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DOI:10.2214/AJR.05.1491
AJR 2006; 187:W498-W502
© American Roentgen Ray Society

Original Research

Comparison of MDCT Radiation Dose: A Phantom Study

William H. Moore1, Michael Bonvento1 and Rosemarie Olivieri-Fitt1

1 All authors: Department of Radiology, Stony Brook University, University Hospital HSC Level 4, Room 120, Stony Brook, NY 11794.

Received August 24, 2005; accepted after revision November 12, 2005.

 
Stony Brook, NY 11794. Address correspondence to W. H. Moore.

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Abstract
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 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
OBJECTIVE. Recently there has been a significant increase in the use of CT imaging resulting in a significant increase in radiation exposure to the population. Few studies have compared the degree of radiation exposure among the currently available MDCT units. Our objective is to make such a comparison.

MATERIALS AND METHODS. Using a Rando anthropomorphic phantom, we placed thermoluminescent dosimeters into the center, anterior, and lateral aspect of the lower chest of the phantom. Standard CT of the chest was performed with the current protocols used at our institution on 4-, 8-, and 16-MDCT GE Healthcare systems. Next, near-identical CT scans of the entire chest were performed on the same CT systems.

RESULTS. The 4-detector array showed statistically significantly higher radiation dose compared with the 16-detector array with near-identical technique (p < 0.01). There is a trend toward decreasing radiation dose with the increasing number of detectors using both standard and near-identical technique. An inverse relationship exists between measured radiation dose and the number of detectors.

CONCLUSION. We theorize that as the number of detectors increases, there is a decrease in the amount of nonutilized radiation exposure, thus resulting in a lower total radiation dose.

Keywords: CT technique • MDCT • radiation dose • radiation exposure


Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
The use of radiologic imaging, especially CT, has seen a significant increase over the past several years [1]. This increase in usage has raised the concern about radiation exposure and the health risks related to increased radiation exposure [2, 3]. Many studies have evaluated radiation exposure in CT [4-8], and it was the topic of a consensus statement by the Fleischner Society, in which a suggestion of an appropriate radiation dose for CT of the chest was made [9]. A study performed in 1989 found that although CT accounted for only 2% of all radiologic studies, CT made up 20% of the effective radiation dose [10]. A follow-up study preformed several years later showed that both the use of CT and CT-related radiation exposure had doubled [11].

A standard range of radiation dose for multiple organs has been reported in several studies [9, 12-14]. Since the advent of MDCT, several studies have evaluated the radiation exposure between single-detector CT and MDCT [6, 9, 10]. These studies have shown that there is an increase in radiation exposure of up to 27-36% with MDCT compared with single-detector CT. However, to date there has been no published study comparing radiation dose among the different available MDCT units. Groves et al. [15] compared the utility of a Monte Carlo-calculated radiation dose to thermoluminescent dosimeter-measured radiation dose and showed that the actual radiation dose was 18% higher than the calculated radiation dose with a 16-MDCT system.

The purpose of this study was to measure the radiation dose on 4-, 8-, and 16-MDCT with both standard and near-identical techniques using thermoluminescent dosimeters placed in or on an anthropomorphic Rando phantom (The Phantom Laboratory).


Materials and Methods
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 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Before all experiments, a total of 60 thermoluminescent dosimeters were annealed at 400°C for 2 hours, then at 100°C for 1 hour. Using a male Rando anthropomorphic phantom, five thermoluminescent dosimeters, wrapped in cellophane, were placed into the predrilled hole at the center of slice 18 of the phantom (near the center of the heart). Additional packages of five thermoluminescent dosimeters were affixed, with silk tape, to the anterior and lateral aspects of the phantom of slice 18, at the level of the heart in the lower chest (Fig. 1). All thermoluminescent dosimeters were kept in small opaque packages to minimize the effects of exposure to ambient light.


Figure 1
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  		Fig. 1 —Axial CT image of chest at section 18 of Rando phantom (The Phantom Laboratory). Packets of thermoluminescent dosimeters wrapped in cellophane (arrowheads) are placed at anterior, lateral, and center positions in this slice.

 
The anthropometric phantom was placed on the CT table and a standard departmental protocol CT of the chest was performed using SMART mA. SMART mA is a proprietary algorithm of GE Healthcare that modulates the tube current (mA) exclusively on the z-axis based on patient habitus and an operator-selected image-quality specification (Noise Index, GE Healthcare). Specific protocols are detailed in Table 1 and were as follows: LightSpeed 4 (GE Healthcare): detector array, 4 x 2.5 mm; field of view, 36 cm; table speed, 15 mm/s; rotation time, 0.6 seconds; 120 kVp; pitch, 1.5:1; and noise index, 11. Light-Speed 8: detector array, 8 x 1.25 mm; field of view, 36 cm; table speed, 13.5 mm/s; rotation time, 0.6 seconds; 120 kVp; pitch, 1.35:1; and noise index, 10.4. LightSpeed 16: detector array, 16 x 1.25; field of view, 36 cm; table speed, 27.5 mm/s; rotation time, 0.6 seconds; 120 kVp; pitch, 1.35:1; and noise index, 11.5. All exposures were performed with SMART mA.


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  		TABLE 1: Parameters for Standard Departmental CT Chest Protocol

 

The second experiment was performed using near-identical techniques. Specific protocols are detailed in Table 2 and were as follows: Light-Speed 4: detector array, 4 x 1.25 mm; field of view, 36 cm; table speed, 7.5 mm/s; rotation time, 0.6 seconds; 120 kVp; pitch, 1.5:1; noise index, 13. LightSpeed 8: detector array, 8 x 1.25 mm; field of view, 36 cm; table speed, 13.5 mm/s; rotation time, 0.6 seconds; 120 kVp; pitch, 1.35:1; noise index, 13. LightSpeed 16: detector array, 16 x 1.25; field of view, 36 cm; table speed, 27.5 mm/s; rotation time, 0.6 seconds; 120 kVp; pitch, 1.35:1; noise index, 11. All exposures were performed with SMART mA. The noise index was chosen based on the SD in the Rando phantom's heart in each CT unit. The SD was set to approximately 8 H for all three scanners. The table speed in the 4-detector unit is the maximal speed allowed in this unit at 4 x 1.25 mm.


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  		TABLE 2: Parameters Used to Create the Near-Identical Chest CT Technique

 

Before interpreting the thermoluminescent dosimeters, a 24-hour waiting period was observed to allow some of the excited electrons to return to a steady state. This has been shown to decrease the number of outlier measurements [8]. The thermoluminescent dosimeters were then interpreted using a Model 3500 thermoluminescent dosimeter reader (Harshaw Chemical), and the output of the reader was recorded in nano-coulombs (nC). After each set of exposures, a separate set of thermoluminescent dosimeters were exposed to known levels of radiation. Calibration was performed using an electron chamber (MDH RadCal). Three separate exposures were made: 11.3 mGy (94.4 nC), 22.4 mGy (201.1 nC), and 45.0 mGy (331.5 nC). These thermoluminescent dosimeters were also interpreted 24 hours after exposure, and conversion of nanocoulombs to grays was generated. This was used to calculate radiation dose for the remaining thermoluminescent dosimeters, which were placed in or on the Rando phantom.


Results
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 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
All 60 thermoluminescent dosimeters used for this study are included in the data set. The method described of waiting 24 hours before interpreting the thermoluminescent dosimeters was highly effective in this study. None of the thermoluminescent dosimeters were felt to be far outside an acceptable range. There was a single thermoluminescent dosimeter in the center of the 8-MDCT arrangement that was higher than any of the other thermoluminescent dosimeters. Exclusion of this thermoluminescent dosimeter did not change the data appreciably, although there was an increase in the SD in this series.

There was no statistically significant difference by Student's t test in radiation dose when comparing the three different scanners using standard departmental protocols, p = 0.06-0.4 (Fig. 2). However, a trend of decreasing radiation dose with increasing number of detectors was observed (Fig. 3). Further, there was no statistically significant difference observed in radiation dose between the center, anterior, and lateral thermoluminescent dosimeters on or in the Rando phantom when using standard departmental protocols or with a near-identical technique, p = 0.09-0.4 by Student's t test (Figs. 2 and 4).


Figure 2
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  		Fig. 2 —Radiation dose for 4-, 8-, and 16-MDCT units using standard departmental protocol for CT of chest. Bars represent average radiation dose of five thermoluminescent dosimeters used at center, anterior, and lateral positions, respectively, in slice 18 (midportion of heart) of Rando phantom (The Phantom Laboratory). Error bars are 2 x SD.

 

Figure 3
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  		Fig. 3 —Trend of radiation dose with standard departmental protocol for 4-, 8-, and 16-MDCT units. Radiation dose is recorded in mGy. Error bars are 2 x SD. Each data point is average of five thermoluminescent dosimeters placed in center, anterior, and lateral aspects of Rando phantom (The Phantom Laboratory).

 

Figure 4
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  		Fig. 4 —Radiation dose for 4-, 8-, and 16-MDCT units using near-identical protocols. Bars represent average of five thermoluminescent dosimeters at each site with error bars representing 2 x SD.

 
When the CT units were all set to the same peak kilovoltage, noise, and collimation, there was approximately a 47% higher radiation dose observed with the 4-detector unit compared with the 16-detector unit (Fig. 4). This difference is statistically significant by Student's t test (p < 0.01). A similar statistical difference was observed between the 4- and the 8-detector systems (p < 0.01). However, no statistically significant difference was observed between the 8- and the 16-detector units (p = 0.26). As with standard departmental protocols, there is a trend toward decreasing radiation dose with an increasing number of detectors (Fig. 5).


Figure 5
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  		Fig. 5 —Trend of radiation dose with near-identical technique for 4-, 8-, and 16-MDCT units. Radiation dose is recorded in mGy; error bars are 2 x SD. Each data point is average of five thermoluminescent dosimeters placed in center, anterior, and lateral aspects of Rando phantom (The Phantom Laboratory).

 
We observed a 55% increase in radiation dose comparing the standard departmental protocol to the near-identical protocol on the 4-MDCT unit. This difference was statically significant (p < 0.01). There was no significant difference in radiation dose between the 8- or 16-detector units when comparing standard departmental protocol to a near-identical technique.

When comparing the measured radiation dose in the Rando phantom from the thermoluminescent dosimeter data to the calculated dose-length product (DLP) and effective dose (ED), we found that the measured radiation dose was higher for all the CT units. The calculated dose was underestimated by 1-30%. This correlates with the Groves et al. [15] data, which showed that the calculated radiation dose was approximately 18% lower than the measured radiation exposure. The trends observed from the thermoluminescent dosimeter data are echoed by the DLP and ED calculations. We see that there is an increased amount of radiation dose seen with the 4-detector unit when compared with both the 8- and 16-detector units when near-identical technique was used. Also, with standard departmental protocols, there is only a small degree of difference between the radiation doses with all three CT units.


Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
When comparing standard technique, there is not a significant difference between any of the detector arrangements we tested. However, there is a trend toward decreasing radiation dose with the increasing number of detectors (Fig. 3). When comparing near-identical techniques, there is a statistically significant decrease in the radiation dose from the 4- to the 8-detector units and the 4- to the 16-detector arrangements. There is a trend toward decreasing radiation dose with the increasing number of detectors (Fig. 5).

When comparing standard technique, the 8-detector unit has a slightly higher radiation dose than the 4-detector unit. Some of the reasons for this finding are related to the unit's configuration. The 4-detector unit is set up to have 4 detectors at 2.5-mm collimation each. There is a nominal prepatient collimation of 10 mm with an actual prepatient collimation of 13 mm. The 8-detector unit is set up to have 8 detectors at 1.25 mm each, also with a nominal prepatient collimation of 10 mm and an actual prepatient collimation of 13 mm. The pitch used on the 8-detector unit was lower than that used on the 4-detector unit, which can account for the slightly higher radiation dose between these two units. In addition, the noise index for the 4-detector unit was higher than that of the 8-detector unit, thus resulting in a higher mA being used at slice 18 of the phantom with the 8-detector unit, which further increases the observed radiation dose (Table 1).

The 16-detector unit is set up with 16 detectors at 1.25-mm collimation each. There is a nominal prepatient collimation of 20 mm with an actual prepatient collimation of 21 mm. Therefore, to generate the same CT beam coverage, the 4- and 8-detector units will require 26 mm of exposure compared with 21 mm with the 16-detector unit. This can explain the decreased radiation dose and the observed trend toward decreasing radiation dose with increasing detector configurations. Although this is an accurate portrayal of how these systems function, the actual implementation of this concept is far more complicated because of the helical nature of the image acquisition, which results in overlapping beams of radiation.

When using a near-identical technique, the 4-detector system was less dose-efficient. The reason for this difference could be related to the size of the prepatient collimation. The 4-detector system has a prepatient collimator opening of 8.0 mm using 4 detectors at 1.25 mm. The nominal size of the detector array with this configuration is 5 mm, resulting in 3.0 mm of wasted or nontarget radiation. We would therefore expect an increase in radiation dose of approximately 66% compared with the 16 x 1.25 mm array. This is close to our observation of an average increased radiation of 47%. The variance of this observation from the calculated result could also be related to several factors: First, the 4 x 1.25 mm configuration had a pitch of 1.5:1, whereas the 16 x 1.25 mm configuration had a pitch of 1.35:1. Second, differences in the noise index and thus the mA used in these images help to account for the differences in the observed radiation dose. Finally, the 16-detector unit is not perfect; there is a 1-mm area of wasted radiation. This additional wasted radiation further accounts for the differences between the calculated and observed radiation dose.

Given these changes in prepatient collimation, we would calculate an increased radiation dose of 23% when comparing the 4 x 2.5 mm configuration to the 4 x 1.25 mm configuration. This is less than the observed 55% increase in radiation dose. Some of the additional change in radiation could be explained by the increasing number of tails of radiation exposure with the smaller detector configuration. In addition, the noise index that was used for the 4-detector unit was based on the 2.5-mm images. Given that noise decreases with increased slice thickness and that the mA chosen by SMART mA on the 1.25-mm images was higher than on the 2.5-mm images, we would expect higher radiation dose with the 4 x 1.25 mm configuration. The 8-detector unit at 1.25 mm has a prepatient collimator opening of 13 mm. Thus, this system has 20% less nontarget radiation than the 4-detector array at 1.25 mm when covering the same volume. In addition, for the same 20-mm volume of coverage, the 4-detector array will have eight overlapping tails of wasted or nontarget radiation, whereas the 8-detector array will have four overlapping tails. Finally, the 16-detector array will only have two overlapping tails. This trend of decreasing overlapping tails of radiation exposure could explain some of the additionally observed decrease in radiation dose with an increasing number of detectors.

This study does have several limitations. First, the use of thermoluminescent dosimeters introduces significant bias. These devices are small and have a tendency to have spurious measurements, requiring exclusion of multiple thermoluminescent dosimeters in most studies. We were able to include all thermoluminescent dosimeters in these data sets.

Second, a standard dose graph was used to estimate the radiation dose obtained from the thermoluminescent dosimeters. This technique can result in miscalibrations at many levels. However, many studies have used this technique to evaluate radiation dose [4-8]. Although the most reliable manner to estimate radiation dose is an ionization chamber, this is not a reasonable alternative with phantom work, primarily for physical reasons.

Third, it is not possible to make identical exposures with each of the different CT units. The 4-detector unit only allows two different pitches, 1.5:1 and 0.75:1. These pitches are not available on the 8- and 16-detector units (8- and 16-detector experiments were performed at 1.375:1). We used the 1.5:1 pitch in the 4-detector unit, which could potentially lower the radiation dose seen in the 4-detector unit.

Fourth, slight differences in generator calibration could have resulted in differences in the overall radiation dose. Finally, although there is a trend toward decreasing radiation dose, statistical significance was only shown between the 4-detector unit and the 8-detector unit and between the 4-detector unit and the 16-detector unit. There was no statistically significant difference between the 8- and 16-detector units. Some of these limitations could explain why there was not a statistically significant difference observed with the standard departmental protocol.

Future studies will be needed to evaluate the dose efficiency of new, higher-detector array units, such as the currently available 64-detector unit. If the trend of decreasing radiation dose continues, this may suggest that higher-detector units should be used on a larger scale in an attempt to decrease radiation dose to the public, especially in centers where a large volume of pediatric CT is performed.


References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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