Feedback control of phased array electromagnetic hyperthermia using MRI temperature mapping

Hyperthermia, a mode of tumor treatment where the tumor temperature is elevated by 6°C over an extended period of time, is an effective means of tumor eradication. Hyperthermia is particularly attractive because it can be used in conjunction with other more aggressive techniques such as chemotherapy and radiation, for a more effective treatment. This improvement results in a reduction of the required treatment dose and the associated side effects. Hyperthermia is commonly induced via ultrasound or electromagnetic energy.; Widespread use of hyperthermia has been hindered due to lack of a real-time, noninvasive temperature imaging technique and an accompanying closed loop control system to automatically control heating of the tumor while sparing healthy tissue from overheating. Such difficulties have initiated many recent investigations to explore the use of MRI thermometry, for noninvasive temperature mapping in closed loop systems. Most of these clinical studies have taken advantage of the desirable property of focused ultrasound that creates a well-defined heating spot inside the tissue. Little attention has been paid to noninvasive closed loop control of microwave hyperthermia in deep tissue heating. Unlike its ultrasound counterpart, the microwave fields are strongly influenced by the heterogeneous electromagnetic properties of the tissue, and particularly at smaller wavelengths the prediction and control of the hot spot becomes increasingly difficult. Due to these difficulties, microwave hyperthermia systems have often employed invasive probes to measure and focus the electric fields into a desired target location. It is desirable to eliminate using such invasive probes.; This work focuses on closed loop control of microwave hyperthermia using noninvasive MRI thermometry. The following developments are reported here: (1) An integrated MR-microwave hyperthermia system intended for small animal studies was designed and built. (2) A new technique is presented that allows noninvasive mapping of the relative phase patterns of the microwave fields inside a phantom. (3) An MRI-based technique is presented for closed loop spatial steering of the hot spot in a phased array system. (4) Noninvasive techniques are demonstrated that allow focusing of small-wavelength microwave fields into a heterogeneous medium. Temporal and spatial control of microwave heating are demonstrated that represent typical clinical applications.