Abstract
Purpose:To determine the feasibility of identifying cortical bone on MR images with a short‐TE 3D fast‐GRE sequence for attenuation correction of PET data in PET/MR.Methods:A water‐fat‐bone phantom was constructed with two pieces of beef shank. MR scans were performed on a 3T MR scanner (GE Discovery™ MR750). A 3D GRE sequence was first employed to measure the level of residual signal in cortical bone (TE1/TE2/TE3=2.2/4.4/6.6ms, TR=20ms, flip angle=25°). For cortical bone segmentation, a 3D fast‐GRE sequence (TE/TR=0.7/1.9ms, acquisition voxel size=2.5×2.5×3mm3) was implemented along with a 3D Dixon sequence (TE1/TE2/TR=1.2/2.3/4.0ms, acquisition voxel size=1.25×1.25×3mm3) for water/fat imaging. Flip angle (10°), acquisition bandwidth (250kHz), FOV (480×480×144mm3) and reconstructed voxel size (0.94×0.94×1.5mm3) were kept the same for both sequences. Soft tissue and fat tissue were first segmented on the reconstructed water/fat image. A tissue mask was created by combining the segmented water/fat masks, which was then applied on the fast‐GRE image (MRFGRE). A second mask was created to remove the Gibbs artifacts present in regions in close vicinity to the phantom. MRFGRE data was smoothed with a 3D anisotropic diffusion filter for noise reduction, after which cortical bone and air was separated using a threshold determined from the histogram.Results:There is signal in the cortical bone region in the 3D GRE images, indicating the possibility of separating cortical bone and air based on signal intensity from short‐TE MR image. The acquisition time for the 3D fast‐GRE sequence was 17s, which can be reduced to less than 10s with parallel imaging. The attenuation image created from water‐fat‐bone segmentation is visually similar compared to reference CT.Conclusion:Cortical bone and air can be separated based on intensity in MR image with a short‐TE 3D fast‐GRE sequence. Further research is required to optimize the strategy to reduce Gibbs artifacts.
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