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Commentary |
1
Department of Radiology, The University of Texas M. D. Anderson Cancer Center,
1515 Holcombe Blvd., Box 57, Houston, TX 77030.
2
Institut fur Medizinische Physik, Friedrich-Alexander-Universitat
Erlangen-Nurnberg, Krankenhausstr. 12, D-91054 Erglangen, Germany.
Received September 21, 2000;
accepted after revision November 1, 2000.
Address correspondence to P.M. Silverman.
Introduction
 Top Introduction References |
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When introduced at the Radiological Society of North America meeting in
1989, spiral CT was easy to comprehend
[1]. There was only one new
scan parameter, the table feed per gantry rotation (d), because there was now
a shift from "step and shoot" technology to one in which there was
continuous table movement with continuous transport of the patient through the
scanner without pausing to perform a slice. Incidentally, this parameter was
equal to the table speed, because there was a fixed rotation time of 1 sec and
only a single row of detectors available. The practical impact of this new
parameter on radiologists and technologists was to introduce the new term
"pitch" (P) as the ratio of table feed and collimated slice- or
beam-width, which were synonymous.
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Subsequently, spiral CT options were offered successively by major manufacturers and the synonymous term, helical CT, was introduced in 1992 without major confusion or controversy in terminology [2,3]. An understanding had developed that dose decreases linearly, and that slice sensitivity profiles deteriorate slightly as a function of increasing pitch, with pitch clinically chosen varying between 1 and a maximum value of 2. Protocols have been developed that optimize these trade-offs between pitch and collimation to improve both routine axial and 3D imaging of various organ systems [4,5]. At approximately the same time, dual-helical technology was released (Elscint Medical System, Haifa, Israel), in which two simultaneously interweaving helices allow the acquisition of two slices for each gantry rotation.
New detector technologies were introduced in 1998, offering more options than the previously known single-detector or the dual-detector introduced in 1992. In fact, different arrays of between eight and 34 rows became available, yielding up to four interweaving helices generating four slices at a time [5,6]. Independent of the number of rows (D), the number of simultaneously acquired interweaving helices (M) is still limited to four. Depending on the scanner, M = 2 (dual) or M = 4 (quad) may be available. Eight-slice scanners are already in production. The number of simultaneously acquired helices will certainly increase in the future; scan modes with M > 4 are in development. For the end user, M is extremely important, because it is directly related to acquisition speed and has significant impact on the development of protocols and the timing of scanning related to IV contrast medium administration.
Many terms have been used to describe this new technology: multidetector-row CT, multirow helical CT, multidetector helical CT, and multislice helical CT. The latter, multislice helical CT, most effectively denotes the fact that the result of a single rotation is not one slice but multiple slices. Conversely, the way that each vendor sets up its detector rows (multidetector row CT) is vastly different and does not clearly convey the end result: more slices for each rotation. The term "multislice helical CT" has been used widely in the technical and clinical literature to describe this new technology [6,7].
Controversies have arisen regarding the definition of the pitch (P) factor
in multislice CT. Unfortunately, manufacturers define pitch in two ways for
multislice CT: "volume or slice pitch" and "beam
pitch." Slice pitch terminology, P = d / S, is used by a single
manufacturer (General Electric Medical Systems, Milwaukee, WI). This results
in a definition of pitch that can be confusing to some end users not familiar
with this equipment, for example:
 |
Pitch has also been defined by manufacturers (Siemens Medical Systems,
Erlangen, Germany; Marconi Medical Systems, Cleveland, OH; and Toshiba Medical
Systems, Tokyo, Japan) as:
 |
As defined by General Electric Medical Systems, a beam pitch of 0.75 is equivalent to a slice pitch of 3 (HQ); a beam pitch of 1.5 is equivalent to a slice pitch of 6, Hi-Speed (HS). In contrast to other manufacturers, General Electric has shown a "sweet spot" where P = 3 and P = 6 have a slightly better quality than other pitches. With P < 1, scanning is performed in an overlapping fashion. The HQ and HS modes are the only two modes available on this vendor's scanner. Thus, a binary decision must be made by the user when protocols are devised. If axial image quality is of primary importance, then the HQ mode is selected. When operated in this mode, coverage is limited by the table speed and the ability of the patient to hold his or her breath. The HS mode is applied when the operator wants to prioritize maximal coverage, with some compromise in image quality being acceptable. Each of these modes may allow different choices in subsequent reconstruction of slice thicknesses in the axial plane. In contrast to General Electric, the other manufacturers have taken a different approach with slice thickness and coverage selected, in which pitch is the result of these selections. Technical considerations related to current amperage and potential kilovoltage are made by all manufacturers in a generic approach.
The respective manufacturers have not redefined the dose index when using CT. Therefore, an accurate comparison of radiation dose can be made between units regardless of vendor, because the formula for this takes into account the slice thickness and the number of simultaneously acquired slices. Just as the definition for dose has remained unaltered, it would seem best if the more commonly used clinical definition of pitch be made uniform so that we can compare scanner specifications more easily.
We propose that scientific journals adopt a uniform way to define pitch to provide for clarity of expression. In the era of multislice scanning (e.g., P = 0.75 [HQ = 3]), a uniform definition would make it easier for all end users to compare various scan protocols and to understand the terminology used by the different vendors. Such uniformity would simplify the ongoing transition from routine single-helix scanners to multislice scanners. In the long run, all radiologists will benefit from a common nomenclature when trying to understand and evaluate research and when developing new clinical protocols.
Acknowledgments
We thank Brenda J. Sommerville, of the M. D. Anderson Cancer Center's
Division of Diagnostic Imaging, for her technical support.
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