Noninvasive temperature estimation in tissue via ultrasound echo-shifts. Part II. In vitro study

Abstract

Time shifts in echo signals returning from a heated volume of tissue correlate well with the temperature changes. In this study the relationship between these time shifts (or delays) and the tissue temperature was investigated in excised muscle tissue (turkey breast) as a possible dosimetric method. Heat was induced by the repeated activation of a sharply focused high-intensity ultrasound beam. Pulse echoes were sent and received with a confocal diagnostic transducer during the brief periods when the high-intensity ultrasonic beam was inactive. The change in transit time between echoes collected at different temperatures was estimated using cross-correlation techniques. With spatial-peak temporal-peak intensities (ISPTP) of less than 950W/cm2, the delay versus temperature relationship was fit to a linear equation with highly reproducible coefficients. The results confirmed that for spatial-peak temperature increases of approximately 10 degrees C, temperature-dependent changes in velocity were the single most important factor determining the observed delay, and a linear approximation could produce accurate temperature estimations. Nonlinear phenomena that occurred during the high-intensity irradiation had no significant effect on the measured delay. At ISPTP of 1115-2698 W/cm2, the delay-temperature relationship showed a similar monotonically decreasing pattern, but as the temperature peaked its slope gradually increased. This may reflect the curvilinear nature of the velocity-temperature relationship, but it may also be related to irreversible tissue modifications and to the use of the spatial-peak temperature to experimentally characterize the temperature changes. Overall, the results were consistent with theoretical predictions and encourage further experimental work to validate other aspects of the technique.

Non-invasive estimation of hyperthermia temperatures with ultrasound

“…In particular, papers by Sun and Ying [23], Seip, Simon, Ebbini and coworkers [54][55][56], Maass-Moreno, Damianou and coworkers [57][58][59] and this group [29,60,61] have reported the changes in received ultrasonic signals due to changes in ultrasonic tissue characteristics with temperature. These changes have been investigated both theoretically and in vitro with an eye towards using these signals for non-invasive monitoring of thermal therapy.…”

“…Most of these efforts have been geared towards high intensity focused ultrasound (HIFU) therapy, which typically heats small volumes of tissue to above 60 C. In investigations by Ebbini and coworkers, tracking echo shifts from scattering volumes was shown to be promising, as was the work of other investigators looking at the echo shifts [56]. Maass-Moreno and coworkers investigated the ability to predict temperature in HIFU therapy from echo shifts in turkey breast muscle [57,58]. They found that results were consistent with their theoretical predictions.…”

Thermal strain imaging: a review

“…As the temperature increases in soft tissue, the tissue expands and the ultrasonic propagation speed changes, introducing physical and apparent shifts in scatterer position, respectively [1 -7]. The latter is dominant over a wide range of operating conditions near body temperature (nominally 378C) [8,9]. The local temporal gradient of scatterer shift, often called the thermal strain, is directly related to tissue composition and temperature.…”

Noninvasive Measurement of Local Thermal Diffusivity Using Backscattered Ultrasound and Focused Ultrasound Heating

“…The parameter estimation technique is based on visualizing the spatiotemporal variation of temperature induced strain (Miller et al 2002(Miller et al , 2004 estimated from the raw ultrasound backscatter data after a short focused ultrasound heating pulse is applied at subablative intensities. The temperature induced strain is caused by changes in the local sound speed of the medium and thermal expansion (Maass-Moreno et al 1996;Simon et al 1998;Varghese et al 2002). The maximum induced temperature rise is less than 10°C to avoid any permanent changes in tissue.…”