Shizuo Ishikawa

A 120-MHz Ultrasound Probe for Tissue Imaging

Ultrasound transducers with center frequency above 100 MHz are expected to be used for future diagnostic tissue characterization because of their high lateral resolution. We have fabricated a 120-MHz transducer that consists of a ZnO piezoelectric film on a sapphire substrate that has a concave acoustic lens. The lateral resolution was calculated as 13 microns. The insertion loss of the transducer, defined as the difference between the received voltage and the transmitted one, was -45 dB. The 6-dB handwidth of the received signal was approximately 40 MHz. The transducer was mounted in a rod-shaped probe to ensure contact with in vivo tissue, because of the low penetration of ultrasound in the high frequency region. While the probe is rotated and moved along its axis mechanically, the transducer receives backscattered ultrasound from the surrounding tissue on a cylindrical plane that is kept a constant distance from the probe surface. The feasibility of this high-frequency tissue imaging probe has been demonstrated by obtaining preliminary images of an in vitro bovine kidney.

Intracorporeal imaging and differentiation of living tissue with an ultra-high-frequency ultrasound probe

Intraoperative diagnostic tissue differentiation is expected to be useful clinically. We have fabricated a 3-mm diameter rod-shaped ultrasound (US) probe mounted with a 120-MHz transducer whose lateral resolution is the same as the cellular size of 13 microm. The probe can image a microscopic structure (i.e., the cellular arrangement inside intracorporeal living tissue). We imaged normal kidney tissue of a living mouse and tumor tissue implanted in another mouse kidney. We anesthetized the mice, exteriorized the kidneys, and punctured the kidneys with the probe. Renal corpuscle-like structures were seen in the healthy kidney, but a wavy spindle-like structure was seen in the tumor. The similarity between the ultrasonic images and histological sections taken from the imaged organs demonstrates the possibility of real-time tissue differentiation by ultra-high-frequency US.

Effects of unequal interval quantization for ultrasonic echo on ultrasonic B-mode imaging

An amplitude quantizing method for analog-to-digital (A/D) conversion of an ultrasonic echo from biological tissue is discussed. The unequal interval quantization (UIQ) method, which precisely quantizes a small amplitude signal and roughly quantizes a large-amplitude signal, is proposed. The B-mode images of sponge and of liver are constructed using 10-b equal interval quantization (EIQ, the quantizing method used in general A/D converters), 6-b UIQ, and 6-b EIQ. There is practically no difference between the images made by 10-b EIQ and 6-b UIQ, whereas weak signals are not displayed in the image made by 6-b EIQ. Using UIQ A/D conversion, one can construct an image as good as that made by a high-resolution A/D converter using the EIQ with a smaller number of output bits.<<ETX>>