Noninvasive Vascular Imaging

Grayscale Ultrasound

Much of the evaluation of blood vessels with ultrasound can be accomplished with conventional grayscale imaging. A transducer that emits high-frequency sound waves (usually 2-7 MHz) is held to the skin using a coupling gel to eliminate the air gap between the transducer and the skin. The sound waves are reflected to variable degrees by the internal structures. A computer measures the time that it takes for the sound waves to return to the transducer and then creates an image. Sound waves that are reflected by a tissue, such as the wall of a blood vessel, are visible as echoic structures. Sound waves that are transmitted by structures, such as the fluid-filled lumen of a blood vessel, have no echoes (anechoic). Grayscale ultrasound is a powerful tool for defining the morphology of blood vessels, but it has distinct limitations. Air, bone, and metal are so highly reflective that sound waves cannot penetrate to visualize underlying tissues. The two areas of the body where this is most problematic are the chest (air in the lungs) and the head (bone in the skull). Another limitation of grayscale ultrasound is that it does not image blood flow. The presence of a vascular disease may be suspected on the basis of the grayscale appearance of the vessel wall, such as a large echogenic plaque in an artery, visualization of wall pulsatile motion, echogenic material in the vessel lumen, or inability to compress the vessel. Fortunately, a very basic principle of ultrasound, Doppler shift, can be used to indirectly measure the velocity of flow.

Doppler Ultrasound

When a sound wave is reflected from a stationary object, the frequency of the returning wave is the same as that of the initial wave. The frequency of a wave that is reflected from an object moving toward the sound source is higher in proportion to the speed of that object. Conversely, the frequency of a wave reflected from an object moving away from the sound source is lower in proportion to the velocity of the object. This is why the tone of a siren drops noticeably lower as an ambulance drives by. The audible difference between the two frequencies is termed the “Doppler shift.” This same phenomenon can be applied to flowing blood during an ultrasound examination using the sound waves emitted from and reflected back to the transducer.

Most diagnostic ultrasound equipment utilizes a thin beam of pulsed ultrasound (known as pulsed - wave Doppler ) because this allows precise spatial localization of the measured velocity within the tissues. Continuous-wave Doppler (e.g., the small handheld units used to detect arterial pulses) measures all flow within the emitted beam, such that overlapping structures are easily confused.

Review of the Doppler equation helps to understand the strengths and limitations of this technique. The simplified equation is as follows:

where F _r is the frequency of the reflected sound, F _t is the frequency of the transmitted sound, V is the velocity of flow, θ is the angle of the ultrasound beam with respect to the long axis of the vessel lumen, and c is the speed of sound in soft tissues. The velocity of flow is useful for detection of disease, so the equation can be rearranged as:

where F _d is the Doppler shift. Notice that the calculated velocity is directly proportional to the Doppler shift, but inversely proportional to cos θ (the angle of the ultrasound beam to the direction of flow). This last fact explains why the best angles for measurement of velocity are less than 60 degrees. Below a θ of 60 degrees the value of 1/cos θ changes at a relatively leisurely rate. At a θ greater than 60 degrees the changes in the value of 1/cos θ are large with only incremental changes in θ . This leads to magnification of errors during the velocity calculation, rendering the results unreliable.

Velocity of flow is most commonly displayed as a tracing on a scale determined by the operator. By convention, flow toward the transducer is displayed above the baseline, and flow away from the transducer is displayed below. The characteristics of the tracing are determined by the type of vessel, the organ which it supplies, and the presence of disease states. Arterial flow varies with the cardiac cycle, with a rapid rise to peak velocity during systole, and gradual decrease in velocity during diastole. Shortly after flow peaks, the aortic valve closes, creating a small secondary peak in flow termed the dicrotic notch. Blood flow in high-resistance structures, such as the leg muscles, is triphasic with a brief period of retrograde flow during diastole ( Fig. 3-1 ). Blood flow in low-resistance structures, such as visceral organs, remains antegrade throughout diastole ( Fig. 3-2 ). With occlusive disease the velocity of flow in the stenosis rises and the Doppler tracing thickens (termed spectral broadening ) as the range of velocities present increases ( Fig. 3-3 ). As the stenosis progresses in severity, the velocity may decrease and the waveform becomes dampened or disappears altogether.

Blood flow in veins characteristically has a lower velocity than in arteries, with a less pulsatile Doppler waveform ( Fig. 3-4 ). The velocity and pulsation vary depending on the proximity of the vein to the heart, the health of the heart, and the volume status of the patient. Phasic changes in velocity with respiration also occurs, with substantial increases in flow during inspiration and dampened flow during expiration. With forced expiration (the Valsalva maneuver) flow may be completely arrested ( Fig. 3-5 ). In general, the more distant the vein is from the central circulation, the slower and more constant the flow.

Two vascular abnormalities with distinct Doppler signatures that are important to angiographers are arteriovenous fistula (AVF) and arterial pseudoaneurysm (PSA). Both lesions can occur following percutaneous vascular procedures. An AVF is a direct communication between an artery and a vein (see Fig. 1-25 ). The Doppler characteristics are accentuated diastolic flow (i.e., a low-resistance pattern) in the artery proximal to the AVF, a jet of extremely high-velocity flow through the point of communication, and an arterialized waveform in the vein central to the fistula ( Fig. 3-6 ). A PSA is a focal disruption of at least one layer of the arterial wall communicating with a blood-filled space that is usually contained by the adjacent soft tissues (see Fig. 1-9 ). The Doppler characteristics are a jet of high-velocity flow through the neck that may reverse during diastole (“to-and-fro”) with swirling flow in the pseudoaneurysm ( Fig. 3-7 ).