JPS62127657A - Primary filter for x-ray ct device - Google Patents

Abstract

PURPOSE:To correct the sensitivity irregularity of an X-ray detector and to reduce a ring artifact by forming a primary filter by using calcium or material whose atomic number is close to that of calcium. CONSTITUTION:The primary filter 2 is used to reduce the low energy of X rays and improve the ratio of image information to the exposure of an object body. The most principal material which generates a difference between an X-ray spectrum obtained by passing the X rays through water in components of the human body and an X-ray spectrum obtained by passing the X rays through other organism is calcium. for the purpose, the primary filter made of calcium is used to reduce the difference between the sensitivity irregularity of water and the sensitivity irregularity of data obtained by passing the X ray through the human body, thereby obtaining an image of excellent quality by the both. Further, the material is not limited to calcium, but may use another material as long as its atomic number is close to that of calcium, thereby obtaining substantially the same effect by using a filter formed of potassium, scandium, or titanium.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention is directed to reducing ring artifacts that tend to occur in X-ray CT devices, especially third-generation X-ray CT devices (CT stands for Computer Tomography). This article relates to improvements to the X-ray filter added to the.

(Prior art) Figure 1 shows gun 1 of the X-1cT device (third generation device).
FIG. Inside the gantry, an X-ray tube 1, a primary filter 2, and a wedge filter 3 are attached to one end of a rotating frame (not shown) that rotates 360 degrees, and an X-ray tube is attached to the other end.
line detector 4 and data acquisition device (abbreviated as DAS) 5
is installed.

X-ray tube 1, primary filter 2. The wedge filter 3 and the X-ray detector 4 rotate integrally around the center O, and their relative positions remain unchanged. A subject is placed in the center, and a fan-shaped beam irradiated from an X-ray tube 1 is detected by an opposing X-ray detector. The detector 4 is composed of a large number of X-ray detection elements that can obtain electrical signals corresponding to the X-ray dose, and the output of each detection element is collected by a data collection device 5 and sent to the main body of the C-type device.

The wedge filter 3 is a correction filter used to adjust the input within the dynamic range of the analog-to-digital converter provided in the data acquisition device 5.
It acts to minimize the dose to the subject.

The primary filter 2 is for reducing the low energy of X-rays and improving the ratio of image information to radiation exposure of the subject. The X-rays emitted from the X-ray tube 1 are a mixture of continuous X-rays and linear beams, and the intensity distribution is as shown in Figure 2. As shown, this distribution has a high proportion of low energy. Low-energy X-rays are attenuated at a large rate as they pass through substances, making it difficult for them to reach the X-ray detector 4. Therefore, X-rays containing a lot of low energy are
Although the radiation exposure to the subject is large, the amount of image information included is small, and a first-order filter is used to reduce these low-energy X-rays. Aluminum is used as the material for this primary filter.

(Problem that the defense attempts to solve) Generally, in X-ray CT, from the definition of CT value (value corresponding to various tissues), the X-ray detector uses one or both of two types of data: water and air. 4 characteristics (mainly variations in sensitivity) are corrected, but the X-ray spectrum that has passed through water or air and the spectrum of X-rays that have passed through the subject (human body) (particularly the X-rays that have passed through the skull) are Since there is a large difference between the two, detector variations cannot be completely corrected.

If variations in sensitivity (@m unevenness) remain, a ring artifact 1- will occur in the back projection image. Therefore, even if the sensitivity unevenness is corrected using water or air and lJ is used, there is a problem in that the back-projection image of the head is covered with Ring Ardifac 1~.

The present invention was made to solve the above problem (see 7),
The purpose of this is to use a first-order filter with similar characteristics to human bones, and to filter the parts of the head and other parts (
A primary filter for X-ray CT equipment that eliminates the difference in X-ray amplitude (1~1) for water (water, abdomen, etc.), effectively compensates for uneven sensitivity of the X-ray detector, and reduces ring artifacts1~. The offer is to cover.

(Means for Solving the Problems) The present invention, which solves the above-mentioned problems, uses calcium or It is characterized by being formed using a substance with an atomic number close to that of calcium.

(Embodiments) Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
− Clarify. For example, the difference in sensitivity unevenness between 1 and t when scanning water and air using X-rays that have already passed through water 200111 and 111 is as low as 17 times after passing through water is the difference in sensitivity unevenness between X-rays that do not pass through anything ( Figure 2〉The difference between the water and the sky that was scanned using ε is small compared to 1). This is due to the fact that the sensitive X-rays are removed first. The X-rays that are easily converted to oxidation vary depending on the substance.

Sensitivity between substances with different X-ray absorption coefficients
In order to reduce the difference in muff, all you need to do is change the X-ray Sven 1-l to a G-type so that the difference is less likely to appear.2')()I
I will be charged 8 yen.

Calcium (Ca >-<, ・There is) is the biggest causative substance that causes a difference between the X-ray spectrum that passes through water in the human body and the X-ray spectrum that passes through the tissues of the human body.

If there is one primary filter made of calcium, the difference between the sensitivity of water and the sensitivity unevenness of data transmitted through the human body will be small, and high-quality images can be obtained with both. However, it is limited to calcium. - Substance C that has an atomic number close to this is also good.

÷ 1- The substances adjacent to calcium (by atomic number) are:

Atomic number: 19 20 21 22 Substance: K Ca Sc
T! If the filter is made of potassium, scandium SC or titanium li, calcium
Substantially the same effect can be obtained.

Note that since potassium, calcium, and scandium are chemically not very stable, it is desirable to form a filter into a compound or an alloy.

(Effect of 'ft brightness) As mentioned above, the first-order filter of the present invention allows g'
) For example, when the head is skinned, the ring arch -j Noah 771~ on the image is reduced.

y, each low knee i' - X-ray is reduced by ♀;];
Utanu (-Katsubino Artinoa/) l~
(cuppin arts) This first-order filter effectively reduces the amount! ”Trokotori (can do 1
゜, IM side q) 100γ11h explanation'! 151N is a diagram illustrating the main part of the gun 1-port of the X-ray (C) apparatus, and FIG. 2 is a diagram illustrating the intensity distribution of the X-ray spectrum.

1...X-ray tube 2...1st-order noise 3.
... Wedge filter 4 ... X-ray detector 5 ... Atta collection system Patent applicant Yokogawa Medical Systems Co., Ltd. 71

Claims (1)

[Claims] 1 of the X-ray CT apparatus, characterized in that the primary filter disposed between the X-ray tube and the subject in the gantry of the X-ray CT apparatus is formed using calcium or a substance with an atomic number close to calcium; Next filter.