JPS6272328A - X-ray ct scanner - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to an X-ray CT scanner that can improve the detection accuracy of a main detector by correcting X-ray fluctuations of a radiation source.

[Technical Problems of the Invention and Problems A] FIG. 5 shows the main structure of a conventional X111 CT scanner, and this conventional example is a second generation three-channel C-type scanner.

The filament 101 of the X-ray tube as a radiation source is connected to an AC power source 1
03, the thermoelectrons are accelerated, collide with the target 105, and emit X-rays. The X-rays emitted from the target 105 are shaped into a pencil beam by the collimator 107 and then transmitted through the object to be examined 109 . Test object 1
The X-rays transmitted through 09 are detected by a main detector 113 provided for each channel. Then, a tomographic image on the transmission line of the object 109 to be examined is obtained.

Further, a comparison detector 115 is provided to face the entire focal point of the target 105, and this monitors periodic fluctuations of X-rays over the entire target, and this detection output is used for detection by the main detector 113. The signal has been corrected.

By the way, in general, periodic fluctuations in X-rays are caused by filament 1
There are fluctuations due to ripples in the tube voltage of the X-ray tube due to the cup around 010, and electric field fluctuations.

The former ripple fluctuation in tube voltage is detected as a fluctuation in the total dose per unit time, but with the recent development of rectifier circuits, the tube voltage ripple has been suppressed to 1% or less. On the other hand, the periodic fluctuation of the electric field means that when the nourament of the X-ray tube 101 is turned on with alternating current, the thermionic focusing mechanism also becomes an alternating electric field, and the focal point size of the target 105 changes due to this electric field fluctuation. With this variation, although the total dose per unit time does not change, the radiation dose per unit area of the target 105 changes. Therefore, both tube voltage ripple fluctuations and voltage fluctuations must be taken into account when correcting the detection signal.

However, in the conventional comparison detector 115 described above,
Since the entire X-ray focal point on the target 105 is visible, it is possible to detect the total fluctuation of the X-ray beam due to the tube voltage ripple fluctuation, but the fluctuation of the radiation dose per unit area due to the electric field fluctuation can be detected. There was a problem that it could not be detected.

[Objective of the Invention 1 The present invention has been made based on the above circumstances, and its object is to provide an X-ray CT scanner that can accurately correct detection signals due to fluctuations in X-rays. .

[Summary of the Invention] In order to achieve the above object, the present invention disposes a remeter between the radiation source and the object to be examined, and between the object to be examined and the main detector, or both. In an x-ray CT scanner in which a beam is irradiated onto an object to be examined via a collimator or directly, and the transmitted dose is detected via a remeter or directly at a main detector, The gist is that the comparison detector is placed so as to face the same position as the focal point.

Embodiment 1 of the Invention FIG. 1 is a diagram showing an embodiment of a CT system to which the present invention is applied. The X-ray CT scanner shown in the figure is a second generation two-channel CT scanner, and the X-rays emitted from the target 1 of the radiation generator are
The beam is shaped into a beam and is incident on the object to be inspected 5. The X-rays that have passed through the object to be examined 5 are incident on a remeter 7, and the transmitted dose of the object to be examined 5 is detected by a main detector 9 provided for each channel.

In this embodiment, a comparison detector 11 is provided close to the surface of the collimator 3 on the object 5 side.

Further, the collimator 3 is provided with a beam hole for guiding the X-rays from the target 1 to the comparison detector 11. Therefore, the comparison detector 1 faces the X-ray focal point of the target 1 through the beam aperture of the collimator 3.

In this way, the comparison detector 11 inputs X to the main detector 9.
Since the X-rays are input from the same X-ray focal position (range) as the X-ray, X-ray density fluctuations due to fluctuations in the X-ray focal spot size due to the electric field fluctuations described above can also be detected, and are detected by the main detection 9. The transmitted dose can be corrected more accurately. In this case, due to problems with the manufacturing accuracy of the collimator, if the X-ray focal point facing the comparison detector 11 and the X-ray focal point facing each detector of the main detector 9 are misaligned, the X-ray It is conceivable that a slight shift (a slight phase difference φi) may occur in the detected value of the rain detector 9.11 due to the periodic fluctuation of the rain detector 9.11. In this case, the influence of this shift can be avoided by reference correction, which will be described later.

The X-ray beams detected by the main detector 9 and the comparison detector 11 are guided to a data acquisition section (DAS).

The data collection unit 13 collects data under the control of the CPU 15, and the collected data is subjected to calculation control by a high-speed calculation unit 17.

FIG. 2 is a flowchart showing the processing procedure executed in the device shown in FIG.
and data from comparator 11 are collected.

In the next step 203, reference correction is performed based on the data collected by the data collection unit 13. As will be described later, this is a process of comparing the X-ray beam detected by the comparison detector 11 and the beam detected by the main detector 9 to correct variations in the X-rays.

In the next step 205, 109 conversion processing is performed. Since the X-ray dose is attenuated by an exponential function of the X-ray absorption coefficient, this is a process of converting it into a log.

In the next step 207, air correction processing is performed.

This is a process for correcting the gain when a plurality of the main detectors 9 are used.

The filtering process in step 209 is a process in which a filter with specific frequency characteristics is applied to the projection data to perform back projection. After this filter processing, back projection processing in step 211 is performed.

Next, reference correction by the comparison detector will be explained.

The periodic fluctuation of the X-ray density can be expressed by a sine wave, and the frequency is considered to be determined by the X-ray power supply period and the integration time of the detected amount.

Ideally, the focal positions of each main detector 9 and comparison detector 11 should be at the same location (range), but 1
, it is expected that there will be slight deviations depending on the manufacturing precision of the collimator. Therefore, each main detector 9 and comparison detector 1
1 may have a certain phase difference. Here, if the comparison detector is taken as a reference, each main detection value is expressed as (1) i=1...the number n of main detectors, and the comparison detection value is IR(t) − It can be expressed as 1Rsin ωt +BR・(2).

Here, if the correction value is calculated by dividing as in general reference correction, Ic (t) = IMi (t)/IR (t)
=Fi (ω, μi (t))+Bi i
=i...n. Here, Fi (ω) is a periodic function with a period ω, Bi is a bias, and μ1(t) is a line integral value of a linear absorption coefficient.

In other words, in the correction by division, Fi becomes a function of ω, and unnecessary fluctuations are added to Fi.

Here, by utilizing the fact that the period ω is known, processing is performed to remove the period ω component in the frequency domain. However, it should be noted that it is necessary to leave the period ω component originally present in the projection data.

First, as a calibration, AIR data is collected and the data obtained by Fourier transformation is obtained in advance, F'i.

J(r=(ω+, O)+Bi)=/f″o!
=.(3) i=1...n F'O1 minus the DC component is the periodic variation and noise, which is expressed as F'o, -. Honey indicates Fourier transform.

Next, the Fourier transform of the actual collected data is similarly obtained as Fui (i-1...n), and from fFu, /e:1. By subtracting , - and performing an inverse Fourier transform, the periodic component ω is removed.

As described above, in the embodiment shown in FIG. 1, the beam size is defined by the source-side collimator 3, but FIG. 3 shows an example in which the beam size is defined by the detector-side collimator 7.

As shown in the figure, in this embodiment, the radiation source side golimeter 3
Therefore, the comparison detector 11 is arranged so as to face the same focal position of the target 1, which the light detector 9 faces, via the collimator 33 for comparison detection. The beam fluctuations of the radiation source are now monitored in the same way as in the first embodiment.

Further, as shown in FIG. 4, a comparison detector 11 may be provided in the detector side collimator 7 in parallel with the main detector 9.

In the above embodiments, the second generation CT scanner has been described as an example, but the present invention can of course be applied to first generation, third generation, and other types of CT scanners.

[Effect of the invention 1] As explained above in detail, according to the present invention, the comparison detector is arranged so as to face the same position as the X-ray focal point in the radiation source that the main detector faces, so that Fluctuations can be accurately grasped, making it possible to accurately compensate for dose fluctuations.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the configuration of an embodiment of the present invention together with the overall configuration, FIG. 2 is a flowchart showing the processing procedure executed by the device in FIG. 1, and FIGS. 3 and 4 are diagrams showing other embodiments of the present invention. FIG. 5 is a diagram showing the configuration of an embodiment, and FIG. 5 is a diagram showing the configuration of a conventional example. 1...Target 3.5...Collimator 9...Main detector 11...Comparison detector

Claims (1)

[Claims] A collimator is disposed between the radiation source and the object to be examined and/or between the object to be examined and the main detector, and the X-ray beam from the radiation source is directed to the object through the collimator or directly to the object to be examined. In an X-ray CT scanner that irradiates a radiation source and detects the transmitted dose through a collimator or directly with a main detector, a comparative detector is arranged so that the main detector faces the same position as the X-ray focal point in the radiation source. An X-ray CT scanner characterized by having: