The development of multislice technology represents a substantial technological advance in CT. The dramatic 4- to 8-fold increase in imaging speed not only makes the examination more comfortable but also provides a higher quality of examination, more flexibility for scanning with thinner sections, increased flexibility in scanning during multiple phases of hepatic enhancement, including the early and late hepatic arterial phase (HAP) and the portal venous phase (PVP) phase, and the ability to perform exquisite vascular, 3-dimensional (3-D) imaging,
When conventional helical CT is utilized, there is a continuous acquisition limited to a single slice. This is related to the intrinsic technology of the scanner, which consists of a detector made up of an array of rectangular channels and usually measuring approximately 1 mm transversely and 20 mm in the z-axis (Fig. 1). The collimation determines the slice thickness. These scanners generally produce slice thicknesses of 1, 3, 5, 7 or 10 mm. With the transformation to multislice scanners, various manufacturers have developed different arrays (Fig. 1). Instead of being long and rectangular the arrays are divided into cross-sectional, rectangular matrices that generally fall into two different categories. One group has detector elements of equal width along the z-axis (matrix detectors) and the second has detector elements of unequal width (adaptive or hybrid array detectors). An example of the equal-width detector array would be the General Electric Medical Systems, whereas the unequal-width detector arrays are represented by Marconi, Siemens and Toshiba Systems. The latter systems tend to have thinner sections in the middle of the detector array and are, in general, sub-second scanners with scans times ranging from 0.8 to 0.5 s. Because of the new type of detector array, one must appreciate that there is reduction in efficiency based on the fact that there are thin septae between detectors in the z-axis that absorb radiation but produce no data, and that the systems generally produce four slices per rotation rather than one and thus the amount of unusable radiation (penumbra) is increased, thus an obligatory increase in the radiation dose (Table 1).
Recent technological advances in hardware and tracking software for conventional helical CT have made us familiar with the use of the term pitch, defined below.
As pitch increases, the table speed increases. The patient’s dose decreases, image noise increases, but there is improved coverage in the z (longitudinal) axis. In view of different vendors’ proprietary technological development, there are now two definitions of pitch for multislice. With the introduction of multislice CT, the definition of pitch depends upon the vendor. The first is the conventional definition as described above which applies to equipment marketed by Siemens, Marconi and Toshiba and a second definition applies to General Electric.
The first definition, based on ‘beam pitch’, is defined below.
In contrast the other manufacturer, General Electric, uses pitch as ‘slice pitch’ thus:
These definitions are synonymous, just different terminology! It is thus important to understand the differences when using or comparing equipment. A pitch of 0.75 (Siemens, Marconi, Toshiba) is equivalent to Pitch = 3:1 (HQ), General Electric. Additionally, the three vendors described have pitches, which are variable and range between one and two, whereas General Electric allows only a ‘binary decision’ using the high quality HQ (3:1) or High Speed HS (6:1) mode (Fig. 2). Using the technology available on this machine, it has been found that image quality related to increasing pitch has certain ‘sweet spots’ at 3:1 and 6:1 where there is better operational performance. Other vendors have found a rather smooth and decreasing curve with increasing pitch, explaining the different approaches in terminology. Understanding this is important in establishing patient protocols.
Using the detector technology available on the GE system, the smallest slice thickness is 1.25 mm and, with the collimator open to expose only these four central detectors, the total coverage is 5 mm. The second detector configuration is 4 × 2.5-mm slices exposing the central eight detectors, 10 mm. A third detector configuration is for 3.75 mm thick slices with the collimator open to the central 12 detectors (minimum slice thickness 3.75 mm). The fourth detector configuration is 5-mm thick slices with the collimator open to all 16 detectors and the minimum achievable slice thickness being 5 mm. While one cannot reconstruct thinner sections than the thinnest detector configuration, thicker slices can be reconstructed and can decrease any artifacts, i.e. posterior fossa between petrous bones, however, thinner slices can be reconstructed. Additionally, thinner sections can be reconstructed using specific choices as long as one realizes that scans cannot be reconstructed thinner than the individual slice thickness. For example, if the abdomen is scanned by the 4 × 5 mm detector configuration, P = 0.75 table speed = 15 mm/rotation, the minimum slice thickness is 5 mm. However, if it is scanned 4 × 2.5 mm, P = 1.5, identical table speed = 15 mm/rotation, the slices can be reconstructed at 5 mm thickness or the raw data used for thinner sections of 2.5 and 3.75 mm.