Extremity Venous Anatomy and Technique for Ultrasound Examination

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.


The three main goals in diagnosing venous thrombosis are to determine the following:



Venous duplex is uniquely suited for each of these tasks.


In order to perform venous duplex ultrasound of the veins, the examiner must understand the venous anatomy14 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.



Anatomy of the Lower Extremity


There are three kinds of veins that will be examined by duplex imaging:







Venous Duplex Imaging Examination Technique and Protocol


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.


There are three factors that will dramatically influence the quality of venous duplex imaging studies:





Patient Positioning for the Lower Extremity


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.



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.






Imaging Normal, Thrombus-Free Veins


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.



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.





Determining the Presence of Thrombus


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.







Characterization of Thrombus


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:



Characteristics usually associated with chronic clot are the following:




Acute Thrombus


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.

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Mar 5, 2016 | Posted by in ULTRASONOGRAPHY | Comments Off on Extremity Venous Anatomy and Technique for Ultrasound Examination
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