The local tissue cooling coefficient: a unified approach to thermal washout and steady-state 'perfusion' calculations

Several investigators have attempted to utilize either transient 'thermal washout' and/or steady-state temperature data from standard hyperthermia thermometry to obtain information regarding both tumour and normal tissue perfusions, and transient data to measure SAR distributions. This paper reviews that literature, presents a unified theoretical basis for all of the temporal 'perfusion' approaches, and shows that both the steady-state and the 'washout' techniques actually measure the same quantity, which is related to, but not equal to, the tissue perfusion. A new nomenclature for the quantity measured by these techniques is proposed, the local tissue cooling coefficient, a name which avoids any use of the term 'perfusion', in order to avoid unwarranted inferences regarding this quantity. The requirements for relating this quantity to the true tissue perfusion are presented, and possible applications of the local tissue cooling coefficient are reviewed. Finally, the techniques used by various investigators for normalizing SAR data are summarized and discussed, and a standard approach suggested.