Bioheat transfer is the study of the transport of thermal energy in living systems. Because biochemical processes are temperature dependent, heat transfer plays a major role in living systems. Also, because the mass transport of blood through tissue causes a consequent thermal energy transfer, bioheat transfer methods are applicable for diagnostic and therapeutic applications involving either mass or heat transfer. This article presents the characteristics of bioheat transfer that distinguish it from nonliving systems, including the effects of blood perfusion on temperature distribution, coupling with biochemical processes, therapeutic and injury processes, and thermoregulation.
The study of bioheat transfer involves phenomena that are not found in systems that are not alive. For example, blood perfusion is considered a three-dimensional (3D) process as fluid traverses in a volumetric manner through tissues and organs via a complex network of branching vessels. Heat transfer is affected by vessel geometry, local blood flow rates, and thermal capacity of the blood (1). One factor that makes modeling blood perfusion difficult is the complex network of pairs of arteries and veins with countercurrent flow (2), as shown in Fig. 1. Arterial and venous blood temperatures may be different, and it is possible that neither is equal to the local tissue temperature. These temperatures may vary as a function of many transient physiological and physical parameters. The regulation of temperature and blood flow is quite nonlinear and has presented a major challenge to understand and model. Nevertheless, these critical processes must be accounted for in the design of many types of systems that interface with humans and animals.
Many scientists view life from either the macroscopic (systems) or the microscopic (cellular) level, but in reality one must be aware that life processes exist continuously throughout the spectrum. In order to better understand life processes at the molecular level, a significant research effort is underway associated with molecular biology. Because temperature and blood flow are critical factors, bioengineers are collaborating with molecular biologists to understand and manipulate the molecular and biochemical processes that constitute the basis of life. Research has found that the rates of nearly all physiological functions are altered 6-10%/°C (3). Similarly, heat can be added or removed during therapeutic or diagnostic procedures to produce or measure a targeted effect, based on the fact that a change in local temperature will have a large effect on rates of biochemical process rates. Thus, the measurement and control of temperature in living tissues is of great value in both the assessment of normal physiological function and the treatment of pathological states.
The study of the effects of temperature alterations on biochemical rate processes has been divided into three broad categories: hyperthermia (increased temperature), hypothermia (decreased temperature), and cryobiology (subfreezing temperature). An extensive review of these domains has been published (4), to which the reader is referred for further details and bibliography.
Effects of Blood Perfusion on Heat Transfer
Blood perfusion through the vascular network and the local temperature distribution are interdependent. Many environmental (e.g., heat stress and hypothermia), pathophysiologic (e.g., inflammation and cancer), therapeutic (e.g., heating-cooling pads) situations create a significant temperature difference between the blood and the tissue through which it flows. The temperature difference causes convective heat transport to occur, altering the temperatures of both the blood and the tissue. Perfusion-based heat transfer interaction is critical to a number of physiological processes, such as thermoregulation and inflammation. The convective heat transfer depends on the rate of perfusion and the vascular anatomy, which vary widely among the different tissues, organs of the body, and pathology. Diller et al. published an extensive compilation of perfusion data for many tissues and organs and for many species (5). Charney reviewed the literature on mathematical modeling of the influence of blood perfusion on bioheat transfer phenomena (6).
The rate of perfusion of blood through different tissues and organs varies over the time course of a normal day's activities, depending on factors, such as physical activity, physiological stimulus, and environmental conditions. Further, many disease processes are characterized by alterations in blood perfusion, and some therapeutic interventions result in either an increase or decrease in blood flow in a target tissue. For these reasons, it is very useful in a clinical context to know what the absolute level of blood perfusion is within a given tissue. Many thermal techniques have been developed that directly measure heat flux to predict blood perfusion by exploiting the coupling between vascular perfusion and local tissue temperature using inverse mathematical solutions.