Ultrasound is an attractive modality for temperature monitoring because it is non-ionizing, convenient, inexpensive and has relatively simple signal processing requirements. This modality may be useful for temperature estimation if a temperature-dependent ultrasonic parameter can be identified, measured and calibrated. The most prominent methods for using ultrasound as a non-invasive thermometer exploit either (1) echo shifts due to changes in tissue thermal expansion and speed of sound (SOS), (2) variation in the attenuation coefficient or (3) change in backscattered energy from tissue inhomogeneities. The use of echo shifts has received the most attention in the last decade. By tracking scattering volumes and measuring the time shift of received echoes, investigators have been able to predict the temperature from a region of interest both theoretically and experimentally in phantoms, in isolated tissue regions in vitro and preliminary in vivo studies. A limitation of this method for general temperature monitoring is that prior knowledge of both SOS and thermalexpansion coefficients is necessary. Acoustic attenuation is dependent on temperature, but with significant changes occurring only at temperatures above 50 C, which may lead to its use in thermal ablation therapies. Minimal change in attenuation, however, below this temperature range reduces its attractiveness for use in clinical hyperthermia. Models and measurements of the change in backscattered energy suggest that, over the clinical hyperthermia temperature range, changes in backscattered energy are dependent on the properties of individual scatterers or scattering regions. Calibration of the backscattered energy from different tissue regions is an important goal of this approach. All methods must be able to cope with motion of the image features on which temperature estimates are based. A crucial step in identifying a viable ultrasonic approach to temperature estimation is its performance during in vivo tests.

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Noninvasive temperature imaging would enhance the ability to uniformly heat tumors at therapeutic levels. Ultrasound is an attractive modality for this purpose. Previously, we predicted monotonic changes in backscattered energy (CBE) of ultrasound with temperature for certain subwavelength scatterers. We also measured CBE values similar to our predictions in bovine liver, turkey breast, and pork muscle in one dimension (1-D). Those measurements were corrected manually for changes in the axial position of echo signals with temperature. To investigate the effect of temperature on CBE in 2-D, we imaged 1cm thick samples of bovine liver, turkey breast, and pork muscle during heating in a water bath. Images were formed by a phased-array imager with a 7 MHz linear probe. Using radio frequency (RF) signals permitted the use of cross correlation as a similarity measure for automatic tracking of feature displacement as a function of temperature. Feature displacement across the specimen was nonuniform with typical total displacements of 0.5 mm in both axial and lateral directions. Apparent movement in eight image regions in each specimen was tracked from 37 to 50 C in 0.5 C steps. Envelopes of motion-compensated image regions were found then smoothed with a 3 3 running average filter before forming the backscattered energy at each pixel. Our measure of CBE compared means of both the positive and negative changes in the backscattered energy (BE) images. CBE was monotonic and differed by about 4 dB at 50 C from its value at 37 C. Relatively noise-free CBE curves from tissue volumes of less than 1 cm 3 supports the use of CBE for temperature estimation.