Dynamics of tissue shrinkage during ablative temperature exposures

Abstract

There is a lack of studies that examine dynamics of heat-induced shrinkage of organ tissues. Clinical procedures such as radiofrequency ablation, microwave ablation or high-intensity focused ultrasound, use heat to treat diseases such as cancer and cardiac arrhythmia. When heat is applied to tissues, shrinkage occurs due to protein denaturation, dehydration, and contraction of collagen at temperatures greater 50ºC. This is particularly relevant for image-guided procedures such as tumor ablation, where pre- and post-treatment images are compared and any changes in dimensions must be considered to avoid misinterpretations of the treatment outcome. We present data from ex vivo, isothermal shrinkage tests in porcine liver tissue, where axial changes in tissue length were recorded during 15 minutes of heating to temperatures between 60 and 95ºC. A mathematical model was developed to accurately describe the time and temperature-dependent shrinkage behavior. Shrinkage dynamics had same characteristics independent of temperature; the estimated relative shrinkage, adjusted for time since death, after 15 min heating to temperatures of 60, 65, 75, 85 and 95ºC, was 12.3, 13.8, 16.6, 19.2, and 21.7%, respectively. Our results demonstrate shrinkage dynamics of organ tissues, and suggest the importance of considering tissue shrinkage for thermal ablative treatments.

Characterisation of tissue shrinkage during microwave thermal ablation

“…Recently, shrinkage of the ablated tissue volume has been reported in clinical practice and in experimental investigation of MW, and to a lesser extent in RF heating [13][14][15][16]. Looking at the spatial and temporal tissue contraction via CT scans performed during a MW ablation procedure, significant shrinkage was observed during the first minutes of heating [13,16].…”

“…Looking at the spatial and temporal tissue contraction via CT scans performed during a MW ablation procedure, significant shrinkage was observed during the first minutes of heating [13,16]. Moreover, expansion of the tissue was also noticed in some samples [14,17].…”

“…The reported results underline that the volume occupied by the ablated tissue as measured at the end of the thermal ablation procedure is smaller than the volume occupied by the equivalent untreated tissue. Moreover, the shrinkage is directly linked to the deployed energy [15], and it increases with the temperature [16]. As a consequence, it is important to be able to adequately characterise the shrinkage of the tissue during a MW thermal ablation procedure ideally in real-time in order to correctly predict the true treatment volume.…”

Microwave ablation of hepatocellular carcinoma

“…Tissue carbonisation is not, therefore, an insurmountable barrier to the MW heating process, and temperatures far higher than 100 ℃ may be reached within the target tumor, allowing enhanced active and passive tissue heating, larger coagulation zones and more effective rejection of heat sinking effects [32,33] . When high power MWA treatments are performed, several qualitative and quantitative differences are observed in terms of RFA: (1) the hyper-echogenic spot around the probe-active tip detectable on ultrasound-scanning during a thermal ablation procedure forms and expands at a much higher rate, providing a visual feedback of the ongoing vaporization process; (2) post-MWA follow-up scans [either computed tomography (CT) or magnetic resonance imaging (MRI)] usually show, in the region surrounding what was the probe active tip position during the ablation, an inner hyper-dense core contrasting with an outer thicker and hypo-dense annulus, the former being charred tissue (not present on RFA) and the latter being the coagulated but not carbonized zone typical of any thermal ablation modality ( Figures 1 and 2); and (3) due to massive water evaporation, MWA treatments induce substantial contraction in target tissues (30%-70% in volume, according to several ex-vivo and in vivo experimental observations [34][35][36] ), far more than their RFA counterparts [37] . If the appropriate shrinkage correction factor is used for accurately calculating the actual ablation volume, the coagulative performance gap between MWA and RFA widens further.…”

Bipolar radiofrequency ablation with 2 × 2 electrodes as a building block for matrix radiofrequency ablation:Ex vivoliver experiments and finite element method modelling

“…Two of them are purely theoretical [63,98] and one has been compared only with microwave ablation [103]. Research into quantification of temperature and time-dependent tissue shrinkage due to evaporation has only just started [105,106] and will be valuable for future modelling.…”