JP5523691B2 - X-ray diagnostic equipment - Google Patents

Description

  The present invention relates to an X-ray diagnostic apparatus capable of low contrast imaging by so-called rotational imaging.

  In low-contrast imaging, for example, a flood image (also referred to as a non-attenuated image) is subtracted from each of a plurality of images (referred to as subject images) having different photographing angles with respect to a subject captured under a tube voltage of 100 kV. This is a technique called CT-like imaging in which a three-dimensional image is reconstructed from a difference image using a reconstruction technique such as an X-ray computed tomography apparatus. In this low contrast imaging, it is necessary to acquire a flood image with the same tube voltage as the subject image.

  The flood image is taken without placing anything between the X-ray tube and the X-ray detector. Therefore, when a flood image is acquired with the same tube voltage as that of the subject image, the dose becomes excessive, halation occurs, and the relationship between the X-ray condition and the image luminance becomes unstable. By taking a flood image at 60 to 80 kV, excessive dose is suppressed, and the relationship between the X-ray condition and the image brightness is stabilized.

  However, if the subject image is taken at 80 kV, for example, in accordance with the flood image, the dose may be insufficient or the image quality may deteriorate due to the effect of beam hardening.

  An object of the present invention is to solve both the excessive dose of a flood image and the insufficient dose of a subject image.

  In one aspect, the present invention provides a substantially C-arm, a support mechanism that rotatably supports the substantially C-arm, a rotation drive unit that drives rotation of the substantially C-arm, and the substantially C-arm. An X-ray tube to be mounted; an X-ray detector mounted on the substantially C-shaped arm facing the X-ray tube; and the X-ray tube and the X-ray detector under the first tube voltage. A storage unit for storing data of at least one frame of the flood image taken in Step 1, and an image of the subject taken by the X-ray tube and the X-ray detector under a second tube voltage higher than the first tube voltage. An X-ray diagnostic apparatus comprising: an image reconstruction unit that reconstructs a three-dimensional image by subtracting the flood image from each of a plurality of images having different angles.

  According to the present invention, it is possible to solve both the excessive dose of the flood image and the insufficient dose of the subject image.

Hereinafter, an X-ray diagnostic apparatus according to the present invention will be described with reference to the drawings according to preferred embodiments.
As shown in FIG. 1, the X-ray diagnostic apparatus has an X-ray imaging mechanism 10. As shown in FIG. 2, the X-ray imaging mechanism 10 includes an X-ray tube 12 and an X-ray detector 14. The X-ray detector 14 includes an image intensifier 15 and a TV camera 16. Alternatively, the detector 14 may be configured by a flat panel detector (FPD: planar X-ray detector) having semiconductor detection elements arranged in a matrix. The X-ray tube 12 and the X-ray detector 14 are mounted on the C-arm 60 so as to face each other. The subject P on the couch top 50 is disposed between the X-ray tube 12 and the detector 14. The C-arm 60 is supported by a column 64 suspended from the ceiling base 63 or a floor stand. The C-shaped arm 60 is rotatable about three orthogonal axes A, B, and C. The rotation drive unit 22 is accommodated inside the support column 64. The rotation drive unit 22 has two power sources for individually rotating the C-arm 60 in the directions of arrows A and B. The C-arm 60 can be rotated like a propeller at high speed by the rotation drive unit 22.

  As described above, while rotating the C-shaped arm 60 like a propeller at high speed with the rotation drive unit 22, imaging is repeated at intervals of, for example, 0.5 degrees while changing the projection angle, and the obtained rotation angle is, for example, 200 degrees. 400 patterns of X-ray intensity distributions (X-ray images) are collected. The X-ray image signal is converted into a digital signal by an analog / digital converter (A / D converter) in the camera controller 21. The image memory 25 is provided to mainly store data related to a plurality of X-ray images of the subject.

  The flood image storage unit 19 is provided for storing at least one frame of flood image data. The flood image is taken at the time of shipment, regular inspection, or immediately before the subject is taken. A flood image is typically taken as a step in calibration that is performed periodically. As shown in FIG. 4, the flood image is taken with only a 1.8 mm thick aluminum quality filter interposed between the X-ray tube 12 and the X-ray detector 14. The imaging condition of the flood image is set to a predetermined tube voltage, for example, 80 kV regardless of the tube voltage set in the imaging condition of the subject. The pulse width of the X-ray is recorded after being converted into α × Ymsec obtained by multiplying the pulse width Ymsec set in the flood image capturing condition by a conversion coefficient α. Furthermore, although the photographing tube voltage of the flood image is set to 80 kV, it is treated as 100 kV for recording. As will be described later, the conversion coefficient α is 0.62 when a flood image captured at a tube voltage of 80 kV is converted into a flood image recorded as captured at a tube voltage of 100 kV. The conversion coefficient α = 0.62 converts a flood image taken at 80 kV so as to be equivalent to a flood image taken at 100 kV, and is equivalent to other flood images taken at 90 kV, 110 kV, etc. In the case of conversion as described above, conversion coefficients corresponding to the respective conversion coefficients are determined and stored in advance. A plurality of conversion coefficients corresponding to different tube voltages are selectively used in the flood image conversion unit 26 according to the tube voltage of the imaging condition of the subject. Here, the pulse width Ymsec is multiplied by the conversion coefficient α. However, the present invention is not limited to this. Instead, either the tube current XmA or the detector gain Z may be multiplied.

  Note that there are the following methods for correcting a flood image so as to be substantially equivalent to that obtained by photographing under the second tube voltage. 1) A method of converting a flood image by applying a conversion coefficient α to each pixel value of the flood image, and converting a collection tube voltage of the flood image into a second tube voltage. 2) A method in which nothing is performed on the flood image itself, and conversion is performed by applying α to the X-ray pulse width, and the collected tube voltage of the flood image is converted to the second tube voltage. That is, if the X-ray pulse width is Ymsec, the pulse width registered in the flood image is changed to Yαmsec. 3) A method in which α is applied to the tube current without doing anything to the flood image itself, and the collected tube voltage of the flood image is converted to the second tube voltage. That is, if the tube current is XmA, the tube current registered in the flood image is changed to XαmA. 4) A method in which the flood image itself is converted to a second tube voltage by performing a conversion by multiplying the detector gain by α without doing anything. That is, if the gain of the detector is Z, the gain of the detector registered in the flood image is changed to Zα. Any one of the above conversions 1) to 4) can be corrected. However, in the first method 1), since the entire image has to be converted, it is preferable to use one of the following three methods 2) to 4).

  The subtraction unit 27 considers the difference in X-ray conditions for each of a plurality of X-ray images having different imaging angles with respect to a subject actually captured at 100 kV from an apparent 100 kV flood image converted from an 80 kV flood image. While making a difference. The three-dimensional reconstruction unit 34 reconstructs a three-dimensional image (CT-like image) based on a plurality of difference images. The three-dimensional image processing unit 35 converts the reconstructed first three-dimensional image into a three-dimensional image by a method such as volume rendering. As an example of the reconstruction method, here is the case of the filtered back projection method proposed by Feldkamp et al. An appropriate convolution filter such as Shepp & Logan or Ramachandran is applied to a 400 frame difference image. Call. Next, reconstruction data is obtained by performing a back projection operation. Here, the reconstruction area is defined as a cylinder inscribed in the X-ray bundle in all directions of the X-ray tube 12. The inside of this cylinder is discretized three-dimensionally with a length d at the center of the reconstruction area projected onto the width of one detection element of the detector 14, for example, and it is necessary to obtain a reconstruction image of discrete point data. . However, although an example of the discrete interval is shown here, since there are various methods, basically the discrete interval defined by the apparatus may be used. Three-dimensional image data is displayed on a display unit 37 via an analog-digital converter (D / A) 36.

  FIG. 3 shows a procedure for determining the conversion coefficient α. Here, an example of a procedure for determining the conversion coefficient α for converting an 80 kV flood image into an apparent 100 kV flood image is shown. The conversion coefficient α is determined at the time of shipment or periodic inspection. First, the tube voltage is set to 100 kV (S11), and only a 1.8 mm thick aluminum quality filter is interposed between the X-ray tube 12 and the X-ray detector 14 as shown in FIG. The dose is repeatedly detected while changing the mAs value (S12). Here, the mAs value is represented by the product of the tube current value (mA) and the X-ray pulse width (sec). A plurality of dose values Dose are distributed as shown in FIG. 5 in relation to a plurality of mAs values (S13). The dose / mAs value relationship is approximated by a linear equation, and its slope A is determined (S14).

  Next, the tube voltage is set to 80 kV (S15), and only a 1.8 mm thick aluminum quality filter is interposed between the X-ray tube 12 and the X-ray detector 14 in the same manner as S12. The dose is repeated while changing the mAs value (S16). A plurality of dose values Dose are distributed as shown in FIG. 6 in relation to a plurality of mAs values (S17). The dose / mAs value relationship is approximated by a linear equation, and its slope B is determined (S18).

The conversion coefficient α is obtained by B / A.
Here, the presence or absence of dependence of the conversion coefficient α on the visual field diameter FOV and the source detector distance SID is verified. The FOV is switched between the normal option of 16 inches and 12 inches. At the same time, the SID is switched between the normal options of 120 cm and 110 cm. FIG. 7 shows the result of the conversion coefficient α obtained by the flowchart of FIG. 3 for each condition. Basically, there is no dependency of the conversion coefficient α on the field-of-view diameter FOV and the source-source-detector distance SID, and they can be used in common.

  FIG. 8 schematically shows a procedure of low contrast imaging (CT-like imaging). First, at the time of calibration, a flood image is taken at a tube voltage of 80 kV with only a 1.8 mm thick aluminum quality filter interposed between the X-ray tube 12 and the X-ray detector 14. The flood image conversion unit 26 converts the pulse width Ymsec set in the shooting conditions of the flood image into α × Ymsec multiplied by the conversion coefficient α, and records it. Further, the tube voltage is converted to 100 kV and recorded.

  As a result, the flood image photographed at 80 kV is converted into a flood image photographed at 100 kV, which is the same as the tube voltage under the photographing condition of the subject. The data of an apparent 100 kV flood image is stored in the flood image storage unit 28.

  Next, the subject is arranged between the X-ray tube 12 and the X-ray detector 14 together with a 1.8 mm thick aluminum quality filter, and a plurality of X-ray images related to the subject are set at a set tube voltage of 100 kV. Images are taken under a so-called rotational shooting procedure. For example, 400 X-ray images whose imaging angles differ by 1 degree are captured.

  A plurality of X-ray images related to the subject are each subtracted from an apparent 100 kV flood image and the subtraction unit 27 while taking into account the difference in X-ray conditions. The three-dimensional reconstruction unit 34 reconstructs a three-dimensional image (CT-like image) based on the plurality of difference images. In this embodiment, the imaging tube voltage of the subject is fixed at 100 kV, but the present invention is not limited to fixed kV, and may be variable between 90 kV and 120 kV, for example. In that case, the collection tube voltage of the flood image, for example, the conversion coefficient for 90 kV to 120 kV for 80 kV is stored in the conversion coefficient storage unit 28, and the conversion for the pulse width Ymsec is not applied to the flood image. When subtracting the X-ray image and the flood image, the calculation is performed in consideration of the contribution of the conversion coefficient α in the calculation of the difference in the X-ray conditions.

  According to the present embodiment, it is possible to solve both the excessive dose of the flood image and the insufficient dose of the subject image.

  In this embodiment, a flood image is taken at 80 kV, and a subject is taken at a higher kV such as 100 kV. Although it is known that shadows generally fade when photographing at high kV, the average X-ray energy transmitted through the subject is more effective than the average energy of X-rays output from a 100 kV X-ray tube. May be even higher energy. As a result, the influence of the shadow of the non-uniform structure such as the scattered radiation removal grid becomes more difficult, and if the subject image and the blood image are subtracted as they are, the influence of the shadow may remain as a residual. Therefore, the flood image is converted so as to reduce the influence of the shadow. One method of conversion is low-pass filtering, and the other method is to fit a luminance pattern of a flood image with a quadratic function or higher order function, and an attenuation coefficient of less than 1 for the residual with the fitting. This is a method of adding the fitting images again after applying.

  The present invention is not limited to the above-described embodiments as they are, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. In addition, various inventions can be formed by appropriately combining a plurality of components disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different embodiments may be appropriately combined.

The block diagram of the X-ray diagnostic apparatus which concerns on this embodiment. FIG. 2 is an external view of the X-ray imaging mechanism of FIG. 1. The figure which shows the determination procedure of conversion coefficient (alpha) for converting the 80 kV flood image into a 100 kV flood image in this embodiment. The figure which shows the imaging | photography state of process S12, S16 of FIG. FIG. 4 is a supplementary diagram of steps S13 and S14 in FIG. 3. FIG. 4 is a supplementary diagram of steps S17 and S18 in FIG. 3. In this embodiment, the figure which shows the evaluation result of the dependence of the conversion factor (alpha) with respect to visual field diameter FOV and the distance SID between radiation source detectors. The figure which shows the process sequence using the conversion factor (alpha) in actual rotation imaging | photography in this embodiment.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... X-ray imaging mechanism, 12 ... X-ray tube, 14 ... X-ray detector, 19 ... Flood image storage part, 20 ... System controller, 21 ... Camera controller, 22 ... Rotation drive part, 23 ... Rotation controller, 25 ... Image memory, 26 ... Flood image conversion unit, 27 ... Subtraction unit, 28 ... Conversion coefficient storage unit, 34 ... Three-dimensional reconstruction unit, 35 ... Three-dimensional image processing unit, 36 ... D / A conversion unit, 37 ... Display unit 60 ... C-arm.

Claims (6)

A substantially C-shaped arm;
A support mechanism for rotatably supporting the substantially C-shaped arm;
A rotation drive unit for driving rotation of the substantially C-arm;
An X-ray tube mounted on the substantially C-shaped arm;
An X-ray detector mounted on the substantially C-shaped arm in a direction facing the X-ray tube;
A storage unit for storing data of a flood image of at least one frame imaged under the first tube voltage by the X-ray tube and the X-ray detector;
A correction unit that corrects at least one of the brightness of the flood image and the X-ray condition on recording of the flood image so as to be substantially equivalent to the image taken under the second tube voltage higher than the first tube voltage. When,
A three-dimensional image again by difference the corrected flood image from a plurality of images each having different imaging angles about a subject taken under the previous SL second tube voltage in the X-ray tube and the X-ray detector An X-ray diagnostic apparatus comprising: an image reconstruction unit configured. The correction unit multiplies a correction coefficient α determined in advance so as to be substantially equivalent to a photograph taken under a second tube voltage higher than the first tube voltage , and records the flood image. X-ray diagnostic apparatus according to claim 1, wherein the collection tube voltage and converting the second tube voltage. The correction unit uses a correction coefficient α that is determined in advance so as to be substantially equivalent to an image captured under a second tube voltage higher than the first tube voltage , which is any of a tube current , an X-ray pulse width, and a gain. The X-ray diagnostic apparatus according to claim 1 , wherein the X-ray diagnostic apparatus multiplies one by one and converts the collected tube voltage on the flood image recording into the second tube voltage. The second tube voltage can be changed according to the state of the subject, and a predetermined correction coefficient for correcting the flood image is stored for each value of the second tube voltage so as to be substantially equivalent. When the difference between the flood images is obtained from each of a plurality of images having different imaging angles with respect to the subject imaged under the second tube voltage, a correction coefficient is entered for the difference in the X-ray conditions. The X-ray diagnostic apparatus according to claim 1, wherein correction is performed. Wherein the correction unit, X-rays diagnostic apparatus according to any one of claims 1 to 4, characterized by applying low-pass filtering to the flood image. The correction unit fits the flood image with an image luminance distribution using a quadratic function or a higher order function, and multiplies the fitting image again after applying an attenuation coefficient of less than 1 to the fitting residual. The X-ray diagnostic apparatus according to any one of claims 2 to 5 , wherein:

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