It has the advantage of imaging a moving column of blood and does not require ionising radiation or iodinated contrast, but the technique has obvious drawbacks in terms of cost efficiency and accessibility to scanners. A variety of imaging sequences are used depending on the vessels being studied and the field strength of the machine. The most commonly used techniques include time of flight, two and three dimensional angiography and phase contrast. Magnetic resonance angiography is well established for examining the cerebral vessels and the carotid arteries, and its role in other territories is being extended. Ultrasound detection of deep vein thrombosis.
Pressure is applied every few centimetres by compressing the transducer head against the skin.
Deep veins and arteries lie together in the leg, and the normal vein appears as an echo-free channel and is usually larger than the accompanying artery. Venous ultrasonography is a very accurate method of identifying deep vein thrombi from the level of the common femoral vein at the groin crease to the popliteal vein but is less reliable for diagnosing calf vein thrombosis. Criteria for diagnosis of deep vein thrombosis Failure of vein to collapse on direct compression Visualisation of thrombus within lumen Absent or abnormal venous pulsation on Doppler scanning Venous reflux Colour duplex scanning has revolutionised the investigation of the lower limb venous system because it allows instant visualisation of blood flow and its direction. Thus, reflux at the saphenofemoral junction, at the saphenopopliteal junction, and within the deep venous system, including the popliteal vein beneath the knee and the gastrocnemius veins, can be detected without invasive techniques. Colour duplex scanning of saphenopopliteal junction.