Photon counting circuit, radiographic imaging apparatus, and threshold setting method
A photon counting circuit, a radiographic imaging apparatus, and a threshold setting method are provided, in which the photon counting circuit is capable of sufficiently reducing a difference between a target threshold value and a threshold value set for a pixel even if each pixel of a photon counting detector is divided into a plurality of subpixels. In the photon counting circuit for counting, for each pixel, electrical charges generated depending on photon energy of radiation applied to an object, a pixel is divided into a plurality of subpixels. When N is a natural number, a threshold value of each of the subpixels is selected from among top N discrete values of a plurality of discrete values arranged in order of proximity to a target threshold value corresponding to the photon energy so as to minimize a difference between the target threshold value and an average of the threshold values of the respective subpixels included in the pixel.
1. A photon counting circuit for counting, for each pixel, electrical charges generated depending on photon energy of radiation applied to an object,
wherein a pixel is divided into a plurality of subpixels, and
when N is a natural number, a threshold value of each of the subpixels is selected from among top N discrete values of a plurality of discrete values arranged in order of proximity to a target threshold value corresponding to the photon energy so as to minimize a difference between the target threshold value and an average of the threshold values of the respective subpixels included in the pixel.
2. The photon counting circuit according to claim 1 , wherein when the number of subpixels included in the pixel is 2×2, N=5.
3. The photon counting circuit according to claim 1 , wherein when the number of subpixels included in the pixel is 3×3, N=11.
4. The photon counting circuit according to claim 1 , wherein N=3.
5. The photon counting circuit according to claim 1 , wherein N=2.
6. The photon counting circuit according to claim 1 , wherein when the number of subpixels included in the pixel is M, N=M+1.
7. The photon counting circuit according to claim 1 , wherein the average is calculated as a weighted average of the threshold values of the respective subpixels, the weighted average being obtained by using an area ratio for each subpixel as a weight coefficient.
8. The photon counting circuit according to claim 1 , wherein the average is calculated as a weighted average of the threshold values of the respective subpixels, the weighted average being obtained by using, as a weight coefficient, a photon number ratio obtained from an energy spectrum of the radiation.
9. The photon counting circuit according to claim 1 , wherein the subpixels are divided into a plurality of groups, and the threshold values of the subpixels are selected so as to minimize a difference between the target threshold value and an average of threshold values of the respective groups.
10. The photon counting circuit according to claim 9 , wherein the groups are three groups of corner subpixels that are subpixels located at corners of the pixel, edge subpixels that are subpixels located on edges of the pixel, and a subpixel except for the corner subpixels and the edge subpixels.
11. The photon counting circuit according to claim 10 , wherein the corner subpixels are divided into two of diagonally opposite corners, and the edge subpixels are divided into two of opposite edges.
12. A radiographic imaging apparatus, comprising:
a radiation source for irradiating an object with radiation; and
a detection element module for detecting the radiation,
wherein the detection element module has the photon counting circuit according to claim 1 .
13. The radiographic imaging apparatus according to claim 12 , further comprising a display portion for displaying a display window that has a pixel map showing a location of a pixel a threshold value of which falls out of an allowable range, and a subpixel map showing a state of threshold values of subpixels making up a pixel selected.
14. A threshold setting method for setting threshold values for subpixels of a radiographic imaging apparatus that includes a radiation source for irradiating an object with radiation and a detection element module having a photon counting circuit for counting, for each pixel, electrical charges generated depending on photon energy of the radiation, the pixel being divided into a plurality of the subpixels,
the threshold setting method comprising the step of:
when N is a natural number, selecting a threshold value of each of the subpixels from among top N discrete values of a plurality of discrete values arranged in order of proximity to a target threshold value corresponding to the photon energy so as to minimize a difference between the target threshold value and an average of the threshold values of the respective subpixels included in the pixel.