FUSIC DVT is the point of care ultrasound assessment of the proximal lower limb deep veins to assess for DVT.
It is important to realise this is a rule-in scan, unlike a departmental lower limb doppler ultrasound which is a rule-out scan for DVT.
Visualisation of thrombus
The most obvious way of DVT identification on ultrasound is direct visualisation of the clot within the lumen of the vein.
It is important to realise that an acute clot may not be visualised on 2D ultrasound within the vein lumen which is why compressibility and colour flow assessment are important.
Right popliteal vein with visualisation of the thrombus
Right common femoral vein with visualisation of the clot in the CFV (on the right of the image) - this image highlights how direct visualisation can be difficult.
The lumen is compressed because if there is no thrombus in the vessel lumen there will be complete wall to wall compression of the vein. As a clot forms in a vein lumen it prevents it being completely compressible. If there is partial compression of the vein this indicates a non-occlusive thrombus and if there is no compression this indicates an occlusive thrombus.
Complete compression of the right common femoral vein - indicating that there is no thrombus
Incomplete compression of the right common femoral vein with visualisation of the clot in the CFV (on the right of the image)
Colour flow doppler
Colour flow doppler can help detect the areas of flow around any clot formed in a vessel lumen. It could also show flow throughout the whole vessel indicating no clot. If there is no flow in the vessel this could indicated an occlusive thrombus.
The colour flow doppler detects flow away and towards the ultrasound probe and this is represented as a colour (Blue Away, Red Towards the probe), so if the probe is at exactly 90 degrees to the vessel there may be a lack of colour flow doppler. This is why colour flow is best interpreted in conjunction with the compression test.
Colour flow also helps distinguish arteries and veins. Colour flow doppler over an artery will appear pulsatile, whereas over a vein will appear phasic with respiratory variation.
A further useful test to assess for flow in a vein using colour flow doppler is augmented flow by squeezing the distal area of the limb being scanned and flow may then be seen on colour flow. This is useful if this augmented flow fills the whole vessel and a clot can be ruled out. It could also cause flow around a clot which margins will then be more clearly seen on colour flow doppler. It is important to not be too vigorous with attempting this as there is a theoretical risk of embolization.
Colour flow over the left common femoral vein indicating blood flow and no obvious thrombus
Colour flow over the right common femoral vein and colour flow can be seen moving around the thrombus indicating a non-occlusive thrombus
Ultrasound differentiation of arteries and veins
Thicker & Brighter
Much less compressible
Colour flow doppler
Phasic flow with respiration
Artery vs veins: Right popliteal fossa
The artery is seen to be pulsatile with pulsatile colour flow.
The vein is more superficial above the artery, is non-pulsatile and initially had little colour flow. The colour flow seen briefly at the end of the clip was augmented by a calf squeeze.
As this is a proximal leg scan the veins of interest run from the common femoral vein (CFV) down to the trifurcation of the popliteal vein distally.
At the level of the inguinal crease the great saphenous vein joins the common femoral vein. Moving down the leg the common femoral vein bifurcates into the femoral and deep femoral vein.
The femoral vein extends further down the leg and passes through the adductor canal just distal to the mid-thigh, where it forms the popliteal vein.
The popliteal vein is located in the popliteal fossa on the back of the knee and extends down to a trifurcation forming the anterior and posterior tibial veins and the peroneal vein. This is the end point for the scan.
The technique for the scan is a continuous compression test – compressing every 1cm from 2cm proximal from the junction of the CFV and saphenous veins to mid-thigh (where the femoral vein passes through the adductor canal), and then compressing in the popliteal fossa every 1cm from the distal 2 cm of the popliteal vein to its trifurcation.
This scanning technique covers three key areas where DVTs are most likely to present in the lower limb. The scanning zones are:
Start at the inguinal crease and identify the sapheno-femoral junction. This has the appearance of ‘Mickey Mouse’ – with the face being the common femoral vein and the ears are the common femoral artery and greater saphenous vein. The orientation of the ears depends on which leg is being scanned, but the common femoral artery is lateral to the vein with the greater saphenous vein attaching medially to the common femoral vein.
Compress with the ultrasound probe every 1cm from 2cm proximal to the sapheno-femoral junction to the junction of the deep and common femoral vein this is the end of zone 1.
The path of the ultrasound probe will have moved from the anterior groin crease down the antero-medial side of the thigh.
Right common femoral vein giving the characteristic 'Mickey Mouse' appearance on ultrasound. This is at the level of the inguinal crease.
This is the junction between the deep femoral and common femoral vein and extending down to the mid-thigh. As you scan down the leg a compression test should be performed every 1cm to the mid-thigh.
The ultrasound probe will have moved from the antero-medial side of the upper thigh to the medial side of the mid-thigh. Better image acquisition may be helped by externally rotating the leg of the patient.
Ultrasound appearance of the femoral artery and vein below the distal to the common femoral vein bifurcation into the femoral and deep femoral vein.
Compressions test of the femoral vein showing complete compression - indicating no thrombus
The third scanning zone is located by placing the ultrasound probe in the popliteal fossa on the posterior side of the leg. By flexing and externally rotating the leg there will be more room to place the ultrasound in the popliteal fossa.
Identify the popliteal artery and popliteal vein and assess the popliteal vein for compressibility. Compress every 1 cm from the distal 2cm of the popliteal vein until it’s trifurcation.
Popliteal fossa ultrasound appearance of the popliteal vein and popliteal artery. Scanning distally brings the popliteal trifurcation into view.
Compression test in the right popliteal fossa - if the pressure applied with the probe isn't perpendicular to the vessel it may not completely occlude as seen above. This is more difficult in the popliteal fossa and could lead to a false positive result.
Ultrasound of the right popliteal fossa - scanning from the popliteal vein down to the popliteal trifurcation.
Pulsatile flow is seen in the femoral artery. To left on the image is the femoral vein which is non-compressible with absent colour flow.
Tracking the femoral vein down the leg there is a lack of compressibility of the vein and absent flow.
Pulsatile colour flow doppler seen in the popliteal artery, to the left on the image is the popliteal vein where the thrombus can be visualised. There is also lack of compressibility - demonstrated below.
Lack of compressibility of the popliteal vein.
The evidence from ESC guidelines 2019
The majority of PE’s originate from lower limb DVTs and rarely from the upper limb.
Compression ultrasonography for DVT diagnosis has a sensitivity >90% and specificity ~95% for proximal symptomatic DVT. It shows a DVT in 30-50% of patients with PE.
For patients with suspected PE a 4-point (bilateral groin and popliteal fossa) compression DVT scan can be performed. A positive proximal DVT scan has a high positive predictive value for PE.
The only validated diagnostic criterion for DVT is incomplete compressibility of the vein – this indicated the presence of thrombus. Flow measurements can generally be unreliable.
For haemodynamically unstable patients with suspicion of PE a combination of echocardiography and point-of-care 4-point DVT scan may further increase specificity.
An echo without signs of RV dysfunction and a normal DVT scan exclude PE with a high negative predictive value (96%).
As mentioned above this is a rule-in scan and so if no DVT is seen, or the scan appears normal then this does not mean that there is no DVT, therefore the term negative scan is avoided. If a thrombus is seen in the vessel, the vessel lacks compressibility or there is absent blood flow on colour flow doppler then this is a positive scan.
It is important to accurately document the results of the ultrasound and save any images as evidence. Depending on your findings or the clinical picture further imaging may be required.
Content created by Ben Stoney
Design by Max Broadbent
The ultrasound images and clips used on this website have be reproduced following the local clinical governance guidance.