JP4908957B2 - X-ray CT system - Google Patents

Description

  The present invention relates to an X-ray CT apparatus that includes air correction in projection data correction processing, and in particular, performs air correction related to X-ray detector sensitivity including output fluctuations due to the tilt angle and rotation speed of the scanner. The present invention relates to an X-ray CT apparatus that obtains an image with little CT value deviation and artifacts depending on tilt angle and rotational speed.

The X-ray CT apparatus has an X-ray tube and an X-ray detector facing each other so as to sandwich the subject (subject), and rotates around the subject while maintaining the positional relationship between the X-ray tube and the X-ray detector. X-rays are exposed to the subject. This is an apparatus capable of measuring X-rays transmitted through a subject a plurality of times in the angle direction and obtaining a cross-sectional image of the subject from a plurality of measured projection data. Particularly in the medical field, it is widely used for diagnosis by easily obtaining a cross-sectional image of the human body.
There exist patent document 1, 2 in a prior art example.
JP-A-9-248301 JP 2003-61948 A

  Patent Document 1 is an example in which correction data is obtained by performing a calibration scan. Correction data is obtained by making the rotation speed of the scanner coincide with the actual rotation speed, and this is used for sensitivity correction. However, there is no description of performing a calibration scan with the tilt angle as one of the parameters.

  Patent Document 2 relates to correction of movement for each detector by the tilt angle, and causes the collected projection data to be corrected for movement. Therefore, there is no description about the collection and accumulation of air correction data based on the tilt angle and its use.

Among various corrections used in the X-ray CT apparatus, there is air correction for correcting variations in sensitivity characteristics of the X-ray detection elements. The flow of air correction will be described with reference to FIG. Before performing the main measurement A for photographing the subject, air is measured in a state B where the subject is not in the scanner. This measurement is called air measurement, and the projection data collected by the data collection circuit 1 is subjected to various corrections 3 (offset correction, logarithmic conversion, etc.) in the processing circuit 2 and stored in the memory 4 as air correction data. deep. The projection data obtained in the main measurement A is subjected to the above various corrections, and then subjected to air correction 5 for subtracting the stored air correction data. After the air correction, various corrections 6 are performed, and the image reconstruction unit 7 performs image reconstruction processing. In order to rotate the scanner during air measurement, output fluctuation due to fluctuation during rotation of the X-ray beam position is included in the measurement data. The following points can be cited as factors that cause the X-ray beam position to fluctuate during measurement.
(1) Effects of centrifugal force and gravity on the anode axis inside the X-ray tube depending on the rotation speed of the scanner and the position of the X-ray tube (2) Expansion and contraction due to heat in the direction of the anode axis due to continuous exposure and exposure suspension (2 ), A method is used in which a beam position is always detected using a detection means such as an X-ray detector, and an X-ray tube or a collimator is moved so as to be a normal position.

Since only the X-ray beam position deviation in the body axis direction is adjusted according to the above method, the obtained air correction data includes output fluctuations due to the X-ray beam position deviation in the channel direction and the centrifugal force direction. FIG. 3 shows an example of data obtained by measuring air in an arbitrary n channel among a plurality of X-ray detector elements. The horizontal axis represents the measurement angle θ (tube position at the time of measurement), and the vertical axis represents the projection data. The dotted curve is the scanner rotation speed S _i , the air measurement data at the tilt angle A _i (Air1 (θ, S _i , A _i )), and the alternate long and short dash line curve is the measurement condition (scanner rotation speed S _j , This is air measurement data (Air2 (θ, S _j , A _j )) measured at a tilt angle A _j ). The curve in the figure is an example, and there are various forms. As described above, the fluctuation of data during one rotation is different between the two, and if the image is reconstructed by performing air correction in all channels, there is a possibility that a CT value shift or an artifact occurs.

The cause of the difference between the two data fluctuations will be described with reference to FIG. When the air correction data Air1 (θ, S _i , A _i ) is measured under the conditions of the scanner rotation speed S _i and the tilt angle A _{i in} the upper diagram (a) of FIG. 4, the lower diagram (b) of FIG. Suppose that Air2 (θ, S _j , A _j ) is measured under conditions of speed S _j and tilt angle A _j , and the direction of gravity and centrifugal force applied to the anode depending on the position of each X-ray tube is shown from the front and side ing. When the scanner is tilted at (A _j -A _i ) ° as compared to the upper diagram (a) of FIG. 4 as shown in the lower diagram (b) of FIG. 4, the direction of gravity applied to the X-ray tube changes, The X-ray beam position fluctuation (output fluctuation) differs between the upper and lower figures in FIG. Further, when the rotation speed of the scanner is faster or slower than the upper diagram (a) in FIG. 4, the magnitude of the centrifugal force applied to the anode changes, and the X-ray beam position fluctuation (output fluctuation) during rotation changes in the vertical direction of FIG. Different in the figure.

Despite these differences, in air correction, air correction data measured with reference scanner rotation speed and tilt angle conditions (eg, S _i , A _i ) is used for all scanner rotation speeds. It is used for the actual measurement data of tilt angle. Therefore, if the rotational speed and tilt angle of the scanner are different between the air measurement and the main measurement, a difference in output fluctuation occurs, which may cause a CT value shift and an artifact. This is a phenomenon that occurs not only when air is measured but also when a subject is measured. In order to correct the fluctuation difference, it is only necessary to have air correction data performed under the conditions of the required scanner rotation speed and tilt angle. However, since the air measurement is performed as a preparation before the main measurement on a daily basis, for example, once a day, an increase in the number of air measurements limits the usage time of the apparatus for the user.

  The present invention has been made in view of such a problem, and provides an X-ray CT apparatus that can obtain a reconstructed image with little CT value deviation and artifacts regardless of the rotation measure and tilt angle of the scanner. Objective.

The present invention provides an X-ray tube that exposes X-rays to a subject;
An X-ray detector that sandwiches the subject and is opposed to the X-ray tube and detects transmitted X-rays of the subject;
A scanner that mounts the X-ray tube and the X-ray detector and rotates each around the subject;
Data collection means for collecting transmitted X-rays detected by the X-ray detector as projection data;
Correction means for performing preprocessing correction including air correction on the projection data collected by the data collection means;
In an X-ray CT apparatus comprising: a reconstruction processing unit that reconstructs a tomographic image of the subject from the projection data corrected by the correction unit;
The correction means includes storage means for storing air correction data obtained for each parameter (measurement angle, settable rotational speed of the X-ray scanner, settable tilt angle of the X-ray scanner) in association with the parameters; Air correction read from the storage means corresponding to actual measurement parameters (measurement angle, set X-ray scanner rotation speed, set tilt angle) for object projection data obtained by actual object measurement An X-ray CT apparatus including an air correction unit that performs air correction based on data is disclosed.

  Further, according to the present invention, the air correction data is obtained by using other parameters (measurement angle, rotation speed, tilt angle) obtained based on air measurement data obtained with reference parameters (measurement angle, reference rotation speed, reference tilt angle). An X-ray CT apparatus having a relative value to the air measurement data is disclosed.

  Further, according to the present invention, when the air correction data is a tilt angle which is one of the parameters, a value obtained by measurement by an intermittent part of the settable tilt angles, and other tilt angles. An X-ray CT apparatus is disclosed that is calculated by the value obtained by interpolation of the value obtained by this measurement.

  According to the present invention, it is possible to provide a reconstructed image with less CT value deviation and artifacts for all conditions by having air correction data suitable for each rotation speed and tilt angle of the scanner.

  The tilt angle is an inclination angle of the scanner (gantry), and various angle settings can be set continuously or discretely by a tilt angle driving mechanism. The angle to be set is determined by an imaging purpose, an imaging method, a position / shape and structure of an imaging part, a part that is difficult to irradiate X-rays, or a part hidden behind another part. Therefore, the tilt angle is a very important control parameter for the X-ray CT apparatus.

  An important parameter along with the tilt angle for the scanner is the rotational speed. Recently, various rotational speeds such as high speed, medium speed, and low speed can be set by a rotational speed driving mechanism.

  The present invention measures air projection data at any time while paying attention to the three parameters of the measurement angle (0 ° to 360 °, 0 ° to 180 °, etc.) together with the tilt angle and rotation speed. In addition, in order to cope with the change in the air correction data over time, the air photographing data is obtained on a daily basis, and the correction data is updated over time.

Here, the change of the air correction data over time will be described.
Scanners and gantry weigh as much as several hundred kilograms to several tons, operate many times a day, and take pictures while frequently changing the rotation speed and tilt angle. Although the rotation speed and tilt angle are controlled by the drive mechanism, the mechanical burden is heavy due to the weight of the scanner and gantry. In addition, because the processing and control by the computer are accurately performed, the set rotational speed and tilt angle can be finally set even if the drive mechanism itself has some aging. Under such circumstances, the air correction data changes with time, which corresponds to changes with time in the rotation speed and tilt angle.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
FIG. 5 shows a basic structure of a general X-ray CT apparatus. Projection data is collected from an X-ray tube 400 that irradiates X-rays, a collimator 401 that determines the beam width of the X-rays, an X-ray detector 402 that detects X-rays and converts them into electrical signals, and an X-ray detector 402 Data acquisition circuit 403, X-ray tube 400, collimator 401, X-ray detector 402, scanner 404 equipped with data acquisition circuit 403, scanner tilt mechanism 405 for tilting the scanner 404 back and forth, bed 406 for placing a subject, data A projection data correction processing device 407 that performs correction processing of a signal (projection data) from the acquisition circuit 403, an image reconstruction processing device 408 that reconstructs a cross-sectional image from the projection data subjected to the correction processing, a projection data correction processing device 407, An image processing device 409 equipped with an image reconstruction processing device 408, a display device 410 that displays the results of image processing, an input device 411 that starts imaging, sets parameters, and inputs, an X-ray tube 400, a collimator 401, and X-ray detection 402 Gosuru controller 412, the beam position becomes a beam position adjustment control and drive unit 413 for moving the X-ray tube 400 and collimator 401 to become a regular position.

FIG. 1 shows a processing flow when the air correction data acquisition and the accompanying update processing are repeatedly performed in the X-ray CT apparatus configured as described above. The scanner rotation speed and tilt angle conditions used as the parameters for air measurement are S ₁ and A _1, and all the settable rotation speeds and all settable tilt angle conditions for other scanners are summarized. S _m and A _m (m = 2, 3,...). In FIG. 1, 500 is an unusual air measurement and processing flow, 510 is an everyday air measurement and processing flow, and 520 is a main measurement and processing flow for photographing a subject.

First, 500 measurements and processing will be described.
[S501] The S ₁ and A _1, S _m and A Air Measurement 2 condition of _m shown in, for example, at the time of pre-shipment inspection is performed in a non-routine timing such time of installation or during maintenance of the apparatus. Here, the timing of performing the air measurement 2 is not limited to the above. The data obtained by the air measurement 2 is subjected to correction 1 in [S502] and stored as air correction data 2 in [S503].

[S504] S ₁ a rotational speed of the scanner, the tilt angle with respect to the air correction data A _1, and calculates a conversion value to convert all of the air correction data measured by the air measurement 2, and stores. The conversion of S ₁ and A ₁ itself is also performed, but the conversion value is naturally zero. This calculation method will be described later.

Next, 510 measurement and processing will be described.
As shown in [S511] of 510, the scanner rotation speed is S ₁ 'and the tilt angle is A ₁ ' (S ₁ '= S ₁ , A ₁ ' = A ₁ However, since the measurement timing is different, the symbols are divided for explanation), air correction 1 is performed, correction 1 is performed in [S512], and air correction data 1 is stored in [S513]. Using the conversion value stored in [S504], the air correction data 1 stored in [S514] is converted into air correction data for the required scanner rotation speed and tilt angle and stored. The conversion method will be described later. Here, “daily” means that air measurement is performed in the morning and noon of CT operation days excluding holidays or every 3 hours of CT operation days.

Next, the measurement and processing of 520 will be described.
The main measurement data measured in [S521] is corrected 1 in [S522], and then in [S523], air correction data corresponding to the rotation speed and tilt angle conditions of the main measurement is read and air correction is performed. . After air correction, correction 2 is performed in [S524], and then an image is created by the image reconstruction process in [S525].

  As shown above, the air measurement (510) of the reference condition is performed on a daily basis, and the air measurement (500) for calculating the conversion value is performed on an irregular basis. It is possible to have air correction data for the rotation speed and tilt angle of the scanner.

Next, the air correction data conversion value calculation means and conversion means described above will be described with reference to FIG. First, the conversion value calculation means ([S504]) will be described. FIG. 6A is an example of air correction data measured by the air measurement 2. Air correction data _{(Air1 (θ, S 1,} A 1)) of the dotted curve the rotational speed and the tilt angle of the scanner S _1, A ₁ is the rotational speed and tilting angle curve scanners dashed line is S _m, air correction data _{a m (Air2 (θ, S} m, a a _m), which shows the data fluctuation during one rotation of the n-channel, respectively. the following first difference value between the two at each measurement angle theta It calculates with the numerical formula 1 of these.

[Formula 1] Sub (θ, S _m -S ₁ , A _m -A ₁ ) = Air2 (θ, S _m , A _m ) -Air1 (θ, S ₁ , A ₁ )
(Fig. 6 (b))

Sub (θ, S _m −S ₁ , A _m −A ₁ ) calculated in this way is stored as a conversion value. The above conversion value calculation means is performed for the measured rotation speed and tilt angle of the scanner.

Next, the conversion means ([S514]) will be described. Air correction data 1 by routine air Measurement 1 (Air1 '(θ, S 1', A 1 ')) is updated. As shown in Equation 2 below, the updated air correction data 1 (Air1 '(θ, S 1', A 1 ')) is converted by adding the conversion value calculated by Equation 1 at each measurement angle The

[Equation 2] Air2 '(θ, S _m ', A _m ') = Air1' (θ, S ₁ ', A ₁ ') + Sub (θ, S _m -S ₁ , A _m -A ₁ )
(Fig. 6 (c))
Air2 ′ (θ, S _m ′, A _m ′) obtained by Equation 2 is stored and used for air correction in this measurement. The above conversion means is performed for the necessary rotation speed and tilt angle of the scanner.
In the above description, the n channels of the plurality of X-ray detector elements are described, but the same processing is performed for all channels. Further, since the number of data points is large when handled for all measurement angles, the number of data points may be reduced when the capacity of the storage device is large.

  The rotational speed S of the scanner of the X-ray CT apparatus varies depending on the apparatus, but is usually limited to several types. On the other hand, it is assumed that there is an X-ray CT apparatus capable of tilting ± 30 ° from an upright state, for example, although the tilt angle A of the scanner varies depending on the apparatus. Since the rotation speed S of the scanner is limited to several types, it is not a problem to measure all. However, assuming that ± 30 ° tilt is possible and the tilt pitch is 1 ° as described above, there are 61 ways to measure all. Naturally in everyday cases, and even if it is an extraordinary measurement, this would result in a huge number of measurements. Therefore, the following means are shown for the purpose of reducing the number of measurements related to the tilt angle of the air measurement 2.

As a first means, for example, in the apparatus capable of tilting ± 30 °, air measurement is performed at a pitch of 10 ° in the ± direction across 0 ° as shown in FIG. Sub for each tilt angle is calculated by Equation 1, and the calculated Sub is applied as a Sub value of measured tilt angle ± 5 °. As an example, the case of A ₁ = 0 and A ₂ = + 20 is shown. From Equation 1

[Formula 3] Sub (θ, S _m -S ₁ , 20-0) = Air2 (θ, S _m , + 20) -Air1 (θ, S ₁ , 0)
Sub (θ, S _m -S ₁ , 20-0) calculated by Expression 3 is treated as a Sub value of + 15 ° to + 25 °.

As a second means, there is a case of calculating a difference as a relative value by interpolation or the like. For example, the measurement is similarly performed at a pitch of 10 °, the Sub for each tilt angle is calculated according to Formula 1, and the calculated Sub (θ, S _m −S ₁ , A _m −A ₁ ) is measured with respect to the tilt angle at each measurement angle θ. A polynomial approximation formula of the difference value Sub is calculated. As an example, the case of A ₁ = 0, A ₂ = ± 10, ± 20 is shown. Five difference values from Sub (θ, S _m −S ₁ , 20-0) to Sub (θ, S _m −S ₁ , −20-0) are calculated by Equation ₁ . An Nth-order polynomial approximate expression App (A) such as Expression 4 is calculated using five Sub values at each measurement angle θ.

[Equation 4] App (A) = C _N A ^N + C _N-1 A ^N-1 +… + C ₁ A + C ₀
The polynomial approximate expression of Formula 4 is calculated at all measurement angles θ, and the Sub value between the measured tilt pitches is calculated from the polynomial approximate expression.

  The difference between the air projection data with the reference parameter and the air projection data with the other parameters is used as the correction data. However, there is an example in which the ratio or average value of both is used as the correction data.

  With the above two means, it is possible to reduce the number of measurements of the air measurement 2 with respect to the tilt angle of the scanner. The possible tilt angle and the tilt angle pitch to be measured are not limited to the above.

It is a flowchart as an Example of this invention. It is a flowchart of a general correction process. It is the schematic which showed the difference of the fluctuation | variation of the air correction data by measurement conditions. It is the schematic which showed the difference of the direction of gravity and the centrifugal force concerning an X-ray tube by measurement conditions. It is the block diagram which showed the basic composition of the X-ray CT apparatus concerning one Embodiment of this invention. It is the schematic which showed an example of the conversion value calculation and conversion means concerning this invention. It is the schematic which showed an example of the air measurement number reduction with respect to the tilt angle concerning this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100 X-ray tube 101 Collimator 102 X-ray detector 102 Data acquisition circuit 103 Scanner 105 Bed 106 Pre-processing 107 Reconstructed image processing 108 Image processing apparatus 109 Display apparatus 110 Input apparatus 111 Control apparatus 112 X-ray tube and collimator drive mechanism 113 Scanner tilt mechanism 500 Extraordinary processing 501 Daily processing

Claims (3)

An X-ray tube that exposes X-rays to the subject;
An X-ray detector that sandwiches the subject and is opposed to the X-ray tube and detects transmitted X-rays of the subject;
A scanner that mounts the X-ray tube and the X-ray detector and rotates each around the subject;
Data collection means for collecting transmitted X-rays detected by the X-ray detector as projection data;
Correction means for performing preprocessing correction including air correction on the projection data collected by the data collection means;
In an X-ray CT apparatus comprising: a reconstruction processing unit that reconstructs a tomographic image of the subject from the projection data corrected by the correction unit;
The correction means is
Various converted values corresponding to the rotational speed S, tilt angle A, and measurement angle θ obtained by air measurement at extraordinary times including inspection or installation or maintenance before shipment of the X-ray CT apparatus (here, The conversion value includes measurement data of the reference rotation speed S ₁ and tilt angle A ₁ and measurement data of all other rotation speeds S _m and tilt angles A _m (m = 2, 3,...). A first memory for storing a difference value for each measurement angle),
About various air measurement data of rotational speed S, tilt angle A, and measurement angle θ obtained by daily air measurement including the working day of the X-ray CT apparatus, conversion values stored in the first memory are A second memory that stores the same rotation speed, tilt angle, and corresponding measurement angle as air correction data;
For the measurement data of the rotation speed S, tilt angle A, and measurement angle θ of the main measurement obtained in the main measurement with the X-ray CT apparatus, the same rotation speed and tilt angle corresponding to the second memory are obtained. Air correction means for performing air correction based on air correction data of the measurement angle;
X-ray CT apparatus provided with The air correction data corresponds to a tilt angle close to the tilt angle set at the time of actual subject measurement from the values obtained by measurement using a part of the tilt angles that can be set. The X-ray CT apparatus according to claim 1, wherein a value obtained by the measurement is applied. For the tilt angle, the air correction data is obtained by interpolation between the values obtained by measurement using a part of the settable tilt angles and the values obtained by this measurement for other tilt angles. The X-ray CT apparatus according to claim 1, wherein the X-ray CT apparatus is calculated by:

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