Vascular Laboratory: Venous Duplex Scanning

Introduction

Duplex scanning is readily available, noninvasive, and inexpensive. It is the primary means for diagnosis of deep vein thrombosis (DVT) and chronic venous insufficiency (CVI) (see Ch. 148 , Acute Lower Extremity Deep Venous Thrombosis: Presentation, Diagnosis, and Medical Treatment; and Ch. 156 , Chronic Venous Disorders: Postthrombotic Syndrome, Natural History, Pathophysiology, and Etiology). There are limitations, however. The accuracy and reliability of venous duplex scanning still depend on the thoroughness of the examination, the skill of the technologist performing the study, and the competence of the interpreting physician.

The use of Doppler ultrasound for the evaluation of lower extremity veins was pioneered in the 1960s. ^, In 1968, David Sumner, Dennis Baker, and D. Eugene Strandness described use of an ultrasonic flow detector to assess venous flow patterns. In 1972, Strandness and Sumner published their observations on the use of Doppler ultrasound for the diagnosis of venous thrombosis. Steven Talbot, a vascular technologist, published his description of the use of B-mode (2-D grayscale) imaging for the diagnosis of deep vein thrombosis (DVT) in 1982. ^,

Early validation studies that compared venous ultrasonography to venography identified limitations of B-mode imaging when it was used as a sole modality. Imaging of deeper structures was limited. Moderate probe compression did not collapse some normal vein segments (e.g., femoral vein at the level of the adductor hiatus) and vein compressibility could also be limited by obesity, edema, or tenderness. It was soon recognized that the accuracy of the examination was improved by combining imaging and Doppler-derived information about the presence and nature of venous flow in the interrogated segments. ^, Within a few years, duplex scanning became the primary diagnostic modality for diagnosis of venous disorders, not just a screening method.

In the modern era, the primacy of duplex scanning for diagnosis is no longer in question. Now, vascular laboratories have a greater focus on how to optimize examination efficiency (tailoring study protocols for individual patients), improving reporting, and reducing variation.

Variation is reduced by exam standardization and by the use of appropriate study procedures and interpretation criteria. The Intersocietal Accreditation Commission (IAC) accredits imaging facilities specific to vascular testing. Accreditation recognizes facilities that provide quality vascular testing services by compliance with relevant standards. It also serves as an educational tool to improve overall quality. IAC accreditation standards are regularly updated; they represent a consensus on what constitutes an appropriate vascular laboratory examination. These standards include appropriate training and credentials of technologists and interpreting physicians, and the use of established, consistent examination protocols ( Tables 25.1 and 25.2 ). Maintaining accreditation requires that facilities implement quality improvement programs that focus on consistency and appropriateness of testing and reporting (see Ch. 18 , Noninvasive Vascular Laboratory Quality Assurance and Accreditation).

TABLE 25.1

Vascular Laboratory Standards for Peripheral Venous Testing

(Modified from: Intersocietal Accreditation Commission - Vascular Testing. IAC Standards and Guidelines for Vascular Testing Accreditation. Section 4B: Peripheral Venous Testing 2020. https://www.intersocietal.org/vascular/standards/IACVascularTestingStandards2020.pdf )

Indications: Peripheral venous testing must be performed for appropriate clinical indications	Testing indication must be documented
Equipment: Equipment must provide accurate data	Imaging equipment: duplex ultrasound with color flow Doppler, with o imaging frequencies appropriate for the structures evaluated o Doppler frequencies appropriate for the vessels evaluated o range-gated spectral Doppler with ability to adjust the depth and position of the range gate in the area of interest o Doppler angle which is measurable and adjustable o visual display and permanent recording of images o visual display, audible output, and permanent recording of Doppler waveforms and corresponding images Equipment quality control: equipment used for diagnostic testing must be maintained in good operating condition
Protocols: There must be a written protocol for each examination, including:	equipment to be used proper technique anatomic extent of a complete examination, generally includes entire course of the accessible portion of each vessel variations in technique following vascular interventions, including dialysis access variations in technique and documentation for limited exams routine documentation and additional documentation that must be acquired to describe abnormalities description of how color Doppler or other flow imaging modes (e.g., power Doppler) are used to supplement grayscale imaging and spectral Doppler measurements
Techniques: Appropriate techniques to document their severity, location, extent and (whenever possible) etiology	Elements of proper technique include: o performance of an examination according to a specific written protocol o proper patient positioning ▪ for assessing reflux: standing, sitting or reverse Trendelenburg (at least 15 degrees) o patient preparation o appropriate equipment and transducer selection o appropriate transducer positioning o identification of vessels by imaging and Doppler o proper sample volume size and positioning o optimization of equipment gain and display settings o proper vein diameter measurements: ▪ acquired with the limb in a dependent position ▪ measured anterior wall to posterior wall ▪ no external pressure is applied to the vein during measurement o transverse grayscale imaging without and with transducer compressions o provocative maneuvers are used to test for reflux in lower extremity vein ▪ the limb must be examined in a dependent position; standing is preferred ▪ manual distal compression or automated rapid cuff inflation/deflation devices should be used to elicit reflux in deep and superficial veins ▪ Valsalva maneuvers may be substituted for distal compression when examining the common femoral vein and the saphenofemoral junction
Documentation	grayscale images color Doppler images Doppler waveforms velocity measurements vein diameter valve closure time, reflux duration abnormalities will require additional images and waveforms that demonstrate the severity, location, extent, and etiology areas of suspected obstruction must include representative Doppler waveforms recorded at and distal to the obstruction superficial reflux must be traced to its source (e.g., saphenous junctions, great, small, anterior accessory saphenous vein, perforating vein, pelvic origin varicose veins), with reflux duration time documented

TABLE 25.2

Components of Venous Duplex Scans: Peripheral Venous Documentation

(Modified from: Intersocietal Accreditation Commission – Vascular Testing. IAC Standards and Guidelines for Vascular Testing Accreditation. Section 4B : Peripheral Venous Testing Peripheral Venous Testing 2020 https://www.intersocietal.org/vascular/standards/IACVascularTestingStandards2020.pdf )

Lower Extremity Venous Duplex for Thrombosis and Patency	Transverse grayscale images without and with transducer compression, including: o common femoral vein o saphenofemoral junction o proximal femoral vein o mid femoral vein o distal femoral vein o popliteal vein o posterior tibial veins o peroneal veins o additional images to document areas of suspected thrombus Spectral Doppler waveforms demonstrating spontaneous venous flow, phasicity and/or flow augmentation, including: o right and left common femoral veins o popliteal vein o additional waveforms, as appropriate o abnormalities require additional images, waveforms and velocity measurements
Lower Extremity Venous Duplex for Reflux	Transverse grayscale images without and with transducer compression, including: o common femoral vein o saphenofemoral junction o mid femoral vein o great saphenous vein o popliteal vein o small saphenous vein Spectral Doppler waveforms with the leg in a dependent position, demonstrating baseline flow and response to distal augmentation; if reflux is present, duration of retrograde flow is measured with calipers: o common femoral vein o saphenofemoral junction o great saphenous vein o mid femoral vein o popliteal vein o small saphenous vein Transverse grayscale images without and with transducer compression, including: o saphenofemoral junction o great saphenous vein at proximal thigh o great saphenous vein at knee o small saphenous vein (at saphenopopliteal junction)
Upper Extremity Venous Duplex for Thrombosis and Patency	Transverse grayscale images without and with transducer compression, including: o internal jugular vein o subclavian vein o axillary vein o brachial vein(s) o basilic vein o cephalic vein o additional images to document areas of suspected thrombus o brachiocephalic vein should be imaged, if possible, but compression maneuvers are not used for intrathoracic veins Spectral Doppler waveforms demonstrating spontaneous venous flow, phasicity and/or flow augmentation, including: o internal jugular vein o right and left subclavian veins o axillary vein o additional waveforms (e.g., brachiocephalic veins), as appropriate
Vein Mapping	assessment of the veins required by the facility protocol vein patency and size
Venous Stents	Spectral Doppler waveforms with color Doppler images, including: o proximal stent o mid stent o distal stent o native vessel adjacent to the proximal end of the stent o native vessel adjacent to distal end of the stent

Normal B-Mode Imaging Findings

Venous duplex scanning may be performed with most ultrasound systems. Transducer selection depends on the depth of the veins to be scanned. A 5- to 10-MHz linear array transducer is sufficient for a most extremity venous examinations, but a 3- to 5-MHz curved linear transducer may be needed to evaluate deeply located vessels, such as the inferior vena cava and iliac veins, or to evaluate obese or edematous extremities. High-frequency transducers may be used for mapping or guiding access to superficial veins.

Blood is not an effective reflector of ultrasound. Thus, with conventional B-mode imaging the lumen of normal vein is typically anechoic (black). The interface between blood and the vein wall is echogenic (bright), especially when specular reflections are created when the ultrasound angle of incidence is close to 90 degrees. The wall of a vein appears thinner than the wall of the adjacent artery. Flowing blood may sometimes be seen within veins, especially when a high-frequency transducer is used to examine a vein that is close to the skin surface. This may be most apparent when slow flow conditions result in red blood cell aggregation with rouleaux formation, creating greater echogenicity.

Extremity veins may vary in size and profile. With hypovolemia, veins may appear ovoid or collapsed. Because of the normally low pressure in veins, they are easily collapsed with extrinsic compression. (Of note, normal arteries may also be compressible if sufficient probe pressure is applied.) When venous pressure is high (heart failure, more central obstruction, etc.), veins may be distended. Extremity veins may appear collapsed when the limb is elevated above the phlebostatic axis (level of the right atrium) and they will be full and dilated when the limb is dependent. Lower extremity examinations are typically done with some reverse-Trendelenburg (foot down) positioning to make the veins easier to identify.

Normal Flow Patterns in Veins

Color flow Doppler and pulsed Doppler spectral waveform analysis are used to evaluate venous flow characteristics ( Fig. 25.1 ). Terms for describing Doppler waveforms have been standardized. Venous waveforms are described by major key descriptors (flow direction, flow pattern, and spontaneity). Additional modifier terms for venous waveforms include augmentation (normal, reduced, or absent), reflux, and fistula flow patterns.

Figure 25.1, Normal flow in the lower extremity veins is spontaneous (occurring without calf pump function or other augmentation maneuvers) and phasic with respiration. Flow patterns differ at proximal and distal locations, with less spontaneous flow observed in the smaller veins in the distal limb. The presence or absence of thrombus within an imaged vein can be confirmed with probe compression.

In the arterial system, pumping action of the heart is the primary driving force for blood flow, but in the low-pressure venous system antegrade movement of venous blood flow is substantially affected by other factors. Normal venous flow is phasic with respiratory movements (respirophasic). During inspiration, the diaphragm moves downward and intraabdominal pressure is increased. This decreases venous return from the lower extremities. Flow increases during expiration. In the upper extremity veins, respiratory phasicity has a different pattern. Intrathoracic pressure is decreased during inspiration; antegrade flow in upper extremity veins is increased. The cardiac cycle has greater effects on venous flow patterns in more central veins.

Spontaneous antegrade venous flow is seen under resting conditions, but flow velocity is dramatically augmented with voluntary muscle contraction or by manual compression by the examiner. Muscle contraction transiently increases compartment pressures, compresses deep veins and venous sinuses within the muscles, and thereby functions as a peripheral pumping mechanism in the circulation. Venous obstruction interferes with the flow augmentation produced by compression or other provocative maneuvers.

Diagnosis of Deep Venous Thrombosis

Direct ultrasound visualization of thrombus within a vein is the most obvious finding associated with deep vein thrombosis (DVT). Deep veins that are acutely occluded by thrombosis are often distended, appearing round and larger than the adjacent artery. Thrombus echogenicity increases as the clot organizes, making it easier to detect with B-mode imaging over time. Acute thrombus may be adherent to the vein wall, or it may be loosely attached and mobile, a “free floating thrombus” or “thrombus tail.”

B-mode imaging alone has limited sensitivity. Fresh thrombus may be hypoechoic. It can also be difficult to recognize intraluminal thrombus if the imaging quality is suboptimal, as can be the case with an uncooperative patient, greater imaging depth (obesity), failure to optimize imaging settings, or other technical factors.

For better detection of thrombus in the lumen, venous scanning is done with compression maneuvers ( Fig. 25.2 ). Pressure is manually applied with the imaging probe. Normal vein walls collapse with modest compression. Vein walls will not coapt when there is thrombus in the lumen, though soft thrombus may deform with firm pressure. Partial compressibility may be seen with non-obstructive acute thrombosis or as recanalization of an occluded segment occurs over time.

Figure 25.2, The presence or absence of thrombus within an imaged vein can be confirmed with probe compression.

It is therefore recommended that B-mode imaging not be used as a stand-alone modality for DVT diagnosis. Imaging of deeper structures can be limited and some normal venous segments may not collapse with probe pressure (e.g., femoral vein at the level of the adductor hiatus). Venous compressibility can also be limited by obesity, edema, or tenderness. Calf veins may be difficult to visualize. Duplex scanning, the combination of imaging and Doppler-derived information about the presence and nature of venous flow, improves the accuracy of the examination ( Fig. 25.3 ). The addition of flow information makes it possible to identify veins that are not well demonstrated with B-mode alone and to determine patency. Color flow Doppler helps confirm patency of venous segments that were incompletely evaluated with B-mode imaging. It can help to distinguish between occluding and non-occluding thrombus and identify anechoic thrombus by visualization of flow around it. The color velocity scale (pulse repetition frequency or PRF) should be at a low setting to detect low velocity venous flow.

Figure 25.3, Color Doppler image of a normal common femoral vein. There is complete color filling of the vein lumen. The flow velocity waveform demonstrates spontaneous flow that is phasic with respiration (highlighted by dashed yellow line). Cardiac effects are also seen in the waveform.

The diagnostic accuracy of duplex ultrasound scanning is good enough that it has replaced contrast venography as the definitive diagnostic test for DVT, except in limited situations. A systematic review found 100 publications from 1966 to 2004 that compared ultrasound to venography in patients with suspected DVT. Meta-analyses of data from these studies confirmed that the best results were obtained with examinations that combined B-mode, pulsed Doppler, and color Doppler imaging. The pooled sensitivity was 96% for proximal limb DVT, 75% for distal DVT and a pooled specificity of 94%. Sensitivity was found to be higher in the more recently published studies, in cohorts with higher pre-test probability of DVT and more proximal DVT, and was lower in cohorts that reported interpretation by a radiologist. Specificity was higher in cohorts that excluded patients with known prior DVT.

Imaging techniques and provocative maneuvers need to be used in combination to correctly distinguish normal flow variations from DVT ( Fig. 25.4 ). The presence of intraluminal echoes from rouleaux formation can be distinguished from thrombus by the disruption of the echogenic erythrocyte aggregates by flow augmentation and by vein wall collapse with compression. The absence of spontaneous flow (pulsed Doppler or color Doppler) may be normal in the distal limb or it may be the result of more central obstruction. Augmentation maneuvers (calf pump, manual distal compression) will demonstrate flow.

Figure 25.4, Examples of Acute Deep Vein Thrombosis.

Despite the cost-effectiveness of using pretest probability scoring and d -dimer testing in the outpatient setting to determine the need for imaging, duplex scanning continues to be used primarily as a stand-alone testing modality. This may be due to the availability of ultrasound in most settings (see Ch. 146 , Acute Deep Venous Thrombosis: Epidemiology and Natural History).

Calf Vein Thrombosis

All lower extremity deep veins should be examined as part of a complete DVT examination, including infrapopliteal veins: peroneal, posterior tibial, soleal, and gastrocnemius veins. Isolated distal deep vein thromboses are common in patients with acute DVT and often occur as an isolated finding. ^, The peroneal and posterior tibial veins are involved in the majority of cases. Anterior tibial vein thrombi are less common.

Duplex scanning can reliably evaluate for calf vein DVT, but there are some differences in examination technique and interpretation. Spontaneous flow is not always detected, but there will be Doppler-detectable flow with augmentation by distal manual compression. Calf veins may be difficult to visualize with B-mode imaging, especially in the obese or those with significant edema. Detection of venous flow with color Doppler, with or without augmentation maneuvers, can confirm calf vein patency ( Fig. 25.5 ).

Figure 25.5, Imaging of the posterior tibial and peroneal veins: left , longitudinal view; right , transverse view. Color flow imaging can confirm patency of the posterior tibial and peroneal veins. This can help to rule out calf DVT when vein wall coaptation with compression maneuvers is difficult to demonstrate.

Isolated calf vein thrombi rarely cause clinically relevant pulmonary emboli (PE), but patients with calf vein DVT are at risk for progression to involve the femoropopliteal veins. Symptomatic calf vein DVT is associated with progression or recurrent thrombosis in 15%–20% of patients within three months. ^, Calf vein DVT is a marker for increased thromboembolic risk, for which anticoagulation may be indicated. Few patients with calf vein DVT are at high risk for progression or PE. Risk is higher when thrombus is isolated to calf veins, if the DVT was a second or subsequent venous thromboembolism (VTE) episode, in patients with recent orthopedic procedures, in patients with malignancy, and in those who are immobile.

When anticoagulation is not used to treat acute, isolated calf vein thrombi, follow-up duplex scanning over at least 2 weeks is recommended to evaluate for progression.

Thrombosis of intramuscular veins, those within the gastrocnemius or soleus muscles, can be associated with localized calf tenderness. In rare cases, there can be propagation and subsequent pulmonary embolism, but isolated muscular calf vein thrombosis, has a low risk of extension. If extension is not observed within two weeks, it is very unlikely. Patients with muscular calf vein thrombosis, with only transient risk factors, do not require anticoagulation, but they should be re-evaluated with duplex scanning 1 week after diagnosis to confirm thrombus resolution or absence of progression.

Iliac Veins and Inferior Vena Cava

The inferior vena cava (IVC) and iliac veins, like other intraabdominal and retroperitoneal vessels, can be difficult to evaluate due to overlying abdominal bowel gas and the decreased resolution of ultrasound when evaluating deeper structures. When possible, patients should fast for 6 hours prior to the examination to help reduce bowel gas. B-mode imaging and transverse probe compression maneuvers are seldom effective when evaluating abdominal or pelvic veins. Therefore, color flow and Doppler spectral waveforms are the primary modalities for evaluation of the IVC and iliac veins. Color scale and gain settings are adjusted for the depth.

IVC flow should be spontaneous and phasic throughout. Greater pulsatility is seen in the inferior vena cava due to its proximity to the heart. Absence of detectable flow by pulsed Doppler and color Doppler imaging suggests IVC occlusion or thrombosis.

External iliac and common iliac veins can be evaluated from the inguinal ligament to the level of the IVC. Augmentation of flow is produced with thigh compressions. Internal iliac veins are identified at their confluence with the external iliac veins. Complete or partial absence of color flow within the iliac veins may indicate intraluminal thrombus. Absence of spontaneous flow or lack of respiratory phasicity (continuous flow) may indicate a more central venous obstruction. Comparison of venous flow characteristics to the contralateral side may show indirect findings that indicate common iliac vein obstruction or compression ( Fig. 25.6 ). Continuous venous waveforms with elevated flow velocities may be seen with venous stenosis.

Figure 25.6, Continuous venous flow (loss of normal respiratory phasic variation) indicates venous outflow obstruction. This indirect finding does not distinguish occlusion, non-occlusive thrombosis, chronic narrowing, or extrinsic compression. When seen in one common femoral vein, as shown, it indicates iliac vein obstruction.

Abnormal flow patterns in lower extremity veins may reflect abnormal (central) cardiovascular abnormalities. Heart failure or tricuspid valve regurgitation can increase venous flow variations with each cardiac cycle.

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