This project investigates the applicability of coherent, linearly frequency modulated (FM) excitation signals for blood velocity measurements. Systems based on such excitation signals are called FM Doppler systems. By utilizing FM signals, it is possible to avoid the high peak energy emitted with conventional pulsed wave Doppler systems. In an echo-ranging system, everything else equal, the range resolution size is inversely proportional to the bandwidth of the emitted signal. Thus, if the range resolution is to be improved in a PW Doppler system, the time duration of the emitted burst must be reduced. On the other hand, if the signal to noise ratio is to remain unchanged, the mean emitted power must also be kept constant, which requires increased peak emitted power. However, from a design point of view, a high peak to average power is undesirable, as the duty cycle diminish and non-linear propagation may occur. In addition, especially for medical applications, the peak power level might in certain situations exceed the regulatory limits for diagnostic ultrasound equipment. We have shown that the signal processing principles, utilized with Pulsed Wave - time shift measurement Doppler (PW-tsm), can be used with FM excitation signals as well. The FM counterpart to the PW-tsm technique is spectral cross-correlation, and such a system is called Frequency Modulation - frequency shift measurement, FM-fsm. Experimental data, recorded at Worcester Polytechnic Institute, has been processed with an improved version of the fsm signal processing technique. The promising results show that the FM technique has a precision comparable to that of the PW technique.
|Effective start/end date||01/05/1991 → 01/01/9999|