CN114399564A

CN114399564A - Cone beam computed tomography imaging method and system based on scattering identification

Info

Publication number: CN114399564A
Application number: CN202210299797.4A
Authority: CN
Inventors: 小野光; 吉振宁; 郭咏梅; 郭咏阳
Original assignee: Kangda Intercontinental Medical Devices Co ltd
Current assignee: Kangda Intercontinental Medical Devices Co ltd
Priority date: 2022-03-25
Filing date: 2022-03-25
Publication date: 2022-04-26
Anticipated expiration: 2042-03-25
Also published as: CN114399564B; WO2023179227A1

Abstract

The invention discloses a cone beam computed tomography imaging method based on scattering identification, which relates to the technical field of image processing and comprises the following steps: controlling the emitter to emit transmission rays with preset spectral width to the target tissue; acquiring first ray intensity of the transmitted rays at each pixel point after the transmitted rays penetrate through the target tissue through a top detector; acquiring second ray intensity of the transmitted ray at each pixel point after the transmitted ray is scattered and attenuated by the collimator through the base layer detector; estimating the scattered ray intensity of the transmitted ray after penetrating the target tissue through the first ray intensity and the second ray intensity at the corresponding pixel points; estimating the intensity of the primary ray after the scattered ray is removed through the second ray intensity based on the estimated scattered ray intensity; and constructing the tomography imaging of the target tissue according to the initial ray intensity at each pixel point. The invention can obtain the imaging result more quickly under the condition of ensuring the safety of the irradiation dose.

Description

Cone beam computed tomography imaging method and system based on scattering identification

Technical Field

The invention relates to the technical field of image processing, in particular to a cone beam computed tomography imaging method and system based on scattering identification.

Background

Cone Beam Computed Tomography (CBCT) is an imaging modality that is widely used in tissue structure research, applications such as targeted controlled release based on image guidance and radiation guidance. By applying the technology to the C-shaped arm, good clinical imaging capacity and mobility are provided for the research of the target tissue structure, the target tissue is easier to access, and the implementation of operations such as stent placement in the target tissue is strongly supported.

However, due to the wide coverage of the cone-beam and the detector of the CBCT, the radiation (X-ray) in the target tissue and the device is severely scattered and detected by the detector during the transmission of the X-ray through the target tissue. A high scattered photon ratio (SPR, the ratio of detected scattered radiation to primary radiation) can seriously degrade the imaging quality, such as low contrast-to-noise ratio and/or inaccurate CT number, which may cause the finally obtained image to show a deviation of the target tissue state from the actual target tissue state, thereby causing irreparable adverse effects on the subsequent study (wherein the primary radiation refers to pure original radiation without any interaction between the radiation emitter and the CBCT detector).

In order to overcome the influence of the scattering phenomenon on the final imaging during the process of penetrating the target tissue by the original ray, in the prior art, there are two main methods. One of them is a hardware method, for example, by adding a collimator to the detector to block scattered rays from the original rays. In principle, this is the best way to obtain an ideal image, but it requires weighing the dose effect and has certain technical challenges, since too high a density of collimators may also block the primary rays, thereby reducing the signal intensity of the primary rays detected by the detector. Another is by a software algorithm that estimates the scattered ray signal from the detected signal. Its main challenge is that the acquired signal contains both scattered and primary rays, which are not necessarily suitable for estimating the true scattered rays due to the complexity of the signal composition.

Disclosure of Invention

In order to obtain a higher-quality tomographic image, the invention introduces an additional hardware modification, and introduces new data information into a software algorithm, so that the estimation difficulty of the software algorithm is reduced, and the accuracy and the acquisition efficiency of final imaging are improved. Specifically, the invention provides a cone beam computed tomography imaging method based on scattering identification, which is characterized in that a detector receives original rays emitted by an emitter, the detector comprises a top detector, a collimator and a base detector which are sequentially arranged, and the method comprises the following steps:

s1: controlling an emitter to emit original rays with preset spectral width to target tissues;

s2: acquiring first ray intensity of original rays at each pixel point after the original rays penetrate through target tissues through a top detector;

s3: acquiring second ray intensity of the original rays at each pixel point after the original rays are scattered and attenuated by the collimator through a base layer detector;

s4: estimating the scattered ray intensity of the original ray after penetrating through the target tissue according to the first ray intensity and the second ray intensity at the corresponding pixel points;

s5: estimating the intensity of the primary ray after the scattered ray is removed through the second ray intensity based on the estimated scattered ray intensity;

s6: and constructing the tomography imaging of the target tissue according to the primary ray intensity at each pixel point.

Furthermore, the top detector has low resistance to radiation, and the base detector has high resistance to radiation.

Further, the top detector has a position sensitivity to the radiation intensity.

Further, in the step S4, the estimation of the scattered ray intensity is expressed as the following formula:

in the formula, S^estIceil is the first ray intensity, Ibase is the second ray intensity, A is the proportion of primary rays absorbed in the top detector,b is the proportion of scattered radiation that is absorbed in the top detector.

Further, in the step S5, the estimation of the primary ray intensity is expressed as the following formula:

in the formula, P^estFor the purpose of the estimated intensity of the primary rays,

is the proportion of primary rays absorbed in the underlying detector,

is the proportion of the scattered radiation that is absorbed in the underlying detector.

The invention also provides a cone beam computed tomography imaging system based on scattering identification, which comprises:

the emitter is used for emitting original rays with preset spectrum width to target tissues;

the top detector is used for acquiring first ray intensity of original rays at each pixel point after the original rays penetrate through target tissues;

the collimator is used for attenuating the scattered ray of the original ray after the original ray penetrates through the target tissue and the top detector;

the base layer detector is used for acquiring second ray intensity of the original rays at each pixel point after the original rays are scattered and attenuated by the collimator;

the data processing unit is used for estimating the scattered ray intensity of the original ray after penetrating through the target tissue according to the first ray intensity and the second ray intensity of the corresponding pixel points, and estimating the primary ray intensity after eliminating the scattered ray through the second ray intensity based on the estimated scattered ray intensity;

the imaging processing unit is used for constructing tomography imaging of the target tissue according to the primary ray intensity at each pixel point;

the top detector, the collimator and the base detector are connected in sequence.

Further, in the data processing unit, the estimation of the scattered ray intensity is expressed by the following formula:

in the formula, S^estFor the scattered ray intensity, Iceil is the first ray intensity, Ibase is the second ray intensity, a is the proportion of primary rays absorbed in the top detector and B is the proportion of scattered rays absorbed in the top detector.

Further, in the data processing unit, the estimation of the primary ray intensity is expressed by the following formula:

is the proportion of primary rays absorbed in the underlying detector,

Compared with the prior art, the invention at least has the following beneficial effects:

(1) according to the cone beam computed tomography imaging method and system based on scattering identification, through the arrangement of the top detector, scattered rays are preferentially detected under the condition of no external interference, and the function relation of primary rays and the scattered rays is established by utilizing the ray intensities detected by the top detector and the base detector, so that the primary ray intensity can be better identified by using additional information from the top detector. In the prior art, only a single detector is used, the detected rays comprise both primary rays and scattered rays, and it is difficult to distinguish the primary rays from the mixed rays;

(2) due to the addition of new hardware, new data is introduced into the software algorithm, so that the finally required primary ray intensity data can be deduced in one measurement by using a simple functional relation, compared with the prior art that the mixed signal of primary rays and scattered rays is used for estimating scattered rays, the estimation is not very accurate, and some methods even increase the dose because they have to give up some data. Most of the scattered radiation can be removed fundamentally by the collimator as a classical method. The invention can further estimate the scattered X-ray mixed in the signal through a top detector so as to reduce estimation error;

(3) the top detector has low resistance to the original ray and cannot block the propagation of the primary ray, so that the ray intensity detected by the final base detector cannot be lost, and the actually required irradiation dose cannot be influenced.

Drawings

FIG. 1 is a diagram of method steps for a cone beam computed tomography imaging method based on scatter identification;

FIG. 2 is a system block diagram of a cone beam computed tomography imaging system based on scatter identification;

FIG. 3 is a schematic diagram of conventional CBCT transmission;

FIG. 4 is a schematic view of a detector according to the present invention;

description of reference numerals: 1-emitter, 2-target tissue, 3-original ray, 4-scattered ray, 5-FPD, 6-top detector, 7-collimator, 8-basal detector.

Detailed Description

The following are specific embodiments of the present invention and are further described with reference to the drawings, but the present invention is not limited to these embodiments.

Example one

In the conventional CBCT, as shown in fig. 3, only one Flat Panel Detector (FPD) is used as a CBCT original ray detector. The FPD is a two-dimensional original ray detector, and has position sensitivity after discretization. The original ray 3 generated by the emitter 1 is transmitted and projected onto the FPD5, the ray intensity of which is recorded as a shadow. Repeating the original radiation exposure around the target tissue 2 and recording at different angles, a cross-sectional image of the target tissue can be reconstructed. However, the original radiation produces scattered radiation 4 in the target tissue, thereby contaminating the data acquired by the FPD. The scattered radiation is then estimated using various scattered radiation estimation algorithms, but the estimation in this process is essentially only possible using the contaminated data obtained on the FPD.

Among them, one of the classical algorithms of the scattered ray estimation algorithm is a filtering convolution, in which a convolution kernel thereof is used to estimate the scattered ray. The intensity of the radiation detected by the flat panel detector is expressed by the following formula, where P and S represent the primary radiation and the scattered radiation, respectively.

Wherein the estimate of the scattered ray intensity at pixel location (i, j) in the image approximates the estimation formula:

（0）

where the summation is a kernel defined by the pixels relative to k and l (i.e., a kernel function)

) Size. However, this estimation method results in a partially estimated residual because it is an approximate solution equation designed by empirical theory. To solve this residual error, the kernel needs to be optimized by extra scanning, but this obviously increases the radiation dose and makes the operation more complicated in practice. In the latest technologyIntraoperatively, there is also a method of adding an obstacle between the X-ray tube and the target tissue in order to block only primary rays (to avoid detection of scattered rays), but this method also increases the radiation dose because it loses part of the signal by blocking primary rays.

In the patent, in order to accurately estimate the scattered ray without increasing the radiation dose, the above-mentioned problem that the prior art is difficult to estimate the scattered ray due to the complex combination of the original rays detected by the single detector at each pixel point is also considered. In the present invention, a new apparatus is proposed to improve the existing method, thereby better reducing the interference of scattered rays in the final imaging. As shown in fig. 1, the present invention provides a cone beam computed tomography imaging method based on scattering identification, which receives original rays emitted by an emitter through a detector, wherein the detector comprises a top detector 6, a collimator 7 and a base detector 8 (as shown in fig. 4) which are arranged in sequence, and the method comprises the following steps:

s6: and constructing the tomography imaging of the target tissue according to the initial ray intensity at each pixel point.

In the present invention, both detectors (top detector and base detector, detector configuration is not limited to FPD, and similar but different configurations can be implemented by connecting the two detectors in parallel and installing a collimator therebetween) can receive the original radiation penetrating or scattered in the target tissue. Part of the original rays reach the top detector, are transmitted through or scattered, are further attenuated by the collimator between the two detectors and reach the base detector. In order to reduce the influence of the blocking of the top detector to the radiation on the base detector, the top detector needs to be made of a material with weak resistance to the radiation. Of these, the most conservative application is to use 200 to 500 microns thick silicon. In a typical energy range (namely, a preset frequency spectrum width, 60-80 kev) emitted by the emitter, the top detector composed of the silicon material with the thickness can transmit 97% of original rays, so that the influence of the top detector on a basic detector can be minimized. Thereby utilizing the data information obtained by the top detector without affecting the intensity of the radiation detected by the base detector.

In a preferred embodiment, the top detector may also be considered to use a material with more powerful blocking properties, such as 150um thick Csl, where 54% of the radiation can react at 60 kev. Csl has the advantage that light absorption dominates low (95% at 60 kev) in the typical energy range. Therefore, the probability of further scattering of the rays in Csl is much less than for other materials such as silicon, and no complex reprocessing between scattered and primary rays occurs in Csl. In other words, the scattered and primary rays are not scattered further in the Csl, but the primary rays can be detected directly by the underlying detector. Due to the physical characteristics of the X-ray, the signals of the top detector and the base detector form a low-energy ray part and a high-energy ray part, so that material analysis can be performed, and further the low-energy ray part and the high-energy ray part are converted into material information of the target tissue.

Meanwhile, due to the following reasons:

(1) compared with the base detector, the top detector has no collimator to block scattered rays;

(2) low energy rays are more easily scattered within the target tissue, while top detectors are generally more resistant at low energies;

(3) scatter radiation andnot perpendicularly incident on the top detector, but rather

At an angle of incidence, the effective detection thickness of the top detector for scattered radiation is therefore proportional to

。

For the three reasons, the top detector in the present invention has the characteristic of preferentially detecting scattered rays, that is, the top detector has a high scattered photon ratio compared with the base detector. Based on the characteristic, the scattered ray and the primary ray can be estimated according to the difference of the ray intensities detected by the top detector and the base detector without increasing the ray dose, and the specific estimation idea is as follows.

In this embodiment, at a certain pixel point, the first ray intensity obtained by the top detector is represented by Iceil, the second ray intensity obtained by the base detector is represented by Ibase, and since both of them are composed of the primary ray intensity (assumed to be defined as P) and the scattered ray intensity (assumed to be defined as S), the following linear combination formula can be used:

（1）

wherein A is the proportion of primary rays absorbed in the top detector, B is the proportion of scattered rays absorbed in the top detector,

is the proportion of primary rays absorbed in the underlying detector,

Then, based on the above formula set (1), the expressions for the intensity of the primary radiation and the intensity of the scattered radiation can be obtained as follows:

（2）

in the formula (I), the compound is shown in the specification,

for simplification of the coefficients of the representation are made of

Obtained through a series of transformations. In order to achieve the simplification of formula (2), the present invention utilizes the following fact. In consideration of energy conservation and attenuation of the original rays by the collimator, in practical cases, the following equation should be satisfied:

（3）

in the present invention, since the top detector is made of a weak resistive material, the values of A and B are much less than 1 (e.g., silicon)<0.03). Wherein

Being the ratio of the blocking of the primary rays by the collimator,

is the ratio of the collimator to the blocking of scattered radiation. When the collimator blocks a small portion of the primary radiation, a large portion of the scattered radiation is also blocked, i.e. the primary radiation is blocked

. Meanwhile, since the primary rays have higher energy than the scattered rays, the top detector has weaker blocking capability to the high-energy rays, and A and B are consistent and even smaller.

In particular, the method comprises the following steps of,

can be derived by tabulating the following formulaThe following steps:

due to A<<1，B<<1，

，

，

，

Therefore, it is

，

Based on this, each simplified coefficient is actually expressed as:

then by eliminating

And

and taking advantage of the fact of equation (3), P and S can be further expressed as follows:

（4）

intuitively, equation (4) is reasonable. Since the intensity of the radiation detected by the base-layer detector contains most of the signal at the pixel of interest, the primary radiation is essentially determined by the intensity of the radiation detected by the base-layer detector. It should be noted that Iceil is related to scattering S, and the second term of each sub-formula in the formula group (4) is a correction factor after the test efficiency normalization.

As can be seen from the formula set (4), the most direct way to obtain the intensity of the primary radiation is to use the first sub-formula P. In this case, the primary ray is estimated directly by a linear combination of the signals from the top and bottom detectors. In practice, since equation (4) is only an approximation, we can obtain C and D in equation (2) by experiment to best estimate the primary ray. To perform the calibration, pure primary rays may be obtained using X-ray blanker techniques when scanning various objects.

The second method is to estimate the scattered ray intensity as employed in the present invention. First, the air is scanned to calibrate the substrate detector. Since there is no scatter when calibrating with air, the estimated primary ray P^estRepresented by the formula (5) below, wherein

Represents the intensity obtained with the substrate probe during the air scan:

（5）

at this time, when scanning an object of limited size and calibrating the underlying detector, the influence of scattered rays is slightly considered, and the estimated primary ray P is obtained^{est,tentative}The following were used:

（6）

in the formula (I), the compound is shown in the specification,

，P^objfor detection in the substrate detector during scanningIntensity of secondary ray, S^objIs the intensity of the scattered radiation detected in the underlying detector during scanning.

Therefore, to further eliminate this effect, the present invention multiplies the second equation S of equation (4) by

The scattered ray is estimated and subtracted to yield the final solution as follows:

（7）

in the formula, S^estRefers to the scattered ray intensity estimated using equation (4). The method requires calibration in equation (7)

、

、

These three coefficients.

Wherein the coefficients of the second sub-formula S of formula (4), i.e. 1/B and a/B, may be determined or calibrated before the start of the experiment. Whereas the classical method of determining scattered radiation is beam-blocking arrays.

Since the calibration of equation (5) is substantially the same as the calibration of CT. However, for a C-arm, due to its mechanical simplicity, the top detector and the collimator can be designed separately in order to perform the calibration in a simpler manner. After scanning the air with and without the top detector plus collimator, respectively, a true primary ray signal can be obtained

And the signal after attenuation in the top detector plus collimator

。

Based on equation (1), the first coefficient a' in equation (7) can be calibrated by the following equation:

in order to determine the coefficients of the second term and the third term in equation (7), an object needs to be scanned. First, the object is scanned and signals are obtained at the top and bottom detectors, i.e.

，

. We now aim to do this by using

、

And estimating the second term and the third term in equation (7) by the following expression:

（8）

in other words, it needs to be determined

And

. To do this, we need to obtain the true value on the left in equation (8). The true value can be obtained by using a beam stop array placed between the X-ray tube and the target tissue. The beam stop array creates shadows on certain pixels, stopping only primary rays and not scattered rays. Thus, the signal of the basic detector with the beam stop array changes from equation (1) to

. Will be provided with

Substituted in formula (6)

Then, the true value on the left side in equation (8) is obtained:

（9）

the formula (8) is combined with two unknown coefficients

And

by comparing the value obtained by equation (9) with the beam stop array, the coefficient can be determined

And

. Since we have two unknown coefficients, we need to scan several different objects with different sizes and/or structures, or to scan only one object with a fixed structure from different angles.

Furthermore, it should be noted that if a weak resistance top probe is used, the statistical data for Iceil may be relatively poor. In this case, the noise of each signal is controlled by Iceil. However, the scatter ray distribution is weakly dependent on the detector position, and by smoothing S at pixel coordinates (i, j) to 100 pixel levels, the noise generated by the top detector due to statistical differences can be eliminated and the noise level can be easily reduced to that of the base detector. I.e. the signal-to-noise level of the final image of each pixel is determined by the signals of the top and bottom detectors. Therefore, poor signal from the top detector significantly degrades the quality of the final image. However, the top detector signal may be smoothed over approximately 100 pixels to improve the statistics, since the scatter distribution is generally smooth.

The third method is to use the conventional method described at the beginning of the present embodiment, because the top detector is substantially the same as the detector used in the conventional method. First, the air is scanned to calibrate the detector, we use equation (5). Also, as in equation (6), when scanning an object of finite size and applying calibration to the detector, the effect of scattered radiation is slightly included. Then, we reuse equation (7), but now we estimate the scattering S therein in a different way^est. Unlike the second approach, we will I_ceilSubstituting into I in equation (0) and estimating the scatter. And finally, obtaining pure scattering data by using the beam stop array, and calibrating the kernel in the formula (0).

Example two

In order to better understand the technical content of the present invention, the present embodiment illustrates the present invention by way of a system structure, as shown in fig. 2, a cone beam computed tomography imaging system based on scatter identification includes:

the imaging processing unit is used for constructing tomography imaging of the target tissue according to the initial ray intensity at each pixel point;

Further, the top detector has low resistance to radiation and the base detector has high resistance to radiation.

Further, in the data processing unit, the estimation of the scattered ray intensity can be expressed as the following formula:

Further, in the data processing unit, the estimation of the primary ray intensity can be expressed as the following formula:

is the proportion of primary rays absorbed in the underlying detector,

In summary, according to the cone-beam computed tomography imaging method and system based on scattering recognition, the top detector is arranged, scattered rays are preferentially detected under the condition of no external interference, and the functional relationship between the ray intensity detected by the top detector and the base detector and the primary rays and the scattered rays is utilized, so that the primary ray intensity is estimated by utilizing the scattered ray intensity which is relatively more stable in computation, and the final imaging quality is better.

Due to the addition of new hardware, new data is introduced into a software algorithm, and finally required primary ray intensity data can be deduced in one measurement by using a simple functional relation.

The top detector has low resistance to the original ray and cannot block the propagation of the primary ray, so that the ray intensity detected by the final base detector cannot be lost, and the actually required irradiation dose cannot be influenced. The function coefficient of the scattered ray intensity in the actual calculation process is only related to the added hardware, and can be confirmed and calibrated before actual operation, so that the influence of the structure and the shape of the target tissue is avoided, and the application range is wider.

It should be noted that all the directional indicators (such as up, down, left, right, front, and rear … …) in the embodiment of the present invention are only used to explain the relative position relationship between the components, the movement situation, etc. in a specific posture (as shown in the drawing), and if the specific posture is changed, the directional indicator is changed accordingly.

Moreover, descriptions of the present invention as relating to "first," "second," "a," etc. are for descriptive purposes only and are not to be construed as indicating or implying relative importance or implicit ly indicating a number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one such feature. In the description of the present invention, "a plurality" means at least two, e.g., two, three, etc., unless specifically limited otherwise.

In the present invention, unless otherwise expressly stated or limited, the terms "connected," "secured," and the like are to be construed broadly, and for example, "secured" may be a fixed connection, a removable connection, or an integral part; can be mechanically or electrically connected; they may be directly connected or indirectly connected through intervening media, or they may be connected internally or in any other suitable relationship, unless expressly stated otherwise. The specific meanings of the above terms in the present invention can be understood by those skilled in the art according to specific situations.

In addition, the technical solutions in the embodiments of the present invention may be combined with each other, but it must be based on the realization of those skilled in the art, and when the technical solutions are contradictory or cannot be realized, such a combination of technical solutions should not be considered to exist, and is not within the protection scope of the present invention.

Claims

1. A cone beam computer tomography imaging method based on scattering identification is characterized in that original rays emitted by an emitter are received through a detector, the detector comprises a top detector, a collimator and a base detector which are sequentially arranged, and the method comprises the following steps:

2. The method of claim 1, wherein the top detector is low-impedance to radiation and the base detector is high-impedance to radiation.

3. A method as claimed in claim 1 wherein said top detector has a positional sensitivity to radiation intensity.

4. The method as claimed in claim 1, wherein the step of S4 is characterized in that the estimate of the scattered ray intensity is expressed by the following formula:

in the formula, S^estFor the estimated scattered ray intensity Iceil is the first ray intensity, Ibase is the second ray intensity, a is the proportion of primary rays absorbed in the top detector and B is the proportion of scattered rays absorbed in the top detector.

5. The method as claimed in claim 4, wherein the step of S5 is characterized in that the estimation of the primary ray intensity is expressed by the following formula:

is the proportion of primary rays absorbed in the underlying detector,

6. A cone beam computed tomography imaging system based on scatter identification, comprising:

7. The scatter-identification-based cone beam computed tomography imaging system of claim 6, wherein the top detector is low-impedance to radiation and the base detector is high-impedance to radiation.

8. The scatter identification-based cone beam computed tomography imaging system of claim 6, wherein the top detector has a positional sensitivity to radiation intensity.

9. A scatter identification based cone beam computed tomography imaging system as claimed in claim 6 wherein the estimate of the scattered ray intensity in the data processing unit is expressed by the formula:

10. A scatter identification based cone beam computed tomography imaging system as claimed in claim 9, wherein the estimate of the primary ray intensity in the data processing unit is expressed by the formula:

is the proportion of primary rays absorbed in the underlying detector,