JP2009142382A - X-ray ct apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an X-ray CT apparatus for generating a tomographic image of a subject based on X-ray transmission images of the subject obtained at a number of positions in the circumferential direction of rotation by relatively rotating an X-ray source and an X-ray detector disposed at opposite positions in the periphery of the subject around the subject. <P>SOLUTION: The X-ray CT apparatus comprises a liquid calibration data storing means for obtaining and storing calibration data based on the X-ray transmission image of a liquid phantom; an air calibration data storing means for obtaining and storing calibration data based on the X-ray transmission image of an air layer, and in addition, an auxiliary calibration data storing means for obtaining and storing calibration data based on the X-ray transmission image of a prescribed object. The X-ray CT apparatus executes the calibration operation using the liquid calibration data, air calibration data and auxiliary calibration data. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

The present invention relates to an X-ray CT apparatus that captures a tomographic image of a human body part.

In recent years, needs for three-dimensional image diagnosis are increasing in the medical field and the dental field, and an X-ray CT apparatus that can be used in a wide medical field is desired. In particular, in the dental field, with the advancement of dental treatment represented by implant treatment, the need for three-dimensional image diagnosis is increasing.

In general, an X-ray CT apparatus causes an object to enter an imaging region in which an X-ray source and an X-ray detector are opposed to each other, irradiates an X-ray beam from the X-ray source, and transmits the X-ray intensity after passing through the object. Is detected by an X-ray detector. Then, measurement data (raw data) from each direction is repeatedly collected while rotating around the subject, and convolution (convolution), back projection (back projection), and other processing are added to each measurement data. Perform reconfiguration. The image data obtained at this time is converted into a CT value based on calibration data acquired in advance, and an image is displayed.

Here, reference is made to the CT value. The CT value is expressed as a relative value where the value of the X-ray absorption coefficient of the living tissue is 0 with water, and is obtained by the following equation. The CT value when the constant K = 1000 is called Hounsfield Unit (HU) or Hounsfield Number, and is generally used in medical CT, and is obtained by the following formula.

CT value = (μt−μw) / μw × K

μt: Value proportional to tissue X-ray absorption rate μw: Value proportional to water X-ray absorption rate K: 1000

However, the logarithm of the X-ray transmission data (raw data) actually captured by the CT apparatus is data indicating the X-ray absorption rate, and in order to obtain the CT value, the calibration data is obtained by scanning the water phantom in advance. It is necessary to perform calculation, that is, calibration (water calibration) using the calibration data from the raw data obtained by photographing the subject. Further, in the calibration with the water phantom, the acquired value is relatively stable, and it takes time for the collection work, so that it is difficult to re-read frequently. Therefore, in general, calibration is performed using a water phantom during maintenance at a manufacturer, and calibration data (air calibration data) usually obtained through only the air layer is obtained, and calibration is performed based on these calibration data. Is going. In other words, in the conventional calibration, it can be said that the calibration is performed based on the photographing data corresponding to the CT value of water = zero and air = −1000 given as the reference value of the CT value.

As described above, normally, in calibration, variations in the X-ray detector are corrected based on calibration data of two imaging objects, water and air, and the X-ray absorption rate in other subject regions is corrected after image reconstruction. CT value is predicted and converted. Specifically, if the CT value of air is -1000 and water is 0, the CT value of a substance having an X-ray absorption coefficient twice that of water is 1000 as is apparent from the above equation, and -1000 for human tissues. The range from 1 to 1000 is sufficient, and a CT value of ± 1000 is used. However, in the CT value conversion with calibration data obtained only with air and water, it is expected that errors will increase in the conversion of CT values far away from the two points of air = −1000 and water = 0. Conventionally, CT images in the medical field are mainly soft tissues such as internal organs, and water and air, in particular, sites that are far from CT values of water are rarely diagnosed, and this point has not been particularly recognized.

On the other hand, the X-ray CT apparatus is also used for industrial purposes. In this case, an object that should be regulated by a CT value far away from water and air is mainly photographed. The error was not regarded as the attention part in the detection result.

On the other hand, in recent years, in the dental diagnosis where the use needs of the X-ray CT apparatus are increasing, it is necessary to accurately diagnose the tissue structure from the air layer such as the maxillary sinus to the bone having a large CT value. It has been found that the two-point calibration of air has a large error as it approaches a tissue having a high X-ray absorption rate, and it is difficult to diagnose the tissue corresponding to the CT value.

In the medical field, the use of an X-ray CT apparatus for cardiovascular diagnosis and inner ear diagnosis has attracted attention, and two needs for accurate CT value measurement and wide-range CT value measurement are increasing. The calibration with the existing X-ray CT apparatus was insufficient.

Japanese Patent Laid-Open No. 4-38937

In view of such circumstances, an object of the X-ray CT apparatus of the present invention is to provide an X-ray CT apparatus capable of measuring a wide range of CT values with high accuracy, particularly a dental CT apparatus.

The X-ray CT apparatus of the present invention relatively rotates an X-ray source and an X-ray detector arranged at opposite positions around the subject around the subject, and makes a number of rotations in the circumferential direction. The X-ray CT apparatus generates a tomographic image of the subject based on the X-ray transmission image of the subject acquired at the position. In this apparatus, a water calibration that acquires and stores calibration data based on an X-ray transmission image of a water phantom disposed within a range in which an X-ray transmission image that is required even in a conventional X-ray CT apparatus can be acquired. Storage means, and air calibration data storage means for acquiring and storing calibration data based on an X-ray transmission image of an air layer within a range where an X-ray transmission image can be acquired. In addition to the calibration data storage means, the X-ray CT apparatus according to the present invention acquires and stores calibration data based on an X-ray transmission image of a target object disposed within a range where an X-ray transmission image can be acquired. Auxiliary calibration data storage means is added.

The X-ray CT apparatus uses the calibration data obtained from the above three calibration data means, specifically, the water calibration data means, the air calibration data means, and the auxiliary calibration data means to perform calibration. An operation is performed.

In general, an X-ray CT apparatus has a limit value of CT values that can be detected in accordance with its performance and measurement target. In one X-ray CT apparatus of the present invention, calibration data is stored in auxiliary calibration data storage means. The target object to be acquired is an object corresponding to a CT value equal to or less than a maximum CT value (hereinafter referred to as “allowable CT value”) that can be detected in advance according to at least the X-ray CT apparatus. It is preferably determined based on the respective X-ray detector and X-ray source and the arrangement of the X-ray detector and X-ray source. For example, the target object is a material corresponding to a CT value greater than 0 and less than or equal to an allowable CT value.

In another X-ray CT apparatus of the present invention, the target object for which the calibration data is acquired by the auxiliary calibration data means is a material having a CT value close to a CT value set in advance as corresponding to the imaging target material. It is preferable that it is comprised. In addition, when there are a plurality of CT values that are set in advance assuming that the target object corresponds to a material for imaging purposes, the target object may be composed of a plurality of objects having a CT value close to each of the plurality of CT values.

Furthermore, in the case of a dental X-ray CT apparatus as an example of the X-ray CT apparatus of the present invention, at least one of the target objects is an object whose main component is hydroxyapatite (hydroxyapatite) or aluminum close to the CT value of bone. Alternatively, it is preferably composed of a contrast agent for CT imaging.

Conventionally, calibration has been performed in which raw data of radiographing is corrected using calibration data based on two substances, water and air, which is the basis for CT value calculation. However, according to the X-ray CT apparatus of the present invention, Calibration is performed by adding calibration data based on at least one substance other than air. Thereby, it is possible to obtain a tomographic image with a smaller CT value error than the calibration performed at two points of water and / or air. In particular, the CT value error in a substance corresponding to a large CT value can be reduced. Therefore, it is possible to detect a precise CT value even in a CT tomographic image including a substance having a high X-ray absorption rate, such as a bone or tooth as a subject part, or an external substance used for medical and dental purposes. Become.

In the X-ray CT apparatus of the present invention, calibration data based on a target object made of a material corresponding to a CT value having a large limit (preferably a CT value near the limit) that can be detected for each performance of the apparatus. To get. The allowable CT value here is determined by the sensitivity and dynamic range (= measurable range) of the X-ray detector, the voltage / current applied to the X-ray source, the distance between the X-ray detector and the X-ray source, and the like. However, the actual set value may be smaller than the value determined from a medical viewpoint.

Since the CT value of water is 0 and the CT value of air is -1000, if there is an error in the CT value within this range, no large error will occur in the range of -1000 to 0, but in the positive direction The detected CT value error increases as the allowable CT value is approached. Therefore, in one X-ray CT apparatus, not only calibration data based on other target objects other than water and air but also an object corresponding to a large CT value in the plus direction is used as the target object for auxiliary calibration. This makes it possible to detect CT values with high accuracy over a wide range. The target object is preferably as far as possible in the plus direction, but as the limit, the target value can be easily selected by setting the CT value detection limit of each X-ray CT apparatus to the CT value corresponding to the target object. Therefore, it is preferable that the CT value corresponding to the target object of the auxiliary calibration is larger than the CT value (= 0) corresponding to water. Furthermore, as a CT value limit peculiar to the human body, the tooth has the largest CT value. When considered, the CT value within a desired range can be precisely detected by selecting a target object corresponding to at least the CT value of enamel.

Further, in the X-ray CT apparatus described above, attention has been paid to increasing the accuracy of CT value detection in a wide range of subject parts. However, in other X-ray CT apparatuses of the present invention, a material that a doctor or the like particularly desires for CT value detection is used. The focus is on improving the accuracy of detected CT values in the vicinity. In another X-ray CT apparatus of the present invention, calibration data of a target object corresponding to a CT value close to the material itself desired to detect the CT value is acquired. As a result, it is possible to detect a CT value with particularly high accuracy for a part (or an external object) that a doctor or the like particularly wants to see. Further, when the X-ray CT apparatus of the present invention is used as a dental X-ray CT apparatus, for example, if a substance having a CT value close to a tooth or bone of particular interest in dentistry is selected, the dentist can easily understand the tissue structure. A tomographic image can be provided.

Hereinafter, an embodiment of a dental X-ray CT apparatus will be described as an example of the X-ray CT apparatus of the present invention.
First, one general configuration of a general dental X-ray CT apparatus will be outlined as a premise. Referring to FIG. 1, a general dental X-ray CT apparatus 10 is illustrated. The main body 10 of the dental X-ray CT apparatus is provided with a base 12 at the bottom, and a support column 14 extends upward from the base 12. The support column 14 is provided with a horizontal arm 16 that is bent at the upper end and extends in the horizontal direction. In addition, a revolving arm 18 that is rotatable with respect to the horizontal arm 16 is attached to a lower portion of the horizontal arm 16. There is also a chair 29 for sitting the patient.

Further, the swivel arm 18 is formed in a suspended shape bent downward at both ends, and an X-ray source 20 and an X-ray detector 22 are mounted at positions facing each other below the both ends. The X-ray source 20 is irradiated with cone-beam X-rays that diffuse toward the X-ray detector 20. The X-ray detector 22 is a two-dimensional X-ray detector (hereinafter referred to as “X-ray detector 22”) that collects X-rays emitted from the X-ray source 20 on the detection surface 22 a on the X-ray source 20 side. Also referred to as a “two-dimensional detector 22”).

In addition, a fixed arm 24 is provided on the support column 14, and the fixed arm 24 extends horizontally from the support column 14 in parallel with the horizontal arm 16. A subject holding means is attached to the distal end portion of the fixed arm 24. For example, a horizontal bar 26 for positioning and fixing the patient's front-rear direction and a jaw receiver 28 for positioning and fixing the patient's jaw downward are provided. It is attached.

When CT imaging is performed with the X-ray CT imaging apparatus 10, the patient's jaw is first fixed to subject holding means such as the horizontal bar 26, and the chair is positioned at a good position in consideration of the patient's height. 29 is moved up and down. When the jaw portion is positioned, the photographing standby state is set. At the time of imaging, the X-ray detector 22 detects X-rays transmitted by the turning arm 18 turning while irradiating X-rays from the X-ray source 20 to the patient's jaw positioned on the subject holding means. In the present embodiment, since the X-ray CT imaging apparatus 10 shown in FIG. 1 has been illustrated and described, the patient 29 is positioned by the chair 29, and the swivel arm 18 is relatively positioned around the patient. Other configurations may be used as long as the positional relationship is rotating. For example, the chair 29 on which the patient is seated may be provided with a turning function as well as a turning function so as to rotate the patient itself between the X-ray source and the X-ray detector.

Next, FIG. 2 shows the overall configuration of the imaging function of the X-ray CT imaging apparatus 10. As described above, the X-ray CT imaging apparatus 10 is arranged such that the X-ray source 20 and the X-ray detector 22 are positioned so as to face each other with respect to the patient's jaw 30 as a subject, and the swivel arm 18. However, the X-ray source 20 and the X-ray detector 22 are suspended from each other. The patient's jaw 30 is fixed by subject holding means constituted by the horizontal bar 26, the jaw holder 28 and the like. The X-ray CT imaging apparatus 10 processes an X-ray image and displays it on the display device 32. As the control configuration, the image processing means 34 for controlling the entire X-ray image processing control as a whole, or an operator Includes an operation unit 36 for performing an imaging operation. The operation unit 36 includes a display unit 36b that easily displays an operation guide for operating the apparatus 10, and includes an operation panel 36a for performing an imaging operation according to the display of the display unit 36b. Yes.

The X-ray source 20 includes an X-ray beam controller 40 that adjusts the energy of the generated X-ray beam and enables the X-ray cone beam 38 having a desired beam width to be emitted. Here, the X-ray detector 22 receives and detects X-rays emitted from the X-ray source 20 and transmitted through the subject's jaw, and X-ray intensity data as X-ray transmission image data is converted into analog electrical data or itself. When an A / D converter is provided, it has a function of outputting as digital data. Specifically, there are a combination of a II tube and a CCD (solid-state imaging device) camera, and a combination of FPD and IP. For example, when using an II tube, X-rays are converted into visible light by the II tube, this visible light is converted into electrons by a photoelectric converter, and then multiplied, and the multiplied electrons are visualized by a phosphor. There is a configuration in which the image is taken with a CCD camera that is two-dimensionally arranged through a lens after being converted into a lens.

Further, the turning arm 18 is provided with an arm rotation motor 42 so as to turn at a constant speed or a variable speed around the turning center 18a. The arm rotation motor 42 constitutes a turning drive means for the turning arm 18, and uses a motor that can freely control its rotation speed and rotation position, such as a servo motor or a pulse motor. The X-ray turning center 18a of the turning arm 18 is directly connected to the axis. Accordingly, the swivel arm 18 can be rotated around the swivel center 18a, and the rotational position can be known along the time axis. The X-ray CT imaging apparatus 10 of the present invention can be effectively implemented because it is convenient for taking out the image and because there is little center deflection.

The portion of the turning arm 18 at the X-ray turning center 18a is a hollow portion 18b. The hollow portion 18b has a connection line between the X-ray source 20 and the X-ray detector 22 suspended from the swivel arm 18 and the operation unit 36 and the image processing unit 34 described above.

Next, the image processing means 34 includes an arithmetic processing means 34a, an A / D conversion means 34c, a frame memory 34d, and an arithmetic memory 34e that are constituted by an arithmetic processor that can operate at high speed for image processing analysis. The display device 32 displays an X-ray tomographic image obtained by the X-ray CT imaging obtained by the image processing unit 34, and specifies a position of a specific portion on the image on the displayed curved X-ray tomographic image. Selection can be made.

The image processing unit 34 is connected to the operation unit 36 and the display device 32 such as the X-ray source 20, the X-ray detector 22, and the arm rotation motor 42, and processes and controls data from these. Specifically, the X-ray intensity data obtained from the X-ray detector 22 is converted into a digital signal by the A / D conversion means 34c and stored in the frame memory 34d. The plurality of X-ray intensity data stored in the frame memory 34d is stored in the calculation memory 34e, and a predetermined calculation process for generating a tomographic image is performed on the stored X-ray intensity data. The three-dimensional X-ray absorption coefficient (CT value) of the imaging region of interest is also calculated, and various images are reconstructed from the three-dimensional X-ray absorption coefficient and displayed on the display device 32. In addition, it is stored in an external storage means (not shown) as necessary. Note that the calibration data which is a problem in the present invention is stored in the external storage means via the frame memory 34d by calibration shooting, but is stored in the calculation memory 34e from the external storage means in the calibration calculation process. Thus, it is used for the calibration calculation executed in the calculation processing means 34a.

  Next, the concept of image reconstruction in the dental X-ray CT apparatus 10 will be described with reference to FIG. 3 as an X-ray CT apparatus. 3 is a flow chart of image processing performed by the arithmetic processing unit 34a in FIG.

First, a subject (corresponding to the dentomaxillofacial surface 30 in FIG. 2; hereinafter, widely described as “subject 30”) is imaged and captured by the X-ray detector 22 by the image processing means 34 (see FIG. 2). Projection data (raw data) is acquired (S1). Next, offset correction (S2) is performed on the projection data. After the offset correction (S2), Log conversion (S3) is performed.

Next, air calibration is applied to the measurement data subjected to the log conversion process (S3) (S3). This calibration corrects that the sensitivity to the X-ray intensity irradiated by the X-ray detector 22 differs for each channel, and corrects based on each channel value in the stored calibration data obtained in advance. It is the arithmetic processing to do.

Thereafter, image reconstruction is performed (S4), and a final tomographic image is reconstructed. At this time, since the reconstructed tomographic image data is X-ray absorption rate data, correction for conversion into CT values is performed. For example, a curve or straight line passing through the calibrated three points is corrected (S5). Then, image display is performed by the display device 32 (see FIG. 2) (S6).

As a premise of this arithmetic processing, calibration data storage means for acquiring and storing each calibration data is required. Here, there are water calibration data storage means, air calibration data storage means, and auxiliary calibration data storage means, respectively, based on the photographing of the water phantom, the air layer, and a predetermined target object.

First, the water calibration data storage means acquires and stores the X-ray intensity data when the water phantom disposed in the imaging range is imaged as water calibration data. The data acquisition / storage process by this means is mainly performed during manufacturer maintenance. Further, the air calibration data storage means acquires and stores the X-ray intensity data when the air layer in the imaging range is imaged as air calibration data. The data acquisition / storage process by this means is usually performed by the user before photographing. Further, the auxiliary calibration data storage means acquires and stores the X-ray intensity data when the predetermined target object arranged in the imaging range is imaged as auxiliary calibration data.

At this time, a predetermined object (referred to as a “target object”) is used as an imaging target to be stored in the auxiliary calibration data storage unit. This target object is separately selected for auxiliary calibration and placed within the imaging range. For example, a CT value having a large limit that can be detected for each performance of the X-ray CT apparatus 10, preferably the limit A target object having a material corresponding to a nearby CT value is selected, and a material having a CT value corresponding to +1000 or more is selected. In addition, as a target object of another example, a material corresponding to a CT value close to a material itself that a doctor or the like desires to detect a CT value is selected.

In the case of the dental X-ray CT apparatus 10 shown in FIG. 1, typically, hydroxyapatite is selected as a material having a CT value close to the dentition tissue or artificial bone of particular interest in dentistry.

The dental X-ray CT apparatus has been described as an example as an embodiment of the X-ray CT apparatus of the present invention. However, the X-ray CT apparatus according to the present invention has added other calibration data as a solution to the problems that have been calibrated with calibration data of water and / or air only, and more specifically, A major feature is that an imaging target (target object) is provided, and the present invention is not limited to a dental X-ray CT apparatus. The X-ray CT apparatus of the present invention is a CT corresponding to (approximating) the CT value of an object corresponding to the maximum CT value that can be imaged according to the standard of each X-ray CT apparatus or an object that a doctor or the like desires to detect. It will be apparent to those skilled in the art that there are various X-ray CT apparatus embodiments that perform a new calibration with the added calibration data of the value object.

It is the perspective view which illustrated one of the common X-ray CT apparatuses. It is the conceptual diagram which showed the outline | summary of the structure of the X-ray CT apparatus of FIG. It is a flowchart of the reconstruction process of the image containing the calibration calculation in the X-ray CT apparatus of this invention.

Explanation of symbols

10 X-ray CT apparatus 20 X-ray source 22 X-ray detector S3 Calibration / Log conversion

Claims (6)

X-ray sources and X-ray detectors arranged at opposite positions around the subject are rotated relative to each other around the subject, and the X of the subject acquired at a number of circumferential positions of the rotation. An X-ray CT apparatus that generates a tomographic image of the subject based on a ray transmission image,
Water calibration data storage means for acquiring and storing calibration data based on an X-ray transmission image of a water phantom disposed within a range capable of acquiring an X-ray transmission image;
Air calibration data storage means for acquiring and storing calibration data based on an X-ray transmission image of an air layer within a range where an X-ray transmission image can be acquired;
Auxiliary calibration data storage means for acquiring and storing calibration data based on an X-ray transmission image of at least one target object disposed within a range where an X-ray transmission image can be acquired;
Means for performing a calibration operation using calibration data obtained from the water calibration data storage means, the air calibration data storage means, and the auxiliary calibration data storage means;
X-ray CT apparatus. The target object for obtaining calibration data in the auxiliary calibration data storage means is an object corresponding to a CT value equal to or lower than the maximum CT value that can be detected in advance according to the X-ray CT apparatus, and the maximum CT value The X-ray CT apparatus according to claim 1, wherein the X-ray CT apparatus is determined based on at least each of the X-ray detector and the X-ray source and an arrangement of the X-ray detector and the X-ray source. The target object for acquiring calibration data in the auxiliary calibration storage means is made of a material having a CT value close to a CT value set in advance as corresponding to a material for imaging purposes. The X-ray CT apparatus according to 1. The X-ray CT apparatus according to claim 2, wherein the target object is an object corresponding to a CT value larger than a CT value corresponding to water. When there are a plurality of CT values set in advance assuming that the target object corresponds to a material for imaging purposes, the target object is composed of a plurality of objects of a material having a CT value close to each of the plurality of CT values. The X-ray CT apparatus according to claim 3. The X-ray CT apparatus is a dental X-ray CT apparatus, and the target object is composed of an object mainly composed of hydroxyapatite (hydroxyapatite) or aluminum or a contrast agent for CT imaging. The X-ray CT apparatus according to any one of claims 3 and 5.