Hybrid hyperthermia controlled by magnetic resonance imaging.
P. Wust, W. Wlodarczyk, J. Nadobny, H. Fahling, J. Gellermann.
Charite – Universitatsmedizin, Campus Virchow-Klinikum, Berlin, Germany.
(Радиология в медицинской диагностике [современные технологии] 2003: 71-72)

Background: Objective of our development was the integration of a multi-antenna applicator for part-body hyperthermia (BSD 2000/3D) in a 1.5 T MR-tomograph (Siemens Magnetom Symphony) in order to perform a noninvasive MR-monitoring in real time to increase reliability and effectiveness of the heat treatment (so-called hybrid hyperthermia).

Methods: The steps of this development are listed and described in the following. A) Mechanical integration: The positioning unit is mechanically coupled to the MR-gantry from the back side, and the body coil is utilised for monitoring. Patient and applicator are moved on a sliding rod into the gantry with special attention to various cables, connectors, catheters, hoses and sensor wires (power supply, phase measurement, thermometry, water flow). B) Decoupling: To increase signal-to-noise ratio, the hyperthermia antenna system (100 MHz, 1.500 W) and the MR-receiver (63.9 MHz) has to be decoupled in terms of high frequency (filter) and electromagnetically (emv). Filter systems are inserted into the high-power part to damp parasitary frequencies generated in the amplifier system (particularly in the range 64 – 100 MHz) as well as in the receiving part of the MR tomograph (around 100 MHz). A simultaneous operation of radiofrequency hyperthermia and MR-system is possible at clinically relevant power levels up to 1600 W. C) MR-monitoring: The processing of MR-data sets is performed in a software platform developed at the Konrad-Zuse Institute (AMIRA) containing the hyperthermia planning system HyperPlan. MR datasets are used for tumour imaging (spin echo standard frequencies), for hyperthermia planning (T1-weighted gradient echo, GRE, frequencies in equal- and opposed-phase techniques), and for temperature and perfusion estimation according to the suitable sequences such as proton resonance frequency method (PRF-method, phase evaluation of a GRE sequence with long echo time), diffusion method (gradient-weighted GRE sequences with different strengths for ADC distributions), flow and perfusion measurements (analysing the contrast media dynamics). Major goal is completion of software modules to make the procedure fast and user-friendly for online monitoring.

Results: We verified the performance of the hybrid system employing a three-dimensional phantom in the SIGMA-Eye applicator. We tested the PRF-method (phase differences), and utilised the water bolus and a few interior reference temperatures for calibration. This results in an accuracy of the MR-temperature of ±0.5 °C in comparison to direct temperature measurements. Clinically, we treated 108 patients since 2002. In 58/108 (54%) patients with tumors in the pelvis or lower extremity MR-thermography was possible. In 14/108 (13%) the (small) tumor was not clearly identified on the temperature scans, but temperature was measurable in the surrounding normal tissues. In 6/108 (6%) a navigation system was required to compensate motion by respiration. Only in 27% of patients MR-thermometry was not achieved because of tumor location (thorax, abdomen), restlessness of the patient and intolerance to the MR and/or hyperthermia.

Discussion: MR-thermography was possible in the hybrid approach for the majority of patients. However, MR-temperatures (derived from phase differences) are a superposition of perfusion and temperature changes. Therefore, further development is strongly needed to use T1-relaxation, diffusion and contrast media dynamics for parameter identification (to separate temperature and perfusion). In the next step, the online registered MR datasets must be utilized to establish a feed-back control in order to optimise the pattern (by variation of phases). This probably requires multiantenna applicators with stable phase conditions in the feed points, online measurement of these actual phases and sufficient efficiency.