The ultimate goal of radiation therapy is to increase tumor control while reducing normal tissue complications. This is accomplished by conforming the radiation dose delivered to a patient to the tumor geometry, permitting the delivery of higher doses to the target volume while decreasing the dose in surrounding normal tissues. In order to best achieve this goal, our group is pursuing the design of a 0.2T biplanar magnetic resonance imager (MRI) coupled with a medical linear accelerator, which will be capable of performing real-time image guided radiotherapy. In a simplified design of the permanent magnet structure for this system, large paramagnetic plates which affect the characteristics of the magnetic field in the imaging volume are used to hold the magnetic material in place. Since the sole purpose of the MRI module of this unit is to provide geometrical information regarding the shape and position of the target volume during irradiation, obtaining distortion-free images is critical. In the present work, we seek a particular surface topology on the pole plates of the permanent magnet structure which minimizes the overall size of the pole plates while maximizing the homogeneity of the magnetic field in the imaging volume. A rose ring design is investigated with the aid of finite element analysis and the results indicate that a significant improvement in field uniformity is obtained as compared to the simplest design possible.