Automatic control of hyperthermic therapy based on real‐time Fourier analysis of MR temperature maps

Abstract

Local hyperthermia is increasingly being used for therapeutic purposes, such as tumor ablation. Heat conduction and energy absorption in vivo during the hyperthermic procedure are largely unknown, thus making feedback temperature control highly desirable. Here, a general method for temperature control based on Fourier transformation (FT) of the bio-heat equation is presented, taking into account heat diffusion ( Within the full spectrum of electromagnetic and sound waves, only focused ultrasound (FUS) can be used to generate local hyperthermia noninvasively (1). The local temperature increase may serve a wide variety of medical interventions, such as ablation of tumoral tissue (2,3) or cardiac tissue to treat arrhythmias (4). In addition, local hyperthermia has been suggested for local drug delivery (5-7) with thermosensitive microcarriers (1), control of gene therapy using heat-sensitive promoters (8), and heatactivated chemotherapy or local contrast agent deposition (9,10). Temperature evolution during hyperthermic treatment is a function of heat deposition and heat diffusion. Heat deposition by FUS depends on the local absorption of ultrasound energy and its transition to molecular (atomic) vibration energy. Pure water and blood hardly absorb ultrasound energy. The absorption of ultrasound depends on tissue composition and the ultrasound wavelength. Therefore, accurate quantification of energy deposition by FUS and absorption by tissue is difficult to predict. Heat conduction also depends on tissue composition. Diffusion and perfusion processes may vary locally as a function of tissue architecture and tissue composition. Physiological events such as temperature-dependent perfusion increases may play a role. In case of ablation procedures, tissue coagulation may significantly modify heat conduction as well as energy absorption. As a consequence, heat losses and energy absorption are difficult to predict in advance of the procedure.Optimal control of the temperature-based treatment requires regulation of the temperature. Recent developments have shown that rapid MRI, followed by on-line data processing and real-time feedback to the FUS output, and combined with new temperature regulation algorithms, may provide such control. Salomir et al. (11) described regulation of temperature evolution of the focal point based on temperature mapping and a physical model of local energy deposition and heat conduction. An essential element was the real-time evaluation of temperature gradients near the focal point. This was achieved by on-line calculation of the Laplacian value from the temperature maps. The method performed well under conditions of FUS energy deposit in a small, well defined region. Recently, it was shown that the use of either multiple FUS focal points (using a phased-array FUS transducer (12,13)) or a spatial trajectory of a single focal point (mechanical displacement (14)) may lead to shorter treatment times. However, heat diffusion estimated from the Laplacian value of temperature is ill defined in such ...

Non‐invasive determination of tissue thermal parameters from high intensity focused ultrasound treatment monitored by volumetric MRI thermometry

“…The spatial distribution of temperature for constant power heating with fixed focal point can thus generally be fitted by a Gaussian function (16) and can be inserted as an initial temperature distribution to solve eqn (2) during the cooling period. The analytical solution of eqn (2) can be derived from Fourier transformation over spatial coordinates (12) as follows:…”

“…The simple BHT model may describe the heat transfer at a macroscopic scale accurately. This model has been successfully used in non-perfused tissue and proposed as a reasonable physical basis for automatic control of temperature in high intensity focused ultrasound (HIFU) therapy, guided by MRI of temperature (12). Experimental studies for estimating the validity of the BHT in perfused tissue have been carried out in the past (7,13), usually using invasive thermometry methods based on multiple thermocouples inserted in the organs.…”

Treatment monitoring and thermometry for therapeutic focused ultrasound

“…Later, online temperature correction, again using MR thermometry, was demonstrated [136] in studies in which a PRF sensitivity of 9.4 ppb/ C was assumed. This group went on to demonstrate the production of a spiral of damage requiring a 1000-s exposure duration under PRF thermometry guidance using a clinical scanner [137,138].…”

Adaptive real-time closed-loop temperature control for ultrasound hyperthermia using magnetic resonance thermometry

“…To avoid large overshoots, the controllers with fixed gains required prolonged rise times (9,18). In applying a physical model of heat conduction, some control methods neglect the effects of blood perfusion and assume the absorption and diffusion of heat to be independent of space and time, which limited the stability and tolerance for spatial deviations (10,19,22). With the assumption that tissue parameters are time invariant, other controllers had less ability to track the system variations during the long period of hyperthermia (21).…”