Duplex Ultrasound

Anatomy and Physiology

The Doppler effect refers to the change in observed sound wave frequencies due to motion. In 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 ultrasound waves and vice versa. The probe measures the frequency shift due to reflections off the underlying tissues.

This Doppler shift is calculated using the following equation: Doppler shift: 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.[4] This equation explains why Doppler images degrade at an angle greater than 70 degrees as the cos(theta) approaches 0 at 90 degrees.[5] 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.[3] 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 clarity or anatomic reasons.

Various methods for wave generation exist, including 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 shift, returning to the detectors. Objects moving towards the transducer result in an increased frequency, while those moving away result in a decreased 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, high reconstruction can be inaccurate, and flow directions can reverse. This phenomenon is known as an 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.[6][7] 

Pulsed wave ultrasound increases the maximum velocity measurable by minimizing the overlap between echo trains.[8] While it employs similar principles to continuous wave ultrasound, the sound waves are generated in regular intervals with pauses. In pulse-wave systems, the maximum velocity measured by the instrument is determined by the pulse repetition frequency (PRF). Therefore, the maximum accurate velocity, V(m), is calculated by using the following equation: V(m) = c² / [8*R*f(t)], where R is the range or distance from the transducer. The V(m) is further increased by the introduction of a high pulse repetition frequency, which utilizes pulse waves at 2 to 5 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.[2] Conversely, power Doppler is influenced very little by the angle.[7] Power Doppler provides excellent anatomic pictures due to reduced background noise but less information regarding flow velocity within vessels. Power Doppler is often utilized to visualize the vasculature of interest before applying other analysis methods.

Indications

Duplex ultrasound has many advantages over other imaging methods, 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, duplex ultrasound 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.

Contraindications

No specific contraindications to Duplex evaluation have been established, although patient tolerance may be limited in areas of inflammation or infection. Patient habitus can make assessment more difficult.

Equipment

The necessary equipment consists of an ultrasound machine with duplex capabilities, including a probe with piezoelectric crystals. The machine and transducers enable varying focus, direction, and adjustment of the ultrasound waves' gain, resolution, and depth. Typical depth adjustments range from <1 cm to 20 cm.

Equipment is also generally compatible with multiple transducers, which provide differing advantages. The linear array transducers are beneficial for the anatomic mapping of arterial and venous systems. Lower frequency transducers, which are typically curved linear or phased arrays, 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 to construct 3-dimensional imaging from ultrasound data.

Two displays are available when using this equipment for duplex ultrasonography: 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.

Personnel

A well-trained and licensed vascular technician is imperative to high-quality results, as duplex ultrasound is highly operator-dependent.

Preparation

Preparations for duplex ultrasound evaluation are specific to the diagnostic location of interest. For example, when examining the torso vessels, having the patient fast for 4 to 6 hours before performing the evaluation may be helpful and increase the yield of the study.

Technique or Treatment

Deep Vein Thrombosis Evaluation with Duplex Ultrasound

In the case of DVT analysis, the examination starts at the mid-calf with the identification of the tibial veins. The tibial vein 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. A DVT should be suspected if augmentation is not demonstrated upon distal compression. Notably, the performance of augmentation maneuvers may require additional training.[9] 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 improved visualization of partial occlusions.

Venous Insufficiency Evaluation with Duplex Ultrasound

In the evaluation for venous insufficiency, the patient is evaluated while standing or with truncal elevation, which 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. Depending on the vein's location, reflux is induced by either Valsalva or manual proximal compression. Reflux of >500 milliseconds is considered pathologic.

Arterial Disease Evaluation with Duplex Ultrasound

Duplex ultrasonography identifies and monitors various arterial diseases, including cerebrovascular, renal, mesenteric, and aortoiliac disease. Examining torso arteries, eg, the aortoiliac, renal, and visceral arteries, can be extremely laborious and time-intensive. Again, these are highly operator-dependent and rely heavily on the appropriate utilization of protocols and well-trained vascular staff.

Given these limitations, duplex ultrasonography is inferior to other imaging techniques and is reserved for specific clinical situations, eg, renal insufficiency patients. If duplex ultrasonography is utilized, these limitations should be considered. Additional diagnostic testing should follow a screening study with positive results. In the case of cerebrovascular arterial analysis, duplex ultrasound is particularly useful for carotid evaluation.[10] This modality is highly sensitive and specific for identifying plaques and can be used to characterize and assess plaques that 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

Complications from duplex ultrasound are usually related to their use during specific procedures and procedural interventions rather than the utilization of the ultrasound.

Clinical Significance

Duplex ultrasound is an important clinical tool. The advantages of this imaging modality, including portability, affordability, safety profile, and tolerance, have led to a continually expanding application field. Understanding the clinical applications and principles behind this important diagnostic modality is essential.[11][12]

Enhancing Healthcare Team Outcomes

Effective utilization of duplex ultrasound in patient-centered care requires a coordinated effort among physicians, advanced practitioners, nurses, pharmacists, and other healthcare professionals. Physicians and advanced practitioners must develop proficiency in performing and interpreting ultrasound to enhance diagnostic accuracy and optimize clinical decision-making. Nurses play a crucial role in patient preparation, positioning, and education, ensuring that patients understand the procedure and its benefits. Radiology technicians contribute by recognizing situations where contrast-free imaging is preferable, especially in patients with renal impairment or contrast allergies. Through ongoing training and collaboration, healthcare teams can maximize the advantages of ultrasound as a first-line imaging modality, improving efficiency and reducing reliance on more costly and invasive techniques.

Interprofessional communication and care coordination are essential for integrating duplex ultrasound into routine clinical practice. Physicians and advanced practitioners must effectively communicate findings to the care team, enabling timely interventions and appropriate treatment plans. Nurses and technologists must relay any procedural concerns or limitations, such as patient positioning challenges or unclear imaging results, ensuring that alternative approaches are considered. Radiology specialists can provide guidance on imaging alternatives when ultrasound findings require further evaluation. By fostering teamwork and shared responsibility, healthcare professionals can enhance patient safety, streamline diagnostics, and improve outcomes while minimizing unnecessary exposure to radiation and contrast agents.