Select a 5–10 MHz linear array transducer for lower extremity scanning (Figure 17-5). Many ultrasound machines have settings that allow the clinician to adjust the frequency of the transducer. Higher frequency settings allow for better spatial resolution and improved viewing of more superficial structures, which maximizes image quality in thin patients. Alternatively, the lower frequency settings can optimize imaging in larger patients or deeper structures. Color Doppler and pulse wave Doppler may also aid in differentiation of arterial versus venous blood flow if the anatomy is difficult to identify. However, unlike the duplex ultrasound performed in most vascular laboratories, Doppler functions are used as an adjunct for identifying anatomy or assessing structures that cannot be compressed, and not as a primary modality in the limited compression ultrasound exam.
A linear array transducer.
While venous compression is the most important method to evaluate the patency of veins in the lower extremity, some clinicians may choose to utilize color flow Doppler or spectral Doppler. Doppler techniques can be used to evaluate spontaneity, phasicity, direction, and augmentation of flow. In normal patent veins, flow is spontaneous and the velocity of flow varies (phasicity) with the changing intrathoracic pressures during the respiratory cycle. Typically, flow is anterograde and unidirectional unless there is venous valvular insufficiency. Additionally, venous flow can be augmented by applying pressure to the calf. Increase flow in the anterograde direction when performing this maneuver indicates patency of the vein. If an abnormality is noted using the Doppler adjuncts, have a heightened suspicion of venous thrombosis.
Place the patient in the supine position with the head of the bed raised 30–45° (Figure 17-6). This increases venous pooling in the lower extremities and distends the veins of the lower extremity. Extend the leg of interest and slightly externally rotate with the knee bent for the examination of the vessels in the proximal thigh (Figure 17-7). Obese patients may have an abdominal pannus that covers the proximal thigh and this should be raised. Usually, the patient can assist with this, but if unable, an assistant may be needed. Apply a liberal amount of ultrasound gel to the proximal thigh and place the ultrasound transducer on the thigh at the level of the inguinal crease.
A bed angle of 30–45° allows lower extremity veins to fill and makes them easier to locate. (Courtesy of James Mateer, MD)
The leg is bent at the knee and rotated outward to allow best exposure of popliteal fossa as well as the junction of the common, deep, and superficial femoral veins.
Locate the femoral artery and vein and adjust the ultrasound transducer so that it is transverse to the long axis of the vessels (Figure 17-8). At the level of the inguinal crease, the common femoral vein is almost always medial to the artery, though this relationship can change as distance from the inguinal ligament increases. Visualize the junction of the common femoral vein and the greater saphenous vein because this is a common location for clot and the proximal starting point for the limited compression ultrasound of the leg veins (Figure 17-9A). Apply firm pressure to the transducer (Figure 17-10). Both the common femoral vein and the proximal portion the greater saphenous vein should be compressible (Figure 7-9B). Complete apposition of the walls of the vein excludes DVT. Adequate force must be applied to the ultrasound transducer to achieve complete compression. In the setting of DVT, the femoral artery can often be seen to compress and is an indication that adequate force has been applied. Novice clinicians may have difficulty fully compressing a normal femoral vein. This is most often due to either inadequate force of compression or compression at an angle oblique, rather than truly transverse, to the vessel.
The approximate position of the linear transducer is shown transversely over the common femoral vein. The transducer handle is being held near the cord for demonstration.
Right femoral vessels—Transverse view, linear array transducer: Baseline view (A) shows that the common femoral artery has already bifurcated into the deep femoral artery (DFA) and superficial femoral artery (SFA). Compression view (B) demonstrates complete collapse of the common femoral vein (CFV). The greater saphenous vein (GSV) can be seen branching anteromedially and is mostly collapsed in this view (slight probe repositioning and pressure should be applied to ensure complete collapse if subtle thrombus is to be excluded in this vessel).
A moderate amount of pressure is applied to the leg as shown on the right in this figure. Inadequate pressure can lead to incomplete collapse of the vein.
Some experts think it is best to follow the common femoral vein distally and compress it every 2 cm until the bifurcation of the superficial and deep femoral veins (Figure 17-11) or 10 cm distal to the inguinal ligament if it is difficult to visualize the bifurcation. However, it may not be necessary to follow the vein distally because most studies are based on just one compression at the level of the inguinal ligament (Figure 17-9).2,3,11,23 Once examination of the common femoral vein is complete, proceed to evaluation of the popliteal vessels.
On the left side of the image, the transducer is positioned over the superficial and deep femoral arteries (SFA and DFA, respectively) and the superficial and deep femoral veins (SFV and DFV, respectively). On the right side, pressure has been applied and both veins have collapsed completely leaving only the two arteries visible.
For evaluation of the popliteal vein, patient positioning is often more challenging (Figure 17-12). Gaining adequate access to the popliteal fossa is more difficult than the inguinal region in many patients, especially the obese. One common approach is to have the patient move to a lateral recumbent position with the examined leg in a dependent position and the knee slightly flexed. This position allows access to the popliteal fossa and optimal venous distention in the extremity.
Ideally, the patient should be placed with their affected side down in reverse Trendelenburg to maximize distension of the popliteal vein (A). The other hand can be used to aid in compression by holding the knee in place (B).
The popliteal vein is located adjacent and superficial to the popliteal artery in the mid popliteal fossa (Figure 17-13A). Complete collapse of the vein lumen demonstrates patency of the vein and an absence of clot (Figure 17-13B). The popliteal fossa is a small area, and compression of the vein in more than one location can be quickly performed. We recommend compression of the proximal, mid, and distal popliteal fossa with the latter being identified as the popliteal vein reaches its trifurcation.
Left popliteal vessels—Transverse view, linear array transducer: Baseline view (A). Pressure applied to the transducer (B) results in complete collapse of the popliteal vein (PV), which is located superficial to the popliteal artery (PA). Showing the femur (F) at the bottom of the screen ensures that the vessels being imaged are the popliteal and not superficial or duplicated vessels.
It is common to see smaller veins adjacent or superficial to the popliteal vessels and 5% of patients have a truly duplicated popliteal vein. Identification of the popliteal vein may be difficult in morbidly obese patients due to vessel depth and poor patient positioning. It is important to recognize that the popliteal vein, whether normal or duplicated, superficial or deep, is always accompanied by the popliteal artery. Additionally, the popliteal vein is the deepest vein in the popliteal fossa when viewed from a posterior approach, so visualization of the femur deep to the vessel is further confirmation that the vein in question is in fact the popliteal and not a more superficial vessel. If this is not appreciated, superficial veins may be mistaken for the popliteal vein and a DVT may be missed.
Controversy exists regarding the need to routinely scan the contralateral leg in patients undergoing limited compression ultrasound for unilateral lower extremity complaints. The incidence of contralateral, asymptomatic leg thrombi is as high as 34% in some patient populations, especially hospitalized patients and those with active malignancy or other significant risk factors for DVT.24 While some authors have argued that bilateral scanning should be the standard, others argue that scanning an asymptomatic extremity is not cost-effective and wastes resources.25–27 It may be reasonable to scan both extremities in very high-risk patients and only the symptomatic extremity in most outpatients.24 One study recommends a unilateral lower extremity scan in outpatients and a scan of the contralateral leg if the patient is found to have a DVT in the symptomatic extremity.28 Limited compression ultrasound is much faster compared to traditional vascular laboratory duplex scans, so the threshold for scanning both legs may be lower. However, in the setting of a busy ED, if only one lower extremity is symptomatic, scanning of the contralateral leg is generally not indicated.
Use a 5–10 MHz linear transducer for upper extremity vascular imaging. Place patients supine or in the Trendelenburg position, which increases neck and upper extremity venous engorgement, aiding in identification of anatomy and image acquisition. Begin the study by locating the distal portion of the internal jugular vein in the transverse plane (Figure 17-14). If there is difficulty locating the internal jugular vein, color Doppler may aid with identification. Apply gentle pressure with the transducer to completely collapse the vein in 1 cm increments while moving proximally. Minimize compression of the internal jugular vein only to that necessary to completely collapse the vein. Examine the internal jugular vein proximally to the point where it enters the brachiocephalic or subclavian vein. The proximal portion of the internal jugular vein becomes difficult to compress due to overlying bony structures; direct visualization of thrombus or color Doppler may be helpful in identifying any abnormality. The proximal subclavian vein can usually be visualized using a supraclavicular window with transducer orientation longitudinal to the vessel. It is not possible to compress the proximal subclavian vein so the exam relies on direct visualization of thrombus or visualization of color Doppler flow (Figure 17-15). It may be difficult to visualize the distal subclavian and proximal axillary veins but they should be compressed where possible. Resume a more direct examination when the distal axillary vein emerges in the axilla (Figure 17-16). Compress the brachial and basilic veins after they bifurcate from the axillary vein, moving distally to the level of the antecubital fossa. Compress these veins individually to ensure complete collapse. If there is a high suspicion of DVT in the axillary or subclavian veins and it cannot be visualized with ultrasound, then consider using CT to assist with the diagnosis.29
A linear transducer is held at the level of the internal jugular in transverse. The entire length of the jugular can be traced and evaluated for thrombus. Aggressive compression is avoided in this region, especially in elderly patients. Interrogation is started higher on the neck than shown here. (Courtesy of James Mateer, MD)
A linear transducer is held just above the clavicle, allowing a longitudinal view of the subclavian vein. (Courtesy of James Mateer, MD)
A linear transducer is held in the axilla. This allows transverse visualization of the vessels and compression. (Courtesy of James Mateer, MD)
Documentation and Billing
Document diagnostic ultrasound examinations for the medical record and billing purposes. Documentation of the study should generally include:30
- Indication for the examination
- Views obtained
- Relevant findings
- Physician interpretation
Obtain and store images of the relevant anatomy. When billing for the study, at least one image of the study must be recorded and stored permanently to satisfy CPT coding criteria. Two-point limited compression ultrasound of the lower extremity for DVT is coded as a limited duplex scan of the lower extremity veins (CPT code 93971).31 Storing video clips of venous compression is the best way to document the study. If video is not available, split screen still images can be used to document vein compression (Figures 17-9 and 17-13).