21 Extremity Venous Anatomy and Technique for Ultrasound Examination
Veins are not arteries. They are cylindrical structures that transport blood like their thicker-walled counterparts. However, they are dissimilar in many other ways. Veins are multitaskers. Their job description includes transporting blood back to the heart, helping to regulate body temperature and cardiac output, and providing a storage reservoir for blood. While being stored, blood can stagnate, making the formation of life-threatening blood clots more likely.
When imaging the veins, the examiner must work from a different mindset than when imaging arteries. When imaging the arteries, the most important thing is to determine how narrowed or obstructed a given vessel is. When imaging the veins, the most important thing is to detect the presence or absence of thrombus and determine, if possible, how well attached the thrombus is to the vein wall. A nonobstructive thrombus, for example, may pose a larger risk for breaking loose and traveling to the lungs than a totally obstructive clot because it may be more poorly attached to the vein wall. Fortunately, duplex imaging is a perfect tool for identifying and evaluating these clots, thus allowing physicians to take actions to minimize the risks of clot embolization and pulmonary embolism.
In order to perform venous duplex ultrasound of the veins, the examiner must understand the venous anatomy1–4 and be fully acquainted with the examination protocols for imaging the veins. In this chapter we will review the venous anatomy and the examination protocols.
The deep veins are accompanied by an artery and are, by definition, surrounded by muscle. Their job is to act as the main conduits that transport blood back to the heart. Clots within the deep veins are more likely to produce a clinically significant pulmonary embolism because these clots are usually larger than those in the superficial system. Also, because they are surrounded by muscle, the chance of the clot being dislodged during muscle contraction is higher than for a clot in the superficial veins. For these reasons, the main focus in a lower extremity venous duplex examination is on the deep system.
The superficial veins are, by definition, located near the skin, superficial to the muscle. The main superficial veins usually travel without an accompanying artery within the border that separates the fascia from the muscle. Their job is not to be the primary source for returning blood to the heart, but rather to get blood close to the skin surface so the veins can help to regulate body temperature. They constrict if the environment is cold to help preserve body heat. When the body needs to cool down, they enlarge to shunt large amounts of warm blood to the skin so that heat escapes the body.
There is a mistaken idea that clots within the superficial veins pose no threat of producing a pulmonary embolism. Clots can and do break loose from superficial veins and travel to the pulmonary arteries. These clots in the superficial system are less likely to cause major, clinically significant pulmonary embolism because they are usually smaller than clots found in the deep veins, and they are less likely to be dislodged because they are not surrounded by muscle. Examination of the superficial veins is still an important part of a complete evaluation of the lower extremity because superficial clots may become large (as the normally small superficial vein is expanded by the contained thrombus) and cause considerable discomfort. There is also potential danger that a superficial vein clot can extend into the deep system.
Perforator veins are veins that connect the deep veins with the superficial veins. Their job is to keep blood moving from the superficial system into the deep system. When they are working properly, they keep blood from pooling at the level of the skin. When they do not function, blood can pool at the skin level and chronic stasis changes and even ulcers may result. While a full examination of the lower extremity need not include detailed evaluation of perforators, more attention is paid to them when the question of chronic swelling or venous stasis is being evaluated.
The biggest disadvantage to venous duplex imaging is its operator-dependent nature. The quality of venous imaging studies varies tremendously from institution to institution and between individual sonographers. This is true because of the lack of standardized training and standardized protocols. Following the protocol described in this chapter will increase your likelihood of performing accurate, reliable venous duplex examinations.
The quality of imaging equipment varies widely. Attempting to do venous duplex imaging with inadequate equipment is frustrating and potentially dangerous. The equipment needed for venous imaging will have excellent gray-scale image quality, with the ability to image from the skin line to a depth of about 6 cm. This may require the use of several transducers: a high-frequency linear probe (10-18 MHz) with a small footprint for superficial veins like the saphenous, distal tibial veins, and most of the arm veins; a midrange linear probe (5-9 MHz) for most of the deep veins of the legs and deeper veins of the arm; and a lower-frequency probe (2-5 MHz) for the iliac veins and inferior vena cava and also to see leg veins in heavy patients.
When imaging the lower extremity, the bed should be tilted to allow blood to dilate the leg veins so they can be imaged clearly. Omitting this seemingly insignificant step is one of the most common reasons for missing small clots, especially in the calf. The bed should be placed in a reversed Trendelenburg’s position (head elevated) at about 20 degrees (Figure 21-1, A). Another technique to distend the calf veins is to allow the patient to sit up during calf evaluation (Figure 21-1, B). This is extremely effective but is clumsy and can make the veins difficult to compress. In this situation, veins filled with stagnant flow may be mistaken for thrombus-filled veins.
(Modified from Zwiebel WJ, Priest DL: Color duplex sonography of extremity veins, Semin Ultrasound CT MR 11:136-167, 1990.)
After the bed is tilted, the patient must be positioned properly. For the lower extremity, this means having the knee slightly bent and the hip externally rotated (Figure 21-2). This allows full access to the medial portion of the thigh and calf and also allows access to the popliteal fossa. Trying to image the leg without proper positioning will lead to inadequate views and errors in diagnosis.
The upper extremity veins are examined with the bed flat and the patient in the supine position. It is especially important that the bed be flat while the jugular and subclavian veins are examined because they will collapse if the head of the bed is up. Once these vessels are imaged, the bed can be raised to allow for imaging of the arm veins. The arm is positioned at the patient’s side for examination of the neck veins and the subclavian vein. When examining the axillary veins, the arm is repositioned with the arm raised to allow access to the axilla. It is repositioned again in a slightly lower position with the arm externally rotated to allow access to the medial portion of the upper and lower arm.
Before the 1980s, venograms were the only method available for looking inside the veins of the extremities. It was thought that ultrasound would not be useful in imaging the veins because clots were thought to be invisible to ultrasound (this was the so-called wisdom of the experts of the day). Despite this false assumption, attempts to image the veins with ultrasound began in the early 1980s. In attempting to find diagnostic criteria for identifying clots, investigators discovered not only that you actually could see clot on ultrasound, but also that normal veins are so easily compressed with light probe pressure that vein compression could be used to unequivocally determine the absence of thrombus in that vein.5 Defining compression of the vein as the primary criterion for duplex imaging for deep venous thrombosis (DVT) was the major factor in its universal acceptance. Using compression in this way allows for the examination to quickly progress in a transverse plane to evaluate the extensive venous anatomy.6–10
Most of the imaging performed during the venous duplex examination is done in a transverse plane. The vein and its accompanying artery (if a deep vessel) are identified alongside each other. The artery is thicker walled but usually the smaller of the two (if the limb being examined is positioned below the heart). Light probe pressure is exerted directly over the vessels. The thicker-walled artery will resist compression (it is also under higher pressure). The vein should compress easily. If there is any confusion about which vessel is the artery or vein, the examiner should use Doppler and color to completely and accurately separate the two structures before proceeding.
If the vein is thrombus-free, it will compress completely so that the inner vein walls actually touch each other. When the vein collapses completely with light probe pressure, it can be determined to be unequivocally thrombus-free at that location (Figure 21-3). This is the key to venous duplex imaging of the veins. The pressure on the vein is released, and the vein will reopen. The examiner then moves the probe along the length of the vessel, compressing every centimeter or so, until the entire vein has been imaged in this way. It is important to ensure that these compression maneuvers are as close together as possible. If the “cuts” are too far apart, a major section of vein containing thrombus can be missed. As a rule, the smaller the cuts, the less chance there is of missing a thrombus.
FIGURE 21-3 A split-screen image. Transverse view demonstrating proper compression of a vein. A, In this example, the arteries and vein are seen side by side. B, Only the arteries are seen because the vein is being collapsed by light probe pressure being exerted over the vessels. Note that the vein is completely collapsed. This is the key maneuver in diagnostic venous imaging. When the vein compresses as shown in this example, the vein can be said to be thrombus-free at this given position. In this example, color is used to show the location of the vessels. However, in practice, transverse compression views are often obtained with the color turned off so that color does not obscure the vessel walls.
After the entire segment of vein has been evaluated in the transverse plane, the examiner can rotate the probe and rescan the segment in the longitudinal plane and add color and pulsed Doppler (Figure 21-4). These views provide additional information regarding blood flow through the venous system. These longitudinal views, however, are only an addition to the information already gathered by the transverse compression views and cannot substitute for them. Substituting longitudinal views with color Doppler for the transverse compression views will result in missing partially obstructive thrombus.
Present accreditation protocols require transverse compression views and longitudinal views with color and pulsed Doppler information to be performed at specific levels. Doppler signals in the normal leg vein will be phasic (suggesting an absence of obstruction of the major veins above the level of the probe). The flow in the vein should stop completely as the patient takes a breath in and should resume spontaneously as the patient exhales. Squeezing of the leg distal to the level of the transducer should produce an augmentation of flow during this maneuver (Figure 21–5). This indicates the lack of an obstruction of the major veins from the level of the squeeze to the level of the transducer. The examiner must understand the limitations of information gathered by pulsed or color Doppler; a partially obstructive thrombus may be missed using these methods—thus the need for the transverse compressions.
Thrombus is present within the vein when echogenic material is identified within the lumen of the vein (Figure 21-6) and when full compression of the vein is impeded. It is crucial to note that both of these things must occur together to definitively make the diagnosis of thrombus in the vein (Figure 21-7). Too many institutions simply look for noncompressibility of the vein (some institutions refer to duplex venous imaging as “compression ultrasound”). Failure to link these two will result in false-positive results in cases where the veins are difficult to compress—not because of the presence of thrombus—but because of a myriad of other factors. For example, proximal compression of the vein causing stagnation and increased echogenicity of blood flow may simulate intraluminal thrombus. Complete compression of the vein lumen excludes thrombus (Figure 21-8). Other pitfalls include incomplete compression from a patient bearing down in response to painful probe compressions (Figure 21-9), compression being limited by a nearby bone, and other factors. In the case where the vein is not compressing but thrombus cannot be seen directly (poor views or views of very small or deep structures), an additional maneuver is essential to determine whether the noncompressibility is due to the presence of thrombus or some other factor. This additional maneuver may involve compressing harder until the artery next to the vein starts to compress. If the artery next to the vein compresses and the vein does not, thrombus is likely to be present within the vein despite the fact that the clot is relatively anechoic or is not directly visualized. Augmentation of flow through the noncompressible segment may also demonstrate patency, although this maneuver may not exclude a partially occlusive clot. If there is uncertainty regarding the presence of DVT, a correlative study with magnetic resonance venography or computed tomographic scanning may be required for further evaluation.
FIGURE 21-7 A, Transverse view of the common femoral vein (CFV) with faintly echogenic material within it. B, Probe pressure is being exerted over the vein, and the thrombus is preventing the compression of the vein. This is the key to positively identifying the presence of thrombus within the vein.
FIGURE 21-8 A, Transverse view of echogenic material seen within the vein (arrow) that appears to be thrombus. B, Transverse view of the same vein with probe compression. Note how the vein compresses completely (arrow), indicating the material previously visualized was only stagnant blood, not a thrombus.
FIGURE 21-9 Compression of the vein (arrow) is being attempted. It is uncertain whether there is echogenic material seen within the vein. However, the vein does not fully compress. This might suggest the presence of thrombus that is lightly echogenic. In this case, the lack of compressibility of the vein is occurring because the patient is bearing down in response to the painful compression, thus not allowing the vein to collapse completely. This is a common cause of a false-positive venous duplex. For this reason, whenever echogenic material is not directly visualized but the vein does not compress fully, the examiner must either equivocate, try another position for compression, perform the augmentation maneuver with color Doppler, or compress harder over the vessels so that the artery compresses. These techniques improve the detection of thrombus within the vein.
Once thrombus has been identified, the next step is to try to gain some information about how fixed or poorly attached the clot is and the likelihood of embolization. Generally speaking, the newer the clot, the more likely it is to embolize. This, as you might imagine, is a very difficult task and not an exact science. However, there are clues to the age and stability of a given thrombus. Characteristics usually associated with acute clot are the following:
When a thrombus has just formed, it is very faintly echogenic—almost invisible. When the clot is acute, it may be detected by limiting the compression of the vein and by the presence of a faintly visible edge to the thrombus (Figure 21-10). The experienced examiner will spot faint echoes within the vein and note the difficulty in compression (Figure 21-11). Thrombi at this stage are extremely spongy in texture, so the vein will deform with probe compression (but still not allow complete collapse of the vein). Thrombi at this stage of formation may be attached to the vein wall over only a small area with the remainder of the clot looking like a snake that sways back and forth in the flow stream (“free-floating” DVT) (Figure 21-12). The fact that these poorly attached clots might be more likely to break loose seems logical, although this seemingly obvious conclusion is not universally accepted.