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 (Fig.2).

Fig 2: Plans and Photograph for the X-ray CT phantom to evaluate cerebral stroke. The phantom was prepared to have a cranium, a composition and a size similar to those of the brain section and the acute cerebral stroke section. ©“Department of Radiology, Kitasato University”

2. Imaging conditions and Reconstruction

 Photon-counting detector CT (NEOTOM Alpha in SIEMENS Healthineers, DE; Fig.3) images were created using virtual monoenergetic reconstructions imaging (VMI) in 60 to 70 keV at 1 keV intervals for 11 images, under conditions of 120 and 140 kV, 500 and 800 mA, rotation speed 0.5 s, slice thickness 5.0 mm.

Fig 3: External appearance of Photon-counting detector CT apparatus used in this experiment. NAEOTOM Alpha in SIEMENS Healthineers. ©“Department of Radiology, Kitasato University”

3. Imaging evaluation

3-1. detectability index (d'²)

   Detectability index (d'²) is an index that enables evaluation including the effect of low-frequency noise, which is a problem in virtual monoenergetic reconstructions X-ray CT images, and provides results more consistent with visual impression [9]. W_task is an index that includes lesion size, shape, and contrast difference, and TTF is an index that takes into account the size and shape of the lesion, referring to the MTF obtained by the radial edge method (Fig.4).

Fig 4: Method of calculating detectability index (d’2). ©“Department of Radiology, Kitasato University”

3-2. Low contrast object specific CNR (CNR_LO)

   The CNR_LO value at the 30-mm-diameter imitation disease section in each VMI image was ability to visualize acute-stage cerebral infarctions were evaluated [10]. In the formula, ROI_M represents CT value and ROI_B represents  SD value, and circular ROI size of 170 mm². 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.5).

Fig 5: Method of calculating Low contrast object specific CNR (CNRLO). ©“Department of Radiology, Kitasato University”

3-3. Figure of Merit (FOM)

   Figure of Merit (FOM) is an indicator to evaluate the degree of image quality improvement per patient's exposure dose. The higher FOM indicates more efficient shooting conditions [11]. FOM was calculated from the formula of CNR² / CTDI_vol. CTDI_vol indicates the absorbed dose value displayed by the calculation of the X-ray CT apparatus (Fig.6).

Fig 6: Method of calculating Figure of Merit (FOM). ©“Department of Radiology, Kitasato University”