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Original Report |
1 Department of Radiology, Royal University Hospital, 103 Hospital Dr.,
Saskatoon, SK S7N 0W8, Canada.
2 Saskatchewan Labour, Occupational Health and Safety Division, Saskatoon, SK
S7K 2H6, Canada.
3 Department of Nuclear Medicine, Regina General Hospital, Regina, SK S4P 0W5,
Canada.
Received March 12, 2004;
accepted after revision June 14, 2004.
Address correspondence to D. Leswick
(davidleswick{at}hotmail.com).
Abstract
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CONCLUSION. Because the total measured radiation dose is 32% greater from a single combination helical HRCT scan of the chest versus separate standard helical plus axial HRCT scans, helical HRCT is not a clinically advisable technique.
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In our department, most HRCT scans of the lungs are performed as a separate axial acquisition in conjunction with a standard helical examination to assess the entire chest. This requires scanning the patient twice. The goal of this study was to determine if radiation exposure could be decreased by performing a single helical scan of the chest from which both 5-mm standard and 1.25-mm HRCT images could be obtained.
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A female RANDO phantom (The Phantom Laboratory) was used to determine radiation exposure during the various scan protocols. The RANDO phantom is built on a human skeleton using material radiologically equivalent to human soft tissues and lungs. A total of 82 thermoluminescent dosimeter (TLD) chips (Harshaw, model 100, Thermo Electron) calibrated at 120 kVp were placed within the predrilled holes in the 2.5-cm axial sections of the phantom. Over the three scans, a total of 58 TLDs were placed in the central axis of the thorax (single location in either the mediastinum or lung in each of the phantom sections 11–22); 12 TLDs, in the lateral aspect of the breasts (phantom section 17); and 12 TLDs, in the ovaries (phantom section 33) (Figs. 1A and 1B).
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A single scan of each of three separate CT protocols was performed on the phantom as follows: protocol 1, standard helical chest (1.25 mm x 8, 185 mA [130 mAs], 120 kVp, 0.7-sec scan rotation, 0.875 pitch) plus posteroanterior and lateral scout images; protocol 2, axial HRCT (1.25 x 10 mm, 170 mAs, 120 kVp, 1.0-sec scan rotation) without scout images; and protocol 3, combination helical (1.25 mm x 8, 320 mA [225 mAs], 120 kVp, 0.7-sec scan rotation, 0.875 pitch) with posteroanterior and lateral scout images. Both contiguous 5- and 1.25-mm HRCT images could be extracted from the combination helical scan. Each TLD used to measure absorbed dose was reviewed twice using a Harshaw model 5500 Automatic TLD Reader (Thermo Electron).
Exposure data in milliroentgens were converted to absorbed dose in milligrays. The conversion factor was calculated on the basis of a peak kilovoltage of 120 (average kiloelectron volt of 60). An estimate of the fat, soft-tissue, and air components of each site (chest, breast, and ovary) was made to calculate the f-factors. The resulting f-factors were 0.89 for chest, 0.84 for breast, and 0.93 for ovarian regions [3].
More than one TLD was located at most sites, which allowed us to determine the SEM for all TLDs at that location. For the few central chest measurements in which there was a single TLD in a section, error was estimated using the SEM for the two reviews of that TLD and manufacturer-specified instrument errors. Overall error for an organ (central chest, breast, and ovaries) was determined using the quadrature summing of the errors for each location within the organ. Error propagation was accounted for in the additive groups using quadrature summing (i.e., standard chest plus axial HRCT). Statistical analysis comparing dose between standard helical plus axial HRCT and combination helical scan TLD data was performed using the Wilcoxon's signed rank test for total measured study dose. Statistical analysis for central chest, ovarian, and breast dose was performed using a t test for independent samples.
The dose–length product was recorded from the CT scanner for each of the protocols. The effective dose (ED) was calculated from the dose–length product using the thorax conversion factor of 0.017 mSv/(mGy x cm).
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The measurement error of the data varied with location and was maximal in the combined data due to quadrature summing. The organ-based error was maximally 5.78, 2.59, and 0.05 mGy in the chest, breast, and ovarian locations, respectively. The organ-based errors for each scan are shown as error bars in Figures 2A and 2B.
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For the standard helical, axial HRCT, and combination helical scans, the dose–length products were 481.9, 181.9, and 882.4 mGy x cm, respectively and the EDs were 8.2, 3.1, and 15.0 mSv, respectively. This indicates 33% greater ED from the single combination helical protocol compared with the standard helical plus axial HRCT protocols.
Average ovarian dose (Fig. 2B) was low (0.21–0.37 mGy) during all scan protocols. Average ovarian radiation dose from the standard helical plus axial HRCT scans was slightly greater by 0.12 mGy (25%) than dose from the combination helical scan alone (p < 0.01).
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Helical HRCT image quality and diagnostic efficacy have been previously investigated. Honda et al. [7] showed that diagnostic efficacy was similar for low-pitch helical multidetector HRCT compared with axial HRCT. However, in the Honda et al. study the same milliampere-second was used in both the helical HRCT and axial HRCT techniques with resultant greater noise in the helical HRCT images [7]. This agrees with other investigators who have shown that reduction of milliampere-second is associated with increased mottle during CT chest examinations [1, 4, 8, 9]. Webb et al. [1] indicate that although helical multidetector HRCT techniques have not yet been well established, their experience has shown that helical HRCT can provide excellent lung detail.
Previous studies have shown that lower dose axial HRCT can have similar diagnostic efficiency with conventional-dose axial HRCT [1, 9]. However, low-dose techniques are not generally recommended because they have increased image noise and may obscure some findings such as interstitial lines or ground-glass opacity [1].
If a helical HRCT technique is to be truly comparable to conventional axial HRCT, image noise must be equivalent. Because noise is defined as the SD of the attenuation value in a homogeneous object [10], we could use the homogeneous water phantom to determine that 225 mAs was the helical milliampere-second at which noise was equivalent with our axial HRCT technique.
The dose–length product and the calculated ED are estimates based only on the selected CT protocol parameters [11]. Although this is an accepted indicator of dose, it cannot assess for distribution of dose within the patient. The use of the phantom with direct TLD measurement enabled us to assess dose to specific organs, in addition to determining the total dose.
The TLD data showed that central chest and breast radiation exposure from a single-scan combination helical HRCT was significantly greater than a standard helical scan plus separate axial HRCT. This correlates well with the dose–length product and ED values. Given this greater radiation dose, helical chest HRCT is not recommended as a replacement of the current technique.
There was relatively greater radiation dose in the mid zones of the chest compared with either the lung apices or bases during both helical techniques. During CT, radiation dose at a specific location is also influenced by tails of radiation profile from adjacent scanned sections [10]. Because there are fewer adjacent sections scanned at the lung apices and bases, radiation dose is lower than at the mid portions of the chest.
Radiation dose was higher in the central core of the chest than in the lateral aspect of the breast (phantom slab, section 17), even though the breast TLDs were closer to the skin surface. This relatively greater central axis radiation dose is hypothesized to be due to the shorter anteroposterior than transverse diameter of the chest of the phantom. Radiation dose at any location in an axial slice is partly determined by beam attenuation of overlying tissues from the multiple beam projections. Although CT dose is lower in the center of a circular object if there is significant radiation absorption from more superficial structures, in objects with shorter diameters, exit radiation is less attenuated by tissue absorption, resulting in relatively greater central dose contribution from exit radiation [10]. For a nonhomogeneous object with a noncircular cross section, attenuation varies strongly [12]. Because milliampere-second was held constant during the helical rotation, the ovoid shape of the thorax of the phantom may have allowed a relatively greater summation of radiation at the core of the chest than the lateral aspect of the breast, thus accounting for the higher central radiation dose.
There was a slightly greater ovarian radiation dose from the standard helical plus axial HRCT scan versus the combination helical scan. Because the ovaries are distant from the chest, their low radiation dose was almost exclusively from internal scatter. The discrepancy with thoracic findings is likely a result of differences between direct and scatter radiation. Ovarian dose was greater for the combination scan compared with the standard helical alone, as expected, because scatter increases with greater milliampere-second and scanning time was the same for both the standard and combination helical scans. The increased total scanning time from the addition of the axial HRCT is the likely source of the higher scatter radiation from the standard helical plus axial HRCT compared with combination helical scan alone. However, the small absolute difference in ovarian radiation dose (0.12 mGy greater from the standard helical plus axial HRCT scans) is a relatively small cost when compared with the radiation savings in the thorax.
In conclusion, radiation absorbed dose is 32% greater using a single combination helical HRCT scan of the chest versus the separate standard helical plus axial HRCT protocol. Combination helical HRCT scanning of the chest is, therefore, not a clinically advisable technique.
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