Duplex ultrasound is a specialized interpretation of ultrasound waves and an integral tool in medical diagnosis and therapy today. Duplex ultrasonography combines the principles of anatomic and flow ultrasonography to deliver diagnostic information to the interpreter. Doppler ultrasonography refers to the utilization and application of the Doppler effect to sound wave information to interpret movement or flow within tissues. It is important to have a basic understanding of the technology and related physical principles to read and interpret duplex ultrasonography. These principles include the Doppler effect, electronic gating, and varying methods of wave generation.
The Doppler effect refers to the change in observed sound wave frequencies due to motion. In the case of duplex ultrasound, the source of the sound waves and the measurement of the change are contained within the transducer probe. The transducer contains piezoelectric crystals which convert electrical activity to the ultrasound waves and vice versa. The probe measures the frequency shift due to reflections off the underlying tissues. This Doppler shift is calculated as:
Equation 1: f(d) = f(t) - f(r) = f(t) * 2 * [u*cos(theta) / c]
Where f(d) is the Doppler shift, f(t) is the transmitted frequency, f(r) is the returning frequency, c is the speed of the ultrasound wave and u*cos(theta) is the velocity component of the reflection in the direction of the ultrasonic beam with angle theta measured between the line of movement of the reflector and the transducer beam. This equation explains why Doppler images degrade at an angle greater than 70 degrees as the cos(theta) approaches 0 at 90 degrees. If the exact angle is known, the Doppler output could be translated directly into velocities, but as this is often, unknown outputs remain as Doppler shift. This shift is interpreted via a frequency analyzer into audible or visual portrayals.
Electronic gating is an important aspect of ultrasonography. All duplex and Doppler ultrasounds are equipped with predetermined gating which governs the depth at which the data is interpreted. This allows for increased and decreased penetration which can be adjusted as needed for anatomic reasons or clarity.
A variety of methods for wave generation exist. These include continuous wave, pulsed wave, high repetition frequency, color, and power. As the name indicates, continuous wave ultrasound is a continuous cyclic generation of waves. As the waves disperse and encounter moving structures, they undergo a shift which returns to the detectors. Objects moving towards the transducer result in a decreased frequency while those moving away from an increased frequency. These are then translated into visual images with red traditionally representing movement towards the transducer and blue representing a movement away. When Doppler shifts become, high reconstruction can be inaccurate and flow directions can become reversed. This phenomenon is known as aliasing artifact, or state of ambiguity, and is governed by the Nyquist limit which states that ambiguity will occur if the Doppler shift is greater than twice the sampling frequency. Pulsed wave ultrasound increased the maximum velocity measurable by minimizing overlap between echo trains. While it employs similar principles to continuous wave ultrasound, the sound waves are generated in a regular interval with pauses. In pulse-wave systems, the maximum velocity measurable by the instrument is determined by the pulse repetition frequency (PRF). Therefore the maximum accurate velocity, V(m), is calculated by:
Equation 2: V(m) = c^2 / [8*R*f(t)]
R is the range or distance from the transducer. This is further increased by the introduction of high pulse repetition frequency which utilizes pulse waves at two to five different ultrasound bursts to increase the sampling frequency. Other Doppler modalities include Color Doppler imaging and Power Doppler imaging. In Color Doppler imaging, the flow rates and direction of flow are depicted as a mean Doppler shift. This method is heavily dependent on the angle of the beam in relation to the vessel and thus open to significant error. Conversely, Power Doppler is influenced very little by the angle. This provides excellent anatomic pictures due to reduced background noise but less information regarding the velocity of flow within vessels. It is often utilized to visualize the vasculature of interest before application of other analysis methods.
Duplex ultrasound has many advantages to other methods of imaging including being noninvasive, comprehensive, mobile, and well tolerated. Additionally, it does not expose the patient to nephrotoxic contrast, or radiation and can be performed on patients with implants. Unfortunately, it is highly operator dependent which can lead to misinterpretation or delayed diagnosis. Body habitus can also cause limitations. Duplex ultrasound is the modality of choice for the diagnosis of deep venous thrombosis (DVT), venous insufficiency, and cerebrovascular, renal, mesenteric, and aortoiliac disease.
There are no specific contraindications to Duplex evaluation although patient tolerance may be limited in areas of inflammation or infection. Patient habitus can make evaluation more difficult.
Necessary equipment consists of an ultrasound machine with duplex capabilities. This will include a probe with piezoelectric crystals. The machine and transducers enable varying focusing, direction, and adjustment of the gain, resolution, and depth of the ultrasound waves. Typical depth adjustments range from less than 1 cm to 20 cm. Equipment is also generally compatible with multiple transducers which provide differing advantages. The linear array transducers are beneficial for anatomic mapping of arterial and venous systems. Lower frequency transducers, which are typically curved linear or phased array, are better for visceral vessels or abdominal imaging studies. These are also utilized for transcranial examinations. Ultrawide bandwidth or harmonic imaging transducers provide increased resolution and decreased artifact particularly at high depths. Two-dimensional transducers are also available which provide the construction of three-dimensional imaging from ultrasound data.
When utilizing this equipment for duplex ultrasonography, two displays are available. These are color-flow Doppler and gray-scale B-mode. The color-flow Doppler displays a flow velocity distribution while the gray-scale B-mode provides an anatomic image.
A well-trained and licensed vascular technician is imperative to high-quality results as this testing modality is highly operator dependent.
Preparations for duplex ultrasound evaluation are specific the diagnostic location of interest. For example, when examining the torso vessels having the patient fast for 4 to 6 hours before evaluation may be helpful and increase the yield of the study.
In the case of DVT analysis, the examination starts at the mid-calf with the identification of the tibial veins. This is followed proximally to the popliteal and femoral veins. Each vein is interrogated with interval compression proximally and distally. Proximal compression should interrupt the flow, while distal occlusion should augment flow. If augmentation is not demonstrated upon distal compression, a DVT should be suspected. In the case of upper extremity DVTs, the compressibility of the vein is the main diagnostic criterion, as well as acoustic shadowing as proximal compression of the brachiocephalic or SVC, is impossible. The addition of Color Doppler to the duplex examination has increased the sensitivity and specificity of the examination and allowed for an increased visualization of partial occlusions.
In the evaluation for venous insufficiency, the patient is evaluated while standing or with truncal elevation. This reverses the venous flow by increasing the pressure gradient. With the patient in position, the probe is placed longitudinally in the groin, and each deep and superficial vein is evaluated for compressibility, venous flow, augmentation of flow and visualization. Reflux is induced by either Valsalva or manual proximal compression depending on the location of the vein. Reflux of greater than 500 milliseconds is considered pathologic.
Various arterial diseases are identified and monitored via duplex ultrasonography. These include cerebrovascular, renal, mesenteric, and aortoiliac disease. Examination of torso arteries such as the aortoiliac, renal and visceral arteries can be extremely labor and time intensive. These are again highly operator dependent and rely heavily on appropriate utilization of protocols and well trained vascular staff. Given these limitations, this modality is inferior to other imaging techniques and is reserved for specific clinical situations such as in renal insufficiency patients. If it is utilized, it should be considered. Additional diagnostic testing should follow a screening study and positive results. In the case of cerebrovascular arterial analysis, duplex ultrasound is particularly useful for carotid evaluation. It is highly sensitive and specific for the identification of plaques and can be used to characterize and identify plaques which are more likely to increase the risk of future strokes. These features include hypoechoic and heterogenous plaques which are more likely to cause cerebrovascular symptoms than hyperechoic plaques. Ulcerated plaques are also considered high risk as this is a strong independent risk factor for stroke.
Complications from Duplex ultrasound are usually related to their use during specific procedures and the procedural intervention rather than the utilization of the ultrasound.
Duplex ultrasound is an important clinical tool. Its portability, affordability, safety profile, and tolerance have led to a continually expanding application field. It is important to understand the clinical applications and principles behind this important diagnostic modality.