Over the past 25 years, CT has become an invaluable diagnostic imaging tool for a wide variety of clinical applications. Helical CT technology [1, 2] and, more recently, multislice helical CT [3, 4] have produced dramatic improvements in scanner capability, in image quality, and in patient throughput. However, a growing concern about the radiation dose delivered to the patient during helical CT examinations has accompanied the rapid diffusion of this technology [5,6,7,8].

Dose reduction strategies used by radiologists and technologists in the daily performance of CT examinations include modifying scanning protocols and manipulating scanning parameters such as milliampere-second, peak kilovoltage, and collimation. In the case of single-slice helical CT systems, the radiation dose to the patient can be reduced by simply increasing the pitch, defined as the table increment per 360° of rotation divided by the nominal scan width, when other scan factors are held constant. We decided to conduct an experiment to determine whether the same relationship between the pitch and radiation dose applies to a multislice helical CT system installed in our department.

Our study focuses on comparing a new Somatom Plus 4 VZ (Volume Zoom) multislice helical CT scanner (version A11A; Siemens Medical Systems, Erlangen, Germany) with a single-slice Somatom Plus 4 (version B40C) helical CT scanner from the same manufacturer. All dose measurements used a 100-mm-long CT pencil ionization chamber and electrometer (model 10 × 5-10.3 CT chamber and MDH model 1015 electrometer; Radcal, Monrovia, CA). The CT chamber was placed 1 cm from the superior surface of a 32-cm-diameter acrylic body phantom. A constant volume (80-mm length) was scanned with 120 kVp and 100 effective mAs for all measurements.

For multislice helical CT scanners, manufacturers use different definitions of pitch, which has resulted in much confusion [9]. For the multislice CT scanner we described, the manufacturer defines “pitch” as the ratio of table movement per 360° rotation to single section thickness (P). We chose to use the definition of pitch [9] as table increment per 360° rotation divided by the total beam width (P′). This definition is applicable to both single- and multislice helical CT scanners, as shown in Table 1. Using slice combinations of 4 × 1 mm and 4 × 2.5 mm, the test volume was scanned on the multislice helical CT scanner at the manufacturer's defined pitch selections of 2, 4, and 8 (P′ = 0.5, 1, 2). At a slice width of 3 mm, the same volume was scanned at pitch selections of 0.5, 1, and 2, respectively, on the single-slice helical CT system for comparison. Three dose measurements were recorded and averaged for each pitch setting we tested.

The multislice and single-slice helical CT dose measurements are summarized in Tables 2 and 3. For the multislice helical CT system we tested, the measured radiation doses for all slice combinations were essentially the same, regardless of the pitch. At a slice collimation of 4 × 2.5 mm, for example, the absorbed dose measurements for pitches 2, 4, and 8 were 9.92 ± 0.09, 9.94 ± 0.15, and 10.12 ± 0.18 mGy, respectively. Also, for the multislice helical CT system, the actual recorded tube current increased proportionately with an increase in pitch despite the nominal setting of 100 mAs that we entered before scanning.

In contrast, for the single-slice helical CT system, as pitch increased, the measured radiation dose decreased proportionately. The dose measurements were 12.72 ± 0.02, 6.68 ± 0.07, and 3.62 ± 0.00 mGy for pitches of 0.5, 1, and 2, respectively. The actual recorded tube current setting remained constant at all pitch settings.

CT examinations are contributing substantially to the radiation burden of the population. Data from the United Kingdom [5] indicate that in 1998, the 370 CT scanners installed there accounted for approximately 4% of the diagnostic radiologic examinations performed but contributed 40% to the collective radiation dose the population received from medical imaging. Strategies to reduce the radiation to patients who undergo CT examinations can be broadly classified into two groups. First are strategies dealing with the basic design features of the CT system, which include the efficiency of the detectors, the X-ray tube characteristics, and the type of collimators used. Novel approaches to reduce dose while maintaining image quality are still under active investigation [10, 11]. Second, radiologists and technologists can modify basic scanning parameters such as peak kilovoltage, milliampere-second, and pitch; they can also alter their scanning protocols by reducing the number of images or data sets acquired during a CT study [8, 12].

Our experiment confirms that increasing pitch proportionately reduces patient radiation dose on a single-slice helical CT scanner when other parameters are held constant. This relationship is expressed mathematically as dose ∝ mAs / pitch. On the multislice helical CT scanner we examined, the same relationship holds true. However, when the pitch selection is increased on this particular system, a proportionate increase in tube current is automatically made, presumably to maintain similar noise conditions in the clinical image. This fact explains why, as shown by our experiment, an increase in pitch produced no reduction in the radiation dose to the phantom, because both pitch and tube current scale in opposite directions (Table 4). Had we been aware of this idiosyncrasy of our multislice CT scanner, perhaps we could have anticipated this unexpected result.

From experience over the past decade, radiologists have become accustomed to the notion that by increasing pitch on helical CT scanners, they can proportionately decrease the radiation exposure to patients being scanned [12]. The purpose of our report is to suggest that radiologists exercise caution when extrapolating this concept to multislice scanners.

Comparing the definition of pitch for single- and multislice helical CT scanners can be confusing. In fact, Silverman et al. [9] have suggested adopting a uniform definition of pitch applicable to both single- and multislice CT systems. We agree with their commentary and used their definition of pitch for this study.

Multislice CT technology offers superb image quality, reduced examination time, and the ability to perform complex multiphase vascular and three-dimensional examinations. However, our experiment suggests that the strategy of increasing pitch for radiation dose reduction on single-slice helical CT scanners may not be safely applied to all multislice helical CT systems. To maximize the clinical benefit of multislice helical CT while limiting the radiation our patients receive, radiologists and physicists must acquire a thorough, machine-specific understanding of the multislice equipment of their chosen manufacturer.

Address correspondence to M. Mahesh.