In addition, a large number of radiological applications have been developed for the Digital Fluoroscopy/ Fluorography image acquisition modes. These include:

• Digital Subtraction Angiography - which involves subtraction of images of opacified and non-opacified blood vessels;
• Digital Cardiac Imaging - which relates to the digitization of images produced in cardiac angiography;
• Digital Spot Imaging - which relates to the digitization of images produced in, for example, Ba studies of the alimentary system.

It should be noted that the term Digital Radiography (DR) could also be used to encompass X-Ray Computed Tomography in that CT also uses an imaging system which is interfaced to a digital computer. From the same viewpoint, DR could also encompass techniques which are used for bone densitometry and which rely on acquiring images at two different x-ray energies, eg Dual-Energy X-Ray Absorptiometry (DEXA). However, these techniques are beyond the scope of the treatment here - although dual-energy imaging methods will be referred to later in the context of a complex DSA imaging process referred to as 'Hybrid DSA'.

DSA, as the name implies, involves an image subtraction technique. As will be seen below, the technique involves more than simply applying a subtraction process in the digital image processor. In addition, it will be seen that variations on the subtraction theme have generated more complex digital angiography processes such as Hybrid DSA and temporal filtering. Furthermore, the type of technology applied, while based on the design of an X-Ray Image Intensifier/Video system needs to incorporate a number of modifications particular to DSA. Before addressing the technology however, some basic physics needs consideration so as to place a perspective on the subsequent discussion.