References

• Poluektova AA, Malskat WS, van Gemert MJ, Vuylsteke ME, Bruijninckx CM, Neumann HA, van der Geld CW. Some controversies in endovenous laser ablation of varicose veins addressed by optical-thermal mathematical modeling. Lasers Med Sci. 2013 Oct 9. PMID: 24105396.
  

  Abstract

  Minimally invasive treatment of varicose veins by endovenous laser ablation (EVLA) becomes more and more popular. However, despite significant research efforts performed during the last years, there is still a lack of agreement regarding EVLA mechanisms and therapeutic strategies. The aim of this article is to address some of these controversies by utilizing optical-thermal mathematical modeling. Our model combines Mordon’s light absorption-based optical-thermal model with the thermal consequences of the thin carbonized blood layer on the laser fiber tip that is heated up to temperatures of around 1,000 °C due to the absorption of about 45 % of the laser light. Computations were made in MATLAB. Laser wavelengths included were 810, 840, 940, 980, 1,064, 1,320, 1,470, and 1,950 nm. We addressed (a) the effect of direct light absorption by the vein wall on temperature behavior, comparing computations by using normal and zero wall absorption; (b) the prediction of the influence of wavelength on the temperature behavior; (c) the effect of the hot carbonized blood layer surrounding the fiber tip on temperature behavior, comparing wall temperatures from using a hot fiber tip and one kept at room temperature; (d) the effect of blood emptying the vein, simulated by reducing the inside vein diameter from 3 down to 0.8 mm; (e) the contribution of absorbed light energy to the increase in total energy at the inner vein wall in the time period where the highest inner wall temperature was reached; (f) the effect of laser power and pullback velocity on wall temperature of a 2-mm inner diameter vein, at a power/velocity ratio of 30 J/cm at 1,470 nm; (g) a comparison of model outcomes and clinical findings of EVLA procedures at 810 nm, 11 W, and 1.25 mm/s, and 1,470 nm, 6 W, and 1 mm/s, respectively. Interestingly, our model predicts that the dominating mechanism for heating up the vein wall is not direct absorption of the laser light by the vein wall but, rather, heat flow to the vein wall and its subsequent temperature increase from two independent heat sources. The first is the exceedingly hot carbonized layer covering the fiber tip; the second is the hot blood surrounding the fiber tip, heated up by direct absorption of the laser light. Both mechanisms are about equally effective for all laser wavelengths. Therefore, our model concurs the finding of Vuylsteke and Mordon (Ann Vasc Surg 26:424-433, 2012) of more circumferential vein wall injury in veins (nearly) devoid of blood, but it does not support their proposed explanation of direct light absorption by the vein wall. Furthermore, EVLA appears to be a more efficient therapy by the combination of higher laser power and faster pullback velocity than by the inverse combination. Our findings suggest that 1,470 nm achieves the highest EVLA efficacy compared to the shorter wavelengths at all vein diameters considered. However, 1,950 nm of EVLA is more efficacious than 1,470 nm albeit only at very small inner vein diameters (smaller than about 1 mm, i.e., veins quite devoid of blood). Our model confirms the efficacy of both clinical procedures at 810 and 1,470 nm. In conclusion, our model simulations suggest that direct light absorption by the vein wall is relatively unimportant, despite being the supposed mechanism of action of EVLA that drove the introduction of new lasers with different wavelengths. Consequently, the presumed advantage of wavelengths targeting water rather than hemoglobin is flawed. Finally, the model predicts that EVLA therapy may be optimized by using 1,470 nm of laser light, emptying of the vein before treatment, and combining a higher laser power with a greater fiber tip pullback velocity.

  
  

• Scott SJ, Prakash P, Salgaonkar V, Jones PD, Cam RN, Han M, Rieke V, Burdette EC, Diederich CJ. Approaches for modelling interstitial ultrasound ablation of tumours within or adjacent to bone: Theoretical and experimental evaluations. Int J Hyperthermia. 2013 Nov;29(7):629-642. PMID: 24102393.
  

  Abstract

  Abstract Purpose: The objectives of this study were to develop numerical models of interstitial ultrasound ablation of tumours within or adjacent to bone, to evaluate model performance through theoretical analysis, and to validate the models and approximations used through comparison to experiments. Methods: 3D transient biothermal and acoustic finite element models were developed, employing four approximations of 7-MHz ultrasound propagation at bone/soft tissue interfaces. The various approximations considered or excluded reflection, refraction, angle-dependence of transmission coefficients, shear mode conversion, and volumetric heat deposition. Simulations were performed for parametric and comparative studies. Experiments within ex vivo tissues and phantoms were performed to validate the models by comparison to simulations. Temperature measurements were conducted using needle thermocouples or magnetic resonance temperature imaging (MRTI). Finite element models representing heterogeneous tissue geometries were created based on segmented MR images. Results: High ultrasound absorption at bone/soft tissue interfaces increased the volumes of target tissue that could be ablated. Models using simplified approximations produced temperature profiles closely matching both more comprehensive models and experimental results, with good agreement between 3D calculations and MRTI. The correlation coefficients between simulated and measured temperature profiles in phantoms ranged from 0.852 to 0.967 (p-value < 0.01) for the four models. Conclusions: Models using approximations of interstitial ultrasound energy deposition around bone/soft tissue interfaces produced temperature distributions in close agreement with comprehensive simulations and experimental measurements. These models may be applied to accurately predict temperatures produced by interstitial ultrasound ablation of tumours near and within bone, with applications toward treatment planning.