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USE OF DOPPLER ULTRASOUND SCANNING
Category: Diagnostic Radiology
Abstract : The use of Doppler ultrasound is an integral part of the examination and should not be considered as a separate entity. Many pathological processes in the abdomen affect the haemodynamics of relevant organs and the judicial use of Doppler is an essential part of the diagnostic procedure. Colour Doppler is used to assess the patency and direction of flow of vessels in the abdomen,to establish the v
The use of Doppler ultrasound is an integral part of the examination and should not be considered as a separate entity. Many pathological processes in the abdomen affect the haemodynamics of relevant organs and the judicial use of Doppler is an essential part of the diagnostic procedure.
Colour Doppler is used to assess the patency and direction of flow of vessels in the abdomen,to establish the vascularity of masses or lesions and to identify vascular disturbances, such as stenoses. Flow information is colour-coded (usually red towards and blue away from the transducer) and superimposed on the image. This
gives the operator an immediate impression of a vascular map of the area. This Doppler information is obtained simultaneously, often from a relatively large area of the image, at the expense of the grey-scale image quality. The extra time taken to obtain the Doppler information for each line results in a reduction in frame rate and line density which worsens as the colour Doppler area is enlarged. It is advisable, therefore, to use a compact colour box in order to maintain image quality.
Power Doppler also superimposes Doppler information on the grey-scale image, but without any directional information. It displays only the amount of energy. The advantage of this is that the signal is stronger, allowing identification of smaller vessels with lower velocity flow than colour Doppler. As it is less angledependent than colour Doppler it is particularly useful for vessels which run perpendicular to the beam, for example the inferior vena cava (IVC).
Pulsed Doppler uses pulses of Doppler from individual elements or small groups of elements within the array. This allows the operator to select a specific vessel, which has been identified on the grey-scale or colour Doppler image, from which to obtain a spectrum. This gives further information regarding the flow envelope, variance, velocity and downstream resistance of the blood flow.
Getting the best out of Doppler : Familiarity with the Doppler controls is essential in order to avoid the pitfalls and increase confidence in the results. It is relatively straighforward to demonstrate flow in major vessels and to assess the relevant spectral waveform; most problems arise when trying to diagnose the lack of flow in a suspected thrombosed vessel, and in displaying low-velocity flow in difficult-to-access vessels. Doppler is known to produce false-positive results for vessel occlusion and the operator must avoid the pitfalls and should ensure that the confidence levels are as high as possible.
Steps to take if you cant detect flow with Doppler :
● Ensure the angle of insonation between the vessel and the transducer is <60°. Colour and pulsed Doppler are highly angle-dependent.
● Ensure the Doppler gain is set at the correct level. (Colour and pulsed Doppler gain settings should be just below background noise level.)
● Ensure the Doppler power/output setting is sufficient.
● Ensure the pulse repetition frequency (PRF) is set correctly. A low PRF (‘range or scale setting) is required to pick up low-velocity flow.
● Ensure the wall thump filter setting is low. (If the setting is too high, real low-velocity flow is filtered out.)
● Use power Doppler, which is more sensitive and is not angle-dependent.
● Know the limitations of your machine. Machines differ in their ability to detect lowvelocity flow.
● If in doubt, test it on a reference vessel you know should contain flow.
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