Introduction

The basic principles of B-mode scanning were introduced in Chapter 1. The way the beam is formed and swept through the patient (scanned) in different types of scanner will now be described in more detail.

The transducer is the device that actually converts electrical transmission pulses into ultrasonic pulses and, conversely, ultrasonic echo pulses into electrical echo signals. The simplest way to interrogate all the scan lines that make up a B-mode image is to physically move the transducer so that the beam is swept through the tissues as the pulse–echo cycle is repeated. This was the original method used but it has been superseded by electronic scanning methods which use multi-element array transducers with no moving parts. Array transducers allow the beam to be moved instantly between positions, and give the additional benefit of allowing the shape and size of the beam to be changed to suit the needs of each examination. The beam-former is the part of the scanner that determines the shape, size and position of the interrogating beams by controlling electrical signals to and from the transducer array elements. In transmission, it generates the electrical signals that drive each individual transducer element, and in reception it combines the individual echo sequences received by all the transducer elements into a single echo sequence.