MR thermal monitoring of ultrasound interstitial thermal therapy

This study investigates the feasibility of using MR to monitor tissue thermal coagulation and necrosis during ultrasound interstitial thermal therapy (USITT). Recent studies have demonstrated significant advantages of these multi-transducer ultrasound applicators, most notably the capability to dynamically tailor the longitudinal and angular heating distribution which are important for conformable treatments and preserving critical non-targeted tissue. In this effort we consider the use of MRI-based monitoring as a means of on-line treatment control and verification of USITT with these applicators. Applicator design strategies were devised which have improved MR compatibility and reduced image degradation. For this initial feasibility study, one MR thermal imaging approach was investigated: fast spoiled gradient echo sequence with temperature elevation dependent on differential phase maps. Thermal lesions were produced using interstitial ultrasound applicators in vitro and monitored in real time using the MR technique. MR-based temperature and lesion maps were correlated to physical examination of thermal lesions and temperature measurements. The temperature sensitivity and contrast of lesion determination as applied to USITT are discussed, and specific recommendations on applicator fabrication and implementation of MR imaging sequences are given.