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Chủ Nhật, 18 tháng 3, 2018

Acute and Chronic DVT and Ultrasound (US)

SOURCE: Advanced imaging in acute and chronic deep vein thrombosis, Gita Yashwantrao Karande , Sandeep S. Hedgire , Yadiel Sanchez , Vinit Baliyan , Vishala Mishra , Suvranu Ganguli , Anand M. Prabhakar © Cardiovascular Diagnosis and Therapy. Cardiovasc Diagn Ther 2016;6(6):493-507

DVT and Ultrasound (US)[extracted]

US is used in evaluation of both symptomatic and asymptomatic DVT (patients at high risk of DVT). It is useful not only in assessing DVT but can also identify other conditions causing signs and symptoms indistinguishable from DVT.

Compression US
Compression US has been procedure of choice for investigation of suspected upper and lower extremity DVT for decades (14). Other modification to this technique like two-point compression US (15), extended compression US (16) and complete compression US (17) are used in different combinations at different institutions.





Venous duplex US
 Lower extremity venous duplex US combines 2 components to assess for DVT: B-mode or gray-scale imaging with transducer compression maneuvers and Doppler evaluation consisting of color-flow Doppler imaging and spectral Doppler waveform analysis. Respiratory phasicity and cessation of flow with the valsalva maneuver offer indirect evidence of patent abdominal and pelvic veins (18). The primary diagnostic US criteria for acute DVT remains non-compressibility of the vein with secondary diagnostic criteria being echogenic thrombus within the vein lumen, venous distention, complete absence of spectral or color Doppler signal within the vein lumen, loss of flow phasicity, and loss of response to valsalva or augmentation (18). US can also be used to differentiate acute from chronic thrombus. In acute thrombosis, vein is distended by hypoechoic thrombus and shows partial or no compressibility without collaterals (Figure 1). In chronic thrombosis, the vein is incompressible, narrow and irregular and shows echogenic thrombus attached to the venous walls with development of collaterals (Figure 2). According to American College of Radiology (ACR) guidelines and technical standards, lower extremity US should include compression, color and spectral Doppler sonography with assessment of phasicity and venous flow augmentation (19,20). Advantages of lower extremity venous Duplex US are that it is readily available, quick, cost effective, noninvasive, devoid of ionizing radiation, lacks need for intravenous contrast and can be portable for critically ill patients prone for developing DVT.








Limitations include that it is difficult and less sensitive in patients with obesity, edema, tenderness, recent hip or knee arthroplasty, cast, overlying bandages and immobilization devices. It also has limitations in patients who had previous DVT and have new symptoms shortly after the treatment. False-positive results include extrinsic compression of a vein by a pelvic mass or other perivascular pathology (21) and thrombosis in the distal popliteal vein. False-negative studies may occur in the presence of calf DVT, proximal DVT in asymptomatic (even high-risk) patients or in the presence of a thrombosed duplicated venous segment. In a systematic review of accuracy of US in diagnosis of DVT in asymptomatic patients, Kassai et al. suggested that US was accurate in proximal veins for diagnosis of DVT in patients hospitalized for orthopaedic surgery (11) with lower sensitivity in other settings.



Sonographic elasticity imaging (SEI)
As described previously venous duplex US is considered primary noninvasive imaging for DVT. However, this method cannot assess the age and maturity of the thrombus i.e., it cannot distinguish pure post thrombotic syndrome (PTS) (which develops in 20–50% of patients after DVT) from new development of acute DVT with or without PTS (22). It is important to distinguish acute on chronic DVT from PTS as the latter doesn’t require anticoagulant therapy with highly potent fast acting anticoagulants associated with high risk of bleeding (23). Also chronic clots are treated with oral warfarin sodium, which has better safety profile than heparin. SEI is the latest promising technique for estimating age of the thrombus. It uses tissue deformation to assess the tissue hardness and hence clot maturity (24,25). It has shown promising results in animals and in a few studies in humans. As SEI requires tissue to be deformed during imaging, it is consistent with venous duplex US which also requires compression. The degree of compression required for standard strain measurements on SEI is lesser than compression US that is beneficial for patients with swollen painful legs (26-28). Thus, even if evaluation of clot age does not work, just the use of SEI to detect thrombi may be an improvement over the present compression US technique. SEI can hence be incorporated into standard duplex US so that thrombus can be diagnosed and presumably aged simultaneously. SEI is in preliminary stages of investigations and there is limited data on its ability to determine the age of DVT in human subjects (26,29,30). It is highly operator dependent. Another limitation is that comparison between the two clots requires an internal standard with the same hardness in both images, as it is difficult to know the force that was used to deform the tissue in each case. Rubin et al. used the wall of the vein as standard but the hardness difference estimate was conservative due to lack of another standard reference (31). Another limitation is that it is difficult to distinguish between subacute and chronic thrombi that are closer in age (29).
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Conclusions



The incidence of DVT is increasing, not just in the lower extremity but also in upper extremity, where malignancy and central venous catheters are the major precipitating factors. While US still has advantages, it has various limitations and in such cases advanced imaging techniques such as MRI, should be considered. CTV, while sensitive, can be incorporated into CTPA in suspected PE but has the disadvantage of radiating sensitive pelvic organs especially in young patients. MRI will almost likely feature more commonly in DVT evaluation in the near future with new “blood pool” contrast agents allowing a comprehensive examination for PE and DVT in the same scan. Although advanced imaging techniques in nanotechnology/ biotechnology, molecular imaging and PET are also being investigated, these may not replace the established first line modalities in diagnosis, but may be useful as adjuncts in patients who are not good candidates for structural imaging like renal disease patients with contraindication for intravenous contrast. Most of these agents are under trial and will hopefully come into routine use in diagnosing this potentially fatal disease.
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