William L. Straube, R. Martin Arthur, Jason W. Trobaugh, Jesse Parry, Yuzheng Guo, and Eduardo G. Moros, "In Vivo Measurement of Changes in Ultrasonic Backscattered Energy for Homogeneous (Water Bath) and Heterogeneous (SAHUS) Heating", Proceedings 2006 Annual Meeting of the Society for Thermal Heating, Bethesda, MD, 6-8 April 2006, p. 39.
Abstract
Ultrasound is an attractive modality for noninvasive temperature imaging to monitor temperature changes during hyperthermia. Previously, we predicted monotonic changes in backscattered energy (CBE) versus temperature for certain sub-wavelength scatterers. We then measured CBE with temperature in bovine liver, turkey breast, and pork muscle and found these measurements to be in agreement with our predictions. The objective of this study was to measure the CBE and to determine its suitability for temperature estimation in perfused, living systems.
We measured CBE in living normal murine tissue and in implanted tumors (HT29 colon carcinoma) in nude-mouse preparations. Measurements were made in degassed water heated homogeneously with the mouse partially submerged in the water. The hind limb of the mouse was imaged with the ultrasound imaging system. A thermistor was placed near the contralateral limb to track the temperature of the tissue. The imaging system was a Terason 2000 (Teratech Corp., Burlingon, MA) laptop-based, phased-array system. The imaging system used a 7-MHz linear array transducer with the focus set at 2 cm, the center of the mouse limb. For the homogeneous heating images were taken in 0.5oC steps from 37 to 45 oC. For images within each preparation, motion was tracked and compensated using cross-correlation of the RF signal at each temperature. Envelopes of motion-compensated image regions were found with the Hilbert transform and smoothed with a 3x3 running average filter. Backscattered energy at each pixel was referred to the energy at 37 oC to find the CBE. Experiments were also performed using a small animal heating ultrasound system (SAHUS) as a heterogeneous heat source. For heterogeneous heating, the mouse was placed in the SAHUS and the images were attained orthogonal to the heating apparatus through ultrasound coupling gel. Images were taken before heating, during heating at different power levels and in rapid sequence after heating.
Motion estimates were typically 0.3 – 0.4 mm axially and 0.1 mm in the lateral direction for the homogeneous heating. In studies of motion-compensated regions from images of normal tissue and in regions of implanted tumor, CBE varied nearly monotonically by about 4dB and the change in tumor tissue was similar to that in normal tissue. The images during heterogeneous heating were contaminated by the ultrasonic radiation from the heat source. Analysis of the images before heating and the rapid sequence after heating (during cool down) is ongoing.
Our findings of the CBE dependence on temperature in living mice are consistent with our previous theoretical predictions and our previous in vitro studies. These results support using CBE for temperature measurements in living systems.
Support: R21-CA90531, R01-CA107558 and
the Wilkinson Trust at Washington University, St. Louis.