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The Impact of Scan Length on the Exposure Levels in 16- and 64-Row Multidetector Computed Tomography

Rationale and Objectives

Higher patient exposure levels have been reported for 64-row multidetector computed tomography (MDCT) compared to 16-row MDCT. The objective of this study was to make a thorough comparison by evaluating the impact of scan length on the exposure levels at 16-row MDCT and 64-row MDCT.

Materials and Methods

Dose-length product (DLP) values were determined to compare exposure levels in 16- and 64-row MDCT. This phantom study does not deal with a possible reduction in image quality induced by an increase in scattered radiation in 64-row MDCT compared to 16-row MDCT.

Results

The exposure levels of 64-row MDCT (scan slice thickness, 0.5 mm) are up to 18% lower than those of 16-row MDCT at slice thickness 0.5 mm when scanning an object larger than 12.3 cm. At this value, the plots of the 16- and 64-row DLP values versus scan length cross. The DLP curves of 1- and 2-mm slice thickness 16-row MDCT are in closer resemblance to those of 0.5-mm 64-row MDCT. The respective exposure levels of 1- and 2-mm slice thickness 16-row MDCT exceed those of 0.5-mm 64-row MDCT by up to 4% and 3%, with intersections of 30 and 25 cm, respectively.

Conclusion

Lower effective doses are obtained in 64-row MDCT compared to 16-row MDCT (0.5-mm slice thickness) provided that scan length exceeds 12.3, 30, and 25 cm, for 16-row MDCT slice thickness of 0.5, 1, and 2 mm, respectively. Reduced effective dosage in 64-row MDCT compared to 16-row MDCT has not been demonstrated before. Differences in object size may thus explain discrepancies between previous studies with regard to the exposure levels at 64-slice CT compared to 16-slice CT.

Multidetector computed tomography (MDCT) has become increasingly important as a noninvasive imaging modality since its introduction in 1998 ( ). Research on patient dosage, a major concern in MDCT examinations, has been ongoing for many years ( ). In young patients in particular, special attention to this matter is required. An accurate analysis of patient dose in MDCT as a function of beam width, often referred to as collimation, is nevertheless still lacking.

The CT dose index (CTDI) value is frequently used as reference for dose exposed under clinical conditions. CTDI can be measured inside a defined phantom (CTDI weighted ) in air (CTDI air ) or in other ways. As CTDI measurements inside a phantom are linearly related to those in air, an analysis of exposure ratios can be done using CTDI air . Furthermore, CTDI represents the output of tube/filter combination under specified conditions. Exposure to the patient, specified as dose-length product (DLP), is used as measure of exposure in the scan procedure ( ).

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Materials and methods

System Parameters

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Effective Exposure Analyses

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CDTIair=1NT∫−50mm50mmd(z)dz[mGy] C

D

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(

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where N s the number of slices, T (mm) is the nominal slice thickness, d(z) is the dose profile for an axial scan, and the integral is taken over the interval of 100-mm scan length. Figure 1 demonstrates that the dose profile varies with the beam width, resulting in different CTDI values. Because the absolute width of the penumbra (hatched areas) and thus additional radiation exposure, due to the penumbra is almost independent of the nominal beam width, the contribution of the penumbra will have the greatest relative effect on both image quality and patient exposure if the nominal band width is small ( Fig. 1 , top).

Figure 1, Top : Typical relative dose profile as a function of beam width (BW). The penumbra is illustrated in the hatched area . Bottom : The relative contribution of penumbra to total exposure becomes less relevant with increasing beam width. Notice that the penumbras of both profiles are approximately the same. FWHM, full-width half-maximum; nom, nominal.

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dlpair=CTDIair×N×T(mGy⋅cm). d

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DLPair=dlpair×R(mGy⋅cm) D

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where R is the total number of rotations, which includes over-ranging (the hatched areas in Fig. 2 ). Mathematically, R can be expressed as:

R=L+overrangingBWnom×P R

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where L is the scan length, BW nom is the nominal beam width, and P is the CT pitch, which is determined by the couch-top movement and the number of slices ( ). The enumerator represents the exposure length. Over-ranging therefore relates to exposure length according to the following equation:

overranging=exposurelength−L(cm). o

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Figure 2, Schematic relation between scan dose profiles of 16-row multidetector computed tomography (MDCT) and 64-row MDCT using the same effective milliamperes. Over-ranging in 16-row MDCT is smaller than that in 64-row MDCT, whereas the CT dose index (CTDI) vol-air in 16-row MDCT exceeds that of 64-row MDCT.

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Radd=overrangingBWnom×P. R

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Figure 3, Measurement setup showing the ionization chamber and table couch. The ionization chamber is positioned free in the air during scanning and the table top outside the scan area.

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Results

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Table 1

Comparison of DLP air in 16- and 64-Row MDCT With Variable Scan Lengths

Exposure: length (cm) Scan: length (cm) DLP air (mGy · cm) 16-row MDCT 64-row MDCT 16-row MDCT 64-row MDCT 6.6 10.4 5 143.9 178.4 11.6 15.4 10 253.7 264.6 13.9 17.7 12.3 ⁎ 304.2 304.2 21.6 25.4 20 473.3 436.9 31.6 35.4 30 692.9 609.3 41.6 45.4 40 912.5 781.7 51.6 55.4 50 1132.1 954.0 61.6 65.4 60 1351.8 1126.4 71.6 75.4 70 1571.4 1298.7 81.6 85.4 80 1791.0 1471.1

DLP, dose-length product.

P = .9375 for 16-row multidetector computed tomography (MDCT) and P = .8281 for 64-row MDCT. Separate experiments had established that the CT dose index (CTDI) varies with beam width. For beam widths of 8 mm (16 × 0.5) and 32 mm (64 × 0.5), the CTDI air values were 20.6 and 14.3 mGy, respectively. For 139 mAs effective, this results in CTDI vol-air = 21.97 mGy (16-row MDCT) and CTDI vol-air = 17.27 mGy (64-row MDCT).

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Figure 4, Top : Dose-length product curves in 16- and 64-row multidetector computed tomography (MDCT). The crossing at 12.3 cm indicates that around that scan length nominal beam width does not play an important role in exposure levels. Bottom : Total scan times of both systems demonstrating that the amount of time saved by 64-row MDCT instead of 16-row MDCT increases with scan length.

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Figure 5, Top : Dose-length product curves using 16 × 1 mm and 64 × 0.5 mm. Bottom : Total scan times of both systems demonstrating increasing 64- versus 16-row multidetector computed tomography (MDCT) time savings as the scan length increases.

Figure 6, Top : Dose-length product curves using 16 × 2 and 64 × 0.5 mm. Bottom : Total scan times of both systems are similar at any scan length.

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Discussion

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Conclusion

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References

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