JP2014176571A - X-ray computer tomography apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce pulsation artifact and radiation dose without using electrocardiographic synchronized radiography.SOLUTION: An X-ray computer tomography apparatus includes: an X-ray tube; a high pressure generating part for generating a tube voltage and a tube current to the X-ray tube; a control part for controlling the high pressure generating part to repeat alternatively an X-ray ON period in which X-rays are generated with a prescribed dosage and an X-ray OFF period in which X-ray radiation is stopped or X-rays are generated with a dosage less than the prescribed dosage; and an image reconfiguration part for reconfiguring image data on the basis of an output of the X-ray detector corresponding to the X-ray ON period. A time length of the X-ray OFF period is predetermined according to a cardiac rate of an analyte.

Description

  Embodiments described herein relate generally to an X-ray computed tomography apparatus.

  In the X-ray computed tomography apparatus, an image in which motion artifacts due to the motion of the heart are reduced can be created by an electrocardiogram synchronization method (an electrocardiogram synchronization scan method, an electrocardiogram synchronization reconstruction method).

  Further, there is a method in which tomographic images at a plurality of times are generated in synchronization with electrocardiogram, the center-of-gravity displacement from the center-of-gravity coordinates of each tomographic image is calculated, and a tomographic image in a motion state is selected from this displacement amount (Patent Document 1) ).

  In order to use the electrocardiogram synchronization method, it is necessary to measure an electrocardiogram, and it is necessary to attach electrodes to the patient. For an engineer, it takes time for preparatory work before photographing, and it is necessary to adjust the position to be pasted by an electrocardiogram. Not only is the patient uncomfortable because it is applied directly to the chest, but the electrodes also cause artifacts during chest imaging.

  In the electrocardiogram synchronous reconstruction method, scans are continuously executed, an electrocardiogram is measured simultaneously with the scans, and projection data corresponding to an arbitrary heartbeat phase is collected after the scan is completed to reconstruct an image. For this purpose, photographing is performed for one cycle of exercise or a corresponding time, and exposure may be increased.

JP 2004-313513 A

  The purpose is to realize reduction of pulsation artifacts together with exposure reduction without using an electrocardiogram synchronous imaging method.

  The X-ray computed tomography apparatus according to the present embodiment includes an X-ray tube, a high-voltage generator that generates tube voltage and tube current for the X-ray tube, an X-ray detector, and an X-ray that generates X-rays at a predetermined dose. A control unit that controls the high-pressure generating unit to alternately repeat a line-on period and an X-ray off period in which X-rays are stopped or generated at a dose lower than the predetermined dose, and X-ray detection corresponding to the X-ray on period An image reconstruction unit that reconstructs image data based on the output of the instrument, and the length of the X-ray off period is determined in advance according to the heart rate of the subject.

It is a figure which shows the structure of the X-ray computed tomography apparatus which concerns on this embodiment. It is a figure which shows the scanning operation | movement by the control part of FIG. It is a figure which shows an example of the heart rate / low body motion period correspondence table memorize | stored in ROM of FIG. In this embodiment, it is a figure which shows the temporal relationship between the low body movement period of a subject and an X-ray ON period by various patterns. In this embodiment, it is a figure which shows the scanning of the high body movement site | part containing a heart and the low body movement site | part. In this embodiment, it is a figure which shows automation of the completion | finish of scanning.

  Hereinafter, an embodiment of the X-ray computed tomography apparatus according to the present embodiment will be described with reference to the drawings. The X-ray computed tomography apparatus includes a rotation / rotation method in which an X-ray tube and an X-ray detector are rotated as one body, and a large number of X-ray detectors are arranged on a ring. A fixed / rotation method in which only the X-ray tube rotates around the subject, a fixed / fixed method in which a plurality of X-ray tubes are arranged on the ring, and a plurality of X-ray detectors are similarly arranged on the ring. There are various methods, and any method is applicable. Regarding the rotation / rotation method, a so-called single-tube type in which a pair of X-ray tubes and X-ray detectors are mounted on a rotating frame, and a so-called multiple pairs of X-ray tubes and X-ray detectors are mounted on a rotating frame. There are multitubular bulb types, but any type is applicable. Regarding the X-ray detector, an indirect conversion type in which X-rays transmitted through a subject are converted into light by a phosphor such as a scintillator and then converted into electric charge by a photoelectric conversion element such as a photodiode, and electrons in a semiconductor by X-rays There is a direct conversion type using the generation of a hole pair and its movement to the electrode, that is, a photoconductive phenomenon, and any type may be adopted.

  FIG. 1 is a block diagram showing the configuration of a computed tomography apparatus according to this embodiment. FIG. 2 shows a schematic structure of the X-ray detector shown in FIG. The gantry unit 100 includes a rotating ring 102. The rotation driving unit 105 rotates the rotating ring 102 at an angular velocity selected from a plurality of angular velocities under the control of the scan control unit 106. An X-ray tube device 101 and an X-ray detector 103 are mounted on the rotating ring 102. The X-ray detector 103 faces the X-ray tube apparatus 101 across an imaging region in which the subject P on the bed apparatus 400 is inserted by the top slide.

  The high voltage generation unit 104 generates a tube voltage to be applied between both electrodes of the X-ray tube apparatus 101, and generates a tube current to be supplied to the cathode filament. Under the control of the scan control unit 106, the application and stop of the tube voltage from the high voltage generation unit 104 to the X-ray tube apparatus 101 are switched at high speed. When a tube voltage is applied from the high voltage generator 104 to the X-ray tube apparatus 101 and the tube current is maintained at a predetermined amount, X-rays are generated with a predetermined intensity effective for image reconstruction. A period during which X-rays are substantially generated is referred to as an X-ray on period. X-rays are not substantially generated when the application of the tube voltage from the high voltage generator 104 to the X-ray tube apparatus 101 is stopped or the tube current is reduced to zero or an approximate value thereof. A period in which X-rays are not substantially generated is referred to as an X-ray off period.

  The X-ray tube apparatus 101 outputs X-rays in a cone beam shape. The X-ray detector 103 employs a multi-slice type in order to receive cone beam X-rays. The X-ray detector 103 is connected to a data acquisition unit 107 generally called a DAS (data acquisition system). The data acquisition unit 107 includes an IV converter that converts a current signal of each channel of the multi-slice X-ray detector 103 into a voltage, and the voltage signal periodically in synchronization with an X-ray exposure cycle. An integrator for integrating, an amplifier for amplifying the output signal of the integrator, and an analog / digital converter for converting the output signal of the preamplifier into a digital signal are provided for each channel.

  A console 200 is connected to the gantry 100. The pre-processing unit 204 of the console 200 corrects non-uniform sensitivity between channels with respect to data received from the data collection unit 107, and extremely reduces signal strength due to an X-ray strong absorber, mainly a metal part. Alternatively, preprocessing such as correction of signal dropout is executed. The projection data storage unit 205 stores the projection data subjected to the preprocessing by the preprocessing unit 204. The reconstruction processing unit 206 reconstructs image data based on the stored projection data. The image storage unit 207 stores reconstructed image data. The image processing unit 208 processes the stored image data to generate image data for display. The display unit 209 receives image data for display and displays an image.

  The heart rate monitor 300 is attached to the subject P, for example, on the wrist or fingertip. The heart rate monitor 300 detects the heart rate of the subject P and calculates the heart rate (average heart rate) from the cycle. The heart rate data is supplied to the scan condition determination unit 201. The heart rate may be measured directly by a doctor or the like from the subject P and the value may be input via the operation unit 202.

The scan condition determination unit 201 determines a scan condition based on the heart rate. The scan condition includes an X-ray on period time length T1, an X-ray off period time period T2, an X-ray tube 101 rotation time Trot, and a reconstruction method of 360 degree projection data. Selection of a full reconstruction method to be performed and a half reconstruction method to reconstruct an image from projection data for (180 degrees + fan angle). The time Trecon required to collect data in the angle range required for reconstruction of one image is determined by one rotation time Trot of the X-ray tube 101 and the selected reconstruction method. Here, the heart rate is HR [bpm], one heart cycle is RR [sec] (= 60 / HR), and a period of relatively small body motion without being affected by pulsation within one heart cycle (low body) If the movement period is T3,
T1> = Trec * 2
(T3-T2) / 2> = Trec f (RR)
The scan condition determination unit 201 determines one rotation time Trot, reconstruction method, T1, and T2 of the X-ray tube 101 so as to satisfy the above.

  Typically, the reconstruction method is selected by the operator. The one-rotation time Trot of the X-ray tube 101 is an apparatus-specific value. When a plurality of one-rotation times Trot are prepared and arbitrarily selected, the shortest one-rotation time Trot corresponding to the fastest angular velocity is set. . The time length T1 of the X-ray on period is initially set to a time required for two rotations (2 × Trot). Therefore, basically, the time length T2 of the X-ray off period other than the above is a period in which the body motion is relatively small and not affected by the heart rate in the heart rate and one heartbeat period (the low body motion period). ) Determined from T3. The heart rate is measured or entered as described above. The low body movement period T3 is unknown.

  As shown in FIG. 2, the overlapping period T5 of the X-ray on period T1 and the low body movement period T3 in which projection data related to the subject P can be actually collected is collected in an angular range required for reconstruction of one image. When the reconfigurable condition that the time required for the time Trecon is exceeded is satisfied, the image can be reconstructed. In this embodiment, without using electrocardiogram synchronization, the X-ray on period T1 and the X-ray off period T2 are alternately repeated regardless of the heart rate. When the probability of satisfying the reconfigurable condition is high, for example, when it exceeds 90%, it is possible to eliminate the artifacts from the electrodes because it does not rely on the ECG synchronization method, and to reduce the occurrence of body movement artifacts while suppressing the exposure dose. It can be said that it will be possible. An important parameter for increasing the probability of satisfying the reconfigurable condition is the low body movement period T3.

  The low body movement period T3 is defined as a period in which the motion index is equal to or less than a threshold value. The motion index is given, for example, as a distance from the initial position of a specific part, or as a difference sum or variance of images or projection data separated by a predetermined number of frames before or after (time). In the present embodiment, in order to reduce exposure, the subject P is not examined in advance before the main scan to obtain a motion index. Prior to product shipment, various heart rates were measured and simulations were repeated, and a standard correspondence between a plurality of heart rates and a plurality of low body motion periods T3 (the heart rate of the low body motion period T3) Dependency curve) is obtained, and the data of the correspondence table is stored in the ROM 203. As shown in FIG. 3, the low body motion period T3 when the heart rate is the lowest, for example, 30 bpm, is set to the maximum value Tmax. The low body movement period T3 is naturally shortened according to the increase in heart rate (shortening of the heartbeat cycle), and the low body movement period T3 is less than the time Trecon required for collecting data in the angular range required for reconstruction of one image. Therefore, since the reconfigurable condition is not satisfied, the minimum time of the low body movement period T3 is given as Trecon. Trecon is applied as the low body motion period T3 when a heart rate equal to or higher than the heart rate HRupper at that time is indicated.

  In this embodiment, two types of dependency curves for the heart rate during the low body movement period T3 are prepared and can be arbitrarily selected. On the other hand, the dependence curve for the heart rate during the low body movement period T3 indicated by the solid line has a property that the low body movement period T3 varies in proportion to the fluctuation of the heart rate. The dependence curve for the heart rate during the low body movement period T3 indicated by the other dotted line has a property that the low body movement period T3 exponentially shortens as the heart rate increases. In accordance with the nature of the dependency curve, the probability of satisfying the reconfigurable condition by scanning of “repeating alternately the X-ray on period T1 and the X-ray off period T2 independently of the heartbeat” according to the present embodiment changes.

  In this embodiment, since the X-ray on period T1 and the X-ray off period T2 are alternately repeated without using electrocardiogram synchronization, as shown in FIG. 4, the body motion change due to pulsation, the X-ray on period T1, The time relationship of is not adjustable. That is, the X-ray on period T1 appears variously with different temporal relationships as illustrated in the patterns (a) to (g) with respect to the body motion change due to pulsation. The reconfigurable condition that the overlapping period T5 of the X-ray on period T1 and the low body movement period T3 is equal to or longer than the time Trecon required for data collection in the angular range required for reconstruction of one image is (T1 is a comparison) If the target is short), the patterns (a), (d), and (e) are satisfied. The patterns (b), (c), (f), and (g) can satisfy the reconfigurable condition by selecting the half reconstruction method as the reconstruction method. That is, in most patterns, even after the scan is completed, even if the operator does not satisfy the reconfigurable condition with the full reconstruction method, by selecting the half reconstruction method as the reconstruction method, The reconfigurable condition can be satisfied.

  As described above, in the present embodiment, the X-ray on period T1 in which X-rays are substantially generated and the X-ray off period T2 in which X-rays are not substantially generated without using the electrocardiogram synchronization. Are alternately repeated, and the X-ray ON period T1 and the X-ray OFF period T2 are adjusted according to the scheduled low body movement period T3 according to the heart rate. It is possible to satisfy the reconfigurable condition that the overlapping period T5 with T3 is equal to or longer than the time Trecon required for data collection in the angle range required for reconstructing one image with most patterns.

  The scanning method according to this embodiment can be extended. For example, as shown in FIG. 5, when scanning the chest region, the X-ray off period T1 and the X-ray off which does not substantially generate X-rays are scanned for a scan of a relatively large body motion region including the heart. A mode in which the period T2 is alternately repeated (first mode) is applied. On the other hand, for a scan of a relatively small region of body movement far from the heart, scanning is performed only once in a state where X-rays are constantly generated. The mode of the normal method (second mode) is applied. As described above, the first and second modes are selected in accordance with the body movement state of the imaging region, and the first and second modes are switched in accordance with the state of body movement in each region even during one imaging period. Thus, since it does not depend on the electrocardiogram synchronization method, artifacts from the electrodes can be eliminated, and generation of body motion artifacts due to pulsation can be reduced while suppressing the exposure dose.

  Further, as shown in FIG. 6, when scanning a region including the heart, imaging can be interrupted when necessary data is obtained. For example, when data corresponding to two heartbeats is required (in the case of a relatively high heartbeat), the timing with the smallest movement is found from the heart rate before photographing or time data obtained from a specified value. A small timing of the next movement within a certain range is found from the amount of movement, and when the data necessary for reconstruction from the data is collected, the photographing is interrupted. Similarly, in the case of arrhythmia, the imaging can be interrupted.

  Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

  DESCRIPTION OF SYMBOLS 100 ... Stand part, 101 ... X-ray tube apparatus, 102 ... Rotating ring, 103 ... X-ray detector, 104 ... High voltage generation part, 105 ... Rotation drive part, 106 scan control part, 107 ... Data collection part, 200 ... Console 201: Scan condition determining unit 202 ... Operation unit 203 ... ROM (Heart rate / low body motion period correspondence table) 204 ... Pre-processing unit 205 ... Projection data storage unit 206 ... Reconstruction processing unit 207 ... Image storage unit, 208 ... Image processing unit, 209 ... Display unit, 300 ... Heart rate monitor, 400 ... Bed apparatus,

Claims (5)

An X-ray tube;
A high voltage generator for generating a tube voltage and a tube current for the X-ray tube;
An X-ray detector for detecting X-rays transmitted through the subject;
Controlling the high-pressure generating unit to alternately repeat an X-ray on period in which the X-ray is generated at a predetermined dose and an X-ray off period in which the X-ray is stopped or generated at a dose lower than the predetermined dose A control unit;
An image reconstruction unit that reconstructs image data based on the output of the X-ray detector corresponding to the X-ray on period;
2. The X-ray computed tomography apparatus according to claim 1, wherein a time length of the X-ray off period is determined in advance according to a heart rate of the subject.   2. The time length of the X-ray on period and the number of occurrences of the X-ray in one heart cycle are determined in advance according to the heart rate together with the time length of the X-ray off period. X-ray computed tomography apparatus.   The time length of the X-ray on period, the time length of the X-ray off period, and the number of occurrences of the X-ray are the time required for data collection in the angle range required for reconstruction of one image, as well as the heart rate. The X-ray computed tomography apparatus according to claim 2, wherein the X-ray computed tomography apparatus is determined on the basis of a period of low body movement. The time length of the X-ray on period is T1, the time length of the X-ray off period is T2, the heart rate is HR, one heart cycle is RR (= 60 / HR), and the angle range required for reconstruction of one image If the time required for data collection is Trecon and the low body movement period is T3,
T1> = Trec * 2
(T3-T2) / 2> = Trec
T3 = f (RR)
4. The X-ray computed tomography apparatus according to claim 3, wherein the T1 and the T2 are determined so as to satisfy the condition. An X-ray tube;
A high voltage generator for generating a tube voltage and a tube current for the X-ray tube;
An X-ray detector for detecting X-rays transmitted through the subject;
An image reconstruction unit for reconstructing image data based on the output of the X-ray detector;
A mode in which the X-ray on period in which the X-ray is generated at a predetermined dose and the X-ray off period in which the X-ray is stopped or generated at a dose lower than the predetermined dose are alternately repeated; An X-ray computed tomography apparatus comprising: a control unit that controls the high-pressure generation unit in order to switch a mode to be generated according to a region of the subject.

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