Diagnostic ultrasound (US) equipment uses a piezoelectric crystal to convert electrical energy into mechanical movement generating focused sound waves. The same crystal is used to detect the reflected sound from soft tissue interfaces.
Ultrasound is suitable for examining soft tissues and bone surfaces although for the latter plain x-rays are the preferred technique. Fluid-containing structures do not reflect sound and are seen as black areas on the image.
Muscle is seen as a ‘herring bone’ pattern and fat produces many bright echoes. The technique is very dependent on the skill of the observer and it is unwise to attempt to interpret images away from the patient.
Uses
Mass Lesions
US is a cheap, rapid and safe method for the diagnosis of lumps. It is highly sensitive to the presence of a mass and in experienced hands is probably as sensitive as MRI and better than computed tomography (CT). with greater certainty than with CT or MRI.
However, the anatomical definition is less clear and demonstration of a solid lesion should lead to CT or preferably MRI to follow. US is therefore a useful screening tool to exclude patients with no mass lesion and to show cysts that may require no further analysis.
It is an easy and cheap way of following the resolution of haematoma, especially in coagulation disorders where the characteristics of the US image may be used to date the origin and to determine whether recent extension of the haemorrhage has occurred.
Tendon
US is the technique of choice in detecting tendon swelling or rupture. Tendonitis, paratendonitis and synovitis are readily apparent. Occasionally MRI will be useful in complex anatomical sites and for measuring the gap between the ends of a torn tendon.
Joints
US is very sensitive to joint effusion. In practice this is of most value in the hip when the exclusion of an effusion is of particular help in children with irritable hips. If fluid is found it is not possible to determine its nature by US but the method may be used to guide aspiration of the joint.
In neonates the unossified femoral head and acetabulum are easily seen by US. This allows detection of dislocation and dynamic instability. US has become an important dynamic technique in screening infants at risk of hip dysplasia and for follow-up of those undergoing treatment.
Deep Venous Thrombosis
US is an accurate method for the detection of femoral and popliteal vein thrombosis especially with the addition of Doppler flow studies. It is less accurate for calf vein thrombosis where venography remains the technique of choice.
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