Photon Counting CT (PCCT), which can obtain the energy information of each X-ray photon, is expected to provide new diagnoses. In the conventional CT, iodine contrast agent is often used to evaluate the tumor malignancy, and the capability of PCCT was evaluated about the discrimination between malignant tumors affected by iodine and some objects without iodine like blood clots, which is not necessarily be feasible without the energy information. The phantom with rods containing iohexol (0 - 3 mgI/mL) as the malignant tumor analogue and sucrose (0 - 200 mg/mL) as the benign tumor was used and material decomposition was performed into tin and PMMA as the basis materials. ROI values were derived in the basis material images and 70 keV virtual monochromatic image (VMI) for each rod. As a result, some rods of different compositions showed almost the same CT values in the 70 keV VMI, while the two-dimensional plot of tin and PMMA values demonstrated that the contribution of iodine and sucrose was clearly separated. This would mean that PCCT may bring improvement of quantitative iodine evaluation and better diagnosis on malignant and benign tumors.
Photon Counting Detector CT (PCD-CT) is expected to provide new possibilities by its capability of energy decomposition. However, because of non-ideal phenomena like the finite widths of individual energy windows of the detector or the scattered radiation, PCD-CT can have artifacts of not only dark but bright bands in some images. Therefore, we developed a correction method to reduce both dark and bright bands and verified it with data acquired with a prototype machine. The correction is performed on the material decomposition images, and acrylic and iodine images were employed. The dark band observed in the iodine image was corrected with the method used in the Energy Integrated Detector CT system. To suppress the bright band seen in the acrylic image, several virtual monochromatic images were created from the material decomposition images, and the energy was sought at which the dark and bright bands were cancelled. The bright band correction was performed using this energy and the correction amount of the dark band in the iodine image. It was demonstrated with the prototype machine data that the dark and bright bands were reduced in the material decomposition image and the virtual monochromatic image, and it was shown that the artifacts of both dark and bright bands can be corrected by the proposed method.
We developed a simple method of many (more than 2) material decomposition by combining 2-material decomposition by using energy information obtained with Photon Counting CT. We started from a theoretical approach and led the equation of the attenuation caused by 3 basis materials. As a result, we confirmed that the equations were degenerated when we used the same combination of the energy ranges among multiple 2-material decompositions, and the right lengths of individual basis materials cannot be obtained. On the other hand, we found that a set of equations can be solved when we used different combinations. To demonstrate our method, we simulated how 3-material decomposition (basis materials: water, iodine and calcium) is carried by 2-material decompositions with the same energy data and the different energy data. As a result, we cannot obtain proper solutions in the case of the same energy ranges as found in the theoretical approach, and we confirmed that the right lengths were provided in the case of the different energy ranges. In conclusion, we demonstrated that 3- material decomposition can be implemented by combining multiple 2-material decompositions by using different sets of the energy ranges.
To understand how the signals are affected by the radiation scattered by the test subject in Photon Counting CT system, the characteristics of the scattered photons were evaluated using Monte Carlo simulation “GEANT” (GEometry ANd Tracking). Cylinder water phantoms with diameters of 165 – 380 mm were examined, and the X-ray energy from 20 to 120 keV was divided into 5 ranges with a width of 20 keV in the detector. With the phantom with the diameter of 380 mm, the ratio of signals which are scattered in the phantom to those of the total X-rays incident on the detector turned out to be more than 50% in the 20 - 40 keV range, while it remained 2% in the 100 - 120 keV one. The profiles of this ratio were approximated by a quadratic function αx2 +β in each energy range where x corresponds to the longitudinal detector position. It was found that α and β can be described with the energy range and phantom size.
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