Microwave thermal ablation (MWA) has been experiencing rapid clinical adoption in cancer therapy due to its advantages relative to RF ablation. 2,3 MWA can generally create larger lesions with less invasiveness and more control than RF ablation as it generates heat based on a special case of dielectric heating instead of electrical conductivity. MWA applicator (probe) insertion has been predominantly guided by ultrasound and the effects of its therapy have been monitored with limited thermocouple measurements and occasionally intraoperative CT. As the models of thermal therapy delivery have been largely developed using RF ablation, an opportunity exists for MRI thermometry to provide the experimental basis for new models that account for MWA’s fundamentally different method of heat generation. Therefore, we present a feasibility study of a MR compatible MWA system based upon a FDA-approved commercial design with real-time applicator guidance during insertion and high frame rate MR thermometry.

Proposed is a scheme to characterize and demonstrate the use of Magnetic Resonance (MR) in imaging Microwave Thermal Ablation (MWA). Generating tissue ablation is achieved by using a MR-compatible microwave applicator modeled after the Neuwave (Madison, WI) CertusPR clinical applicator but constructed from copper components. The prototype applicator is consistent with power and frequency of the clinically used Neuwave technology. 4,5 A 2.45 GHz microwave generator (Cober Muegge LLC, CT) was used as the power source with the MR-compatible applicator. Power was carried by a coaxial cable extended approximately 3 m from the MR console room to the scanner. All experiments were performed with excised bovine liver tissue and the applicator was inserted to the tissue horizontally and parallel to the bulk magnetic field.

The MR imaging was performed on a GE HDx 1.5 T scanner with 8-Ch cardiac coil (GE Healthcare, Waukesha, WI). A 3 minute time sequence of stacks of 2D T1-weighted spin echo images was first used to monitor the ablation zone with each individual acquisition requiring 30 seconds. Imaging parameters include: 14 ms TE, 1150 ms TR, 1 mm slice thickness, 26 cm by 14 cm FOV, 192 x 192 matrix.

Real-time MRI was also utilized to monitor the applicator’s insertion and then quantitatively map the ablation zone temperature using the MR Proton Resonance Frequency (PRF) method. A 2D fast gradient-echo spiral imaging sequence with 5 interleaves with the temporal resolution of 150 ms (TE = 13 ms, TR = 30 ms, slice thickness = 5 mm, FOV = 32 cm, acquisition matrix = 140 x 140) provided on the RTHawk platform (HeartVista, CA) was used for both purposes.

The high-powered (85W) microwave ablation system was able to quickly ablate a large teardrop-shaped zone.2, 3 Progression of this zone over a three minute ablation period can be seen in Figure 1. Encouragingly, the T1-weighted SE images in Figure 1 correlate well with the gross ablation regions of approximately 28cc observed via dissection in Figure 2.

Real-time imaging of over 6 frames per second (fps) provides visualization of the insertion of the microwave applicator was able to be tracked throughout insertion, as shown in Figure 3. Quantitative thermal mapping of the formation of the ablation zone, also at over 6 fps, is presented in Figure 4. Phase wrapping within the ablation zone can be observed 50-60 seconds after heating has started near the very hot center of the ablation zone. The large temperature increases possible with microwave ablation require more attention for an accurate dynamic range of the temperature map.

Currently microwave ablation applicators are guided with one modality while other modalities provide quite limited monitoring capabilities during and after ablation. An MRI system outfitted with high performance, real-time capabilities offers an opportunity to guide applicator insertion and quantitatively map temperature in the same setting. The high PRF performance makes this an ideal tool for generating insight to augment the limited models for predicting the progression of microwave ablation zones today.

We have demonstrated the feasibility of a MR compatible microwave ablation system based on currently FDA approved ablation technology. Our results demonstrate that rapid MRI guidance and monitoring provides an opportunity to expand the applications of microwave ablation into other body regions that require greater precision and control. MRI also presents a capable window for augmenting and validating future, more accurate models of microwave ablation.