Magnetic resonance image-guided thermal therapy with a radio frequency phased array

Magnetic resonance imaging (MRI) can be used to noninvasively detect temperature changes in tissue. Several MR measurable parameters have been shown to have temperature coefficients with sufficient sensitivity to be useful for application in thermal therapy (Parker 1983, LeBihan 1989, Ishihara 1995). Consequently, there have been several reports of the use of MRI to guide, monitor or assess thermal therapeutic procedures (Hynynen 1996, Carter 1998). Most of these applications have been associated with thermal ablation which involves high tissue temperature (< 6OC) and small volumes. In these procedures, a highly localized energy distribution is induced in the target volume for a period of several seconds. Rapid imaging techniques can localize the energy distribution and track the treatment volume. These energy distributions are produced using focused ultrasound (US), lasers, or radiofrequency (RF) electrodes. Other applications are geared to produce elevated temperatures (< 50') for longer time periods in larger volumes of tissue in an effort to sensitize tissue to other cytotoxic agents such as ionizing radiation and drugs. These procedures require tissues at elevated temperatures for times on the order of tens of minutes to hours (Carter 1998). For these procedures, microwave (MW) RF and US sources can be used. This paper concerns this latter application and is specific to malignant target sites that are located in the lower extremity of the body (Leopold 1989, Carter 1998). The heating technique used in combination with MRI is described