Measuring RF field distributions in MR coils with IR sensors

The uniformity of the MRI B/sub l/ field has a strong relation to image quality. Although computational methods offer the (very desirable) ability to predict these fields, experimental measurements of B/sub l/ are still essential for validating computational predictions and for confirming coil performance. We present a method of probing the field distribution over any unoccupied region of a coil. The technique is adapted from a method used previously for measurements of fields and current distributions in other applications (e.g., see Will, J.E. et al., 1996). The method involves placing into the coil a very thin dielectric film having a small amount of loss. Components of the electric field in the plane of the film produce ohmic currents, which deposit thermal energy in the film. The resulting local temperature increase can be detected remotely with an infrared (IR) camera, and from it the local electric field can be inferred. For coils based on TEM fields, the electric field is directly proportional to the B/sub l/ field. We describe the theory behind the method, its implementation, and some example results.