抄録
As Ultrasonic Doppler signal is very low in amplitude, it is necessary that signal-to-noise ratio of the Doppler signal be improved by use of transmission and reception circuit and transducer with higher sensitivity. Therefore, the transmission and reception system with single crystal was improved by, the authors. The principle of this system is as follows.
A bridge circuit, one element of which consists of the transducer with single crystal, is used for connecting three blocks, i. e., transmission circuit, transducer and reception circuit, and balanced against the transmitting signal, so that ultrasound can be simultaneously transmitted and received by a single crystal.
In actual use, however, the impedance of the transducer may fluctuate to unbalance the bridge circuit according to the change of acoustic load. As a result, the transmitting signal is fed to the input terminal of the receiving circuit, and the receiving amplifier may be saturated. Such an impedance fluctuation must be suppressed. Therefore, the following arrangement of transducer was newly employed. An acoustic coupler is set in between the crystal and acoustic load (skin, blood vessel, etc.), not including two surfaces parallel with each other in the pathway of ultrasound in order to prevent the standing wave. Therefore, the crystal is constantly loaded by the coupler, and the impedance of the transducer can become free from the load variation.
As compared with the transmission and reception system with dual crystal, this system was found to have improved the signal-to-noise ratio by 10 dB.
