Automatic exposure control (AEC) systems have been developed by computed tomography(CT) manufacturers to improve the consistency of image quality among patients and to control the absorbed dose. Since a multichannel helical CT scan may easily increase individual radiation doses, this technical improvement is of special interest in children who are particularly sensitive to ionizing radiation, but little information is currently available regarding the precise performance of these systems on small patients. Our objective was to assess an AEC system on pediatric dose phantoms by studying the impact of phantom transmission and acquisition parameters on tube current modulation, on the resulting absorbed dose and on image quality. We used a four-channel CT scan working with a patient-size and -axis-based AEC system designed to achieve a constant noise within the reconstructed images by automatically adjusting the tube current during acquisition. The study was performed with six cylindrical poly(methylmethacrylate) (PMMA) phantoms of variable diameters (10–) and one 5 years of age equivalent pediatric anthropomorphic phantom. After a single scan projection radiograph (SPR), helical acquisitions were performed and images were reconstructed with a standard convolution kernel. Tube current modulation was studied with variable SPR settings (tube angle, mA, kVp) and helical parameters (6–noise indices, 80– tube potential, 0.8–. tube rotation time, 5–x-ray beam thickness, 0.75–1.5 pitch, 1.25–image thickness, variable acquisition, and reconstruction fields of view). CTdose indices (CTDIvol) were measured, and the image quality criterion used was the standard deviation of the CT number measured in reconstructed images of PMMA material. Observed tube current levels were compared to the expected values from Brooks and Di Chiro‘s [R.A. Brooks and G.D. Chiro, Med. Phys.3, 237–240 (1976)] model and calculated values (product of a reference value multiplied by a dose ratio measured with thermoluminescent dosimeters). Our study demonstrates that this AEC system accurately modulates the tube current according to phantom size and transmission to achieve a stable imagenoise. The system accurately controls the tube current when changing tube rotation time, tube potential, or image thickness, with minimal variations of the resulting noise. Nevertheless, CT users should be aware of possible changes of tube current and resulting dose and quality according to several parameters: the tube angle and tube potential used for SPR, the x-ray beam thickness (tube current decreases and imagenoise increases when doubling x-ray beam thickness), the pitch value (a pitch decrease leads to a higher dose but also to a higher noise), and the acquisition field of view (FOV) (tube current is lower when using the small acquisition FOV compared to the large one, but the use of small acquisition FOV at leads to a peculiar increase of tube current and CTDIvol).
This work was supported in part by the Fondation Curie and the Association pour la Recherche sur le Cancer (Grant No. 3704).
II. MATERIALS AND METHODS
II.A. CT equipment and automatic exposure control system
II.C. Dose measurements
II.C.1. CTDI determination
II.C.2. Thermoluminescent dosimetry
II.D. Image quality assessment
II.E. Dose-quality relationship
II.F. Experimental process
II.F.1. Modulated tube current profile analysis
II.F.2. Influence of SPR acquisition parameters on tube current modulation
II.F.3. Modulation response to variations of phantom size, shape, and attenuation
II.F.4. Modulation response to variations of noise indices
II.F.5. Tube current modulation response and image quality according to variations of helical acquisition parameters
III.A. Modulated tube current profile analysis
III.B. Influence of SPR acquisition parameters on tube current modulation
III.C. Modulation response to variations of phantom size, shape, and attenuation
III.C.1. Comparison between SPR pixel value profile and z-axis modulation
III.C.2. Correlation between expected and observed absorbed doses according to PMMA phantom diameter
III.D. Modulation response to variations of noise indices
III.E. Tube current modulation response and image quality according to variations of helical acquisition parameters
III.E.1. Variations of tube rotation time
III.E.2. Variations of tube potential
III.E.3. Variations of x-ray beam thickness
III.E.4. Variation of reconstructed image thickness
III.E.5. Variation of pitch factor
III.E.6. Variation of acquisition field of view
III.E.7. Variation of reconstruction field of view
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