Temporal Shaping of the PWD Pulse
In order to make more accurate measurements of the blood velocity, the temporal pattern of the pulse is typically repeated. This will result in a narrower bandwidth of the transmitted pulse, which can make more accurate velocity measurements. The number of repetitions is determined by Pulse Repeat. If set to 0, the size of the gate will determine the pulse repeat, limited by the Tx Max Pulses (PW).
- Pulse Repeat
- Tx Max Pulses (PW)
The transmit frequency of the PWD pulse is set through:
Since the PWD line is repeatedly scanned at a high PRF, localized heat is deposited into the tissue. In order to meet the safety limits, the amplitude of the pulse should be reduced. The following voltage indexes determine how the amplitude will be adjusted for increased PRFs:
- Voltage- HighPRF
- Voltage- MedPRF
- Voltage- LowPRF
The conversion between the indices and actual voltages can be found here.
Transmit Beam Forming
The focal point of the PW transmit is determined by Tx Focus Distance. If set to 0, the focal point would be at the center of the gate. Either way, the focus distance can be limited to a minimum set by Tx Focus Min Distance.
- Tx Focus Distance
- Tx Focus Min Distance
Since the PWD line is repeatedly scanned at a high PRF, localized heat is deposited into the tissue. In order to meet the safety limits, the aperture used for the transmission is reduced. Tx Aperture Cutoff cuts the aperture for PW transmissions. This is to reduce the acoustic output when the default calculated B-mode transmit aperture is too big for a PW transmission.
- Tx Aperture Cutoff
The beam angle for the PWD scan line can be adjusted through:
Temporal Shaping of the RF Data
The downsampling of the RF data collected from the PWD gate is determined by:
- PW RF Decimation
The analog TGC for PWD RF data collection is determined through a number of parameters. The main TGC curve is specified by TGC (see System Curves). The maximum depth that the TGC curve is used to is determined by Gn Curve Depth. Depths beyond will use the last calculated value of the curve.
- Gn Curve Depth
A digital TGC is also applied to PWD RF data. The TGC curve is specified by TGC Digital (see System Curves). The maximum depth for the TGC Digital curve is the same as the analog TGC
- TGC Digital
The choice between mono (only PWD) and duplex (PWD and B-mode) sequences are made through Mode. A value of 0 only runs a B-mode sequence. A value of 1 runs a PWD mono sequence, and a value of two runs a PWD duplex sequence.
The scan line on which the gate is located is the PWD Line. This line will be repeated in the sequence. The PWD line and the location and span of the gate are determined through:
- PW Line
- Gate Depth
- Gate Size
When using the interrupted PWD duplex sequences, three parameters determine the sequence of PWD and B-mode scan-lines that are programmed. The Frames Before Skip is the number of PWD scan lines acquired before a B-mode line is acquired. The Frames Skipped is the number of PWD scan lines which are skipped to open up time for acquiring B-mode data. The system automatically segments the B-mode image into smaller sectors which would fit in this interval. The ratio of the PWD scan lines which are acquired to the ones which are skipped is determined by the Interrupt Ratio. As can be seen the three parameters are dependent by:
Frames Before Skip = Interrupt Ratio * Frames Skipped.
Although this dependency is not enforced in the user interface, internally, Frames Before Skip gets determined based on the other two parameters as shown above.
- Interrupt Ratio
- Frames Before Skip (Duplex)
- Frames Skipped (Duplex)
The repetition period (and thus frequency) of the the scanning for the PWD scan line is determined by:
The parameters of the wall filter are adjustable. The cutoff frequency, as a percentage of the pulse repetition frequency (one over PRP) is determined by WF. The wall filter is an IIR filter. The order of the wall filter is 8 by default in the mono mode. In the interrupted duplex (and triplex) mode only, the order of the filter is determined by WF Order (Duplex) parameter.
- WF Order (Duplex)
In the duplex (and triplex) mode, because of the missed PWD scan lines, the wall filter should be adjusted. The order is determined by Num Init Samples (Duplex WF) and Frames Before Skip. The wall filter is still an IIR filter. Initial in this context means the maximum order of the filter tried for the filter design. This usually is the filter order, however the design process may end up with a lower order filter. The actual order is determined from:
int numOfPoles = 1 + (Num_Init_Samples_Duplex_WF >= Frames_Before_Skip) ? Frames_Before_Skip - 1 : Num_Init_Samples_Duplex_WF;
- Num Init Samples (Duplex WF)
When the direction of blood flow (long axis of the blood vessel) has an angle with the PWD scan line direction, the measured blood velocity numbers are the projection of the actual velocity along the scan line. A correction factor (1/cos of the angle) can be applied to get the correct velocities displayed.
- Correction Angle
The position of the time axis in the PWD spectrum image can be shifted up and down through Baseline. This also changes the range of speeds displayed. Note that because of the aliasing in the frequency domain, the PWD spectrum image has a wrap around property. The Invert parameter, inverts the velocity axis, so positive and negative velocities are switched.
The time-extent of the PWD spectrum image can be changed through Line Repetition Period (LRP). The area showing this image consists of 629 pixels on my system. If the LRP is set to 10,000 micro seconds, each line in this area represents 10,000 microseconds. The total extent would be 6.29 seconds for my system.
The layout of the screen (relative position of the B-mode and PWD images) is determined by the provisional Screen Layout. This parameter does not do anything in this version of the Exam software. The zoom of the B-mode image can be changed through the Initial Zoom. The tint of the PWD image can be changed through Chroma.
- Screen Layout
- Initial Zoom
- Audio Gain
- Audio Shift
- Audio Mask Num Bit
- Audio Filter Size