阅读说明：本技术 用于使用并行发射射频线圈阵列的同时多切片磁共振指纹成像的系统和方法 (System and method for simultaneous multi-slice magnetic resonance fingerprint imaging using parallel transmit radio frequency coil arrays ) 是由 B·B·梅达 S·科波 M·特威格 M·A·格里斯沃尔德 于 2018-04-06 设计创作，主要内容包括：提供了用于从一个或多个共振物种采集成像数据的系统和方法,该共振物种在多个不同的切片中同时产生单独的磁共振信号。通过使用pTX RF线圈阵列同时激励多个不同的切片采集数据,使得该多个不同的切片中的至少一些通过从pTX RF线圈阵列中的发射通道的子集发射RF能量来激励。该方法还包括将数据与信号演化的字典进行比较,以至少部分地基于将数据与存储在字典中的一组已知信号演化进行匹配确定该共振物种的两个或更多个参数的定量值。该方法包括至少部分地在定量值上为多个不同的切片的位置中的每一个产生图像。(Systems and methods are provided for acquiring imaging data from one or more resonant species that simultaneously generate separate magnetic resonance signals in a plurality of different slices. Data is acquired by simultaneously exciting a plurality of different slices using the pTX RF coil array such that at least some of the plurality of different slices are excited by transmitting RF energy from a subset of transmit channels in the pTX RF coil array. The method also includes comparing the data to a dictionary of signal evolutions to determine quantitative values for two or more parameters of the resonant species based at least in part on matching the data to a set of known signal evolutions stored in the dictionary. The method includes generating an image at least partially quantitatively for each of a plurality of different slice locations.)

1. A method of simultaneous multi-slice (SMS) Magnetic Resonance Fingerprinting (MRF) imaging using a parallel transmit (pTX) Radio Frequency (RF) coil array, the method comprising:

a) acquiring data with a Magnetic Resonance Imaging (MRI) system in a series of variable sequence blocks that cause one or more resonant species in a subject to simultaneously generate separate magnetic resonance signals in a plurality of different slices, wherein the series of variable sequence blocks comprises simultaneously exciting the plurality of different slices using the pTX RF coil array such that at least some of the plurality of different slices are excited by transmitting RF energy from a subset of transmit channels in the pTX RF coil array;

b) comparing the acquired data to a dictionary of signal evolutions to determine quantitative values for two or more parameters of the resonant species based at least in part on matching the separated magnetic resonance data to a set of known signal evolutions stored in the dictionary; and

c) displaying, based at least in part on the quantitative values, an image depicting each of the plurality of different slice locations of the subject at a slice location.

2. The method of claim 1, wherein step b) comprises comparing the acquired data for individual slices as the plurality of different slices to the dictionary to determine quantitative values for two or more parameters of the resonant species in each individual slice.

3. The method of claim 2, wherein the dictionary comprises at least a first dictionary and a second dictionary, and comparing the acquired data for individual slices as the plurality of different slices to the dictionary comprises comparing the acquired data for individual slices as the plurality of different slices to each of the first dictionary and the second dictionary.

4. The method of claim 1, wherein the series of variable sequence blocks comprises temporally varying the subset of transmit channels in the pTX RF coil array.

5. The method of claim 4, wherein said subset of transmit channels in said pTX RF coil array are varied in time in said series of variable sequence blocks such that an entire field of view is excited by said series of variable sequence blocks for each of said plurality of different slices.

6. The method of claim 4, wherein the series of variable sequence blocks includes temporally varying the subset of transmit channels in the pTX RF coil array such that aliasing in a through-plane direction is minimized.

7. The method of claim 1, wherein the comparing of step b) comprises pattern recognition based on template matching.

8. The method of claim 1, wherein the dictionary comprises a separate sub-dictionary for each spatial location, wherein the sub-dictionaries are generated using spatial distributions of RF energy measured in separate scans.

9. The method of claim 1, wherein the dictionary is generated by including B1+ terms for different channels as additional dimensions within the dictionary.

10. A system for simultaneous multi-slice (SMS) Magnetic Resonance Fingerprinting (MRF) imaging using a parallel transmit array (pTX), the system comprising:

a magnet system configured to generate a polarizing magnetic field around at least a region of interest of a subject arranged in the system;

a plurality of gradient coils configured to apply gradient fields to the polarizing magnetic field;

a Radio Frequency (RF) system comprising a multi-channel transmit RF coil array, the RF system configured to apply an excitation field to the subject and acquire MR image data from a region of interest (ROI) within the subject; and

a computer system comprising a processor and a memory, the memory having stored thereon instructions that, when executed by the processor, cause the processor to perform the method of any preceding claim.

Background

Characterizing tissue species using nuclear magnetic resonance ("NMR") may include identifying different properties of the resonant species (e.g., T1 spin-lattice relaxation, T2 spin-spin relaxation, proton density). Other properties (e.g., superposition of tissue types and properties) may also be identified using the NMR signals. These properties and others can be identified simultaneously using Magnetic Resonance Fingerprinting ("MRF"), for example, as described in "Magnetic Resonance Fingerprinting" by d.ma et al (nature, 2013, 495 (7440): page 187-192).

Conventional magnetic resonance imaging ("MRI") pulse sequences include a similarly repetitive preparation phase, a waiting phase and an acquisition phase that successively produce signals from which images can be made. The preparation phase determines when a signal is acquired and determines the nature of the acquired signal. For example, a first pulse sequence may produce a T1-weighted signal at a first echo time ("TE"), while a second pulse sequence may produce a T2-weighted signal at a second TE. These conventional pulse sequences typically provide qualitative results in which data is acquired with various weightings or contrasts that emphasize particular parameters (e.g., T1 relaxation, T2 relaxation).

When generating magnetic resonance ("MR") images, radiologists and/or surgeons may view these images, which they interpret qualitative images for particular disease features. The radiologist may examine multiple image types (e.g., T1-weighted, T2-weighted) acquired in multiple imaging planes to make a diagnosis. A radiologist or other individual reviewing qualitative images may require specific skills to be able to assess variations between sessions, between machines, and between machine configurations.

Unlike conventional MRI, MRF employs a series of varying sequence blocks that simultaneously produce different signal evolutions in different resonant species (e.g., tissues) to which radio frequency ("RF") is applied. However, the signals from different resonating tissues will be different and can be distinguished using MRF. Different signals may be collected over a period of time to identify signal evolution of the volume. The resonant species in the volume can then be characterized by comparing the signal evolution with known evolutions. Characterizing the resonant species may include identifying a material or tissue type, or may include identifying MR parameters associated with the resonant species. For example, a "known" evolution may be a simulated evolution calculated from physical principles and/or previously acquired evolutions. A number of known evolutions may be stored in a dictionary.

Disclosure of Invention

The present disclosure provides systems and methods for performing simultaneous multi-slice (SMS) techniques using a parallel transmit (pTX) array to perform MRF imaging. More specifically, systems and methods are provided to perform SMS using an MRF framework by temporally varying transmit channels exciting various slices using an MRF pulse sequence to capture the entire field of view of multiple slices.

According to one aspect of the present disclosure, a method is provided that includes a) acquiring data with a Magnetic Resonance Imaging (MRI) system in a series of variable sequence blocks that cause one or more resonant species in a subject to simultaneously generate separate magnetic resonance signals in a plurality of different slices. The series of variable sequence blocks includes simultaneously exciting the plurality of different slices using the pTX RF coil array such that at least some of the plurality of different slices are excited by transmitting RF energy from a subset of transmit channels in the pTX RF coil array. The method also includes comparing the acquired data to a dictionary of signal evolutions to determine quantitative values for two or more parameters of the resonant species based at least in part on matching the separated magnetic resonance data to a set of known signal evolutions stored in the dictionary. The method further includes generating an image depicting each of the plurality of different slice locations of the subject at the slice location based at least in part on the quantitative values.

In accordance with another aspect of the present disclosure, a system is provided that includes a magnet system, a plurality of gradient coils, a Radio Frequency (RF) system, and a computer system. The magnet system may be configured to generate a polarizing magnetic field around at least a region of interest of a subject arranged in the system. The plurality of gradient coils may be configured to apply gradient fields to the polarizing magnetic field. The RF system may include a pTX RF coil array. The RF system may be configured to apply an excitation field to the subject and acquire MR image data from the ROI. The computer system may include a processor and a memory. The memory may have instructions stored thereon that, when executed by the processor, cause the processor to perform the methods described herein.

The foregoing and other aspects and advantages of the present disclosure will appear from the following description. In the present description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown by way of illustration preferred embodiments. This embodiment does not necessarily represent the full scope of the invention, however, and reference is therefore made to the claims herein for interpreting the scope of the invention.

Drawings

Fig. 1 is a block diagram of an example magnetic resonance imaging ("MRI") system in which methods described in this disclosure may be implemented.

Fig. 2 is a flow chart illustrating a method according to the present disclosure.

Fig. 3 is an illustration of the use of a pTX array for exciting different regions of multiple slices to avoid aliasing.

FIG. 4A is an example pulse sequence that may be used in accordance with some embodiments of methods described in this disclosure;

fig. 4B illustrates an example pattern of varying sequence parameters, including temporally varying a subset of transmit channels used to excite different ones of a plurality of slices, in accordance with some embodiments of the present disclosure.

Detailed Description

Magnetic resonance fingerprinting ("MRF") is a technique that facilitates mapping of tissue or other material properties based on random or pseudorandom measurements of the subject or object being imaged. In particular, MRF can be conceptualized as employing a series of varying "sequence blocks" that simultaneously produce different signal evolutions in different "resonant species" to which RF is applied. The term "resonant species" as used herein refers to a material that can be made to resonate using NMR, such as water, fat, bone, muscle, soft tissue, and the like. By way of illustration, when radio frequency ("RF") energy is applied to a volume having both bone and muscle tissue, both bone and muscle tissue will produce nuclear magnetic resonance ("NMR") signals; however, the "skeletal signal" represents the first resonant species and the "muscle signal" represents the second resonant species, and thus the two signals will be different. The different signals from different species may be collected simultaneously over a period of time to collect the overall "signal evolution" of the volume.

Measurements acquired in MRF techniques are achieved by varying acquisition parameters from one repetition time ("TR") period to the next, which creates a time series of signals with varying contrast. Examples of acquisition parameters that may be varied include flip angle ("FA"), RF pulse phase, TR, echo time ("TE"), and sampling pattern (such as by modifying one or more readout encoding gradients). The acquisition parameters are varied in a random, pseudo-random, or other manner, which results in signals from different materials or tissues being spatially incoherent, temporally incoherent, or both. For example, in some instances, the acquisition parameters may be varied according to a non-random or non-pseudo-random pattern that would otherwise result in signals from different materials or tissues being spatially incoherent, temporally incoherent, or both.

Based on these measurements (which, as noted above, may be random or pseudo-random, or may contain spatially incoherent, temporally incoherent, or both signals from different materials or tissues), the MRF process may be designed to map any of a variety of parameters. Examples of such parameters that may be mapped may include, but are not limited to, longitudinal relaxation time (T)₁) Transverse relaxation time (T)₂) Main magnetic field or static magnetic field pattern (B)₀) And proton density (ρ). MRF is generally described in U.S. patent No.8,723,518 and published U.S. patent application No.2015/0301141, both of which are incorporated herein by reference in their entirety.

Data acquired using MRF techniques is compared to a dictionary of signal models or templates generated from a magnetic resonance signal model, such as a physical simulation based on Bloch's equation, for different acquisition parameters. This comparison allows for the estimation of physical parameters such as those mentioned above. By way of example, the comparison of the acquired signals to the dictionary may be performed using any suitable matching or pattern recognition technique. The parameters of the tissue or other material in a given voxel are estimated to be the values that provide the best signal match. For example, comparison of the acquired data with a dictionary may result in selection of a signal vector from the dictionary that best corresponds to the observed signal evolution, which may constitute a weighted combination of signal vectors. The selected signal vector includes values for a plurality of different quantitative parameters, which may be extracted from the selected signal vector and used to generate an associated quantitative parameter map.

The stored signals and information derived from the reference signal evolution may be associated with a potentially very large data space. The data space for signal evolution can be described in part by:

wherein SE is the signal evolution; n is a radical of_SIs the number of spins; n is a radical of_AIs the number of sequence blocks; n is a radical of_RFIs the number of RF pulses in the sequence block; alpha is the turning angle; phi is the phase angle; r_i(α) is rotation due to detuning;

rotation due to RF differences; r (G) is the rotation due to the magnetic field gradient; t is₁Is the longitudinal or spin-lattice relaxation time; t is₂Is the transverse or spin-spin relaxation time; d is diffusion relaxation; e_i(T₁,T₂D) signal attenuation due to relaxation differences; and M₀Is the magnetization in the default or natural alignment that the spins align with when placed in the main magnetic field.

Although E is provided by way of example_i(T₁,T₂D), but in a different case the attenuation term E_i(T₁,T₂D) may also include additional items, E_i(T₁,T₂D.,) or may include fewer terms, such as by not including diffusion relaxation, as E_i(T₁,T₂) Or E_i(T₁,T₂,...). Also, the sum over "j" may be determined by the product over "jAnd (6) replacing.

The dictionary may store signals described by the following equation,

S_i＝R_iE_i(S_i-1) (2)；

wherein S is₀Is a default or balanced magnetization; s_iIs shown in the i^thDifferent components M of the magnetization during one acquisition block_x、M_yAnd M_zThe vector of (a); r_iIs at the i^thA combination of rotational effects occurring during each acquisition block; and E_iIs at the i^thThe combination of the effects of varying the amount of magnetization in different states of the individual acquisition blocks. In this case, the ith^thThe signal at each acquisition block is the acquisition block (i.e., (i-1))^thIndividual acquisition blocks) of the previous signal. Additionally or alternatively, the dictionary may store the signals as a function of current relaxation and rotation effects and previous acquisitions. Additionally or alternatively, the dictionary may store signals such that the voxel has multiple resonance species or spins, and the effect may be different for each spin within the voxel. Still further, the dictionary may store signals such that a voxel may have multiple resonance species or spins, and for spins within a voxel, the effect may be different, and thus the signals may be a function of the effect and previous acquisition blocks.

As will be described, the present disclosure provides an MRF framework for simultaneous multi-slice (SMS) MRF imaging using a parallel transmit (pTX) RF coil array. That is, systems and methods are provided to perform SMS using an MRF framework by temporally varying transmit channels exciting various slices using an MRF pulse sequence to capture the entire field of view of multiple slices.

Referring now particularly to FIG. 1, there is shown an example of an MRI system 100 in which the methods described herein may be implemented. The MRI system 100 includes an operator workstation 102, which operator workstation 102 may include a display 104, one or more input devices 106 (e.g., keyboard, mouse), and a processor 108. Processor 108 may include a commercially available programmable machine running a commercially available operating system. The operator workstation 102 provides an operator interface that facilitates the entry of scan parameters into the MRI system 100. The operator workstation 102 may be coupled to various servers including, for example, a pulse sequence server 110, a data acquisition server 112, a data processing server 114, and a data storage server 116. The operator workstation 102 and the servers 110, 112, 114, and 116 may be connected via a communication system 140, and the communication system 140 may include wired or wireless network connections.

The pulse sequence server 110 is used to operate a gradient system 118 and a radio frequency ("RF") system 120 in response to instructions provided by the operator workstation 102. Gradient waveforms for performing a prescribed scan are generated and applied to a gradient system 118, which gradient system 118 then excites the gradient coils in assembly 122 to produce magnetic field gradients G_x、G_yAnd G_zThese magnetic field gradients are used to spatially encode the magnetic resonance signals. The gradient coil assembly 122 forms part of a magnet assembly 124 that includes a polarizing magnet 126 and a whole-body RF coil 128.

RF waveforms are applied to the RF coil 128 or a separate local coil by the RF system 120 to perform the specified magnetic resonance pulse sequence. The responsive magnetic resonance signals detected by the RF coil 128 or a separate local coil are received by the RF system 120. The responsive magnetic resonance signals may be amplified, demodulated, filtered, and digitized under the direction of commands generated by the pulse sequence server 110. The RF system 120 includes an RF transmitter for generating a variety of RF pulses for an MRI pulse sequence. The RF transmitter responds to prescribed scans and directions from the pulse sequence server 110 to generate RF pulses of the desired frequency, phase and pulse amplitude waveform. The generated RF pulses may be applied to the whole-body RF coil 128 or to one or more local coils or coil arrays.

The RF system 120 also includes one or more RF receiver channels. The RF receiver channel includes an RF preamplifier that amplifies the magnetic resonance signals received by the coil 128 connected thereto; and a detector that detects and digitizes the I and Q quadrature components of the received magnetic resonance signal. The amplitude of the received magnetic resonance signal can thus be determined at the sampling point by the square root of the sum of the squares of the I and Q components:

and the phase of the received magnetic resonance signals may also be determined according to the following relationship:

the RF system 120 may also include one or more RF transmit channels that generate a specified RF excitation field. The fundamental or carrier frequency of the RF excitation field is generated under the control of a frequency synthesizer that receives a set of signals (e.g., digital signals) from the pulse sequence server 110. These signals indicate the frequency and phase of the RF carrier signal. An RF carrier is applied to the modulator and upconverter, where the amplitude of the RF carrier is modulated in response to a signal r (t) (also received from the pulse sequence server 110). The signal r (t) defines the envelope of the RF excitation pulses to be generated. For example, the amplitude of the RF excitation pulses is attenuated by an exciter attenuator circuit, which receives digital commands from the pulse sequence server 110. The attenuated RF excitation pulses are then applied to a power amplifier driving the pTX RF coil array.

The pulse sequence server 110 may receive patient data from the physiological acquisition controller 130. For example, the physiological acquisition controller 130 may receive signals from a number of different sensors connected to the patient, including electrocardiograph ("ECG") signals from electrodes, or respiratory signals from a respiratory bellows or other respiratory monitoring device. These signals may be used by the pulse sequence server 110 to synchronize or "gate" the performance of the scan with the subject's heartbeat or respiration.

The pulse sequence server 110 may also be connected to a scan room interface circuit 132, which scan room interface circuit 132 receives signals from various sensors associated with the condition of the patient and the magnet system. Through the scan room interface circuit 132, the patient positioning system 134 may receive commands to move the patient to a desired position during the scan.

The digitized magnetic resonance signal samples produced by the RF system 120 are received by the data acquisition server 112. In response to instructions downloaded from the operator workstation 102, the data acquisition server 112 operates to receive real-time magnetic resonance data and provide a buffer storage device so that no data is lost due to data overrun. In some scans, the data acquisition server 112 communicates the acquired magnetic resonance data to the data processor server 114. In scans where information derived from the acquired magnetic resonance data is needed to control further performance of the scan, the data acquisition server 112 may be programmed to generate and transmit such information to the pulse sequence server 110. For example, during a pre-scan, magnetic resonance data may be acquired and used to calibrate the pulse sequence performed by the pulse sequence server 110. As another example, navigator signals may be acquired and used to adjust operating parameters of the RF system 120 or gradient system 118, or to control the view order of sampling k-space. In yet another example, the data acquisition server 112 may also process magnetic resonance signals used to detect the arrival of contrast agent in a magnetic resonance angiography ("MRA") scan. For example, the data acquisition server 112 may acquire magnetic resonance data and process the magnetic resonance data in real time to generate information that is used to control the scan.

The data processing server 114 receives magnetic resonance data from the data acquisition server 112 and processes the magnetic resonance data in accordance with instructions provided by the operator workstation 102. Such processing may include, for example, reconstructing a two-dimensional or three-dimensional image by performing a fourier transform of the raw k-space data; performing other image reconstruction algorithms (e.g., iterative or backprojection reconstruction algorithms); applying a filter to the raw k-space data or the reconstructed image; generating a functional magnetic resonance image; or computing a motion or flow image.

The images reconstructed by the data processing server 114 are transmitted back to the operator workstation 102 for storage. The real-time images may be stored in a database memory cache from which the real-time images may be output to the operator display 102 or display 136. The batch mode image or the selected real-time image may be stored in a host database on the disk storage device 138. When such images have been reconstructed and transferred to a storage device, the data processing server 114 may notify the data storage server 116 on the operator workstation 102. The operator may use the operator workstation 102 to archive images, produce movies, or send images to other facilities via a network.

The MRI system 100 may also include one or more networked workstations 142. For example, the networked workstation 142 may include a display 144, one or more input devices 146 (e.g., keyboard, mouse), and a processor 148. The networked workstation 142 may be located within the same facility as the operator workstation 102, or in a different facility, such as a different medical facility or clinic.

The networked workstations 142 may obtain remote access to the data processing server 114 or the data storage server 116 via the communication system 140. Thus, a plurality of networked workstations 142 can access the data processing server 114 and the data storage server 116. In this manner, magnetic resonance data, reconstructed images, or other data may be exchanged between the data processing server 114 or data storage server 116 and the networked workstation 142 such that the data or images may be processed remotely by the networked workstation 142.

Referring to fig. 2, the present disclosure provides a method 200 of SMS MRF using a pTX RF coil array. At process block 202, the method 200 may include acquiring data from a subject using a series of variable sequence blocks, wherein a plurality of different slices are simultaneously excited using a pTX RF coil array. As described above, a series of variable sequence blocks may be implemented for this purpose. In the series of variable sequence blocks, a subset of the transmit channels available to the pTX RF coil array is used to excite a given slice, and the subset may vary in time in the variable sequence blocks. Typically, for a pTX RF coil array having N coils, the subset used to excite a given slice will contain M < N coils. As a non-limiting example, when two slices are simultaneously excited using a pTXRF coil array having four transmit channels, two of the channels may be used to excite a first slice while the other two of the channels may be used to excite a second slice. For example, a first RF excitation pulse may be generated using a first subset of the coils to excite spins in a first slice, while a second RF excitation pulse may be generated using a second subset of the coils to simultaneously excite spins in a second slice.

In some embodiments, multiband RF pulses may be generated using a subset of coils such that multiple slices are excited simultaneously by the subset of coils. For example, a first multiband RF excitation pulse may be generated using a first subset of coils to simultaneously excite spins in a first set of slices, while a second RF pulse (which may be a single band RF pulse or a multiband RF pulse) may be generated using a second subset of coils such that one or more slices are excited simultaneously with the first set of slices.

In general, for a pTX RF coil array having N coils used to simultaneously excite K slices, each subset of RF coils may include N/K coils, such that each coil in the pTX RF coil array is uniquely assigned to one of the subsets for each excitation. In other embodiments, a different number of RF coils may be included in each subset of RF coils. The number of coils may be the same or different in each subset used for a particular excitation.

The particular channel used in each excitation may then be temporally varied between the available channels such that the entire field of view is excited for each slice by executing a series of variable sequence blocks. For example, in the above example where a pTX RF coil array with a coil count N-4 is used to simultaneously excite two different slices, the first subset set may include using coils #1 and #2 to excite slice #1, and coils #3 and #4 to excite slice # 2. Then, in the subsequent variable sequence block, the subset group can be modified such that slice #1 is excited using coils #3 and #4 and slice #2 is excited using coils #1 and # 2. In some embodiments, the coils in the subset may be further arranged, such as by exciting slice #1 using coils #1 and #3, and slice #2 using coils #2 and # 4. In the latter example, the same coil may be used in different subsets to excite the same slice in different variable sequence blocks. However, in other examples, the subsets may be designed such that each coil is used only once to excite a particular slice. It will be understood that any variable combination of such number of coils is used in the subset.

In still other examples, the number of coils in a subset may also vary in time. For example, in the case of using a pTX coil array having a coil number N-8, in some variable sequence blocks each subset of coils may include 2 coils, while in subsequent variable sequence blocks each subset of coils may include more than 2 coils. Similarly, in some embodiments, the number of slices that are simultaneously excited may vary in time. As an example, the first series of variable sequence blocks may include simultaneously exciting two slices at a time, with four different coils used in each subset of coils. Then, in the following series of variable sequence blocks, four different slices can be excited simultaneously, with two different coils being used in each subset of coils. It will be appreciated that the number of coils used in the subset, the number of slices being simultaneously excited, or other such combinations of the two may be varied.

At process block 206, the method 200 may include comparing the data collected at process block 202 to a dictionary of signal evolutions. At process block 206, method 200 may include generating images of a plurality of different slices based at least in part on quantitative values estimated by comparing acquired data to a dictionary.

In one particular but non-limiting example application, SMS MRF imaging may be performed using a 16-channel pTX array. In this method, separate transmit channels are used to excite different slices. Since each transmit channel has a different B1+ profile, the transmit channels can be driven in such a way that the merged image of the multiple slices exhibits minimal aliasing.

Referring to fig. 3, a diagram illustrating the use of a pTX array for exciting different regions of multiple slices to avoid aliasing is shown. FIG. 3 shows an example in which 4 transmit channels (two top channels 302 and two bottom channels 304) are used to acquire a gradient echo (GRE) image. The image of slice 1 in fig. 3 shows a GRE image in which all four channels 302, 304 are used to excite slice 1. The image of slice 2 in fig. 3 shows a GRE image in which all four channels 302, 304 are used to excite slice 2. The images of half-slice 1 and half-slice 2 in fig. 3 show GRE images in which the top two channels 302 are used to excite slice 1 and the bottom two channels 304 are used to excite slice 2. Thus, the images of half-slice 1 and half-slice 2 of fig. 3 show the top half from slice 1 and the bottom half from slice 2.

The features shown in fig. 3 may be utilized in the MRF framework to perform SMS. The method involves temporally varying the transmit channels of excitation slice 1 and slice 2 to ensure that the entire field of view of all slices is captured. In some cases, the selection of a channel for a given time frame may be made in a similar manner to that shown in the images of half-slices 1 and 2 of fig. 3, such that each frame exhibits minimal aliasing of multiple slices.

Referring to fig. 4A, an example pulse sequence that may be implemented in one or more variable sequence blocks is shown. Fig. 4B shows a pattern for changing sequence parameters in an example series of variable sequence blocks. As shown, the transmit channels used to excite different slices in each sequence block vary in time.

For reconstruction, MRF template matching may be performed based on pattern recognition. The dictionary may be generated using any suitable technique for MRF. In one example approach, a separate dictionary for each spatial location may be generated using B1+ measured in a separate scan. In another example approach, B1+ terms for different channels may be included as additional dimensions in the dictionary.

The present disclosure has described one or more preferred embodiments, and it is to be understood that many equivalents, substitutions, variations, and modifications, aside from those expressly stated, are possible and within the scope of the present invention.