Comparisons between PW Doppler system and enhanced FM Doppler system

This paper presents a new implementation of an formance, relative to the FM Doppler system reported echo-ranging FM Doppler system with improved perpreviously. The use of long sweeps provides a significant reduction in peak to average power ratio compared to pulsed wave (PW) emission. A PW Doppler system exploits the direct relationship between arrival time of the received signal and range from the transducer. In the FM Doppler systems, a similar relationship exists in the .spectral domain of the demodulated received signals, so that range is represented by frequency. Thus, a shfl in location of moving scatferers between consecutive emissignature. The improvement relative to the earlier version sions corresponds to a frequency shifi in the spectral of the FM Doppler system is attained by utilizing cross-correlation ofreal spectra rather than of magnitude spectra for assessing flow velocity. This approach requires a priori knowledge of the envelope of the received sweep from a point scatterer. 1 . INTRODUCTION The non-invasive assessment of blood flow with PW Doppler ultrasound is today a standard technique in power of the emitted PW signal, but at the same time hospitals and clinics. However, to reduce the high peak maintain a wide bandwidth, Doppler systems emitting coherent, repetitive frequency modulated (FM) signals have been de~eloped.".~' This paper presents an improved version of the FM frequency shifi measurement (FM-fsm) Doppler technique"' which in several respects is analotechnique. Simultaneous emission and reception is gous to the PW time shift measurement,"' PW-ism, generally utilized in the FM Doppler system, thus requiring a dual transducer system. 2 . EMITTED AND RECEIVED SIGNAL For a given measurement of a velocity profile, the Gaussian envelope, denoted n = 10; N l ] . The time entire emitted signal consists of N sweep signals with a interval between consecutive sweeps is T,. Each sweep signal has the form over the interval 0 < t < t,. The parameter I, is the length of the signal (typical values ranges from 50 ps to 2M) ps), a is a form factor for the Gaussian envelope (normally a = 31.h is the start frequency at t = 0 and S, is the sweep rate in Hds. If the stop frequency at t = t, is called h, then S , = (T,-f,)it,. In this paper, the emitting and receiving transducers are assumed ideal, except for a into ( I ) by the Gaussian envelope. bandlimited frequency response that is modelled directly Assuming that the received sweep from a scatterer moving axially away from the transducer with speed v and located at range d"' at t = 0 and n = 0 can be written as an amplitude modulated, time delayed and Doppler compressed replica of the emitted signal, the nth received signal can be written as