抄録
M-sequence method and pulsed method giving range resolution to Doppler method are being applied to the measurement of blood velocity profile in a human heart. In these methods, however, the product of maximum Doppler frequency and measurable range is limited to less than half the sound velocity in the medium. To remove this limitation, two methods named double M-sequence Doppler method and M-sequence pulsed Doppler method were devised. In experiments with a model target, Doppler signals of reflected wave from desirable distance were successfully obtained, separating these signals from signals of targets at undesirable distances. An elementary ultrasonic Doppler system with range resolution is shown in Fig. 1. The carrier signal generated by an oscillator is modulated with a pseudo-random signal, and applied to a transmitter. The received signal from reflective bodies is demodulated with a delayed pseudo-random signal. After filtering the output of the demodulator, a Doppler signal is detected. An M-sequence signal shown in Fig. 2 can be used as a pseudo-random signal. Owing to the power spectrum of this signal, shown in Fig. 3, the product of maximum Doppler frequency and measurable range is restricted by the relation (5). In case of pulsed Doppler method, the same relation holds. When this limitation is disregarded, undesirable signals will be dominant. In order to overcome this difficulty, two methods are devised. These methods reduce undesired signals to a level enabling measurement of the Doppler velocity of the desired object. One is the double M-sequence Doppler method with the signal shown in Fig. 4(c). This method expels a great part of the undesired signal power from the measuring Doppler frequency band (Fig. 5). The other is the M-sequence pulsed Doppler method with the signal shown in Fig. 8(b). In this method, a gate does not let great undesired signals into the receiver. For the test of these methods, a steel ball of 10mm in diameter was moved back and forth from 6. 5cm to 10. 5cm in front of the oscillator surface. The Doppler signal from the ball was analysed. Fig. 9(CW) shows the Doppler frequency in case that a continuous sinusoidal wave is transmitted. With the double M-sequence method, the Doppler signal at the range of 9cm is obtained intensively as shown in Fig. 9(DM1). Fig. 9(MP1) and (MP2) show the signal from the ball at 9cm, and the signals from other ranges disappear or are dispersed.
