Large blood vessel cooling in heated tissues: a numerical study.

Large blood vessels can produce steep temperature gradients in heated tissues leading to inadequate tissue temperatures during hyperthermia. This paper utilizes a finite difference scheme to solve the basic equations of heat transfer and fluid flow to model blood vessel cooling. Unlike previous formulations, heat transfer coefficients were not used to calculate heat transfer to large blood vessels. Instead, the conservation form of the finite difference equations implicitly modelled this process. Temperature profiles of heated tissues near thermally significant vessels were calculated. Microvascular heat transfer was modelled either as an effective conductivity or a heat sink. An increase in perfusion in both microvascular models results in a reduction of the cooling effects of large vessels. For equivalent perfusion values, the effective conductivity model predicted more effective heating of the blood and adjacent tissue. Furthermore, it was found that optimal vessel heating strategies depend on the microvascular heat transfer model adopted; localized deposition of heat near vessels could produce higher temperature profiles when microvascular heat transfer was modelled according to the bioheat transfer equation (BHTE) but not the effective thermal conductivity equation (ETCE). Reduction of the blood flow through thermally significant vessels was found to be the most effective way of reducing localized cooling.