Thermal ablation is a clinical procedure that aims at destroying pathological tissue minimally invasively through temperature changes. Temperature monitoring during the treatment is instrumental to achieve a precise and successful ablation procedure: ensuring a complete target ablation while preserving as much healthy tissue as possible. Ultrasound (US) is a promising low cost and portable modality, that could provide real-time temperature monitoring. However, the validation of such a technique is challenging. It is usually done with thermometers. They provide temperature measurements with good temporal resolution but only at a few local points. Magnetic Resonance Imaging (MRI) is the gold standard in term of temperature monitoring nowadays. It could also be used for validation of other thermometry techniques with a more accurate spatial resolution, but it requires MR-compatible devices. In this paper, we propose to leverage the use of a novel bipolar radiofrequency (RF) ablation device that provides 10 different ablation shapes to validate an ultrasound-based temperature monitoring method. The monitoring method relies on an external ultrasound element integrated with the bipolar RF ablation probe. This element send through the ablated tissues ultrasound waves that carry time-of-flight information. The ultrasound waves are collected by a clinical diagnostic ultrasound probe and can be related to the changes in temperature due to the ablation since ultrasound propagation velocity in biological tissue changes as temperature increases. We use this ultrasound-based method to monitor temperature during RF ablation. First on simulation data and then on two ex-vivo porcine liver experiments. Those dataset are used to show that we can validate the proposed temperature reconstruction method using the novel conformal radiofrequency ablation device by generating different ablation shapes.
High Intensity Focused Ultrasound (HIFU) is a non-invasive ablative therapy. It is usually performed under MR monitoring, which provides reliable real-time thermal information to ensure a complete tumor ablation while preserving as much healthy tissue as possible. Unfortunately, many patients do not necessarily have access to this expensive and cumbersome cutting-edge technology, which is prohibitive for a widespread use of MRI to guide thermal ablation procedures. Ultrasound (US) is a promising low cost and portable alternative, that allows real-time monitoring and can easily be deployed outside hospitals. However, US-based thermometry alone is not robust enough for the monitoring of in-vivo tissue ablation, and its feasibility is demonstrated only on in-vitro cases for small range of temperatures, up to 50°C. Computational models can simulate the biophysical phenomena and mechanisms which govern this complex thermal therapy. The US wave propagation, the temperature evolution as well as the resulted necrotic lesion can be modeled. A method integrating those sources of information to intra-operative US data would allow to recover the accurate temperature in a wider range. Therefore, US thermometry could be improved and provide an inexpensive yet comprehensive method for intra-procedural monitoring of the ablative process through HIFU. In this paper, we propose to study the rise in temperature induced by high intensity US propagation in biological tissue, which is particularly difficult to simulate due to the complexity of the involved phenomena. The physics-based HIFU model simulates the nonlinear US propagation using a k-space model coupled with the heat propagation in biological tissue using a reaction-diffusion equation. We analyze numerically the model to evaluate its accuracy and related computational cost. Finally, our simulation approach is validated against MR thermometry, the gold-standard monitoring tool used in clinical setting. Three consecutive HIFU ablations were performed on a 2% agar and 2% silicon phantom using the Sonalleve V2 MR-HIFU system (Profound Medical, Toronto, Canada).
KEYWORDS: Chemical elements, Error analysis, High power microwaves, Ultrasonography, Tissues, Liver, Transducers, Temperature metrology, Ultrasound tomography
Thermal monitoring for ablation therapy has high demands for preserving healthy tissues while removing malignant ones completely. Various methods have been investigated. However, exposure to radiation, cost-effectiveness, and inconvenience hinder the use of X-ray or MRI methods. Due to the non-invasiveness and real-time capabilities of ultrasound, it is widely used in intraoperative procedures. Ultrasound thermal monitoring methods have been developed for affordable monitoring in real-time. We propose a new method for thermal monitoring using an ultrasound element. By inserting a Lead-zirconate-titanate (PZT) element to generate the ultrasound signal in the liver tissues, the single travel time of flight is recorded from the PZT element to the ultrasound transducer. We detect the speed of sound change caused by the increase in temperature during ablation therapy. We performed an ex vivo experiment with liver tissues to verify the feasibility of our speed of sound estimation technique. The time of flight information is used in an optimization method to recover the speed of sound maps during the ablation, which are then converted into temperature maps. The result shows that the trend of temperature changes matches with the temperature measured at a single point. The estimation error can be decreased by using a proper curve linking the speed of sound to the temperature. The average error over time was less than 3 degrees Celsius for a bovine liver. The speed of sound estimation using a single PZT element can be used for thermal monitoring.
Radiofrequency ablation (RFA) is the most widely used minimally invasive ablative therapy for liver cancer, but it is challenged by a lack of patient-specific monitoring. Inter-patient tissue variability and the presence of blood vessels make the prediction of the RFA difficult. A monitoring tool which can be personalized for a given patient during the intervention would be helpful to achieve a complete tumor ablation. However, the clinicians do not have access to such a tool, which results in incomplete treatment and a large number of recurrences. Computational models can simulate the phenomena and mechanisms governing this therapy. The temperature evolution as well as the resulted ablation can be modeled. When combined together with intraoperative measurements, computational modeling becomes an accurate and powerful tool to gain quantitative understanding and to enable improvements in the ongoing clinical settings. This paper shows how computational models of RFA can be evaluated using intra-operative measurements. First, simulations are used to demonstrate the feasibility of the method, which is then evaluated on two ex vivo datasets. RFA is simulated on a simplified geometry to generate realistic longitudinal temperature maps and the resulted necrosis. Computed temperatures are compared with the temperature evolution recorded using thermometers, and with temperatures monitored by ultrasound (US) in a 2D plane containing the ablation tip. Two ablations are performed on two cadaveric bovine livers, and we achieve error of 2.2 °C on average between the computed and the thermistors temperature and 1.4 °C and 2.7 °C on average between the temperature computed and monitored by US during the ablation at two different time points (t = 240 s and t = 900 s).
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.