Method for reducing the deposited power in magnetic resonance imaging using multi-band pulses and multi-channel transmission

Abstract

A method for substantially simultaneously exciting spins in a plurality of slice positions using a magnetic resonance imaging (MRI) system including an RF coil assembly composed of a plurality of radio frequency (RF) coil elements, comprising: a) selecting a plurality of slice positions for substantially simultaneous manipulation with the MRI system; b) Providing an RF transmission card (B1 + ) for each of the plurality of RF coil elements in the RF coil assembly; c) identifying those of the plurality of RF coil elements to be used to manipulate corresponding ones of the plurality of slice positions selected in step a) using the RF transmission cards provided in step b); d) designing an RF pulse to be transmitted by each of the RF coil elements identified in step c); and e) transmitting the pulses designed in step d) substantially simultaneously to manipulate spins in each of the plurality of slice positions.

Description

Reference to similar applications

[0001] This application relies on U.S. Provisional Patent Application No. 61 / 425,511, filed on December 21, 2011, entitled “Power Reduction with Multiband Pulses using Multichannel Transmit.” Reference to government-funded research

[0002] This invention was made with government funding under grant RR008079 through the National Institutes of Health. The government has certain rights in the invention. Background of the invention

[0003] The present invention relates to the field of magnetic resonance imaging ("MRI") systems and methods. More specifically, the invention relates to systems and methods for reducing the deposited power, sometimes referred to as the specific absorption rate ("SAR"), generated during a substantially simultaneous multi-slice acquisition.

[0004] Since their original application, the acquisition time for volume acquisition using echo-planar imaging (EPI) or spiral MRI data acquisition has not substantially decreased. Nearly all successful efforts to reduce EPI acquisition time have aimed to reduce the number of refocused echoes required for spatial encoding to generate an image, for example, through partial Fourier imaging, parallel imaging, or sparse sampling techniques. Although these approaches reduce the acquisition time for spatially encoding a single slice in EPI, they do not necessarily reduce the time required for image acquisition by a significant amount.

[0005] Recently, it has been shown that further accelerations are possible through slice multiplexing, whereby multiple image slice positions are simultaneously excited and acquired by a multi-band ("VB") radio frequency ("RF") pulse, commonly referred to as multi-band imaging. The VB technique is limited in part—particularly at high magnetic fields—by considerations of deposited power and SAR, and in part by the peak power and / or voltage that can be tolerated by the RF coil switching technique or the peak power and / or voltage that can be generated by the amplifier in an MRI system. These limitations arise because multi-band RF pulses applied in VB imaging are partly the sum of individual single-band RF pulses that manipulate only spins in a single slice position.This sum of individual single-band RF pulses leads to a linear increase in the voltage of the multi-band RF pulse with the number of different, simultaneously manipulated slice positions. This quantity is also referred to as the multi-band factor. Due to this voltage increase, the power applied to the RF coil increases quadratically with the multi-band factor if the pulse duration and, if present, the number and duration of repetitions are the same as in a single-band RF pulse application. With the multi-band technique, the multiple applications of multi-band RF pulses to image a volume of interest (“VOI”) increase the power applied to the RF coil and the power deposited in the subject quadratically with the multi-band factor of the multi-band RF pulses used. This increase in power is in turn quadratic with respect to the multi-band factor.of the applied or deposited power when the same general pulse shape is used to achieve single-layer rather than multi-layer excitations, and the time required to acquire the entire VOI is reduced accordingly by the multi-band factor.

[0006] SAR, which is a measure of the rate at which energy is absorbed by the body when exposed to an RF electromagnetic field and is measured in watts / kilogram of tissue ("W / kg"), is a factor to consider when conducting MRI experiments on humans. As mentioned above, SAR is particularly important when exciting multiple slice positions simultaneously because, when multiple RF pulses are applied simultaneously, the local electric fields generated by each RF pulse could locally overlap, creating local extremes in the electric field strength that lead to peaks in local and global SAR, which are of concern to regulatory authorities in both the United States and Europe.A discussion of these regulatory concerns in the United States can be found, for example, in “Guidance for the Submission of Premarket Notifications for Magnetic Resonance Diagnostic Devices,” Center for Devices and Radiologic Health, Rockwell, MD: Food and Drug Administration (1998), or - for Europe - in “International Standard, Medical Equipment - Part 2: Particular Requirements for the Safety of Magnetic Resonance Equipment for Medical Diagnosis, Second Revision,” International Electrotechnical Commission, Geneva: International Electrotechnical Commission (2002).

[0007] The need to stay below safe SAR limits often leads to unfavorable compromises in acquisition parameters, such as increased repetition time ("TR") or reduced excitation angle. SAR is particularly problematic at high field strengths of 3 Tesla and above, where the power required for a given excitation angle increases approximately quadratically with the magnetic field strength. This can result in a quadrupling of the power compared to a 1.5 Tesla application.

[0008] The use of multi-band RF pulses generally increases the RF peak power requirements. Furthermore, the average SAR is also increased when multi-band RF pulses are applied to shorten the TR for acquiring the same volume compared to a single-band acquisition because the RF pulses are repeated more quickly. Similarly, the average SAR is also increased when the same TR is used but the volume acquisition is extended. The potentially significant increase in SAR therefore limits the use of multi-band imaging sequences to pulse sequences with low SAR, such as gradient-echo EPI, especially in high magnetic fields. However, even in such low-power applications, the SAR and / or the tolerance of the RF coils and the electrical components of the RF chain may reach their limits.

[0009] It would therefore be desirable to provide a method for simultaneous spin manipulation using RF pulses that manipulate multiple slice positions, with a reduced deposition of the power generated by the application of RF pulses and / or a reduced peak power and voltage applied to the RF coil. Spin manipulation includes, for example, excitation, inversion, and refocusing. Such a method would extend the applicability of multi-band RF pulses to sequences other than the known low-SAR sequences and would allow for the improvement of further imaging parameters, such as TR and volume acquisition, without risking critically high SAR values. Summary of the invention

[0010] The present invention overcomes the above-mentioned disadvantages by providing a method for transmitting a plurality of RF pulses substantially simultaneously from different radio frequency (RF) coil elements in an RF coil assembly.

[0011] According to one aspect, the invention provides a method for substantially simultaneously manipulating spins in a plurality of slice positions using a magnetic resonance imaging (MRI) system comprising an RF coil arrangement composed of a plurality of RF coil elements. A plurality of slice positions to be manipulated substantially simultaneously with the MRI system are selected, and a B l + (RF transmission) card is provided for each of the plurality of RF coil elements. Using the provided B1 +A subset of slice positions to be manipulated with each of the RF coil elements is then selected from the maps. Using the selected subset of slice positions, an RF pulse is designed to be transmitted by each of the RF coil elements. The designed RF pulses are then transmitted substantially simultaneously with the MRI system to manipulate spins in each of the plurality of slice positions substantially simultaneously.

[0012] According to a further aspect, the invention provides an MRI system comprising a magnet system configured to generate a polarizing magnetic field over at least a portion of a subject arranged in the MRI system, a plurality of gradient coils configured to apply a gradient field to the polarizing magnetic field, and an RF coil arrangement having a plurality of RF coil elements. Each of these RF coil elements is configured to apply an RF field to the subject and to acquire magnetic resonance image data therefrom. The MRI system further comprises a computer system programmed to select a plurality of slice positions to be manipulated substantially simultaneously with the RF coil arrangement, and an RF transmission card, B1 +, for each of the plurality of RF coil elements in the RF coil assembly. The computer system is further programmed to identify a subset of slice positions to be manipulated by each of the RF coil elements using the provided RF transmission maps, to design an RF pulse for each of the plurality of RF coil elements using the identified subset of slice positions, and to provide each designed RF pulse to the corresponding one of the plurality of RF coil elements. Furthermore, the computer system is programmed to control operation of the plurality of gradient coils and the RF coil assembly to manipulate spins in the selected plurality of slice positions using the provided RF pulses substantially simultaneously, and to acquire MR image data from the subject.

[0013] The foregoing and other aspects and advantages of the invention will become apparent from the following description. In the description, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, and therefore, reference should be made to the claims and description for a determination of the scope of the invention. Short description of the drawings Fig. 1 shows a block diagram as an example of a magnetic resonance imaging (MRI) system; Fig. Figure 2 shows a block diagram as an example of a radio frequency (RF) system that is part of the Fig. 1 shown MRI system; Fig. 3a is an exemplary pictorial representation of the relationship between RF transmission profiles along a slice selection direction for a plurality of RF coil elements and a plurality of slice positions to be manipulated by these RF coil elements substantially simultaneously, and in which there is substantially no overlap between the RF transmission profiles; and Fig. 3b is an exemplary pictorial representation of the relationship between RF transmission profiles along a slice selection direction for a plurality of RF coil elements and a plurality of slice positions to be manipulated by these RF coil elements substantially simultaneously, and in which there is an overlap between the RF transmission profiles; and Fig. Figure 4 is a flowchart illustrating an example of a method for reducing the power generated by an MRI system when performing a pulse sequence with substantially simultaneous manipulation of spins in a plurality of slice positions. Detailed description of the invention

[0014] Methods are provided for reducing power deposition during magnetic resonance imaging (MRI) scanning, in which spins at multiple slice positions are manipulated substantially simultaneously. Generally, an array of radio frequency (RF) coils, such as a multi-channel parallel transmission coil array, is used to generate an electromagnetic RF field that manipulates the spins present at different slice positions substantially simultaneously. The "manipulation" of spins is generally understood to mean the rotation of the principal magnetization formed by these spins by any angle around an axis, which may be accompanied by variable angular excitation, refocusing, or reversal.The power deposited in the subject during essentially simultaneous manipulation of slice positions can be reduced by spatially distributing the generation of the RF fields used for spin manipulation across different RF coil elements for different physical slice positions. This reduction in power deposition can be measured as a reduction in the specific absorption rate (SAR).

[0015] The methods of the present invention also enable a reduction in the amount of energy required to produce a particular B l +Amplitude of the peak and total power required at a given location in a volume of interest, allowing a reduction in the power deposited in each coil element and corresponding circuitry, as well as in the subject, by reducing the power delivered to the RF system. This power reduction has additional benefits. In particular, reducing the peak power applied to the RF system can enable the generation of RF fields that would otherwise require peak power unattainable with current RF systems.For example, if an RF system is limited to a peak voltage of one kilovolt (KV), but a voltage of 4 kilovolts is required, a multi-channel transmit coil arrangement according to the methods described herein could be applied to distribute this voltage using several different amplifiers, each coupled to one or a few of the elements of a multi-channel transmit coil, such that the available peak power satisfies the one kilovolt peak voltage limitation.

[0016] In Fig. 1 shows an example of a magnetic resonance imaging (MRI) system 100. The MRI system 100 includes a workstation 102 with a display 104 and a keyboard 106. The workstation 102 includes a processor 108, such as a commercially available programmable machine running a commercially available operating system. The workstation 102 provides the user interface that allows acquisition instructions to be entered into the MRI system 100. The workstation 102 is connected to four servers: a pulse sequence server 110, a data acquisition server 112, a data processing server 114, and a data storage server 116. The workstation 102 and each of the servers 110, 112, 114, and 116 are connected to communicate with each other.

[0017] The pulse sequence server 110 operates according to instructions from the workstations 102 to operate a gradient system 118 and a radio frequency (RF) system 120. The gradient waveforms required to perform the prescribed recording are generated and applied to the gradient system 118, which excites the gradient coils in an array 122 to generate the magnetic field gradients G X , G Y and G Z to generate signals used for position encoding of the MR signals. The gradient coil assembly 122 is part of a magnet assembly 124, which includes a polarization magnet 126 and a whole-body RF coil 128.

[0018] RF waveforms are applied by the RF system 120 to the RF coil 128 or a (in Fig. 1) to perform the prescribed magnetic resonance pulse sequence. The pulses generated in response by the RF coil 128 or a Fig. 1) are received by the RF system 120 and amplified, demodulated, filtered, and digitized according to instructions from the pulse sequence server 110. The RF system 120 includes an RF transmitter for generating a wide variety of RF pulses used in MR pulse sequences. The RF transmitter responds to the acquisition instructions and instructions from the pulse sequence server 110 to generate RF pulses with the desired frequency, phase, and pulse amplitude waveforms. The generated RF pulses can be applied to the whole-body RF coil 128 or to one or more local coils or coil assemblies (not shown in Fig. 1 shown).

[0019] The RF system 120 also includes one or more RF receiver channels. Each RF receiver channel includes an RF preamplifier that amplifies the MR signal received by the coil 128 to which it is connected, and a detector that detects and digitizes the I and Q quadrature components of the received MR signal. The strength of the received MR signal can thus be determined at each sampled point by the square root of the sum of the squares of the I and Q components: M=I2+Q2

[0020] The phase of the received MR signal can also be determined: φ=tan−1(QI)

[0021] Pulse sequence server 110 optionally also receives patient data from a physiological acquisition controller 130. Controller 130 receives signals from a number of different sensors connected to the patient, such as electrocardiogram (ECG) signals from electrodes or respiratory signals from a breathing bellows or other respiratory monitoring device. Such signals are typically used by pulse sequence server 110 to synchronize or interleave the acquisition with the patient's heartbeat or respiration.

[0022] The pulse sequence server 110 is also connected to a room scan interface 132, which receives signals from various sensors related to the patient's condition and the magnet system. The room scan interface 132 also provides instructions to the patient positioning system 134 to move the patient into the desired position during the scan.

[0023] The digitized MR signal samples generated by the RF system 120 are received by the data acquisition server 112. The data acquisition server 112 operates under the instruction of the workstation computer 102 to receive the real-time MR data and provide a buffer memory to prevent data loss due to data overwriting. For some acquisitions, the data acquisition server 112 does little more than forward the acquired MR data to the data processing server 114. However, for acquisitions in which information derived from the acquired MR data is needed to control the further execution of the acquisition, the data acquisition server 112 is programmed to generate this information and forward it to the pulse sequence server 110. For example, during pre-acquisitions, MR data is acquired and used to calibrate the pulse sequence performed by the pulse sequence server 110.During an acquisition, navigation signals can also be acquired and used to adjust the operating parameters of the RF system 120 or the gradient system 118, or to control the image sequence in which k-space is scanned. For example, the data acquisition server 112 acquires MR data and processes it in real time to generate information that can be used to control the acquisition.

[0024] The data processing server 114 receives MR data from the data acquisition server 112 and processes it according to the instructions downloaded from the workstation. This processing includes, for example, Fourier transforming the raw k-space MR data to generate two- or three-dimensional images; applying filters to a reconstructed image; performing backprojection image reconstruction of acquired MR data; generating functional MR images; and calculating motion or flow images.

[0025] The images reconstructed by the data processing server 114 are transmitted back to the workstation 102, where they are stored. Real-time images are stored in a database cache (in Fig. 1 not shown), from where they can be output to a user screen or to a screen located near the magnet assembly 124 for use by the attending physicians. Batch mode images or selected real-time images are stored in a host database on a storage disk 138. The data processing server 114 notifies the data storage server 116 on the workstation 102 when such images have been reconstructed and transferred to storage. The workstation 102 can be used by a user to archive the images, generate movies, or send the images to other facilities over a network.

[0026] As in Fig. 1, the radio frequency (RF) system 120 may be connected to the whole-body RF coil 128. However, it may also be connected to the whole-body RF coil 128, as shown in Fig. 2, a transmit section of the RF system 120 may be connected to one or more transmit channels 202 of an RF coil assembly 204, and a receive section of the RF system 120 may be connected to one or more receive channels 206 of the RF coil assembly 204. The transmit channels 202 and the receive channels 206 are connected to the RF coil assembly 204 by one or more transmit / receive combiners 208. The receive channel 206 may also be a separate arrangement of coils from the transmit coil assembly. In this case, the T / R combiners 208 are not required. The transmit coil elements are detuned or otherwise rendered inoperative during the receive application, and the receive coil elements are similarly detuned or otherwise rendered inoperative during the transmit coil application. Detuning can be achieved, for example, by suitable logical control signals.

[0027] According to Fig. 2, the RF system 120 comprises one or more transmission channels 202 that generate a prescribed electromagnetic RF field. The generation of the fundamental or carrier frequency of this RF field is controlled by a frequency generator 210, which receives a set of digital signals from the pulse sequence server 110. These digital signals indicate the frequency, amplitude, and phase of the RF carrier signal generated at output 212. The RF carrier is applied to a modulator and, if necessary, an upconverter 214, where its amplitude and phase are modulated according to a signal R(t), also coming from the pulse sequence server 110. The signal R(t) defines the envelope of the RF pulse to be generated and is generated by continuously reading a series of stored digital values. These stored digital values can be modified to enable the generation of any desired RF pulse envelope.

[0028] The strength of the RF pulse generated at output 216 is attenuated by an attenuator circuit 218, which receives digital instructions from the pulse sequence server 110. The phase of the RF pulse can also be varied using phase shifters (not shown). The modulated RF pulses are then applied to a power amplifier 220, which drives one or more electrically coupled elements of the RF coil assembly 204. Multiple transmission channels then drive further elements of the multi-channel transmission coil assembly.

[0029] The MR signal generated by the subject is detected by the RF coil assembly 202 and applied to the inputs of the set of receive channels 206. A preamplifier 222 in each receive channel 206 amplifies the signal, which is then attenuated, if necessary, by a receiver attenuator 224 by an amount determined by a digital attenuation signal from the pulse sequence server 110. The received signal is at or near the Larmor frequency. This high-frequency signal can be converted to a lower frequency in a two-stage process by a downconverter 226. For example, in such a process, the downconverter 226 first mixes the MR signals with the carrier signal on line 212 and then mixes the resulting difference signal with a reference signal on line 228 generated by a reference frequency generator 230.The MR signal is applied to the input of an analog-to-digital (A / D) converter 232, which samples and digitizes the analog signal. As an alternative to downconverting the high-frequency signal, the received analog signal can also be directly acquired using a suitable fast analog-to-digital converter and / or suitable subsampling. The sampled and digitized signal can then be applied to a digital detector and signal processor 234, which generates the phase (I) and quadrature (Q) values corresponding to the received signal. The resulting data stream of digital I and Q values of the received signal is output to the data acquisition server 112. In addition to generating the reference signal on line 228, the reference frequency generator 230 also generates a sample signal on line 236, which is applied to the A / D converter 232.

[0030] The power, measurable as the specific absorption rate (SAR), deposited in a subject undergoing an MRI scan in which multiple slice positions are manipulated essentially simultaneously can be reduced by using multiple RF coils to transmit the electromagnetic RF fields. In an ideal configuration, the multiple RF coils are arranged so that the spatial B1 transmission profile (B1 + ) for a given coil or set of coils is spatially correlated with one of the layer positions in the multilayer spin manipulation. In such a configuration, a different coil—or different sets of coils—produces the desired B1 +-field across different slice positions. With this technique, the different slice positions can be manipulated essentially simultaneously using a multi-channel transmission in which different slice-selective pulses are transmitted using the RF coils with the corresponding B1 + profiles are applied to the different layer positions. Using this technique, and if the B1 + -Profiles of different coil elements in the coil array do not overlap, the power deposited in the subject will not exceed the power that would be deposited when manipulating spins in a single slice position with a volume RF coil that covers an equivalent volume as the multiple transmission coils and has an equal B1 + -field is created across all layer positions.

[0031] In general, the ideal situation described above is not achieved. For example, the B1 + -Profile of a coil generally partially corresponds to the B1 + profile of another RF coil. As a result of this overlap, the corresponding electric fields will add up in a complex way, so that the SAR could be increased or decreased. Furthermore, in some cases, a single RF coil corresponding to a specific slice position will not exist. Instead, it will be necessary to identify a combination of RF coils or sets of RF coils that achieve an optimal B1 + -field corresponding to a specific slice position. However, as long as some different spatial B1 +profiles exist, a reduction in SAR will occur compared to essentially simultaneously applied multi-band pulses with a single transmit coil covering the entire volume of interest.

[0032] In general, a multiband RF pulse is a composite RF pulse composed of multiple RF pulses that share a common temporal footprint. Each of these component RF pulses has a frequency matched to the Larmor frequency, modified by an excited slice selection magnetic field gradient associated with a specific slice position. Thus, each multiband RF pulse contains a component pulse for each slice position where spins are manipulated by the multiband RF pulse. In a typical multiband RF application, the relative amplitude of the component RF pulse is the same for each individual slice because only a single RF coil is used to transmit the multiband RF pulse. However, this is not the case with the present invention, as described below.

[0033] As in the Fig. 3A and Fig. 3B, for example, four slice positions S1-S4 are manipulated essentially simultaneously using an RF coil arrangement with at least four coil elements C1, C2, C3, and C4 distributed along a slice selection axis. The B1 + Maps of these coil elements along the slice selection axis are shown at 302, 304, 306 and 308. In this illustrative example, only the strength of the B1 + -field and for simplicity it is assumed that the B1 + -field vectors from all coils have the same direction everywhere. As in Fig. As shown in Figure 3A, it is possible to uniquely assign a particular coil element to a particular slice position if there is no or only little overlap of the B1 +Profiles 302, 304, 306, 308 exist with respect to the selected slice positions S1-S4. For example, coil element C1 can be uniquely assigned to slice position S1. In this way, it is possible to manipulate the spins in slice positions S1-S4 essentially simultaneously by transmitting a single-band RF pulse with each corresponding coil element essentially simultaneously.

[0034] In Fig. 3B, nine slice positions S1-S9 are manipulated essentially simultaneously by an RF coil arrangement with at least four coil elements C1, C2, C3, and C4 distributed along a slice selection axis. If there is an overlap between the B1 +-profiles 302, 304, 306, 308 of neighboring coil elements with respect to the desired slice positions, it will not always be possible to unambiguously assign a particular coil element to the manipulation of spins in a particular slice position solely during a substantially simultaneous excitation. Rather, the electromagnetic field generated by a particular coil element will manipulate multiple slice positions, and for some slice positions, the optimal RF pulse will use two neighboring coil elements. In this case, a subset of slice positions can be selected for spin manipulation by a single coil element by transmitting a suitably designed multi-band RF pulse with that coil element. Fig. 3B, for example, layers S1, S2, S4, S6, S8, and S9 can each be manipulated by a single coil element alone. More specifically, layers S1 and S2 can be manipulated by coil C1 alone, layer S4 can be manipulated by coil C2 alone, layer S6 can be manipulated by coil C3 alone, and layers S8 and S9 can be manipulated by coil C4 alone. In contrast, layer S3 is affected by both coil elements C1 and C2, layer S5 is affected by both coil elements C2 and C3, and layer S7 is affected by both coil elements C3 and C4.Thus, in this example, the spins in layer S3 can best be manipulated by a linear combination of coils C1 and C2, the spins in layer S5 can best be manipulated by a linear combination of coils C2 and C3, and the spins in layer 7 can best be manipulated by a linear combination of coils C3 and C4.

[0035] Consequently, a multi-band RF pulse intended to manipulate layers S2, S4, and S6, for example, would be divided into three single-band component RF pulses whose slice-selective properties are concentrated on layers S2, S4, and S6. Within such a pulse, each single-band component RF pulse can be directed to a single coil element. For example, the single-band component RF pulse for layer S2 would go to coil C1 alone, the single-band component RF pulse for layer S4 would go only to coil C2, and the single-band component RF pulse for layer S6 would go only to coil C3. In this case, each coil sees a single slice-selective RF pulse, in contrast to current practice, where the entire coil array, and consequently the entire sample acquired by the coil array, experiences the sum of these three pulses.Consequently, in current use, three times the voltage is required and delivered to the entire volume to be imaged compared to the possibility of generating each individual slice-selective RF pulse in separate coils as described above.

[0036] For a multi-band pulse aimed at essentially simultaneously manipulating layers S3, S5, and S7, the situation is different. In this case, the multi-band RF pulse is again subdivided into three single-band component RF pulses, each suitable for selecting layers S3, S5, and S7. With regard to layer S3, it will be advantageous to apply the layer-selective RF pulse to coils C2 and C3 (possibly with a phase difference depending on the coil geometry), with each coil experiencing a lower power compared to the power applied to a single coil suitable for manipulating layer S2, S4, or S6. For a layer-selective RF pulse, the power is therefore applied to coils C1 and C2 such that the vector sum of the B1 +-The fields of the two individual coils add up to a suitable value required for the desired rotation of the main magnetization. Similarly, for layer S5, power will go to coils C2 and C3, and for layer S7, power will go to coils C3 and C4.

[0037] In general, the relative amplitudes and phases of the component RF pulses used to form the multi-band RF pulses for the substantially simultaneous manipulation of multiple slice positions are determined according to the B generated for the coil element transmitting the component RF pulse to the slice position affected by the component RF pulse. l + weighted. This B1 +is measured or calibrated in the form of amplitude and phase at each point in space within the volume of interest for each coil element in the transmit coil array. Typically, this calibration is performed at the beginning of a recording session for a given subject and is used to decide how the single-band or multi-band RF pulses should be generated for each slice position. For example, it is desirable to achieve the same excitation angle for the spins in each slice position. It is known that the excitation angle generated by an RF pulse is proportional to the amplitude of the B1 + -field at the locations of the affected spins. The amplitude of the B1 + -field is determined by the vector sum of the B1 generated by each coil element + -fields at this location. That is, the vector sums of the B1 + -fields is determined by the amplitude and phase of the B1 + -field. The phase of the B1+ -field is defined by the B1 + -cards.

[0038] The flowchart in Fig. 4 shows, by way of example, the steps of a method for reducing the deposited power generated by an MRI system when performing a pulse sequence that includes the substantially simultaneous manipulation of spins in a plurality of slice positions. In particular, the MRI system comprises an RF coil arrangement that is suitable for transmitting electromagnetic RF fields through a plurality of RF coil elements connected in parallel. First, in step 402, a plurality of slice positions are selected for the slices to be imaged in a subject. The slice positions can, for example, be contiguous or separated by a certain distance. Next, in step 404, a B1 + -card is provided for each coil element in the RF coil assembly. This B1 +-Cards can be replaced by a B1 + mapping technique or stored maps from a previous mapping process. It should be considered that the generation of B1 + cards for each recording session delivers better results. The B1 + -Maps contain information about the size and direction of the B1 + -field as a function of spatial coordinates over the volume to be imaged. Because the B1 + -field is a vector quantity in the plane perpendicular to the direction of the main magnetic field, B0, it can be characterized by an amplitude and a phase. Consequently, the B1 + -Maps an amplitude and direction of the B1 + field generated by each coil. This B1 +-Maps are used to determine the appropriate combination of RF coil elements for manipulating spins in each slice position. Next, as shown in step 406, the coil elements used to manipulate spins in each of the prescribed slice positions are selected using the provided B1 + -cards. For example, the overlap between the B1 + -Cards are identified and used to determine whether each slice position can be uniquely manipulated by a single RF coil element, or whether multiple RF coil elements will manipulate the same slice position.

[0039] After the individual RF coil elements of the transmission coil arrangement relating to each of the slice positions have been determined, the RF pulses thus transmitted for the substantially simultaneous manipulation of spins in the slice position are designed, as shown in step 408. For example, if a particular RF coil element is identified that is used to manipulate the spins in only one slice position, the RF pulse to be transmitted by that RF coil element can be designed as a single-band RF pulse specifically adapted to the corresponding slice position. On the other hand, if a given RF coil element is identified that is used to manipulate spins in more than one slice position, the appropriate multi-band RF pulse can be designed.For example, if a subset of three slice positions is affected, a multi-band RF pulse composed of three component RF pulses, each tailored to a slice position, is designed. The relative amplitudes of the component RF pulses that comprise the multi-band RF pulse can be determined using information about the spatial sensitivity of the RF coil element with respect to the corresponding slice position. This information is provided in Figure B1. + -Cards included. Once the appropriate RF pulses are designed, they are transmitted differently in the RT system and used to perform an imaging sequence during which the plurality of slice positions are manipulated substantially simultaneously by transmitting the designed RF pulses with the corresponding RF coil elements substantially simultaneously.

[0040] The present invention has been described with reference to one or more preferred embodiments. It should be noted that, in addition to those expressly mentioned, many equivalent solutions, alternatives, variations, and modifications are possible and fall within the scope of the invention.

Claims

[1] A method for substantially simultaneously exciting spins in a plurality of slice positions using a magnetic resonance imaging (MRI) system including an RF coil assembly composed of a plurality of radio frequency (RF) coil elements, comprising: a) selecting a plurality of slice positions for substantially simultaneous manipulation with the MRI system; b) Providing an RF transmission card (B1 + ) for each of the plurality of RF coil elements in the RF coil assembly; c) identifying those of the plurality of RF coil elements to be used to manipulate corresponding ones of the plurality of slice positions selected in step a) using the RF transmission cards provided in step b); d) designing an RF pulse to be transmitted by each of the RF coil elements identified in step c); and e) transmitting the pulses designed in step d) substantially simultaneously to manipulate spins in each of the plurality of slice positions. [2] The method of claim 1, wherein step c) comprises identifying overlaps between the RF transmission maps provided in step b). [3] The method of claim 2, wherein an RF coil element identified in step c) as having an RF transmission map that does not overlap with the RF transmission map of an adjacent RF coil element is intended to manipulate a slice position. [4] The method of claim 3, wherein step d) comprises designing a single-band RF pulse for each RF coil element associated with a slice position. [5] The method of claim 2, wherein an RF coil element identified in step c) as having an RF transmission map that overlaps with the RF transmission map of an adjacent RF coil element is assigned to at least two slice positions. [6] The method of claim 5, wherein step d) comprises designing a multi-band RF pulse for each RF coil element associated with at least two slice positions. [7] The method of claim 6, wherein the multi-band RF pulse is composed of a number of RF pulse components equal to a number of slice positions to be manipulated by the multi-band RF pulse. [8] Magnetic resonance imaging (MRI) system, comprising: a magnet system configured to generate a polarizing magnetic field over at least a portion of a subject disposed in the MRI system; a plurality of gradient coils configured to apply a gradient field to the polarized magnetic field; a radio frequency (RF) coil assembly comprising a plurality of RF coil elements, each RF coil element configured to apply an RF field to the subject and acquire magnetic resonance (MR) image data therefrom; a computer system programmed to: selecting a plurality of slice positions for substantially simultaneous manipulation with the RF coil assembly; an RF transmission card (B1 + ) for each of the plurality of RF coil elements in the RF coil assembly; identify those of the plurality of RF coil elements that are used to manipulate corresponding ones of the plurality of slice positions using the provided RF transmission cards; design an RF pulse for the identified ones of the plurality of RF coil elements; deliver each designed RF pulse to the corresponding one of the plurality of RF coil elements; and to control the operation of the plurality of gradient coils and the RF coil assembly to manipulate spins in the selected plurality of slice positions using the provided RF pulses substantially simultaneously and to acquire MR image data from the subject. [9] The MRI system of claim 8, wherein the computer system is further programmed to identify those of the plurality of RF coil elements used to manipulate the corresponding one of the plurality of slice positions by identifying overlaps between the provided RF transmission maps. [10] The MRI system of claim 9, wherein the computer system is further programmed to assign an RF coil element identified as having an RF transmission map that does not overlap with the RF transmission map of an adjacent RF coil element to a slice position. [11] The MRI system of claim 10, wherein the computer system is further programmed to design the RF pulse for each RF coil element associated with a slice position as a single-band RF pulse. [12] The MRI system of claim 9, wherein the computer system is further programmed to assign an RF coil element identified as having an RF transmission map that overlaps with the RF transmission map of an adjacent RF coil element to at least two slice positions. [13] The MRI system of claim 12, wherein the computer system is further programmed to design the RF pulse for each RF coil element associated with at least two slice positions as a multi-band RF pulse. [14] The MRI system of claim 13, wherein the computer system is programmed to design the multi-band RF pulse as composed of a number of RF pulse components equal to a number of slice positions to be manipulated by the multi-band RF pulse.

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