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X-ray CT measurements

X-ray CT is a radiological imaging system first developed by Hounsfield (1973). X-ray CT is a technique to reconstruct a CT image showing the spatial distribution of X-ray linear absorption coefficients of samples. This technique allows us to observe and nondestructively analyze 3-D internal structures. Linear absorption coefficient depends on density and chemical composition of samples and X-ray energy used. Higher density and higher atomic numbers result in higher attenuation of X-rays. X-ray CT produces images in which grayscale intensity corresponds to the X-ray linear attenuation coefficient. Individual CT images are referred to as slices. By acquiring a contiguous set of slices, data for a complete volume can be obtained.

A medical X-ray CT scanner (Hitachi CT-W2000) at Geological Survey of Japan, AIST, was used to acquire the CT images. The medical CT system was originally designed for use on human subjects to image soft tissue and bone. Therefore, medical CT systems are required to use a limited dose of relatively low energy X-rays to minimize patient exposure. Cores used in this study consist mainly of sediments with densities of ~2 g/cm3, which is consistent with that of human bone. It might be reasonable to use medical X-ray for 3-D mapping of corals within bulk sediments.

All samples were scanned with an X-ray peak energy of 120 kV with 150 mA current. Slice acquisition times were 4 s per scan. A 160 mm field of reconstruction was captured in a 512 pixel × 512 pixel image. The resolution of CT scanning is 0.313 mm/pixel × 0.313 mm/pixel. Contiguous two-dimensional 16-bit CT images at intervals of 1 mm sequentially from core top to core bottom of each half-round core were acquired. These data were used to reconstruct the 3-D images. Grayscale images from X-ray CT scanning were constructed using histogram equalization using the lower limit value of water, which provides contrastive images for structure observations.