MAGNETIC-RESONANCE-IMAGING CAN CAUSE FOCAL HEATING IN A NONUNIFORM PHANTOM

To test if the radiofrequency fields of a magnetic resonance imager co uld cause focal heating, two cylindrical phantoms were made from a mix ture of agar and saline. The first phantom was uniform; the second was nonuniform in that a narrow bridge of agar was produced. Both phantom s were exposed to high levels of radiofrequency power (140 W) at 63 MH z and the tempature rises were measured. In the uniform phantom, the t emperature increased as the radius increased. In the bridge phantom, t he narrow bridge heated three times greater than at the opposite unifo rm periphery, and over five times the average of the uniform phantom. This experiment demonstrates that the radiofrequency fields of magneti c resonance imagers can cause focal heating if the exposed object is n onuniform. Since nonuniformity is present in the human body, as the ra diofrequency power of magnetic resonance imaging techniques increases, focal heating in patients is a concern.