JP3663798B2 - X-ray CT system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an X-ray CT apparatus including an X-ray detector in which a large number of fluorescent elements that receive X-rays and emit light and photoelectric conversion elements that convert the light emission amount of the fluorescent elements into electric signals are combined.
[0002]
[Prior art]
In recent years, solid X-ray detectors using scintillator elements and the like have been widely used as X-ray detectors used in X-ray CT apparatuses in order to improve detection accuracy in place of conventional ionization chamber methods using xenon gas. It has been.
[0003]
Such a solid-state X-ray detector includes an X-ray tube that includes an X-ray detection element comprising a scintillator element that converts incident X-rays into light, and a photodiode that detects light converted by the scintillator element and outputs it as an electrical signal. It has a configuration in which about 500 to 1000 channels are arranged in a circular arc shape as the center.
[0004]
[Problems to be solved by the invention]
However, an X-ray solid state detector using a scintillator element or the like is characterized in that the sensitivity of the detection element, particularly the emission intensity of the fluorescent element, is reduced by irradiating the X-ray as shown in FIG.
[0005]
In such a case, the X-ray detector is not irradiated with X-rays for a long time, and the rate of decrease in sensitivity increases immediately after the start of X-ray irradiation, and the sensitivity variation also increases for each X-ray detection element.
[0006]
For this reason, in an X-ray CT apparatus using a scintillator element or the like as an X-ray detector, even if calibration or the like is performed prior to imaging, variations due to sensitivity deterioration of each detection element cannot be corrected, and a virtual image such as an artifact is added to the captured image. There was a problem that occurred.
[0007]
In order to solve these problems, an object of the present invention is to provide an X-ray CT apparatus capable of obtaining a good image without a virtual image even when a scintillator element or the like is used in an X-ray detector.
[0008]
[Means for Solving the Problems]
In order to achieve the above-described object, the present invention provides an X-ray generation unit, a fluorescent element that emits light by receiving X-rays, and a combination of a number of photoelectric conversion elements that convert the amount of light emitted from the fluorescent element into an electrical signal. An X-ray CT apparatus having a detector, wherein the X-ray detector is irradiated with X-rays from the X-ray generator so as to stabilize the output sensitivity due to radiation damage to the X-ray detector prior to subject imaging. Control means is provided.
[0009]
The amount of X-ray irradiation is 5000 mAs or more .
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an embodiment of the present invention will be described with reference to FIGS.
[0011]
FIG. 1 is a schematic view of an X-ray CT apparatus according to an embodiment of the present invention. An X-ray generation unit 1 and an X-ray detection unit 2 are arranged to face a subject 3 and face each other. The X-ray tube and the X-ray detection unit 2 are rotated integrally while irradiating the subject 3 with X-rays from the X-ray tube of the unit 1, so that the X-ray detection unit 2 rotates the subject from about 360 ° or 180 °. X-ray transmission data of the specimen 3 is acquired.
[0012]
The X-ray generator 1 mainly includes an X-ray tube and a high voltage source, and the high voltage source is usually installed at a position away from the X-ray CT apparatus main body.
[0013]
The X-ray detection unit 2 includes an X-ray generation unit that converts an X-ray detection element including a scintillator element that converts incident X-rays into light, and a photodiode that detects light converted by the scintillator element and outputs it as an electrical signal. It has a configuration in which about 500 to 1000 channels are arranged in a circular arc shape as the center, and after normally performing processing such as amplification on the data obtained from each channel, it is AD converted and sent to the arithmetic control unit 4 To do.
The calculation control unit 4 controls the operation of the entire X-ray CT apparatus, and obtains a tomographic image of the subject 3 by calculation based on the data for each channel sent from the X-ray detection unit 2. In addition, the arithmetic control unit 4 generates X-rays so that the sensitivity of each scintillator element is stabilized when the X-ray CT apparatus is started up or when there is no X-ray irradiation for a long time. Part 1 is given.
[0014]
Next, the effect | action of this invention is demonstrated based on the flowchart shown by FIG.2 and FIG.3 which show the operation | movement of the calculation control part 4. FIG.
[0015]
First, when an instruction to operate the apparatus (usually once a day because it is used when using the apparatus) is given, the arithmetic control unit 4 sets up the entire X-ray CT apparatus in an operable state. (S1) Prior to imaging, the X-ray generation unit 1 is instructed to emit X-rays with an irradiation dose M sufficient to stabilize the sensitivity of each X-ray detection element of the X-ray detection unit 2. Here, the X-ray irradiation dose M sufficient to stabilize the sensitivity is obtained in advance and stored in the calculation control unit 4. The process of obtaining the irradiation dose M will be described based on the flowchart shown in FIG.
[0016]
The arithmetic control unit 4 instructs the X-ray generation unit 1 to irradiate X-rays (S11), monitors the data obtained from the X-ray detection unit 2 for each X-ray detection element, and detects all X-rays. X-rays are irradiated until the change rate of the element output reaches a predetermined value α (a value close to 1) (S12). FIG. 4 represents the outputs of the two X-ray detection elements with sensitivity indicating the ratio at the beginning of irradiation. The portion where the change rate is equal to or less than the predetermined value α is where the sensitivity of the X-ray detection element is constant.
[0017]
When the rate of change of all X-ray detection element outputs is equal to or less than the predetermined value α, the accumulated irradiation dose M at that time is calculated and stored (S13).
[0018]
Thereby, the irradiation amount of X-rays to be irradiated before the sensitivity of each X-ray detection element becomes constant is calculated.
[0019]
In FIG. 2, when the irradiation of the X-ray with the irradiation dose M is completed, the output of each X-ray detection element is calibrated (S3).
[0020]
As shown in FIG. 5, the calibration is performed by irradiating X-rays absorbed by a known absorption amount Z and performing predetermined data processing (usually Y = −logX, X: X-ray detection element output). The following correction is performed on a function obtained in advance showing the relationship between the output Y and the absorbed amount Z.
[0021]
Z = (Y1 / Y0) × Y
Such calibration is performed in order to correct variation for each X-ray detection element in the relationship between each X-ray detection element output Y and the X-ray absorption amount Z. Since the approximate variation in sensitivity is corrected by the X-ray irradiation shown in step 2 (S2) above, highly accurate calibration can be performed.
[0022]
When calibration is completed, the imaging mode of the subject 3 is entered, and normal tomographic imaging is performed (S4).
[0023]
As a result, prior to imaging the subject 3, X-rays that always have a constant detector sensitivity are irradiated to each X-ray detection element. Therefore, when imaging the subject 3, the X-ray detection element sensitivity is always constant. It is made in the state. For this reason, in an X-ray CT apparatus having a scintillator element or the like as an X-ray detection element, a good image with few artifacts can be obtained.
[0024]
In addition, when the X-ray CT apparatus is operated, it can also serve as so-called aging for stabilizing the X-ray tube (it is longer to stabilize the scintillator element etc. than the so-called aging time). It does not take much time.
[0025]
In the above-described embodiment, the X-ray irradiation dose at which the sensitivity of each X-ray detection element is constant is obtained in advance. However, if a normal X-ray detection element, for example, a scintillator element is used, 5000 mAs is used. Since the sensitivity is stabilized if X-rays are irradiated to the extent, the X-ray irradiation amount performed prior to imaging may be fixed to a predetermined value of about 5000 mAs or more.
[0026]
In the above-described embodiment, an example in which X-ray irradiation is performed when the apparatus is in operation has been described. However, when X-rays are not irradiated, the sensitivity of each X-ray detection element is restored to around 1, so that X-rays are irradiated for a predetermined time. Even when it is not performed, it may be configured to perform the imaging of the subject.
In the above-described embodiment, the X-ray detector is mounted on the X-ray CT apparatus and X-rays are irradiated to stabilize the sensitivity of each detection element. If the X-ray detector is irradiated with sufficient X-rays in advance, the recovery of the sensitivity of each X-ray detection element is reduced, and the X-ray irradiation time performed prior to the operation of the X-ray CT apparatus is also reduced.
[0027]
【The invention's effect】
According to the present invention, prior to imaging of the subject, X-rays that always have a constant detector sensitivity are irradiated to each detection element, so that imaging of the subject is always performed with the detection element sensitivity being constant. Thus, in an X-ray CT apparatus having an X-ray detection element as a fluorescent element such as a scintillator element, a good image with little artifact can be obtained.
[0028]
In the present invention, when the X-ray CT apparatus is operated, it can also serve as so-called aging for stabilizing the X-ray tube, so that it does not take a long time for imaging compared to the conventional case.
[Brief description of the drawings]
FIG. 1 is a schematic view of an X-ray CT apparatus according to the present invention.
FIG. 2 is a diagram illustrating an operation of an arithmetic control unit according to the present invention.
FIG. 3 is a diagram showing an operation of an arithmetic control unit for obtaining an X-ray irradiation amount that makes the output sensitivity of a detector constant.
FIG. 4 is a diagram showing a change in sensitivity with respect to an X irradiation amount of a detector.
FIG. 5 is a diagram illustrating a calibration operation.
FIG. 6 is a diagram showing a sensitivity change with respect to an X irradiation amount of a detector.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... X-ray generation part 2 ... X-ray detection part 3 ... Subject 4 ... Calculation control part

Claims (2)

In an X-ray CT apparatus comprising an X-ray generator, an X-ray detector that combines a fluorescent element that emits light upon receipt of X-rays, and a photoelectric conversion element that converts the amount of light emitted from the fluorescent element into an electrical signal. An X-ray CT apparatus comprising: control means for irradiating the X-ray detector from the X-ray generation unit with X-rays enough to stabilize output sensitivity due to radiation damage to the X-ray detector prior to specimen imaging .  The X-ray CT apparatus according to claim 1, wherein the X-ray dose is 5000 mAs or more.

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