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1 Department of Radiology and Center for Imaging Science, Samsung Medical
Center, Sungkyunkwan University School of Medicine, 50, Ilwon-Dong,
Kangnam-Ku, Seoul 135-710, Korea.
2 Biostatistics Unit, Samsung Biomedical Research Institute, Samsung Medical
Center, Seoul 135-710, Korea.
Received October 17, 2002;
accepted after revision February 12, 2003.
Address correspondence to K. S. Lee.
Abstract
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SUBJECTS AND METHODS. Image clusters, composed of nine images, using MDCT (120 kVp, a 2.5-mm collimation, a pitch of 6, and 2.5-mm reconstruction intervals) were obtained at each of two levels—the azygous arch and the right inferior pulmonary vein—at 170, 100, 70, 40, 20, and 10 mA. Independently, two chest radiologists assessed and compared the quality of the images obtained at the six milliamperage exposures. Image quality was graded using a 5-point scale with lung and mediastinal window settings. Radiation doses were measured at each of the six milliamperage settings while scanning the whole lung of a thoracic phantom using MDCT.
RESULTS. The mean image quality scores at exposures of 170, 100, 70, 40, 20, 10 mA were as follows: 3.9, 3.7, 3.8, 3.2, 2.5, 1.6 at lung window settings and 4.1, 4.3, 4.0, 3.4, 2.3, 1.3 at mediastinal window settings, respectively. Images obtained at 70 mA were rated significantly better than those obtained at 40 mA or less (p < 0.01). The mean radiation dose at 170, 100, 70, 40, 20, 10 mA was 23.72, 14.39, 10.54, 5.41, 2.74, and 1.50 mGy, respectively.
CONCLUSION. With a tube current setting as low as 70 mA, MDCT provides images of acceptable quality and volumetric data sets for the evaluation of bronchiectasis. The trade-off of using MDCT rather than conventional high-resolution CT is that the radiation dose is five times higher with MDCT (10.54 mGy) than with conventional high-resolution CT (2.17 mGy with parameters of 120 kVp, 170 mA, 1-mm collimation, and 10-mm intervals).
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In helical volumetric CT, all data for a complete anatomic volume are acquired in one contiguous scan [2]. Theoretically, helical volumetric CT can eliminate potential weaknesses associated with high-resolution CT [1–3]. In particular, the continuous acquisition is faster, and data for the whole lung can be recorded during one breath-hold. The clinical use of helical volumetric CT in the assessment of airway disease has been limited by concern over the high dose of radiation delivered to the thorax [4, 5]. Jung et al. [6] showed that low-dose helical CT may offer more information than does high-resolution CT in the evaluation of bronchiectasis with acceptable image quality and similar radiation dose.
With the advent of multidetector CT (MDCT), we have the advantages of shorter acquisition times and greater anatomic coverage. The nearly isotropic matching of inplane resolution and section thickness for MDCT means that alternative multiplanar imaging and axial imaging is feasible [7, 8].
The objectives of our study were to assess the quality of MDCT scans obtained using different tube currents and to compare the actual radiation dose of MDCT at these different tube currents.
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Image Acquisition
All chest scans were obtained with a four-channel MDCT scanner (LightSpeed
Advantage QX/i Scanner, General Electric Medical Systems, Milwaukee, WI). IV
contrast medium was not injected in any of the study subjects. At our
institution, diagnostic imaging for bronchiectasis is based on helical
volumetric CT scan data using four-channel MDCT (120 kVp, 70 mA, 2.5-mm
collimation, and pitch of 6 [total active detector length pitch of 1.5]),
which are reconstructed into both axial (2.5-mm thickness) and coronal (1.2-
to 2.0-mm thickness) images. Before obtaining the diagnostic images, we
obtained additional image clusters covering 20 mm along the z-axis at
two levels—the azygous arch and the right inferior pulmonary
vein—with the same scanning parameters at six tube currents: 170, 100,
70, 40, 20, and 10 mA. All scanning data were reconstructed using a bone
algorithm. The scanning data were directly displayed on four monitors (1536
x 2048 image matrices, 8-bit viewable gray-scale, and 60-foot-lamberts
luminescence) of a PACS ([picture archiving and communication system]
PathSpeed, General Electric Medical Systems Integrated Imaging Solutions, Mt.
Prospect, IL). On the monitors, both mediastinal (width, 400 H; level, 20 H)
and lung (width, 1500 H; level, –700 H) window settings were
evaluated.
Image Analysis
Two chest radiologists, who were unaware of the tube currents,
independently assessed and compared the image quality of clusters of axial
images obtained at six exposures (170, 100, 70, 40, 20, 10 mA). Images
obtained at both lung and mediastinal window settings and at both the azygous
arch and the right inferior pulmonary vein were graded in terms of quality
with a 5-point scale (5 = excellent, 4 = good, 3 = fair, 2 = poor, and 1 =
nondiagnostic). In grading image quality at lung window settings, the
observers focused on the morphologic appearances of bronchi and pulmonary
parenchymal vessels (distinction of bronchovascular margins and distinction of
intraluminal air from mural soft tissue in the bronchi) at lobar, segmental,
subsegmental, and sub-subsegmental levels where there is no parenchymal or
airway disease. In the grading of the image quality at mediastinal window
settings, attention was given to the morphology of the aorta, pulmonary
vessels, pleura, and chest wall.
Circular regions of interest (mean ± SD, 108.9 ± 18.9 mm2) were placed in the descending thoracic aorta at the azygous arch and the right inferior pulmonary vein, respectively. The mean attenuation and SD values were measured on images obtained at six tube currents at mediastinal window settings. The SD values were regarded to represent the extent of image noise.
Measurement of Radiation Dose
The absorbed dose of radiation to the lungs was obtained from direct
measurements with thermoluminescence dosimeters using an anthropomorphic Rando
phantom (model RAN-110, Churchin Associates, Smithtown, NY)
[9,
10]. Thermoluminescence
dosimetry was performed using lithium fluoride chips (TLD-100 [3.2 x 3.2
x 0.9 mm3], Thermo RMP, Solon, OH). Two lithium fluoride
chips were placed in the center of the irradiated lungs of the phantom along
its main axis, one in the right lung and the other in the left lung. The
measurements were performed four times at each of the six tube currents. Organ
and tissue doses were assessed by calculating the average of the absorbed
doses that were measured with the thermoluminescence dosimeter chips placed
inside the phantom. The Rando phantom is suitable for determining the
radiation dose because this phantom consists of a human skeleton embedded in
tissue-equivalent material, and the lungs are simulated by lung-equivalent
material [11].
Data Analysis and Statistics
The correlation between tube current and subjective image quality was
assessed with Spearman's rank correlation coefficient. The Friedman test was
used to evaluate the statistical differences between images in terms of
quality and amount of noise at various amperages, and the Fisher protected
least significant difference test using ranks was used for multiple
comparisons between various amperages. Interobserver agreement on CT findings
for the image qualities was measured with the intraclass correlation
coefficient.
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The noise level of images showed a negative linear correlation with the six milliamperages (Spearman's correlation coefficient, r = –0.742; p < 0.01). The image noise was significantly increased in accordance with a decrease in the milliamperages (p < 0.05) (Table 3). The average image noise was 39.00 H at 170 mA, 42.65 H at 100 mA, 53.57 H at 70 mA, 69.23 H at 40 mA, 98.45 H at 20 mA, and 157.24 H at 10 mA.
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The radiation doses associated with the tube currents are summarized in Table 4. The radiation dose to the organs at 70 mA was less than half the dose at 170 mA.
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Controversies exist about the radiation hazard of relatively low levels of ionizing radiation exposure. The relationship of this radiation exposure to biologic risk for patients is determined by extrapolation based on changes observed after exposure to higher levels of radiation [14]. In addition, the age and sex of patients should be considered when analyzing a patient's risk. For example, the delivery of 1 rad (10 mGy) to the breast of a woman younger than 35 years old is estimated to increase her risk of breast cancer by approximately 14% over the spontaneous rate for the general population [15].
The radiation doses of MDCT measured in the current study (10.16–10.96 mGy at 70 mA) slightly exceed those reported by Lucidarme et al. [1] (7.0–8.0 mGy for single-detector helical CT [150 mA, pitch of 1.6]) and are similar to those reported by Jung et al. [6] (8.93–12.10 mGy for single-detector helical CT [150 mA, pitch of 2]), although the scanning techniques of Lucidarme et al. and Jung et al. differed from those of our study. Our scanning protocol used a thinner collimation and lower tube current than those of Lucidarme et al. and Jung et al. Nevertheless, the radiation dose in our study exceeded that described for a conventional high-resolution CT protocol using 120 kVp, 170 mA, 1-mm collimation, and 10-mm intervals (2.17 mGy [range, 1.90–2.67 mGy]) [6]. However, the higher radiation dose inherent with MDCT applications compared with high-resolution CT must be regarded as the compromise required to overcome some of the limitations associated with high-resolution CT [1–3].
A reduction in milliamperage causes a proportionate reduction in radiation dose to the patient because radiation dose is linearly correlated to amperage at a fixed kilovoltage [6, 16, 17]. In 1990, Naidich et al. [16] described low-dose CT of the lungs and showed acceptable diagnostic quality on conventional chest CT scans obtained with a setting as low as 10 mA for 2-sec scans (20 mAs). This study focused on pulmonary parenchymal lesions in 12 patients without statistical analysis. Mayo et al. [17], using a conventional CT technique (10-mm collimation), reported a twofold reduction in tube current (from 400 to 140 mA) did not cause a significant change in subjective image quality in the detection of mediastinal or lung abnormalities. Although diagnostic images of the lung parenchyma can be obtained using 20 mA, Mayo et al. concluded that 140 mA is the minimal tube current required to provide good image quality for examinations of patients of average weight, because lower dose techniques produce images with significant noise. We have shown that subjective image quality of MDCT scans (2.5-mm collimation, continuous data acquisition) obtained using 70 mA is comparable to MDCT images acquired using 170 mA and otherwise identical technical parameters. This finding indicates that a significant reduction in radiation dose is possible without compromising perceived image quality.
A potential problem associated with reducing milliamperage is that resolution is limited by quantum mottle; in other words, increased artifacts and noise could cause subsequent image degradation [18]. In the study of Zwirewich et al. [19], linear streak artifacts were more prominent on high-resolution CT images acquired with a low-dose (20 mA) technique than those acquired with a high-dose technique, even though both were judged equally diagnostic in most cases. Because the lung is aerated and thus is of low attenuation, the lung has higher contrast than solid organs such as the liver. Therefore, detection of pathologic changes should depend less on image noise in the lung than in the solid organs [16].
In our study, the lower signal-to-noise ratio of low-dose CT did not significantly affect subjective image quality. At 70 mA, good-quality images could be obtained at both lung and mediastinal window settings (mean score, near 4.0 [good]) (Table 1). At 40 mA, image quality deteriorated and image noise increased (mean score, near 3.0 [fair]) (Table 1). We found that an abrupt increase in noise (53.57–69.23 H) was associated with a perceived decrease in image quality (from near 4.0 to 3.3) when the milliamperage was reduced from 70 to 40 mA. Therefore, an MDCT protocol with 40 mA may have some limitations in terms of diagnostic quality compared with images acquired with 70 mA.
One possible limitation of low-dose MDCT is the increased amount of scanning data that results from volumetric acquisitions using narrow collimation. Low-dose MDCT scans consisted of 175–211 images for each patient, including coronally reformatted images (mean, 204 images; the resulting total image data, 512 x 512 x 204 = 53.5 MB) for each patient. Nevertheless, advances in computer software and hardware applications and improved image compression techniques may overcome the problem of storing large amounts of data. Another issue resulting from the data explosion associated with MDCT is the greater amount of time required for the radiologists to review such data. These increased time requirements include not only the time required to review the increased number of images that comprise MDCT data sets, but also the time required for the image postprocessing needed to produce multiplanar reformatted images.
One limitation of our study is the fact that we evaluated patients with normal airways, although the patients were clinically suspected to have bronchiectasis. Another limitation is that we assessed only six discrete milliamperage values, as opposed to using a greater range of discrete milliamperages or continuous milliamperages. Therefore, the selection of discrete milliamperage values (e.g., 40 mA, 70 mA) for the purposes of this study does not define the lowest tube current at which diagnostically useful images for the assessment of bronchiectasis may be obtained. Nevertheless, our data show that MDCT may be performed for the assessment of suspected bronchiectasis using a significantly decreased radiation dose without compromising perceived image quality.
In conclusion, with a tube current setting as low as 70 mA, MDCT provides images of acceptable quality and volumetric data sets for the evaluation of bronchiectasis. The trade-off between using MDCT rather than conventional high-resolution CT is that the radiation dose is five times higher with MDCT (10.54 mGy) than with conventional high-resolution CT (2.17 mGy with parameters of 120 kVp, 170 mA, 1-mm collimation, and 10-mm intervals); however, the radiation exposure at 70 mA is reduced to less than half that at 170 mA.
Acknowledgments
We sincerely thank Moon Chan Kim for helping us in measuring radiation
doses of various CT techniques.
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