Optimization of chest CT protocols based on pixel image matrix, kernels and iterative reconstruction levels - A phantom study.

INTRODUCTION: This study investigated the impact of high matrix image reconstruction in combination with different reconstruction kernels and levels of iterative reconstructions on image quality in chest CT. METHODS: An anthropomorphic chest phantom (Kyoto Kagaku Co., Ltd., Kyoto, Japan), and a Catphan® 600 (The Phantom Laboratory, Greenwich, NY, USA) phantom were scanned using a dual source scanner. Standard institutional protocol with 512 × 512 matrix was used as a reference. Reconstructions were performed for 768 × 768 and 1024 × 1024 matrices and all possible combinations of three different kernels and five levels of iterative reconstructions were included. Signal difference to noise ratio (SdNR) and line pairs per cm (lp/cm) were manually measured. A Linear regression model was applied for objective image analysis (SdNR) and inter-and intra-reader agreement was given as Cohen's kappa for the visual image assessment. RESULTS: Matrix size did not have a significant impact on SdNR (p = 0.595). Kernel (p = 0.014) and ADMIRE level (p = 0.001) had a statistically significant impact on SdNR. The spatial resolution ranged from 7 lp/cm to 9 lp/cm. The highest spatial resolution was achieved using kernel Br64 and ADMIRE 1, 2 and 3 in both 768- and 1024-matrices, and with Br59 with ADMIRE 2 and 4 and 768-matrix, all visualizing 9 lp/cm. Both readers scored kernel Br59 highest, and the scoring increased with increasing levels of Iterative Reconstruction. CONCLUSION: Matrix size did not influence image quality, however, the choice of kernel and degree of IR had an impact on objective and visual image quality in 768 - and 1024-matrices, suggesting that increased degree of IR may improve diagnostic image quality in chest CT. IMPLICATIONS FOR PRACTICE: Image quality in CT of the lung may be improved by increasing the level of IR.

New diagnostic reference level for full-field digital mammography units.

The diagnostic reference level (DRL) has generally been defined as the 75th percentile of the distributions of mean doses observed on a sample of standard-sized patients, but for mammography, the 95th percentile has been used. In this study, the 75th and 95th percentiles are estimated for 26 full-field digital mammography units, representing six models from four manufacturers. Systematic differences between categories of manufacture/models are investigated with respect to mean glandular dose (MGD) and figure of merit (FOM), defined as signal difference to noise ratio squared divided by the MGD. The MGDs per unit range from 0.7 to 2.1 mGy, with overall 75th and 95th percentiles of 1.4 and 2.0 mGy, respectively. The different manufacture/models show differences in both dose distributions and FOMs. As national DRL, the 95th percentile is proposed to determine which units can be accepted for use. To identify the need for optimisation, it is proposed to use the 75th percentile for the different manufacture/models, along with the FOM.

Uncertainties involved in the estimation of mean glandular dose for women in the Norwegian Breast Cancer Screening Program (NBCSP).

The aim of this study was to reflect on the estimation of the mean glandular dose for women in Norway aged 50-69 y. Estimation of mean glandular dose (MGD) has been conducted by applying the method of Dance et al. (1990, 2000, 2009). Uncertainties in the thickness of approximately ±10 mm adds uncertainties in the MGD of approximately ±10 %, and uncertainty in the glandularity of ±0 % will lead to an uncertainty in the MGD of ±4 %. However, the inherent uncertainty in the air kerma, given by the European protocol on dosimetry, will add an uncertainty of 12 %. The total uncertainty in the MGD is estimated to be ∼20 %, taking into consideration uncertainties in compressed breast thickness (±10 %), the air kerma (12 %), change in HVL by -0.05 mm (-9.0 %), uncertainty in the s-factor of ±2.1 % and changing the glandularity to an age-dependent glandularity distribution (+8.4 %).

Patient doses from screen-film and full-field digital mammography in a population-based screening programme.

The aim of this study was to compare mean glandular dose (MGD) in all full-field digital mammography (FFDM) and screen film mammography (SFM) systems used in a national mammography screening program. MGD from 31 screening units (7 FFDM and 24 SFM), based on an average of 50 women at each screening unit, representing 12 X-ray models (6 FFDM and 6 SFM) from five different manufacturers were calculated. The MGD was significantly lower for FFDM compared with SFM (craniocaudal): 1.19 versus 1.27 mGy, respectively, mediolateral oblique: 1.33 versus 1.45 mGy, respectively), but not all of the FFDM units provided lower doses than the SFM units. Comparing FFDMs, the photon counting scanning-slit technology provides significantly lower MGDs than direct and indirect conversion digital technology. The choice of target/filter combination influences the MGD, and has to be optimised with regard to breast thickness.