ABSTRACT
A primary, if not the primary, source of clinical CT data is helical CT. In helical CT, also called spiral CT, the CT gantry (i.e., the x-ray source and detectors) rotates about the patient as the patient is moved forward along the axis parallel to the axis of rotation of the CT gantry (z-axis). As a result, the projection data are obtained along a helix of a cylinder. The data on the helix are then interpolated to generate CT slice data with which standard CT reconstructions can be performed (Calhoun et al. 1999; Schaller et al. 2000). Over the past decade, multiple detector rows have been added to allow faster scanning of the patients. Faster scans have opened the door for many applications, including imaging of moving structures, such as the heart or the lungs. In addition, helical CT changed CT presentation and analysis substantially and significantly. Before helical CT, individual slices were obtained as the gantry rotated about the patient, the patient was moved, and the data for the next slice were obtained. Thus, the CT data resembled a stack of pennies, with each slice being “independent” of the other slices, often disrupting the continuity of the various structures. With helical CT, the data in the slices are coupled due to the interpolation of the acquired data (see Chapters 2 and 3). Indeed, CT slices can be reconstructed for arbitrarily small sampling distances between slices. However, improved sampling does not in general improve resolution that is determined by the detectors and the pitch (distance moved along z during one rotation). Because of this coupling, the structures in helical CT data sets are continuous (when there are no artifacts; see the following section and Chapter 4 and 11). In addition, the voxel size along the z-axis can be chosen relatively arbitrarily, so that isotropic voxel sizes (the same along all three axes) can be chosen. The coupling of the CT data across slices and the “isotropy” of the voxel dimensions give rise to a continuity of the structures that was not possible in standard CT.
