Radiation Oncology

MR Simulation

MR only simulation for radiation therapy – DRR generation

Y Yang*1, F Han2, M Cao1, K Sheng1, M Selch1, T Kaprealian1, N Agazaryan1, D Low1, P Hu2
1. Department of Radiation Oncology, UCLA School of Medicine, Los Angeles, CA
2. Department of Radiology, UCLA School of Medicine, Los Angeles, CA

Background

  • 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.
  • 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.

Objectives

  • To develop a technique which can potentially enable high quality MR bone imaging for radiation therapy based on MRI simulation.

Methods

  • Four meningioma patients were scanned at 1.5 Tesla using a 3D radial UTE (Ultrashort Echo Time) gradient echo sequence in addition to their standard clinical MRI and CT simulation.
  • The UTE sequence acquires two sets of images at two different echo times (0.07ms and 3.61ms). The two echoes are acquired asymmetrically within the same TR.
  • The two images were processed using software built in house, which performs pixel-by-pixel magnitude subtraction of the two images. Further post-processing includes mask based noise removal and scaling for better visualization.
  • The resultant bone images were subsequently loaded into commercial treatment planning system to generate DRRs.
  • For each patient, orthogonal kV/kV pair was simulated from patient CT which was well registered with UTE bone MRI.
  • Known shifts in three translational directions were applied to simulated kV/kV pair.
  • Auto matching between MRI-DRR and simulated kV/kV pair was done with commercial imaging registration system.