Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

.~! ~i !j t--------------Ij• •ssFIGURE 10-1. The homogeneous gray image (left) illustrates a radiographicprojection void of contrast. The chest radiograph (right) demonstrates a widedegree of contrast, as illustrated by the profile of image density below theimage. The profile under the left image is flat, demonstrating no contrast.a number of different steps that occur during image acquisition, processing, anddisplay. Different definitions of contrast occur for each step in the imaging process,and these different mechanisms are described below.Subject contrast is the difference in some aspect of the signal, prior to its beingrecorded. Subject contrast can be a consequence of a difference in intensity, energyfluence, x-ray energy, phase, radionuclide activity, relaxation characteristics, and soon. While most two-dimensional (2D) medical images are spatial in nature, and subjectcontrast is mediated through spatially dependent events, time-dependent phenomenacan produce spatially dependent subject contrast in some modalities such asin digital subtraction angiography or magnetic resonance imaging. Subject contrastis a result of fundamental mechanisms, usually an interaction between the type ofenergy ("carrier wave") used in the modality and the patient's anatomy or physiology.While subject contrast (like all concepts of contrast) is defined as the difference insome aspect of the signal, the magnitude of the difference (and the amount of con-