KR101347254B1 - Correction method for non-uniformity of photon counting detector - Google Patents

Abstract

Disclosed is a method for correcting images obtained with a photon counting-based X-ray detector. The method includes a step of measuring the number of detection photons detected in each pixel of the photon counting-based X-ray detector after passing through a medium while the thickness of the medium is changed; a step of analyzing a relation between each thickness of the medium and the number of incident photons in each pixel based on the measurement; a step of estimating response properties of each pixel according to the analyzed relation; and a step of correcting the number of detection photons detected from each pixel by applying the estimated response properties of each pixel to images on which a subject is photographed. [Reference numerals] (AA) Start; (BB) End; (S310) Measurement of the number of detection photons detected at each pixel of a detector after passing through a medium while the thickness of the medium is changed; (S320) Analysis of a relation between the thickness of the medium and the number of incident photons on each pixel; (S330) Estimation of the response properties of each pixel; (S340) Determination of the equivalent medium thickness corresponding to an object by each pixel based on the response properties of each pixel; (S350) Determination of the number of the detection photons of each pixel based on the ideal number of the detection photons of each pixel corresponding to the thickness of the equivalent medium

Description

Non-uniformity correction method of radiation detector {CORRECTION METHOD FOR NON-UNIFORMITY OF PHOTON COUNTING DETECTOR}

The present disclosure relates to a method for correcting nonuniformity of a radiation detector, and more particularly, to a method for effectively correcting a photographed image obtained from a radiation detector and an apparatus used therefor.

Recently, medical imaging technology has advanced dramatically, and new convergent imaging equipment has been continuously developed.

Conventional X-ray detectors convert visible light into electrical signals using scintillation crystal detectors. However, in the recent photon counting system, electron-electron pairs are detected using semiconductor detectors, and then, after passing through the discriminator, not only the number of photons but also energy information is passed. I can get it.

CdZnTe (cadmium zinc telluride) and CdTe (cadmium telluride), which are new semiconductor detectors capable of photon counting, have the advantage of detecting both X-rays and gamma rays. As a result, research is being actively conducted.

In addition, the photon count-based CdTe detector has a high atomic number and a wide band spacing, and thus has high detection efficiency and can be operated at room temperature. However, there was a problem of crystal defects caused by the difficulty of growing a CdTe crystal largely, and an unevenness of the image due to the distance between the edge of the CdTe detector and application specific integrated circuits (ASIC).

 Conventionally, a flat-field correction method has been used as a method for correcting image non-uniformity, but this may cause a problem that an incorrect corrected image is obtained by distorting noise information of the image.

The present invention has been proposed to solve the above problems, and an object of the present invention is to provide a new method for correcting an image by deriving a response characteristic for each pixel of a CdTe detector.

According to an embodiment of the present disclosure, a method of correcting an image acquired by a photon count based X-ray detector is disclosed. The method includes measuring the number of detected photons detected at each pixel of the photon count based X-ray detector after passing through the medium with varying thickness of the medium; Analyzing a relationship between each thickness of the medium and the number of photons incident on each pixel based on the measurement; Estimating a response characteristic of each pixel according to the analyzed relationship; The method may include correcting the number of detected photons detected by each pixel by applying the estimated response characteristic of each pixel to an image photographed by an arbitrary subject.

At this time, the estimated response of each pixel is

이며,

Lt;

는 상기 매질의 두께이고, 상기

와

는 (i,j) 픽셀의 응답 특성 계수이고, 상기

는 검출 광자 수 일 수 있다.remind

Is the thickness of the medium, and

Wow

Is the response characteristic coefficient of the pixel (i, j),

May be the number of photons detected.

Preferably the medium may be aluminum.

The photon-based X-ray detector may be a cadmium telluride (CdTe) detector.

The correcting the number of detected photons may include determining an equivalent medium thickness corresponding to the subject for each pixel based on the estimated response of each pixel; And determining the number of detected photons for each pixel based on the ideal number of detected photons for each pixel corresponding to the equivalent medium thickness, wherein the ideal number of detected photons for each pixel is Monte Carlo simulation (Monte). Carlo Simulation).

A computer readable recording medium according to another embodiment of the present specification may include instructions for performing each step of the method according to any one of the above methods.

According to another embodiment of the present disclosure, an apparatus for correcting an image obtained from a photon count based X-ray detector is disclosed. The apparatus includes: a detector for measuring the number of detected photons detected at each pixel of the photon count-based X-ray detector after passing through the medium while changing the thickness of the medium; A response characteristic estimator for analyzing a relationship between each thickness of the medium and the number of photons detected in each pixel based on the measured number of detected photons, and estimating a response characteristic of each pixel according to the analyzed relationship Wow; And an image compensator for correcting the number of detection photons detected at each pixel by applying the estimated response of each pixel to an image photographed by an arbitrary subject.

Preferably the medium may be aluminum.

The photon-based X-ray detector may be a cadmium telluride (CdTe) detector.

The image compensator determines an equivalent medium thickness corresponding to the object for each pixel based on the estimated response of each pixel, and based on the ideal number of detected photons for each pixel corresponding to the equivalent medium thickness. The number of detected photons for each pixel can be determined.

According to the embodiment disclosed herein, more accurate image correction can be performed by deriving and applying a response characteristic for each pixel.

1 is a conceptual diagram illustrating a radiation imaging apparatus according to the prior art.
2A and 2B illustrate a method of correcting nonuniformity of a photon count detector according to an exemplary embodiment of the present specification.
3 is a flowchart illustrating an image correction method according to an exemplary embodiment of the present specification.
4 is a graph showing the distribution of photons per pixel of the original image and the original image.
FIG. 5 is a graph illustrating photon number distribution of each pixel of an image in which the original image of FIG. 4 is corrected according to an exemplary embodiment of the present specification.
6 is a graph illustrating uniformity of an image corrected according to an exemplary embodiment of the present specification.
7 is a subject used for capturing an image for applying an embodiment of the present specification.
8 is an original image and a corrected image of the subject of FIG. 7.
9 and 10 are diagrams evaluating the original image and the corrected image of FIG. 8.
11 is a block diagram illustrating an image calibrating apparatus according to an exemplary embodiment of the present specification.

It is noted that the technical terms used herein are used only to describe specific embodiments and are not intended to limit the invention. It is also to be understood that the technical terms used herein are to be interpreted in a sense generally understood by a person skilled in the art to which the present invention belongs, Should not be construed to mean, or be interpreted in an excessively reduced sense. In addition, when the technical terms used herein are incorrect technical terms that do not accurately represent the spirit of the present invention, it should be replaced with technical terms that can be understood correctly by those skilled in the art. In addition, the general terms used in the present invention should be interpreted according to a predefined or prior context, and should not be construed as being excessively reduced.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings, wherein like reference numerals refer to like or similar elements throughout the several views, and redundant description thereof will be omitted. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail. It is to be noted that the accompanying drawings are only for the purpose of facilitating understanding of the present invention, and should not be construed as limiting the scope of the present invention with reference to the accompanying drawings. The spirit of the invention should be construed to extend to all changes, equivalents, and substitutes in addition to the accompanying drawings.

1 is a conceptual diagram illustrating a radiation imaging apparatus according to the prior art.

In general, the radiographic imaging apparatus includes a radiation source 10 that emits radiation toward an object B, a radiation detector 20 that detects radiation transmitted through the object B, and an object B and a radiation detector 20. It may be configured to include a collimator 30 disposed in the to adjust the path of the radiation transmitted through the object (B). In this case, the detector 20 may be configured as a photon count detector. In general, a photon counting detector refers to a detector that counts the number of photons of incident light. The photon count detector can detect photons included in incident light and output a pulse signal corresponding to the detected photons. The number of photons can correspond to this pulse signal. When a photon count detector is used, the photon count detector counts the photons of the X-rays that have passed through the object and reach each pixel of the detector and provide them to the image reader, the image reader based on the number of detected photons per pixel. It is common to implement

In this case, an error may occur in the number of photons incident on each pixel due to various reasons (for example, defects or edges of the crystal itself). In other words, a number of photons different from the number of photons that should ideally be incident under specific conditions may be incident, and a flat-field correction method has been conventionally used to correct this error. However, the conventional flat field correction method has a problem in that an incorrect corrected image may be obtained by distorting noise information of an image.

Therefore, the following proposes a method for effectively correcting the error.

2 is a diagram illustrating a method of correcting nonuniformity of a photon count detector according to an exemplary embodiment of the present specification.

First, FIG. 2A is a schematic diagram showing an outline of the method. A radiation source 100, a photon count detector 200 and a transmission medium 400 are provided for carrying out this embodiment.

In this case, the radiation source may be an X-ray emitter. The photon count detector 200 may be a cadmium telluride (CdTe) detector or a cadmium zinc telluride (CdZnTe) detector. Radiation emitted from the radiation source 100 is provided to penetrate the medium 400 and enter the detector 200.

The correction method proposed in the present specification is to acquire response characteristics of each pixel of the detector and to correct a photographed image based on the response characteristics of each pixel. The response characteristic of each pixel may be obtained in the form of a function including a response characteristic coefficient, and a detailed description thereof will be described with reference to FIG. 2B.

The response characteristic of each pixel may be obtained by repeatedly measuring the number of photons incident on each pixel of the detector 200 through a specific medium. That is, it can be obtained by experimentally measuring how the number of photons incident on each pixel changes under changing medium conditions.

 In order to obtain the response characteristics of each pixel, it is preferable to change the thickness of the medium and measure the number of photons incident on each pixel.

Referring to the correction method proposed in this specification in more detail with reference to Figure 2b as follows. For convenience of description, the medium located between the radiation source and the detector is exemplarily described as being aluminum.

When the number of photons (equivalently energy) incident on the detector is larger than the area of the detector and the uniform aluminum thickness is changed, the number of photons detected at each pixel of the detector is Beer-Lambert. According to the law of Equation 1 may be expressed as

Where Io (E) is the number of incident photons, I (E) is the number of photons detected by the detector, μ is the linear attenuation coefficient of the medium (aluminum), and t is the thickness of the aluminum.

In an ideal case, the number of photons passing through aluminum having a specific thickness t and reaching the pixel of the detector would be the same and can be obtained using Equation 1 above.

In practice, however, the number of photons reaching each pixel is often different from the ideal case, and thus there is a need for appropriate correction of the number of photons detected for each pixel.

The method of correcting the number of detected photons of each pixel proposed in the present specification starts with experimentally obtaining a response characteristic of each pixel of the detector. Therefore, in order to obtain the response characteristic of each pixel of the detector, the photon number detected in each pixel is experimentally measured according to the change in the thickness t of the passing medium (aluminum).

2B graphically illustrates the number of photons detected in any four pixels, each represented by a different color (red, blue, black, green). The graph in FIG. 2B (right) shows the number of photons that have passed through aluminum of a certain thickness, detected at any of the four pixels, where photons emitted from the radiation source enter each pixel after passing through aluminum of the same thickness. Even if the number of photons detected is different for each pixel (i.e., ideally the number of photons incident on each pixel) is different.

However, although the number of photons detected for each pixel is different when the same thickness of aluminum is transmitted, the number of photons detected at each pixel tends to decrease according to Beer-Lambert's law as described above. This tendency can be approximated with a logarithm function. This decreasing tendency can be seen as a response characteristic of each pixel, and the response characteristic of each pixel can be expressed as Equation 2 below.

는 알루미늄의 두께를 나타내고,

와

는 (i,j)픽셀의 응답 특성 상수(계수)이며,

는 검출된 광자 수(또는 대응되는 에너지)이다. 이때

와

를 각 픽셀의 응답 특성 맵(map) 또는 각 픽셀의 응답 특성 계수라고 정의할 수 있다.

Represents the thickness of aluminum,

Wow

Is the response characteristic constant (coefficient) of (i, j) pixels,

Is the number of photons detected (or the corresponding energy). At this time

Wow

May be defined as a response characteristic map of each pixel or a response characteristic coefficient of each pixel.

와

를 더 정확하게 추정할 수 있다.By measuring the I of each pixel while varying the thickness T of the known aluminum,

Wow

Can be estimated more accurately.

와

)를 획득하게 되면, 임의의 물체를 촬영하였을 때, 상기 촬영된 영상의 각 픽셀에서 검출된 광자 수(

) 및 상기 추정된 해당 픽셀의 응답특성 계수(

와

)를 이용하여 각 픽셀에서 검출된 광자 수를 보정할 수 있다. 즉, 촬영된 영상에 상기 획득된 각 픽셀의 응답특성을 적용하여 촬영 영상을 보정할 수 있는 것이다.As such, the response characteristics of each pixel (especially

Wow

), The number of photons detected at each pixel of the photographed image when an object is photographed.

) And the estimated response characteristic coefficient of the corresponding pixel (

Wow

) Can be used to correct the number of photons detected at each pixel. That is, the captured image may be corrected by applying the response characteristics of the acquired pixels to the captured image.

If this correction is described in more detail, it can be described by the following steps.

) 및 해당 픽셀의 응답특성 계수(

와

)를 상기 수학식 1에 적용하면, 각 픽셀마다 상기 임의의 물체와 등가(equivalent)인 알루미늄 두께(

)를 구할 수 있다. The number of photons detected at each pixel of the image in which

) And response coefficients for that pixel (

Wow

) Is applied to Equation 1, and for each pixel, an aluminum thickness (equivalent) equivalent to the arbitrary object (

) Can be obtained.

)에 상기 구해진 각 픽셀에 대한 등가 알루미늄 두께(

)에 대응되는 보정 가중치를 가감하여, 해당 픽셀의 광자 수를 보정할 수 있다. 이때 상기 보정 가중치는, 각 픽셀에 대해 특정 두께의 알루미늄(즉, 각 픽셀에서 피촬영 물체와 등가인 알루미늄 두께(

)을 통과했을 때의 이상적인(ideal) 검출 광자 수와 실제 검출 광자 수(

)의 차이로 정해질 수 있다. 상기 이상적인(ideal) 검출 광자 수는 별도의 과정을 통하여 구해 질 수 있다.The number of photons detected at each pixel since

Is the equivalent aluminum thickness for each pixel

) Can be corrected by adding or subtracting a correction weight corresponding to). In this case, the correction weight is, for each pixel, aluminum having a specific thickness (that is, an aluminum thickness equivalent to the object to be photographed at each pixel)

The number of ideal detected photons and the actual number of detected photons when passing through

) Can be determined. The ideal number of detected photons can be obtained through a separate process.

As one exemplary method for obtaining the correction weight, i.e., the ideal number of detected photons, a method using a Monte Carlo Simulation tool is described.

)의 알루미늄을 통과했을 때 이상적인 검출 광자 수를 산출하는 관계식이다. 하기 수학식 3은 몬테칼로 시뮬레이션 툴로 GATE(GEANT4 Application for Tomographic Emission)를 사용했을 때의 산출 관계식이다. GEANT4는 입자와 물질 사이의 physics를 모사할 수 있으며, GATE는 GEANT4에 기반을 두는 응용프로그램으로, X선 촬영(X-ray radiography), 컴퓨터 단층촬영(computed tomography, CT) 및 방사선 요법(radiation therapy) 시스템을 모델링 할 수 있다.Equation 3 is a specific thickness using a Monte Carlo Simulation tool (tool)

The equation for calculating the ideal number of detected photons when passing through aluminum. Equation 3 below is a calculation relation when GATE (GEANT4 Application for Tomographic Emission) is used as a Monte Carlo simulation tool. GEANT4 can simulate the physics between particles and materials, and GATE is an application based on GEANT4, with X-ray radiography, computed tomography (CT), and radiation therapy Model the system.

는 특정 픽셀에서 특정 두께(T)의 알루미늄을 통과 후 이상적으로 검출되는 광자 수이고,

는 알루미늄 두께이고,

,

,

는 시뮬레이션 상에서 알루미늄 두께과 이상적 검출 광자 수 간의 관계를 나타내는 지수적 근사화(exponential fitting) 계수이다.here

Is the number of photons ideally detected after passing aluminum of a certain thickness (T) in a particular pixel,

Is aluminum thickness,

,

,

Is an exponential approximation factor that represents the relationship between aluminum thickness and ideal number of detected photons in the simulation.

,

,

를 획득할 수 있다. 그 후 특정 픽셀에 대해 임의의 피촬영 물체와 등가인 알루미늄 두께(

와

,

,

을 상기 수학식 3에 적용하면

를 구할 수 있다. 이와 같이 구해진

는 특정 두께의 알루미늄을 투과한 후 이상적으로 검출되는 광자 수가 되는 것이다.By using Equation 3, the number of photons ideally detected after passing aluminum having a specific thickness may be derived. That is, for each thickness of aluminum using the GATE tool

,

,

Can be obtained. Thereafter, for a particular pixel, the aluminum thickness equivalent to any photographed object (

Wow

,

,

Is applied to Equation 3 above

Can be obtained. Thus obtained

Is the number of photons ideally detected after passing through aluminum of a certain thickness.

)에 각 픽셀의 응답 특성(예컨대 상기

와

)을 적용하여 각 픽셀에 대해 피촬영 물체와 등가인 알루미늄 두께(

)를 산출하고,(예컨대, 수학식 2)In summary, (1) the number of photons detected at each pixel of any captured image (

Response characteristics of each pixel (e.g.,

Wow

) So that for each pixel, the equivalent aluminum thickness (

), (E.g., Equation 2)

(2) the equivalent aluminum thickness ( Calculating the ideal number of detected photons at each pixel for

(3) The calculated number of detected photons becomes the corrected number of detected photons in each pixel.

Through the above correction of the present specification it is possible to obtain a more accurate image than the correction by the existing method, thereby reducing the risk of image misreading.

3 is a flowchart illustrating an image correction method according to an exemplary embodiment of the present specification.

The image correction method proposed in the present specification is to obtain a response characteristic of each pixel of the detector and to correct a photographed image based on the response characteristic of each pixel. The response characteristic of each pixel may be obtained in the form of a function including a response characteristic coefficient. For details, refer to FIG. 2.

In order to obtain a response characteristic of each pixel, an embodiment of the image correction method proposed in this specification is performed through the following steps.

Referring to FIG. 3, the image correction apparatus according to the present disclosure measures the number of detected photons detected at each pixel of the X-ray detector based on the photon count after passing through the medium (S310).

In this case, the photon-based X-ray detector may be a cadmium telluride (CdTe) detector, and the medium may be aluminum.

Next, the image correction apparatus analyzes the relationship between each thickness of the medium and the number of photons incident on each pixel based on the measurement (S320), and according to the analyzed relationship, the response characteristic of each pixel is determined. It is estimated (S330).

로 표현될 수 있으며, In this case, the estimated response of each pixel is

Can be expressed as

는 상기 매질의 두께이고, 상기

와

는 (i,j) 픽셀의 응답 특성 계수이고, 상기

는 검출 광자 수이다.remind

Is the thickness of the medium, and

Wow

Is the response characteristic coefficient of the pixel (i, j),

Is the number of photons detected.

Next, the image correction apparatus corrects the number of detection photons detected in each pixel by applying the estimated response characteristic of each pixel to an image photographed by an arbitrary subject. The correction of the number of photons is performed by determining an equivalent medium thickness corresponding to the subject for each pixel based on the estimated response of each pixel (S340) and the ideal angle corresponding to the equivalent medium thickness. The detection photon number for each pixel may be determined based on the number of photons detected for each pixel (S350).

On the other hand, the ideal number of detection photons for each pixel can be determined through Monte Carlo Simulation.

4 is a graph illustrating the distribution of photons per pixel of an original image and an original image, and FIG. 5 is a graph illustrating the distribution of photons per pixel of an image of the original image of FIG. 4 corrected according to an exemplary embodiment of the present specification.

FIG. 4 (a) is an image of aluminum having a uniform thickness (1 mm) using 60 kVp and 10 A, and FIG. 4 (b) shows photons detected in the pixels of each row of the image (a). Mean counts of each row, (c) of FIG. 4 is a mean counts of each colum detected in the pixels of each column of the image (a).

In order to acquire the images of FIGS. 4 and 5, 60 kVp and 10 A were used, and the distance between the X-ray tube and the detector was 100 cm.

4 and 5 are images of aluminum having a uniform thickness (1 mm), and FIG. 4 (a) is an original image obtained before correction, as shown in FIGS. 4 (b) and (c). It can be seen that the detection efficiency is lowered toward the edge of, and although the aluminum of uniform thickness is imaged, the difference in the number of detected photons for each pixel is large.

FIG. 5A is an image obtained by correcting the image of FIG. 4A using the correction method of the present specification, and FIG. 5B is detected from the pixels of each row of the image of FIG. Mean counts of each row, and FIG. 5C is a mean counts of each colum detected in the pixels of each column of the image.

As shown in FIGS. 5B and 5C, it can be seen that the number of photons detected in the pixels of each row and each column falls within ± 2% of the average value.

That is, using the correction method of the present specification, it can be seen that the number of photons detected at each pixel in the image photographing a predetermined subject is corrected to be close to the ideal number of detected photons.

6 is a graph illustrating uniformity of an image corrected according to an exemplary embodiment of the present specification.

FIG. 6A is a graph showing a coefficient of variation (COV) of an original image and an image obtained by correcting the original image according to an embodiment of the present specification, and FIG. 6B is an original image. The graph shows the COV of the image corrected using raw and the prior art correction method (flat-field method).

6 (a) and 6 (b) evaluate uniformity characteristics of an image using a coefficient of variation (COV), and the COV may be expressed by Equation 4 below.

Where σ is the standard deviation and m is the mean value.

6 (a) shows the COV of the original image (indicated by 'Raw') and the COV of the image corrected by the correction method of the present specification (indicated by 'Corrected by RPC'), and FIG. 6 (b) shows the original COV of the raw image (indicated by 'Raw') and COV of the image corrected by the conventional flat-field method (indicated by 'Corrected by flat-field with xx mm Al').

The flat-field method, which is a conventional correction method, uses a correction equation such as Equation 5 below.

는 검출기의 모든 픽셀에서 검출된 광자수의 평균값이고,

는 검출기의 각 픽셀에서 검출된 광자 수이다. 이때 영상을 획득하기 위해서는 피사체 없이 X-ray를 조사하거나, 또는 균일한 두께(예, Al 1mm, Al 5mm, Al 10mm)의 피사체를 두고 X-ray를 조사한다.here

Is the average value of the number of photons detected in all pixels of the detector,

Is the number of photons detected at each pixel of the detector. At this time, to obtain an image, X-ray is irradiated without a subject or X-ray is irradiated with a subject having a uniform thickness (eg, Al 1mm, Al 5mm, Al 10mm).

Referring to FIG. 6, in FIG. 6A, the image corrected using the correction method of the present specification has a lower COV value than the original image. This means that the average detected photon number variation of all the pixels in the corrected image is very low.

The reason why the COV value increases as the thickness of the subject (aluminum, Al) increases is that as the thickness of the subject increases, the average number of photons (m) detected by the detector decreases. This is because the specific gravity is larger than the value of σ decreases. In addition, even after the correction using the correction method of FIG. 6A in FIG. 6A, the increase in COV is similar, which means that the correction method of the present specification does not lose attenuation information of the X-ray.

FIG. 6 (b) shows a result of correction using the flat-field method of the prior art, and as shown in FIG. 6 (b), a subject having a specific thickness (5, 10, 15, 22 mm) ( If a flat-filed map is obtained and applied using aluminium (Al), the COV value is almost zero at the specific thickness used, and the COV value increases again at the remaining thickness. see. This suggests that the flat-field correction method can distort the image. For example, when 1mm thick Al is used for flat-field correction, when correcting images of low contrast material (Al 1mm) and high contrast material (Al 10mm), Al 1mm Since a flat-filed map was obtained using, a low-contrast material (Al 1mm) image was acquired to obtain a noise-free image. Instead, the distortion was more visible than a high contrast material (Al 10mm). Can cause.

7 is a subject used to capture an image for applying an embodiment of the present disclosure, and FIG. 8 is an original image and a corrected image photographing the subject of FIG. 7.

7 is a cylindrical phantom manufactured to confirm the effect of the correction method of the present specification. The model consists of poly methyl methacrylate (PMM), and has 20 mm height, 18 mm diameter, and includes four 5 mm diameter holes. The holes are filled with different concentrations of gadolinum, and the gadolinium concentrations are 92.85, 185.70, 278.55 and 371.40 mg / ml, respectively.

The original image obtained by photographing the model of FIG. 7 is the same as that of FIG. 8A, and FIG. 8B is an image corrected using the flat-field method, and FIG. 8C. ) Is an image corrected using the correction method of the present specification.

Referring to FIG. 8, it can be seen that an image corrected using the correction method of the present specification has a higher uniformity. Evaluation of each image is described in more detail with reference to FIGS. 9 and 10.

9 and 10 are diagrams evaluating the original image and the corrected image of FIG. 8.

(A), (b), and (c) of FIG. 9 respectively show contrast and noise of the original image of FIG. 8, the image corrected using the prior art, and the image corrected using the correction method of the present specification. (noise) and CNR (contrast-to-noise ratio) items are shown.

In each graph, 'RAW' is the original image, 'Corrected by flat-field' is the image corrected by the conventional flat-field correction method, and 'Corrected by RPC' is the evaluation result of the image corrected by the correction method of the present specification, respectively. Indicates.

As shown in FIG. 9, the corrected image according to the method of the present specification shows the same or lower contrast than the original image, but is corrected by the original image or the flat-field method. The noise is significantly reduced, resulting in an increase in the contrast-to-noise ratio (CNR).

10 is a graph illustrating a CNR increase rate of an image corrected by a conventional correction method and a correction method of the present specification.

Corrected by flat-field and corrected by RPC 'herein yield a CNR improvement of 1.00-1.55 and 1.67-2.25, respectively, when compared to the original image. Referring to FIG. 10, it can be seen that an image corrected using the correction method of the present specification has a higher CNR improvement than an image corrected by the conventional flat field method.

11 is a block diagram illustrating an image calibrating apparatus according to an exemplary embodiment of the present specification.

The image correction device 100 of the present specification performs the image correction method described with reference to FIGS. 2 to 10. That is, the image calibrating apparatus 100 may acquire a response characteristic of each pixel of the detector and correct the photographed image based on the response characteristic of each pixel. The response characteristic of each pixel may be obtained in the form of a function including a response characteristic coefficient. For details, refer to FIGS. 2 and 3.

The image calibrating apparatus 100 may include a detector 101, a response characteristic estimator 102, and an image compensator 103.

The detector 101 may detect the number of photons incident on each pixel of the X-ray detector based on the photon count. In particular, the detector 101 may measure the number of detected photons detected by each pixel of the X-ray detector based on the photon count after passing through the medium while changing the thickness of the medium. In this case, the photon-based X-ray detector may be a cadmium telluride (CdTe) detector, and the medium may be aluminum.

The response characteristic estimator 102 analyzes the relationship between the thickness of the medium and the number of photons detected in each pixel based on the number of photons detected by the detector 101, and the analyzed relation. The response characteristic of each pixel can be estimated according to the above.

The image correction unit 103 may correct the number of detected photons detected by each pixel by applying the estimated response characteristic of each pixel to an image photographed by an arbitrary subject. In this case, the image correction unit 103 determines an equivalent medium thickness corresponding to the subject for each pixel based on the estimated response characteristics of each pixel, and detects an ideal photon for each pixel corresponding to the equivalent medium thickness. Image correction may be performed by determining the number of detected photons for each pixel based on the number.

The detector 101, the response characteristic estimator 102, and the image compensator 103 may further include an application-specific integrated circuit (ASIC), another chipset, a logic circuit, and / or a data processing device.

When the embodiments of the present invention are implemented in software, the above-described techniques may be implemented with modules (processes, functions, and so on) that perform the functions described above. The module may be stored in the storage 104 and executed by the detector 101, the response characteristic estimator 102, and the image compensator 103.

The embodiments described above are those in which the elements and features of the present invention are combined in a predetermined form. Each component or feature shall be considered optional unless otherwise expressly stated. Each component or feature may be implemented in a form that is not combined with other components or features. It is also possible to construct embodiments of the present invention by combining some of the elements and / or features. The order of the operations described in the embodiments of the present invention may be changed. Some configurations or features of certain embodiments may be included in other embodiments, or may be replaced with corresponding configurations or features of other embodiments. The present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Accordingly, the foregoing detailed description should not be construed in a limiting sense in all respects, but should be considered as illustrative. The scope of the invention should be determined by reasonable interpretation of the appended claims, and all changes within the equivalent scope of the invention are included in the scope of the invention. In addition, claims that do not have an explicit citation in the claims may be combined to form an embodiment or included in a new claim by amendment after the application.

Claims (11)

A method of correcting an image obtained by a photon count based X-ray detector,
Measuring the number of detected photons detected at each pixel of the photon count based X-ray detector after passing through the medium with varying thickness of the medium;
Analyzing a relationship between each thickness of the medium and the number of photons incident on each pixel based on the measurement;
Estimating a response characteristic of each pixel according to the analyzed relationship; And
And correcting the number of detection photons detected at each pixel by applying the estimated response of each pixel to an image photographed by an arbitrary subject. The method of claim 1,
The response characteristic of each estimated pixel is

Lt;
remind

Is the thickness of the medium, and

Wow

Is the response characteristic coefficient of the pixel (i, j),

Is the number of photons detected. The method of claim 1,
The medium is aluminum. The method of claim 1,
And the photon count based X-ray detector is a cadmium telluride (CdTe) detector. The method of claim 1,
Correcting the number of detected photons,
Determining an equivalent medium thickness corresponding to the subject for each pixel based on the estimated response of each pixel;
Determining the number of detected photons for each pixel based on the ideal number of detected photons for each pixel corresponding to the thickness of the equivalent medium. The method of claim 5,
The ideal number of detected photons for each pixel is determined by Monte Carlo Simulation. An apparatus for correcting an image obtained from a photon count based X-ray detector,
A detector for changing the thickness of the medium and measuring the number of detected photons detected at each pixel of the X-ray detector based on the photon count after passing through the medium;
A response characteristic estimator for analyzing a relationship between each thickness of the medium and the number of photons detected in each pixel based on the measured number of detected photons, and estimating a response characteristic of each pixel according to the analyzed relationship ; And
And an image corrector for correcting the number of detected photons detected by each pixel by applying the estimated response characteristic of each pixel to an image photographed by an arbitrary subject. The method of claim 7, wherein
The medium is an image correction device, characterized in that aluminum (aluminum). The method of claim 7, wherein
The photon count based X-ray detector is a cadmium telluride (CdTe) detector, characterized in that the image. The method of claim 7, wherein
The video correction
Determine an equivalent medium thickness corresponding to the subject for each pixel based on the estimated response of each pixel;
And determining the number of detected photons for each pixel based on the ideal number of detected photons for each pixel corresponding to the equivalent medium thickness. A computer-readable recording medium comprising instructions for performing each step of the method according to any one of claims 1 to 6.

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