METHOD AND DEVICE FOR RAPIDLY ACQUIRING AND RECONSTRUCTING A SEQUENCE OF MAGNETIC RESONANCE IMAGES COVERING A VOLUME

Abstract

A method for creating, in particular acquiring and reconstructing, a sequence of magnetic resonance (MR) images of an object (1), said sequence of MR images representing a series of cross-sectional slices (2) of the object (1), comprises (a) providing a series of sets of image raw data including an image content of the MR images to be reconstructed, said image raw data being collected with at least one radiofrequency receiver coil of a magnetic resonance imaging (MRI) device, wherein each set of image raw data includes a plurality of data samples being generated in an imaging plane with a gradient-echo sequence that spatially encodes an MRI signal received with the at least one radiofrequency receiver coil using a non-Cartesian k-space trajectory, each set of image raw data comprises a set of homogeneously distributed lines in k-space with equivalent spatial frequency content, the lines of each set of image raw data cross the center of k-space and cover a continuous range of spatial frequencies, the positions of the lines of each set of image raw data differ in successive sets of image raw data, and the number of lines of each set of image raw data is selected such that each set of image raw data is undersampled below a sampling rate limit defined by the Nyquist-Shannon sampling theorem, and (b) subjecting the sets of image raw data to a regularized nonlinear inverse reconstruction process to provide the sequence of MR images, wherein each of the MR images is created by a simultaneous estimation of a sensitivity of the at least one receiver coil and the image content and in dependency on a difference between a current estimation of the sensitivity of the at least one receiver coil and the image content and a preceding estimation of the sensitivity of the at least one receiver coil and the image content, wherein said cross-sectional slices (2) of the object (1) are contiguous cross-sectional slices (2) with a predetermined slice thickness, each set of said image raw data represents one of said contiguous cross-sectional slices (2), and the position of each cross-sectional slice is shifted by a slice shift A perpendicular to the imaging plane in order to cover a volume of the object (1).

Claims

1. A method for creating a sequence of magnetic resonance images of an object under investigation, said sequence of magnetic resonance images representing a series of cross-sectional slices of the object, comprising the steps of: (a) providing a series of sets of image raw data including an image content of the magnetic resonance images to be reconstructed, said image raw data being collected using at least one radiofrequency receiver coil of a magnetic resonance imaging device, wherein each set of the image raw data includes a plurality of data samples being generated in an imaging plane with a gradient-echo sequence that spatially encodes magnetic resonance imaging signal received with the at least one radiofrequency receiver coil using a non-Cartesian k-space trajectory, each set of the image raw data comprises a set of homogeneously distributed lines in k-space with equivalent spatial frequency content, the lines of each set of the image raw data cross a center of k-space and cover a continuous range of spatial frequencies, positions of the lines of each set of the image raw data differ in successive sets of image raw data, and a number of lines of each set of image raw data is selected such that each set of the image raw data is undersampled below a sampling rate limit defined by the Nyquist—Shannon sampling theorem, and (b) subjecting the sets of the image raw data to a regularized nonlinear inverse reconstruction process to provide the sequence of magnetic resonance images, wherein each of the magnetic resonance images is created by a simultaneous estimation of a sensitivity of the at least one receiver coil and the image content and in dependency on a difference between a current estimation of the sensitivity of the at least one receiver coil and the image content and a preceding estimation of the sensitivity of the at least one receiver coil and the image content, wherein said cross-sectional slices of the object are contiguous cross-sectional slices, with a predetermined slice thickness, each set of said image raw data represents one of said contiguous cross-sectional slices, and the position of each cross-sectional slice is shifted by a slice shift in a direction perpendicular to the imaging plane in order to cover a volume of the object under investigation.

2. The method according to claim 1, wherein the method comprises a further step of (c) combining the magnetic resonance images for creating a three-dimensional image of the object.

3. The method according to claim 1, wherein the reconstruction process includes a filtering process suppressing image artefacts.

4. The method according to claim 3, wherein the filtering process includes at least one of a median filter for a number of successive frames, and a spatial filter for each frame.

5. The method according to claim 4, wherein the filtering process includes said spatial filter for each frame, and said spatial filter is a non-local means filter.

6. The method according to claim 1, wherein the slice shift of successive slices in the perpendicular direction is equal to the slice thickness of the cross-sectional slices.

7. The method according to claim 1, wherein the slice shift of successive slices in the perpendicular direction is selected in a range from 10% to 80% of the slice thickness of the cross-sectional slices.

8. The method according to claim 1, wherein the gradient-echo sequence comprises a single-echo FLASH sequence, a multi-echo FLASH sequence, a FLASH sequence with refocusing read gradients, a FLASH sequence with reversely refocusing read gradients, or a FLASH sequence with fully balanced read and slice gradients.

9. The method according to claim 1, wherein the number of lines of each set of the image raw data is selected such that a resulting degree of undersampling is at least a factor of 5.

10. The method according to claim 1, wherein the number of lines of each set of the image raw data is at most 30.

11. The method according to claim 1 wherein a duration of collecting each set of the image raw data is at most 100 ms.

12. The method according to claim 1, wherein the lines of each set of the image raw data are selected such that the lines of successive sets of die image raw data are rotated relative to each other by a predetermined angular displacement.

13. The method according to claim 1, wherein the collection of each set of the image raw data or a selectable number of sets of the image raw data is interleaved with a radiofrequency and gradient module for spatial pre-saturation, or a radiofrequency and gradient module for frequency-selective saturation.

14. The method according to claim 1, wherein steps (a) and (b) are repeated for monitoring dynamic changes of the object.

15. The method according to claim 1, wherein the sets of the image raw data are provided by at least one of arranging the object in the magnetic resonance imaging device including the at least one receiver coil, subjecting the object to the gradient-echo sequence, and collecting the series of sets of the image raw data using the at least one receiver coil, and receiving the sets of the image raw data by a data transmission collected from a distant magnetic resonance imaging device.

16. A magnetic resonance imaging device being configured for creating a sequence of magnetic resonance images of an object under investigation, comprising a magnetic resonance imaging scanner including a main magnetic field device, at least one radiofrequency excitation coil, three magnetic field gradient coils and at least one radiofrequency receiver coil, and a control device being configured for controlling the magnetic resonance imaging scanner for collecting the series of sets of image raw data and reconstructing the sequence of magnetic resonance images with the method according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Further details and advantages of the invention are described with reference to the attached drawings, which show in

[0041] FIG. 1: a schematic illustration of a preferred embodiment of the MR image reconstruction method according to the invention;

[0042] FIG. 2: a schematic illustration of a preferred embodiment of an MRI device according to the invention;

[0043] FIG. 3: examples of T1-weighted MR images of the human abdomen with different slice shifts;

[0044] FIG. 4: examples of T2/T1-weighted MR images of the human brain with different slice shifts;

[0045] FIG. 5: examples of T2/T1-weighted MR images of the human brain selected from a volume coverage scan in 5.0 seconds;

[0046] FIG. 6: examples of T1-weighted MR images and a 3D reconstruction of the human carotid arteries selected from a volume coverage scan in 6.4 seconds;

[0047] FIG. 7: examples of T1-weighted MR images with interleaved fat saturation of the human liver selected from a volume coverage scan in 6.0 seconds; and

[0048] FIG. 8: examples of T2/T1-weighted MR images with interleaved fat saturation of the human prostate selected from a volume coverage scan in 6.0 seconds.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0049] Preferred embodiments of the invention are described in the following with particular reference to the data flow of the inventive reconstruction process, the basic components of an inventive MRI device and practical application examples.

[0050] The details of the design of gradient-echo sequences, k-space trajectories, raw data acquisition and the mathematical formulation and implementation of the regularized nonlinear inverse reconstruction are provided as disclosed in [6]. In particular, the regularized nonlinear inverse reconstruction process is implemented as disclosed in [6] for the reconstruction of time series of MR images of an object under investigation. Thus, [6] is incorporated to the present specification by reference in its entirety, in particular with regard to all details of data acquisition and image reconstruction of the sequence of cross-sectional gradient-echo MR images of the object under investigation. All procedural steps applied to time series of sets of raw data and sequences of MR images in [6] can be applied in the same manner to series of sets of raw data and sequences of MR images representing contiguous cross-sectional slices.

[0051] Further details of the MRI device, the construction of gradient echo sequences and their adapta-tion to a particular object to be imaged, the numerical implementation of the mathematical formulation using available software tools and optional further image processing steps are not described as far as they are known from conventional MRI techniques. Furthermore, exemplary reference is made in the following to parallel MR imaging wherein the image raw data comprise MRI signals received with a plurality of radiofrequency receiver coils. It is emphasized that the application of the invention is not restricted to parallel MR imaging, but rather possible even with the use of one single receiver coil.

[0052] Reconstruction Process and MRI Device

[0053] FIG. 1 summarizes a complete data flow of the inventive reconstruction process, as described in [6], comprising a first step S1 of collecting measured data, a second step S2 of preprocessing the measured data, and a third step S3 of iteratively reconstructing a sequence of MR images. FIG. 2 schematically shows an MRI device 100 with an MRI scanner 10 including a main magnetic field device 11, at least one radiofrequency excitation coil 12, three magnetic field gradient coils 13 and radiofrequency receiver coils 14. The object 1 to be investigated is accommodated in the MRI device 100. Furthermore, the MRI device 100 includes a control device 20 being adapted for controlling the MRI scanner 10 for collecting the series of sets of image raw data and reconstructing the sequence of MR images with the method according to FIG. 1. The control device 20 includes at least one GPU 21, which is preferably used for implementing the regularized nonlinear inversion.

[0054] With step S1, a series of sets of image raw data including an image content of the MR images to be reconstructed is collected with the use of the radiofrequency receiver coils 14 of the MRI device 100. The object 1, e. g. a tissue or organ of a patient, is subjected to a slice-selective radiofrequency excitation pulse and a gradient-echo sequence encoding the MRI signal received with the radiofrequency receiver coils 14. The gradient-echo sequence is constructed such that data samples are collected along non-Cartesian k-space trajectories. The slice shift is accomplished by changing the radiofrequency excitation pulse.

[0055] Examples of gradient-echo sequence are disclosed in FIGS. 3A, 3B and 4B of [6]. Deviating from [6], each set of the image raw data represents another one of contiguous cross-sectional slices 2 as shown in the schematic insert of FIG. 2.

[0056] With step S2, the image raw data are subjected to an optional whitening and array compression step S21 and to an interpolation step S22, wherein an interpolation of the non-Cartesian data onto a Cartesian grid is conducted. Steps 21 and 22 are implemented as disclosed in [6].

[0057] Finally, with step S3, the sequence of MR images of the object 1 is reconstructed by the regularized nonlinear inverse reconstruction process, which is described in [6]. Starting from an initial guess S31 for the MR image of a first cross-sectional slice and the coil sensitivities, each of the MR images is created by an iterative simultaneous estimation S32 of sensitivities of the receiver coils and the image content. Step S32 comprises the nonlinear inverse reconstruction using an iteratively regularized Gauss-Newton method including a convolution-based conjugate gradient algo-rithm S33. The number of iterations (Newton steps) is selected in dependency on the image quality requirements of a particular imaging task. Finally, the reconstructed series of MR images is output (S35). Further steps of conventional processing, storing, displaying, or recording of image data can follow.

EXPERIMENTAL EXAMPLES

[0058] Experimental examples of the invention are described in the following with particular reference to applications in medical imaging. All examples refer to studies of healthy human subjects.

[0059] FIG. 3 shows T1-weighted images (50 ms acquisition time, 1.2×1.2 mm.sup.2 in-plane resolution, 4.0 mm slice thickness) of the abdomen at the level of the kidneys which were obtained in separate volume coverage scans with a single-echo FLASH sequence and increasing slice shifts of 25% (1.0 mm), 50% (2.0 mm), 75% (3.0 mm), and 100% (4.0 mm), respectively, of the cross-sectional slice thickness. The comparison demonstrates the range of usable slice shifts for T1-weighted images which goes up to 100% of the slice thickness (i.e., directly neighbouring slice positions). The images also demonstrate robustness against peristaltic or breathing movements (i.e., the absence of motion artefacts). Slight differences are due to the fact that all 4 image series were obtained during free breathing which naturally affects the position of abdominal organs such as liver, pancreas and small bowel.

[0060] Complementary to the aforementioned example, FIG. 4 shows T2/T1-weighted images of the brain (50 ms acquisition time, 1.0×1.0 mm in-plane resolution, 6.0 mm slice thickness) which were obtained in separate volume coverage scans with a FLASH sequence with refocused read gradients and increasing slice shifts of 10% (0.6 mm), 25% (1.5 mm) and 50% (3.0 mm), respectively. These images are compared to a reference image at the same position which was obtained as a single image with full radial sampling and conventional Fourier transform reconstruction. The example images reveal signal changes as a function of slice shift, which are most prominent for long-T2 components such as cerebrospinal fluid in the brain ventricles (bright signal). The effect is due to the fact that the establishment of T2/T1-like contrasts requires the proton spins to experi-ence a sufficiently large number of radiofrequency excitations. This is more easily accomplished for small slice shifts which ensure a longer period of overlapping excitations.

[0061] FIG. 5 depicts selected (every 15th) T2/T1-weighted images of a volume coverage scan of the brain obtained with a FLASH sequence with refocused read gradients in only 5.0 s (150 mm volume, 50.0 ms acquisition time, 1.0×1.0×6.0 mm.sup.3 resolution, slice shift 25%=1.5 mm, total number of images=100). The example demonstrates excellent image quality from (upper left) top of the brain to (lower right) bottom of the brain (e.g., negligible sensitivity to magnetic field inhomogeneity).

[0062] Another application of the invention is demonstrated in FIG. 6 which shows selected (every 20th) T1-weighted images of a volume coverage scan of the carotid arteries obtained with a sin-gle-echo FLASH sequence in only 6.4 s (128 mm volume, 40.0 ms acquisition time, 0.8×0.8×4.0 mm.sup.3 resolution, slice shift 20%=0.8 mm, total number of images=160). The lower right picture is a magnetic resonance angiogram of the carotid arteries (single side) obtained by a maximum intensity projection of the combined series of 160 cross-sectional images.

[0063] The robustness of the invention against movements is demonstrated in FIG. 7 which shows selected (every 20th) T1-weighted images of a volume coverage scan of the liver obtained with a single-echo FLASH sequence and interleaved fat suppression (each image) in only 6.0 s (180 mm volume, 50.0 ms acquisition time, 1.2×1.2×6.0 mm.sup.3 resolution, slice shift 25%=1.5 mm, total number of images=120). The scan runs from (upper left) the bottom of the beating heart to (lower right) the kidneys during free breathing. Neither cardiac pulsations nor respiratory and peristaltic movements cause any visible motion artefacts in individual images.

[0064] FIG. 8 depicts selected (every 15th) T2/T1-weighted images of a volume coverage scan of the prostate obtained with a FLASH sequence with refocused read gradients and interleaved fat suppression (every third image) in only 6.0 s (90 mm volume, 66.7 ms acquisition time, 1.0×1.0×4.0 mm.sup.3 resolution, slice shift 25%=1.0 mm, total number of images=90). The scan runs from (upper left) below the prostate to (lower right) the upper part of the bladder during free breathing. The example demonstrates insensitivity of the invention to motion and magnetic field inhomogeneity as well as the possibility to integrate and combine clinically important features such as T2/T1-contrast and fat suppression.

[0065] The application of the invention is not restricted to medical imaging, like in the above examples, but correspondingly possible for imaging other objects, like workpieces or other technical objects.

[0066] The features of the invention disclosed in the above description, the drawings and the claims can be of significance individually, in combination or sub-combination for the implementation of the invention in its different embodiments.