Garron Deshazer with Dr. Derek Merck, Dr. Damian Dupuy, Dr. Edward Walsh, Dr. Jeom Soon Kim, Dr. William Park
Goal
Model and validate thermal profiles produced by a physically based generative model of various microwave ablation procedures (MWA) and correlate these profiles with 3D ablation zone size and shape.
Importance
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We are proposing that thermal models are necessary margin estimation in ablation therapy, and therefore necessary for predictive planning.
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4D (time driven) thermal profiles are necessary to develop and validate time dependent thermal models in order to ensure robust accuracy of proposed generative based modeling approach, as well as eventually provide clinicians with more pre-procedural planning advantages
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Intra-operative 4D thermometry could allow physicians to directly observe when a target tissue region has been completely treated (reached a target temperature). Thus far MR thermography has not been used for MWA due to the limitations of not being able to bring the generator or metallic antenna into the room, however we now have a research MWA generator and MR safe antenna that we can bring into the room for testing. We also plan to try to perform validation of MWA ablation experiments using CT thermometry.
Methods
In order to propose a generative model, a simulation-validation pipeline must be established that can evaluate the change in volume and shape of the ablation zone during the procedure. Moreover, the literature states that once cells have reached a specific temperature during an ablation procedure ( > ~50 ◦C) then irreversible cell damage occurs (as opposed to typical hyperthermia temperatures in which the damage can be reversed). Thus it is becoming increasingly relevant to provide accurate absolute temperature mapping during the ablation procedures.
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There have been several manuscripts that have proposed ways of doing CT thermometry or changes in Hounsfield Unit (HU) /◦C. However, nothing has been formally established nor used clinically to validate effective treatment s a function of surrounding temperature of the treatment margins. One of the main reasons from the literature and our own preliminary tests is that it is difficult to find proper function to model for the temperature change due to artifact from the MWA antenna and limited range of HU change during the procedure. We think that this is a limitation of using a single energy CT scanner and using dual energy (DE) CT scanner would . We propose collecting DE CT images every minute during MWA experiments and focus on correlating HU change with direct point-measured thermography and artifact reduction near treatment site. We plan to start with ex vivo- experiments on bovine liver to test this methodology on the DE CT scanner at Brigham Women and Infants Hospital that has 80 kV and 140 kV energies. (with Dr. Damian Dupuy and Scott Collins from Rhode Island Hospital and Dr. Paul Shyn from Brigham and Women’s Hospital)
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MR thermography is a useful validation took that has been used more traditionally during High frequency ultrasound (HIFU) and other forms of external hyperthermia procedures. Due to the limitations of strong magnetic field in the MR suite, the current metallic components of the antenna and the generator for commercial MWA devices make it not only unsafe but incompatible during the actual image acquisition process due to major artifact. BSD is a commercial collaborator of ours that makes these antenna and generators for the MWA procedure that Dr. Dupuy performs, and has provided us with an antenna and research generator that we can test in the MR. Preliminary tests performed at Brown University’s MRF center have shown that we can bring the generator inside the bore at a specific distance from the bore. This is due to the magnetic shielding that the 3T scanner has up to a specific distance in the suit. Given this data we are going to perform MR data collection on ex- vivo ablations on several agarose phantoms and focus on the image sequencing and post-processing necessary to accurately map post ablation cool down of the phantom. It would be more relevant to due both the heating and cool down mapping, however the antenna is still metallic and cannot be inside the bore when it is acquiring images. In addition, from the preliminary data we should be able to pinpoint what can be done from both a materials standpoint and the frequency interference from the cable in order to create an MWA antenna that is both MR safe and MR image compatible within the year (Dr. Edward Walsh at Brown MRI Research Facility).
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The generative models I am currently running provide temperature data for all time points for several of the preliminary ex-vivo ablations we have performed at RIH and Brown. Visually the simulation is 2D cut plain of axial symmetry in which the antenna characteristics and placement are taken into consideration as well as the dielectric properties of the homoegenous ex-vivo phantoms are taken into account. For preliminary estimations of the thermal profile this is fine especially since computation time is not too long, however, it is known from the literature that the dielectric properties of the presented phantoms (especially liver) changes with temperature. Incorporating the temperature-dependent dielectric property functions increases computational time and provides more realistic data that can be compared to actual experiments. Very soon I will do complete 3D modeling in which no symmetry is assumed.
Academic Plan (Spring 2014)
Participate in Brown Medical Image Analysis course, and continue with course of study that will be finalized soon. Continue to gather research mentors that will provide me with necessary guidance and finalize my committee with URI advisers.
Goals through Summer 2014 (next 4-6 months)
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Create a base model that can accurately simulate the final size and time-dependent heat profile for some of the ex-vivo ablation experiments we are doing at RIH, Brigham, and Brown. In order to accomplish this I am currently learning Comsol multiphysics finite element modeling software package. Need to figure out which variables are most important to incorporate in the model (relative permittivity, electric conductivity, perfusion, etc.)
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Help Dr. Walsh with preliminary MR data and create an MR safe microwave ablation antenna to continue MR thermography studies. If successful we will have a valuable validation tool for my model. Possibly get BSD involved in making it
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Trip to BWH to study DE CT during ablation
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Prepare abstracts for for submission to RSNA (February), AAPM (March), NEAAPM (April), MICCAI workshop (July), and a manuscript for Med Phys (Red Book).