Abstract
Ultrasound is an attractive modality for
noninvasive temperature imaging to monitor hyperthermia treatment of cancer.
Previously, we predicted monotonic changes in backscattered energy (CBE) of
ultrasound with temperature for certain sub-wavelength scatterers then measured
CBE with temperature in bovine liver, turkey breast, and pork muscle in
agreement with our predictions. The objective of this study was to measure CBE
in vivo 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 cancer line) on nude-mouse preparations. Measurements were made in
degassed water heated homogeneously. Four mice, one with an implanted tumor were
anesthetized with Ketamine Xylazine and heated. Temperature was measured with a
thermistor at the hind limb contralateral to the one imaged. Imaging was done
with a Terason 2000 (Teratech Corp., Burlington, MA), laptop-based, phased-array
system. The imaging system used a 7-MHz linear probe (model 10L5) focused at 2
cm, the center of the mouse leg. Images were taken in 0.5oC steps from 37.0 to
45.0oC. For image regions within each preparation, motion was tracked and
compensated using cross-correlation of RF signals at adjacent temperatures.
Envelopes of motion-compensated image regions were found with the Hilbert
transform then smoothed with a 3x3 running average filter. Backscattered energy
at each pixel was referred to the value at 37oC to find CBE.
Motion estimates were typically 0.3 – 0.4 mm axially, the propagation direction
of the transducer, and < 0.1 mm in the lateral direction. In studies of 4
motion-compensated regions from images of normal tissue and 3 motion-compensated
regions of implanted tumor, CBE varied nearly monotonically by about 4 dB. CBE
in tumor was similar to that in normal tissue.
Our findings of the CBE dependence on temperature in living mice are consistent
with both our theoretical predictions and our previous in vitro studies. These
results support using CBE for temperature estimation in living systems.
Support: R21-CA90531, R01-CA107558 and the Wilkinson Trust at Washington
University, St. Louis.