Ultrasonic colour Doppler imaging

Abstract

Ultrasonic colour Doppler is an imaging technique that combines anatomical information derived using ultrasonic pulse-echo techniques with velocity information derived using ultrasonic Doppler techniques to generate colour-coded maps of tissue velocity superimposed on grey-scale images of tissue anatomy. The most common use of the technique is to image the movement of blood through the heart, arteries and veins, but it may also be used to image the motion of solid tissues such as the heart walls. Colour Doppler imaging is now provided on almost all commercial ultrasound machines, and has been found to be of great value in assessing blood flow in many clinical conditions. Although the method for obtaining the velocity information is in many ways similar to the method for obtaining the anatomical information, it is technically more demanding for a number of reasons. It also has a number of weaknesses, perhaps the greatest being that in conventional systems, the velocities measured and thus displayed are the components of the flow velocity directly towards or away from the transducer, while ideally the method would give information about the magnitude and direction of the three-dimensional flow vectors. This review briefly introduces the principles behind colour Doppler imaging and describes some clinical applications. It then describes the basic components of conventional colour Doppler systems and the methods used to derive velocity information from the ultrasound signal. Next, a number of new techniques that seek to overcome the vector problem mentioned above are described. Finally, some examples of vector velocity images are presented.

Keywords: clinical applications; imaging; ultrasonic colour Doppler.

Figure 1.

Pulse-echo image of the superficial femoral artery (SFA) and the superficial femoral vein (SFV) in the thigh of a healthy subject. The scan direction is vertical to generate the best view of the arterial walls. The scale to the right and the top of the image is calibrated in centimetres.

Figure 2.

Colour flow image obtained by superimposing Doppler information on the pulse-echo image. The subject's head is to the left of the scan, and so the arterial flow is from left to right and the venous flow from right to left. The colour scale on the left of the image is calibrated in cm s⁻¹, and shows that flow towards the probe is coloured as red-orange-yellow, while flow away from the probe is coloured in shades of blue. Note that the area of the colour box indicating the region from which velocity information is extracted occupies only part of the image. Note also that the angle of the colour box is inclined at an angle to the vertical to ensure that the Doppler angle is different from 90°.

Figure 3.

General layout of the receive Doppler path for a colour flow imaging system (RF, radio frequency; TGC, time gain compensation; I, in-phase components of each signal; Q, quadrature components of each signal).

Figure 4.

Signal obtained from a single moving scatterer crossing a beam from a concave transducer (from Jensen [29]). Here, the measurement time t_m is indicated on the left panel as the dotted line.

Figure 5.

Position of a rotating signal vector during two successive samples i − 1 and i, showing the in-phase components I(i − 1) and I(i) and quadrature components Q(i − 1) and Q(i).

Figure 6.

Vector velocity image of the carotid artery bifurcation shortly (a) before and (b) after peak systole (from Udesen et al. [2]). The arrows indicate both direction and magnitude of the velocity and colour intensity indicates velocity magnitude.

Figure 7.

Simple unidirectional flow in the jugular vein (top) and the carotid artery (bottom) (modified figure from Hansen et al. [68]).

Figure 8.

The velocity vector are encoded as both arrows and in a colour scale where the brightness indicates magnitude and the colour direction.

Figure 9.

Longitudinal scan of the femoral vein with disturbed flow at the passage of a venous valve. A vortex is formed in the pocket behind the valve (modified figure from Hansen et al. [68]).

Figure 10.

Secondary flow in the abdominal aorta (modified figure from Hansen et al. [68]).