There are two ways of presenting the information gathered from an ultrasound probe - A and B scans. The A scan (amplitude modulated scan) presents the information as signal strength against time. The B scan (brightness modulated scan) uses the signal strength to vary the brightness of the bits of the image on a screen.
A scans are used where the arrangement of the internal organs is understood and a precise measurement of distance is needed. If several B scans are taken of the same part of the body, all at once, all the scans can be assembled into images representing sections of the body.