Page 46 - Libro vascular I

Chap-04.qxd 29~8~04 13:21 Page 37
CREATION OF A COLOR FLOW IMAGE
37
appear at different places on the clock face. The time delay between the two clocks will be 30 min, or alternatively this delay could be measured as a phase shift, which in this case would be half a cycle of the clock face.
The delay between the returning ultrasound signals from the first and second pulses shown in Figure 4.3 can be measured in terms of a phase shift. We can see that the second signal is the same shape as the first but is delayed. This is analogous to the clock hands travelling at the same speed but with the second clock being half a cycle behind the first. A similar phase shift can be measured between signals 2 and 3. These phase shifts can be used to estimate the velocity of the blood. This method does not actually measure the Doppler shift frequency; however, the shape of the detected signal is similar to the Doppler shift that would be obtained from a continuous wave system and there- fore can be described by the Doppler equation.
Modern color flow imaging scanners use the phase shift approach, employing a process known as autocorrelation detection to estimate the mean Doppler shift frequency. Autocorrelation compares two consecutive pulses returning from a given sam- ple volume to produce an output that is dependent on the phase shift (i.e., dependent on the Doppler frequency). If the echoes are returning from sta- tionary objects there will be no phase shift. The phase shifts between four or more pulses are used to estimate the frequency. The more pulses used, the more accurate is the result, as long as the time taken is not so great that the velocity of the blood cells has changed. This relatively small number of pulses required to estimate the mean relative velocity (Doppler shift frequency) enables several color images to be produced every second.
Another method, not currently used by ultra- sound manufacturers, uses time-domain processing, which employs time delay rather than phase shift to estimate the velocity of blood. From the opera- tor’s perspective, both the frequency and time- domain processes produce similar color images.
ELEMENTS OF A COLOR FLOW
SCANNER
Figure 4.4 shows the basic elements of a color flow scanner. Before any analysis of the returning
Pulse 1
Pulse 2
Pulse 3
Figure 4.3 Signals from three consecutive pulses returning from a given group of cells as they move farther away from the transducer. Note the short delay introduced. (After Ferrara & DeAngelis 1997, with permission.)
the group of cells does not change significantly during the short time over which the blood flow velocity is being estimated. Figure 4.3 shows the shapes of three consecutive returning pulses as a group of blood cells passes through the sample vol- ume. A short delay is introduced into the signal returning from a given group of cells as they move farther away from the transducer between pulses. An estimate of the velocity can be made by detec- ting this delay in the echo complex. This shift in the waveform can be understood in two ways. First, it can be considered as a time delay introduced between returning pulses as the group of cells moves. Second, it can be considered as a phase shift of the signal between two pulses scattered from the same sample volume. To understand the concept of a phase shift, consider an example using two clocks. If both the clocks are set to read the same time, both minute hands will be moving at the same speed, giving a frequency of one complete revolu- tion of the clock face an hour. Both clocks will also be in phase with each other (i.e., the minute hand will appear at the ‘12’ on both clocks at the same time), giving a phase shift of zero. If, however, one clock has the minute hand set 30 min behind the other, the minute hands will still complete one rev- olution of the clock face an hour, but the two clocks will now be out of phase and the clock hands will