Performing<unk> <unk> <unk>  is based on the principle that sound can pass through human body tissues and is reflected by the tissue interfaces in the same way that light can reflect back on itself, from a mirror. Tissue in the body will offer varying degrees of resistance, known as acoustic impedance, to the path of the ultrasound beam. When there is a high impedance difference between two tissues, the interface between them will strongly reflect the sound. When the ultrasound beam meets air, or solid tissue such as bone, their impedance difference is so great that most of the acoustic energy is reflected making it impossible to see any underlying structures. The examiner will see just a shadow, instead of the image expected. Air will impede sound waves which is why a gel is used. The gel prevents air bubbles from forming between the probe, and the patient's skin and so helps the conduction of the sound waves from the transducer into the body. The watery medium also helps to conduct the sound waves. Liquids, including blood have a low impedance, which means that little energy will be reflected and no visualization possible. One of the important exceptions is that when the blood flow is very slow it can in fact be seen, in what is termed "spontaneous contrast". This technology is widely used in confirming<unk>  pathology diagnoses. The imaging capability needed, was made possible with the developments of Doppler and color Doppler. Doppler measurements using Doppler effect can show the direction of the blood flow and its relative velocity and color Doppler is the provision of color to help interpret the image, showing for example, the blood flow towards the probe in one color and that flowing away another. Whilst the equipment itself is costly, the procedure is not. Apart from the scanner, different probes are required according to the depth to be studied. A gel is used with the probe to make a good acoustic impedance contact. The training and expertise of the examiner is important because of the many technical complications that can present.<unk>  anatomy for example, is not constant, for example a patient's vein layout of the right limb is not identical to that of the left limb. The probe is an ultrasonic sensor, generally known as a transducer, which functions to send and receive acoustic energy. The emission is generated on piezoelectric crystals by the piezoelectric effect. The reflected ultrasound is received by the probe, transformed into an electric impulse as voltage and sent to the engine for signal processing and conversion to an image on the screen. The depth reached by the ultrasound beam is dependent on the frequency of the probe used. The higher the frequency the lesser the depth reached.
Ultrasonography of chronic venous insufficiency of the legs
Ultrasonography of deep vein thrombosis
Peripheral vascular examination
Coronary CT calcium scan
Assessment of kidney function
Cardiac magnetic resonance imaging perfusion
Intracoronary optical coherence tomography
Pressure–volume loop analysis in cardiology