JPWO2020225291A5 - - Google Patents

Description

The present invention relates to the field of magnetic resonance (MR) imaging. The present invention relates to an MR imaging method for an object. The invention also relates to an MR device and a computer program running on the MR device.

Imaging MR methods that utilize the interaction between a magnetic field and nuclear spins to form a 2D or 3D image are in many respects superior to other imaging methods for soft tissue imaging. Since it does not require ionizing radiation and is generally non-invasive, it is widely used today, especially in the field of medical diagnostics.

Generally, according to the MR method, the object, for example the patient's body to be examined, is placed in a strong, homogeneous magnetic field, the direction of which is simultaneously the axis of the coordinate system on which the measurements are based (usually the z-axis). stipulate. The magnetic field produces different energy levels for individual nuclear spins, depending on the magnetic field strength (spin resonance), which can be excited by application of an electromagnetic alternating magnetic field (RF field) of defined frequency (the so-called Larmor frequency or MR frequency). do. From a macroscopic point of view, the distribution of individual nuclear spins produces an overall magnetization that can be deflected out of equilibrium by the application of an electromagnetic pulse (RF pulse) of appropriate frequency, so that the magnetization is centered around the z-axis. Perform precession motion. Precession describes the surface of a cone whose opening angle is called the flip angle. The magnitude of the flip angle depends on the strength and duration of the applied electromagnetic pulse. In the case of the so-called 90° pulse, the spins are deflected in the transverse plane (flip angle 90°) from the z-axis.

After termination of the RF-pulse, the magnetization returns to its original equilibrium state, where the magnetization in the z-direction is re-accumulated with a first time constant T ₁ (spin-lattice or longitudinal relaxation time) and the magnetization in the direction perpendicular to the z-direction is The magnetization relaxes with a second time constant T ₂ (spin-spin or transverse relaxation time). Magnetization variations can be detected by means of a receive RF coil positioned and oriented within the examination volume of the MR apparatus such that the magnetization variations are measured in a direction perpendicular to the z-axis.

Within the context of this specification, MR device and MR imager are used interchangeably. The decay of the transverse magnetization is e.g. the transition of the nuclear spins (local magnetic field inhomogeneity induced by). Dephasing can be compensated for by means of refocusing pulses (eg 180° pulses). This generates an echo signal (spin echo) in the receiving coil. To achieve spatial resolution within the body, constant magnetic field gradients extending along the three principal axes were superimposed on a uniform magnetic field, resulting in a linear spatial dependence of the spin resonance frequency. The signal picked up in the receive coil then contains components at different frequencies that can be associated with different locations within the body. The signal data obtained through the receiving coils corresponds to the spatial frequency domain and is called k-space data. K-space data typically includes multiple lines acquired with different phase encodings. Each line is digitized by collecting a number of samples. A set of k-space data is transformed into an MR image by means of image reconstruction methods. MR images are diffusion sensitive.

Known diffusion weighted imaging (DWI) techniques are generally performed by using an imaging sequence that includes a diffusion gradient in which the diffusion of protons (of water molecules) along the direction of the diffusion gradient reduces the amplitude of the acquired MR signal. be done. DWI techniques are particularly vulnerable to macroscopic physiological motion in whole-body applications, as signal attenuation resulting from motion can confound measurements of interest. Subject movement during an MR examination can be particularly problematic in populations such as children, the elderly, or patients with medical conditions such as Parkinson's disease, who are not yet lying down. Motion affects the data in two main ways, shifting the imaged tissue (resulting in ghosting artifacts in reconstructed MR images) and exposure to inaccurate diffusion coding.

To avoid significant artifacts resulting from motion, DWI data have generally been acquired using single-shot imaging sequences such as single-shot echoplanar imaging (EPI). However, single-shot DWI can result in poor image quality and limited spatial resolution. The limited spatial resolution as well as the significant geometric distortion due to the echo-planar technique combined with the main magnetic field inhomogeneity make it difficult to measure the diffusion properties with high accuracy. Although these distortions are still acceptable or well correctable in brain applications, whole-body DWI MR images are seriously compromised. This is due to the larger scale of the main magnetic field inhomogeneity effect in the body (due to the tissue/air interface) as well as motion effects leading to both direct motion artifacts and dynamic changes in the magnetic field inhomogeneity effect.

As a result, several other DWI techniques have been developed, some of them using steady-state free precession (SSFP) imaging techniques. In general, SSFP imaging sequences are based on gradient echo imaging sequences with short repetition times. The SSFP sequence includes lateral coherence tomography from overlapping multi-order spin echoes and stimulated echoes. This is usually accomplished by refocusing the phase-encoding gradient at each repetition interval to keep the phase integral (or gradient moment) constant. A fully balanced SSFP imaging sequence achieves zero phase by refocusing all imaging tilts. Diffusion sensitivity can be induced in the SSFP imaging sequence by adding a diffusion gradient. Diffusion-weighted dual-echo steady-state (DW-DESS) MR imaging has been proposed as a distortion-free alternative to the conventional single-shot EPI approach (Gras V, Farrher E, Grinberg F, Shah NJ, “Average Diffusivity, 3 Diffusion Weighted DESS Protocol Optimization for Simultaneous Mapping of Proton Density and Relaxation Times at Tesla”, Magn Reson Med, 2017, 78(1), 130-141). DESS produces two MR signals, a free induction decay (FID) signal and an echo signal from steady-state free precession, independently at each iteration. The phase-encoding field gradients are balanced to maintain a steady state of transverse magnetization. A steady-state signal enables acquisition with good signal-to-noise ratio (SNR) efficiency (comparable to EPI). This indicates that DW-DESS has been used, for example, in knee imaging applications (cartilage), due to its short T2 relaxation ₍ Miller KL, Hargreaves BA, Gold GE, Pauly JM, “Stability of knee cartilage in vivo. State Diffusion Weighted Imaging”, Magn Reson Med 2004, 51, 394-398). Two-signal (FID and echo signal acquisition provides a means to correct for relaxation weighting, and additionally allows quantitative apparent diffusion coefficient (ADC) assessment with only two scans with different diffusion weightings. (Bieri O, Ganter C, Scheffler K, "Quantitative in vivo diffusion imaging of cartilage using dual-echo steady-state free precession," Magn Reson Med, 2012, 68, 720-729). However, DW-DESS has not become a routine sequence to date, partly because the full potential of signal gain in the steady state is not balanced against tilting moments, especially diffusion-weighted tilting, and corresponding Another reason is that the conventionally used unipolar diffusion gradient introduces a very strong sensitivity to bulk motion. be.

From the foregoing, it can be readily appreciated that improved DWI techniques are needed.

Consequently, it is an object of the present invention to minimize motion-induced artifacts to enable distortion-free high-quality DWI.

According to the present invention, a method for MR imaging of an object located in an examination volume of an MR apparatus is disclosed. The method is
exposing the object to a dual-echo steady- state imaging sequence, wherein a free induction decay signal and an echo signal are generated at each interval between two successive RF pulses, equal-phase integral and opposite-polarity paired diffusion a gradient waveform is applied to the interval between the free induction decay signal and the echo signal;
acquiring the free induction decay signal and the echo signal over multiple iterations of the imaging sequence while varying phase encoding;
reconstructing diffusion-weighted MR images from the acquired free induction decay and echo signals.

In other words, the present invention proposes a diffusion-weighted dual-echo steady-state (DESS) sequence using bipolar diffusion gradients, as opposed to the unipolar diffusion gradients used conventionally. This allows a perfect balance of tilting moments such that different signal coherence paths are preserved and contribute to the acquired MR signal. Maximum signal gain of the steady-state sequence is obtained and therefore maximum SNR efficiency is achieved. Moreover, the bipolar diffusion gradient makes the method of the present invention motion insensitive. The additional advantages of DW-DESS mentioned above are preserved.

The method of the present invention has no net effect on the phase of the fixed spins while the criteria for a perfectly balanced imaging sequence are met, but any phase difference between the fixed and mobile spins. A bipolar diffusion gradient is used that includes a gradient waveform of .

The approach of the present invention has not previously been considered, as sufficiently strong bipolar diffusion gradients lead to imaging sequence repetition times (TR) on the order of 10 ms or more. Combined with a perfectly balanced steady-state free precession readout, this typically results in closely spaced dark band artifacts that preclude clinical use of the technique.

In the (single-echo) balanced SSFP, the dark band spacing depends only on the repetition time TR and the field inhomogeneity-induced off-resonance. The dark bands of the FID and echo signals in the fully balanced DW-DESS also depend on the moment of the diffusion gradient, which causes additional off-resonance effects in the gradient direction. It is the insight of the present invention that the orientation and spacing of the dark bands are strongly dominated by diffusion tilting at high tilting moments, while the actual off-resonance plays only a minor role. This is exploited in preferred embodiments of the present invention by selecting the 0th order moments of the individual diffusion-weighted gradient waveforms such that the spatial distance of the dark band artifacts is less than the voxel size in the reconstructed diffusion-weighted MR image. be done. The 0th order moment is equal to the area under the gradient waveform as a function of time. By making the 0th order gradient moment of the diffusion gradient waveform sufficiently high, the spatial distance of the dark band artifacts can be reduced to a value smaller than the imaging voxel size. Dark bands within voxels can partially reduce the overall signal strength, but are not visible as artifacts. Therefore, by combining all coherence tomography paths for the overall FID and echo signals of the DESS acquisition, strong diffusion weighting can be induced with high SNR efficiency .

Diffusion weighting imposed on the FID and echo signals is used in accordance with the present invention to reconstruct diffusion-weighted MR images. In a preferred embodiment, reconstruction of MR images includes derivation of diffusion coefficients. Acquisition of FID and echo signals is preferably repeated two or more times with different diffusion gradient waveforms (having different spatial directions and/or different gradient moments) applied in different iterations. Images are derived from fractional anisotropy (FA) maps, mean diffusivity (MD) maps, radial diffusivity (RD) maps, or axial diffusivity (AD) maps commonly used in clinical studies, or diffusion weighting. It may be a map of any other derived scalar metric. For example, reconstruction of a diffusion-weighted MR image may simply involve calculating the ratio of a first MR image reconstructed from FID signals and a second MR image reconstructed from echo signals. In addition, reconstruction of diffusion-weighted MR images can involve the derivation of maps of apparent diffusion coefficients (ADC) from the acquired FID and echo signals, as proposed by Bieri et al. (see above).

In a preferred embodiment of the invention, the tilting moment is maximized by applying diffusion gradients in all spatial directions simultaneously. In this way, the capacity of the tilting system of the MR apparatus used can be optimally utilized .
In a further preferred embodiment of the invention, the FID and echo signals are acquired with opposite readout gradients. In this embodiment, the tilt is switched between opposite directions in the readout direction throughout the interval between two successive RF pulses.

The methods described so far include at least one main magnet coil for generating a homogeneous static magnetic field within the examination volume and a magnetic field gradient for generating switched magnetic field gradients in various spatial directions within the examination volume. several gradient coils and at least one RF coil for generating RF pulses in the examination volume and/or for receiving MR signals from an object located in the examination volume, RF pulses and switching It can be implemented by means of an MR apparatus including a control unit for controlling the temporal succession of the applied magnetic field gradients and a reconstruction unit. The method of the invention can for example be implemented by a corresponding program of the reconstruction unit and/or the control unit of the MR apparatus.

The method of the present invention can be advantageously implemented in most MR devices currently in clinical use. For this purpose it is only necessary to make use of a computer program by which the MR apparatus is controlled to carry out the above-described method steps of the invention. The computer program may reside on a data carrier or within a data network so as to be downloaded for installation in the control unit of the MR device.

According to a further aspect, the invention provides at least one main magnet coil (2) for generating a homogeneous static magnetic field within the examination volume and magnetic field gradients switched in different spatial directions within the examination volume. a plurality of gradient coils (4, 5, 6) for and at least one RF coil (9) for generating RF pulses from an object (10) located in the examination volume and a temporal succession of the RF pulses and a control unit (15) for controlling the switched magnetic field gradients, and a reconstruction unit (17) , the MR device (1) for dual-echo steady- state imaging of a subject (10). A step of exposing to a sequence in which a free induction decay signal (FID) and an echo signal (ECHO) are generated at each interval between two successive RF pulses, and a pair of spreading gradient waveforms (GDIF) of equal phase integral and opposite polarity. ) is applied to the interval between the FID signal and the echo signal; acquiring the FID signal and the echo signal at multiple repetitions of the imaging sequence with varying phase encoding ; and acquiring the FID signal and the echo signal. reconstructing a diffusion-weighted MR image from the MR device.
The accompanying drawings disclose preferred embodiments of the invention. It is understood, however, that the drawings are designed for purposes of illustration only and are not intended as a definition of the limits of the invention.

1 shows an MR apparatus for carrying out the method of the invention; FIG. 2 shows a diagram of an imaging sequence used in one embodiment of the present invention; DW-DESS brain images acquired at different diffusion gradient moments are shown. Figure 2 shows the reconstruction of a DW-DESS brain image according to the invention; DW-DESS brain images acquired with bipolar and unipolar diffusion gradients are shown.

Referring to FIG. 1, an MR device 1 is shown. The apparatus comprises superconducting or resistive main magnet coils 2 such that a substantially uniform and temporally constant main magnetic field is generated through the examination volume along the z-axis. A magnetic resonance generation and manipulation system applies a series of RF pulses and switched magnetic field gradients to flip or excite nuclear magnetic spins, induce magnetic resonance, refocus magnetic resonance, manipulate magnetic resonance, Magnetic resonance is spatially and otherwise encoded, spins are saturated, and the like to perform MR imaging .
More specifically, a gradient pulse amplifier 3 applies current pulses to selected ones of whole body gradient coils 4, 5 and 6 along the x, y and z axes of the examination volume. A digital RF frequency transmitter 7 transmits RF pulses or pulse packets to a whole body volume RF coil 9 via a transmit/receive switch 8 to transmit RF pulses to the examination volume. A typical MR imaging sequence consists of a packet of short duration RF pulse segments which, together with each other, allow any applied magnetic field gradient to selectively manipulate nuclear magnetic resonance. Achieve. The RF pulses are used to saturate, excite resonance, reverse magnetization, refocus resonance, or manipulate resonance and select portions of body 10 that are located within the examination volume. MR signals are also picked up by a whole body volume RF coil 9 .
For the generation of MR images of a limited area of body 10, a set of local array RF coils 11, 12, 13 are placed adjacent to the area selected for imaging. Array coils 11, 12, 13 can be used to receive MR signals induced by body coil RF transmissions .
The resulting MR signals are picked up by whole body volume RF coil 9 and/or array RF coils 11, 12, 13 and demodulated by receiver 14, which preferably includes a preamplifier (not shown). Receiver 14 is connected to RF coils 9 , 11 , 12 and 13 via transmit/receive switch 8 .
Host computer 15 controls gradient pulse amplifier 3 and transmitter 7 to generate any of a number of MR imaging sequences, such as diffusion-weighted dual-echo steady-state (DW-DESS) imaging sequences. For selected sequences, receiver 14 receives single or multiple MR data lines in rapid succession following each RF excitation pulse. Data acquisition system 16 performs an analog-to-digital conversion of the received signal, converting each MR data line into a digital format suitable for further processing. In today 's MR machines, data acquisition system 16 is a separate computer dedicated to the acquisition of raw image data.

Ultimately , the digital raw image data is reconstructed into an image representation by a reconstruction processor 17 that applies a Fourier transform or other suitable reconstruction algorithm such as SENSE, SMASH, or GRAPPA. MR images can represent planar slices through a patient, arrays of parallel planar slices, three-dimensional volumes, and the like. The image is then stored in an image memory, where slices, projections, or other portions of the image representation can be visualized, for example, via a video monitor 18 providing a human-readable display of the resulting MR image. can be accessed to convert it into the appropriate format for composing .
With continued reference to FIG. 1 and further reference to FIGS. 2-5, embodiments of the method of the present invention are described below.

Body 10 undergoes multiple iterations of the diffusion-weighted steady- state imaging sequence, as shown in FIG. The sequence consists of two acquisitions (indicated by ADC) of the free induction decay and echo signals, FID and ECHO, respectively, with equal phase integrals and opposite polarity paired spreading gradient waveforms GDIF, i.e. equal magnitudes in the depicted embodiment. are modified fully balanced DESS sequences separated by bipolar trapezoidal diffusion gradients with FID and echo acquisition are performed with reverse readout gradient GX. Both readouts are combined with prealigned and realigned phase ramps for a perfectly balanced readout. The phase-encoding gradients in the GY and GZ directions are perfectly balanced as well. For maximum diffusion weighting, all gradient directions Gx, Gy, Gz are used simultaneously to generate bi-directional diffusion gradients .
As detailed above, if the diffusion gradient moment is insufficient, dark band artifacts in the FID and echo signals will occur in perfectly balanced DW-DESS. The orientation and spacing of the dark bands are strongly dominated by the diffusion tilt, especially at high tilting moments.

Figure 3a shows a DW-DESS brain image acquired with insufficient tilting moment. A typical dark band artifact can be seen.

In FIG. 3b, the diffusion gradient is chosen according to the invention to be sufficiently strong that the spatial distance of the dark bands is smaller than the voxel size. Dark bands within each voxel partially reduce the overall signal strength, but are no longer visible as artifacts. Applications of the present invention exist in all regions of diffusion-weighted MR imaging, not only in the brain, but especially in the body, where magnetic field inhomogeneities cause large geometric distortions in conventional DWI based EPI acquisitions. , or even signal cancellation. Especially for whole-body applications, the present invention uses a fat-saturated (or water-only) bSSFP/TrueFISP technique (Scheffler K, Heid O, Hennig J, "Magnetization preparation during steady state: fatsaturated 3D TrueFISP", Magn Reson Med, 2001, 45(6), 1075-80.

Figure 4 shows a further brain imaging example (repetition time TR = 30 ms). Two FID and echo signals in the figure. 4a and 4b show different degrees of diffusion weighting because the FID and echo signal coherence tomography paths are affected differently by the diffusion gradient.

In FIG. 4c, the ratio FID/ECHO is calculated for each voxel emphasizing the difference in diffusion weighting .

The diffusion weighted fully balanced DESS technique of the present invention is particularly robust with respect to motion, as can be seen in FIG. FIG. 5 compares DW-DESS images using bipolar diffusion gradients (FIGS. 5a, 5b) and unipolar diffusion gradients (FIGS. 5c, 5d). Although the bipolar variant according to the invention provides stable image quality for both FID and ECHO signals , in FIG . clearly destroyed by
Furthermore, a combined acquisition of distortion-free diffusion-weighted images and tissue conductivity maps is provided using a fully balanced dual-echo steady-state (DESS) sequence. Banding artifacts are avoided by using a sufficiently high gradient moment of the diffusion gradient so that the banding is contained within a single voxel. The stability of B1 transmit-receive phase measurements with balanced DESS sequences allows the derivation of quantitative tissue conductivity based on the second derivative using standard EPT (Electrical Property Tomography ) methods. The simultaneous feasibility of DWI and EPT was demonstrated in phantom and volunteer experiments (head) on a 3T MRI system .
Diffusivity and tissue conductivity, for example, are physiological parameters with diverse applications in tumor characterization, typically assessed in separate sequences by diffusion weighted imaging (DWI) and electrical property tomography (EPT). . EPI-based DWI sequences often suffer from geometric distortions (magnetic field inhomogeneities). Diffusion-weighted dual-echo stationary (DWDESS) MRI with unipolar gradients offers a distortion-free alternative, but is inherently sensitive to motion and does not utilize steady- state signals due to unbalanced gradients.

In this study, we developed a balanced DW-DESS sequence using bipolar DW gradients while avoiding dark band artifacts. EPT is based on the transmit-receive phase φ purely related to B1 (unaffected by B0), as in spin-echo (SE)-based sequences and balanced steady-state sequences. The use of φ from balanced DW-DESS as a basis for EPT was investigated. This synergistically allows assessing two related physiological parameters from a single MR acquisition. A fully balanced DW-DESS sequence is used in combination with a bipolar DW gradient.

In the (single-echo) balanced SSFP, the dark band spacing is TE = TR/2 and the echoes are fully refocused , so it depends only on TR and off-resonance (frequency spacing 1/TR). The dark bands of the S+ and S- perfectly balanced DESS also depend on the moments of the diffusion-weighted gradient lobes, which appear as additional off-resonance effects in the gradient direction. At high tilting moments, the orientation and spacing of the dark bands are dominated by tilting effects and are less affected by real off-resonances. This work applies a sufficiently high gradient moment of the bipolar gradient lobes so that the spatial distance of the dark band artifacts is reduced to a value smaller than the imaging voxel size. Dark bands within voxels partially reduce the overall signal strength, but are not visible as artifacts. Therefore, by combining all coherence tomography paths for the overall FID (Echo 1, S+) and ECHO (Echo 2, S−) of the DESS acquisition, it is possible to induce strong diffusion weighting with high SNR efficiency. can. From the resulting DESS signal, the conductivity σ is calculated by the following equation combined with a bilateral denoising filter (with vacuum permeability μ and Larmor frequency ω).

σ=▽ ² φ/2μω

The phantom has different diffusion and conductivity values for the outer and inner compartments (inner: D = 1.04 × 10 mm /S, σ = 0.66 S/m, P/G/S/W = 5/3/ 0.5/91.5 mass%, outside: D=0.8×10mm/S, σ=0.42 S/m, P/G/S/W=25/3/0.3/71.7m% ), gelatin (G), NaCl (S), and H2O (W) .
A combined DW-DESS and EPT acquisition was performed on a phantom and in a volunteer head study (male, age 50) using the following imaging parameters with written consent using a 3T MRI system (Achieva TX, Philips, NL). 3D balanced dual echo SSFP, 8-channel head coil, TR/TE/TE = 31/1.8/26ms (phantom: 53/1.85/50.8ms), FOV 224 x 224 x 120mm, pixel 1.8 × 1.8 mm, reconstruction 224 × 224, 24 slices (in vivo 5 mm, 1.8 mm phantom), pixel bandwidth 1.3 kHz, bipolar or unipolar diffusion gradient (3 simultaneous directions, duration 2 × 11 ms (phantom: 2 × 22 ms, gradient 0.4 ms, intensity 18 mT/m), two signal averages (phantom: 6), total scan time 2 min 55 sec (phantom: 7 min) Diffusion-weighted images in the ratio S+/S− calculated as

The phantom results confirm that the diffusion weightings can be compared with ADC maps obtained using the standard DWI sequence (EPI, 8b values 0...1400, Fig. 2d). Conductivity maps obtained from S+ and measured σ values, inside/outside = (0.77±0.02)/(0.31±0.06) S/m, correspond to phantom preparation.

Balanced D W-DESS acquisition can be successfully implemented using large bipolar DW tilts to avoid banding artifacts and exhibit low motion sensitivity. Although the dark band content within the voxels degrades the SNR, the image quality is clearly improved compared to the unipolar gradient. A drawback of bipolar DW-DESS is given by the limitation of achievable b-values. In this first demonstration, the diffusion weighted images also contain significant T2 weighting due to the long _second echo time (26 or 51 ms). Multiple b values with the same echo time can be used to reduce the T2 weighting ( _b =0 cannot be used due to banding artifacts). The transmit and receive phases of DW-DESS can be used for EPT, yielding conductivity maps with comparable quality to those previously obtained in the brain. Due to the low overall SNR in S images (DW and long TE), the EPT reconstruction is preferably computed from the first echo S+.

DW-DESS can generate distortion-free diffusion-weighted images and conductivity maps simultaneously. Thus, it is expected to be a valuable sequence, especially for tumor characterization.
Various forms of the present invention are described below.
[Item 1]
A method of MR imaging of an object positioned within an examination volume of an MR apparatus, comprising:
exposing the object to a dual-echo steady-state imaging sequence, wherein a free induction decay signal and an echo signal are generated at each interval between two successive RF pulses, equal-phase integral and opposite-polarity paired diffusion a gradient waveform is applied to the interval between the free induction decay signal and the echo signal;
acquiring the free induction decay signal and the echo signal over multiple iterations of the imaging sequence while varying phase encoding;
reconstructing diffusion-weighted MR images from the acquired free induction decay and echo signals;
has
the dual-echo steady-state imaging sequence is perfectly balanced;
the zeroth order moment of the diffusion gradient is selected such that the spatial distance of dark band artifacts is less than the voxel size in the reconstructed diffusion-weighted MR image;
the free induction decay signal and the echo signal are acquired with a reverse readout gradient;
Method.
[Item 2]
The method of item 1, wherein the diffusion gradient is applied simultaneously in all spatial directions.
[Item 3]
3. The method of any one of items 1-2, wherein the free induction decay signal and the echo signal are acquired using a reverse readout gradient.
[Item 4]
reconstructing the diffusion-weighted MR image comprises calculating a ratio of a first MR image reconstructed from the free induction decay signal and a second MR image reconstructed from the echo signal; 4. The method of any one of items 1-3.
[Item 5]
5. The method of any one of items 1-4, wherein reconstruction of the diffusion-weighted MR image comprises derivation of a map of apparent diffusion coefficients from the acquired free induction decay and echo signals.
[Item 6]
6. The method of any one of items 1-5, wherein the reconstruction of the MR image includes derivation of diffusion coefficients.
[Item 7]
7. The method of any one of items 1-6, wherein the acquisition of the free induction decay signal and the echo signal is repeated two or more times, and different diffusion gradient waveforms are applied in the different repetitions.
[Item 8]
8. The method of any of items 1-7, further comprising reconstructing a conductivity image from phase information obtained from the acquired free induction decay and/or echo signals.
[Item 9]
1. An MR imaging apparatus comprising at least one main magnet coil for generating a homogeneous static magnetic field within an examination volume and a plurality of gradient coils for generating switched magnetic field gradients in different spatial directions within said examination volume, comprising: The MR device is
exposing the object to a dual-echo steady-state imaging sequence, wherein a free induction decay signal and an echo signal are generated at each interval between two successive RF pulses, equal-phase integral and opposite-polarity paired diffusion a gradient waveform is applied to the interval between the free induction decay signal and the echo signal;
acquiring the free induction decay signal and the echo signal over multiple iterations of the imaging sequence while varying phase encoding;
reconstructing diffusion-weighted MR images from the acquired free induction decay and echo signals;
is configured to run
the dual-echo steady-state imaging sequence is perfectly balanced;
The MR imaging device adjusts the diffusion gradient such that the 0th order moment of the diffusion gradient waveform is sufficient to reduce the spatial distance of dark band artifacts to less than the voxel size of the reconstructed diffusion-weighted MR image. configured to operate all the gradient coils simultaneously during the application of
The MR imaging device is configured to acquire the free induction decay signal and the echo signal with a reverse readout gradient.
MR imaging device.
[Item 10]
A computer program executed on an MR apparatus, the computer program comprising:
generating a dual-echo steady-state imaging sequence, wherein a pair of equal-phase integral and opposite-polarity diffusion gradient waveforms are applied in the interval between the free induction decay signal and the echo signal;
acquiring the free induction decay signal and the echo signal over multiple iterations of the imaging sequence while varying phase encoding;
reconstructing diffusion-weighted MR images from the acquired free induction decay and echo signals;
has an order for
the dual-echo steady-state imaging sequence is perfectly balanced;
the zeroth order moment of the diffusion gradient waveform is selected such that the spatial distance of dark band artifacts is less than the voxel size of the reconstructed diffusion-weighted MR image;
the free induction decay signal and the echo signal are acquired with a reverse readout gradient;
computer program.

Claims (10)

A method of MR imaging of an object located in an examination volume of an MR apparatus, comprising:
exposing the object to a dual-echo steady- state imaging sequence, wherein a free induction decay signal and an echo signal are generated at each interval between two successive RF pulses and are composed of equal-phase integral and opposite-polarity paired diffusion; a gradient waveform is applied in the interval between the free induction decay signal and the echo signal;
acquiring the free induction decay signal and the echo signal over multiple iterations of the dual-echo steady-state imaging sequence while varying phase encoding;
reconstructing diffusion-weighted MR images from the acquired free induction decay and echo signals;
the dual-echo steady- state imaging sequence is perfectly balanced;
selecting the zeroth order moment of the diffusion gradient waveform of the dual-echo steady-state imaging sequence such that the spatial distance of dark band artifacts is less than the voxel size in the reconstructed diffusion-weighted MR image;
the free induction decay signal and the echo signal are acquired with a reverse readout gradient;
Method. 2. The method of claim 1 , wherein the diffusion gradient is applied simultaneously in all spatial directions. 3. The method of claim 1 or 2 , wherein the free induction decay signal and the echo signal are acquired using a reverse readout gradient. reconstructing the diffusion-weighted MR image includes calculating a ratio of a first MR image reconstructed from the free induction decay signal and a second MR image reconstructed from the echo signal; 4. A method according to any one of claims 1-3 . 5. The method of any one of claims 1-4 , wherein reconstruction of the diffusion - weighted MR image comprises derivation of a map of apparent diffusion coefficients from the acquired free induction decay and echo signals. 6. The method of any one of claims 1-5 , wherein reconstructing the MR image comprises deriving diffusion coefficients. 7. The method of any one of claims 1-6 , wherein the acquisition of the free induction decay signal and the echo signal is repeated two or more times, and different diffusion gradient waveforms are applied in the different repetitions. 8. A method according to any preceding claim, further comprising reconstructing a conductivity image from phase information obtained from the acquired free induction decay and/or echo signals. 1. An MR imaging apparatus comprising at least one main magnet coil for generating a homogeneous static magnetic field within an examination volume and a plurality of gradient coils for generating switched magnetic field gradients in different spatial directions within said examination volume, The MR device is
exposing the object to a dual-echo steady-state imaging sequence, wherein a free induction decay signal and an echo signal are generated at each interval between two successive RF pulses and are composed of equal-phase integral and opposite-polarity paired diffusion; a gradient waveform is applied in the interval between the free induction decay signal and the echo signal ;
acquiring the free induction decay signal and the echo signal over multiple iterations of the dual-echo steady-state imaging sequence while varying phase encoding;
reconstructing a diffusion-weighted MR image from the acquired free induction decay and echo signals;
the dual-echo steady- state imaging sequence is perfectly balanced ;
The MR imager selects a zeroth order moment of the diffusion gradient waveform of the dual-echo steady-state imaging sequence such that the spatial distance of dark band artifacts is less than the voxel size of the reconstructed diffusion-weighted MR image. and configured to simultaneously operate all the gradient coils during application of the diffusion gradient waveform,
The MR imager is configured to acquire the free induction decay signal and the echo signal with a reverse readout gradient.
MR imaging device. A computer program executed on an MR apparatus, said computer program comprising:
generating a dual-echo steady-state imaging sequence, wherein a pair of equal-phase integral and opposite-polarity diffusion gradient waveforms are applied in the interval between the free induction decay signal and the echo signal;
acquiring the free induction decay signal and the echo signal over multiple iterations of the dual-echo steady-state imaging sequence while varying phase encoding;
reconstructing a diffusion-weighted MR image from the acquired free induction decay and echo signals;
the dual-echo steady- state imaging sequence is perfectly balanced ;
selecting the zeroth order moment of the diffusion gradient waveform of the dual-echo steady-state imaging sequence such that the spatial distance of dark band artifacts is less than the voxel size of the reconstructed diffusion-weighted MR image;
the free induction decay signal and the echo signal are acquired with a reverse readout gradient;
computer program.