MR Only Simulation for Radiation Therapy _ DRR Generation
Y Yang*, F Han, M Cao, K Sheng, M Selch, T Kaprealian, N Agazaryan, D Low, P Hu, UCLA School of Medicine, Los Angeles, CASU-E-J-31 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
Purpose:MRI only radiation therapy has been increasingly gaining research interests. Conventionally, MRIs are fused with CT to take advantage of its superior soft tissue contrast. Such an approach has drawbacks such as additional uncertainty from fusion and extra cost. To achieve MR-based simulation for radiation therapy, bone imaging is an important challenge that needs to be overcome. The ultra-short T2 and T1 of cortical bone result in low signal intensity, which presents difficulty for both dose calculation and patient setup in terms of digitally reconstructed radiograph (DRR) generation. Current solutions will either require manual bone contouring or multiple MR scans. We present a technique to generate brain DRR using bone MRI with a single Ultra Short Echo Time (UTE) sequence.
Methods: Healthy volunteers were scanned at 1.5 Tesla using a radial UTE sequence. The sequence acquires two images at two different echo times (0.07ms and 3.6ms). The two images were processed using in-house software, which performs pixel-by-pixel magnitude subtraction of the two images. The difference image was subsequently divided pixel-by-pixel to the long TE (3.6ms) image. As the cortical bone has larger percentile difference between the UTE and long TE image compared with other tissues, the bone is bright in the processed image. The resultant bone images were subsequently loaded into commercial treatment planning system to generate DRRs.
Results: The majority of cranial, facial and vertebral bones are well visualized in both volunteers. The fused image of the long TE image with the processed bone image demonstrates the accuracy of automatic bone identification using our technique. The generated DRR is of acceptable quality.
Conclusion: This study shows the potential of DRR generation with single MR sequence. Further work will be needed on MR sequence development and post-processing procedure to achieve MR bone image with comparable quality to CT.