Major pitfalls in Doppler investigations. Part II. Low flow velocities and colour Doppler applications

Many pitfalls result from the limited ability of Doppler instruments to record low flow velocities. These include a misleading resistance or pulsatility index due to diastolic cut-off and taking no signal to equal no flow assuming that no signal means no flow. Comparison of actual flow velocities as measured in an in-vitro system (range: 0.8 to 3.4 cm/s) with the lowest recordable spectral or colour signals in 3 Duplexscanners showed that reduced sensitivity to low flow velocities is not only dependent on the high pass ("wall") filter setting, Doppler frequency or angle of incidence, but also on factors such as vessel diameter, impairing the signal to noise ratio. Characteristic errors of colour flow mapping (misleading vascular anatomy, imitation of pathological findings, erroneous exclusion of flow) are due to partial volume effect, limited temporal and velocity resolution, changing angle of incidence, aliasing and failure to detect low flow velocities.