain circumstances. However, other embodiments may also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful and is not intended to exclude other embodiments from the scope of the disclosure.

Claims

CLAIMSWhat is claimed is:

1. A method of magnetic resonance imaging (MRI) an object comprising: providing a magnetic field, Bo, along the object; applying a radio frequency (RF) pulse sequence to induce transverse signals within a region of the object, wherein the RF pulse sequence comprises an off-resonant RF pulse that is spatially varying in the region of the object and induces a spatially varying phase in the region of the object; acquiring the transverse signals induced by the RF pulse sequence; determining a phase shift in the transverse signals caused by the off-resonant RF pulse; determining a Bi field map based on the phase shift; and reconstructing an image from the transverse signals and determined Bi field map.

2. A magnetic resonance imaging (MRI) system to image an object comprising: a polarizing magnet assembly configured to provide a magnetic field, Bo, along the object; radio frequency (RF) coil assembly comprising at least one RF coil to transmit a RF field orthogonal to the magnetic field of the polarizing magnet; and a computing apparatus operably coupled to the polarizing magnet assembly and the RF coil assembly and comprising one or more processors, the computing apparatus configured to: apply a RF pulse sequence using the RF coil assembly to induce transverse signals within a region of the object, wherein at least one pulse of the RF pulse sequence is spatially varying in the region of the object and comprises an off-resonant RF pulse to induce a spatially varying phase in the region of the object; acquire the transverse signals induced by the RF pulse sequence and the off- resonant RF pulse; determine a phase shift in the transverse signals caused by the off-resonant RF pulse; determine a Bi field map based on the phase shift; and reconstruct an image from the transverse signals and determined Bi field map.

3. The method as in claim 1 or the system as in claim 2, wherein reconstructing the image from the transverse signals and determined Bi field map comprises reconstructing the image from the transverse signals and determined Bi field map without applying a Bo gradient magnetic field to the object.

4. The method as in claim 1 or the system as in claim 2, wherein the reconstructing the image from the transverse signals and determined Bi field map comprises reconstructing the image from the transverse signals and determined Bi field map without applying a plurality of Bo gradient magnetic fields to the object.

5. The method or system as in any one of claims 1-4, wherein the RF pulse sequence further comprises: a RF excitation pulse to excite spins in the region of the object; and at least one frequency-modulated RF pulse to dephase or rephase the spins excited by the RF excitation pulse that spatially vary in the region of the object.

6. The method or system as in claim 5, wherein the off-resonant RF pulse is applied prior to the at least one frequency-modulated RF pulse.

7. The method or system as in claim 5, wherein the off-resonant RF pulse is applied after the at least one frequency-modulated RF pulse.

8. The method or system as in claim 5, wherein the off-resonant RF pulse is applied after a first frequency-modulated RF pulse of the at least one frequency-modulated RF pulse and before a second frequency-modulated RF pulse of the at least one frequency-modulated RF pulse.

9. The method as recited in claim 1 or the system as in claim 2, wherein reconstructing the image from the transverse signals comprises removing phase induced by the Bo field using a reference scan or a spin-echo sequence.

10. The method or system as in any one of claims 1-9, wherein the RF pulse sequence further comprises a spoiling off-resonant RF pulse to remove unwanted signal coherence pathways in the region of the object.

11. A method of magnetic resonance imaging (MRI) an object comprising: providing a magnetic field, Bo, along the object; applying a radio frequency (RF) pulse sequence to induce transverse signals within a region of the object, wherein the RF pulse sequence comprises a spoiling off-resonant RF pulse to remove unwanted signal coherence pathways in the region of the object; acquiring the transverse signals induced by the RF pulse sequence; and reconstructing an image from the transverse signals.

12. A magnetic resonance imaging (MRI) system to image an object comprising: a polarizing magnet assembly configured to provide a magnetic field, Bo, along the object; a radio frequency (RF) coil assembly comprising at least one RF coil to transmit a RF field orthogonal to the magnetic field of the polarizing magnet; and a computing apparatus operably coupled to the polarizing magnet assembly and the RF coil assembly and comprising one or more processors, the computing apparatus configured to: apply a RF pulse sequence using the RF coil assembly to induce transverse signals within a region of the object, wherein the RF pulse sequence comprises at least one spoiling off-resonant RF pulse to remove unwanted signal coherence pathways in the region of the object; acquire the transverse signals induced by the RF pulse sequence; and reconstruct an image from the transverse signals.

13. The method as in claim 11 or the system as in claim 12, wherein reconstructing the image from the transverse signals and determined Bi field map comprises reconstructing the image from the transverse signals without applying a Bo gradient magnetic field to the object.

14. The method or system as in any one of claims 11-13, wherein the RF pulse sequence further comprises:a RF excitation pulse to excite spins in the region of the object; and at least one frequency-modulated RF pulse to dephase or rephase the spins excited by the RF excitation pulse that spatially vary in the region of the object.

15. The method or system as in claim 14, wherein the at least one spoiling off-resonant RF pulse comprises: a first spoiling off-resonant RF pulse applied prior to the at least one frequency-modulated RF pulse, and a second spoiling off-resonant RF pulse applied following to the at least one frequency- modulated RF pulse.

16. The method or system as in claim 14, wherein the at least one spoiling off-resonant RF pulse is applied after the at least one frequency-modulated RF pulse.

17. The method or system as in claim 14, wherein the at least one spoiling off-resonant RF pulse is applied prior to the at least one frequency-modulated RF pulse.

Images