Patent Description:
Ultra-high field magnetic resonance imaging offers improved signal-to-noise ratios and enhanced resolution compared to lower field strengths. However, at 7T, the wavelength of the radiofrequency (RF) pulses which are necessary to excite the spins within the field-of-view, approach the dimensions of the human head, resulting in increased B1+ inhomogeneity and signal dropouts.

One way to combat this increased inhomogeneity of the B1+ field generated by the RF coils, is to use parallel transmission (pTx), which makes use of RF coils comprising multiple independent transmitters, such as an array coil. Each transmitter may correspond to an RF channel. In a pTx system, each RF channel is independent and can play RF pulses with different shapes, amplitudes and phases. These additional degrees of freedom can be manipulated to improve excitation homogeneity.

For slice-selective excitation, so-called spokes RF pulses are sometimes used. Spokes pulses consist of one or more slice-selective pulses played consecutively, interleaved with gradient blips. These gradient blips determine the transmit k-space locations of the spokes and have to be optimised to maximise excitation homogeneity. The principle of spokes is disclosed in <NPL>) as well as in the thesis by <NPL>). A spokes pulse can also be applied in single channel RF transmission using a single RF coil, though it is often combined with pTx, i.e. multi-channel transmission.

In traditional single-band excitation, where each RF pulse excites one slab or slice, the positions of the spokes can be determined via an inverse Fourier transform method. This is a fast, computationally simple algorithm suited to designing subject-specific spokes pulses in real-time on the scanner. It has also been demonstrated that slice-specific optimisation performs better than a whole-volume-based optimisation when acquiring multiple slices across a volume. The inverse Fourier transform method is disclosed in <NPL>).

However, this approach breaks down when considering simultaneous multi-slice (SMS) excitation, also known as multi-band (MB) excitation or pulses. This is because different slices will have different optimal spokes locations and different RF shim weightings, i.e. the amplitudes and phases with which the individual channels of a parallel transmission Rf coil are played. <NPL>), outlines a design method for a general parallel spectral-spatial excitation that achieves a targeterror minimization simultaneously over a bandwidth of frequencies and a specified spatial-domain. TSE <NPL>) addresses the challenges of B1 inhomogeneity and long acquisition time. Flip angle homogenized excitations were achieved by parallel transmission (pTx) of <NUM>-spoke pulses, designed by magnitude least-squares optimization in a slice-by-slice fashion.

Previous works have optimized static RF shims (i.e. amplitude and phase of the individual RF channels) for MB imaging in the brain, as described in <NPL>), and <NPL>). A similar approach for MB imaging in the heart is disclosed in <NPL>)). However, these works do not use spokes RF pulses, but only one sub-pulse without involving in-plane gradients.

It is an object of the invention to provide a method to optimise multi-band RF excitation pulses to compensate for patient-specific B1+ inhomogeneity. It is a further object to provide a method for calculating slice specific multi-band spokes RF pulses with optimal spokes locations to be designed.

These objects are met or exceeded the method according to claim <NUM>, a computer program according to claim <NUM>, a non-transient computer-readable medium according to claim <NUM>, a control unit according to claim <NUM> and a magnetic resonance system according to claim <NUM>. These claims define the invention.

According to a first aspect of the invention, a method, carried out by a computer or a control unit for a magnetic resonance imaging system, for subject-specific optimisation of a multi-band RF pulse for exciting spins within a field-of-view to obtain a target magnetisation in a magnetic resonance imaging examination of a subject is provided.

The RF pulse to be optimised is a so-called "spokes RF pulse", as described in the reference by Padormo et al. , in which a number of sub-pulses or spokes, which each provide slice selectivity, are interleaved with gradient blips, so that the transverse magnetisation follows a defined trajectory in transmit k-space. Each sub-pulse and therefore the RF pulse as a whole is a multi-band pulse, i.e. is designed to excite several slices or slabs in the slice select direction. This makes it difficult to optimise the spokes locations in k-space, since the optimal k-space trajectory for each slice may be different. The term "slice" is meant to also incorporate "slabs", i.e. thicker slices. The slice excited by each sub-pulse may have a thickness between <NUM> and <NUM>, preferably between <NUM> and <NUM>, most preferred between <NUM> and <NUM>. Each sub-pulse may be a summation of sinc pulses at different frequencies to realise a multi-band pulse.

The RF pulse may be used in the context of a magnetic resonance imaging examination of a subject such as a patient, in particular of a certain body part of the subject. The body part may be the brain, heart, lungs, thorax, leg, arm, shoulder, or any other organ or body part of human. The RF pulse may be used in any type of magnetic resonance image acquisition sequence, such as spin-echo sequences and gradient-echo sequences, e.g. turbo spin-echo or FLASH.

The field-of-view excited by the multi-band RF pulse may comprise at least <NUM> slices through the body part, for example to <NUM> to <NUM> slices through the human head. The several slices excited in one multi-band RF pulse is termed slice group herein.

The method performs a subject specific optimisation, which means that the RF pulse is optimised for each magnetic resonance imaging session, taking into account at least one of a B0-field map and a B1+ field map, which has been obtained during the same imaging session on the field-of-view to be imaged using the multi-band RF pulse. In other words, the RF pulse is not optimised on a phantom or based on simulation, but the method performs a subject-specific spokes location optimisation.

The method of the invention presents an algorithm which extends the inverse Fourier method to multi-band pulses, allowing for slice specific multi-band spokes RF pulses with optimal spokes locations to be designed.

In detail, the method of the invention uses a starting k-space position, which may be predetermined, as the current k-space position. It then performs an inverse Fourier optimisation method for each slice separately. This is performed, for each slice in the slice group, by calculating a sub-pulse based on the current k-space position and calculating the expected magnetisation, b, which is expected to result from this sub-pulse. The term "magnetisation" is used herein to designate the transverse magnetisation distribution over the respective slice. The magnetisation b may be a 2D matrix or may be represented by a vector. The calculation of the sub-pulse may be done by a magnitude least squares (MLS) method, as described in the review article by Padormo et al. Therein, a cost function term is minimized, which is based on the sensitivity matrix S, which is constructed from the B1+ field information of all RF channels within the field-of-view, a vector w containing the complex weights to each channel, and a vector btarget containing the desired B1+ field distribution. Other possible methods are the linear least squares method. These methods are known in the art and are sometimes termed "static" pTx optimization, because they optimize the complex weights with which each channel of a multichannel RF coil is to be driven, using the sensitivity matrix S derived from B1+ view maps, and using a certain constraint, in particular a static constraint.

Once the expected magnetisation distribution has been calculated, an inverse Fourier transform of the difference between the expected mechanisation and the target magnetisation is calculated. Again, each of expected magnetisation and target magnetisation is a distribution over the field-of-view, and may be represented by a vector. The distribution of the differences may be termed bdiff. A fast inverse Fourier transform (IFFT) algorithm may be used to perform the inverse FT. From the difference vector, the optimal k-space position for the next spoke for this slice is determined to be at the position where the absolute value of the inverse Fourier transform has a maximum. In other words, for the next sub-pulse, it is best to move to the k-space location of maximum distance to the target magnetisation, so that the next sub-pulse will be applied at the k-space location where maximum correction is needed.

This is repeated for each slice in the slice group, wherein possibly a different optimal k-space location for the next spoke will be determined for each slice.

The method of the invention therefore determines the next k-space position (which must be the same for all slices) on the basis of the optimal k-space positions determined for each slice individually. Preferably, the optimal k-space position of every slice in this slice group is taken into account. This may be done in several ways, wherein preferably each slice contributes to the next k-space position.

So far, the method has determined the k-space positions of two spokes, namely the spoke at the starting k-space position and a second spoke. In order to determine the k-space positions of further spokes, steps b and c may be repeated.

In useful embodiments, the number of spokes of the multi-band RF pulse is predetermined, and thus steps b and d are repeated a predetermined number of times, for example <NUM> to <NUM> times. In another embodiment, the method stops when the difference between the expected magnetisation and the target magnetisation has reached a minimum. This minimum may be predetermined, and may relate to the absolute value of inverse Fourier transform of the difference. It may for example be an average or a maximum of the absolute value of inverse Fourier transform over the difference magnetisation distribution. In another embodiment, the method stops when the relative change of the expected magnetisation between two successive iterations is below a threshold. The threshold may be predetermined.

Finally, the complete RF pulse, including all sub-pulses, is calculated/optimised based on the previously determined k-space positions of the spokes. This may be done by first optimizing the RF pulse for each slice using the determined spoke positions, and then combining the optimized single-band spokes pulses to give the final RF pulse. A suitable calculation method is for example disclosed in the above-cited paper by <NPL>). The optimisation step generates a set of per-channel complex weights (amplitude and phase) for each spoke. According to an embodiment of the invention, a magnitude least square cost function may be used, but it is possible to use other cost functions (see table <NUM> of the Padormo paper).

The method of the invention thereby applies an iterative inverse Fourier method to multi-band spokes RF pulse design.

According to a preferred embodiment, the next k-space position for all slices together is determined by calculating a weighted mean of the optimal k-space positions determined for each slice individually. In other words, the method takes a weighted mean of the position of the spokes in the individual slices and uses it for the final multi-band RF pulse design. This has the advantage that each slice contributes to the final multi-band RF pulse design. The weighting may be adjusted such that the overall RF pulse is optimised to obtain the best possible B1+ homogeneity over all slices in the slice group.

According to an embodiment, the weight of each individual slice in the weighted mean is the same, it may for example be (<NUM>/Number of Slices). In other words, the mean position of all slices is calculated from the optimal k-space position determined for each slice. Thereby, each slice is weighted equally, which is a good approach if each slice is considered equally important.

According to an alternative embodiment, the weight of each individual slice in the weighted mean is proportional to the number of tissue pixels in the individual slice. Thus, each slice may be weighted by the number of pixels in the image mask for that slice, wherein the image mask may represent the outer boundary of the body part, as visible on that slice. This embodiment is useful when slices through the body part have considerably differing sizes, as for example in axial slices through the head. The slices nearer the crown of the head have a smaller image mask, and therefore may be given less weight in the weighted mean.

According to another embodiment, the weight of each individual slice in the weighted mean is proportional to the amplitude of the maximum inverse Fourier transform of that slice. For example, the IFFT amplitude-weighted mean of the positions of the spokes across all slices in the slice group may be taken. Thereby, those slices which have the largest amplitude of the maximum inverse Fourier transform, and thus are furthest from the target magnetisation, are given more weight than those where the difference between the expected magnetisation and the target magnetisation is smaller. This method may also provide very fast optimisation, requiring few spokes in the multi-band RF pulse.

According to yet another embodiment, the next k-space positions for all slices together is determined in step c by taking the optimal k-space position of the slice which has the largest maximum inverse Fourier transform. This may be considered a simplified version of the previous embodiment, since the slice having the largest maximum is used to determine the next spoke position. "Large" may refer to the absolute value of the maximum inverse Fourier transform of that slice.

The method of the invention may be useful in single-channel RF pulse design. More preferred, it is used in multi-channel RF pulse design. The method may include a step of receiving a subject-specific B1 field map or B1+ field map of the field-of-view, wherein the RF pulse is a parallel transmission pulse and wherein the sub-pulses of the RF pulse are calculated by optimizing the weights, with which individual channels of a parallel transmission RF coil are driven, based on the B1+ field map.

According to an embodiment, the starting k-space position is predetermined. Thereby, no further optimisation has to be done. In a preferred embodiment, the starting k-space position is at the centre of k-space. Most spokes RF pulses include one spoke at the centre of k-space, however this is no absolute requirement. One may also start at a predetermined position close to the centre of k-space, or possibly two or three spokes may have predetermined positions close to the centre of k-space.

According to an embodiment, the method stops once a predetermined number of spokes positions has been determined (including the starting position), wherein the predetermined number may be <NUM> to <NUM>, preferably <NUM> to <NUM>, more preferably <NUM> to <NUM>. It has been shown that very good B1+ field homogeneity can be reached even with as few spokes as that.

According to an embodiment, the predetermined number of slices to be excited by the multi-band RF pulse is <NUM> to <NUM>, preferably <NUM> to <NUM>, more preferred <NUM> to <NUM>. These are numbers of slices which may be realised within one slice group with still satisfactorily homogenous RF excitation over the slice group.

According to a further aspect of the invention, a computer program is provided, which includes program code which causes a computer to carry out the method as described herein, when the computer program is executed on a computer. The computer may include a control unit for a magnetic resonance imaging system, as described herein below. The computer program may be written in any known programming language. It may be a programming language which is used on MRI systems to calculate/optimise RF pulses.

According to a further aspect of the invention, a non-transient computer-readable medium comprising a computer program as described herein is provided. The non-transient computer-readable medium may for example be any digital storage medium, such as a hard drive, a server, a cloud, a computer, an optical and/or magnetic storage medium, a CD-ROM, an SSD, an SD card, a DVD, a Blu-ray disc and/or a USB stick. All features and advantages of the method may be adapted to the computer-readable medium and vice versa.

According to further aspect of the invention, a control unit for a magnetic resonance imaging system is provided, wherein the control unit is configured to carry out the method according to the invention. The control unit may be any calculating unit, such as a CPU or GPU, and may be part of the computer, server, cloud computer et cetera. The control unit may particular be part of the hardware used to control the MRI system. The control unit may also be part of a mobile device such as a laptop, tablet computer or smartphone.

Finally, the invention is directed to a magnetic resonance imaging system comprising a control unit as described herein. In useful embodiments, the MRI system operates at high static magnetic field strength, such as 3T or above, such as 3T to 12T, more preferred 5T to 7T. However, a low field scanner, for example operating at <NUM> T, may also profit from the invention.

All features and advantages of the method may be adapted to the control unit and the MRI system, and vice versa.

The invention shall now be described by means of embodiments with reference to the attached drawings, in which:.

<FIG> depicts the k-space trajectory of a spokes RF pulse <NUM>. The slice select direction is z, so each sub-pulse <NUM> is played simultaneously with a slice select gradient in z- direction, and therefore the trajectory during the sub-pulses <NUM> runs parallel to the direction kz in k-space, as depicted in <FIG>. The depicted spokes RF pulse <NUM> has <NUM> spokes, one at the centre of k-space where kx and ky are <NUM>. In between the spokes <NUM>, gradient blips in the in-plane direction kx and ky are played, resulting in trajectory segments <NUM>, which connect the spokes <NUM> in the kx-ky-plane. According to the method of the invention, the exact position in the kx-ky-plane of the spokes <NUM>, in particular of the spokes <NUM> which are not in the centre of k-space, are optimised in a patient specific way for multiple slices, which are excited with the spokes RF pulse <NUM> simultaneously.

An example of an algorithm which embodies a method according to an embodiment of the invention is depicted in <FIG>. The method is applied to optimizing a multi-band RF pulse with an MB factor of NMB, which is the number of slices excited simultaneously with one RF pulse. If the total number of slices to be imaged is Nsl, there will be a total of Nsl/NMB = Nslgp 'slice groups', each requiring an RF excitation pulse. For each slice group, i, where i ∈ Nslgp, NMB slices have to be excited at the same time. The method described may be considered as a workflow for designing an MB pulse for a single slice group.

In the <NUM>st step <NUM>, i is set to <NUM>, i.e. the first spoke. The starting k-space position is taken as the current k-space position (also referred to as spoke position) for i = <NUM>, in this case it is predetermined to be at the centre of k-space: <MAT>.

The next steps <NUM> to <NUM> will be repeated for each slice j in the slice group.

In step <NUM>, the spoke position for the slice j, <MAT>, is set to equal the current spoke position <MAT>.

In step <NUM>, a sub-pulse is calculated by designing an RF shim based on spoke position, <MAT>, and the resultant expected magnetisation, b, is calculated.

In step <NUM>, the inverse Fourier transform of the difference between resultant expected magnetisation and target magnetisation is calculated for slice j: IFFT(b - btarget) = IFFT(bdiff).

In step <NUM>, the optimal new spoke location (kx,ky) for slice I is determined by the location coordinates of max(abs(IFFT(bdiff))). The optimal new spoke position for slice j is set to this k-space location <MAT>.

At <NUM>, if j ≠ NMB, the algorithm increments j by <NUM> and goes back to step <NUM>, to repeat steps <NUM> to <NUM> for the next slice in the slice group.

If j = NMB, i.e. if the optimal new spoke location has been determined for all slices, the algorithm proceeds to step <NUM>. In step <NUM>, the next k-space position for all slices together is determined based on the optimal k-space positions for each slice. There are several ways of doing this. According to one embodiment, the mean of the optimal new spoke positions for all slices is calculated and used for the final multi-band pulse design: <MAT>.

In step <NUM>, the counter i for the spokes is increased by <NUM>, i.e. i = i + <NUM>.

In step <NUM>, it is determined whether the maximum predetermined number of spokes for the pulse has been reached, i.e. if i = Nsp. If no, the algorithm jumps back to step <NUM>, using the determined optimal new spoke position of all slices <MAT> as the current k-space position.

This is repeated until i = Nsp, wherein Nsp spokes is a predetermined number of spokes of the RF pulse.

The method then moves on to step <NUM>, in which, for each slices j in the slice group, the RF pulse, Pj, is optimized for Nsp spokes with spoke positions <IMG>.

In step <NUM>, the RF pulses Pj corresponding to each slice j in the slice group are combined Pj∀j ∈ [<NUM>,NMB] to give the final MB pulse, P.

Step <NUM> has several different embodiments. According to a <NUM>nd embodiment, not the mean of the optimal spokes position in the individual slices is taken as next spoke position. Rather, the spokes location corresponding to the largest Fourier transform residue in the slice group is taken.

An example workflow corresponding to this embodiment is described below.

Two further embodiments of the MB spokes design algorithms are described below, in which step <NUM> may be varied as follows:
According to a <NUM>rd embodiment, the ROI-weighted mean of the positions of the spokes are taken across all slices in the slice group, i.e. setting <MAT>, where Wj is the number of pixels in the image mask for slice j, and <MAT> <MAT>.

According to a <NUM>th, embodiment, the IFFT amplitude-weighted mean of the positions of the spoke across all slices in the slice group is taken, i.e. setting <MAT>, where Wj is the number of pixels in the image mask for slice j, and <MAT>.

The four embodiments for determining the next k-space position based on the optimal k-space positions determined for each slice individually were compared with each other and with the performance of an optimized single-band RF pulse. Magnetic resonance imaging of the brain was performed on six healthy volunteers using the multi-band RF pulses calculated according to the four embodiments of the invention, and a single-band RF pulse. The optimisation method was performed using whole-brain per-channel B1+ and ΔB<NUM> maps obtained from the six volunteers. The following pulse design parameters were used: MB factor = <NUM>; target flip angle (FA) = <NUM>°; number of spokes = <NUM> (<NUM> per pulse, Hanning-filtered sinc). No constraints were applied to the overall peak voltage of the pulse.

The result of the comparison is shown in <FIG>, which depicts the root-mean-square error (RMSE) across the whole brain for all volunteers across five different optimisation methods. Each dot represents a volunteer, while the black crosses and error bars represent the mean and standard deviation RMSE values respectively. Taking the mean spoke locations across the simultaneously excited slices yielded the smallest RMSE values out of the four embodiments of the MB spokes location optimisation method. To quantify the differences, especially between the prior art single-band optimisation method and the inventive embodiments, four paired Student's t-tests were performed using 'mean' as a reference. There were no significant differences between the single-band optimisation method and taking the mean spokes location in MB optimisation, implying that it has a good performance.

In other words, the method of the invention has been shown to have extremely good performance, since the RMSE is comparable to the single-band pulse optimisation, which naturally results in RF pulses which require much longer acquisition times, since multi-slice excitation is not possible.

<FIG> shows an embodiment of a magnetic resonance imaging system <NUM> according to the invention. The magnetic resonance imaging system <NUM> comprises a magnetic resonance scanner <NUM> and a control unit <NUM>. The control unit <NUM> is configured to receive B1+ field maps and to perform the multiband RF pulse optimisations described herein. A User may interact with the control unit <NUM> via a user interface <NUM>, which may include a screen and a keyboard, and the control unit <NUM> is configured to output information and/or suggestions via an output device of the user interface <NUM> and to receive user input via a user input device of the user interface <NUM>. A computer program according to the invention may be stored on disc <NUM> and loaded into the control unit <NUM>.

Claim 1:
A method, carried out by a computer or a control unit for a magnetic resonance imaging system, for subject-specific optimisation of a multiband RF pulse (<NUM>) for exciting spins within a field-of-view to obtain a target magnetisation in a magnetic resonance imaging examination of a subject (<NUM>),
wherein the RF pulse is a spokes RF pulse comprising a train of sub-pulses (<NUM>), called spokes, interleaved with gradient blips, wherein the gradient blips determine the trajectory of a magnetisation in transmit k-space so that each sub-pulse is played at a specific position in transmit k-space, and wherein each sub-pulse (<NUM>) is to excite a pre-determined number of slices simultaneously, the method comprising the steps of
a1. Receiving a subject-specific B1+-field map of the field-of-view,
a2. Receiving a starting k-space position and using that as the current k-space position,
b. For each of the slices to be excited by the RF pulse, performing the following steps (i) to (iii):
i. calculating (<NUM>) a sub-pulse (<NUM>) based on the current k-space position, taking into account the B1+-field map, and calculating the expected magnetization after that sub-pulse (<NUM>);
ii. calculating (<NUM>) the inverse Fourier transform of the difference between the expected magnetization and the target magnetisation;
iii. determining (<NUM>) an optimal k-space position for the next spoke for this slice to be at the position where the absolute value of the inverse Fourier transform has a maximum;
c. determining (<NUM>) the next k-space position for all slices together based on the optimal k-space positions determined for each slice individually;
e. calculating (<NUM>, <NUM>) the multi-band RF pulse (<NUM>) based on the k-space positions determined in step c.