WO2018184056A1 - Procédé et appareil d'imagerie par résonance magnétique - Google Patents

Procédé et appareil d'imagerie par résonance magnétique Download PDF

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WO2018184056A1
WO2018184056A1 PCT/AU2017/050294 AU2017050294W WO2018184056A1 WO 2018184056 A1 WO2018184056 A1 WO 2018184056A1 AU 2017050294 W AU2017050294 W AU 2017050294W WO 2018184056 A1 WO2018184056 A1 WO 2018184056A1
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readout
trajectories
acquisition
trajectory
echo signals
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PCT/AU2017/050294
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Daniel Stäb
Markus Barth
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The University Of Queensland
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    • 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
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/05—Detecting, measuring or recording for diagnosis by means of electric currents or magnetic fields; Measuring using microwaves or radio waves 
    • A61B5/055—Detecting, measuring or recording for diagnosis by means of electric currents or magnetic fields; Measuring using microwaves or radio waves  involving electronic [EMR] or nuclear [NMR] magnetic resonance, e.g. magnetic resonance imaging
    • 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
    • G01R33/50—NMR imaging systems based on the determination of relaxation times, e.g. T1 measurement by IR sequences; T2 measurement by multiple-echo sequences
    • 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
    • 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
    • G01R33/5615—Echo train techniques involving acquiring plural, differently encoded, echo signals after one RF excitation, e.g. using gradient refocusing in echo planar imaging [EPI], RF refocusing in rapid acquisition with relaxation enhancement [RARE] or using both RF and gradient refocusing in gradient and spin echo imaging [GRASE]
    • G01R33/5616—Echo train techniques involving acquiring plural, differently encoded, echo signals after one RF excitation, e.g. using gradient refocusing in echo planar imaging [EPI], RF refocusing in rapid acquisition with relaxation enhancement [RARE] or using both RF and gradient refocusing in gradient and spin echo imaging [GRASE] using gradient refocusing, e.g. EPI

Definitions

  • the present invention relates to a magnetic resonance imaging (MRI) method and apparatus, and in one particular example, to a magnetic resonance imaging method and apparatus suitable for use in susceptibility mapping.
  • MRI magnetic resonance imaging
  • QSM Quantitative Susceptibility Mapping
  • the current standard acquisition technique for structural T 2 * weighted, susceptibility weighted MRI and QSM is a three-dimensional (3D) spoiled gradient-echo (GRE) sequence with Cartesian sampling. It can be used with high spatial resolution to detect small structures and lesions and has performed well with respect to S R and stability. However, the long measurement time can be problematic in terms of head motion during the scan. This is particularly the case in clinical populations, such as Alzheimer's and Parkinson's disease, where involuntary head motion is more common than in healthy control participants.
  • the image acquisition can be accelerated using partial Fourier imaging, elliptical scanning and parallel imaging. However, these methods come with a reduction in SNR due to the acquisition of a reduced amount of &-space data.
  • parallel imaging such as simultaneous acquisition of spatial harmonics (SMASH), sensitivity encoding (SENSE) or generalized auto-calibrating partially parallel acquisitions (GRAPPA) - introduce some noise amplification during the reconstruction process, characterized by the so-called geometry factor. To a certain extent, this effect can be mitigated by shifting the sampling positions in &-space so that the coil sensitivity variations can be exploited more efficiently in multiple dimensions, resulting in a more robust parallel imaging reconstruction.
  • SMASH spatial harmonics
  • SENSE sensitivity encoding
  • GRAPPA generalized auto-calibrating partially parallel acquisitions
  • EPI echo-planar imaging
  • T E desired echo time
  • T 2 * of the tissue of interest can be restricting, since long readout trains can lead to spatial blurring.
  • Further limitations include geometric distortions and signal dropouts, which are typically worse at higher fields and become more severe at ultra-high field strength.
  • US2012/0220858 describes an MRI-based system includes an MRI scanner having a first axis and a first plane perpendicular to the first axis, a pulse sequence module configured to provide a 3D pulse sequence to the MRI scanner, and a control module configured to instruct the MRI scanner to conduct radial A space samples having N second planes that each are perpendicular to the first plane and through which the first axis passes, N being an integer greater than 1.
  • the 3D pulse sequence instructs the MRI scanner to a radio-frequency (RF) pulse, conduct a gradient readout in the first plane, and conduct a gradient readout in one of the N second planes.
  • RF radio-frequency
  • an aspect of the present invention seeks to provide a magnetic resonance imaging method for imaging an anatomical region of a subject, the method comprising: generating an imaging field, the anatomical region being provided in an acquisition space within the imaging field; generating multiple acquisition sequences, each acquisition sequence including an RF excitation pulse, and gradient field pulses configured to generate a readout trajectory in &-space, each readout trajectory including a number of scan lines extending in a frequency encoding direction and spaced in a second phase encoding direction along a phase encoding axis, and the multiple acquisition sequences generating a plurality of readout trajectories extending over at least part of an acquisition space, the readout trajectories being rotated at least partially about the phase encoding axis; acquiring echo signals from the readout trajectories for each of the multiple acquisition sequences to generate &-space data; and, processing the &-space data to generate at least one image representation of the anatomical region.
  • the method includes undersampling in at least one of a radial and a phase encoding direction.
  • the readout trajectories are undersampled with an undersampling factor of RpE > 1 to shorten echo signal acquisition times.
  • the method includes undersampling to acquire echo signals from selected scan lines of a trajectory, thereby missing at least some scan lines.
  • the method includes acquiring echo signals from missing scan lines in subsequent acquisitions.
  • the method includes acquiring echo signals from at least one of: interleaved scan lines of multiple trajectories; and, block-wise segmented trajectories.
  • the method includes performing accelerated imaging by reconstructing data from missing scan lines.
  • the method includes reconstructing data using at least one of: echo signals acquired from different scan lines in the trajectory; echo signals acquired from scan lines of different trajectories; echo signals acquired from scan lines of adjacent trajectories; echo signals acquired from equivalent different scan lines of adjacent trajectories and, echo signals acquired from different scan lines of adjacent trajectories.
  • the method includes sampling at least one common scan line in two different readout trajectories with different echo times.
  • the multiple readout trajectories are asymmetrically arranged with respect to the phase encoding axis.
  • the degree of asymmetry changes during acquisition.
  • the method includes applying multiple acquisition sequences to generate a homogeneous azimuthal distribution of readout trajectories.
  • the readout trajectories are distributed by an azimuthal angle ⁇ given by:
  • N denotes the number of traj ectories
  • the method includes generating readout trajectories spaced by a golden angle.
  • the method includes generating readout trajectories spaced by an azimuthal angle according to:
  • the method includes generating readout trajectories spaced by a random azimuthal angle increment.
  • the readout trajectory is rotated about the phase encoding axis as the phase encoded signals are acquired to define a curved readout trajectory.
  • the phase encoding axis is orientated at least one of: along a shortest dimension of the anatomical region; perpendicular to a trajectory of the at least one image representation; and, perpendicular to a transverse trajectory of the subject.
  • a variation of the azimuthal angle of the readout trajectory is defined by a variation of the amplitude of pulses in readout gradient fields.
  • an asymmetry about a phase encoding axis is controlled based on an amplitude of dephaser pulses in readout gradient fields.
  • At least one acquisition sequence includes at least one of: an RF excitation pulse; and, a preparation module including one or more pulses.
  • the method of reconstructing data includes at least one of: performing phase correction; at least one of: combining echo signals from multiple trajectories; and, reconstructing missing data from undersampled trajectories; performing a Fourier transform along a phase encoding axis; regridding to a uniform grid; and, combining individual data subsets into a combined dataset.
  • the method of reconstructing data includes exploiting the symmetry of &-space.
  • the method of acquiring and reconstructing data uses a partial Fourier techniques.
  • the method of reconstructing includes filtering of &-space data. [0033] In one embodiment the method of reconstructing uses at least one of: prior knowledge; prior knowledge of different echo-times of individual trajectories; prior knowledge of contrasts of individual trajectories; and, a model -based reconstruction.
  • the method includes processing the &-space data by interpolating echo signals onto equidistant sampling points along a readout axis to remove non-linearities.
  • the method includes processing the &-space data by performing, for echo signals from at least one readout trajectory, at least one of: gradient delay; Nyquist ghost correction; physiologic noise correction; and, eddy current corrections.
  • the method includes processing the &-space data by: acquiring one or more non-phase-encoded navigator echo signals between each RF excitation and acquiring echo signals from the readout trajectories; estimating a gradient delay induced shift along a readout axis; and, correcting for gradient delays using the estimated gradient delay.
  • the method includes processing the &-space data by: obtaining shifts at multiple azimuthal readout trajectory angles; fitting shift values to a gradient delay model; and, using delay model fit values to correct for the linear phase errors between the odd and even scan lines.
  • the method includes processing the &-space data for at least one of susceptibility mapping and relaxation time mapping.
  • an aspect of the present invention seeks to provide a magnetic resonance imaging apparatus for imaging an anatomical region of a subject, the apparatus comprising: a magnetic resonance imaging device; and, one or more processing devices that: control the magnetic resonance imaging device to cause the magnetic resonance imaging device to: generate an imaging field, the anatomical region being provided in an acquisition space in the imaging field; generate multiple acquisition sequences, each acquisition sequence including an RF excitation pulse, and gradient field pulses configured to generate a readout trajectory in &-space, each readout trajectory including a number of scan lines extending in a frequency encoding direction and spaced in a second phase encoding direction along a phase encoding axis, and the multiple acquisition sequences generating a plurality of readout trajectories extending over at least part of the acquisition space, the readout trajectories being rotated at least partially about the phase encoding axis; acquire echo signals from the readout trajectories for each of the multiple acquisition sequences to generate k- space data
  • an aspect of the present invention seeks to provide a magnetic resonance imaging method for imaging an anatomical region of a subject, the method comprising: generating an imaging field, the anatomical region being provided in an acquisition space in the imaging field; generating multiple acquisition sequences, each acquisition sequence including an RF excitation pulse, and gradient field pulses configured to generate a readout trajectory in &-space, each readout trajectory including a number of scan lines extending in a frequency encoding direction and spaced in a second phase encoding direction along a phase encoding axis, and the multiple acquisition sequences generating a plurality of readout trajectories extending over at least part of the acquisition space, wherein at least one of the acquisition sequences is undersampled in at least one of a frequency and a phase encoding direction; acquiring echo signals from the multiple readout sub -trajectories for each of the multiple acquisition sequences to generate &-space data including reconstructed readout trajectories; and, processing the
  • an aspect of the present invention seeks to provide a magnetic resonance imaging apparatus for imaging an anatomical region of a subject, the apparatus comprising: a magnetic resonance imaging device; and, one or more processing devices that: control the magnetic resonance imaging device to cause the magnetic resonance imaging device to: generate an imaging field, the anatomical region being provided in an acquisition space in the imaging field; generate multiple acquisition sequences, each acquisition sequence including an RF excitation pulse, and gradient field pulses configured to generate a readout trajectory in &-space, each readout trajectory including a number of scan lines extending in a frequency encoding direction and spaced in a second phase encoding direction along a phase encoding axis, and the multiple acquisition sequences generating a plurality of readout trajectories extending over at least part of the acquisition space, wherein at least one of the acquisition sequences is undersampled in at least one of a frequency and phase encoding direction; acquire echo signals from the multiple readout sub -trajectories for each of the multiple acquisition
  • Figure 1 A is a schematic diagram of an example of an imaging apparatus
  • Figure IB is a schematic diagram of an example of a processing system
  • Figure 2 is a flow chart of an example of an image generation process
  • FIG. 3 is a schematic diagram of an example of an acquisition pulse sequence
  • Figure 4A is a schematic diagram of an example of a readout trajectory
  • Figure 4B is a schematic diagram showing a perspective view of an example of a rotating EPI readout trajectory arrangement
  • Figure 4C is a schematic diagram showing a plan view of the rotating EPI readout trajectory arrangement of Figure 4B;
  • Figure 4D is a schematic diagram of a perspective view of an example of an asymmetric rotating EPI readout trajectory arrangement
  • Figure 4E is a schematic diagram of a plan view of the asymmetric rotating EPI readout trajectory arrangement of Figure 4D;
  • Figures 5A and 5B are positive and negative images reconstructed from an in vivo 3D rotating EPI dataset
  • Figures 6 A and 6B are positive and negative images reconstructed from 3D multi- echo GRE measurement and a first repetition of the threefold segmented 3D rotating EPI measurement;
  • Figures 7 A and 7B are positive and negative images illustrating geometric distortion and effects of averaging and acceleration in the 3D rotating EPI data
  • Figure 8 shows susceptibility maps obtained from the 3D GRE and the first 3D rotating EPI measurements
  • Figure 9 shows graphs of susceptibility values (mean and standard deviation) measured within the caudate, pallidum, putamen and corpus callosum of one volunteer
  • Figure 10 shows graphs of susceptibility measured with 3D rotating EPI as a function of the susceptibility obtained using 3D GRE;
  • Figures 11A and 11B are positive and negative images showing the effects of motion observed for the threefold segmented 3D rotating EPI measurement
  • Figure 11C is a graph showing a calculated susceptibility within the pallidum plotted against the overall acquisition time
  • Figures 12A and 12B are positive and negative images of 3D rotating EPI and 3D rotating asymmetric EPI;
  • Figure 13 is an image showing a comparison between the resolution of small anatomical features 3D rotating EPI and 3D rotating asymmetric EPI.
  • the MRI system 100 includes a main or primary shimmed magnet 101, three principal axis gradient coils 102, a patient/sample bed 108 and MR instrumentation 109.
  • the main magnet 101 is adapted to generate a substantially homogeneous static magnetic imaging field B 0 , whilst the gradient coils apply gradient fields G x , G y , G z , over an acquisition space 111, which contains a subject, such as at least part of a patient or sample.
  • the subject is also exposed to an RF field, generated by an RF coil (not shown) allowing MRI to be performed.
  • the workings of these components are substantially identical to those of contemporary systems, and will not therefore be described in any further detail.
  • the above described process is performed at least in part utilising a processing system, which may be coupled to, or form part of, the MR instrumentation 109.
  • a processing system which may be coupled to, or form part of, the MR instrumentation 109.
  • the processing system 120 includes a processor 121, a memory 122, an input/output device, such as a keyboard and mouse 123, and an optional external interface 124 coupled together via a bus 125.
  • the optional external interface may be coupled to a database 126, allowing the processing system 120 to store data and/or access previously stored data.
  • the processor 121 typically executes applications software stored in the memory 122, to allow the processor 121 to control the MRI system, perform required calculations and/or display results. This can include, for example, performing analysis of the imaging field and collected sample data in order to generate image data, as well as optionally to assist in generating transform functions. It will be appreciated that these processes can be performed automatically, but typically involve at least some input or other control by the user.
  • the processing system 120 may be one or more suitably programmed computer systems, such as a desktop and/or server, or the like, although alternatively the processing system 120 may be formed from specialised hardware forming part of the MRI system.
  • the MRI system is used to generate an imaging field B 0 , with the anatomical region of the subject being provided in the acquisition space and hence in the imaging field.
  • each acquisition sequence includes an RF excitation pulse 300, and encoding gradient field pulses G x , G y , G z 302, which are applied to the anatomical region of the subject in the acquisition space.
  • Each pulse sequence is configured to generate a readout trajectory in &-space, with each readout trajectory including a number of scan lines extending in a frequency encoding direction and spaced in a second phase encoding direction along a phase encoding axis.
  • FIG. 4 A An example of a planar readout trajectory is shown in Figure 4 A, with the frequency encoding direction corresponding to k x for this orientation of the planar trajectory, and the phase encoding direction k y , with the phase encoding axis being shown at 401.
  • readout trajectories having other shapes could be used, including but not limited to: helical, staircase, random, star, curved, spiral, cork-screw, cylindrical, or the like.
  • the following examples will focus on the use of a planar trajectory, but it will be appreciated that this is not intended to be limiting.
  • the multiple acquisition sequences are configured so as generate a plurality of readout trajectories extending over at least part of the acquisition space, the readout trajectories being rotated about the phase encoding axis 401, as shown in Figures 4B and 4C.
  • an azimuthal angle of the readout trajectory can be controlled by adjusting the relative amplitude of pulses in readout gradient fields G x , G z .
  • rotation is only about the phase encoding axis in this example, it will be appreciated that rotation could also be performed about other axes, as long as at least a component of the rotation is about the phase encoding axis.
  • the readout trajectory can be generated asymmetrically arranged with respect to the phase encoding axis, so that the trajectory is offset in the frequency encoding direction, to thereby acquire higher frequencies during the acquisition, although this is not essential.
  • Echo signals are acquired for each of the acquisition signals at step 220, allowing k- space data to be obtained. This is typically performed synchronously with the gradient pulses G x , G z , as shown by the pulses ADC in Figure 3, which refer to the acquisition of MRI signal by an analog-to-digital converter (ADC) forming part of the MRI system.
  • ADC analog-to-digital converter
  • This process is performed multiple times using respective acquisition sequences, with the resulting &-space data being used to construct at least one image representation of the anatomical region image at step 230, which can be performed using known processing techniques.
  • the above described process uses an echo imaging type approach in which readout trajectories are rotated about a phase encoding axis in order to generate images of the anatomical region.
  • This approach has a number of benefits over more traditional Cartesian based approaches.
  • this provides greater coverage about the phase encoding axis in &-space, which helps improve the contrast of the acquired images.
  • This in turn allows data to be collected more rapidly, whilst maintaining a good signal-to-noise ratio.
  • the technique can be used to provide mapping at an isotropic resolution of 1mm with a sufficiently high image quality and signal-to-noise ratio in the phase data to allow it to be used for reliable QSM processing.
  • the approach can easily be utilised with various undersampling and accelerated imaging protocols, as will be described in more detail below, thereby further reducing image acquisition time.
  • Such rapid image acquisition is particularly suited for whole brain susceptibility mapping, as the reduction in imaging time helps minimise subject movement during imaging.
  • the imaging methodology can also have applicability to a wide range of other applications including, but not limited to: T2* weighted imaging, Susceptibility Weighted Imaging (SWI), QSM, Arterial Spin Labelling (ASL), functional MRI (fMRI), or the like.
  • the readout can also be used for T2 weighted imaging by including a 180° RF refocussing pulse as part of a spin echo preparation module, whilst RF preparation modules can be added to enable fast 77-mapping, Magnetisation Transfer (MT) contrast imaging, or the like.
  • MT Magnetisation Transfer
  • the method includes undersampling in at least one of a radial and an axial direction, by omitting trajectories or scan lines within individual trajectories or a combination of both. In general, this can be used to reduce the length of time over which data is collected, which in turn can help reduce movement artefacts.
  • undersampling the readout trajectories with an undersampling factor of R PE > 1 can be used to shorten echo times and reduce signal decay artefacts.
  • this can involve acquiring echo signals from selected scan lines of a trajectory, thereby missing at least some scan lines, for example by acquiring data from every second or third scan line or by skipping a whole block of scan lines.
  • Data from missing scan lines can then be collected using subsequent acquisitions, for example by interleaving scan lines of different trajectories.
  • multiple readout trajectories at a given azimuthal position could be used in order to collect data over a complete set of scan lines. This is often referred to as segmentation. It will be appreciated that other segmentation strategies could also be used, such as block-wise segmented trajectories, with all scan lines in a first block being collected separately from all scan lines in a subsequent block, at the same azimuthal angle.
  • the readout trajectories can be configured to overlap so that at least some scan lines are sampled with different echo times.
  • the method could include performing accelerated imaging by exploiting symmetry of the echo signals in &-space.
  • the concept widely known as partial Fourier, skips echo signals from up to half of a readout trajectory, in turn allowing the acquisition to be performed faster, in turn potentially allowing to reduce the echo-time.
  • the skipped lines can then be filled with zero values or specific partial Fourier reconstruction techniques can be employed.
  • the method could include performing accelerated imaging by reconstructing data from missing scan lines.
  • data could be reconstructed based on echo signals acquired from different scan lines in the same trajectory, or from scan lines in different trajectories, or both.
  • the different trajectories would typically be adjacent trajectories, although this is not essential, and extrapolation from non-adjacent trajectories could be performed.
  • the reconstruction can use echo signals acquired from different scan lines of adjacent trajectories, so that for example, the reconstruction of scan line 6 of a first trajectory uses echo signals acquired from scan line 5 of a second trajectory.
  • reconstruction can be performed using echo signals acquired from equivalent scan lines, so that for example, the reconstruction of scan line 6 of a first trajectory uses echo signals acquired from scan line 6 of a second trajectory.
  • acquisition skips scan lines during the measurement, with the undersampling patterns being interleaved in adjacent readout trajectories.
  • the approach skips scan lines 2, 4, 6, ... in first, third, fifth,... readout trajectories and scan lines 1, 3, 5, ... in the second, fourth, sixth... readout trajectories. This can significantly improve the image reconstruction, while minimising acquisition time.
  • the concept is not limited to an acceleration factor of 2, but rather any number of scan lines could be skipped, with data being reconstructed from one or more adjacent trajectories as needed.
  • reconstruction can be performed taking into account prior knowledge, such as knowledge of sample response or image properties (e.g. sparsity in transform domains), as well performing reconstruction in image space as opposed to &-space.
  • reconstruction involves obtaining echo signals and using k- space filtering or model-based reconstructions including prior knowledge, to allow images or tissue specific parameters (relaxation times, susceptibility or the like) to be derived from either a single acquisition, or one or more partial or fully sampled acquisitions.
  • &-space filtering takes into account the fact that the contrast information of an image is stored in the centre of &-space, while the resolution information is found in its outer parts.
  • rotational acquisition scheme means that the centre of &-space is regularly sampled, and so it is possible to filter &-space before the reconstruction, so that specific scan lines carrying specific contrast information are weighted higher than other lines.
  • Using an acquisition that incorporates trajectories with different echo-times or contrast information allows certain trajectories with a specific echo-time or contrast to be given a higher weighting than other trajectories in the &-space centre. It will also be possible to only use the information of a single trajectory in the centre of &-space to constrain the echo-time or contrast of the reconstruction, with information from other trajectories being used to obtain information about the spatial resolution.
  • a reconstruction like this can, for example, be used to obtain images with different echo-times or contrasts from less than one acquisition for each echo-time or contrast. It will be appreciated that this can reduce the amount of sampling required, which in turn can reduce imaging time.
  • &-space filtering it is also possible to incorporate prior knowledge on different echo-times or contrasts of individual trajectories in the reconstruction or to use a model-based reconstruction. If for example, a-priori information about the contrast at the time of acquisition of a trajectory is known or the contrast is given by a certain signal behaviour, such as an exponential decay, incorporating a model of this decay into the reconstruction (e.g. an iterative optimization), enables the parameters that describe the signal decay (e.g. one or more tissue specific parameters) to be determined for each pixel. In addition to this, it can be possible to obtain the images at different echo-times or contrasts from this reconstruction.
  • a model-based reconstruction e.g. an iterative optimization
  • echo-time shifting can be performed while rotating the readout trajectories. In effect this involves not only rotating the readout trajectories about the phase encoding axis, but also shifting them along the phase encoding axis. For example, from a first readout trajectory scan lines 1-40 could be sampled, whilst in subsequent trajectories, scan lines 2-41, 3-42, and the like can be sampled. Again, the echo will always occur in line 21, but the time that passes between excitation and echo formation will differ in every trajectory, again helping to avoid signal drop-out artefacts, or the like. In practice this could be achieved by adjusting a dephaser pulse for the phase encoding field G y for the different readout trajectories.
  • the multiple readout trajectories are asymmetrically arranged with respect to the phase encoding axis to acquire higher spatial frequencies during the acquisition, or to reduce the scan and echo-time while maintaining spatial resolution, as shown for example in Figures 4D and 4E.
  • the asymmetry is controlled based on an amplitude of dephaser pulses in readout gradient fields G x , G z .
  • the degree of asymmetry can change during acquisition, thereby providing an effective elliptical or partial coverage of &-space.
  • the acquisition sequences are typically configured to generate a homogeneous azimuthal distribution of readout trajectories.
  • the trajectories are spaced so that over 360°, readout trajectories are interleaved as shown in Figure 4C.
  • the readout trajectories are distributed by an azimuthal angle ⁇ given by:
  • the approach can use readout trajectories spaced by any suitable angle ⁇ 0. This includes randomly chosen angle increments, but in one example this is a golden angle or tiny golden angle, given by where Q can be any integer number.
  • Such arrangements achieve a very uniform coverage of the 3D cylindrical volume with a small number of trajectories, which is particularly advantageous for time-resolved measurements, such as functional MRI.
  • a random readout trajectory acquisition order can also be used, thereby effectively selecting or increasing the azimuthal angle randomly, which can be used to avoid the need to use spoiler gradient fields.
  • spoiler gradient fields are normally used to dephase and remove magnetization after the acquisition of a readout trajectory is finished, thereby removing artefacts in the image.
  • using a random scheme allows the spoiling gradient field to be avoided, thereby shortening the acquisition time for each trajectory and reducing potential eddy-current effects.
  • phase encoding axis is orientated along a shortest dimension of the anatomical region, perpendicular to a trajectory of the at least one image representation or perpendicular to a transverse trajectory of the subject.
  • orientating the phase encoding axis along a shortest dimension of the anatomical region can minimise the time taken to acquire data from a readout trajectory, in turn reducing the echo-time and the acquisition bandwidth along the phase encoding axis, which can help reduce distortions and signal dropouts.
  • orientating the phase encoding axis perpendicularly to the image plane of the images to be displayed this means that distortions arising within the trajectory are less pronounced as the readout trajectory is oriented perpendicular to the image plane.
  • the image reconstruction process will vary depending on the particular approach used. Typically this will involve one or more of the following steps: performing phase correction; either combining echo signals from multiple undersampled trajectories or reconstructing missing data from undersampled trajectories; performing a Fourier transform along a phase encoding axis; regridding to a uniform grid; and, combining individual data subsets into a combined dataset.
  • the method of processing the &-space data includes interpolating echo signals onto equidistant sampling points along a readout axis to remove non-linearities and specifically ramp sampling induced non-linearities.
  • the method can also include processing the &-space data by performing any one or more of gradient delay, Nyquist ghost correction, physiologic noise correction, eddy current corrections, measuring the trajectory, or the like, for echo signals from each readout trajectory.
  • the method includes processing the &-space data by acquiring optional non-phase-encoded navigator echo signals between each RF excitation and acquiring echo signals from the readout trajectories, as shown at 301 in Figure 3.
  • a gradient delay induced shift along a readout axis is then estimated, for example using an average delay between the first and third and the second navigator echo signals or any other suitable approach, with this being used to correct for gradient delays using the estimated gradient delay.
  • the navigator pulses could be omitted, with the radial symmetry being used to correct for gradient delays, which would shorten the echo time and reduce the acquisition time for each trajectory.
  • the method includes processing the &-space data by obtaining shifts at multiple azimuthal readout trajectory angles, fitting shift values to a gradient delay model and using delay model fit values to correct for the linear phase errors between the odd and even scan lines.
  • susceptibility weighted imaging When used for phase imaging, susceptibility weighted imaging, or susceptibility mapping, known processing techniques can be used, which may include determining phase data for each coil channel individually, deriving a brain mask from combined magnitude data and obtaining susceptibility mapping images by calculating the mean of all channels, although channels could be combined initially depending on the preferred approach.
  • the magnetic resonance imaging method comprises generating an imaging field, the anatomical region being provided in the imaging field; generating multiple acquisition sequences, each acquisition sequence including an RF excitation pulse, and phase encoding gradient field pulses configured to generate a readout trajectory in &-space, each readout trajectory including a number of scan lines extending in a frequency encoding direction and spaced in a second phase encoding direction along a phase encoding axis, and the multiple acquisition sequences generating a plurality of readout trajectories extending over at least part of the acquisition space, wherein the acquisition sequences are undersampled in at least one of a frequency and phase encoding direction; echo signals from the multiple readout sub-trajectories for each of the multiple acquisition sequences to generate &-space data including reconstructed readout trajectories; and, processing the k- space data to generate at least one image representation of the anatomical region.
  • typically accelerated acquisition techniques include the selection of the orientation of the phase encoding axis relative to the subject, the use of interleaved or segmented readout trajectories, or the like.
  • a 3D EPI sequence was equipped with a POP readout scheme sampling a cylinder shaped &-space.
  • the 3D POP scheme consists of multiple standard 2D EPI readout trajectories rotating about the phase encoding axis. Per excitation a single trajectory on this paddlewheel is sampled, with the 3D slab selective excitation performed along the rotation/phase encoding axis.
  • segmentation or regular undersampling is used to allow a reduction of the echo train length.
  • the azimuthal acquisition order used is shown in Figure 4C, with the numbers indicating the order of sampling.
  • the azimuthal angle ⁇ is constantly increased within [0; 2 ⁇ ].
  • the second half of the projections are interleaved between the first half.
  • the &-space for each trajectory was fully covered using three interleaved shots before incrementing the azimuthal angle. Acquisition times per volume were 35 s, 39 s and 43 s. Repeating the measurements nine, nine and eight times resulted in overall acquisition times of 312 s, 347 s and 349 s, respectively.
  • brain imaging was performed using a 3D multi-echo GRE sequence. Again, the images were acquired at an isotropic spatial resolution of 1 mm 3 .
  • Image reconstruction for the 3D POP EPI data was performed using MATLAB (MathWorks, Natick, MA, USA). First, all ramp-sampling induced non-linearities along the readout axis were removed by interpolating the data onto equidistant sampling points. Hereafter, a gradient delay and Nyquist ghost correction was performed: for each trajectory, the gradient delay induced shift along the readout axis was estimated using known techniques based on an average of the first and third and the second navigator as the two opposed calibration lines. To minimize the influence of noise and eddy currents in the estimation, the shifts obtained at all angles ⁇ were fitted using the appropriate gradient delay model.
  • the fit values were finally used to correct for the linear phase errors between the odd and even phase encoding lines.
  • the phase correction was carried out individually for each acquired data subset, i.e. before combining individual segments into a measurement or reference dataset.
  • GRAPPA was employed to reconstruct the missing phase encoding lines with the weight sets determined trajectory -wise from the combined acquired data subsets.
  • the non-Cartesian data of each axial slice was reconstructed using non-uniform fast Fourier transform (NUFFT) software.
  • NUFFT non-uniform fast Fourier transform
  • the images for the 3D GRE measurement were calculated online using the reconstruction pipeline provided by the manufacturer.
  • GRAPPA was utilized with 36 auto-calibration lines for weight set calculation. Again, a root- sum-of-squares receiver channel combination was employed to finalize the reconstruction.
  • the image phase was extracted from the uncombined single-channel data and processed using a total generalized variation (TGV) method.
  • the mapping procedure incorporates phase unwrapping, background field removal and dipole inversion in a single step.
  • the required brain mask was generated based on root-sum-of- squares-combined magnitude data using the segmentation and image calculator module of SPM12 (Wellcome Trust Centre for Neuroimaging, London, UK).
  • the single-channel susceptibility maps were finally combined by computing the mean across all channels.
  • phase correction not only removes Nyquist ghost artefacts, it also decreases signal inhomogeneities with low spatial frequencies. Additionally, the combination of individual segmentation subsets significantly benefits from the correction. Similar effects are observed for GRAPPA reconstructed datasets. The improved combination of segmentation subsets in the TGRAPPA calibration data leads to substantially reduced residual undersampling artefacts in the sagittal views.
  • Distortions are only observed along the phase encoding direction. Arrows indicate areas with major geometric distortions. The same axial slice is also depicted for the ninefold &-space average (avg) of all 3D POP EPI repetitions (right-hand column, top) as well as the threefold accelerated and GRAPPA reconstructed 3D POP EPI measurement (right-hand column bottom). The overall acquisition time is given in the bottom right of each image.
  • the number of averages basically does not affect the mean or standard deviation.
  • the corresponding susceptibilities calculated from the 3D GRE data are depicted as well, and match those measured with the 3D POP EPI approach. Significant differences between the different echo times are not observed.
  • the standard deviation which represents the variability within a region of interest, is very similar between the 3D GRE and 3D POP EPI measurements.
  • FIG. 10 depicts the susceptibility measured with 3D POP EPI as a function of the susceptibility obtained using 3D GRE for three subjects and the different protocols, with a line of identity being given by the dashed line.
  • Figure 10 depicts the susceptibility measured with 3D POP EPI as a function of the susceptibility obtained using 3D GRE for three subjects and the different protocols, with a line of identity being given by the dashed line.
  • the Figure shows a generally high correspondence between the different sequences.
  • Axial slice of the &-space averaged reconstruction of all volumes left-hand image
  • single-volume reconstructions of Repetition 1 central image
  • Repetition 9 right-hand image
  • calculated susceptibility within the pallidum plotted against the overall acquisition time of the data included in the evaluation (graph, right).
  • Head motion during the acquisition leads to significant blurring and loss of details in the averaged reconstruction and an increasing drift of the calculated susceptibility.
  • the overall acquisition time is given in the bottom right of each image.
  • phase correction was based on additional navigator lines that were acquired for every trajectory prior to the actual echo-planar readout.
  • an azimuthal distribution of the trajectories over 2 ⁇ rather than ⁇ was chosen.
  • the phase correction successfully removed Nyquist ghosting and increased the signal homogeneity across the brain. It also had a major positive effect on the image reconstruction. Skipping the phase correction leads to discontinuities in &-space when combining the segmentation or GRAPPA calibration subsets, introducing residual artefacts either directly or through miscalibration of the GRAPPA kernel. Minor other effects that are considered to be a result of improving the trajectory alignment were a slight reduction in background noise and some slight removal of blurring artefacts. However, these effects appeared to be rather small.
  • the above examples demonstrate that high resolution accelerated QSM can be performed using non-Cartesian 3D POP EPI at ultra-high field.
  • the proposed technique is considerably faster than the conventional Cartesian 3D multi-echo GRE approach, while yielding comparable susceptibility values in subcortical structures.
  • the proposed non-Cartesian POP readout scheme allows for an echo time suitable for susceptibility mapping, reduced echo train lengths and reduced distortions with respect to conventional Cartesian EPI. Providing high flexibility in terms of undersampling renders POP EPI also interesting for other applications such as functional MRI.
  • FOV 212 x 212 x 108mm 3
  • T R 47 ms
  • T E 24 ms
  • ES 1.0 ms
  • flip angle 13°
  • ramp sampling s 20% 3 ⁇ fold segmentation along phase encoding direction
  • acquisition time 51 seconds.
  • 3 additional navigator echoes were acquired and an interleaved radial projection order was used to achieve a homogeneous azimuthal distribution of the trajectories as shown in Figure 4E.
  • Image reconstruction was performed offline using MATLAB (MathWorks, Natick, MA, USA). After combination of the segmentation subsets and phase correction, the individual readout trajectories were Fourier transformed along the phase encoding axis and gridded onto a Cartesian grid using the non-uniform fast Fourier transform (NUFFT) software. The ramp sampling interpolation was incorporated in the final gridding step.
  • NUFFT non-uniform fast Fourier transform

Abstract

L'invention concerne un procédé d'imagerie par résonance magnétique permettant d'obtenir une image d'une région anatomique d'un sujet, le procédé consistant : à générer un champ d'imagerie, la région anatomique étant disposée dans un espace d'acquisition à l'intérieur du champ d'imagerie ; à générer de multiples séquences d'acquisition, chaque séquence d'acquisition comprenant une impulsion d'excitation RF, et des impulsions de champ de gradient conçues pour générer une trajectoire d'indicateur dans un espace k, chaque trajectoire d'indicateur comprenant un certain nombre de lignes de balayage s'étendant dans une direction de codage de fréquence et espacées dans une seconde direction de codage de phase le long d'un axe de codage de phase, et les multiples séquences d'acquisition générant une pluralité de trajectoires d'indicateur s'étendant sur au moins une partie d'un espace d'acquisition, les trajectoires d'indicateur étant tournées au moins partiellement autour de l'axe de codage de phase ; à acquérir des signaux d'écho à partir des trajectoires d'indicateur pour chacune des multiples séquences d'acquisition afin de générer des données d'espace k ; et à traiter les données d'espace k pour générer au moins une représentation d'image de la région anatomique.
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