Dual-echo steady-state MR imaging using bipolar diffusion gradients

Abstract

The present invention relates to a method for MR imaging of an object (10) placed in an examination volume of an MR device (1). One object of the present invention is to achieve high-quality diffusion-weighted imaging (DWI) without distortion while minimizing artifacts caused by motion. The method of the present invention comprises the following steps: subjecting the object (10) to a dual-echo steady-state imaging sequence, generating a free induction decay signal (FID) and an echo signal (ECHO) in each interval between two consecutive RF pulses, wherein a pair of diffusion gradient waveforms (G) with equal phase integrals and opposite polarities are applied in the interval between the FID signal and the echo signal. DIF ); acquiring the FID signal and the echo signal in several repetitions of an imaging sequence with varying phase encoding; and reconstructing a diffusion-weighted MR image based on the acquired FID signal and echo signal. The present invention also relates to an MR device for performing the method and a computer program to be run on the MR device.

Description

Technical Field

[0001] The present invention relates to the field of magnetic resonance (MR) imaging. It relates to a method for MR imaging of an object. The invention also relates to an MR device and to a computer program to be run on an MR device. Background Art

[0002] Image-forming MR methods, which utilize the interaction between magnetic fields and nuclear spins to form two- or three-dimensional images, are widely used today, particularly in the field of medical diagnostics, because they have many advantages over other methods for imaging soft tissues, do not require ionizing radiation, and are generally non-invasive.

[0003] According to a general MR method, an object, such as the body of a patient to be examined, is placed in a strong, uniform magnetic field, the direction of which simultaneously defines the axis of the coordinate system on which the measurement is based (usually the z-axis). The magnetic field generates different energy levels for individual nuclear spins, depending on the strength of the magnetic field, which can be excited (spin resonance) by applying an electromagnetic alternating field (RF field) with a defined frequency (the so-called Larmor frequency, or MR frequency). From a macroscopic perspective, the distribution of individual nuclear spins produces an overall magnetization, which can be deviated from the equilibrium state by applying electromagnetic pulses (RF pulses) of a suitable frequency, causing the spins to precess about the z-axis. The precession describes a conical surface, the aperture angle of which is called the flip angle. The magnitude of the flip angle depends on the intensity and duration of the applied electromagnetic pulse. In the case of a so-called 90° pulse, the spins are deflected from the z-axis to the transverse plane (flip angle 90°).

[0004] After the RF pulse ends, the magnetization relaxes back to its initial equilibrium state, with the z-direction magnetization reestablishing itself with a first time constant, T1 (spin-lattice relaxation or longitudinal relaxation time), and the magnetization perpendicular to the z-direction relaxing with a second time constant, T2 (spin-spin or transverse relaxation time). Changes in magnetization can be detected with the aid of receive RF coils, which are arranged and oriented within the examination volume of the MR system so that changes in magnetization are measured perpendicular to the z-axis. In the context of this specification, MR system and MR imaging system are used interchangeably. The decay of transverse magnetization is accompanied by a transition of nuclear spins (caused by magnetic field inhomogeneities) from an ordered state with identical phases to a state where all phase angles are uniformly distributed (dephasing), for example, after applying a 90° pulse. This dephasing can be compensated with the aid of a refocusing pulse (e.g., a 180° pulse). This generates an echo signal (spin echo) in the receive coils.

[0005] To achieve spatial resolution in the body, a constant magnetic field gradient extending along the main axis is superimposed on the uniform magnetic field, resulting in a linear spatial dependence of the spin resonance frequency. The signal picked up in the receiving antenna then includes components of different frequencies, which can be associated with different positions in the body / object. The signal data obtained via the receiving coil corresponds to the spatial frequency domain and is called k-space data. The k-space data typically includes multiple lines acquired with different phase encodings. Each line is digitized by collecting several samples. The collection of k-space data is converted into an MR image with the aid of an image reconstruction algorithm.

[0006] MR imaging is sensitive to diffusion. Known diffusion-weighted imaging (DWI) techniques are typically performed using imaging sequences that include a diffusion gradient, wherein the diffusion of protons (water molecules) along the direction of the diffusion gradient reduces the amplitude of the acquired MR signal.

[0007] DWI techniques are particularly susceptible to macroscopic physiological motion when applied to the whole body, as motion-induced signal attenuation can confound measurements of interest. Subject movement during MR examinations can be particularly problematic in populations such as children, the elderly, or patients with medical conditions that prevent them from lying down (e.g., Parkinson's disease). Motion affects the data in two primary ways: shifting of the tissue being imaged (leading to ghosting artifacts in the reconstructed MR images) and exposure to incorrect diffusion encoding.

[0008] To avoid significant artifacts caused by motion, single-shot imaging sequences, such as single-shot echo-planar imaging (EPI), are often used to acquire DWI data. However, image quality can be low, and spatial resolution in single-shot DWI is limited. Significant geometric distortion caused by echo-planar techniques combined with main magnetic field inhomogeneities and limited spatial resolution make it difficult to measure diffusion properties with high accuracy.

[0009] While these distortions are still acceptable or adequately corrected in brain applications, whole-body DWI MR imaging is severely impaired due to the large main magnetic field inhomogeneity effects in the body (due to the tissue / air interface) and motion effects, which result in both direct motion artifacts and dynamic variations of field inhomogeneity effects.

[0010] Therefore, several other DWI techniques have been developed, some of which use steady-state free precession (SSFP) imaging technology. In general, SSFP imaging sequences are based on gradient echo imaging sequences with short repetition times. SSFP sequences include transverse coherence from overlapping multi-order spin echoes and stimulated echoes. This is usually achieved by refocusing the phase encoding gradient in 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 gradients. Diffusion sensitivity can be induced in SSFP imaging sequences by adding a diffusion gradient. Diffusion-weighted dual echo steady-state (DW-DESS) MR imaging has been proposed as a distortion-free alternative to conventional single-shot EPI methods (see Gras V, Farrer E, Grinberg F, Shah NJ, “Diffusion-weighted DESS protocol optimization for simultaneous mapping of the mean diffusivity, proton density and relaxation times at 3 Tesla”, Magn Reson Med, 2017, 78(1), 130-141). DESS generates two MR signals individually in each repetition: the free induction decay (FID) signal and the echo signal from the steady-state free precession. The phase-encoding magnetic field gradient is balanced to maintain the steady state of the transverse magnetization. The steady-state signal allows acquisition with good signal-to-noise ratio (SNR) efficiency (comparable to EPI). Because it is suitable for short T2 relaxation, DW-DESS has been used in knee joint imaging applications (cartilage) for example (see Miller KL, Hargreaves BA, Gold GE, Pauly JM, "Steady-state diffusion weighted imaging of in vivo knee cartilage", MagnReson Med 2004, 51, 394–398). The collection of two signals (FID and echo signal) provides a method to correct for relaxation weighting and further allows quantitative apparent diffusion coefficient (ADC) assessment using only two scans using different diffusion weights (see Bieri O, Ganter C, Scheffler K, "Quantitative in vivo diffusion imaging of cartilage using double echo steady-state free precession", Magn Reson Med, 2012, 68, 720–729).

[0011] Despite these advantages, DW-DESS has not become a routine sequence to date. One reason is that the full potential of signal gain in the steady state cannot be utilized because the gradient moments, especially for diffusion-weighted gradients, are unbalanced, and consequently, many signal coherence paths are lost. Another reason is that the conventionally used monopole diffusion gradients introduce strong sensitivity to body motion. Summary of the Invention

[0012] It is readily apparent from the foregoing that there is a need for improved DWI techniques. It is therefore an object of the present invention to achieve high-quality DWI without distortion while minimizing artifacts caused by motion.

[0013] According to the present invention, a method for performing MR imaging on an object placed in an examination volume of an MR device is disclosed. The method comprises the following steps:

[0014] The target is subjected to a dual-echo steady-state imaging sequence, generating an FID signal and an echo signal in each interval between two consecutive RF pulses, wherein a pair of diffusion gradient waveforms with equal phase integrals and opposite polarity are applied in the interval between the FID signal and the echo signal.

[0015] Acquire FID signals and echo signals in several repeated imaging sequences with varying phase encoding; and

[0016] Diffusion-weighted MR images are reconstructed based on the acquired FID signals and echo signals.

[0017] In other words, compared to the conventionally used unipolar diffusion gradients, the present invention proposes a diffusion-weighted dual-echo steady-state (DESS) sequence using bipolar diffusion gradients. This achieves a perfect balance of gradient moments, ensuring that different signal coherence paths are preserved and contribute to the acquired MR signal. This maximizes the signal gain of the steady-state sequence and, consequently, the highest signal-to-noise ratio (SNR) efficiency. Furthermore, the bipolar diffusion gradients render the present method insensitive to motion, preserving the further advantages of DW-DESS as described above.

[0018] The method of the present invention uses bipolar diffusion gradients, which encompass any gradient waveform that produces no net effect on the phase of stationary spins but creates a phase difference between stationary and moving spins while satisfying the criteria for a fully balanced imaging sequence.

[0019] The present method has not been considered previously because sufficiently strong bipolar diffusion gradients result in imaging sequences with repetition times (TRs) on the order of 10 ms and more. When combined with a fully balanced steady-state free precession readout, this typically results in closely spaced dark band artifacts that would prohibit clinical use of the technique.

[0020] For (single-echo) balanced SSFP, the spacing of dark bands depends solely on the repetition time (TR) and off-resonance effects caused by field inhomogeneities. Dark bands in the FID and echo signals in a fully balanced DW-DESS also depend on the moment of the diffusion gradient, which produces additional off-resonance effects in the gradient direction. The present invention recognizes that at high gradient moments, the orientation and spacing of dark bands are strongly dominated by the diffusion gradient, with the actual off-resonance effects playing only a minor role. This is exploited in a preferred embodiment of the present invention by selecting the zero-order moment of the individual diffusion-weighted gradient waveforms so that the spatial distance of the dark band artifacts is smaller than the voxel size in the reconstructed diffusion-weighted MR image. The zero-order moment is equal to the area under the gradient waveform as a function of time. With a sufficiently high zero-order gradient moment of the diffusion gradient waveform, the spatial distance of the dark band artifact can be reduced to values ​​smaller than the imaging voxel size. Dark bands within a voxel may partially reduce the overall signal intensity but are not visible as artifacts. Therefore, by combining all coherent paths of the overall FID and echo signals acquired for the DESS, strong diffusion weighting can be introduced with high SNR efficiency.

[0021] According to the present invention, diffusion weighting applied to the FID and echo signals is used to reconstruct a diffusion-weighted MR image. In a preferred embodiment, the reconstruction of the MR image involves the derivation of a diffusion coefficient. The acquisition of the FID and echo signals is preferably repeated two or more times, wherein different diffusion gradient waveforms (in different spatial directions and / or with different gradient moments) are applied in different repetitions. The image can be a fractional anisotropy (FA) map, a mean diffusivity (MD) map, a radial diffusivity (RD) map, an axial diffusivity (AD) map, or any other scalar metric map derived from diffusion weighting, as commonly used in clinical research. For example, the reconstruction of a diffusion-weighted MR image can simply involve calculating the ratio of a first MR image reconstructed from the FID signal and a second MR image reconstructed from the echo signal. In addition, the reconstruction of a diffusion-weighted MR image can involve deriving an apparent diffusion coefficient (ADC) map from the acquired FID and echo signals, as proposed by Bieri et al. (see the references cited above).

[0022] In a preferred embodiment of the invention, the gradient moment is maximized by applying diffusion gradients in all spatial directions simultaneously. In this way, the capabilities of the gradient system of the MR system used can be optimally utilized.

[0023] In another preferred embodiment of the present invention, the FID signal and the echo signal are acquired with opposite readout gradients. In this embodiment, the gradient is switched between opposite directions of the readout direction during the entire interval between two consecutive RF pulses.

[0024] The method of the present invention described so far can be performed with the aid of an MR device comprising: at least one main magnet coil for generating a uniform static magnetic field within an examination volume; a plurality of gradient coils for generating switched magnetic field gradients in different spatial directions within the examination volume; at least one RF coil for generating RF pulses within the examination volume and / or for receiving MR signals from an object positioned within the examination volume; a control unit for controlling the temporal sequence of RF pulses and the switched magnetic field gradients; and a reconstruction unit. The method of the present invention can be implemented, for example, by corresponding programming of the reconstruction unit and / or the control unit of the MR device.

[0025] The method of the present invention can be advantageously implemented in most MR devices currently used clinically. To do this, it is only necessary to use a computer program that controls the MR device so that it performs the method of the present invention explained above. The computer program can be stored on a data carrier or on a data network, enabling it to be downloaded and installed in the control unit of the MR device.

[0026] According to another aspect of the invention is an MR device comprising: at least one main magnet coil (2) for generating a uniform static magnetic field in an examination volume; several gradient coils (4, 5, 6) for generating switched magnetic field gradients in different spatial directions in the examination volume; at least one RF coil (9) for generating RF pulses in the examination volume and / or for receiving MR signals of an object (10) positioned in the examination volume; a control unit (15) for controlling the time sequence of RF pulses and the switched magnetic field gradients; and a reconstruction unit (17), wherein the MR device (1) is arranged to perform the following steps:

[0027] The target (10) is subjected to a dual-echo steady-state imaging sequence, generating a free induction decay signal (FID) and an echo signal (ECHO) in each interval between two consecutive RF pulses, wherein a pair of diffusion gradient waveforms (G) with equal phase integrals and opposite polarities are applied in the interval between the FID signal and the echo signal. DIF ).

[0028] Acquiring FID signals and echo signals in a plurality of repeated imaging sequences with varying phase encoding; and

[0029] Diffusion-weighted MR images are reconstructed based on the acquired FID signals and echo signals. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The accompanying drawings disclose preferred embodiments of the present invention. However, it is to be understood that the drawings are designed for purposes of illustration and description only and are not intended to define the limits of the present disclosure. In the drawings:

[0031] Figure 1 An MR device for carrying out the method of the present invention is shown;

[0032] Figure 2 shows a diagram of an imaging sequence used in an embodiment of the present invention;

[0033] Figure 3 DW-DESS brain images acquired with different diffusion gradient moments are shown;

[0034] Figure 4 Schematic diagram of the reconstruction of a DW-DESS brain image according to the present invention;

[0035] Figure 5 DW-DESS brain images acquired using bipolar and unipolar diffusion gradients are shown. DETAILED DESCRIPTION

[0036] refer to Figure 1 , shows an MR device 1. The device comprises superconducting or resistive main magnet coils 2, such that a substantially uniform, spatially constant main magnetic field is created along a z-axis through an examination volume.

[0037] Magnetic resonance generation and manipulation systems apply a series of RF pulses and switched magnetic field gradients to invert or excite nuclear magnetic spins, induce magnetic resonance, refocus magnetic resonance, manipulate magnetic resonance, spatially or otherwise encode magnetic resonance, saturate spins, etc., to perform MR imaging.

[0038] More specifically, gradient amplifiers 3 apply 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 deliver RF pulses to the examination volume. A typical MR imaging sequence includes a packet of short-duration RF pulse segments that, along with any applied magnetic field gradients, achieves selected manipulation of nuclear magnetic resonance. The RF pulses are used to saturate, excite resonance, invert magnetization, refocus resonance, or manipulate resonance and select portions of a body 10 positioned in the examination volume. MR signals are also picked up by the whole-body volume RF coil 9.

[0039] To generate MR images of a limited region of the body 10, a set of local array RF coils 11, 12, 13 are placed adjacent the region selected for imaging. The array coils 11, 12, 13 can be used to receive MR signals induced by body coil RF transmissions.

[0040] The resulting MR signals are picked up by the whole body volume RF coil 9 and / or by the array RF coils 11, 12, 13 and demodulated by a receiver 14 preferably including a preamplifier (not shown). The receiver 14 is connected to the RF coils 9, 11, 12 and 13 via a transmit / receive switch 8.

[0041] A host computer 15 controls the gradient pulse amplifier 3 and transmitter 7 to generate any of a number of MR imaging sequences, such as a dual-echo steady-state (DW-DESS) imaging sequence. For the selected sequence, the receiver 14 receives single or multiple MR data lines in rapid succession following each RF excitation pulse. A data acquisition system 16 performs analog-to-digital conversion of the received signals and converts each MR data line into a digital format suitable for further processing. In modern MR devices, the data acquisition system 16 is a standalone computer dedicated to acquiring raw image data.

[0042] 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). The MR image can represent a planar slice through the patient, an array of parallel planar slices, a three-dimensional volume, etc. The image is then stored in an image memory where it can be accessed for conversion of the slice, projection, or other portion of the image representation into a suitable format for visualization, for example, via a video monitor 18 that provides a human-readable display of the resulting MR image.

[0043] Continue to refer Figure 1 And further reference Figure 2-5 , an embodiment of the method of the present invention is explained below.

[0044] like Figure 2 As shown in FIG, a body 10 is subjected to a plurality of repetitions of a diffusion weighted steady-state imaging sequence. The sequence is a modified fully balanced DESS sequence in which the two acquisitions of free induction decay and echo signals (indicated by ADC) FID and ECHO are respectively composed of a pair of diffusion gradient waveforms G with equal phase integration and opposite polarity. DIF Separate, i.e., in the depicted embodiment, bipolar trapezoidal diffusion gradients of equal magnitude. FID and ECHO acquisition use opposite readout gradients G X Both readouts are combined with pre-phasing and rephasing gradients to obtain a fully balanced readout. Y and G Z The phase encoding gradients in the direction are also completely balanced. For the maximum diffusion weight, all gradient directions G x , G y , G z Also used to generate bipolar diffusion gradients.

[0045] As described in detail above, if the diffusion gradient moment used is insufficient, dark band artifacts appear in the FID and echo signals in a fully balanced DW-DESS. The orientation and spacing of the dark bands are mainly determined by the diffusion gradient, especially at high gradient moments. Figure 3 a shows a DW-DESS brain image obtained with insufficient gradient moment. Typical dark band artifacts can be seen. Figure 3 In b, according to the present invention, the diffusion gradient is chosen to be so strong that the spatial distance of the dark band is smaller than the voxel size. The dark band portion within each voxel reduces the overall signal intensity but is no longer visible as an artifact.

[0046] Applications of the present invention exist in all areas of diffusion-weighted MR imaging, in the brain, but also particularly in the body, where magnetic field inhomogeneities can lead to large geometric distortions or even signal cancellation in conventional DWI based on EPI acquisition. In particular, for whole-body applications, the present invention can be combined with fat-saturated (or water-only) bSSFP / TrueFISP technology (see Scheffler K, Heid O, Hennig J, "Magnetization preparation during the steady state: fat-saturated 3D TrueFISP", Magn Reson Med, 2001, 45(6), 1075-80) to reduce high signal levels from tissue containing fat.

[0047] Figure 4 Another brain imaging example is shown (repetition time TR = 30 ms). Figure 4 The two FID and ECHO signals in 4a and 4b show different degrees of diffusion weighting because the coherence paths of the FID and ECHO signals are affected differently by the diffusion gradient. Figure 4 In c, the FID / ECHO ratio was calculated for each voxel, highlighting the differences in diffusion weights.

[0048] The diffusion-weighted fully balanced DESS technique of the present invention is particularly robust to motion, such as in Figure 5 Visible in. Figure 5 Compared to the use of bipolar diffusion gradient ( Figure 5 a, 5b) and unipolar diffusion gradient ( Figure 5 c, 5d) DW-DESS imaging. Although the bipolar variant according to the present invention provides stable image quality for both FID and ECHO signals, the ECHO signal Figure 5 The approximately three-minute acquisition period in d is clearly corrupted by overall motion.

[0049] In addition, the use of a fully balanced dual-echo steady-state (DESS) sequence provides the combined acquisition of distortion-free diffusion-weighted images and tissue conductivity maps. Banding artifacts of the diffusion gradient are avoided using sufficiently high gradient moments so that the bands are contained within a single voxel. The stability of B1 transmit-receive phase measurements performed with the balanced DESS sequence allows the use of standard EPT (electrical property tomography) methods to derive second-order derivatives based on quantitative tissue conductivity. In phantom and volunteer experiments (head), the feasibility of simultaneous DWI and EPT has been demonstrated on a 3T MRI system.

[0050] Diffusivity and tissue conductivity are physiological parameters with various applications, e.g. in tumor characterization, and are commonly assessed by diffusion-weighted imaging (DWI) and electrical property tomography (EPT) in different sequences. DWI based on EPI sequences often suffers from geometric distortions (magnetic field inhomogeneities). Diffusion-weighted dual-echo steady-state (DWDESS) MRI using monopolar gradients offers a distortion-free alternative but is inherently motion sensitive and does not utilize the steady-state signal because the gradients are unbalanced. In this study, a balanced DW-DESS sequence was developed using bipolar DW gradients while avoiding dark band artifacts. EPT is based on pure B1-related transmit and receive phases as in spin-echo (SE) based sequences as well as in balanced steady-state sequences. (Not affected by B0). The use of DW-DESS from equilibrium was studied. As a basis for EPT, this would synergistically allow the assessment of two related physiological parameters from a single MR acquisition.

[0051] A fully balanced DW-DESS sequence was used in combination with a bipolar DW gradient.

[0052] For (single-echo) balanced SSFP, the dark band spacing depends only on TR and off-resonance (frequency spacing 1 / TR) because the echoes are fully refocused at TE = TR / 2. The dark bands of S+ and S- in fully balanced DESS also depend on the moment of the diffusion-weighted gradient lobes, which manifests as additional off-resonance effects in the gradient direction. At high gradient moments, the direction and spacing of the dark bands are dominated by gradient effects and are less affected by the actual off-resonance. In this study, sufficiently high gradient moments of bipolar gradient lobes were applied so that the spatial distance of the dark band artifacts was reduced to values ​​smaller than the imaging voxel size. The dark band portion within the voxel reduces the overall signal intensity but is not visible as an artifact. Therefore, by combining all coherent paths of the overall FID (echo1, S+) and ECHO (echo2, S-) acquired for DESS, strong diffusion weighting can be induced with high SNR efficiency.

[0053] From the acquired DESS signal, the conductivity σ is calculated by combining the following formula (wherein, vacuum permeability μ and Larmor frequency ω) with a bilateral denoising filter:

[0054]

[0055] The phantom consisted of polyvinylpyrrolidone (P), gelatin (G), NaCl (S), and H2O (W) with different diffusion and conductance values ​​in 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 %; outer: D = 0.8 × 10 mm / s, σ = 0.42 S / m, P / G / S / W = 25 / 3 / 0.3 / 71.7 m %).

[0056] Combined DW-DESS and EPT acquisition was tested on a 3T MRI system (Achieva TX, Philips, NL) and a volunteer head was examined (male, 50 years old) with written consent. The following imaging parameters were used: 3D balanced dual-echo SSFP, 8-channel head coil, TR / TE / TE = 31 / 1.8 / 26 ms (phantom: 53 / 1.85 / 50.8 ms), FOV 224 × 224 × 120 mm, pixel size 1.8 × 1.8 mm, reconstruction 224 × 224, 24 slices (5 mm in vivo, 1.8 mm in phantom), pixel bandwidth 1.3 kHz, bipolar or monopolar diffusion gradients in 3 simultaneous directions, duration 2 × 11 ms (phantom: 2 × 22 ms), slope 0.4 ms, intensity 18 mT / m), two signal averages (phantom: 6), and total scan time 2 min 55 s (phantom: 7 min). Diffusion-weighted images were calculated as the ratio S+ / S-.

[0057] The phantom results confirmed that diffusion weighted imaging can be compared with that obtained using standard DWI sequences (EPI, 8 b values ​​0…1400, Figure 2 d) ADC maps obtained for comparison. Conductivity maps obtained from S+ and measured σ values, inside / outside = (0.77 ± 0.02) / (0.31 ± 0.06) S / m, corresponding to the phantom preparation.

[0058] Balanced DW-DESS acquisitions can be successfully achieved using large bipolar DW gradients, avoiding banding artifacts and showing low motion sensitivity. Although the SNR is reduced due to the dark band content within the voxel, the image quality is significantly improved compared to monopolar gradients. One disadvantage of bipolar DW-DESS is its limitation on the achievable b-values. In this initial demonstration, the diffusion-weighted images also included considerable T2 weighting because of the long second echo time (26 or 51 milliseconds). 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-receive phase of DW-DESS can be used for EPT, producing conductivity maps of comparable quality to those previously obtained in the brain. Due to the lower overall SNR in S images (DW and long TE), EPT reconstructions are preferably calculated from the first echo S+.

[0059] DW-DESS is able to simultaneously produce distortion-free diffusion-weighted images and conductivity maps and is therefore expected to be a valuable sequence, particularly for tumor characterization.

Claims

1. A method for MR imaging of an object (10) placed in an examination volume of an MR device (1), the method comprising the following steps: A target (10) is subjected to a dual-echo steady-state imaging sequence, generating a free induction decay signal (FID) and an echo signal (ECHO) in each interval between two consecutive RF pulses, wherein a pair of diffusion gradient waveforms (G) with equal phase integrals and opposite polarities are applied in the interval between the free induction decay signal and the echo signal. DIF ); acquiring the free induction decay signal and the echo signal in a plurality of repetitions of the imaging sequence with varying phase encoding; and Diffusion-weighted MR images are reconstructed based on the acquired free induction decay signals and echo signals. wherein the dual-echo steady-state imaging sequence is fully balanced, and wherein the zeroth-order moment of the diffusion gradient is selected such that the spatial distance of the dark band artifact is smaller than the voxel size in the reconstructed diffusion-weighted MR image, and The free induction decay signal and the echo signal are collected using opposite readout gradients.

2. The method according to claim 1, wherein The diffusion gradient is applied simultaneously in all spatial directions.

3. The method according to any one of claims 1 to 2, wherein: The free induction decay signal and the echo signal are acquired using opposite readout gradients.

4. The method according to any one of claims 1 to 2, wherein: The reconstructing of 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.

5. The method according to any one of claims 1 to 2, wherein: The reconstruction of the diffusion-weighted MR image includes deriving a map of apparent diffusion coefficients from the acquired free induction decay and echo signals.

6. The method according to any one of claims 1 to 2, wherein: The reconstruction of the MR image includes deriving a diffusion coefficient.

7. The method according to any one of claims 1 to 2, wherein: The acquisition of the free induction decay signal and the echo signal is repeated two or more times, wherein different diffusion gradient waveforms are applied in different repetitions.

8. The method according to any one of claims 1-2, further comprising the step of reconstructing a conductivity image based on phase information obtained from the acquired free induction decay and / or echo signals.

9. An MR imaging device comprising: at least one main magnet coil (2) for generating a uniform static magnetic field within an examination volume; Several gradient coils (4, 5, 6) for generating switched magnetic field gradients in different spatial directions within the examination volume, wherein the MR imaging device (1) is arranged to perform the following steps: A target (10) is subjected to a dual-echo steady-state imaging sequence, generating a free induction decay signal (FID) and an echo signal (ECHO) in each interval between two consecutive RF pulses, wherein a pair of diffusion gradient waveforms (G) with equal phase integrals and opposite polarities are applied in the interval between the free induction decay signal and the echo signal. DIF ); acquiring the free induction decay signal and the echo signal in a plurality of repetitions of the imaging sequence with varying phase encoding; and Diffusion-weighted MR images are reconstructed based on the acquired free induction decay signals and echo signals. wherein the dual-echo steady-state imaging sequence is fully balanced, and wherein the MR imaging apparatus (1) is arranged to operate all gradient coils (4, 5, 6) simultaneously during application of the diffusion gradient such that the zero-order moment of the diffusion gradient waveform is sufficient to reduce the spatial distance of dark band artifacts to below the voxel size of a reconstructed diffusion-weighted MR image, and The MR imaging device is arranged to acquire the free induction decay signal and the echo signal with opposite readout gradients.

10. A computer program product comprising a computer program to be run on an MR device, the computer program comprising instructions for: Generate a dual-echo steady-state imaging sequence where A pair of diffusion gradient waveforms (G) with equal phase integrals and opposite polarities are applied in the interval between the free induction decay signal (FID) and the echo signal (ECHO). DIF ); acquiring the free induction decay signal and the echo signal in a plurality of repetitions of an imaging sequence with varying phase encoding; and Diffusion-weighted MR images are reconstructed based on the acquired free induction decay signals and echo signals. wherein the dual-echo steady-state imaging sequence is fully balanced, and wherein the zeroth-order moment of the diffusion gradient is selected such that the spatial distance of the dark band artifact is smaller than the voxel size in the reconstructed diffusion-weighted MR image, and The free induction decay signal and the echo signal are collected using opposite readout gradients.

Images