The clarity and accuracy of images produced by CT scanners have enabled CT to become one of the most widespread modalities for diagnostic imaging. Between November 1998 and October 1999, 26.3 million CT procedures were performed in the United States alone, based on a census survey of 90% of the nation's imaging centers. This represents a 16% increase over the 1996 survey result. It was estimated that the number of CT procedures jumped to about 62 million in the United States in 2006. Many books are dedicated to the subject of the clinical utility of CT scanners, and these books cover different types of recommended clinical protocols for different human anatomies. The goal of this chapter is not to replicate or summarize these protocols; instead, this chapter will address new clinical applications that have evolved in recent years and have had a significant impact on CT scanner design.
The definition of ânewâ clinical applications is vague. Many applications that were considered new and challenging to CT not long ago have become much less demanding on the scanners. One good example is CT angiography, the imaging of blood vessels opacified by a contrast medium. With helical and multislice CT, the scan of the entire vascular structure can be routinely completed in a single breath-hold. Many advanced visualization tools such as curved multiplanar reformation, MIP, surface rendering, and VR are readily available on many commercial scanners or workstations. The volume coverage speed is no longer the most important criterion for the performance evaluation of a CT scanner.
Three general trends have emerged in CT scanner usage. First, with increased scanner capability, radiologists are more inclined to use thinner slices for scanning. Thin slices have many technical advantages, such as reduced partial-volume effect and improved spatial resolution. In the past, thin slices were prohibitive due to limitations with the x-ray tube and volume coverage speed. For example, a conventional single-slice step-and-shoot scanner takes 60 sec to cover a 150-mm cervical spine with 1-mm collimation and 0.8-sec gantry speed. The time span clearly exceeds a single breath-hold limit for most patients. The same volume can be covered by a 4-slice scanner with the same gantry speed in 17 sec, and by a 16-slice scanner in less than 2.3 sec.