M.Sc Thesis

M.Sc StudentRazon Alon
SubjectNoninvasive Monitoring of Thermal Ablation of
Needle-Type Applicators Using Ultrasonic
DepartmentDepartment of Biomedical Engineering
Supervisor PROF. Haim Azhari
Full Thesis textFull thesis text - English Version


   Minimal invasive procedures are the current trend in modern surgery. In particular, tumor destruction via thermal ablation which is conducted by deploying energy into a specific target zone, has gained popularity. Commonly, needle-like applicators are used in thermal ablation procedures. These systems emit energy at the applicator’s tip in the form of radio frequency (RF), microwave (MW) or laser radiation, leading to substantial temperature elevation and consequently to the destruction of the treated tissue. This study, was focused on Radiofrequency Ablation (RFA) which has been proven as a safe minimal-invasive option in the treatment for different kinds of tumors.

   Real-time thermal monitoring of such procedures is essential, preferably in a non-invasive manner.  Current methods are based on imaging modalities such as MRI which is expensive and X-ray CT which involves ionizing radiation. The common pulse-echo ultrasound does not provide a good solution either. Through transmission (TT) ultrasound on the other hand, can detect temperature and tissue properties related changes in the speed of sound (SoS). Hence, the aim of this study was to investigate the use of TT ultrasound for non-invasive RF ablation size estimation.

    Initially, temperature distribution maps around the electrode during RFA of ex-vivo tissues were obtained by thermocouple measurements. The maps were validated by calculating the diffusivity-temperature dependence using Penn’s heat equation. The ablation zone size was estimated by applying a threshold to the temperature maps. 

   On the second stage, SoS-temperature relation was quantified by measurements of SoS and temperature of gradually heated specimens. Analytic equations were obtained for chicken breast and lamb fat by curve fitting the measured experimental data. Using these equations, the temperature maps were converted into SoS maps. The obtained SoS maps were then projected onto time of flight (TOF) images.

   On the third stage, chicken breast specimens were scanned during RFA using TT ultrasound. The obtained changes in the time of flight, i.e.  images, were mapped by subtracting the pre-heating image from the current image. Then, by implementing an image processing algorithm to these maps, the ablation size in each tissue slice was estimated.

   Finally, the tissue was cut along planes matching the image slices. The ablated zone was manually traced on the gross-pathology cuts to yield reference ablation sizes.  

The obtained SoS-temperature relations matched previously published patterns. The estimated ablation size calculated from the temperature profiles highly correlated with the values yielding an exponential behavior. The results obtained from the ultrasonic TT scan images also yielded similar exponential relation between the mean values and the measured ablation diameters.

   The results obtained here support the validity of the hypothesis that the ablation zone size can be noninvasively estimated from TT ultrasound scans. Potentially, the results of this study may lead to the development of a new clinical tool for noninvasive real time and cost effective ablation monitoring of needle-type applicators using US.