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Free Breathing Radial Magnetic Resonance Elastography1Diagnostic Imaging, St. Jude Children's Research Hospital, Memphis, TN, United States, 2Siemens Healthcare, Erlangen, Germany
Synopsis
Liver magnetic resonance elastography (MRE) to this point has clinically relied on using breath holds to produce reliable artifact free images. Here we present initial work adapting recent advances in motion compensated abdominal imaging for use in MRE. Specifically, we take advantage of a golden angle radial sampling scheme combined with a self-navigation approach for motion correction to perform free breathing MRE of the liver. Resulting images show enhanced detail compared to the standard breath hold technique while producing comparable image stiffness values.
Introduction
Clinical liver magnetic resonance elastography (MRE) uses breath holds to avoid imaging artifacts caused by respiratory motion. However, not all patients, particularly children, are able to perform breath holds reliably. Forgoing breath holds can still produce a measure of liver stiffness, but it comes at the cost of decreased accuracy and repeatability1. Many methods have been developed that enable free breathing abdominal imaging2-9, but they have not yet been applied to liver MRE. In this work we present initial results showing that a radial acquisition with a golden-angle radial sparse parallel7 (GRASP) like motion correction can be applied to free breathing liver MRE.Methods
A radial GRE pulse sequence was modified to perform golden angle radial MRE. Flow compensated motion encoding gradients (MEGs) were added to the slice direction and sequence timings were adjusted to acquire 4 motion offsets and both positive and negative MEG polarities. This golden angle radial MRE sequence was carried out in free breathing in 2 volunteers and 2 patients on a 1.5T clinical scanner. Sequence parameters included 402 radial lines for a 128 matrix size, a TR of 50 ms with 60 Hz vibration frequency, and a total scan time of 160 sec for one slice. For comparison, a breath hold Cartesian GRE MRE was acquired for the same slice using a 50 ms TR with a vibration frequency of 60 Hz, 4 phase offsets, and positive and negative MEG polarities. All MRE processing was done on the scanner by the manufacturer’s processing pipeline.
While radial acquisitions are inherently more robust than Cartesian scans due to oversampling at the center of k-space, an additional post processing step was used for the radial MRE data to further improve image quality. Motion compensation was improved by using a GRASP like procedure. A self-navigator signal for respiratory motion was extracted by using the central k-space point from each radial readout. To ensure that the navigator primarily contains information on respiration, the coil with the highest signal in the respiratory range of 0.1 to 0.5 Hz was selected to represent all the coils, ignoring other types of motion such as cardiac. The data corresponding to inspiration was then discarded, leaving only k-space data related to expiration, which was then passed to the reconstruction routine for image calculation and MRE inversion.
Results
An example of an extracted respiratory signal is shown in figure 1. This signal was used to distinguish between expiration and inspiration breathing states. Images from the breath hold Cartesian GRE, free breathing radial GRE, and free breathing radial GRE motion correction for a healthy breath hold compliant volunteer are shown in figure 2. Resulting images show enhanced detail compared to the standard breath hold technique while producing comparable image stiffness values. Table 1 gives a summary of the obtained stiffness values and confidence region sizes.Discussion
The use of a radial trajectory produced high quality MRE images of the liver without using breath holds. This, by itself, is a meaningful improvement for children, sick patients, and sedated patients. Visual inspection of the wave pattern shows subtle qualitative improvement in the radial images over the conventional Cartesain GRE. The free breathing radial approach and breath hold GRE produce comparable values for confidence regions and stiffness values, except for patient 2 who was not able to hold her breath consistently. In this patient, an increase of 30% in confidence was observed by using radial MRE. Further, the motion correction technique enabled visualization of several regions of the liver on the magnitude images that were not visible on the radial images without motion correction (figure 2). This data suggests that using a radial trajectory is a viable method for performing free breathing MRE of the liver. Future work with this approach is needed to show reliability across a range of clinical stiffness values and to create statistical measures of the improvement by using radial MRE.Conclusion
This work will improve clinical workflow and patient compliance by providing an alternative MRE examination that does not necessitate patients controlling their breathing. MRE will therefore be more accessible to patient populations that are unable to hold their breath or are sedated for their examination.Acknowledgements
No acknowledgement found.References
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