Keywords:
Not applicable, Image verification, Screening, CT-Quantitative, CT, Radiation physics, Liver, Abdomen
Authors:
K. Kitazawa, T. Kitamura, A. Matuda, K. Togashi; Kyoto/JP
DOI:
10.26044/ecr2020/C-01979
Methods and materials
To evaluate low-contrast detectability, we calculated low-contrast object specific contrast-to-noise ratio (CNRLO) from CT images. CNR is general to evaluate low contrast detectability but frequency characteristic is not taken into account. Then CNR is not able to use for evaluation of images using IR. CNRLO is considered frequency characteristic for evaluation low contrast detectability in image using IR [5].
Fig. 1 shows the formula of CNRLO and axial slice of the human phantom depicting the regions of interest (ROI). ROIM and ROIB are CT values measured in the simulated nodule and liver parenchyma, respectively, and NPS(u) denotes the noise power spectrum (NPS) at the spatial frequency (u).
CNRLO is an index that reflects contrast of the lesion, image frequency characteristics and the frequency components corresponding to lesion size. NPS(u) represents the amount of noise at the spatial frequency that involves detection of the target lesion.
We scanned CT images of the human phantom using a 160 slice CT scanner. Fig. 2 shows the liver placed simulated nodules in human phantom and the slice that was scanned.
Measuring NPS
・Image acquisitions were performed at a normal dose used automatic exposure control and 4 different tube current levels : 20, 110, 220 and 380 mA with 120 kVp of tube voltage.
・Image noise was evaluated with each current level on the liver parenchyma, and we found the scan condition the same as each image noise on the water phantom.
・These images were performed 10 times for each current level and reconstructed with adaptive iterative dose reduction enhanced 3D (eAIDR) and AiCE.
・NPS were measured with a radial frequency method using CT measure Ver. 0.98f [6].
Calculating CNRLO
・CT values of the simulated nodule and 3 points of liver parenchyma as background in human phantom images were measured from each current level and reconstructed with eAIDR and AiCE.
・CNRLO were calculated at the spatial frequency component corresponding to the diameter of the nodule (0.05 mm-1).