A-SCANNER

Typical Number in Hospital: 3

Cost Bands: 4

References: 1

An ultrasonic scanning device employing ultrasonic pulses in the megahertz range is used to detect the depth of reflecting structures within the body. The distance into the body is displayed on the X-axis of a cathode ray tube (CRT) and the returning echoes are displayed as vertical movements (Y-axis), the echo amplitude being shown by the extent of the vertical movement. A-scanners were first used in the 1950s to detect the correct position of the 'midline echo' in the brain, which originates from the falx cerebri. Early A-scanners were converted industrial echo-sounders intended for detecting micro- cracks in metals.

Now, medical A-scanners exist which are intended purely for midline detection and incorporate 'swept gain' correction circuits to compensate for the attenuation of ultrasound which occurs in human tissue. These are used in the casualty or in the X-ray department as a cheap and non-invasive method of identifying the possible cause of concussion where this may be due to the existence of 'space occupying lesions' (tumours, haematomas, etc.) which may be displacing the midline from its normal position.

In addition many other ultrasonic scanners include an A- scan display to show that the 'swept gain' circuit is set correctly and to enable accurate distance measurements to be made between echoes. This measurement facility has proved particularly useful in the measurement of bi-parietal diameter (BPD) in the foetus to establish gestational age.

An A-scanner would normally consist of a single disc piezo- electric damped transducer resonating at a chosen frequency between 2 and 15 MHz, a high voltage pulse generator to energize the transducer (at about 1 kHz), a tuned radio frequency amplifier, and the usual CRT drive circuitry. It is only really different from industrial versions by having a circuit to sweep the gain of the R.F. amplifier to compensate for absorption of the ultrasound in tissue.