A-scan
/ˈeɪskan/noun
A radar or ultrasound scan in which the echo signals are used to generate a linear trace on an oscilloscope screen, the height of a peak being proportional to the strength of the echo; an image obtained by such a scan. Frequently attributive Compare B-scan.
Origin
1940s; earliest use found in British Patents. From A + scan, probably originally with reference to A as the first of a set, although later often reinterpreted as showing the initial letter of amplitude.