Page 38 - Libro vascular I

DOPPLER ULTRASOUND 29
Wall filter 50 Hz
10ms 10ms 10ms 10ms 10ms 10ms
Wall thump
A
B
C
fast Fourier transform (FFT). Figure 3.6C shows a typical spectral display produced by a Doppler system. In this case, each vertical line of data is pro- duced every 5–10ms (i.e., 100–200 lines of data per second).
New line of Doppler spectrum every 5–10 ms
Time Brightness proportional to back-scattered
power of Doppler signal at each frequency
Figure 3.8 Spectral analysis of the Doppler signal enables the frequencies present within the signal to be displayed as consecutive spectra. This produces a display of the changes in blood velocity over time.
Continuous wave (CW) Doppler
Continuous wave (CW) Doppler continuously emits a single frequency while the receiving ele- ment continuously detects any echoes from the sensitive region of the beam (i.e., where transmit- ted and received beams overlap) (shaded region in Fig. 3.5). This region usually covers a depth of a few centimeters, and any flow within this area will be detected. This means that CW Doppler is unable to provide information about the depth from which the Doppler signal is returning. CW Doppler is therefore said to have poor range reso- lution. Veins often lie adjacent to arteries and so, in many cases, the CW Doppler will simultaneously detect arterial and venous flow.
Pulsed Doppler
The poor range resolution of CW Doppler can be overcome by using a pulse of ultrasound energy
Wall filter 75 Hz
Wall filter 550 Hz
A: Wall thump gives a low-frequency, high- amplitude signal. B: A filter can be used to remove wall thump from the Doppler signal, but if the filter cut-off frequency is set too high (as in C), this can alter the appearance of the waveform.
Figure 3.7
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Doppler frequency