FIELD OF THE INVENTION

The invention relates to the field of magnetic resonance (MR) imaging. It concerns a method of MR imaging of an object. The invention also relates to a MR apparatus and to a computer program to be run on a MR apparatus.

BACKGROUND OF THE INVENTION

Image-forming MR methods which utilize the interaction between magnetic fields and nuclear spins in order to form two-dimensional or three-dimensional images are widely used nowadays, notably in the field of medical diagnostics, because for imaging of soft tissue they are superior to other imaging methods in many respects, do not require ionizing radiation and are usually not invasive.

According to the MR method in general, the object, for example the body of the patient to be examined, is arranged in a strong, uniform magnetic field whose direction at the same time defines an axis (normally the z-axis) of the co-ordinate system on which the measurement is based. The magnetic field produces different energy levels for the individual nuclear spins in dependence on the magnetic field strength which can be excited (spin resonance) by application of an electromagnetic alternating field (RF field) of defined frequency (so-called Larmor frequency, or MR frequency). From a macroscopic point of view the distribution of the individual nuclear spins produces an overall magnetization which can be deflected out of the state of equilibrium by application of an electromagnetic pulse of appropriate frequency (RF pulse), so that the magnetization performs a precessional motion about the z-axis. The precessional motion describes a surface of a cone whose angle of aperture is referred to as flip angle. The magnitude of the flip angle is dependent on the strength and the 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°).

After termination of the RF pulse, the magnetization relaxes back to the original state of equilibrium, in which the magnetization in the z direction is built up again with a first time constant Ti (spin lattice or longitudinal relaxation time), and the magnetization in the direction perpendicular to the z direction relaxes with a second time constant T ₂ (spin-spin or transverse relaxation time). The variation of the magnetization can be detected by means of receiving RF coils which are arranged and oriented within an examination volume of the MR apparatus in such a manner that the variation of the magnetization is measured in the direction perpendicular to the z-axis. The decay of the transverse magnetization is accompanied, after application of, for example, a 90° pulse, by a transition of the nuclear spins (induced by local magnetic field inhomogeneities) from an ordered state with the same phase to a state in which all phase angles are uniformly distributed (dephasing). The dephasing can be compensated by means of a refocusing RF pulse (for example a 180° pulse). This produces an echo signal (spin echo) in the receiving coils.

To realize spatial resolution in the body, constant magnetic field gradients extending along the three main axes are superposed on the uniform magnetic field, leading to a linear spatial dependency of the spin resonance frequency. The signal picked up in the RF receiving coils then contains components of different frequencies which can be associated with different locations in the body. The signal data obtained via the RF receiving coils corresponds to the spatial frequency domain and is called k-space data. The k-space data usually includes multiple lines acquired with different phase encoding. Each line is digitized by collecting a number of samples. A set of k-space data is converted to a MR image by means of an image reconstruction algorithm.

MR imaging is sensitive to diffusion. Known diffusion weighted imaging (DWI) techniques are commonly performed by using imaging sequences comprising diffusion gradients, wherein the diffusion of protons (of water molecules) along the direction of the diffusion gradient reduces the amplitude of the acquired MR signals. Diffusion tensor imaging (DTI) is a more sophisticated form of DWI, which allows for the determination of both the magnitude and the directionality of diffusion. For example, DTI enables to visualize white matter fibers in MR brain imaging and can map subtle changes in the white matter associated with diseases like brain infarction, multiple sclerosis, epilepsy etc.

Brain DWI/DTI techniques are particularly vulnerable to macroscopic head motion, as the signal attenuation resulting from the motion can confound the measurement of interest. Subject motion during an MR examination can be particularly problematic in populations like children, the elderly, or patients with medical conditions that prevent them from lying still, such as Parkinson's disease. Motion affects the data by shifts of the brain tissue to be imaged resulting in ghosting artifacts in the reconstructed MR images. To avoid significant artifacts resulting from motion, DWI data have commonly been acquired using single-shot imaging sequences, such as single-shot echo-planar imaging (EPI). However, the image quality can be low and the spatial resolution is limited in single- shot DWI. The significant geometric distortions and limited spatial resolution make it difficult to measure diffusion properties at high precision.

Recent efforts have been made to address the limitations of single-shot DWI.

For example, US 2014/0002078 Al describes a multi-shot DWI technique (termed multiplexed sensitivity encoding - MUSE) which uses parallel acquisition and inherently corrects shot-to-shot phase variations due to motion and thus avoids ghosting artifacts.

Jeong et al. (Magnetic Resonance in Medicine, volume 69 (3), pages 793-802, 2013) propose a multi-shot DWI technique using a modification of the standard SENSE algorithm commonly used for fast parallel image acquisition. The modification accounts for shot-to-shot motion-induced phase errors. This known technique is termed image

reconstruction using image-space sampling functions (IRIS).

In the IRIS technique, navigator echo signals are acquired to generate a motion- induced phase map. The imaging echo signals and the navigator echo signals are acquired with different bandwidths along the phase encoding direction due to different k- space sampling intervals. This difference in the acquisition bandwidths generates unequal susceptibility- induced geometric distortions between the MR images reconstructed from the imaging echo signals and the phase maps derived from the navigator echo signals. Because the motion- induced phase terms and the true image intensities are to be multiplied voxel by voxel, a relatively large spatial mismatch can result in inaccurate reconstruction of diffusion weighted MR images. To apply the phase information derived from the navigator echo image data at the correct positions in the imaging echo data, the navigator echo data needs to be transformed to the image space of the imaging echo data. This transformation depends on the main magnetic field distribution and thus requires Bo mapping. Hence, an additional measuring step for acquiring the Bo map is obligatory. This takes additional scan time.

Moreover, the Bo mapping is a source of error and can have a negative impact on image quality. Furthermore, the MR image reconstruction according to the IRIS technique uses a particular column by column scheme, which is time consuming.

US2004/039276 Al describes a magnetic imaging apparatus that enables parallel imaging even when a navigator echo is used to phase-correct an imaging echo. SUMMARY OF THE INVENTION

From the foregoing it is readily appreciated that there is a need for an improved DWI technique. It is consequently an object of the invention to enable DWI (and DTI) with efficient image acquisition and reconstruction.

In accordance with the invention, a method of MR imaging of an object placed in an examination volume of a MR apparatus is disclosed. The method comprises the steps of:

subjecting the object to a number N of shots of a multi-echo imaging sequence, a train of imaging echo signals being generated by each shot, wherein the multi- echo imaging sequence comprises diffusion gradients;

acquiring the imaging echo signals in parallel via a number M of RF receiving coils, wherein each RF receiving coil has a spatial sensitivity map;

subjecting the object to a navigator sequence, a train of navigator echo signals being generated by the navigator sequence;

acquiring the navigator echo signals;

computing N sets of M modified spatial sensitivity maps by incorporating phase information derived from the navigator echo signals into the spatial sensitivity maps of the RF receiving coils, wherein one set of modified spatial sensitivity maps is attributed to each shot of the multi-echo imaging sequence; and

reconstructing a motion-compensated diffusion weighted MR image from the imaging echo signals using a parallel reconstruction algorithm, wherein the acquired imaging echo signals are combined based on a spatial encoding according to the computed N sets of M modified spatial sensitivity maps.

The invention uses parallel imaging. Common parallel MR imaging methods are SENSE (Pruessmann et al, "SENSE: Sensitivity Encoding for Fast MRI", Magnetic Resonance in Medicine 1999, 42 (5), 1952-1962) and SMASH (Sodickson et al,

"Simultaneous acquisition of spatial harmonics (SMASH): Fast imaging with radio frequency coil arrays", Magnetic Resonance in Medicine 1997, 38, 591- 603). SENSE and SMASH use sub-sampled k-space data acquisition obtained from multiple RF receiving coils in parallel. In these methods, the (complex) signal data from the multiple coils are combined with complex weightings in such a way as to suppress sub-sampling artefacts (aliasing) in the finally reconstructed MR images. This type of complex array combining is sometimes referred to as spatial filtering, and includes combining which is performed in the k-space domain (as in SMASH) or in the image domain (as in SENSE), as well as methods which are hybrids. In either SENSE or SMASH, it is essential to know the proper weightings or spatial sensitivities of the RF receiving coils with sufficient accuracy. To obtain the coil sensitivities, i.e. the spatial sensitivity maps of the receiving coils used for signal detection, a calibration pre-scan is typically performed prior to and/or after the actual image acquisition. In the pre- scan, the MR signals are usually acquired at a resolution which is significantly lower than the resolution required for the diagnostic MR image. The low-resolution pre-scan typically consists of an interleaving of signal acquisition via the array of RF receiving coils and via a volume RF coil, for example the quadrature body coil of the MR apparatus. Low resolution MR images are reconstructed from the MR signals received via the array RF receiving coils and via the volume RF coil. Sensitivity maps indicating the spatial sensitivity profiles of the receiving coils can then computed, for example, by division of the RF receiving coil images by the volume RF coil image.

Parallel imaging is used according to the invention for acquisition of (at least) the imaging echo signals using the multi-echo imaging sequence. The acquisition of the navigator echo signals is interleaved with the acquisition of the imaging echo signals. Parallel imaging may be used for the acquisition of the navigator echo signals as well.

The invention uses the parallel imaging reconstruction algorithm (such as, e.g., SENSE or SMASH reconstruction) as an efficient tool for the correction of motion- induced phase errors in multi-shot diffusion weighted imaging. The imaging echo data acquired in each shot of the multi-echo imaging sequence is a subset of full k-space modulated by a motion- induced phase variation. The parallel image reconstruction algorithm is used to produce a full (unfolded) diffusion weighted MR image from the imaging echo data. The phase variations between the shots are compensated by incorporating phase information derived from the navigator echo signals into the modified spatial sensitivity maps (e.g., by multiplication of the spatial sensitivity maps and the corresponding phase maps derived from the navigator data). One set of modified spatial sensitivity maps is attributed to each shot of the multi-echo imaging sequence such that, for M RF receiving coils used in parallel in N shots of the imaging sequence, N ^x M modified spatial sensitivity maps (N sets, each comprising M modified spatial sensitivity maps) are made available to the parallel reconstruction algorithm for computing the diffusion weighted MR image and simultaneously correcting the motion-induced phase errors.

In other words, the diffusion weighted MR image is reconstructed using a parallel reconstruction algorithm treating the imaging echo signal acquisitions of the N shots as parallel acquisitions using N sets of M RF receiving coils, each set of RF receiving coils being determined by the set of modified spatial sensitivity maps attributed to the respective shot.

In a preferred embodiment of the invention, the method further comprises the step of adapting the parameters of the navigator sequence, such as, e.g., the temporal spacing of the navigator echo signals and/or the readout length of the navigator acquisition, so as to reduce unequal geometric distortions between the navigator echo signals and the imaging echo signals. According to the invention, the navigator echo signals are acquired to generate a motion- induced phase map. Typically, SENSE acceleration is applied to the navigator acquisition so that a phase map can be reconstructed for each shot of the multi-echo imaging sequence. The imaging echo signals will typically be acquired with SENSE acceleration and multi-shot sub-sampling. Hence, the imaging echo signals and the navigator echo signals are acquired with different acquisition bandwidths along the phase-encoding direction due to different k-space sampling intervals. This difference in the acquisition bandwidths generates unequal susceptibility- induced geometric distortions between the imaging echo data and the navigator echo data. This impairs the direct use of the phase information probed by the navigator echo signals in the parallel reconstruction. A 'mini '-navigator acquisition with adaptively calculated (SENSE) acceleration factor may be used according to the invention to address this issue. The readout length of the navigator echo acquisition may be reduced and/or the acquisition bandwidth (in the phase-encoding direction) may be adaptively increased to reduce the inconsistencies between the imaging echo data and the navigator echo data. Ideally, the same acquisition bandwidths of the imaging sequence and the navigator sequence in the phase encoding direction are used. In practice, the timing of the navigator sequence should be adaptively optimized because, e.g., the gradient ascending and descending ramp times of the used phase encoding gradients influence the acquisition bandwidth. In contrast to the above-mentioned IRIS technique, no Bo mapping is required according to the invention to apply the phase information derived from the navigator echo data to the imaging echo data for motion compensation.

In a preferred embodiment of the invention, the imaging echo signals are acquired using signal averaging for improving the signal-to-noise ratio (SNR). This enables to compute one set of modified spatial sensitivity maps also for each averaging step which is then used for image reconstruction in the above-described fashion. Each averaging step can correspond to a shot of the multi-echo imaging sequence wherein the same k-space sampling is applied in each averaging step. In other words, from the perspective of the reconstruction algorithm, each shot and each averaging step adds a set of 'virtual' RF receiving coils to the parallel acquisition, wherein each set of virtual RF receiving coils corresponds to the real 'physical' receiving coils, but having modified spatial sensitivities according to the motion- induced phase information derived from the navigator echoes.

In a further preferred embodiment of the invention, the imaging sequence is a multi-shot EPI sequence, which allows a very efficient data acquisition in combination with parallel imaging. One 'shot' of the multi-echo imaging sequence according to the invention comprises an initial RF pulse for excitation of magnetic resonance followed by at least one (typically 180°) refocusing RF pulse, wherein diffusion gradients are applied prior to and after the refocusing RF pulse. This sequence of pulses generates a diffusion-encoded spin echo which is measured as a train of differently phase- and frequency-encoded gradient- recalled echo signals. These echo signals are acquired, wherein each echo signal represents a k-space profile. One single shot or a plurality of shots of the multi-echo sequence may be applied for completely sampling k-space in order to be able to reconstruct a full MR image from the acquired signal data. A multi-shot multi-echo imaging sequence is applied according to the invention to achieve high image quality and to measure diffusion properties at high precision.

The method of the invention described thus far can be carried out by means of a MR apparatus including at least one main magnet coil for generating a uniform static magnetic field within an examination volume, a number 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 in the examination volume, a control unit for controlling the temporal succession of RF pulses and switched magnetic field gradients, and a reconstruction unit. The method of the invention can be implemented, for example, by a corresponding programming of the reconstruction unit and/or the control unit of the MR apparatus.

The method of the invention can be advantageously carried out in most MR devices in clinical use at present. To this end it is merely necessary to utilize a computer program by which the MR apparatus is controlled such that it performs the above-explained method steps of the invention. The computer program may be present either on a data carrier or be present in a data network so as to be downloaded for installation in the control unit of the MR apparatus. BRIEF DESCRIPTION OF THE DRAWINGS

The enclosed drawings disclose preferred embodiments of the present invention. It should be understood, however, that the drawings are designed for the purpose of illustration only and not as a definition of the limits of the invention. In the drawings:

Figure 1 shows a MR apparatus for carrying out the method of the invention;

Figure 2 shows a diagram of an imaging sequence used in an embodiment of the invention;

Figure 3 shows diffusion weighted MR images acquired and reconstructed conventionally and according to the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

With reference to Figure 1 , a MR apparatus 1 is shown. The apparatus comprises superconducting or resistive main magnet coils 2 such that a substantially uniform, temporally constant main magnetic field is created along a z-axis through an examination volume.

A magnetic resonance generation and manipulation system applies 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 and otherwise encode the magnetic resonance, saturate spins, 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 x, y and z-axes of the

examination volume. A digital RF frequency transmitter 7 transmits RF pulses or pulse packets, via a send/receive switch 8, to a whole-body volume RF coil 9 to transmit RF pulses into the examination volume. A typical MR imaging sequence is composed of a packet of RF pulse segments of short duration which taken together with each other and any applied magnetic field gradients achieve a selected manipulation of nuclear magnetic resonance. The RF pulses are used to saturate, excite resonance, invert magnetization, refocus resonance, or manipulate resonance and select a portion of a body 10 positioned in the examination volume. The MR signals are also picked up by the whole-body volume RF coil 9.

For generation of MR images of limited regions of the body 10, a set of local array RF coils 11, 12, 13 are placed contiguous to the region selected for imaging. The array coils 11, 12, 13 can be used to receive MR signals induced by body-coil RF transmissions. The resultant MR signals are picked up by the whole body volume RF coil 9 and/or by the array RF coils 1 1 , 12, 13 and demodulated by a receiver 14 preferably including a preamplifier (not shown). The receiver 14 is connected to the RF coils 9, 1 1 , 12 and 13 via send/receive switch 8.

A host computer 15 controls the gradient pulse amplifier 3 and the transmitter

7 to generate any of a plurality of MR imaging sequences, such as diffusion weighted echo planar imaging or the like. For the selected sequence, the receiver 14 receives a single or a plurality of 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 to a digital format suitable for further processing. In modern MR apparatuses the data acquisition system 16 is a separate computer which is specialized in acquisition of raw image data.

Ultimately, the digital raw image data is reconstructed into an image representation by a reconstruction processor 17 which applies a Fourier transform or other appropriate reconstruction algorithms, such as SENSE or SMASH. The MR image may represent a planar slice through the patient, an array of parallel planar slices, a three- dimensional volume, or the like. The image is then stored in an image memory where it may be accessed for converting slices, projections, or other portions of the image representation into appropriate format for visualization, for example via a video monitor 18 which provides a man-readable display of the resultant MR image.

With continuing reference to Figure 1 and with further reference to Figures 2 and 3 embodiments of the method of the invention are explained in the following.

The body 10 is subjected to multiple shots of a multi-echo imaging sequence as illustrated in Figure 2. The imaging sequence is a spin echo diffusion weighted multi-shot EPI sequence. The second 180° RF refocusing pulse is followed by a navigator echo acquisition NAV. The imaging echo signal acquisition IMG used for the actual diffusion imaging as well as the navigator echo acquisition NAV are accelerated by SENSE. The imaging echo signals and the navigator echo signals are acquired in parallel via M RF receiving coils 1 1 , 12, 13 (M = 3 in the embodiment) having different spatial sensitivities. The dotted lines in Figure 2 represent the diffusion gradients applied before and after the first 180° RF refocusing pulse. One single shot of the imaging sequence is depicted in Figure 2. A plurality of shots of the multi-echo sequence is applied for completely sampling k-space in order to be able to reconstruct a full MR image from the acquired imaging echo signal data using SENSE reconstruction. The imaging echo signals are acquired using signal averaging for improving the SNR. A total number of N shots of the depicted sequence is performed for acquisition of the full imaging echo signal data required for reconstruction of a diffusion weighted MR image with the desired SNR.

Motion of the object is detected using the navigator echoes. The acquisition of the navigator echo signals NAV is interleaved with the acquisition of the imaging echo signals IMG in each shot of the sequence. The invention uses the SENSE reconstruction algorithm for the correction of motion- induced phase errors. Motion- induced phase variations between the shots are compensated by incorporating phase information derived from the navigator echo signals into modified spatial sensitivity maps of the used RF receiving coils 11, 12 ,13. The SENSE reconstruction then treats the imaging echo signal acquisitions of the N shots as parallel acquisitions using N sets of RP receiving coils, each set of RF receiving coils being determined by the set of modified spatial sensitivity maps attributed to the respective shot. Hence, from the perspective of the reconstruction algorithm, each shot and each averaging step adds a set of 'virtual' RF receiving coils to the parallel acquisition, wherein each set of virtual RF receiving coils corresponds to the real 'physical' receiving coils with modified spatial sensitivities according to the motion-induced phase information derived from the navigator echoes. Preferably, the phase maps derived from the navigator echo signals are spatially smoothed before further processing for computing the modified spatial sensitivity maps.

According to the invention, an accelerated navigator echo acquisition is used to reduce unequal geometric distortions between the imaging echo data and the navigator echo data. Ideally, the same acquisition bandwidths of the imaging sequence and the navigator sequence in the phase encoding direction should be used. The timing and/or the readout length of the navigator sequence is adaptively optimized to enable direct application of the phase information derived from the navigator echo data to the imaging echo data for motion compensation.

Figures 3 a and 3b show diffusion weighted MR images of the human brain acquired and reconstructed using conventional techniques. In Figure 3a, a single-shot diffusion weighted EPI sequence has been applied. This technique allows very time-efficient data acquisition. However, the obtainable spatial resolution is limited by the acquisition window in a single shot. The single-shot technique is also very sensitive to main magnetic field inhomogeneities. In Figure 3b, a multi-shot diffusion weighted EPI sequence has been used. Acquiring k-space in multiple excitations reduces the echo train length, so the multi- shot technique provides an MR image of high spatial resolution and less blurring. A problem of multi-shot diffusion weighted EPI is that the image is typically corrupted by ghosting artifacts due to the motion- induced phase variations from shot to shot. Even the effect of a tiny motion is amplified by the diffusion gradients. In Figure 3b, ghosting artifacts due to the phase variations between different shots have been made invisible by averaging the signal magnitudes from multiple shots. However, the use of multiple signal averages adds considerably to the acquisition time. Moreover, this approach suffers from reduced SNR due to spatially varying noise amplification characterized by the g-factor of SENSE imaging. Figure 3 c shows the diffusion weighted MR image reconstructed using the technique of the invention. SENSE was used to reconstruct phase maps from the navigator echo signals and to estimate the shot-to-shot phase variations induced by motion. The reconstructed phase maps should be spatially smoothed. Thereafter, the spatially smoothed phase values and known spatial sensitivities of the used RF receiving coils were used to calculate 'virtual' (modified) spatial sensitivity maps. Finally, the SENSE framework was used to reconstruct the diffusion weighted MR image from all 'virtual' parallel acquisitions. The superior image quality is immediately recognizable in Figure 3c. In all images (Figures 3a-3c), the SENSE reduction factor equals 2, the number of shots is 3, the number of signal averages is 12, and the b value for diffusion weighting is 1000 s/mm ² .