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Archived Issues of Radiology Rounds
MGH Department of Radiology Website
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Diagnosis of Lower Extemity Deep Vein Thrombosis
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- Compression ultrasound (US) is highly sensitive and specific for the detection of deep vein
thrombosis (DVT) in the upper leg
- Lower extremity US can give indirect evidence of pelvic DVT. However, MR venography is
recommended for direct diagnosis of suspected pelvic DVT
- US is not sufficiently sensitive to rule out thromboses below the knee and, if clinical suspicion
remains high, US examination should be repeated after a week because of the danger of thrombus
propagation into the thigh veins
- CT pulmonary angiography combined with CT venography of lower extremity is recommended for
patients with symptoms of pulmonary embolism to detect emboli in the lung and to screen for DVT
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Deep
vein thrombosis (DVT) is not only a common disorder, affecting some
250,000 people in the USA each year, but is also difficult to diagnose
clinically and potentially life threatening. In cases of untreated DVT,
it has been estimated that up to 50% develop pulmonary embolism.
Therefore, it is important to diagnose DVT. However, when patients are
symptomatic the cause may be Baker’s cyst, cellulitis, lymphedema,
chronic venous disease, or a musculoskeletal disorder rather than DVT.
Alternatively, patients may be asymptomatic but have DVT, especially
after major surgical procedures such as total knee or hip arthroplasty.
The D-dimer assay, which detects one of the products of fibrin
breakdown in the blood, is commonly used as a rapid initial test for
DVT as well as for pulmonary embolism. Studies have shown that a value
of <500 ng/mL from an ELISA D-dimer test rules out DVT and pulmonary
embolism in the vast majority of cases.
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Sensitivity and Specificity for Diagnostic Imaging
of Lower Extremity DVT
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Sensitivity |
Specificity |
| Compression Ultrasound |
93% |
98% |
| MR Venography |
100% |
100% |
| CT Venography |
89-100% |
94-100% |
Sonography
US imaging has both high sensitivity and specificity for the detection
of thromboses in the proximal leg veins. In gray scale (B-mode) US,
normal veins appear dark, whereas a blood clot is more echogenic. In
addition, the veins are typically dilated if DVT is present. The
definitive test is compression US, as pressure applied by the scanning
probe over the region of interest will compress a normal vein but not
one that contains thromboses. Color Doppler imaging provides more
information on both the speed and direction of flow. If there is an
occlusive blood clot, it will show no flow within the vein segment or,
if partially occlusive, there may be limited flow surrounding the blood
clot.
In addition, there are ultrasound techniques that give indirect
evidence of thromboses above and below the site of examination. During
inspiration and expiration, there are changes in pressure within the
body that |
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Compression
US of the right leg in (A) a patient with and (B) a patient without
common femoral vein thrombus. Note that a vein with acute thrombus
demonstrates no intraluminal echoes. Thrombus is only detected by lack
of compression. In contrast, a normal vein is easily compressible.
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modulate the flow rate in healthy veins, which can be detected by
Doppler ultrasound. If no fluctuations in flow rate are observed, there
may be an obstruction present. The Valsalva maneuver will increase
the pressure within the body and will affect venous blood flow in a
normal system, except in cases of pelvic or inferior vena cava
obstructions in which lower extremity venous pressures is greater than
that produced by the Valsalva maneuver. Thus, if there is no change in
blood flow, there is indirect evidence of an obstruction above the
level of the ultrasound transducer. In addition, pressure on the calf
muscles or thigh muscles will normally increase the blood flow through
the deep veins of the thigh. If no increase in blood flow is detected
in the femoral vein, then this is indirect evidence that that there is
an obstruction between the site of manual compression and the
transducer.
Direct US examination of the calf veins is only performed if there are
indirect indications of DVT below the knee. US is less sensitive for
detecting thromboses in the deep veins of the calf because it is not
always possible to visualize all three of the major veins in this
region. Therefore, if no DVT is detected but symptoms or suspicion
persists, the US examination should be repeated after a week to detect
formerly occult calf vein thrombus that might have propagated into the
deep popliteal or femoral veins.
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Right
lower extremity (A) transverse and (B) saggital images from color
Doppler ultrasound demonstrates blood flow in the femoral artery but
not in the common femoral vein (arrows). This is an indirect finding
that suggests common femoral DVT.
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Techniques for the diagnosis of DVT
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Advantages |
Disadvantages |
| D-Dimer assay |
Rapid
Inexpensive
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Sensitivity depends on assay method |
| Ultrasound |
Sensitive and specific for proximal lower extremity DVT
Least expensive imaging method
Can be done at patient’s bedside
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Lower sensitivity for DVT in calf and pelvis
Pain, bandages, or casts may limit or prevent examination
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| MR Venography |
Direct imaging of pelvic veins and vena cava
Highly sensitive and specific
No toxicity problems from contrast agent
Examination not affected by bandages or casts
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Contraindicated for patients with pacemakers and other metallic implants (see MRI Safety issue, Radiology Rounds, February 2005)
Not readily available |
| CT Venography |
Can be performed at same time as pulmonary angiography
Can be used when MRI is contraindicated
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Large volume of contrast agent required to achieve opacification of veins
Contrast agent toxicity
Radiation exposure
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MR Venography
MR
venography will directly detect thromboses in the deep veins of the
pelvis, abdomen, and extremities but it is not used as an initial
examination because it is expensive, MR scanners are not available on
short notice, and the examination is time consuming. However, if US
imaging indicates a suspicion of DVT downstream of the upper leg, the
patients should be further evaluated by MR venography of the pelvis and
abdomen. MRI has several advantages over CT for this purpose.
Visualization is better because MRI is sensitive to low concentrations
of contrast agents. In addition, MR contrast agents are well tolerated
and there is no exposure to ionizing radiation. Furthermore, although
the spatial resolution of MRI is lower than that of CT, this is not an
issue because of the size of the deep pelvic veins. However, MRI may be
contraindication for some patients, such as those with pacemakers or
other metallic implants. In these patients, CT venography can be used
to further evaluate the extent of DVT.
In cases of suspected pulmonary embolism (see Diagnosis of Pulmonary Embolism, Radiology
Rounds, July 2003), CT angiography of the pulmonary arteries followed
by CT venography is recommended. The additional examination time
required for CT venography is only approximately 2 minutes and no extra
contrast agent is injected for the venography phase of the examination. |
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Scheduling
Ultrasound
evaluation for DVT is performed on the main MGH campus, Mass General
West Imaging in Waltham and in the MGH Chelsea Healthcare Center.
Appointments can be scheduled at all locations by calling 617-724-9729
or, for the main campus only, through the Radiology Order Entry system, http://mghroe
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Further Information
For further questions, please contact Suhny Abbara, M.D.
,
(617-726-0796) Director of Cardiovascular Imaging in the Cardiovascular
Imaging and Intervention Division. We would like to thank Michael
Laposata, M.D., Ph.D., Director of Clinical Laboratories, for his
assistance and advice for this issue.
This article provided useful information about the appropriate use of imaging studies:
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References
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American College of Radiology, (ACR). (1999). Suspected lower extremity deep vein thrombosis. Available at http://www.acr.org/ac_pda
Fraser, DG, Moody, AR, Davidson, IR, Martel, AL and Morgan, PS. (2003) Deep venous thrombosis: diagnosis by using venous enhanced subtracted peak arterial MR venography versus conventional venography. Radiology 226: 812-820
Kanne, JP and Lalani, TA. (2004) Role of computed tomography and magnetic resonance imaging for deep venous thrombosis and pulmonary embolism. Circulation 109 Suppl I: 1-21
Katz, DS, Loud, PA, Bruce, D, Gittleman, AM, et al. (2002) Combined CT venography and pulmonary angiography: a comprehensive review. Radiographics 22: S3-24.
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