1. MR Guided Microwave Thermal Ablation: Guidance and Monitoring for Extending the
Physics of Thermal Therapy
Michael T. Simonson1
, Peng Wang2
, Christopher Brace1-3
, Walter F. Block1-3
(1) Department of Biomedical Engineering, University of Wisconsin, Madison,
Wisconsin, USA (2) Department of Medical Physics, University of Wisconsin, Madison,
Wisconsin, USA (3) Department of Radiology, University of Wisconsin, Madison,
Wisconsin, USA
Synopsis:
Monitoring microwave ablation with thermometry could extend the application of this clinical
procedure into territories where greater control is necessary to protect healthy tissue, such as the
spine. MRI thermometry could also provide useful insights to generate and validate new thermal
models based on the fundamentally different mode of dielectric heating utilized by microwave
ablation. We present here a platform consisting of 1) a high power MR compatible microwave
ablation system based on a FDA-approved design, 2) real-time applicator guidance during
applicator insertion and 3) high frame rate MR thermometry and tissue monitoring.
Purpose:
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.
Methods:
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

2. 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.
Results and Discussion:
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.
Conclusion:
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.

3. Figures:
[1]
Figure 1: A 3-minute long time sequence of T1-weighted Spin Echo images demonstrates the rapid
lesion expansion that high power microwave ablations can generate.
[2]

4. Figure 2: Gross anatomical view of ablation zone confirms that the ablation zone correlates well with the
hypo-intense region in the T1 weighted SE sequence. Notice the regular teardrop shape, which is typically
hard to achieve in RF ablation due to interactions with liver tissue heterogeneity.
[3]
Figure 3: Using the RTHawk MRI App Development package, real-time tracking of the microwave
applicator during insertion into a bovine liver sample (large arrows) is demonstrated.
Figure 4: Rapid thermal mapping is achieved (6.67 fps) using a spiral-based Proton Resonance
Frequency MR Thermometry acquisition provided by RTHawk. Snapshots at a variety of time points
show the ablative thermal progression. Phase wrap due to very high temperatures cause an erroneous
result adjacent to the applicator.

5. Acknowledgements:
This research is based on work supported by a Wisconsin State Economic Engagement and Development
Research Program grant. The authors would like to thank Miles Olson and Jim White from UW-Madison
Medical Physics for his contributions of time and resource.
References:
[1] Dong J, Zhang L, Li W, et al. 1.0 T open-configuration magnetic resonance-guided
microwave ablation of pig livers in real time. Scientific Reports. 2015;5:13551.
[2] Chiang J, Wang P, Brace CL. “Computational Modelling of Microwave Tumour
Ablations.” International journal of hyperthermia. 2013;29(4):308-317.
[3] Brace, Christopher L., et al. "Pulmonary Thermal Ablation: Comparison of Radiofrequency
and Microwave Devices by Using Gross Pathologic and CT Findings in a Swine Model
1." Radiology 251.3;(2009): 705-711.
[4] “Certus 140 – 2.45 GHz Ablation System – Probe Specifications”. Neuwave Medical. ,
<http://www.neuwave.com/images/pdfs/Probe.pdf> Accessed on 1 Sept 2015.
[5] Bertram, John M. et al. “A Review of Coaxial-Based Interstitial Antennas for Hepatic
Microwave Ablation.” Critical Reviews in Biomedical Engineering. 34(3); (2006). 187-213.