This was a retrospective study approved by the institutional review board at our institution, and the need for informed consent was waived.
Study population
We included all consecutive pediatric patients who underwent chest or abdomen CT at our institution between February 2020 and October 2020 with the same CT system (Revolution CT; GE Healthcare), which has a routine protocol including DLR. We retrospectively reviewed 51 patients. There were 34 boys and 17 girls with a mean age of 11.5 ± 4.6 years (range: 1–18 years). Non-enhanced chest CT (n = 16), contrast-enhanced chest CT (n = 12), and contrast-enhanced abdomen CT (n = 23) images were included. Height and weight was recorded at the time of CT examination and BMI was calculated. Body weight group was divided as < 20 kg, 20–60 kg, and > 60 kg.
Scanning technique and radiation dose measurements
All patients were examined using a 256-slice CT (Revolution CT; GE Healthcare). Peak kilovoltage (kVp) was divided in to three groups by weight: 100 kVp for > 40 kg, 80 kVp for 15–40 kg, and 70 kVp for < 15 kg. An automatic dose modulation technique (Smart mA; GE Healthcare) was used with a range of 50–200 mAs. The noise index was 33 for abdomen CT and 22 for chest CT. Other parameters used to generate images were as follows: gantry rotation time, 0.35 s; coverage speed, 226.79 mm/s; pitch, 0.992:1; and slice thickness, 2.5 mm.
Weight-based IV contrast injection was used with settings of 1.5-2.0 ml/kg with a maximum of 100 ml, using 300 mg iodine/ml concentration intravenous contrast iobitridol (Xenetix; Laboratoires Guerbet). The contrast was injected through an upper extremity peripheral intravenous line, followed by a saline chaser of 0.5 ml/kg. Injection speed was adjusted for a total injection time of 15 sec or less. For contrast-enhanced abdomen CT, a fixed time interval of 60 sec after contrast injection for portal phase without bolus tracking was used. For contrast-enhanced chest CT, a circular region of interest (ROI) was placed at the main pulmonary artery and the CT scan began 4 sec after the threshold attenuation of 100 Hounsfield units (HU) was reached.
Four axial reconstructions were generated for each patient with a 2.5 mm slice thickness and 2.5 mm slice interval according to the standard algorithm: 50% ASIR-V, 100% ASIR-V, medium- and high-strength DLR (TrueFidelity; GE Healthcare). We set the blending factors to 50% and 100% according to previous experience [3, 4]. DLR provides three selectable reconstruction strength levels (low, medium, and high) to control the amount of noise reduction with a standard reconstruction kernel. We chose medium and high based on our preliminary experience.
The CT dose index volume (CTDIvol, mGy) and dose-length product (DLP, mGy×cm) of all patients were recorded in both CT examinations. CTDIvol was converted to size-specific dose estimates (SSDE) on the basis of the American Association of Physicists in Medicine Report 204 [19]. Patient-specific dimensions were obtained from axial CT images at the carina on chest CT and at the main portal vein on abdomen CT. We used the sum of anteroposterior and lateral dimensions to determine patient effective diameter and conversion factors. The following equation was used to calculate the effective dose (ED, mSv): ED = DLP × WT (tissue-weighting factor; variable according to kVp, organ, and age [20]). Tissue-weighting factors of less than 80 kVp are unknown, so a tissue-weighting factor of 80 kVp was adopted for 70 kVp studies.
Quantitative image analysis
Quantitative analysis of axial images was performed by a board-certified radiologist with 9 years of experience. The mean attenuation (HU) and standard deviation (SD) were measured by manually placing the round ROI (8–10 mm in diameter) using a picture archiving and communication system (PACS) workstation (Centricity Radiology RA1000; GE Healthcare) in the mediastinal/soft tissue window setting (window level, 50 HU; window width, 350 HU). On chest CT images, ROIs were placed in lung and paraspinal muscles at the level of the carina. On abdomen CT images, ROIs were placed in liver, aorta, and paraspinal muscles at the level of the main portal vein on axial images. To obtain reliable measurements for the areas, each ROI was positioned to encompass the homogeneous portion and did not include surrounding structures or vessels. Image noise was defined as the SD of the pixel values obtained from paraspinal muscle. Both contrast- and signal-to-noise ratios (CNR and SNR) were defined as CNR=│HUobject – HUmuscle│/SDnoise and SNR = HUobject /SDnoise [21].
Qualitative image analysis
CT images were independently reviewed by two board-certified pediatric radiologists with 17 and 9 years of experience who were blinded to the clinical findings and the CT reconstruction methods. Images were displayed on the PACS in random order and two radiologists independently recorded their opinions on overall image quality, noise, and motion or beam hardening artifacts. A four-point scale was used: 4 was superior, 3 was average, 2 was suboptimal, and 1 was unacceptable.
Statistical analysis
All statistical analyses were performed using MedCalc software (version 12.1.0; MedCalc Software). Patient demographic characteristics and dose descriptors (CTDIvol, DLP, SSDE, and ED) are summarized and presented as the mean and SD. Repeated measures ANOVA with pairwise comparisons and Bonferroni correction were performed to compare the reconstructions with respect to attenuation, noise, CNR, and SNR. Wilcoxon signed rank and Cohen kappa tests were performed to compare qualitative evaluation and to assess interobserver agreement. Agreement between reviewers is expressed as κ values: κ values of 0–0.20, 0.21–0.40, 0.41–0.60, 0.61–0.80, and greater than 0.81 indicated poor, fair, moderate, good, and excellent agreements, respectively. A p-value of less than 0.05 was considered statistically significant.