Room: AAPM ePoster Library
Purpose: In common practice for surface high-dose-rate (HDR) brachytherapy, CT is employed for treatment planning including catheter detection. Some clinical cases may benefit from the better soft tissue contrast provided by MRI for detecting depth of plaques and spread of disease, but applicators are not visible on standard MR protocols. We aimed to optimize MR sequences for detection of spheres and catheters in Freiburg flap (FF) skin brachytherapy applicators.
Methods: 3D Dixon (TR\TE 4.02\1.23 ms, FOV 200x200 mm², 1.6x1.6x1.4 mm³ voxels, bandwidth 1347 Hz/px) and 3D Pointwise Encoding Time Reduction with Radial Acquisition (PETRA) sequences (TR\TE\TI 3.32\0.07\100 ms, FOV 280*280 mm², 0.8x0.8x0.8 mm³ voxels, bandwidth 400 Hz/px) were optimized on phantoms and a volunteer bearing FF applicators. Images were acquired using an UltraFlex18 and a spine matrix array coil in a 3T Siemens Vida (acquisition time: 16s for DIXON, 6.3min for PETRA) and utilized for generation of treatment plans. Signal intensity ratios (IR) of spheres and catheters relative to the underlying forearm muscle were calculated for the human subject.
Results: FF silicon spheres appeared bright on both fat-only and water-only Dixon images but darker on PETRA (IR sphere/muscle: 0.44±0.51, 14.67±0.64, 1.19±0.39, respectively). Inserted catheters could be traced from their lack of signal on Dixon fat-only and water-only and from their higher signal on PETRA images (IR catheter/muscle: 0.19±0.55, 2.80±0.56, 1.35±0.19, respectively). 3D acquisition allowed for accurate reconstruction in all three planes and for treatment plan generation without the need to use any other imaging modality.
Conclusion: We demonstrated that the positions of spheres and catheters of Freiburg flap applicators for surface HDR brachytherapy can be determined using optimized Dixon and PETRA sequences, employing either negative or positive contrast, in a clinically acceptable acquisition time. This application paves the way for future MR-only treatment planning.