Image reconstruction

Abstract

Coronary CT angiography (CTA) is a three-dimensional imaging technique. The data obtained during image acquisition, however, are not immediately evident to the radiologist, as the huge amount of digital information acquired is used to reconstruct three-dimensional images of the coronary arteries, according to dedicated and complex software and protocols. The coronary arteries are tortuous, moving objects and are thus often not easily visualized. For this reason, in CTA the movements of these arteries must be frozen, through cardiosynchronization, during image acquisition. In order to visualize the coronary arteries in CTA, a very high density of the internal lumen must be created. As specified in the previous chapter, this is accomplished by the dynamic injection of contrast agent. The vessels become evident because of the large difference in the density of the vessel lumen (high density: 300-500 HU, due to the iodine content of the blood during passage of contrast agent) and that of the surrounding tissues, such as epicardial fat (low density:-50-100 HU). The difference is important because during three-dimensional image reconstruction only structures with a very high density will be clearly visualized. The higher the density, the more evident the anatomic structures will be. This is the reason why the first three-dimensional images presented for clinical use were related to bone, a structure with a very high radiologic density. Starting from volume data, there are two ways to construct three-dimensional images of the anatomic structures of interest: the first is to explore the volume using curved or orthogonal bi-dimensional planes with so-called planimetric technique; the second is to consider and image the entire package of three-dimensional data, using so-called volumetric techniques. In this chapter, we offer a simple explanation of the difference in these visualization techniques and their relative importance in clinical use for the evaluation of coronary artery disease.