1. Introduction  Image Guided Adaptive Radiotherapy (IGAR) involves the use of an imaging modality to minimize planning treatment volumes required to deliver a specified dose.  A major problem in the radiotherapy process involves patient movement during treatment and day-to-day organ movement.  We propose the use of real time imaging during radiotherapy treatment combining an Magnetic Resonance Imaging (MRI) system with a linac. - However a linac is a source of Radio Frequency (RF) power; these RF signals can interfere with the operation of the MRI because the MRI uses RF signals in its transmitting and receiving coils. - Interference of the magnetic field from the MRI with the electron-beam steering process in the linac just before the target (location of radiation field production for treatment) may also be a concern. Objectives Combine a magnetic resonance imaging system with a linac to image during treatment. The short term goal of the project is to determine the necessary amount of shielding needed such that the operation of the two devices in close proximity will not affect one another deleteriously. Materials and Methods  We will solve Maxwell’s equations for the magnetic field of the system and the RF noise through the use of finite element analysis (see Figure 1).  The system to be modeled is broken into multiple linked simplified regions of the original object, i.e. finite elements. Physical conditions, such as boundary values, along with equations of equilibrium are applied to each element and a system of equations is constructed. The system of equations is then solved.  This finite element analysis technique will be used to simulate the magnetic field and RF interference between the linac and MR in the system. Results  Magnetic field simulations of the proposed magnet have been completed using the finite element simulation package, results are shown in Figure 2.  An antenna has been designed in order to investigate the effects of the RF noise on the MR system, see Figures 1 and 3.  Initial simulation work of the electric potential of the antenna is shown in Fig 4. Conclusions  Simulations will allow us to compare RF measurements (B. Burke et al.) with simulated results.  We will also determine the necessary shielding requirements such that magnetic field interference will not alter the operation of the linac. Future Work  Determine the antenna radiation pattern and input impedance as a function of signal frequency.  Compare measurements and finite element analysis simulations of the RF signal produced by the linac. Simulation and Background Noise Investigation of an MRI-linac Treatment Unit M. Lamey , , M. Carlone , , S. Steciw ,  and G. Fallone , ,   Department of Physics, University of Alberta, Edmonton, Alberta, Canada  Department of Medical Physics, Cross Cancer Institute, Edmonton, Alberta, Canada  Department of Oncology, Cross Cancer Institute, Edmonton, Alberta, Canada References 1. S. Lim et al. A Tunable Electrically Small Antenna for Ground Wave Transmission IEEE Trans. Ant. Prop. 54 (2006) 417-421. 2. C. Balanis Antenna Theory: Analysis and Design John Wiley and Sons, New York, 2005. 3. COMSOL multiphysics www.comsol.com Figure 2. Magnetic flux density of the proposed 0.2T permanent magnet.. Figure 1. Antenna design using COMSOL multiphysics and the meshing (finite elements) used to solve the radiation field pattern. Figure 3. Blowup of the spherical-conical antenna which will be used to study the RF noise from the linac. Figure 4. Electric potential isosurface around the antenna (initial studies).