Patent Application
20250235167
The present application claims priority from Japanese Patent Application JP 2024-006632 filed on Jan. 19, 2024, the content of which is hereby incorporated by reference into this application.
The present invention relates to a photon counting computed tomography (PCCT) apparatus that is an X-ray CT apparatus comprising a photon counting detector, and particularly relates to expansion processing of calibration data used for calibration of the photon counting detector.
An X-ray CT apparatus uses projection data from multiple directions that is obtained by rotating, around a subject, an X-ray source that emits an X-ray to the subject and an X-ray detector that detects the X-ray transmitted through the subject to generate a tomographic image of the subject. The generated tomographic image is used as a medical image used for image diagnosis of the subject.
In the PCCT apparatus that uses a photon counting detector as an X-ray detector, the number of X-ray photons can be counted for each energy, and thus a substance in the subject can be discriminated by using projection values acquired in each of a plurality of energy bins. In order to obtain a tomographic image in which the substance is discriminated, it is necessary to acquire calibration data for substance discrimination in advance using a plurality of basal substances that are substances having known compositions and thicknesses.
US2016/0363442A discloses that a thickness of a basal substance in a sample is estimated by comparing a calibration spectrum which is an energy spectrum of X-ray photons transmitted through a combination of basal substances having known compositions and thicknesses with a spectrum transmitted through the sample. That is, a calibration spectrum most similar to the spectrum of the sample is searched for from among a plurality of calibration spectra measured for each combination of basal substances having different thicknesses, and a thickness of the basal substance corresponding to the searched calibration spectrum is estimated as the thickness of the basal substance in the sample. In addition, it is disclosed to limit the number of calibration spectra by interpolating the calibration spectra. In addition, it is disclosed that in order to estimate a thickness of a sample having an effective atomic number outside a range of an effective atomic number of the basal substance, a spectrum transmitted through a basal substance having a negative thickness is obtained by measuring a spectrum of a substance other than the basal substance.
However, in US2016/0363442A, in order to obtain a spectrum transmitted through a basal substance having a negative thickness, it is necessary to measure a spectrum of a substance other than the basal substance.
Therefore, an object of the present invention is to provide a PCCT apparatus and a control method thereof capable of obtaining a spectrum transmitted through a basal substance having a negative thickness without measuring a spectrum of a substance other than the basal substance.
In order to achieve the above object, according to an aspect of the present invention, there is provided a PCCT apparatus comprising: a scanner that rotates an X-ray source that irradiates a subject with X-rays and a photon counting detector that detects the X-rays transmitted through the subject for each of a plurality of energy bins around the subject; an image generation unit that generates a tomographic image using projection data calculated based on an output of the photon counting detector; a map creation unit that creates a calibration data map based on the number of X-ray photons transmitted through each of a plurality of calibration phantoms having a first basal substance and a second basal substance of which a material and a thickness are known; an expansion unit that creates an expanded data map in which the calibration data map is expanded to a negative region by performing extrapolation processing on the number of X-ray photons; and a substance discrimination unit that performs substance discrimination based on the expanded data map.
In addition, according to another aspect of the present invention, there is provided a control method of PCCT apparatus including a scanner that rotates an X-ray source that irradiates a subject with X-rays and a photon counting detector that detects the X-rays transmitted through the subject for each of a plurality of energy bins around the subject, and an image generation unit that generates a tomographic image using projection data calculated based on an output of the photon counting detector, the control method comprising: a map creation step of creating a calibration data map based on the number of X-ray photons transmitted through each of a plurality of calibration phantoms having a first basal substance and a second basal substance of which a material and a thickness are known; an expansion step of creating an expanded data map in which the calibration data map is expanded to a negative region by performing extrapolation processing on the number of X-ray photons; and a substance discrimination step of performing substance discrimination based on the expanded data map.
According to the present invention, it is possible to provide a PCCT apparatus and a control method thereof capable of obtaining a spectrum transmitted through a basal substance having a negative thickness without measuring a spectrum of a substance other than the basal substance.
Hereinafter, examples of a photon counting computed tomography (PCCT) apparatus and a control method thereof according to the present invention will be described with reference to the accompanying drawings. In the following description and the accompanying drawings, components having the same functional configuration will be denoted by the same reference numerals, and duplicated descriptions will not be repeated.
An overall configuration of a PCCT apparatus of Example 1 will be described with reference to
The scanner 100 comprises an X-ray source 101, a rotating plate 102, a collimator 103, a photon counting detector 106, a data collection unit 107, an examination table 105, a rotating plate controller 108, an examination table controller 109, an X-ray controller 110, and a high-voltage generation unit 111. The X-ray source 101 is a device that irradiates a subject 10 placed on the examination table 105 with X-rays and is, for example, an X-ray tube device. The collimator 103 is a device that limits an irradiation range of X-rays. The rotating plate 102 includes an opening portion 104 through which the subject 10 placed on the examination table 105 enters, is equipped with the X-ray source 101 and the photon counting detector 106, and rotates the X-ray source 101 and the photon counting detector 106 around the subject 10.
The photon counting detector 106 is a device that is disposed to face the X-ray source 101 and acquires a spatial distribution of X-rays by comprising a plurality of detection elements that detect the X-rays. The detection elements of the photon counting detector 106 are arranged two-dimensionally in a rotation direction and a rotation axis direction of the rotating plate 102, individually count incident X-ray photons, and discriminate each X-ray photon into a plurality of energy bins.
The rotating plate controller 108 is a device that controls rotation and inclination of the rotating plate 102. The examination table controller 109 is a device that controls upward, downward, forward, backward, leftward, and rightward movements of the examination table 105. The high-voltage generation unit 111 is a power source that generates a tube voltage, which is a voltage applied to the X-ray source 101, and a tube current, which is a current supplied to the X-ray source 101. The X-ray controller 110 is a device that controls an output of the high-voltage generation unit 111. The rotating plate controller 108, the examination table controller 109, and the X-ray controller 110 are, for example, a micro-processing unit (MPU) or the like.
The operation unit 120 comprises an input unit 121, an image generation unit 122, a display unit 125, a storage unit 123, and a system controller 124. The input unit 121 is a device that is used to input examination data such as a name of the subject 10, an examination date and time, and an imaging condition, and is, for example, a keyboard, a pointing device, a touch panel, or the like. The image generation unit 122 is a device that generates a tomographic image by using digital data collected by the data collection unit 107, and is, for example, an MPU, a graphics processing unit (GPU), or the like. The display unit 125 is a device that displays the tomographic image or the like generated by the image generation unit 122, and is, for example, a liquid crystal display, a touch panel, or the like. The storage unit 123 is a device that stores the digital data collected by the data collection unit 107, the tomographic image generated by the image generation unit 122, a program to be executed by the system controller 124, data to be used by the program, and the like, and is, for example, a hard disk drive (HDD), a solid state drive (SSD), or the like. The system controller 124 is a device that controls each unit such as the rotating plate controller 108, the examination table controller 109, and the X-ray controller 110, and is, for example, a central processing unit (CPU).
With the generation of the tube voltage and the tube current by the high-voltage generation unit 111 based on the imaging condition set via the input unit 121, the X-rays according to the imaging condition are emitted from the X-ray source 101 to the subject 10. The photon counting detector 106 detects the X-rays emitted from the X-ray source 101 and transmitted through the subject 10 with multiple detection elements to acquire the spatial distribution of the transmitted X-rays. The rotating plate 102 is controlled by the rotating plate controller 108 and rotates based on the imaging condition input through the input unit 121, particularly a rotation speed or the like. The examination table 105 is controlled by the examination table controller 109 and moves relative to the rotating plate 102 to move an imaging position designated with respect to the subject 10 to an imaging field of view, which is a range in which the transmitted X-rays are detected.
With repetition of the emission of the X-rays by the X-ray source 101 and the detection of the X-rays by the photon counting detector 106 together with the rotation of the rotating plate 102, projection data of the subject 10 is measured at various projection angles. In the projection data, a view representing each projection angle is associated with a channel (ch) number and a column number which are detection element numbers of the photon counting detector 106. The measured projection data is transmitted to the image generation unit 122. The image generation unit 122 performs back-projection processing on a plurality of projection data to generate the tomographic image. The generated tomographic image is displayed on the display unit 125 or stored in the storage unit 123 as a medical image.
Since the PCCT apparatus comprising the photon counting detector 106 can acquire the projection data of the subject 10 for each energy bin, a medical image for each energy bin or a medical image discriminated into substances having different compositions can be generated. In order to obtain the medical image discriminated into the substances having different compositions, it is necessary to acquire calibration data of the photon counting detector 106 in advance.
The calibration data 210 of the photon counting detector 106 will be described with reference to
An example of the plurality of calibration phantoms 201 will be described with reference to
The calibration data 210 acquired using the plurality of calibration phantoms 201 is compared with the projection data of the subject 10, and the calibration data 210 similar to a photon energy spectrum of the projection data of the subject 10 is selected from a plurality of the calibration data 210. Then, the subject 10 is subjected to substance discrimination based on the thicknesses of the first basal substance 202 and the second basal substance 203 of the calibration phantom 201 corresponding to the selected calibration data 210.
In a case where the subject 10 includes a substance having an effective atomic number outside a range Zeff1<Z<Zeff2 determined by the effective atomic number Zeff1 of the first basal substance 202 and the effective atomic number Zeff2 of the second basal substance 203, the thickness of one of the first basal substance 202 and the second basal substance 203 may be a negative value. On the other hand, since the thicknesses of the first basal substance 202 and the second basal substance 203 included in the calibration phantom 201 used for acquiring the calibration data 210 are not negative values, it is necessary to obtain a negative thickness value by extrapolation processing. In a case of acquiring the negative thickness value by the extrapolation processing using the calibration data 210, an amount of calculation increases and a processing time becomes longer.
Therefore, in Example 1, a calibration data map, which is a set of the calibration data 210, is expanded to a negative region in which the thickness of the basal substance is a negative value, and the substance discrimination is performed based on an expanded data map, which is the calibration data map expanded to the negative region. Since the extrapolation processing is not required by discriminating the substance based on the expanded data map, the amount of calculation is not increased and the processing time is also maintained.
An example of a flow of processing of expanding the calibration data map, which is a set of the calibration data 210, to the negative region will be described for each step with reference to
The calibration phantom 201 is set between the X-ray source 101 and the photon counting detector 106.
The system controller 124 counts the X-ray photons transmitted through the calibration phantom 201 by the X-ray irradiation from the X-ray source 101 and the X-ray detection by the photon counting detector 106. In S402, the rotating plate 102 is not rotated.
Whether or not the counting for all the calibration phantoms 201 has been completed is determined. In a case where the counting for all the calibration phantoms 201 is completed, the process proceeds to S404. In a case where there is any uncounted calibration phantom 201, the process returns to S401, and the uncounted calibration phantom 201 is set between the X-ray source 101 and the photon counting detector 106.
The system controller 124 creates a calibration data map 500 illustrated in
The system controller 124 expands the calibration data map 500 created in S404 to a negative region 601 illustrated in
In a case where the thickness of the basal substance is extremely small and the number of X-ray photons incident on the photon counting detector 106 is large, the count value may be saturated, and the Log value of the count value with respect to the thickness of the basal substance may be non-linear. Therefore, as illustrated in
By the flow of the processing illustrated in
The system controller 124 that executes S404 functions as a map creation unit that creates the calibration data map 500 based on the number of X-ray photons transmitted through each of the plurality of calibration phantoms 201 having the first basal substance 202 and the second basal substance 203 of which a material and a thickness are known. In addition, the system controller 124 that executes S405 functions as an expansion unit that creates the expanded data map 600 in which the calibration data map 500 is expanded to the negative region 601 by performing extrapolation processing on the number of X-ray photons transmitted through each of the plurality of calibration phantoms 201.
An example of a flow of processing of generating a tomographic image in which a substance is discriminated based on the expanded data map 600 will be described for each step with reference to
The projection data of the subject 10 is acquired through the control of the system controller 124. More specifically, in a state in which the rotating plate 102 is rotated, the subject 10 is irradiated with X-rays from the X-ray source 101, and the X-rays transmitted through the subject 10 are detected by the photon counting detector 106, so that the projection data at various projection angles are acquired. The projection data of the subject 10 is acquired while being divided into a plurality of energy bins.
The system controller 124 discriminates the projection data of the subject 10 acquired in S801 into a plurality of substances, for example, the first basal substance 202 and the second basal substance 203, based on the expanded data map 600 read out from the storage unit. In a case of substance discrimination, the thickness of any one of the first basal substance 202 or the second basal substance 203 may be a negative value. In the negative region 601, since a relationship between the thickness of the basal substance and the count value is non-linear, it is preferable that non-linear interpolation using a high-order function is performed. In addition, it is preferable that the order of the high-order function used for the non-linear interpolation is increased according to the strength of the non-linearity of the expanded data map 600. As the non-linearity is stronger, the order is increased, and thus an interpolation accuracy can be further improved.
The system controller 124 reconstructs the tomographic image for each substance using the projection data subjected to the substance discrimination in S802. That is, the tomographic image of the first basal substance 202 and the tomographic image of the second basal substance 203 are reconstructed.
The tomographic image of the subject 10 is generated for each substance using the projection data subjected to the substance discrimination based on the expanded data map 600 according to the flow of processing described with reference to
The examples of the present invention have been described above. The present invention is not limited to the above examples, and the components can be modified and embodied without departing from the spirit of the invention. In addition, a plurality of the components disclosed in the above examples may be combined as appropriate. Further, some components may be deleted from all the components shown in the above examples.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2024-006632 | Jan 2024 | JP | national |
