Method for designing a pulse sequence for parallel transmission MRI and method for performing parallel transmission MRI using such a pulse sequence - Patents.com

Abstract

To provide a method of designing a pulse sequence for parallel-transmission MRI and a method of performing parallel-transmission MRI.SOLUTION: The present disclosure provides a method comprising the steps :a) for each one of a plurality of subjects, estimating a linear adjustment transformation, and converting amplitude maps of RF fields generated by respective transmit channels of a MRI apparatus into respective standardized maps; and b) determining RF waveforms minimizing a discrepancy between subject-specific distributions of flip-angles of nuclear spin and a target distribution averaged over the subjects, the subject-specific distributions corresponding to the flip-angle distributions achieved by applying a superposition of RF fields, each having a temporal profile described by one of the RF waveforms and a spatial amplitude distribution described by a respective standardized map determined for the subject. A method and an apparatus for performing parallel-transmission MRI using such a pulse sequence are also provided.SELECTED DRAWING: Figure 1A

Description

[Technical field]

[0001] The present invention relates to a method for designing parallel transmission magnetic resonance imaging pulse sequences and a method for performing parallel transmission magnetic resonance imaging using such sequences. The pulse sequence includes one or more radio frequency (RF) waveforms or pulses and at least one magnetic field gradient waveform, allowing manipulation of the nuclear magnetization of a sample immersed in a static magnetic field due to the reorientation of nuclear spins. [Background technology]

[0002] High-field magnetic resonance imaging (MRI) has proven its usefulness in clinical routine, thanks to the high signal-to-noise ratio (SNR) it offers, allowing finer temporal and / or spatial resolution. However, several problems inherent to high magnetic fields still prevent widespread use of high-field (3T-7T) scanners in hospitals around the world. Among these are the "B1 artifacts" that occur when the RF wavelength approaches or is smaller than the imaging volume ([1]). In such cases, the transmitted radio frequency "B1 + "The magnetic field becomes inhomogeneous within the region of interest, resulting in an inhomogeneous distribution of the nuclear magnetization flip angles. This leads to the appearance of shadow areas and reduced contrast, which can affect the diagnosis by masking the lesions or by changing the observed enhancement ratio in contrast injection sequences. At 3T, this artifact has particularly serious consequences in abdominal imaging and is of major concern in brain imaging at very high fields (UHF-7T and above).

[0003] Passive RF shimming involves the use of dielectric pads to compensate for B1 inhomogeneities ([2]). Its effectiveness is, for some reason, limited.

[0004] Active RF shimming utilizes parallel transmission (pTx), ie, multiple RF coils, to excite the nuclear spins. There are two main types of active RF shimming techniques: static and dynamic shimming.

[0005] In static active RF shimming, multiple radio frequency RF coil elements all transmit the same RF pulse with different complex weights optimized to homogenize the B1 field [3]. For abdominal imaging at 3T, this technique is satisfactory for most patients, but in about 10-20% of cases it does not provide a sufficiently homogeneous excitation and is unacceptable. For higher field values, static active RF shimming is usually not satisfactory.

[0006] Dynamic RF shimming [4] consists of using multiple RF coils to simultaneously transmit RF pulses with different temporal waveforms, usually defined by a complex time-varying envelope. This technique allows for better uniformity than static shimming, but its complexity makes it essentially a research tool.

[0007] Reference [5] demonstrates the superiority of dynamic RF shimming at kT [6] in clinical routine for non-selective excitation of the abdomen at 3T.

[0008] Optimization of the weighting coefficients (static shimming) or RF waveform (dynamic shimming) requires at least B1 + Moreover, for optimal dynamic RF shimming, an off-resonance frequency △f0 map (corresponding to the static magnetic field inhomogeneity △B0 map in the absence of chemical shifts such as in adipose tissue) is also required before the calculation of the RF pulses. These calculations are time consuming. For example, for a two-channel 3T scanner, the whole calibration process takes almost 2 minutes, i.e., 10 minutes for the B1 map and 10 minutes for the B2 map. + Mapping can take 30 seconds, Δf0 mapping can take 15 seconds, and the pulse design itself can last from 5 seconds (static RF shimming) to 60 seconds (kT points). This gets even worse as the number of channels increases.

[0009] The "universal pulse" introduced in [7] enables calibration-free dynamic RF shimming: instead of designing a unique pulse for each subject, calibration data from a population of subjects is used to create a pulse once that is robust to inter-subject variability. The universal pulse has been successfully implemented in the brain at 7T, using a wide variety of sequences and doses, as well as different underlying pulse designs.

[0010] However, universality comes at the expense of individual homogeneity: a universal pulse, even if it provides acceptable results for most subjects, fails to provide sufficiently homogeneous excitation in a significant number of cases.

[0011] A compromise solution proposed in [8] consists of calculating different "semi-universal" pulses tailored to different groups of imaging subjects. More precisely, according to [8], the set of subjects is grouped into groups according to morphological features - for example, head size in the case of brain imaging - and a "semi-universal" pulse sequence is designed for each group. The same morphological features are then used to select the most suitable pulse sequence for additional subjects. The drawback of this approach is that the groups are formed on empirical grounds and there is no guarantee of the validity of the features used to classify the imaging subjects.

[0012] To overcome this shortcoming, [9] presents a method called "Smart Pulse", in which a population of subjects is divided into clusters and one quasi-universal pulse sequence is designed for each cluster. A machine learning algorithm then classifies new subjects and assigns each subject the best possible pulse based at least on morphological features. In contrast to the "semi-universal" approach of [8], the clusters are not empirically defined based on somewhat arbitrary morphological features, but are instead defined systematically using a clustering algorithm. [Prior art documents] [Patent documents]

[0013]

Patent Document 1

Non-licensed literature

[0014] [Non-licensed document 1] 1.Bernstein MA, Huston J, Ward HA.Imaging artifacts at 3.0TJ Magn Reson Imaging.2006;24(4):735-746.doi:10.1002 / jmri.20698

Non-licensed Document 2

Non-licensed Document 4

Non-Patent Document 5

Non-Patent Document 6

Non-Patent Document 7

Non-Patent Document 8

[0015] The present invention aims to provide a pulse design methodology that offers superior performance in terms of fidelity to the target excitation pattern compared to "smart pulse" approaches, without the need for session-specific pulse engineering, e.g., as in [4]-[6], but with only a much simpler calibration process. [Means for solving the problem]

[0016] The present invention is

number

number

number

number

[0017] Here, the calculation of the rectification transformation (matrix) is object-specific.

number

number

number

[0018] As mentioned above, knowing the session-specific adjustment matrix is ​​easily accessible using fast calibration techniques, so that

number

[0019] The inventors have experimentally verified that the covariance matrix of the standardized TX magnetic field distribution measured across several different individuals is substantially reduced compared to the covariance matrix of the actual physical TX magnetic field. In other words, the rectification transformation allows the mapping of the TX magnetic field distribution in a virtual space, reducing the inter-subject variability of the resonant TX magnetic field.

[0020] The idea behind the present invention is to design a "universal" pulse sequence for multiple subjects using a standardized magnetic field instead of the actual physical TX magnetic field.When a subject needs to undergo pTX MRI, a simple calibration method is used to estimate the adjustment transformation of the subject, and the pulse sequence suitable for this subject is then calculated by applying the inverse adjustment transformation of the subject to the "universal" sequence.Unless otherwise stated, the (inverse) adjustment transformation allows the "customization" of the universal sequence derived from the standardized magnetic field.

[0021] It is therefore an object of the present invention to provide a computer-implemented method for designing a parallel transmission magnetic resonance imaging pulse sequence, the pulse sequence including at least magnetic field gradient waveforms and a set of radio frequency waveforms, each of the radio frequency waveforms being associated with a respective transmit channel of a parallel transmission magnetic resonance imaging device, the method comprising: a) estimating, for each of a plurality of magnetic resonance imaging subjects, a linear transformation, called a rectification transformation, to transform amplitude maps of radio frequency magnetic fields generated by respective transmit channels of the magnetic resonance imaging device in a region of interest comprising a body part of the subject into respective standardized maps, the rectification transformation being chosen in such a way as to minimize a first cost function representing the average difference between the standardized map of the subject and respective reference amplitude maps of reference subjects; b) determining at least the radio frequency waveforms in such a way as to minimize a second cost function representing the difference between a subject-specific distribution of nuclear spin flip angles and a target distribution averaged over the plurality of magnetic resonance imaging subjects, the subject-specific distribution corresponding to the distribution of flip angles being achieved by applying a superposition of magnetic field gradient waveforms and radio frequency magnetic fields, each of the radio frequency magnetic fields having a temporal profile described by one of the radio frequency waveforms and a spatial amplitude distribution described by a respective standardized map determined for the subject; The method includes:

[0022] Another object of the present invention is to provide a method of performing parallel transmission magnetic resonance imaging of an object using a parallel transmission magnetic resonance imaging apparatus including a set of gradient coils, a plurality of transmit channels, and a plurality of receive channels, the method comprising the steps of: i) using receive channels of the device to perform measurements representative of the spatial distribution of radio frequency magnetic fields generated by the respective transmit channels within a region of interest that includes a body part of the subject; ii) using the results of said measurements to estimate a linear transformation, called the adjustment transformation, to transform the amplitude maps of the radio frequency magnetic field generated in the region of interest by each transmit channel of the magnetic resonance imaging device into a respective standardized map, the adjustment transformation being chosen in such a way as to minimize a cost function that represents the average difference between the standardized map of the subject and each reference amplitude map of a reference subject; iii) calculating a set of subject-specific radio frequency waveforms by inverting the adjustment transform and applying the inverted adjustment transform to a set of predetermined reference radio frequency waveforms; iv) applying a respective subject-specific radio frequency waveform to a respective transmit channel while applying a magnetic field gradient waveform to the gradient coils, and receiving parallel transmitted magnetic resonance imaging signals using the receive channels; The method includes:

[0023] Yet another object of the present invention is to provide a parallel transmission magnetic resonance imaging apparatus including a set of gradient coils, a plurality of transmit channels, a plurality of receive channels, and a data processing unit, comprising: - further comprising a memory device for storing data defining a set of predetermined reference radio frequency waveforms and magnetic field gradient waveforms; - a data processing device, - programmed or configured to drive the receive channels to perform measurements representative of the spatial distribution of the radio frequency magnetic fields generated by the respective transmit channels within a region of interest that includes a body part of the subject; - programmed or configured to use the results of said measurements to estimate a linear transformation, called the adjustment transformation, to transform the amplitude maps of the radio frequency magnetic field generated by each transmission channel in the region of interest into a respective standardized map, the adjustment transformation being chosen in such a way as to minimize a cost function representing the average difference between the standardized map of the subject and each reference amplitude map of a reference subject, - programmed or configured to calculate a set of subject-specific radio frequency waveforms by inverting the adjustment transform and applying the inverted adjustment transform to a set of predefined reference radio frequency waveforms; and - programmed or configured to drive the transmit channels to play respective subject-specific radio frequency waveforms, the gradient coils to play magnetic field gradient waveforms, and the receive coils to receive parallel transmitted magnetic resonance imaging signals; The present invention relates to a parallel transmission magnetic resonance imaging apparatus.

[0024] Additional features and advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings. [Brief description of the drawings]

[0025] [Figure 1A] 1 is a flowchart of a method for designing a "standardized" pulse sequence according to an embodiment of the present invention and a flowchart of a method for applying such a pulse sequence in an MRI session according to an embodiment of the present invention. [Figure 1B] 1 is a flowchart of a method for designing a "standardized" pulse sequence according to an embodiment of the present invention and a flowchart of a method for applying such a pulse sequence in an MRI session according to an embodiment of the present invention. [Figure 2A] This is the second order statistics of the radio frequency magnetic field generated by the transmit channel of a parallel transmission MRI machine. [Figure 2B] This is the second order statistics of the radio frequency magnetic field generated by the transmit channel of a parallel transmission MRI machine. [Figure 3A] 2C is a second order statistic of the normalized magnetic field corresponding to the radio frequency magnetic field of FIGS. 2A and 2B. [Figure 3B] 2C is a second order statistic of the normalized magnetic field corresponding to the radio frequency magnetic field of FIGS. 2A and 2B. [Figure 4A] 1 is a graphical representation of the adjustment matrix for one particular subject. [Figure 4B] 1 is a graphical representation of the adjustment matrix for one particular subject. [Figure 4C] 1 is a graphical representation of the adjustment matrix for one particular subject. [Diagram 5] 1 is a plot illustrating the effect of reducing the number of measurements used to estimate the adjustment matrix. [Figure 6] 1 is a plot illustrating the performance of various embodiments of the method of the present invention compared to the prior art. [Figure 7] 1 is a schematic representation of a parallel transmission magnetic resonance imaging apparatus suitable for implementing the present invention; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] The parallel transmission magnetic resonance imaging system implements N independently driven transmit channels. c It contains an array of one RF coil (or resonator), often cylindrical, that surrounds a region into which a body part of an MRI subject (e.g., the head) can be inserted.

[0027] FIG. 1A illustrates various steps of a method for designing a pulse sequence according to one embodiment of the present invention.

[0028]

number

number

number

[0029] It is important to recall that the spatial distribution of the RF magnetic field is not only a function of the geometry of the array, but is also influenced by the presence of body parts and therefore depends on the MRI subject. The control field vector for a given realization, and therefore of the subject, can be measured using several known techniques. See, for example,

[10] .

[0030] For a given realization of the control field ω1 corresponding to a particular MRI subject -R specifies the set of positions r for which ω1(r) is known. Since the distribution of the control field is usually obtained by MR (Magnetic Resonance) imaging, in practice, R is limited in space (there is no MR signal in air,

number

[0031] Control magnetic field distribution

number

number

number

[0032] The control magnetic fields measured from N>1 (typically at least 10, and typically several tens) subjects plus a reference subject constitute the input data for a pulse design method according to one embodiment of the invention, as illustrated in FIG. 1A.

[0033] In some embodiments of the present invention, some or all of the control magnetic field may be calculated by performing a full electromagnetic simulation that takes into account the anatomical structure of a real or virtual subject.

[0034] R ref Let L denote the region where the reference control magnetic field is defined. (n) is N that satisfies the following formula c ×N c Let us denote the complex matrix (rectification transform, or matrix) of

number

number

[0035] By adjusting the conversion, Ω 1,ref It becomes possible to define the standardized control magnetic field (or simply the "standardized magnetic field") as follows:

number

[0036] We assume that both the control field and the standardized control field are random variables following the Gaussian law defined by their second order statistics: ω1(r)~N(μ(r),C(r)) and

number

number

[0037] A similar formula allows the calculation of the second order statistics of the standardized control field (the "standardized control statistics").

[0038] If N is not large enough, the Ledoit-Wolf estimate may be used for the covariance matrix.

[0039] The set of standardized control fields - or simply the corresponding standardized control statistics - is then used to perform a statistically robust design (SRD) of a pulse sequence (the simpler term "pulse" is also used) according to a target control function f, which is also provided as input to the design method.

[0040] Before describing specific SRD approaches, it is useful to recall some basic theoretical results.

[0041] A spin excitation pulse sequence (or simply "pulse") is a collection of time-discretized radio frequency (RF) waveforms.

number

number

[0042] For clarity, the pulse P given by the definitions of the RF waveform U and the MFG waveform H is

number

[0043] Relaxation during excitation and off-resonance (which occurs when T ≪ T 1,2 and off-resonance

number

number

number

number

number

number

[0044] mapping

number

number

[0045] Given a pulse P and a control field distribution ω1, we can solve (at least numerically) the Bloch equations and express the magnetization at time T as:

number

number

number

[0046] The pulse design engineering problem, in turn, is to condition on desired control objectives of the form:

number

[0047] Since in MRI it is generally not possible to perfectly satisfy all conditions simultaneously for all positions, this problem is usually solved by minimizing a metric of the form

number

[0048] If the control field ω1(r) is stochastic, then

number

number

[0049] Then, the optimization problem (14) becomes:

number

[0050] In practice, computing △(P,θ) is difficult, and therefore problem (14) is simplified to a more tractable optimization problem whose optimum is assumed to be close to that of the above problem, see e.g. (14) and (15).

[0051] One such pulse engineering problem is given by:

number

[0052] We have assumed that the statistical distribution of the control field ω(r) is Gaussian with mean μ(r) and covariance C(r), so that for all positions r, the term <FA P (r)-FA t > 2 is a function of μ(r) and C(r). Thus, an interesting feature of (17) is that the optimization does not require perfect knowledge of the control field, but only second order statistics, as illustrated in FIG. 1A. Pulse engineering problems taking into account the statistical distribution of the control field ω1

number

[0053] Typically, the desired control objective function f is a target distribution of nuclear spin flip angles F t In most cases, but not always, this target distribution corresponds to a constant flip angle across the region of interest (e.g., the subject's brain).

[0054] Flip angle distribution

number

number

[0055] In other words, here we use local performance metrics

number

number

[0056] For simplicity, neither pulse energy nor MFG slew rate constraints are taken into account here, however these constraints can be explicitly added to the pulse engineering problem in (16) by replacing it with a constrained optimization problem.

number

number

[0057] It should be understood that equation (16) is just one example of a "relaxed" optimization problem. Problems of various forms, and even the true SRD optimization problem (16), can be applied to various embodiments of the present invention, in some cases requiring deeper knowledge of the control fields.

[0058] Moreover, according to various embodiments of the present invention, both the RF waveform U and the gradient waveform H can be optimized, or only the RF waveform can be optimized, with the gradient waveform being predefined.

[0059] A crucial feature that distinguishes the present invention from the prior art is the "physical" magnetic field.

number

number

number

number

number

[0060] Direct exposure of an MRI subject to standardized pulses would not produce the desired results, i.e., a distribution of spin-flip angles that closely matches the target. However, for a subject with tuning matrix “L”,

number

number

[0061] Unless otherwise stated, the RF components of P' are modified by right multiplying them with the inverse adjustment matrix of interest.

number

number

number

[0062] To perform spin excitation on an object, we then calculate an estimate of its tuning matrix

number

number

[0063] As discussed above, given that the least-squares problem given by Eq. (1) is largely overdetermined, the estimator

number

number

number

[0064] We will now discuss the advantages provided by the present invention using specific numerical examples illustrated by FIGS. 2A-6.

[0065] MRI data were acquired on a parallel-transmission-enabled whole-body Siemens 7T system (Siemens Healthineers, Erlangen, Germany) equipped with an SC72 whole-body gradient insert (200 mT / m / ms and 70 mT / m maximum gradient slew rate, and maximum gradient amplitude). The parallel transmission system consisted of eight transmitters (1 kW peak power per channel). Measurements were performed with an 8Tx-32Rx Nova head coil (Nova Medical, Wilmington, MA, USA). Data were collected from N=36 separate adult subjects (age=40±20 years) measured.

number

[0066]

number

[13] –

[15] , 5 mm isotropic resolution, matrix size 40 × 64 × 40, TR = 20 s, TA = 4 min 40 s).

[0067] In Figures 2A and 2B, the second order statistics (mean: Figure 2A; covariance: Figure 2B) of the control field for the Nova TX array are displayed for one axial slice through the brain. The mean control field μ thus gives rise to eight axial images, one for each TX channel.

[0068] P←A and R→L identify the front←back and right→left orientation of the object.

[0069] 3A and 3B are reduced statistics of the normalized magnetic field statistics for the same axial slice.

number

number

[0070] As explained above, the adjustment matrix of the object can be calculated using the reduced data set. A particularly suitable choice for the subsampling operator F in equation (23) is to use the reduced set of imaged slices (N denoting the number of imaged slices). s ) is an operator that selects a subset of m. s In the case of F n is the optimization area (in this example, the brain volume) and

number

[0071] In Fig. 4A and Fig. 4B, m=40 (L 40 , Fig. 4A) and m=3 (L3, Fig. 4B) 36 alignment matrices (one per object, the projection matrix of the first object, which is an identity matrix) are shown. The real and imaginary parts of the matrices are concatenated horizontally and an 8x8 identity matrix I8 is subtracted from every matrix to make it easier to visualize. Visually, the L3 (only 3 slices are used) and L 40 (where all slices are used) there is little difference.

number

[0072] The difference in size between the two adjustment matrices |L3-L 40 | is shown in Figure 4C.

[0073] Figure 5 shows the relationship between d(L m ,L 40 ) values. When m ≥ 3, d(L m ,L 40 It can be seen that d(L)<0.2. However, for m<3, the estimates start to diverge. This confirms that a 3-slice TX field mapping protocol is quite satisfactory to correctly estimate the projection matrix. However, if we were to employ, for example, only the central slices, this would be insufficient (d(L)<0.2 for some subjects). m ,L 40 )>0.5).

[0074] We optimized the universal kT point pulse ([6]) of the Nova TX array, designed according to the SRD approach described above, to produce a uniform 10° flip angle excitation throughout the brain. For this purpose, the planning space R design is defined as follows:

number

number

number

number

[0075] For the design, the spins were assumed to be on-resonant (ω0 = 0) everywhere throughout the head. The optimization was performed by varying the flip angle (measured by how far the magnetization is tilted from the z-axis along the initial z axis) distribution.

number

[0076] For simplicity, neither pulse energy nor MFG slew rate constraints were taken into account here. However, once the problem is solved, it can be adapted to satisfy all hardware (peak RF power, average RF power, or MFG slew rate limits) or safety (specific absorption rate) constraints, possibly by increasing the sub-pulse and MFG blip durations. In this numerical application, the number of kT points was set to 5, and their positions in k-space were determined by the SRD approach described above.

[0077] A statistically robust solution is obtained using Equation (16) (the “relaxed” SRD) with the natural control field statistics (μ, C) and the reduced control field statistics (

number

number

[0078] For each subject, a subject-tailored (ST) pulse was also calculated using the same parameterization as for UP and SUP. The cost function to be minimized was

number

[0079] Here we are:

number

[0080] The tailored solution obtained from m slices is

number

[0081] The performance analysis of UP,SUP consisted of simulating the flip angle distribution for each subject (36 simulations) and calculating the Normalized Root Mean Square (NRMS) deviation of the latter from the target, which is called Flip Angle NRMS Control Error (FA-NRMSE) in the following. Here, for every control field realization (i.e. for every subject), five pulses were tested, the five pulses being: i) P UP ii) P SUP / L 40 , iii) P SUP / L3, iv)P ST and v)

number

[0082] The results of the FA-NRMSE analysis are presented in Figure 6, where for each pulse type, the error bars identify the lowest and highest NRMSE, the middle horizontal bar is the median value, and the box contains 75% of the subjects. The plot displays an FA-NRMSE equal to 8% on average for the UP of 5-kT points, compared to about 5% for the equivalent SUP of 5-kT points, i.e., nearly the best UP performance. Optimal tuning of the SUP

number

number

[0083] In conclusion, the method of the present invention (SUP pulse) provides better uniformity than the universal pulse (UP) approach, while the calibration required to determine the subject-specific tuning transformation is fairly simple (e.g., using only 3-slice MRI). Subject-tailored (ST) pulses achieve even better uniformity, but require much more stringent calibration (ST 3 Attempting to reduce the complexity of the calibration step, as was done in the example, produces unacceptable results).

[0084] The calibration step may be further simplified by deriving the user's inverse adjustment transform from measurements other than MR imaging. For example, the measured noise covariance matrix of multiple receive channels of an MRI scanner is affected by the subject's body part in the scanner. Machine learning algorithms may then be used to estimate the inverse adjustment matrix from the noise measurements. Several different types of measurements (e.g., MR and noise) may also be combined.

[0085] Biometric and / or anaglyphic data of the user (eg head size, gender, age...) may also be taken into account in combination with the measurements to estimate the adjustment transformation.

[0086] FIG. 7 shows a highly simplified representation of a parallel transmission MRI device (or scanner) suitable for implementing the invention. Reference M denotes a magnet for generating a strong (e.g. 3T or even 7T) static magnetic field B0 oriented along a "longitudinal" direction z, which in the case of FIG. 7 is horizontal. This magnet is cylindrical and hollow to allow the insertion of a subject (typically a human being) or a body part thereof (e.g. the head). Additional magnets called gradient coils (reference GC) are provided outside the main magnet M and can be driven with slow (i.e. non-radio frequency) waveforms (gradient waveforms) that generate slowly varying magnetic fields exhibiting intensity gradients along three orthogonal spatial dimensions. In the figure, only one pair of gradient coils is represented, which allows the generation of a gradient along a single spatial dimension. A number of radio frequency coil elements are arranged around the inner volume of the main coil M, of which there are eight in the example of FIG. 7, namely RFC1, RFC2, RFC3, RFC4, RFC5, RFC6, RFC7, RFC8. These coil elements are part of respective transmit channels, which also include devices not shown, such as amplifiers, and are independently driven by a computer to emit respective RF pulses (RF waveforms) generally having the same carrier frequency (at the Larmor frequency) and different time-varying amplitudes and phases that may be defined by complex envelopes. Receive channels (not shown) for detecting magnetic resonance signals are also provided.

[0087] The computer also drives the gradient coil GC to generate the gradient waveforms. + is generated by RF coil elements, the assembly of which is sometimes called a (RF) coil or array.

[0088] According to the invention, the complex envelopes of "standardized" pulse sequences, including one RF and gradient waveform for each transmit channel of the scanner, are stored in a memory device DB to which the computer CP has access. The computer CP is further programmed to drive the scanner to perform measurements representative of the spatial distribution of the radio frequency magnetic field generated by the RF array, to use the results of said measurements to estimate a subject-specific adjustment matrix, to apply said adjustment matrix (or rather its inverse adjustment matrix) to the standardized pulse to calculate a subject-specific modified pulse, and to drive the RF and gradient coils to reproduce the modified pulse waveform.

[0089] References 1.Bernstein MA, Huston J, Ward HA.Imaging artifacts at 3.0TJ Magn Reson Imaging.2006;24(4):735-746.doi:10.1002 / jmri.20698 2.Franklin Kendra M.,Dale Brian M.,Merkle Elmar M.Improvement in B1-inhomogeneity artifacts in the abdomen at 3T MR imaging using a radiofrequency cushion.J Magn Reson Imaging.2008;27(6):1443-1447.doi:10.1002 / jmri.21164 3.JVHajnal, SJMalik, DJLarkman, D.O'Regan, K.Nehrke, U.Katscher, I.Graesslin, and P.Boernert, Initial Experience with RF shimming at 3T using a whole body 8 channel RF system, in Proceedings of the 16th Annual Meeting of ISMRM, Toronto, Ontario, Canada, 2008.p.496. 4. Katscher U, Boernert P, Leussler C, van den Brink JS. Transmit SENSE. Magn Reson Med. 2003;49(1):144 - 150. doi:10.1002 / mrm.10353 5. Tomi - Tricot R, Gras V, Mauconduit F, et al. B1 artifact reduction in abdominal DCE - MRI using kT - points: First clinical assessment of dynamic RF shimming at 3T. J Magn Reson Imaging.:n / a - n / a. doi:10.1002 / jmri.25908 6. Cloos MA, Boulant N, Luong M, et al. kT - points: Short three - dimensional tailored RF pulses for flip - angle homogenization over an extended volume. Magnetic Resonance in Medicine 67:72 - 80(2012). 7. Gras V, Vignaud A, Amadon A, Bihan DL, Boulant N. Universal pulses: A new concept for calibration - free parallel transmission. Magn Reson Med. 2017;77(2):635 - 643. doi:10.1002 / mrm.26148 8. International Publication No. WO 2017 / 060142 Pamphlet 9.Raphael Tomi-Tricot,Vincent Gras,Bertrand Thirion,Franck Mauconduit,Nicolas Boulant,et al..SmartPulse,a Machine Learning Approach for Calibration-Free Dynamic RF Shimming:Preliminary Study in a Clinical Environment.Magnetic Resonance in Medicine,Wiley,2019,Magn Reson Med.,82,pp.2016-2031. 10.Brunheim et al.,“Fast and accurate multi-channel mapping based on the TIAMO technique for 7T UHF body MRI,’’Magnetic Resonance in Medicine,vol.79,no.5,pp.2652-2664,2018. 11.P.Alotto,C.Magele,W.Renhart,A.Weber,and G.Steiner,“Robust target functions in electromagnetic design,’’COMPEL,vol.22,no.3,pp.549-560,Sep.2003. 12.G.Steiner,D.Watzenig,C.Magele,and U.Baumgartner,“Statistical robust design using the unscented transformation,’’COMPEL-The international journal for computation and mathematics in electrical and electronic engineering,vol.24,no.2,pp.606-619,Jun.2005. 13. H.-P. Fautz, MHVogel, P. Gross, A. Kerr, and Y. Zhu, “B1 mapping of coil arrays for parallel transmission,'' In Proceedings of the 16th Annual Meeting of ISMRM, p. 1247, 2008. 14. DOBrunner and KPPruessmann, “B1+ interferometry for the calibration of RF transmitter arrays,'' Magn Reson Med, vol. 61, no. 6, pp. 1480-1488, 2009. 15. A. Amadon, MACloos, N. Boulant, M.-F. Hang, CJ Wiggins, and H.-P. Fautz, “Validation of a very fast B1-mapping sequence for parallel transmission on a human brain at 7T,'' In Proceedings of the 20th Annual Meeting of ISMRM, p. 3358, 2012. [Explanation of symbols]

[0090] CP Data Processing Unit DB Memory Device GC Gradient Coil H magnetic field gradient waveform L Adjustment Transformation P Pulse Sequence P' Subject-specific radio frequency waveform RFC1~RFC8 sending channel U Radio frequency waveform Omega 1 (1) -Ω1 (1) Amplitude Map

Claims

1. 1. A computer-implemented method for designing a parallel transmission magnetic resonance imaging pulse sequence (P), the pulse sequence including at least a magnetic field gradient waveform (H) and a set of radio frequency waveforms (U), each of the radio frequency waveforms being associated with a respective transmit channel (RFC1-RFC8) of a parallel transmission magnetic resonance imaging device, the method comprising: a) estimating, for each of a plurality of magnetic resonance imaging subjects, a linear transformation, called an adjustment transformation (L), and applying said adjustment transformation to amplitude maps of radio frequency magnetic fields generated by respective transmit channels of said magnetic resonance imaging device in a region of interest comprising a body part of said subject to obtain respective standardized maps, said adjustment transformation being chosen in such a way as to minimize a first cost function representing the average difference between said standardized map of said subject and respective reference amplitude maps of reference subjects; b) determining at least the radio frequency waveforms (U) in such a way as to minimize a second cost function representing the difference between a subject-specific distribution of nuclear spin flip angles and a target distribution averaged over the plurality of magnetic resonance imaging subjects, the subject-specific distribution being a distribution of the flip angles achieved by applying to the subject a superposition of magnetic field gradient waveforms and radio frequency magnetic fields, each of the radio frequency magnetic fields having a temporal profile described by one of the radio frequency waveforms and a spatial amplitude distribution described by a respective standardized map determined for the subject; The method includes:

2. The method of claim 1 , wherein step b) further comprises determining the magnetic field gradient waveform (H) so as to minimize the second cost function.

3. 3. The method of claim 1, further comprising a preliminary step of acquiring all or part of the amplitude map using the magnetic resonance imaging device, the acquired amplitude map being used to estimate the rectification transformation.

4. 4. The method according to claim 1, further comprising a preliminary step of calculating all or part of the amplitude map by performing an electromagnetic simulation using a computer, the calculated amplitude map being used to estimate the rectification transformation.

5. 5. The method according to claim 1, further comprising a preliminary step of measuring noise covariance matrices of a plurality of receive channels of the parallel transmission magnetic resonance imaging device, the measured noise covariance matrices being used to estimate the conditioning transformation.

6. 6. The method according to claim 1, wherein step a) further comprises calculating second order statistics of the standardized map, and wherein step b) comprises determining at least the radio frequency waveform from the second order statistics.

7. The method according to any one of claims 1 to 6, wherein the target distribution is homogenous over the entire region of interest.

8. 1. A method for performing parallel transmission magnetic resonance imaging of an object using a parallel transmission magnetic resonance imaging apparatus including a set of gradient coils (GC), a plurality of transmit channels (RFC1-RFC8), and a plurality of receive channels, comprising: i) using the receive channels of the device to perform measurements representative of the spatial distribution of radio frequency magnetic fields generated by the respective transmit channels within a region of interest that includes a body part of the subject; ii) using the results of the measurements to estimate a linear transformation (L), called the adjustment transformation, to transform the amplitude maps of the radio frequency magnetic field generated in the region of interest by each transmit channel of the magnetic resonance imaging device into respective standardized maps, the adjustment transformation being chosen in such a way as to minimize a cost function representing the average difference between the standardized map of the object and each reference amplitude map of a reference object; iii) calculating a set of radio frequency waveforms (P') specific to the subject by inverting the adjustment transformation and applying the inverted adjustment transformation to a set of predefined reference radio frequency waveforms of the reference subject; iv) applying each radio frequency waveform specific to the subject to a respective transmit channel while applying magnetic field gradient waveforms to the gradient coils, and receiving parallel transmitted magnetic resonance imaging signals using the receive channels; The method includes:

9. A method according to claim 8, wherein said set of predetermined reference radio frequency waveforms is obtained using a method according to any one of claims 1 to 7.

10. 10. The method according to claim 8 or 9, wherein step i) comprises acquiring an amplitude map of the radio frequency magnetic field with a spatial resolution lower than the spatial resolution of the parallel transmitted magnetic resonance imaging signals acquired during step iv).

11. A method according to any one of claims 8 to 10, wherein step i) comprises measuring a noise covariance matrix of the receive channel.

12. In a parallel transmission magnetic resonance imaging apparatus including a set of gradient coils (GC), a plurality of transmit channels (RFC1-RFC8), a plurality of receive channels, and a data processing unit (CP), - further comprising a memory device (DB) for storing data defining a set of predetermined reference radio frequency waveforms and magnetic field gradient waveforms; said data processing device, - programmed or configured to drive the receive channels to perform measurements representative of the spatial distribution of radio frequency magnetic fields generated by the transmit channels within a region of interest that includes a body part of a subject; - programmed or configured to use the results of said measurements to estimate a linear transformation (L), called the adjustment transformation, to transform the amplitude maps of the radio frequency magnetic field generated in said region of interest by each transmission channel into a respective standardized map, said adjustment transformation being chosen in such a way as to minimize a cost function representing the average difference between said standardized map of the subject and each reference amplitude map of a reference subject, - programmed or configured to calculate a set of subject-specific radio frequency waveforms by inverting the adjustment transform and applying the inverted adjustment transform to a set of predefined reference radio frequency waveforms; and - programmed or configured to drive the transmit channels to play respective subject-specific radio frequency waveforms, the gradient coils to play magnetic field gradient waveforms, and the receive channels to receive parallel transmitted magnetic resonance imaging signals; A parallel transmission magnetic resonance imaging apparatus comprising:

Images