Introduction

The B-mode image-forming processes described so far have assumed an ideal imaging system operating in an ideal medium. As described in Chapter 2, real ultrasound beams have significant width and structure, which change with distance from the transducer, and ultrasound pulses have finite length. The speed of sound and the attenuation coefficient are not the same in all tissues. These real properties give rise to imperfections in the image, which are essentially all artefacts of the imaging process. However, those that are related primarily to the imaging system (beam width, pulse length, etc.) are usually considered as system performance limitations, as they are affected by the design of the system. Those that arise due to properties of the target medium (e.g. changes in attenuation and speed of sound) are considered as artefacts of propagation.

Imaging system performance

The performance of a particular B-mode system can be characterized in terms of image properties which fall into three groups, i.e. spatial, amplitude and temporal. At the simplest level, spatial properties determine the smallest separation of targets which can be resolved. The amplitude properties determine the smallest and largest changes in scattered or reflected echo amplitude which can be detected. The temporal properties determine the most rapid movement that can be displayed. However, the ability to differentiate between neighbouring targets, or to display targets clearly, may depend on more than one of these property types.