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This article describes the principles and evolution of multislice CT (MSCT), including conceptual differences associated with slice definition, cone beam effects, helical pitch, and helical scan technique. MSCT radiation dosimetry is described, and dose issues associated with MSCT-and with CT in general-as well as techniques for reducing patient radiation dose are discussed. Factors associated with the large volume of data associated with MSCT examinations are presented.
Key Words: CT; multislice CT; radiation dosimetry
J Nucl Med Technol 2008; 36:57-68
DOI: 10.2967/jnmt. 107.044826
This article, the third in a series of continuing education articles on the principles of CT, focuses on multislice CT (MSCT).
PRINCIPLES OF MSCT
Limitations of Single-Slice Slip Ring and Helical Scanners
Soon after their introduction in the late 1980s, slip ring scanners and helical (spiral) CT were rapidly adopted and soon became the de facto standard of care for body CT. However, a significant problem became evident: helical CT was very hard on x-ray tubes. For example, an abdomen-pelvis helical CT covering 60 cm (600 mm) of anatomy with a 5-mm slice thickness, a pitch of 1.0 (thus requiring 120 rotations), and typical technique factors (120 kilovolts [peak] [kVp], 250 mA, 1-s rotation time) deposits a total of 3.6 × 10^sup 6^ J of heat in the x-ray tube anode. Before slip ring CT, individual slices obtained with an equivalent technique (120 kVp, 250 mA, 1-s scan) would deposit only 30,000 J, much of which could be dissipated during the relatively lengthy (several seconds) interscan delay.
A limitation imposed by tube heating was that the thin slices (<3 mm) desired for acceptable-quality reformatting into off-axis images (coronal, sagittal, or oblique) or 3-dimensional reconstructions were impractical unless the scanned region was very limited or the scan technique was severely constrained. It was not uncommon for scanners to limit a helical technique with thin slices to 100 mAs (tube current in milliamperes x scan time in seconds) or less per rotation, yielding low-quality, noisy images.
A straightforward solution to this heat issue, of course, is to develop x-ray tubes with a higher heat capacity; such tubes have been asnd continue to be developed. Another approach is to more effectively use the available x-ray beam: if the x-ray beam is widened in the...