Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
  • G01R33/48—NMR imaging systems
  • G01R33/54—Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console
  • G01R33/56—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution
  • G01R33/565—Correction of image distortions, e.g. due to magnetic field inhomogeneities
  • G01R33/5659—Correction of image distortions, e.g. due to magnetic field inhomogeneities caused by a distortion of the RF magnetic field, e.g. spatial inhomogeneities of the RF magnetic field
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
  • G01R33/48—NMR imaging systems
  • G01R33/4818—MR characterised by data acquisition along a specific k-space trajectory or by the temporal order of k-space coverage, e.g. centric or segmented coverage of k-space
  • G01R33/482—MR characterised by data acquisition along a specific k-space trajectory or by the temporal order of k-space coverage, e.g. centric or segmented coverage of k-space using a Cartesian trajectory
  • G01R33/4822—MR characterised by data acquisition along a specific k-space trajectory or by the temporal order of k-space coverage, e.g. centric or segmented coverage of k-space using a Cartesian trajectory in three dimensions
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
  • G01R33/48—NMR imaging systems
  • G01R33/4818—MR characterised by data acquisition along a specific k-space trajectory or by the temporal order of k-space coverage, e.g. centric or segmented coverage of k-space
  • G01R33/4824—MR characterised by data acquisition along a specific k-space trajectory or by the temporal order of k-space coverage, e.g. centric or segmented coverage of k-space using a non-Cartesian trajectory
  • G01R33/4826—MR characterised by data acquisition along a specific k-space trajectory or by the temporal order of k-space coverage, e.g. centric or segmented coverage of k-space using a non-Cartesian trajectory in three dimensions
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
  • G01R33/48—NMR imaging systems
  • G01R33/483—NMR imaging systems with selection of signals or spectra from particular regions of the volume, e.g. in vivo spectroscopy
  • G01R33/4833—NMR imaging systems with selection of signals or spectra from particular regions of the volume, e.g. in vivo spectroscopy using spatially selective excitation of the volume of interest, e.g. selecting non-orthogonal or inclined slices
  • G01R33/4835—NMR imaging systems with selection of signals or spectra from particular regions of the volume, e.g. in vivo spectroscopy using spatially selective excitation of the volume of interest, e.g. selecting non-orthogonal or inclined slices of multiple slices
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
  • G01R33/48—NMR imaging systems
  • G01R33/54—Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console
  • G01R33/56—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution
  • G01R33/561—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution by reduction of the scanning time, i.e. fast acquiring systems, e.g. using echo-planar pulse sequences
  • G01R33/5611—Parallel magnetic resonance imaging, e.g. sensitivity encoding [SENSE], simultaneous acquisition of spatial harmonics [SMASH], unaliasing by Fourier encoding of the overlaps using the temporal dimension [UNFOLD], k-t-broad-use linear acquisition speed-up technique [k-t-BLAST], k-t-SENSE
  • G01R33/5612—Parallel RF transmission, i.e. RF pulse transmission using a plurality of independent transmission channels

Definitions

• the invention relates to the field of magnetic resonance imaging (MRI) and relates more particularly to the phase of excitation of nuclear spins. More particularly, it relates to the design and application of selective nuclear spin excitation pulse sequences (“excitation sequences”) of the “multi-spoke” (“multi-spokes” or “fast-kz”) type. in English-language literature). It applies in particular to high magnetic field MRI (several Teslas, for example 3 Teslas (T) or more or even 5 Teslas or more or even 7 Teslas or more).
• the spin excitation profile depends on the distribution of the spin excitation radiofrequency field in the part of the body to be imaged.
• the magnetic resonance frequency (Larmor frequency) of the proton spin being low, a very uniform and relatively independent distribution of the object to be imaged can be obtained with a suitable antenna design transmitting RF.
• the wavelength of radiation at the Larmor frequency becomes comparable to the characteristic dimensions of the body to be imaged; this results in interference effects that cause significant inhomogeneities. This is verified around 7T for cerebral imaging and from 3T for that of the abdomen. Under these conditions, it is no longer possible to ensure homogeneous excitation by means of a suitable design of the transmitting antenna and it is necessary to resort to alternative techniques.
• a first technique consists in replacing the volumic RF transmission antenna with an array of antennas, each element of which can be excited by a signal which is specific to it, independently of the other elements of the array. This is referred to as parallel transmission or, in English, “RF-shimming”.
• a second technique directly affects the dynamics of the spins (Bloch's equation) and aims to expose the spins to B fields (emitted radiofrequency field) and to variable magnetic field gradients over time of so as to create in a region of interest an angle of tilting of the nuclear spins in accordance with a setpoint, except for an error that we seek to minimize.
• the invention aims to provide a solution to these two problems.
• this object is achieved by defining a regular (that is to say continuous) parametrization of the RF and gradient signals as a function of the inclination and position parameters of the cut.
• This parametrization being calculated, it is possible to implement any multi-cut protocol by simply evaluating the parametrization on the N inclinations/cut positions defined by the protocol, which requires an extremely simple and almost instantaneous calculation.
• the proposed approach is fully compatible with the two excitation homogenization techniques that are parallel transmission and optimal control. It is also compatible with the “universal pulses” disclosed by EP 15306569 or with the method for designing and adjusting universal radiofrequency pulses in MRI in parallel transmission disclosed by EP3594710.
• the invention is based on the technique of multi-ray pulses (in English “multi-spokes” or “Fast-kz”) - known in particular from [Setsompop 2008] - which consists in constructing sequences of excitations made up of RF pulses separated by the application of magnetic gradient pulses which allow the contributions of the different RF pulses to interfere.
• multi-ray pulses in English “multi-spokes” or “Fast-kz”
• [Setsompop 2008] which consists in constructing sequences of excitations made up of RF pulses separated by the application of magnetic gradient pulses which allow the contributions of the different RF pulses to interfere.
• By optimizing the moments (area under the curve) of the magnetic gradient pulses it is thus possible to create composite pulses offering good excitation homogeneity.
• this approach is used in conjunction with “RF shimming”: each RF pulse is in fact made up of several independent sub-pulses emitted by emission channels (or transmission, the two terms will be used interchangeably) distinct
• An object of the invention is a method for determining an overall parameterization of a family of sequences of spin excitation pulses in magnetic resonance imaging, each sequence of pulses of said family being a sequence of multi-ray type adapted to selectively excite nuclear spins in a respective section of a volume of interest of a body immersed in a stationary magnetic field and comprising the same predetermined number of radiofrequency pulses at a Larmor frequency of said spins pulses alternated with magnetic gradient pulses, the method comprising the steps of:
• A) Acquire at least one intensity map of radiofrequency radiation at the Larmor frequency in said volume of interest;
• B Define a first set of parameters characterizing the radio frequency pulses and a second set of parameters characterizing the magnetic gradient pulses, the value of each parameter of each said set being expressed by a truncated series of functions of a position coordinate and d a pair of angular orientation coordinates of a said section, each series being determined by its coefficients;
• Another object of the invention is a method for designing a sequence of spin excitation pulses of the multi-ray type in magnetic resonance imaging, said sequence of pulses being adapted to selectively excite spins nuclear spins in a respective section of a volume of interest inside a body immersed in a stationary magnetic field and comprising the same predetermined number of radio frequency pulses at a Larmor frequency of said nuclear spins alternating with gradient pulses magnetic, the section being identified by values of a position coordinate and of a pair of angular coordinates of orientation, the method being characterized in that it comprises the steps consisting in: a) Procuring a global parametrization of a family of such excitation sequences, said global parametrization being constituted by a first and a second plurality of sets of coefficients of respective truncated series of functions of said coordinates of position and orientation of a said cut, each set of the first plurality of sets consisting of the coefficients of a truncated series of functions defining a parameter of a radio frequency pulse and each
• the global parametrization can in particular be obtained by a method as outlined above.
• Yet another object of the invention is a magnetic resonance imaging method comprising the following steps: i) designing a plurality of sequences of multi-ray type spin excitation pulses, each said sequence of pulses being adapted to selectively excite nuclear spins in a respective section of a volume of interest of a body to be imaged immersed in a stationary magnetic field and comprising radiofrequency pulses at a Larmor frequency of said nuclear spins alternated with pulses of magnetic gradient, each cut being identified by respective values of a position coordinate and a pair of angular orientation coordinates; ii) Applying to said body, in succession, said sequences of spin excitation pulses, a magnetic cut selection gradient being applied at the same time as each radio frequency pulse; iii) After each said sequence of spin excitation pulses, acquiring a magnetic resonance signal emitted by the nuclear spins; and iv) processing the signals thus acquired to reconstruct an image of a portion of said reference volume defined by the union of said sections; characterized in that step i) is implemented by
• Yet another object of the invention is a computer programmed to: a) receive as input at least one intensity map of radiofrequency radiation in a volume of interest of a body immersed in a stationary magnetic field , said radiofrequency radiation being at a Larmor frequency of nuclear spins of said body; b) Define a global parametrization of a family of sequences of spin excitation pulses of the multi-ray type in magnetic resonance imaging, each sequence of pulses of said family being adapted to selectively excite nuclear spins in a section of said volume of interest and comprising radiofrequency pulses at a Larmor frequency of said nuclear spins alternating with magnetic gradient pulses, said global parameterization comprising a first set of parameters characterizing said radiofrequency pulses and a second set of parameters characterizing said gradient pulses magnetic, the value of each parameter of each said set being expressed by a truncated series of functions of a position coordinate and a pair of angular orientation coordinates of a said section, each series being determined by its coefficients; g) From said intensity map or
• a further object of the invention is a magnetic resonance imaging device equipped with such a computer.
• FIG.1 an MRI device or “scanner” suitable for implementing the invention
• FIG. 2 the coordinates making it possible to identify a section of a body to be imaged.
• FIG. 8 a flowchart of a method according to one embodiment of the invention.
• FIG. 1 very schematically illustrates an IS MRI device (or “scanner”) that may be suitable for implementing the invention.
• This device comprises: a magnet AL for generating a stationary magnetic field B 0 of magnetization, called longitudinal magnetic field, oriented in a “z” direction; coils BGx (not shown), BGy and BGz for generating magnetic field gradients along mutually orthogonal “x”, “y” and “z” directions; a plurality of radiofrequency transmission and reception antennas CTx/CRx surrounding a region or volume of interest ROI which can contain a body C to be imaged (the term “body” is understood to mean any material object, biological or not, which can be imaged by MRI; this will typically be a human body or part of a human body, for example a head); in particular, the region of interest contains the IC isocenter (reference visible in [Fig.
• the computer OC drives the gradient coils BGx, BGy and BGz so that they generate gradient pulses and the RF antennas CTx/CRx so that they generate pulses radiofrequency so as to apply to the body C an excitation sequence obtained by the method of the invention. Furthermore, the OC computer receives from the CTx/CRx antennas magnetic resonance signals emitted by the nuclear spins of the object following their excitation, and processes these signals to reconstitute - in a conventional manner - an MRI image of the body vs.
• the apparatus of FIG. 1 comprises a plurality of RF antennas making it possible to use parallel transmission techniques, but the use of a single antenna is also possible, although less advantageous. Furthermore, the transmitting and receiving antennas may be distinct.
• the [Fig. 2] allows to define the coordinates that identify a CP section of the body C.
• n be the unit vector perpendicular to the cut.
• the orientation of this vector is defined by the spherical angular coordinates Q (azimuth or longitude) and f (colatitude) having for origin the isocenter IC.
• Q azimuth or longitude
• f colatitude
• r be the position vector of any point of the section with respect to the isocenter; the scalar product n r gives z, the distance from the cut to the origin.
• the scalar z can be considered a position coordinate of the cut.
• the [Fig. 3] is a diagram of a simple excitation sequence according to one embodiment of the invention, comprising three radiofrequency pulses IRF1, IRF2 and IRF3 and three gradient pulses IG1, IG2 and IG3 alternated with the pulses radio frequency. GS cut selection gradients are applied at the same time as the radio frequency pulses.
• FIG. 3 shows only the radiofrequency pulses associated with a single emission channel, identified by an index “i”.
• the IRF1, IRF2 and IRF3 radiofrequency pulses have the same temporal shape (a cardinal sine) and different complex amplitudes
• the real and imaginary parts of the pulses - of amplitude Re(aj, j ) and lm(aj, j ), - are represented in continuous and dashed lines, respectively.
• the selection gradients GS are applied at the same time as the radiofrequency pulses. Only one component, identified by an index i' e ⁇ x,y,z ⁇ , and of maximum amplitude G maxj' is represented. The amplitude of the resultant of the three components determines the thickness of the cut.
• the gradient pulses have an alternating polarity; in a known way, this makes it possible to dispense with phase "rewind" gradient pulses which would be necessary if all the gradient pulses had the same polarity ([Gras 2017 B]).
• the polarity of the gradient pulses is changed, it is necessary to time-reverse the radiofrequency pulses, but in the case considered here the latter are symmetrical.
• the position in k-space determined by a gradient pulse depends only on the area Br of each of its three components. It should be noted that gradient pulses are much less intense than selection gradients; in [Fig. 3], for the sake of visibility, they have been amplified by a factor of 100.
• N c be the number of channels and N s the number of radio frequency pulses used.
• the information on the timing of each RF pulse and each gradient pulse) can be represented in matrix form, namely a complex matrix A e (C NcXNs and a matrix B e ® > 3 x ( N si) respectively.
• each column indicates the moments per transmission channel (by axis - x, y, z) of the RF pulse (of the gradient pulse).
• the last RF sub-pulse is followed by a gradient pulse whose components along x, y, and z is equal to :
• each element of the vector is equal to the sum of the areas of the gradient pulses along an axis.
• B ⁇ (r) e (C lxNc the values of the resonant magnetic field in r on each transmission channel. These values are normalized to 1 Volt transmitted on each channel; they are therefore expressed in Tesla per Volt (T /V).
• the position of the RF pulses in the transmission space K is defined by: the lower triangular matrix:
• FA r the operator (which depends on the distribution of the field radio frequency in the body to be imaged, i.e. one or more (in case of parallel transmission) B cards
• each term of each truncated series expressing fy and g ⁇ y is given by the product of a monomial of the position coordinate z and a finite sum of complex or real spherical harmonics (more generally, of functions belonging to a basis of continuous functions of the angular coordinates of orientation).
• any multi-cut protocol is fully characterized as soon as the matrices:
• the root mean square deviation from the setpoint can be defined as follows:
• constraints are added to this problem to obtain acceptable solutions. These constraints relate in particular to the energy of the RF pulses.
• s [0,1] ® IR the temporal form of a generic RF pulse and T its duration.
• M 1 (s) the moments of order 1 and 2 of the pulse s(t).
• the SQP algorithm is used for calculating the non- optimal n z s (52 independent optimizations).
• P 0 (A 0 , 0) where A 0 is a constant matrix and which satisfies:
• the SQP algorithm converges to a value of 0.04 in approximately 180 iterations.
• the [Fig. 6] allows to compare that is to say two sequences defining multi-cut protocols (52 cuts) obtained in accordance with the invention (global parameterization) and the prior art (optimization cut by cut), respectively.
• the left panel represents the coefficients An (real part) of the matrix A and the left panel the coefficients Bu of the matrix B.
• the tilt angle does indeed have a regular profile in the section planes but, on the other hand, shows discontinuities when it is observed in an orthogonal plane, for example sagittal or coronal. This problem is solved with the global parametrization according to the invention.
• FIG. 8 is a flowchart of a magnetic resonance imaging method implementing various aspects of the invention.
• a first step i of the method consists in designing a plurality of sequences of spin excitation pulses of the multi-ray type, each said sequence of pulses corresponding to a respective slice.
• this first step is broken down into two sub-steps: a first sub-step a) consisting in determining a global parametrization or “meta-pulse” by the method described above; and a second step b) consisting in obtaining the sequences of excitations corresponding to each cut from this global parameter setting and the position and orientation coordinates of the chopped off.
• Sub-step a) comprises three phases:
• the second stage consists in applying to the body, in succession, the sequences of spin excitation pulses thus calculated, with the corresponding selection gradients.
• the third step iii) consists in acquiring the magnetic resonance signals after each excitation sequence.
• step iv) consists in processing the signals thus acquired to reconstruct an image of a portion of said reference volume defined by the union of the slices.
• a single transmission channel can be used, instead of a plurality of channels performing parallel transmission.
• the form of the parametrization chosen (polynomial in z and spherical harmonics in q, f) is particularly suited to the problem but is not the only one that is eligible.
• the truncation in z is a useful artifice to compensate for the fact that the polynomial functions diverge at infinity.
• other approaches are potentially possible.
• the optimization of the mean square deviation of the excitation from the target is a usual criterion and advantageous from a mathematical point of view, but other functions of the deviation from the target can be used. Moreover, although this is the most common case, it is not essential that the target excitation be constant over the entire slice.
• the invention does not presuppose a particular type of pulse.
• the embodiment described in detail is the simplest, involving a constant magnetic gradient combined with an apodized cardinal sine type radio frequency waveform. Nevertheless, the invention can be implemented with other pulses, for example of the SLR type [Shinnar1994, Sharma2015] and/or VERSE [Conolly1988]

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• 2021
  • 2021-03-17 FR FR2102654A patent/FR3120949B1/fr active Active
• 2022
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