Kazuaki Sasaki

Animal study of high-speed iterative refraction calibration method for ultrasound computed tomography

We are developing an ultrasound computed tomography (USCT) system for early breast-cancer screening. USCT has great advantages over mammographies because of its lack of X-ray exposure and compression pain. USCT can show both the reflection boundary (structure) distribution and the sound speed (hardness) distribution in a subject, which is estimated from the time-of-flight (TOF) information of transmitted ultrasound waves on the basis of an X-ray CT algorithm. Considering the nature of ultrasound waves, improving the image quality generally increases the calculation burden. To achieve both high-quality images and high throughput, we developed an iterative refraction calibration method. The measured TOF sinogram was iteratively calibrated by the difference between the fastest wave arrival time and the arrival time of the wave along the geometrically shortest path in a section. This method was applied to the data of a gel phantom and a dog’s tumor extirpated at Tokyo University of Agriculture and Technology, which was measured by a USCT prototype with a 10 cm-diameter ring array. As a result, we achieved a calculation speed seven times faster than that of a conventional bent-ray reconstruction with the same contrast as that of a sound-speed image.

HIGH-INTENSITY FOCUSED ULTRASOUND TREATMENT ENHANCED BY MICROBUBBLES

The bioeffect of ultrasound can be enhanced by an order of magnitude with microbubbles. Enhancement of ultrasonic tissue absorption with a microbubble agent is demonstrated both in theory and in an in vivo experiment. The ultrasonic power absorbed by a microbubble in its continuous wave response is estimated through numerically solving a version of the Rayleigh-Plesset equation. This estimation predicts that the tissue ultrasonic absorption at an ultrasonic frequency of 3 MHz will be increased by twice when resonant microbubbles, approximately 1.1 μm in radius, are at a concentration of about 8 bubbles /mm in tissue. An exteriorized murine kidney was exposed to focused ultrasound at 3.2 MHz in degassed saline and the tissue temperature change was measured. With an intravenous bolus administration of 0.2 ml/kg Optison, the ultrasonically induced temperature elevation was enhanced up to five times. Microbubbles generated by focused ultrasound pulses at an extremely high intensity (trigger pulses) can also enhance the heating effect of immediately following ultrasonic waves (heating waves). This effect was demonstrated with a polyacrylamide gel phantom as well as with an excised chicken breast tissue at an ultrasonic frequency of 1 MHz. A large contiguous volume was coagulated in a short time by the trigger pulses focused at three adjacent points in combination with the following heating waves with a widened focus covering all the three points.