We developed a prototype photon-counting mammography unit with a cadmium zinc telluride detector, which provides a new type of image with physical analysis parameters. Using the X-ray attenuation information obtained from this device, we examined the ability of this technique in discriminating substances and estimating their compositions. To estimate the substance compositions, we used resected breast tissues immediately after a surgical operation for invasive carcinoma of no special type, and used phantoms to reproduce mammary glands and adipose tissue. In our system, the spectrum penetrating the substance was measured with three energy bins in each pixel. The products of linear-attenuation coefficient and thicknesses for each bin were calculated. Using these three values, the scatterplots displaying all the values calculated from each pixel inside the region of interest (ROI) on the image were created. The scatterplot displaying only gravity values calculated for each ROI on the image was created for evaluating the separation of plot points to discriminate between different substance compositions. The gravity points placed on the malignant tumor tissue were plotted separately from those on the normal tissue. Furthermore, a fusion image was created by overlapping an X-ray image and values of this scatterplot points represented on a 10-step color scale. The fusion image was highlighting the differences in substance compositions using color tone, such as malignant tumor or mammary gland tissue, by adjusting the color scale level.
A new energy-resolved photon-counting mammography (ERPCM) device with a cadmium telluride (CdTe) series detector (JOB Corporation, Japan) is currently being developed. The CdTe series detector can detect higher-energy photons with high sensitivity, enabling the use of high-energy X-rays for imaging. Our previous research, in which we compared ERPCM using high-energy X-rays (tube voltage 50 kV) with general mammography using low-energy X-rays (tube voltage about 30 kV), reported that ERPCM had a higher CNR (contrast-to-noise ratio) than general mammography. The purpose of this study was to examine the magnitude of the CNR using a simulation and ERPCM; especially we would like to examine the CNR when the tube voltage of higher than 50 kV was adopted. In the comparison of the CNRs, It was necessary to pay attention to equalizing the average glandular dose (AGD). Using the simulation and ERPCM, we compared the CNR between images taken at 50 kV and 75 kV under a constant AGD. The simulation phantom was composed of 50% mammary gland and 50% adipose tissue, and contained tumor regions. The thickness of the simulation phantom was varied. We put an acrylic plate (1 mm thickness) on an RMI-156 phantom. Furthermore, we placed the thicker acrylic plate (10, 20, 30, 40 mm) on the 156 phantom and 1 mm-thick acrylic plate to simulate thicker breast. Based on the results from the simulation, in the phantom thickness of 80 mm, the CNR of image taken by 75kV got extremely closer to that taken by 50kV. The advantage of the image taken at 75 kV for the thicker breast was also confirmed in ERPCM.
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