Here the strongly reflecting pleura (bright reflection) mirrors the subclavian artery (A), producing the impression of a second artery (“ghost vessel”) posterior to the true artery. Mirroring can be prevented by oblique interrogation of the mirroring structure (longitudinal (left) and transverse (right) scans of subclavian artery)
The pulse repetition frequency (PRF) limits the maximum Doppler shift frequency for which precise determination of the direction and amplitude is possible. The PRF is the rate at which ultrasound pulses are emitted from the transducer. In pulsed Doppler (conventional and color duplex), only echoes reflected froma predefined depth are accepted and processed (determination of mean flow velocity or FFT analysis) in the interval between transmissions. In color duplex ultrasound this is accomplished by using multiple gates to which the returning Doppler shift frequency packets are assigned. The relatively low Doppler shift frequencies (some kHz with a period of about 1 ms) are extracted from short, successive ultrasound pulses less than 1 μs in duration. A minimum number of ultrasound pulses is necessary for correct frequency determination. The Nyquist limit states that only Doppler shifts below half the pulse repetition frequency can be unambiguously processed in terms of direction and flow velocity.Doppler shifts exceeding the Nyquist limit lead to aliasing. Depending on the degree of aliasing, the high frequencies “wrap around” and are displayed on the opposite side of the baseline, so that blood appears to be flowing in the wrong direction. Under these conditions, the color duplex image likewise shows an apparent flow reversal by a change in the color coding. In aliasing, the transition to the opposite color progresses from increasingly lighter shades to yellow, followed by a light shade of the opposite color (e.g. from light red, through yellow, to light blue). In contrast, true flow reversal is characterized by a color change froma dark shade of the initial color (or black for a transient period during which no flow information is obtained) to the opposite color. Aliasing ,can be prevented by a higher pulse repetition frequency or various other options such as repositioning of the baseline, reduction of the transmit frequency, and interrogation of the vessel at a larger angle. The lower the transmit frequency, the lower the Doppler shift frequency. The latter can thus be reduced to below half the pulse repetition frequency by lowering the transmit frequency. Larger angles between the ultrasound beam and the blood vessel will also decrease the Doppler shift frequency however, the extent to which this can be done is limited by the fact that errors in velocity measurement will increase unacceptably at angles above 70°. Shifting of the baseline enables doubling of the Doppler frequencies that can be displayed above or below the line, depending on the direction of the shift. Enlarging the positive frequency range in this way automatically reduces the negative frequency range by the same amount and vice versa. Another parameter is the scanning depth, but this is difficult to manipulate. The greater the penetration depth, the longer the pulse delay. As the echo arrival time increases, the pulse repetition frequency must be reduced as successive pulses can only be emitted after all reflected echoes of the preceding pulse have been received. Mathematically, the Nyquist limit is expressed in the following equation:
Fmax = 12PRF
The maximum flow velocity that can be measured is then calculated using the Doppler equation:
Vmax = c4 · T · F0 · cos
The maximum flow velocity is thus inversely proportional toecho arrival time T, or to scan depth and transmit frequency F0
The options available to overcome these limitations can be summarized as follows:
Use of a higher pulse repetition frequency in order to register higher Doppler shift frequencies, but this will reduce the scan depth
Use of a lower transmit frequency, but this will reduce the spatial resolution of the gray-scale scan. Shifting of the baseline, both in the spectral display and on the color scale, by which the frequency range in one direction can be doubled while flow in the opposite direction is no longer depicted. Some ultrasound machines have a high PRF option for the detection of faster flow. The higher frequency is achieved by emitting ultrasound pulses before the preceding signals have been processed. Processing is done using additional Doppler lines. However, this approach is associated with some unreliability in terms of spatial resolution as Doppler information from a larger number of gates must be processed to generate the spectral display. CW Doppler using separate crystals for continuously transmitting and receiving signals is not limited by an upper Doppler shift threshold. This technique does not allow local differentiation of the reflected signals since all frequencies reflected back from along the beam path are processed to generate the Doppler waveform.
Examination of the politeal artery with the pulse repetition frequency set too low. There is aliasing because the Doppler shift frequencies are above the Nyquist limit. Aliasing is seen in the color duplex image as a color change from light red, through yellow, to blue. In the Doppler waveform, the peak velocity is cut off and these signals are displayed below the baseline. Aliasing can be prevented by repositioning the baseline and increasing the PRF Aliasing Nyquist limit (upper frequency threshold) Baseline and Nyquist limit
Pulse repetition frequency (PRF)
In color duplex scanning, a positive Doppler shift is displayed in red and a negative shift in blue with lighter shades indicating higher flow velocities. When the upper velocity threshold set on the color scale is exceeded, aliasing will occur and the faster flow is depicted in the color of the opposite flow direction. In aliasing, the color change from red to blue or vice verse progresses through a lighter shade (faster flow) and a yellow to white zone of transition. In contrast, true flow reversal is characterized by a dark transition zone. Absence of color (black) may be due to a short cessation of flow or change in flow direction relative to the transducer (curved-array transducer) with failure to detect flow at a Doppler angle of 90°
The pulse repetition frequency (PRF) limits the maximum Doppler shift frequency for which precise determination of the direction and amplitude is possible. The PRF is the rate at which ultrasound pulses are emitted from the transducer. In pulsed Doppler (conventional and color duplex), only echoes reflected froma predefined depth are accepted and processed (determination of mean flow velocity or FFT analysis) in the interval between transmissions. In color duplex ultrasound this is accomplished by using multiple gates to which the returning Doppler shift frequency packets are assigned. The relatively low Doppler shift frequencies (some kHz with a period of about 1 ms) are extracted from short, successive ultrasound pulses less than 1 μs in duration. A minimum number of ultrasound pulses is necessary for correct frequency determination. The Nyquist limit states that only Doppler shifts below half the pulse repetition frequency can be unambiguously processed in terms of direction and flow velocity.Doppler shifts exceeding the Nyquist limit lead to aliasing. Depending on the degree of aliasing, the high frequencies “wrap around” and are displayed on the opposite side of the baseline, so that blood appears to be flowing in the wrong direction. Under these conditions, the color duplex image likewise shows an apparent flow reversal by a change in the color coding. In aliasing, the transition to the opposite color progresses from increasingly lighter shades to yellow, followed by a light shade of the opposite color (e.g. from light red, through yellow, to light blue). In contrast, true flow reversal is characterized by a color change froma dark shade of the initial color (or black for a transient period during which no flow information is obtained) to the opposite color. Aliasing ,can be prevented by a higher pulse repetition frequency or various other options such as repositioning of the baseline, reduction of the transmit frequency, and interrogation of the vessel at a larger angle. The lower the transmit frequency, the lower the Doppler shift frequency. The latter can thus be reduced to below half the pulse repetition frequency by lowering the transmit frequency. Larger angles between the ultrasound beam and the blood vessel will also decrease the Doppler shift frequency however, the extent to which this can be done is limited by the fact that errors in velocity measurement will increase unacceptably at angles above 70°. Shifting of the baseline enables doubling of the Doppler frequencies that can be displayed above or below the line, depending on the direction of the shift. Enlarging the positive frequency range in this way automatically reduces the negative frequency range by the same amount and vice versa. Another parameter is the scanning depth, but this is difficult to manipulate. The greater the penetration depth, the longer the pulse delay. As the echo arrival time increases, the pulse repetition frequency must be reduced as successive pulses can only be emitted after all reflected echoes of the preceding pulse have been received. Mathematically, the Nyquist limit is expressed in the following equation:
Fmax = 12PRF
The maximum flow velocity that can be measured is then calculated using the Doppler equation:
Vmax = c4 · T · F0 · cos
The maximum flow velocity is thus inversely proportional toecho arrival time T, or to scan depth and transmit frequency F0
The options available to overcome these limitations can be summarized as follows:
Use of a higher pulse repetition frequency in order to register higher Doppler shift frequencies, but this will reduce the scan depth
Use of a lower transmit frequency, but this will reduce the spatial resolution of the gray-scale scan. Shifting of the baseline, both in the spectral display and on the color scale, by which the frequency range in one direction can be doubled while flow in the opposite direction is no longer depicted. Some ultrasound machines have a high PRF option for the detection of faster flow. The higher frequency is achieved by emitting ultrasound pulses before the preceding signals have been processed. Processing is done using additional Doppler lines. However, this approach is associated with some unreliability in terms of spatial resolution as Doppler information from a larger number of gates must be processed to generate the spectral display. CW Doppler using separate crystals for continuously transmitting and receiving signals is not limited by an upper Doppler shift threshold. This technique does not allow local differentiation of the reflected signals since all frequencies reflected back from along the beam path are processed to generate the Doppler waveform.
Examination of the politeal artery with the pulse repetition frequency set too low. There is aliasing because the Doppler shift frequencies are above the Nyquist limit. Aliasing is seen in the color duplex image as a color change from light red, through yellow, to blue. In the Doppler waveform, the peak velocity is cut off and these signals are displayed below the baseline. Aliasing can be prevented by repositioning the baseline and increasing the PRF Aliasing Nyquist limit (upper frequency threshold) Baseline and Nyquist limit
Pulse repetition frequency (PRF)
In color duplex scanning, a positive Doppler shift is displayed in red and a negative shift in blue with lighter shades indicating higher flow velocities. When the upper velocity threshold set on the color scale is exceeded, aliasing will occur and the faster flow is depicted in the color of the opposite flow direction. In aliasing, the color change from red to blue or vice verse progresses through a lighter shade (faster flow) and a yellow to white zone of transition. In contrast, true flow reversal is characterized by a dark transition zone. Absence of color (black) may be due to a short cessation of flow or change in flow direction relative to the transducer (curved-array transducer) with failure to detect flow at a Doppler angle of 90°
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