# US 10,869,646 B2 · Method and apparatus for computed tomography (CT) and material decomposition with pile-up correction calibrated using real pulse pileup effect and detector response

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- Number: US 10,869,646 B2
- Document ID: US10869646B2
- Status: Granted
- Kind: B2
- Application number: 15/951,329
- Filed: 2018-04-12
- Published: 2019-10-17
- Granted: 2020-12-22
- Earliest claimed: 2018-04-12
- Assignees: Canon Medical Systems Corporation
- Applicants: Canon Medical Systems Corporation
- Inventors: Jian Zhou; Zhou Yu; Yan Liu
- Web page: https://patentgrep.com/patent/US10869646B2

## CPC

- A61B 6/5205: {involving processing of raw data to produce diagnostic data}
- A61B 6/025: {Tomosynthesis}
- A61B 6/032: {Transmission computed tomography [CT]}
- A61B 6/0407: {Supports, e.g. tables or beds, for the body or parts of the body}
- A61B 6/4208: {characterised by using a particular type of detector}
- A61B 6/4241: {using energy resolving detectors, e.g. photon counting}
- A61B 6/4266: {characterised by using a plurality of detector units (A61B6/4014 takes precedence)}
- A61B 6/4275: {using a detector unit almost surrounding the patient, e.g. more than 180°}
- A61B 6/482: {involving multiple energy imaging}
- A61B 6/502: {for diagnosis of breast, i.e. mammography}
- A61B 6/5258: {involving detection or reduction of artifacts or noise}
- A61B 6/582: {Calibration}
- A61B: DIAGNOSIS; SURGERY; IDENTIFICATION (usefulness limited to only animals A61D)

## Abstract

An apparatus and method are described using a forward model to correct pulse pileup in spectrally resolved X-ray projection data from photon-counting detectors (PCDs). To calibrate the forward model, which represents each order of pileup using a respective pileup response matrix (PRM), an optimization search determines the elements of the PRMs that optimize an objective function measuring agreement between the spectra of recorded counts affected by pulse pileup and the estimated counts generated using forward model of pulse pileup. The spectrum of the recorded counts in the projection data is corrected using the calibrated forward model, by determining an argument value that optimizes the objective function, the argument being either a corrected X-ray spectrum or the projection lengths of a material decomposition. Images for material components of the material decomposition are then reconstructed using the corrected projection data.

## Claims

### Claim 1

An apparatus to process projection measurements, the apparatus comprising:
processing circuitry configured to
obtain projection data comprising recorded counts that (i) correspond to energy bins of an X-ray detector, (ii) are affected by pileup of X-rays within a detection time window, and (iii) have a pileup-affected spectrum that is distorted relative to a true spectrum of the detected X-rays, wherein the recorded counts represent X-rays detected at respective photon-counting detector elements comprising the X-ray detector, and the detected X-rays have been transmitted through an object being imaged,
obtain a forward model of pulse pileup that estimates the pileup-affected spectrum of the recorded counts based on an input of a pileup-corrected spectrum proposed to represent the true spectrum of the detected X-rays, the forward model including parameters specific to the respective photon-counting detector elements, and the forward model representing two or more orders of pulse pileup and including a respective pileup response matrix for each one of the two or more orders of pulse pileup, and
correct the projection data using the forward model to generate corrected projection data, wherein
an nth-order of pulse pileup of the two or more orders of pulse pileup represents n+1 X-rays being detected at one of the photon-counting detector elements within a detection time window, and n is a positive integer.

### Claim 2 (depends on 1)

The apparatus according to claim 1, wherein the processing circuitry is further configured to correct the projection data by
optimizing a value of an objective function to determine an optimal value of an argument of the objective function, the objective function representing agreement between the pileup-affected spectrum of the obtained projection data and the estimated pileup-affected spectrum generated using the forward model.

### Claim 3 (depends on 2)

The apparatus according to claim 2, wherein the processing circuitry is further configured to obtain the forward model, wherein
the argument of the objective function, which is being used to optimize the objective function, is one of the input of the pileup-corrected spectrum in the forward model and projection lengths of a material decomposition, which are used to determine the input of the pileup-corrected spectrum in the forward model, wherein,
when the argument is the projection lengths,
the correcting of the projection data using the forward model to generate the corrected projection data is performed such that the corrected projection data includes the projection lengths, and
the processing circuitry is further configured to reconstruct material-component images using the projection lengths of the corrected projection data, and,
when the argument is the spectrum of X-rays detected at the one of the photon-counting detector elements,
the correcting of the projection data using the forward model to generate the corrected projection data is performed such that the corrected projection data includes corrected counts corresponding to other energy bins, and
the processing circuitry is further configured to decompose the corrected counts into projection lengths of the material decomposition and reconstruct material-component images using the projection lengths.

### Claim 4 (depends on 2)

The apparatus according to claim 2, wherein the processing circuitry is further configured to correct the projection data using the objective function that is one of a Poisson likelihood function, a least-squares difference function, and a weighted least-squares difference function.

### Claim 5 (depends on 1)

The apparatus according to claim 1, wherein the processing circuitry is further configured to obtain the forward model, wherein, in the forward model, the input spectrum of the detected X-rays is determined using a multiplication between an attenuation coefficient of a material component and a projection length for the material component, for each material component of a material decomposition.

### Claim 6 (depends on 1)

The apparatus according to claim 1, wherein the processing circuitry is further configured to obtain the forward model, wherein, in forward model, the spectrum of the recorded counts is partitioned according to the energy bins of the X-ray detector, and the input spectrum representing the detected X-rays is partitioned according to other energy bins having one or more different partitions than the energy bins of the X-ray detector.

### Claim 7 (depends on 1)

The apparatus according to claim 1, wherein the processing circuitry is further configured to obtain the forward model, wherein, in the forward model for each one of the photon-counting detector elements, each nth-order of pulse pileup of the two or more orders of pulse pileup is scaled by a Poisson-distributed probability of n+1 X-rays being detected at the one of the photon-counting detector elements within the detection time window.

### Claim 8 (depends on 1)

The apparatus according to claim 1, wherein the processing circuitry is further configured to
decompose the corrected projection data into material components to generate a material decomposition, and
reconstruct material-component images of the object being imaged using the material decomposition.

### Claim 9 (depends on 1)

The apparatus according to claim 1, wherein the processing circuitry is further configured to
correct the projection data using the forward model to generate the corrected projection data, wherein the correcting of the projection data using the forward model integrates a material decomposition with correcting the pileup-affected spectrum of the recorded counts of the obtained projection data to generate the corrected projection data, which includes projection lengths of the material decomposition, and
the processing circuitry is further configured to reconstruct material-component images of the object being imaged using the corrected projection data.

### Claim 10 (depends on 1)

The apparatus according to claim 1, wherein the processing circuitry is further configured to
generate the forward model using calibration data representing projection data generated by the X-ray detector in an absence of the object being imaged, the parameters specific to the respective photon-counting detector elements being determined as an argument that optimizes an objective function that represents agreement between the pileup-affected spectrum of the obtained projection data and the estimated pileup-affected spectrum generated using the forward model, the argument of the objective function being the parameters specific to the respective photon-counting detector elements,
store, in a non-transitory computer readable medium, the parameters specific to the respective photon-counting detector elements that optimize the objective function, and
perform the obtaining of the forward model by reading, from the non-transitory computer readable medium, the stored parameters specific to the respective photon-counting detector elements.

### Claim 11 (depends on 1)

The apparatus according to claim 1, wherein the processing circuitry is further configured to obtain the forward model, wherein, in forward model, a number of the energy bins of the X-ray detector is equal to a number of the other energy bins, which is less than seven.

### Claim 12 (depends on 1)

The apparatus according to claim 1, wherein the processing circuitry is further configured to obtain the projection data, wherein the photon-counting detector elements are semiconductor detectors comprising one or more of cadmium telluride (CdTe), cadmium zinc telluride (CZT), silicon (Si), mercuric iodide (HgI2), and gallium arsenide (GaAs).

### Claim 13

An imaging method, comprising:
obtaining projection images comprising recorded counts that (i) correspond to energy bins of an X-ray detector, (ii) are affected by pileup of X-rays within a detection time window, and (iii) have a pileup-affected spectrum that is distorted relative to a true spectrum of the detected X-rays, wherein the recorded counts representing X-rays detected at respective photon-counting detector elements comprising the X-ray detector, and the detected X-rays have been transmitted through an object being imaged;
obtaining a forward model of pulse pileup that estimates the pileup-affected spectrum of the recorded counts based on an input of a pileup-corrected spectrum representing the detected X-rays, the forward model including parameters specific to the respective photon-counting detector elements, and the forward model representing two or more orders of pulse pileup and including a respective pileup response matrix for each one of the two or more orders of pulse pileup; and
correcting the projection images using the forward model to generate corrected projection images, wherein
an nth-order of pulse pileup of the two or more orders of pulse pileup represents n+1 X-rays being detected at one of the photon-counting detector elements within a detection time window, and n is a positive integer.

### Claim 14 (depends on 13)

The method according to claim 13, wherein the correcting of the projection data further includes optimizing a value of an objective function to determine an optimal value of an argument of the objective function, the objective function representing agreement between the pileup-affected spectrum of the obtained projection data and the estimated pileup-affected spectrum generated using the forward model.

### Claim 15 (depends on 14)

The method according to claim 14, wherein the obtaining of the forward model further includes that
the argument of the objective function, which is being used to optimize the objective function, is one of the input of the pileup-corrected spectrum in the forward model and projection lengths of a material decomposition, which are used to determine the input of the pileup-corrected spectrum in the forward model, wherein,
when the argument is the projection lengths of the material components,
the correcting of the projection data using the forward model to generate the corrected projection data is performed such that the corrected projection data includes the projection lengths of the material components, and
the method further includes reconstructing material-component images using the projection lengths of the corrected projection data, and,
when the argument is the spectrum of X-rays detected at the one of the photon-counting detector elements,
the correcting of the projection data using the forward model to generate the corrected projection data is performed such that the corrected projection data includes corrected counts corresponding to other energy bins, and
the method further includes decomposing the corrected counts into projection lengths of the material decomposition and reconstruct material-component images using the projection lengths.

### Claim 16 (depends on 14)

The method according to claim 14, wherein the correcting of the projection data further includes using the objective function that is one of a Poisson likelihood function, a least-squares difference function, and a weighted least-squares difference function.

### Claim 17 (depends on 13)

The method according to claim 13, wherein
the correcting of the projection data is performed using the forward model to generate the corrected projection data, wherein the correcting of the projection data using the forward model integrates a material decomposition with correcting the pileup-affected spectrum of the recorded counts of the obtained projection data to generate the corrected projection data, which includes projection lengths of the material decomposition, and
the method further includes reconstructing material-component images of the object being imaged using the corrected projection data.

### Claim 18 (depends on 13)

The method according to claim 13, further comprising:
generating the forward model using calibration data representing projection data generated by the X-ray detector in an absence of the object being imaged, the parameters specific to the respective photon-counting detector elements being determined as an argument that optimizes an objective function that represents agreement between the pileup-affected spectrum of the obtained projection data and the estimated pileup-affected spectrum generated using the forward model, the argument of the objective function being the parameters specific to the respective photon-counting detector elements,
storing, in a non-transitory computer readable medium, the parameters specific to the respective photon-counting detector elements that optimize the objective function, and
the obtaining of the forward model including reading, from the non-transitory computer readable medium, the stored parameters specific to the respective photon-counting detector elements.

### Claim 19 (depends on 13)

A non-transitory computer-readable medium storing executable instructions, wherein the instructions, when executed by processing circuitry, cause the processing circuitry to perform the method according to claim 13.

### Claim 20 (depends on 13)

The method according to claim 13, further comprising:
decomposing the corrected projection data into material components to generate a material decomposition, and
reconstructing material-component images of the object being imaged using the material decomposition.