3D MR Imaging with Intrinsic Motion Detection

Abstract

The invention relates to a method of MR imaging of an object (10) placed in an examination volume of an MR apparatus (1). It is an object of the invention to enable fast 3D MR imaging that provides motion-compensation and also allows a precise compensation for system imperfections. The method of the invention comprises the steps of: —subjecting the object (10) to a number of shots (S1-S4) of a 3D imaging sequence, wherein a train of MR signals is generated by each shot (S1-S4), each MR signal representing a k-space profile, wherein the set of k-space profiles of each shot (S1-S4) comprises at least one navigator profile and a number of imaging profiles; —acquiring the MR signals; —deriving motion information from the at least one navigator profile; and —reconstructing an MR image from the imaging profiles, wherein a motion-compensation is applied based on the motion information. Motion-induced phase errors can be derived from the navigator profiles, wherein the motion-compensation involves a corresponding phase-correction. Further, phase errors caused by magnetic field gradient imperfections and/or eddy currents can be derived from the navigator profiles and a corresponding phase-correction can be applied during image reconstruction. Moreover, the invention relates to an MR apparatus (1) for carrying out this method as well as to a computer program to be run on an MR apparatus (1).

Claims

1. A method of magnetic resonance (MR) imaging of an object placed in an examination volume of an MR apparatus, the method comprising: subjecting the object to a number of shots of a 3D imaging sequence, wherein a train of MR signals is generated by each shot, each MR signal representing a k-space profile, wherein the set of k-space profiles of each shot comprises at least one linear k-space navigator profile employed as projections in respective directions and a number of imaging profiles; acquiring the MR signals; deriving motion information from the linear k-space navigator profiles; and arranging for a reconstruction of an MR image from the imaging profiles, wherein a motion-compensation is applied based on the motion information.

2. The method of claim 1, wherein the set of k-space profiles of each shot comprises at least three linear k-space navigator profiles oriented in different directions.

3. The method of claim 1, wherein the motion information is derived from the at least one linear k-space navigator profile as motion-induced phase errors, wherein the motion-compensation involves a phase-correction according to the motion-induced phase errors.

4. The method of claim 1, wherein the set of k-space profiles of each shot comprises at least one pair of linear k-space navigator profiles, wherein the linear k-space navigator profiles of each pair are oriented in opposite spatial directions.

5. The method of claim 4, wherein the reconstruction of the MR image involves deriving phase errors caused by magnetic field gradient imperfections and/or eddy currents from the at least one pair of linear k-space navigator profiles and a corresponding phase-correction.

6. The method of claim 1, wherein a three-dimensional volume of k-space is sampled by the sets of k-space profiles associated with the number of shots of the imaging sequence.

7. The method of claim 1, wherein the derivation of motion information from the linear k-space navigator profiles involves the detection of motion-induced displacements and/or deformations of the object during the acquisition of the MR signals and assigning each of the sets of k-space profiles to a motion state.

8. The method of claim 7, wherein each of the motion states corresponds to one of a plurality of contiguous ranges of motion-induced displacements and/or deformations of the object.

9. The method of claim 7, wherein the frequency of the occurrence of each motion state is determined to apply a weighting to the k-space profiles in the step of reconstructing the MR image, with a stronger weighting being applied to k-space profiles associated with more frequent motion states and a weaker weighting being applied to k-space profiles associated with less frequent motion states.

10. The method of claim 1, wherein the derivation of motion information from the navigator profiles involves the determination of the position of the center of mass of the object from projection images reconstructed from the linear k-space navigator profiles.

11. The method of claim 1, wherein the motion information is supplemented by motion data acquired from the object via one or more external motion sensors.

12. The method of claim 1, wherein the MR signals are acquired using a stack-of-stars or stack-of-spirals acquisition scheme.

13. The method of claim 12, wherein the rotation angle of the radial or spiral k-space profiles is incremented according to a golden angle scheme during the acquisition of the MR signals.

14. A magnetic resonance (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, wherein the MR apparatus is configured to perform the following steps: subjecting the object to a number of shots of a 3D imaging sequence, wherein a train of MR signals is generated by each shot, each MR signal representing a k-space profile, wherein the set of k-space profiles of each shot comprises at least one linear k-space navigator profile employed as projections in respective directions and a number of imaging profiles; acquiring the MR signals; deriving motion information from the at least one linear k-space navigator profile; and arranging for a reconstruction of an MR image from the imaging profiles, wherein a motion-compensation is applied based on the motion information.

15. A computer program to be run on an MR apparatus, which computer program comprises instructions stored on a non-transitory computer readable medium such that when executed by a processor of the MR apparatus causes: executing a number of shots of a 3D imaging sequence, wherein a train of MR signals is generated by each shot, each MR signal representing a k-space profile, wherein the set of k-space profiles of each shot comprises at least one linear k-space navigator profile employed as projections in respective directions and a number of imaging profiles; acquiring the MR signals; deriving motion information from the at least one linear k-space navigator profile; and arranging for a reconstruction of an MR image from the imaging profiles, wherein a motion-compensation is applied based on the motion information.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] 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:

[0029] FIG. 1 shows an MR apparatus for carrying out the method of the invention;

[0030] FIG. 2 shows a diagram k-space illustrating the sampling scheme of the invention;

[0031] FIG. 3 shows a diagram illustrating the soft gating scheme adopted according to the invention;

[0032] FIG. 4 shows a slice of 3D MR image (left: conventional motion-compensation using a respiratory belt, right: motion-compensation using the intrinsic navigator technique of the invention).

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0033] With reference to FIG. 1, an 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] The resultant 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 send/receive switch 8.

[0038] A host computer 15 controls the gradient pulse amplifier 3 and the transmitter 7 to generate any of a plurality of MR imaging sequences. 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.

[0039] 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.

[0040] With continuing reference to FIG. 1 and with further reference to FIGS. 2, 3, and 4 embodiments of the method of the invention are explained in the following.

[0041] The body 10 is subjected to multiple shots of a 3D imaging sequence. The k-space sampling in the first four shots S1-S4 is illustrated in FIG. 2. The imaging sequence may be, e.g., a 3D TFE (turbo field echo) sequence. A train of echo signals is generated in each shot S1-S4, each echo train comprising six navigator profiles (depicted as solid arrows) of which pairs are oriented in opposite spatial directions along the three orthogonal coordinate axes k.sub.x, k.sub.y, k.sub.z, followed by a number of imaging profiles (depicted as dashed arrows). The imaging profiles are acquired according to a stack-of-stars scheme as radial k-space profiles from a number (five in the embodiment of FIG. 2) of parallel planes. The planes are arranged at different positions along the k.sub.z-direction. Cartesian phase-encoding is performed in the k.sub.z-direction, while the radial orientation of the k-space profiles is rotated around the center (k.sub.x=k.sub.y=0) from shot to shot. This results in a cylindrical k-space coverage composed of stacked discs. For the angular ordering of the radial imaging profiles, a golden angle-scheme is employed. Successive acquisition of the phase-encoding steps along the k.sub.z-direction is performed before sampling imaging profiles at different golden angular positions which is essential to ensure high data consistency and general motion-robustness.

[0042] According to the invention, motion of the examined body 10 is detected by deriving motion information from the navigator profiles. To this end, the center of mass of the object is determined from projection images reconstructed from the navigator profiles. The projection images are reconstructed by Fourier transformation of the navigator profiles to obtain projections of the imaging volume onto the direction of the respective navigator profile. The center of mass of these projections is then determined as the average position weighted by signal intensity. The changes in this intensity-weighted position, i.e. the changes of the position of the center of mass of the object, reflects motion of the object, such as, e.g. breathing motion or cardiac motion occurring during the MR signal acquisition. Since these motions have different frequencies (e.g. 0.1-0.5 Hz for respiratory motion and 0.6-3 Hz for cardiac motion) they can be separated by filtering according to the respective frequency bands.

[0043] On the basis of the detected breathing motion, each of the acquired shots S1-S4 can be assigned to a motion (breathing) state. Each of the motion states M1-M4 is defined to correspond to one of a plurality of contiguous ranges of the breathing motion-induced displacement D (see FIG. 3). On this basis, a 3D soft gating approach is implemented by weighting the imaging profiles in the reconstruction of an MR image from the acquired imaging profiles. A stronger weighting is applied to imaging profiles acquired in more frequent motion states M1-M4, while a weaker weighting is applied to imaging profiles in less frequent motion states M1-M4. Imaging profiles acquired with the body 10 assuming its most frequently taken positions during breathing are given a stronger weight while MR signals acquired in rarely assumed positions are suppressed in the reconstructed MR image. The frequency of the occurrence of each motion state M1-M4 as a basis for the weighting can be derived from a histogram. The histogram can be set up during or after MR signal acquisition. It reflects the number of shots per motion state M1-M4. The weighting factors can be derived from the histogram, wherein a user-specified gating percentage is taken into account. The gating percentage defines the proportion of the imaging profiles suppressed by the weighting as a global parameter that can be tuned by the user according to the needs (wherein image noise and artefact level are balanced). When determining the weighting factors, the compliance with the Nyquist criterion must be taken into account in order to avoid aliasing artefacts. In radial k-space sampling schemes, a broader range of weighting factors can be applied to the k-space profiles in the central portion of k-space as compared to the peripheral portions of k-space. The less pronounced weighting in peripheral k-space has the effect that streaking artefacts from k-space sub-sampling can be avoided.

[0044] A challenge associated with radial k-space sampling is the imperfect alignment of each sampled k-space profile with the center of k-space (k.sub.x=k.sub.y=0). Eddy currents, gradient imperfections and timing delay errors cause the sampled k-space profiles to shift from the intended trajectory. This shift changes as the readout direction is rotated. Methods exist to align miscentered radial k-space profiles by comparing radial k-space profiles that have (nearly) opposite readout directions. The opposite readout direction causes the shift in k-space to be in opposite directions. Hence, the k-space shift can be determined from the phase differences between acquisitions having opposite readout directions and the acquired MR signal data can be corrected accordingly. The acquisition of anti-parallel pairs of navigator profiles as shown in FIG. 2 thus enables the derivation of the phase differences caused by magnetic field gradient imperfections and/or eddy currents and a corresponding phase-correction in all three directions. Because of the temporally close acquisition of the navigator profiles at the beginning of each shot S1-S4, the phase-correction is not affected by motion. The navigator profiles of each pair are acquired during (essentially) the same motion state of the body 10 such that inconsistencies in the phase-correction leading to image artefacts are effectively avoided.

[0045] The result of the intrinsic navigation/phase correction approach of the invention is an MR image reconstructed from a 3D radial acquisition having a low level of artefacts in the presence of motion of the body 10. This can be seen in FIG. 4 showing a slice MR image acquired from the abdominal region using a 3D radial acquisition method. The left image has been acquired and reconstructed conventionally using motion-detection by a respiratory belt and corresponding motion-compensation, while the right image has been reconstructed using the soft gating and phase correction approach of the invention.