Interior Tomography Approach for MRI-guided Radiation Therapy Xun Jia1, Steve Jiang1, Gary Pickrell2, Ge Wang3 1. Department of Radiation Oncology, UTSW, USA 2. Department of Materials Science and Engineering, Virginia Tech, USA, 3. Department of Biomedical Engineering, RPI, USA Introduction Image guidance is critical in radiation therapy to ensure treatment accuracy. To enhance image guidance, great efforts are being made to combine MRI with a medical linear accelerator (LINAC). Existing approaches combine a regular MRI scanner with a radiotherapy machine [1-4], which are bulky and expensive. Here we propose to use an interior MRI (iMRI) approach [5] for a volume of interest (VOI) to be specifically imaged. The iMRI-LINAC system can be further enhanced with cone-beam CT to acquire a global image for radiation therapy applications such as adaptive treatment re-planning. Technical requirements of the iMRI may be relaxed in terms of hardware and compatibility with LINAC. It seems feasible to make a compact iMRI plugin retrofitable to an LINAC system. The iMRI component can be integrated into an existing LINAC platform with minimal adaption, as shown in Fig. 3. Also, we simulated data acquisition and iMRI reconstruction using a tight-frame-based iterative reconstruction model: where F is the Fourier transform operator, A is an under-sampling operator in the k-space, and g denotes data. The results are in Fig. 4. Method The iMRI setup is in Fig. 1, which relaxes a global magnetic field to a local one for system compactness. By optimizing the currents in our novel superconducting fibers, we achieved a homogeneous (<50 ppm) main magnetic field B0 in an VOI, as illustrated in Fig. 2. Figure 3. iMRI-LINAC integration. Figure 1. Concept of iMRI with major components displayed. Figure 4. Simulation results. (a) Ground truth and a circular VOI, (b1-3) FBP results with full, 1/2, and 1/3 undersampling data, and (c1-3) iterative reconstruction results. References [1] S. Mutic, et al., Seminars Rad Onc, 24, 196, (2014). [2] J. Lagendijk, et al., J Inter Med., 280, 203,( 2016). [3] G. Fallon, et al., Seminars Rad Onc, 24, 200, (2014). [4] P. Keall, et al., Seminars Rad Onc, 24, 203, (2014). [5] G. Wang, et. al., PMB, 58, R161 (2013). Figure 2. Main magnetic field B0 (T) shown in the x-y plane.