Distortion compensation in MR images for robotic stereotactic procedures

For medical applications, such as stereotactic surgery and radiation treatment, accurate information about the size and location of the lesion is very crucial. Although magnetic resonance (MR) images does convey more clinical information than computerized tomography (CT), the geometry of MR image is distorted because of the inhomogeneous main magnetic field and nonlinar gradient fields. This paper deals with the correction of the MR image distortions by modeling the distortion function as certain high order polynomial equations. The coefficients of the polynomial equations can be determined by mapping a set of distorted reference points to its a priori locations. A special phantom is designed to correct the distortion in all three directions. To avoid round-off error, a subpixel mapping scheme is used which corrects not only the physical distortions but also the intensities of the image as well. Finally a special subharmonic condition is evaluated which when satisfied, will make the use of MR images for robotic stereotactic procedures feasible.