JPH08266532A - X-ray ct system - Google Patents

Abstract

PURPOSE: To correct dispersion due to the reduction in sensitivity of an individual detector element without performing calibration in an X-ray CT system equipped with an array type solid-state detector in which a plural pairs each consisting of a fluorescent element and a photoelectric conversion element are arranged in parallel. CONSTITUTION: A first storage means 11 which stores a sensitivity deterioration characteristics function for the radiation integrated exposure quantity of a detector, and a second storage means 12 which stores the exposure quantity of radiation at every channel of the detector are provided, and the output of a radiation detector is corrected and calculated at every channel by utilizing data stored in the first and second storage means. The dispersion of sensitivity generated in the individual detector element is corrected by the irradiated history of the radiation, and a satisfactory X-ray CT image without a virtual image is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an X-ray CT apparatus, and more particularly to a technique for correcting deterioration and variation in sensitivity of each detecting element of an array type radiation detector used for detecting X-rays in the X-ray CT apparatus. .

[0002]

2. Description of the Related Art As a detector of an X-ray CT apparatus for medical use, there have conventionally been arranged a large number of fluorescent elements which emit light when radiation enters and photoelectric conversion elements which convert light from the fluorescent elements into electric signals. Array type solid-state radiation detectors are used. It is known that when a solid-state detector is used, the output of the detector gradually decreases due to a decrease in the emission intensity of the fluorescent element in particular, depending on the amount of radiation hit, and a device for correcting it is used. Calibration is performed at the start of use, and in some cases, calibration is often performed even during diagnosis to correct the magnitude of the detector output so that there is no variation among the detection elements.

It is also known that the above-mentioned deterioration of the sensitivity of the detector can be recovered to some extent by keeping the detector in a rest state so that the detector is not exposed to radiation. When using for diagnosis, there is no choice but to perform calibration by interrupting the diagnosis in order to compensate for the deterioration of sensitivity.

[0004]

Since the deterioration of the sensitivity of the detector occurs according to the radiation exposure amount to each detecting element,
Further, since the radiation exposure amount of each channel changes depending on the absorption amount by the subject, the sensitivity of the detector varies depending on each channel of the detector element, and when it becomes severe, a virtual image appears in the obtained X-ray CT image. was there. Generally, as in the prior art, it is possible to correct the variation in the sensitivity of each channel by calibration, but the diagnosis is interrupted in order to perform the calibration, which is very inefficient.

The present invention has been invented in view of the above-mentioned problems of the prior art. The problem to be solved by the present invention is to overcome the deterioration of each detector element depending on the radiation exposure dose, and to perform a medical examination. An object of the present invention is to provide an X-ray CT apparatus that can always obtain a good image without performing extra calibration on the way.

[0006]

In order to solve the above-mentioned problems, in the present invention, a large number of channels are provided for a fluorescent element that emits light when radiation enters and a photoelectric conversion element that converts light from the fluorescent element into an electric signal. In an X-ray CT apparatus equipped with arrayed array type radiation detectors, a first sensitivity storing characteristic function with respect to an integrated radiation dose of the detectors is stored.
Storage means and second storage means for accumulating and storing the radiation exposure amount for each channel of the detector, and using the data stored in the first and second storage means. The output of the radiation detector is corrected and calculated for each channel.

Here, the exposure dose of the detector means that the detector output is proportional to the intensity of the irradiated radiation, so that it can be replaced by the detector output, and the detector output can be stored in the second storage means. The value obtained by integrating the sizes is stored. The data stored in the first storage means can also be stored in the form of the detector output when receiving the radiation of the same intensity with respect to the integrated amount of the detector output.

[0008]

The decrease in the sensitivity of the detector, which is mainly caused by the decrease in the amount of light emitted from the fluorescent element, is measured by measuring the relationship between the cumulative dose of radiation and the decrease in the output of the detector in advance as a characteristic function indicating the decrease in sensitivity. It is stored in the first storage means. The exposure dose of each detection element in the array-type detector is accumulated and stored in the second storage means immediately after execution of calibration performed at the beginning of daily use. Since the detector output currently being measured is corrected and calculated from the data stored in the two storage means, the calculated value correctly represents the measured radiation dose. Therefore, the variations in sensitivity caused by the individual detector elements due to the radiation exposure history are correctly corrected, and a good CT image without a virtual image or the like can be obtained.

[0009]

1 shows an embodiment of the X-ray CT apparatus of the present invention. The X-ray beam 2 radiated in a fan shape from the X-ray tube 1 passes through the collimator 7 after being absorbed by the subject 3.
The entire area radiated in a fan shape is simultaneously detected by the array type solid-state detector 4. Array type solid state detector 4
Is a fluorescent element 5 made of CdWO ₄ or CsI (Tl)
The photoelectric conversion elements 5 made of silicon photodiodes and the like are arranged in an arc shape at a pitch of 1 mm, for example.

When the subject 3 is not present, the output of each element (channel) of the array type detector 4 must be the same among the elements, so in an ordinary X-ray CT apparatus, calibration is performed every day when the apparatus is started up. Is performed so that the output of each element is adjusted to the same level. It can be said that this is an operation for adjusting the sensitivity of each element to radiation. In this way, the sensitivity of each element is adjusted every morning, but the output of the detector gradually decreases due to the decrease in the emission intensity of the fluorescent element in particular, depending on the amount of radiation hit. That is, the sensitivity of each element gradually deteriorates as the measurement progresses, and the radiation dose is not constant for each element, so the degree of sensitivity deterioration varies from element to element.

FIG. 2 is an example of the sensitivity deterioration characteristic showing the state of the change in the output of the detector element according to the radiation exposure amount.
Since the size and material of each element of the detector are the same for all elements, one sensitivity deterioration characteristic function can represent the characteristics of all elements. When the radiation dose is x, the detector output is y, and the detector output immediately after calibration is 1 and a graph is drawn, the detector output monotonically decreases in accordance with the amount of irradiation of radiation after calibration. come. The function is assumed to be y = f (x).

In FIG. 1, the signal from the array type detector 4 is A
An output for each element is obtained as a digital signal by a signal processing circuit 13 including a / D converter, and an X-ray CT image is formed by a tomographic image data processor 15 via a correction calculator 14 which will be described later. Is displayed on the display device 16. Reference numeral 11 is a first memory that stores the sensitivity deterioration characteristic function of the detector described with reference to FIG. 2, and reference numeral 12 is a second memory that stores the integrated radiation exposure dose for each channel of the detector. Is. In order to correct the output of one channel, by applying the integrated exposure dose data stored in the second memory 12 to the characteristic function stored in the first memory 11, It is possible to know f (x), and it is possible to obtain a corrected detector output by multiplying the output of the channel obtained by the signal processing circuit 13 in the correction calculator 14 by the reciprocal of f (x).

Since the output of each detector element is proportional to the intensity of the irradiated X-ray, the accumulated exposure dose stored in 12 is the output of each channel obtained by the signal processing circuit 13. It can be obtained by adding up for each. More accurately, the outputs of the respective channels corrected by the correction calculator 14 may be integrated to obtain the integrated exposure dose. At this time, not only when an X-ray CT image is taken, but also while the X-ray is being received by the detector, the correction calculator 14 must be operated to update the integrated exposure dose. 11
The sensitivity deterioration characteristic function stored in is obtained by an experiment in advance and stored in the form of an equation or a table. The horizontal axis at that time is the integrated output of the signal processing circuit 13 or the correction arithmetic circuit 14 described above, and the vertical axis is the raw detector output at that time.

[0014]

EFFECTS OF THE INVENTION Since the sensitivity deterioration of the detector elements is individually corrected by the data of the integrated exposure dose and the data of the deterioration characteristic function, the variations in the sensitivity generated by the individual detector elements due to the radiation exposure history are correct. Corrected, it is possible to obtain a good X-ray CT image without a virtual image. Therefore,
Even if the calibration is not performed during the diagnosis, a good X-ray CT image can be continuously taken during the day and day by only performing the calibration at the time of starting the apparatus every day.

[Brief description of drawings]

FIG. 1 shows an embodiment of an X-ray CT apparatus of the present invention.

FIG. 2 is an example of a sensitivity deterioration characteristic of a detector.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... X-ray tube 2 ... X-ray beam 3 ... Subject 4 ... Array type detector 5 ... Fluorescent element 6 ... Photoelectric conversion element 11 ... First memory 12 ... Second memory 13 ... Signal processing circuit 14 ... Correction calculator 15 ... Tomographic image data processor 16 ... Display device

Claims (1)

[Claims] 1. An X-ray CT apparatus provided with an array type radiation detector in which a plurality of channels are arranged with a fluorescent element that emits light when radiation enters and a photoelectric conversion element that converts light from the fluorescent element into an electrical signal. First storage means for storing a sensitivity deterioration characteristic function with respect to the cumulative radiation exposure dose of the detector, and second storage means for accumulating and storing the radiation exposure dose for each channel of the detector are provided. An X-ray CT apparatus characterized in that the output of the radiation detector is corrected and calculated for each channel by utilizing the data stored in the first and second storage means.