Ultrasound imaging principles of image formation

Ultrasound is an imaging modality which relies upon the use of sound waves which are transmitted into the body and then reflected back again from the structures being examined. The device which transmits the sound pulses is called a transducer. This contains a piezo-electric crystal which is able to convert electrical signal into sound and then convert the returning sound wave back into electrical signals again. Different reflecting surfaces or interfaces within the body reflect sound to different degrees. A strong reflector appears as a bright white area (echogenic) on the image, whereas areas from which no echoes have been obtained appear as black (anechoic).

Every ultrasound image is composed of a discrete number of lines of echo data placed side by side to appear continuous. Multiple lines of data are built up as the ultrasound beam sweeps through the field of view. In order to build up each of the single lines of data, the time taken for the returning echo is measured. This allows the depth of the reflecting interface to be determined. Each sweep of the ultrasound beam produces one frame of data (composed of multiple lines). Many complete sweeps are performed every second which produces the frame rate (frames per second). The operation is analogous to the operation of a television camera. At any one moment, the ultrasound beam is scanning along one of the many lines of sight which will ultimately form the image. The image is constantly updated at the prevailing frame rate. This can be frozen and hard copy images produced.

The resolution of the picture produced depends on the wavelength of the sound wave used - this can be varied. There is a trade off between resolution and penetration. Higher frequency transducers allow better resolution, but as the frequency is increased penetration is reduced. As a general rule, the highest frequency transducer should be used to achieve the penetration required.

An ultrasound image is a map of reflectivity of the body part scanned.

(Organs containing multiple interfaces will produce multiple echoes and this is characteristic of solid organs like liver, spleen and kidneys. Structures containing no interfaces will appear echo-free, such as liquid or urine in the bladder.

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