Propeller MR imaging with relaxation weightings varying between k-space blades and k-space data lines
A method of MR imaging of a body ( 10 ) of a patient reduces contrast blurring in PROPELLER imaging combined with multi-echo acquisitions. The method includes the steps of: generating MR signals by subjecting at least a portion of the body ( 10 ) to a MR imaging sequence including a number of RF pulses and switched magnetic field gradients; acquiring the MR signals as a plurality of k-space blades ( 21 - 26 ) in temporal succession according to a PROPELLER scheme, each k-space blade ( 21 - 26 ) including a number of substantially parallel k-space lines, wherein the k-space blades ( 21 - 26 ) are rotated about the center of k-space, so that a total acquired data set of MR signals spans at least part of a circle in k-space, wherein a common central circular region of k-space is covered by all k-space blades ( 21 - 26 ), wherein a relaxation weighting of the MR signals varies between different k-space blades ( 21 - 26 ); estimating the relaxation weighting of the MR signals; compensating the acquired MR signals according to the estimated relaxation weighting; and reconstructing a MR image from the compensated MR signals.
1. A method of magnetic resonance (MR) imaging of a body of a patient placed in an examination volume of a MR device, the method comprising:
generating MR signals by subjecting at least a portion of the body to a MR imaging sequence comprising a number of RF pulses and switched magnetic field gradients;
acquiring the MR signals as a plurality of k-space blades in temporal succession according to a PROPELLER scheme, each k-space blade comprising a number of substantially parallel k-space lines, wherein the k-space blades are rotated about the center of k-space, so that the total acquired data set of MR signals spans at least part of a circle in k-space, wherein a common central circular region of k-space is covered by all k-space blades, wherein a relaxation weighting of the MR signals varies between different k-space blades and k-space lines of a particular relaxation weighting are distributed irregulary in k-space;
estimating the relaxation weighting of the MR signals;
compensating the acquired MR signals according to the estimated relaxation weighting; and
reconstructing a MR image from the compensated MR signals.
2. The method of claim 1 , wherein the individual k-space lines are T 1 -weighted and/or the T 2 -weighted which weighting varies between the different k-space blades.
3. The method of claim 1 , wherein a T 1 -map and/or a T 2 -map and/or a B 0 map and/or a water map and/or a fat map are derived from the acquired MR-signals, wherein the acquired MR signals are compensated for variations according to the derived T 1 -map, T 2 -map, B 0 -map, water map and/or fat map.
4. The method of claim 1 , wherein the relaxation weighting of the MR signals is varied by variation of echo times of the imaging sequence and/or by varying a time delay after a preparation pre-pulse.
5. The method of claim 1 , wherein the imaging sequence is a multi-echo imaging sequence, including a turbo spin echo (TSE) sequence or a turbo field echo (TFE) or an echo planar imaging (EPI) sequence for generating a train of echo signals by each shot, wherein each k-space line of each k-space blade represents an echo signal.
6. The method of claim 5 , wherein the relaxation weighting of the MR signals is varied by variation of an acquisition order of the k-space lines between the k-space blades.
7. The method of claim 5 , wherein the relaxation weighting of the MR signals is varied by variation of echo shifting of the echo signals between the k-space blades.
8. The method of claim 1 , wherein a relaxation weighted and/or a spectrally weighting MR image is reconstructed from the acquired MR signals for each individual relaxation weighting.
9. The method of claim 8 , wherein a relaxation time map is derived from the reconstructed relaxation weighted MR images.
10. The method of claim 8 , wherein the relaxation weighted MR images are reconstructed using compressed sensing or a parallel image reconstruction algorithm.
11. The method of claim 1 wherein a T 1 -weighted MR image, a T 2 -weighted MR image, a proton density-weighted MR image, a water MR image and/or a fat MR image are reconstructed from the compensated MR signals.
12. The method of claim 1 , wherein the MR image has a combined T 1 -weighted, T 2 -weighted, proton density-weighted, water and/or fat contrast synthesized from the compensated MR signals.
13. The method of claim 1 , wherein the MR imaging sequence includes a flip angle sweep of the RF pulses, wherein the flip angle attributed to each acquired MR signal is considered in the estimation of the relaxation weighting.
14. A magnetic resonance (MR) device comprising:
at least one main magnet coil for generating a uniform, steady magnetic field B 0 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 a body of a patient positioned in the examination volume;
one or more computer processors configured for controlling the plurality of gradient coils and the at least one RF coil and for performing the following steps:
generating MR signals by subjecting at least a portion of the body to a MR imaging sequence including a plurality of RF pulses and switched magnetic field gradients;
acquiring the MR signals as a plurality of k-space blades in temporal succession according to a PROPELLER scheme, each k-space blade including a plurality of substantially parallel k-space lines, wherein the k-space blades are rotated about the center of k-space, so that a total acquired data set of MR signals spans at least part of a circle in k-space, wherein a common central circular region of k-space is covered by all k-space blades, wherein a relaxation weighting of the MR signals varies between different k-space blades and k-space lines of a particular relaxation weighting are distributed irregularly in k-space;
estimating the relaxation weighting of the MR signals;
compensating the acquired MR signals according to the estimated relaxation weighting; and
reconstructing a MR image from the compensated MR signals.
15. A non-transitory computer-readable medium configured to control one or more processors to control a MR device to:
generate MR signals by performing a MR imaging sequence comprising a number of RF pulses and switched magnetic field gradients;
acquire the MR signals as a plurality of k-space blades in temporal succession according to a PROPELLER scheme, each k-space blade comprising a number of substantially parallel k-space lines, wherein the k-space blades are rotated about the center of k-space, so that the total acquired data set of MR signals spans at least part of a circle in k-space, wherein a common central circular region of k-space is covered by all k-space blades, wherein a relaxation weighting of the MR signals varies between different k-space blades and k-space lines of a particular relaxation weighting are distributed irregularly in k-space;
estimate the relaxation weighting of the MR signals;
compensate the acquired MR signals according to the estimated relaxation weighting; and
reconstruct a MR image from the compensated MR signals.