Ultrasound is an attractive modality for non-invasive temperature imaging to enhance the ability to target tumor heating at therapeutic levels.  Previously, we predicted monotonic changes in ultrasonic backscattered energy (CBE) for certain sub-wavelength scatterers.  Recent advances include: 1) measurement of CBE in 2D and in 3D in vitro, 2) measurement of CBE in 2D in vivo, 3) simulation of CBE from multiple scatterers, and 4) estimation of temperature from CBE in simulated images.

            Accurate measurement of CBE requires compensation for apparent motion of image features due changes in speed of sound. We measured CBE in 2D in motion-compensated images of four 1-cm thick samples of bovine liver, two of turkey breast, and one of pork muscle during uniform heating in a water bath from 37 to 50^oC.  Images were formed by a Terason 2000 imager with a 7 MHz linear probe focused at 4.5 cm, the center of each tissue specimen. Employing RF signals from the Terason 2000 (courtesy Teratech Corp.) permitted the use of cross-correlation as a similarity measure for automatic feature tracking with temperature.  Tissue motion in 8 image regions of each specimen was tracked from 37 to 50^oC in 0.5^oC steps.  Maximum displacement in all specimens was about 0.5 mm in both axial and lateral directions.  Motion compensated image regions were demodulated and smoothed.  Pixel values were squared to form the backscattered energy.  We compared means of both the positive and negative changes in the BE images.  CBE was monotonic.  BE differed by about 4 dB at 50^oC from its value at 37^oC.

            We have conducted preliminary experiments in 3D by taking seven 2D images separated by 0.6 mm in elevation at each temperature.  We applied our motion detection and correction methods to a region in the resulting 3D volume then made movies of slices in elevation.  As expected the dominant motion was in the axial direction.  The apparent motion encountered in elevation was much smaller than the beam width in elevation, so that the percentage of scatterers that move in or out of the beam in elevation is expected to be smaller than the percentage in the lateral direction. Motion within a wide beam will change the backscattered energy less than that in a narrow beam. Thus, we expect CBE to support temperature estimation in 3D.

            We have extended this research to an in vivo system for measuring CBE in living, perfused tissue.  The animal system consists of nude mice with implanted tumors (HT29 colon cancer line) on their hind quarters.  Mice were anesthetized with Ketamine Xylazine prior to being secured to a platform.  The platform and lower section of the mouse were submerged in a water bath filled with degassed water and  heated in a method similar to our in vitro experiments.  The temperature of the mouse was measured with a thermistor in a contralateral limb that has a similar implanted tumor and is similarly submerged in the water bath.  The transducer from the Terason 2000 system was coupled to the target tumor through the water bath, and images of the heated tumor were taken at 0.5 ^oC increments from 37.0 ^oC to 45.0 ^oC.  Thus far we have completed the experimental design and have run experiments on a sacrificed mouse and on two living mice.

            Theoretical results for a single scatterer showed that backscattered energy increases or decreases monotonically, depending on the lipid or aqueous nature of the scatterer.  To extend our theory to a more realistic tissue composition, we have developed methods for simulating ultrasonic images of thousands of randomly distributed scatterers.  In the simulations, the imaging system was described by its point-spread function, and the tissue medium was represented by discrete aqueous and lipid scatterers.  Images were simulated to represent temperatures from 37 to 50^oC by changing the scatterer amplitudes according to curves predicted previously for single scatterers.  CBE was computed for each image pixel, referenced to the initial image. To characterize CBE for a region, the means of the positive- and negative-changing pixels and the standard deviation of all pixels were computed.  CBE showed the same monotonic increase and decrease as in experimental results and covered ranges similar to both prediction and experiment. Subsequent simulations included additive noise and showed striking agreement with experimental CBE measurements, replicating both an initial jump and noise throughout the range. These results support the use of CBE for noninvasive temperature estimation, showing that our model for the temperature dependence of CBE can be successfully applied to measurements from multiple scatterers.  These simulation methods also provide a means for exploring limits on temperature accuracy and spatial resolution with varying imaging systems and tissue types. They could also be useful in studying the effects of apparent and bulk tissue motion.

            As an example of methods that could be used for calibration and estimation, we have extended our image simulation methods to the initial development of calibration curves and use of those curves for estimating temperature from CBE.  Calibration curves were generated by fitting the average CBE from multiple simulations with a polynomial.  This polynomial represents the average standard deviation of the CBE for images of a simulated population of lipid and aqueous scatterers with added noise. Estimates of temperature were then generated from that same population using the calibration curve.  Results showed an error of approximately ± 1^oC for these images of 1 cm² or approximately 0.3 cm³ (based on a 3 mm elevation ultrasonic beam width).  This error for a small volume with typical measurement noise levels suggests 0.5^oC or better accuracy may be attainable in 1 cm³ volumes with noise reduction techniques.

Acknowledgement: This work was supported in part by NIH grant R21-CA90531 from the National Cancer Institute and the Wilkinson Trust at Washington University in St. Louis