Patent Application
20050031088
(1) Field of the Invention
This invention relates to a radiographic apparatus for medical or industrial use and a radiation detection signal processing method, for obtaining radiographic images based on radiation detection signals fetched at predetermined sampling time intervals by a signal sampling device from a radiation detecting device as radiation is emitted from a radiation emitting device. More particularly, the invention relates to a technique for improving an image quality vulnerable to impairment of DSA (subtraction process) images due to time lags occurring with the radiation detecting device.
(2) Description of the Related Art
Conventionally, a type of radiographic apparatus is designed for use in digital subtraction angiography (DSA) to observe the conditions of blood vessels of a patient. This apparatus is operable to perform X-ray radiography of a predetermined site of the patient before injection of a contrast medium, and then radiograph the same site of the patient after injection of the contrast medium. An X-ray image (i.e. a live image) of the patient with the contrast medium injected is an image clearly visualizing a blood vessel. From this X-ray image an X-ray image (i.e. a mask image) obtained before injection of the contrast medium and not showing the blood vessel definitely is subtracted, to obtain a subtraction image enhancing only the blood vessel. While the subtraction process is a deducting operation, an arithmetic mean may be determined of mask images obtained through a plurality of radiographic operations, or a weighted arithmetic mean may be determined of live images obtained continually, in order to improve the signal to noise ratio, as disclosed in Japanese Unexamined Patent Publication No. 2000-41973.
However, where a flat panel X-ray detector (hereinafter called “FPD” as appropriate) having numerous X-ray detecting elements arranged longitudinally and transversely on an X-ray detecting surface is used as a radiation detector (radiation detecting device) for detecting such images, time delays of the FPD could cause after-images. Thus, a problem of after-images arises unless lag-behind parts are fully eliminated.
This invention has been made having regard to the state of the art noted above, and its object is to provide a radiographic apparatus and a radiation detection signal processing method for fully eliminating time lags, due to a radiation detecting device, of radiation detection signals taken from the radiation detecting device, thereby obtaining a subtraction image with high accuracy.
To fulfill the above object, Inventors have noted that after-images and the like due to time delays of the FPD correspond to lag-behind parts included in radiation detection signals taken at sampling time intervals. The following technique is conceivable to remove such lag-behind parts. In dealing with the time lags of the FPD, this technique removes a lag-behind part due to an impulse response based on the following recursive equations A-C:
Xk=Yk−Σn=1N{αn·[1−exp(Tn)]·exp(Tn)·Snk} A
Tn=−Δt/τn B
Snk=Xk−1+exp(Tn)·Sn(k−1) C
where Δt: the sampling time interval;
In the above recursive computation, coefficients of the impulse response of the FPD, N, αn and τn, are determined in advance. With the coefficients fixed, X-ray detection signal Yk is applied to equations A-C, thereby obtaining a lag-free X-ray detection signal Xk.
A specific example of the above technique will be described with reference to
As shown in
Based on the above findings, this invention provides a radiographic apparatus having a radiation emitting device for emitting radiation toward an object under examination, a radiation detecting device for detecting radiation transmitted through the object under examination, and a signal sampling device for taking radiation detection signals from the radiation detecting device at predetermined sampling time intervals, to obtain a live image and a mask image based on the radiation detection signals outputted from the radiation detecting device at the predetermined sampling time intervals as radiation is emitted to the object under examination, the live image and the mask image being subjected to a subtraction process to obtain a subtraction image, the apparatus comprising:
With the radiographic apparatus according to this invention, radiation detection signals are outputted from the radiation detecting device at predetermined sampling time intervals as radiation is emitted from the radiation emitting device to an object under examination. A live image and a mask image are obtained from these radiation detection signals, and are subjected to a subtraction process to obtain a subtraction image. A lag-behind part included in each of the radiation detection signals taken at the sampling time intervals is regarded as due to an impulse response formed of one exponential function or a plurality of exponential functions with different attenuation time constants. Such lag-behind parts are removed from the radiation detection signals by a recursive computation to obtain corrected radiation detection signals. In order to pick up a live image and a mask image continually, radiation detection signals for the live image and radiation detection signals for the mask image are continually detected at the sampling time intervals. Thus, the lag-behind parts of these signals are linked in time. When an image accompanying the lag-behind parts is picked up and thereafter a different image is picked up, the lag-behind parts influence the latter image also. Such lag-behind parts influencing one another are used to eliminate fully the time delays of the radiation detection signals due to the radiation detecting device. The live image and mask image are obtained from the corrected detection signals having the mutually influencing lag-behind parts removed. Consequently, the lag-behind parts are fully removed from the subtraction image obtained by performing the subtraction process on the live image and mask image.
In the above radiographic apparatus, the time lag removing device, preferably, is arranged to perform the recursive computation for removing the lag-behind part from each of the radiation detection signals, based on the following equations A-C:
Xk=Yk−Σn=1N{αn·[1−exp(Tn)]·exp(Tn)·Snk} A
Tn=−Δt/τn B
Snk=Xk−1+exp(Tn)·Sn(k−1) C
where Δt: the sampling time interval;
Where the recursive computation for removing the lag-behind part from each of the radiation detection signals is based on equations A-C, the corrected, lag-free radiation detection signal Xk may be derived promptly from equations A-C constituting a compact recurrence formula.
The mask image and live image may be obtained by using the corrected, lag-free radiation detection signals Xk derived from the recurrence formula, as follows.
The mask image may be created by deriving an arithmetic mean of the corrected radiation detection signals Xk from the following equation D:
where M: mask image; and
The live image may be created by a recursive process based on the following equation E showing a weighted mean of the corrected radiation detection signals Xk:
Rk=(1/K)·Xk+(1−1/K)·Rk−1 E
where Rk: live image after a k-th recursive process;
In the radiographic apparatus, one example of the radiation detecting device is a flat panel X-ray detector having numerous X-ray detecting elements arranged longitudinally and transversely on an X-ray detecting surface.
The radiographic apparatus according to this invention may be a medical apparatus, and an apparatus for industrial use as well. An example of medical apparatus is a fluoroscopic apparatus. Another example of medical apparatus is an X-ray CT apparatus. An example of apparatus for industrial use is a nondestructive inspecting apparatus.
In another aspect of the invention, a radiation detection signal processing method is provided for taking, at predetermined sampling time intervals, radiation detection signals generated by irradiating an object under examination, creating a live image and a mask image based on the radiation detection signals outputted at the predetermined sampling time intervals, and performing a signal processing to obtain a subtraction image through a subtraction process, the method comprising the steps of:
This radiation detection signal processing method allows the radiographic apparatus according to the invention to be implemented in an advantageous manner.
In the above radiation detection signal processing method, the recursive computation for removing the lag-behind part from each of the radiation detection signals, preferably, is performed based on the following equations A-C:
Xk=Yk−Σn=1N{αn·[1−exp(Tn)]·exp(Tn)·Snk} A
Tn=−Δt/τn B
Snk=Xk−1+exp(Tn)·Sn(k−1) C
where Δt: the sampling time interval;
Where the recursive computation for removing the lag-behind part from each of the radiation detection signals is based on equations A-C, the radiographic apparatus that performs the recursive computation based on equations A-C may be implemented advantageously.
The mask image and live image may be picked up as follows. In one example, after the mask image is picked up, a contrast medium is given to the object under examination and the live image is picked up. In another example, the mask image and the live image are picked up by switching between a focus voltage and a defocus voltage to be applied to a radiation emitting device that emits radiation toward the object under examination. Further, examples of picking up the mask image and the live image by switching between the focus voltage and defocus voltage include the following modes. In one mode, with a contrast medium given to the object under examination, the defocus voltage is applied to the radiation emitting device to pick up the mask image, and thereafter the focus voltage is applied to the radiation emitting device to pick up the live image. In another mode, with a contrast medium given to the object under examination, the focus voltage is applied to the radiation emitting device to pick up the live image, and thereafter the defocus voltage is applied to the radiation emitting device to pick up the mask image.
For the purpose of illustrating the invention, there are shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangement and instrumentalities shown.
Preferred embodiments of this invention will be described in detail hereinafter with reference to the drawings.
As shown in
The X-ray tube 1 and FPD 2 are opposed to each other across the patient M. In time of X-ray radiography, the X-ray tube 1 is controlled by an X-ray emission controller 6 to emit X rays in the form of a cone beam to the patient M. At the same time, penetration X-ray images of the patient M produced by the X-ray emission are projected to an X-ray detecting surface of FPD 2.
The X-ray tube 1 and FPD 2 are movable back and forth along the patient M by an X-ray tube moving mechanism 7 and an X-ray detector moving mechanism 8, respectively. In moving the X-ray tube 1 and FPD 2, the X-ray tube moving mechanism 7 and X-ray detector moving mechanism 8 are controlled by an irradiating and detecting system movement controller 9 to move the X-ray tube 1 and FPD 2 together as opposed to each other, with the center of emission of X rays constantly in agreement with the center of the X-ray detecting surface of FPD 2. Of course, movement of the X-ray tube 1 and FPD 2 results in variations in the position of the patient M irradiated with X rays, hence movement of a radiographed site.
As shown in
The analog-to-digital converter 3 continually takes X-ray detection signals for each X-ray image at sampling time intervals (t, and stores the X-ray detection signals for X-ray image creation in a memory 10 disposed downstream of the converter 3. An operation for sampling (extracting) the X-ray detection signals is started before X-ray irradiation.
That is, as shown in
The memory 10 is arranged to store also corrected X-ray detection signals obtained by a time lag remover 11 described hereinafter, and stores the corrected X-ray detection signals as detection signals for live images and mask images. Alternatively, a memory for live images and mask images may be provided separately from the memory 10.
As shown in
With the FPD 2, an X-ray detection signal generated at each point of time, as shown in
Specifically, the time lag remover 11 performs a recursive computation for removing a lag-behind part from each X-ray detection signal by using the following equations A-C:
Xk=Yk−Σn=1N{αn·[1−exp(Tn)]·exp(Tn)·Snk} A
Tn=−Δt/τn B
Snk=Xk−1+exp(Tn)·Sn(k−1) C
where Δt: the sampling time interval;
Xk: a corrected X-ray detection signal with a lag-behind part removed from the signal Yk;
The second term in equation A “Σn=1N{αn·[1−exp(Tn)]·exp(Tn)·Snk}” corresponds to the lag-behind part. Thus, the apparatus in the first embodiment derives the corrected, lag-free X-ray detection signal Xk promptly from equations A-C constituting a compact recurrence formula.
In this embodiment, the analog-to-digital converter 3, detection signal processor 4, X-ray emission controller 6, irradiating and detecting system movement controller 9, time delay remover 11 and a DSA (subtraction) processor 14 described hereinafter are operable on instructions and data inputted from an operating unit 12 or on various commands outputted from a main controller 13 with progress of X-ray radiography.
As shown in
Next, an operation for performing X-ray radiography with the apparatus in this embodiment will particularly be described with reference to the drawings.
[Step S1] The analog-to-digital converter 3 starts taking X-ray detection signals Yk for one X-ray image from the FPD 2 at each period between the sampling time intervals Δt (={fraction (1/30)} second) before X-ray emission. The X-ray detection signals taken are stored in the memory 10.
[Step S2] In parallel with a continuous or intermittent X-ray emission to the patient M initiated by the operator, the analog-to-digital converter 3 continues taking X-ray detection signals Yk for one X-ray image at each period between the sampling time intervals At and storing the signals in the memory 10.
The collection and storage in the memory 10 of the X-ray detection signals Yk are both carried out in time of image pickup for a mask image and image pickup for a live image. When the operation moves from step S1 to step S2, step S2 and subsequent steps are executed to perform the image pickup for a mask image without using a contrast medium. When the operation moves from step S4 [injection of contrast medium] described hereinafter to step S2, step S2 and subsequent steps are executed to perform the image pickup for a live image. Also in a state of non-X-ray emission, such as in time of injection of the contrast medium during a shift from the image pickup for a mask image to the image pickup for a live image, the image detection signals Yk remain, while attenuating, because of lag-behind parts as shown in
[Step S3] When the X-ray emission is completed, the operation proceeds to step S4. When the X-ray emission is uncompleted, the operation returns to step S2.
[Step S4] When the X-ray emission for a mask image has been completed, that is when the image pickup for a mask image has been completed, the contrast medium is injected into the patient M to perform the next, image pickup for a live image in parallel with step S5. Then, the operation returns to step S2, and executes steps S2 and S3 as done for the mask image.
[Step S5] In parallel with step S4, X-ray detection signals Yk for one X-ray image collected in one sampling sequence are read from the memory 10.
[Step S6] The time lag remover 11 performs the recursive computation based on the equations A-C, and derives corrected X-ray detection signals Xk, i.e. pixel values, with lag-behind parts removed from the respective X-ray detection signals Yk.
[Step S7] When unprocessed X-ray detection signals Yk remain in the memory 10, the operation returns to step S5. When no unprocessed X-ray detection signals Yk remain, the operation proceeds to step S8. [Step S8] When the corrected X-ray detection signals Xk correspond to the X-ray detection signals Yk collected before the contrast medium injection and with lag-behind parts removed therefrom, these corrected signals Xk are determined to be for a mask image. The corrected X-ray detection signals Xk are read from the memory 10, and the DSA processor 14 creates a mask image. The mask image is created based on an arithmetic mean in the following equation D:
where M: mask image; and
When the corrected X-ray detection signals Xk correspond to the X-ray detection signals Yk collected after the contrast medium injection and with lag-behind parts removed therefrom, these corrected signals Xk are determined to be for a live image. The corrected X-ray detection signals Xk are read from the memory 10, and the DSA processor 14 creates a live image. The live image is created based on a weighted mean in the following equation E (hereinafter called “recursive process” where appropriate):
Rk=(1/K)·Xk+(1−1/K)·Rk−1 E
where Rk: live image after a k-th recursive process;
Rk−1: Rk at a preceding point of time; and
K: weight factor for the recursive process.
The recursive process in this embodiment will particularly be described assuming K=4. First, K is set to 0, and R0 in equation E set to 0 as initial values before X-ray emission. In equation E, k=1 is set. A live image R1 after a first recursive process is derived from equation E, i.e. R1=(¼)·X1+(¾)·R0.
After incrementing k by 1 (k=k+1) in equation E, Rk−1 of a preceding point of time is substituted into equation E, and a live image Rk after a k-th recursive process is calculated.
[Step S9] When the mask image and live image have been created, the DSA processor 14 performs a DSA process on the mask image and live image to obtain a subtraction image.
[Step S10] The subtraction image created is displayed on the image monitor 5.
In this embodiment, the time lag remover 11 computes the corrected X-ray detection signals Xk corresponding to the X-ray detection signals Yk for one X-ray image, and the detection signal processor 4 creates an X-ray image, both at each period between the sampling time intervals Δt (={fraction (1/30)} second). That is, the apparatus is constructed also for creating X-ray images one after another at a rate of about 30 images per second, and displaying the created X-ray images continuously. It is thus possible to perform a dynamic display of X-ray images.
Next, the process of recursive computation carried out in step S6 in
[Step Q1] A setting k=0 is made, and X0=0 in equation A and Sn0=0 in equation C are set as initial values before X-ray emission. Where the number of exponential functions is three (N=3), S10, S20 and S30 are all set to 0.
[Step Q2] In equations A and C, k=1 is set. That is, S11, S21 and S31 are derived from equation C, i.e. Sn1=X0+exp(Tn)·Sn0. Further, a corrected X-ray detection signal is obtained by substituting S11, S21 and S31 derived and X-ray detection signal Y1 into equation A.
[Step Q3] After incrementing k by 1 (k=k+1) in equations A and C, Xk−1 of a preceding time is substituted into equation C, thereby obtaining S1k, S2k and S3k. Further, corrected X-ray detection signal Xk is obtained by substituting S1k, S2k and S3k derived and X-ray detection signal Yk into equation A.
[Step Q4] When there remain unprocessed X-ray detection signals Yk, the operation returns to step Q3. When no unprocessed X-ray detection signals Yk remain, the operation proceeds to the next step Q5.
[Step Q5] Corrected X-ray detection signals Xk for one sampling sequence (for one X-ray image) are obtained to complete the recursive computation for the one sampling sequence.
According to the fluoroscopic apparatus in this embodiment, as described above, a live image and a mask image are obtained from the X-ray detection signals Yk outputted from FPD 2 at sampling time intervals Δt (={fraction (1/30)} second) as the patient M is irradiated with X rays emitted from the X-ray tube 1. A subtraction image is obtained by performing a subtraction process on the live image and mask image. The lag-behind part included in each of the X-ray detection signals Yk taken at sampling time intervals Δt is considered due to an impulse response formed of a plurality of exponential functions. The time lag remover 11 performs the recursive computation based on the equations A-C to remove the lag-behind parts from the respective X-ray detection signals Yk, thereby obtaining corrected X-ray detection signals Xk. In order to pick up a live image and a mask image continually, X-ray detection signals Yk for the live image and X-ray detection signals Yk for the mask image are continually collected at sampling time intervals Δt. Thus, the lag-behind parts of these signals are linked in time. When the live image is picked up after the mask image with lag-behind parts (
This invention is not limited to the foregoing embodiment, but may be modified as follows:
This invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification, as indicating the scope of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| JP2003-290331 | Aug 2003 | JP | national |
