Rationale and Objectives
Efforts to decrease radiation exposure during pediatric high-resolution thoracic computed tomography (HRCT), while maintaining diagnostic image quality, are imperative. The objective of this investigation was to compare organ doses and scan performance for pediatric HRCT using volume, helical, and noncontiguous axial acquisitions.
Materials and Methods
Thoracic organ doses were measured using 20 metal oxide semiconductor field-effect transistor dosimeters. Mean and median organ doses and scan durations were determined and compared for three acquisition modes in a 5-year-old anthropomorphic phantom using similar clinical pediatric scan parameters. Image noise was measured and compared in identical regions within the thorax.
Results
There was a significantly lower dose in lung (1.8 vs 2.7 mGy, P < .02) and thymus (2.3 vs 2.7 mGy, P < .02) between volume and noncontiguous axial modes and in lung (1.8 vs 2.3 mGy, P < .02), breast (1.8 vs 2.6 mGy, P < .02), and thymus (2.3 vs 2.4 mGy, P < .02) between volume and helical modes. There was a significantly lower median image noise for volume compared to helical and axial modes in lung (55.6 vs 79.3 and 70.7) and soft tissue (76.0 vs 111.3 and 89.9). Scan times for volume, helical, and noncontiguous axial acquisitions were 0.35, 3.9, and 24.5 seconds, respectively.
Conclusion
Volumetric HRCT provides an opportunity for thoracic organ dose and image noise reduction, at significantly faster scanning speeds, which may benefit pediatric patients undergoing surveillance studies for diffuse lung disease.
Pediatric diffuse lung disease encompasses a broad range of heterogeneous disorders affecting interstitial, airspace, and small airways . High-resolution computed tomography of the thorax (HRCT) is increasingly used in children for the evaluation of a wide range of diffuse lung diseases . HRCT allows for detailed, noninvasive imaging of the lung parenchyma, thus facilitating demonstration of interstitial lung disease patterns and differential diagnosis, guiding further diagnostic workup and therapy. However, the primary disadvantages of HRCT in children are the image degradation from motion, especially since young children are unable to suspend respiration, and ionizing radiation exposure. Radiation exposure, from all sources, is of paramount concern in children. Compared to adults, children are at greater risk for cancer development at similar radiation exposures due to longer life expectancy and increased radiosensitivity from rapidly growing tissues . Radiation exposure to the female breast is of particular concern with pediatric HRCT. It has been estimated that exposure of the female breast before 35 years of age to 10 mGy increases the risk of cancer development by 14% over the spontaneous rate in the general population . Therefore, efforts to decrease radiation exposure during pediatric HRCT, while maintaining diagnostic image quality are imperative.
The two most widely used methods of acquiring HRCT involve either sampling the lungs with thin (typically 1 mm), noncontiguous axial sections at varying intervals (typically 10–20 mm) or retrospectively deconstructing a helically acquired volumetric data set in thin axial sections at desired intervals. There are several reports in the medical literature comparing image quality between noncontiguous axial and helical HRCT in adults and children , but none to our knowledge have compared true volumetric acquired versus helical and noncontiguous axial acquired HRCT. In addition, while Bastos et al compared total effective dose (using the relatively inaccurate method of multiplying published conversion factors by console-displayed dose–length product [DLP]) in pediatric HRCT performed using helical versus noncontiguous axial acquisitions, they did not measure and compare thoracic organ doses .
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Figure 1
Graphic illustration of the three acquisition methods available to obtain high-resolution thoracic computed tomography. During noncontiguous axial acquisition, the X-ray tube is collimated to a narrow nominal beam width (1 mm in this investigation’s case) and rotates around the patient for one rotation. The tube is then turned off while the patient table is translated 10 mm, at which point the tube is turned back on and undergoes another rotation. This process repeats until the desired scan length is covered. During helical acquisition, a wider nominal beam width is used (32 mm in this investigation’s case) and the tube rotates continuously while the patient table simultaneously translates. During volume mode, a single rotation covers the entire anatomy (up to 160 mm) and there is no table translation during the acquisition.
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Materials and methods
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Phantom and Detector Placement
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Table 1
Locations of organ metal oxide semiconductor field effect transistor detectors in the 5-year-old anthropomorphic phantom
Dosimeter number Organ/structure Slice number Location number 1 Lung–left upper lobe 10 29 2 Thymus 10 34 3 Lung–right upper lobe 11 47 4 Lung–left upper lobe 11 49 5 Lung–right upper lobe 12 53 6 Lung–left upper lobe 12 56 7 Lung–left lower lobe 12 57 8 Right breast 12 58 9 Left breast 12 59 10 Thymus 12 60 11 Lung–right middle lobe 13 67 12 Lung–right lower lobe 13 68 13 Lung–lingula 13 71 14 Thoracic spine 13 75 15 Heart 13 H4 16 Lung–right lower lobe 14 78 17 Lung–left lower lobe 14 81 18 Esophagus 14 85 19 Lung–left lower lobe 14 83 20 Lung–left lower lobe 14 84
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Scanning Parameters
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Table 2
Pediatric high-resolution thoracic computed tomography (CT) protocols
Scanner Scan mode Nominal collimation (channel No. × width) Tube current (mA) Rotation time (sec) Pitch 64-Detector CT Noncontiguous axial 1 × 1 mm 100 0.5 N/A 320-Detector CT Volume 320 × 0.5 mm Min 20/max 120 0.35 N/A 320-Detector CT Helical 64 × 0.5 mm Min 20/max 120 0.5 0.828
All scans performed at 100 kVp, small field of view (240 mm), and 16-cm scan length. Noncontiguous axial scan performed with 1-mm slices every 10 mm. Volume and helical scans deconstructed at 1-mm slice thickness every 10 mm. Volume and helical scans performed with tube current modulation at noise setting (SD) of 7.5. All scans reconstructed with same kernel (FC5) and with same noise reduction algorithms (QDS and BOOST3D) and without iterative reconstruction techniques.
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Anthropomorphic Phantom Measurement Tabulation
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Cylindrical Phantom Measurement Tabulation
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Image Noise Measurement Tabulation
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Statistical Analysis
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Results
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Table 3
Mean and median organ dose comparison for high-resolution thoracic computed tomography performed in three different acquisition modes using average doses for the noncontiguous axial mode
Organ Organ dose (mGy) Volume mode Helical mode Noncontiguous axial mode Mean ± SD Mean ± SD Median (25%, 75%) Lung 1.8 ± 0.1 ∗ 2.3 ± 0.1 ‡ 2.7 (2.5, 3.0) Thymus 2.3 ± 0.2 ∗ 2.4 ± 0.2 ‡ 2.7 (1.8, 3.6) Breast 1.8 ± 0.1 † 2.6 ± 0.1 2.2 (1.2, 3.3) Esophagus 2.0 ± 0.1 2.0 ± 0.2 2.3 (1.8, 3.4) Heart 1.9 ± 0.2 2.1 ± 0.3 2.6 (2.0, 4.3) T-spine 5.7 ± 0.5 6.6 ± 0.7 6.4 (5.2, 9.7)
SD, standard deviation.
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Table 4
Mean and median organ dose comparison for high-resolution thoracic computed tomography performed in three different acquisition modes using peak doses for the non-contiguous axial mode
Organ Organ dose (mGy) Volume mode Helical mode Noncontiguous axial mode Mean ± SD Mean ± SD Median (25%, 75%) Lung 1.8 ± 0.1 ∗ 2.3 ± 0.1 † 4.5 (3.2, 5.0) Thymus 2.3 ± 0.2 † 2.4 ± 0.2 † 4.6 (2.9, 5.0) Breast 1.8 ± 0.1 ∗ 2.6 ± 0.1 3.9 (2.5, 4.9) Esophagus 2.0 ± 0.1 † 2.0 ± 0.2 3.3 (2.9, 3.8) Heart 1.9 ± 0.2 2.1 ± 0.3 † 4.5 (3.2, 4.9) T-spine 5.7 ± 0.5 6.6 ± 0.7 11.5 (6.6, 11.8)
SD, standard deviation.
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Table 5
Median image noise comparison for high-resolution thoracic computed tomography performed in three different acquisition modes
Organ Median image noise (Hounsfield unit standard deviation) Volume Helical Noncontiguous axial Lung 55.6 ∗ 79.3 † 70.7 Soft tissue 76.0 ∗ 111.3 † 89.9 Bone 86.8 120.7 121.1
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Discussion
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Acknowledgments
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