|ECR 2019 / C-1060||
|A New Iterative Reconstruction CT Technique of Forward Projected Model-based IR Solution (FIRST): Evaluation of Image Quality at Head CT using a Cerebral Stroke Phantom Model|
Methods and materials
1. Development of phantom to cerebral infarction
Using a Cerebral Infarction phantom, which had been developed by the authors (Kyoto kagaku, JPN), an imitation disease with 34 HU was set to be visualized. The phantom, prepared with polyurethane resin and epoxy resin, the developed phantom mimicked the head shape and was composed of three sections: brain, cranium, and imitation disease. In the brain section, sphere-shaped imitation diseases (acute-stage cerebral infarction) of 30 mm in diameter were allocated in the middle cerebral artery region. The CT value of the brain section was 36 HU and that of the cranium was 900 HU. The imitation disease section was composed of two balls. The CT values of the two balls were 32, 34HU reflected acute-stage cerebral infarction and their CT values were lower than that of the brain by 4 and 2 HU, respectively ( Fig. 2 ).
2. Imaging conditions
Using tube voltages at 120 kV, which were assumed to be used clinically in X-ray CT imaging (Aquilion ONE Vision FIRST Edition in Canon Medical Systems, JPN; Fig. 3 ) and a Tube current at 200, 400, and 600 mA, and 1.0 s, Tube current time products at 200, 400, and 600 mAs in images with a slice thickness of 1, 3, and 5 mm were obtained.
Full IR in Forward projected model-based Iterative Reconstruction Solution (FIRST) images were created using X-ray CT Reconstruction from mild, standard, and strong in image Strength, under conditions of 120 kV, 200, 400, 600 mA, 1.0 s. Low contrast Object specific CNR (CNRLO) values allowed us to evaluate the visualization of acute-stage cerebral infarction. In addition, a new attempt was evaluate Improvement of image quality in exposure dose in IR image was evaluated by FOM.
4. Imaging evaluation
4-1. Low contrast Object specific CNR (CNRLO)
The CNRLO value at the 30-mm-diameter imitation disease section in each FIRST image was obtained using the formula (|ROIM-ROIB|) / √NPS(‾u) and the ability to visualize acute-stage cerebral infarctions were evaluated10). In the formula, ROIM represents the CT value, obtained of circular ROI size of 170 mm2 at the center of the imitation disease section, ROIB represents the CT value, obtained of circular ROI size 170 mm2 at the normal brain parenchyma section, and NPS(‾u) represents the Noise power spectrum (NPS) value of the normal brain parenchyma six section in virtual slit (60 * 24 pixels). Still more, ‾u represents the spatial frequency (0.17 cycle / mm) contributing to the detection of diameter of diseased part 30 mm ( Fig. 4 ).
4-2. Figure of Merit (FOM)
Figure of Merit (FOM) is an indicator to evaluate the degree of image quality improvement per patient's exposure dose. It is possible to compare and evaluate images in consideration of exposure dose under imaging conditions giving different exposure doses. The higher FOM indicates more efficient shooting conditions11).
FOM was calculated from the formula of CNR2 / CTDIvol. In the formula, The CNR value at the 30-mm-diameter imitation disease section in each FIRST image was obtained using the formula (|ROIM-ROIB|) / SDB, and the ability to visualize acute-stage cerebral infarctions were evaluated. In the formula, ROIM represents the CT value, obtained of circular ROI size of 170 mm2 at the center of the imitation disease section, ROIB represents the CT value, obtained of circular ROI size 170 mm2 at the normal brain parenchyma section, and SDB represents the standard deviation (SD) value of the normal brain parenchyma section12). Further, CTDIvol indicates the absorbed dose value displayed by the calculation of the X-ray CT apparatus, and this was used ( Fig. 5 ).