IMPROVED EQUIVALENT CIRCUIT AND FINITE ELEMENT METHOD MODELING OF CAPACITIVE MICROMACHINED ULTRASONIC TRANSDUCERS

to be increased while retaining image resolution and avoiding the need to augment the transmitted peak pressure amplitude. Such approach minimizes the potential for bioeffects even if the total amount of transmitted energy is enhanced. The goal of this work was to examine the bandwidth and noise immunity requirements of Golay coded waves used to determine frequency dependent attenuation of highly absorbing and scattering biological tissues.The measurements were performed in transmission and in pulse-echo mode using both conventional, narrowband PZT ceramic transducers and wideband PZT composites operating at frequencies of 0.5 MHz, 1MHz and 2 MHz. The measurement system included custom-built Golay code transmitter, receiver electronics and PC based, off-line decoder. In vitro human heel bones with a cortical layer removed were tested and in vivo the measurements were carried out on volunteers′ heels. The results of the measurements obtained with 8 bit and 16 bit Golay codes were compared with the reference ones acquired using 2 cycles tone burst and indicated that coding faithfully retained all information related to frequency dependent attenuation. At the same time, advantageously, the coded transmission doubled the frequency range in which ultrasonic attenuation could be determined. The attenuation data obtained using narrowband and wideband composite transducers were compared and found almost identical. That indicates that the Golay coded signals are very robust, because despite their degradation due to limited bandwidth, they were successfully decompressed without loosing their coding properties. Finally, the desirable noise immunity of the Golay system was verified by performing experiments with signal-to-noise ratio of 0dB. The results indicated that under these conditions the sine-burst excitation mode failed completely while Golay coded signals allowed attenuation properties of the bone to be correctly determined. This work was supported by the KBN grant 4T11E01922