Basics for performing a high-quality color Doppler sonography of the vascular access

Abstract

In the last years, the systematic use of ultrasound mapping of the upper limb vascular network before the arteriovenous fistula (AVF) implantation, access maturation, and clinical management of late complications is widespread and expanding. Therefore, a good knowledge of theoretical outlines, instrumentation, and operative settings is undoubtedly required for a thorough examination. In this review, the essential Doppler parameters, B-Mode setting, and Doppler applications are considered. Basic concepts on the Doppler shift equation, angle correction, settings on pulse repetition frequency, operative Doppler frequency, gain are reported to ensure adequate and correct sampling of blood flow velocity. A brief analysis of the Doppler inherent artefacts (as random noise, blooming, aliasing, and motion artefacts) and the adjustment setting to minimize or eliminate the confounding artefacts are also considered. Doppler aliasing occurs when the pulse repetition frequency is set too low. This artefact is particularly frequent in vascular access sampling due to the high velocities range registered in the fistula's different segments. Aliasing should be recognized because its correction is crucial to analyse the Doppler signals correctly. Recent advances in instrumentation are also considered about a potential purchase of a portable ultrasound machine or a top-of-line, high-end, or mid-range ultrasound system. Last, the pulse wave Doppler setting for vascular access B-Mode and Doppler assessment is summarized.

Keywords: B-mode ultrasound; Doppler parameters; Doppler/B-mode setting; arterio-venous fistula; pulse wave Doppler.

Figure 1.

Piezoelectric effect. A piezoelectric transducer generates a pulsed ultrasound beam that is launched into the soft tissues (a). Echo signals received by the probe are converted into a continuous radiofrequency signal analyzed by the receiver and displayed on the screen in different image modalities (B-Mode, color, and spectral Doppler) (b).

Figure 2.

B-Mode image of native arteriovenous fistula. Side-to-end well-functioning radiocephalic fistula at the wrist. (*) Anastomosis (a). Side-to-end radiocephalic AVF in a diabetic patient. Maturation of vascular access as well as the remodeling and dilation of the radial artery (Ra), anastomotic chamber (*), and draining vein are insufficient (b). Atherosclerotic and calcified radial artery in elderly diabetic patient with inflow stenosis. Brachial artery Q_a in this patient was 180 ml/min (c). Coarse wall calcifications with shadowing (white arrows) into the anastomosis (*) of a well-functioning long-term radiocephalic fistula (d).

Figure 3.

B-Mode image of an arteriovenous graft. Typical grey scale image of a linear radial-cephalic antecubital linear graft (double-track sign) (a). Loop graft are usually placed within the forearm and anastomosed to the brachial artery and the basilic or brachial vein. Linear bridge in PTFE between antecubital radial artery and basilic vein after the thrombosis of vein draining proximal radio-cephalic fistula (*anastomotic chamber) (b). Intraluminal thrombotic material downstream of the venous anastomosis between prosthesis (*) and native cephalic vein (c).

Figure 4.

Doppler effect defines the variation of frequency observed when a US beam from a fixed source strikes red blood cells (RBCs) moving in the vessels towards or away from the transducer. Doppler shift (ΔF) is proportional to operative frequency (F₀), blood flow velocity and cosine function (Θ) of the angle between ultrasound beam and blood flow direction. Thus it is inversely proportional to the pulse velocity propagation through biological tissues. Doppler-Mode software can solve and represent this equation for the velocity-factor.

Figure 5.

Spectral representation of flow-velocities. Placing a volume sample in the vessel lumen as in the tributary artery of a radio cephalic AVF (a) or brachial artery (b) we can record a spectral Doppler trace. Velocities of RBCs, crossing the gate, are plotted versus time and represented as a velocity over time curve. The “y axis” represents velocity (m/sec) and “x axis” the time. Spectral curve provides a measurement of maximal and mean velocity enveloping the curve and integrating the velocities below the curve. Above the “zero line” the velocities are towards the transducer while below the “zero line” are away from the transducer.

Figure 6.

Pulse-echo pulsing. Pulsing characteristics for piezoelectric transducer are similar in all pulse-echo systems. The pulse duration (PD) is the fraction of time when the transducer actively transmits ultrasound. The PD is usually less than 1 μs to generate a broadband pulse and enhance spatial resolution. On the other hand, pulse repetition period (PRP) is the transducer’s receiving time and it is much greater than PD. Pulse repetition frequency (PRF) defines the frequency of acoustic pulses transmitted per second (KHz/s) and it is equal to inverse value of PRP (1/PRP).

Figure 7.

Color Doppler imaging. Color Doppler imaging depicts inreal-time the flow-velocity changes in the vessels. Red and blue hues represent flow moving towards or away from the transducer. Desaturation of red or blue is an indirect measure of the velocity variations related to Doppler angle varying from 90° to 0° and 90° to 180°. Higher flow rates appear whiter than lower flow rates.

Figure 8.

Color Doppler imaging. CD data are evaluated in a region of interest superimposed to the B-Mode field (color-box or color field). Stationary echoes in the color box are represented as brightness points as in B-mode field.

Figure 9.

Spectral inflections in V/t curve. Diagrams illustrate how normal waveforms can be characterized based on the number and phase inflections. The first is a triphasic trace with a reversal flow. It represents the high resistance flow rate of peripheral arteries at rest. The second is a low resistance monophasic trace such as in kidney, liver, brain; the third is a monophasic pulsatile flow such as in superior mesenteric artery. The diagram called “to-and-from” flow can be registered at the neck of a pseudoaneurysm. Artery reversal flow is the typical trace of the renal artery in a transplant with renal vein thrombosis; the last one is the hepatic veins trace in chronic heart failure.

Figure 10.

Doppler criteria of critical stenosis. Vein stenosis in a graft. The draining vein is reduced in diameter at the anastomosis level and shows an increase of PSV (>450 cm/s), spectral dispersion, aliasing and oversaturation at CD sampling. The Doppler angle and color box alignment are perfect.

Figure 11.

Brachial flow rate calculation in AVF. (a) High-flow rate in brachial-cephalic fistula. Q_b = 1770 ml/min; brachial artery diameter 6.2 mm; TAV_m 96.6 cm/s; RI 0.29; S/D ratio is 1.4. (b) High-flow rate in proximal brachial-cephalic fistula. Q_b = 2224 ml/min; brachial artery diameter 6.3 mm; TAV_m 118 cm/s; S/D ratio 2.1. (c) Normal flow-rate in distal brachial-cephalic fistula with multiple no significant draining vein stenoses. Q_b = 511 ml/min; brachial artery diameter 5.0 mm; TAV_m 43,1 cm/s; TAMV 68,9 cm/s; RI 0.59; S/D ratio is 2.4. (d) Low flow rate indistalbrachial-cephalic fistula with stenosis of outflow. Q_b = 259 ml/min; brachial artery diameter 4.5 mm; TAV_m 24.9 cm/s; RI 0.72; S/D ratio is 3.5.

Figure 12.

Aliasing. Aliasing is the major Doppler artifact. It occurs when transducer PRF is less than twice the frequency of Doppler shift. In other words, the PRF setting must be great enough to sample at least two times for each cycle the Doppler signal. If the PRF is less than twice the maximum Doppler shift, the aliasing will occur. PRF value equal to 2fD is known as the Nyquist limit on the basis of the Shannon theorem. In the upper part of figure, a sampling of two sine waves is represented. The black curve is sampled at discrete times indicated by arrows. The dotted red lines on the bottom are the resultant sample signal. The sampling is adequate on the right side because the sampling rate is over the Nyquist limit. On the left side, the sampling rate is below this limit and the frequency of the resultant is an alias of the real signal. CD image represents critical stenosis of the cephalic arch at the level of infra-clavicular Mohrenheim’s fossa. Aliasing of spectral analysis appears as an abrupt cut and inversion of waveform peak. The table lists the useful procedures to correct the aliasing.

Figure 13.

Color Doppler aliasing. When sampling-rate is outside Nyquist limit, color Doppler aliasing appears as an altered progression of chromatic scale.