JP4010584B2 - X-ray CT system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an X-ray CT apparatus that performs X-ray beam shift correction.
[0002]
[Prior art]
The X-ray CT apparatus has an X-ray tube and a collimator that collimates the emitted X-ray in a fan beam shape. X-rays passing through the collimator are turned into a fan beam and irradiated onto the subject. A multi-channel X-ray detector having an arrangement relationship facing the X-ray tube detects X-rays transmitted through the subject. The output of the X-ray detector is AD converted and input to the image processing apparatus. The X-ray tube and the X-ray detector rotate in a positional relationship facing each other. During this rotation, the X-ray tube continuously or intermittently emits X-rays, and a predetermined pitch angle (this) Is called the projection angle), the X-ray detector detects the X-ray transmitted through the subject. The output of the X-ray detector obtained by one rotation or half rotation 180 ° (+ α) under the facing positional relationship is subjected to various pre-processing and reconstruction processing by the image processing apparatus. Obtain a composition image.
[0003]
[Problems to be solved by the invention]
For image reconstruction, it is necessary that the fan beam X-rays accurately enter the X-ray sensitive region of the X-ray detector. By aligning the X-ray tube, the collimator and the X-ray detector, the X-rays emitted from the X-ray tube are turned into a fan beam by the collimator, and the fan beam X-ray is converted into an X-ray sensitive region of the X-ray detector. Incident to.
Such X-ray tubes, collimators, and X-ray detectors are designed and assembled to be mechanically aligned with each other.
[0004]
However, even under such design and assembly, the fan beam X-rays may move and cause a positional shift (shift). The main cause of the displacement is movement due to thermal drift of the X-ray tube itself.
[0005]
Due to the movement of the X-ray tube, the focal point is shifted, and a planned fan beam X-ray cannot be obtained.
Even if there is movement of the X-ray tube due to thermal drift, a shift correction method that detects the movement amount and cancels the movement amount is conceivable. One example is JP-A-4-227238. Japanese Laid-Open Patent Publication No. 4-227238 performs position control so as to cancel the movement amount with respect to a collimator as a means for obtaining a fan beam.
In order to cancel the movement amount due to thermal drift, it is necessary to accurately detect the movement amount.
[0006]
In addition, the position of the collimator is controlled in order to eliminate the shift caused by the thermal drift of the X-ray beam. However, there is a problem that noise is superimposed on the signal for detecting the shift and the shift amount cannot be detected correctly.
[0007]
An object of the present invention is to provide an X-ray CT apparatus that can accurately detect the amount of movement of an X-ray tube due to thermal drift or the like.
[0008]
It is a further object of the present invention to provide an X-ray CT apparatus that can cancel the movement of X-rays without using position control on the collimator.
[0009]
[Means for Solving the Problems]
The present invention comprises an X-ray tube;
An X-ray tube drive mechanism;
A collimator that converts the emitted X-rays from the X-ray tube into a fan beam;
A multi-channel X-ray detector that rotates around the top plate on which the subject is mounted, facing the X-ray tube,
A two-channel configuration that outputs two signals A and B, and two channel signals A and B having a relationship of A = B without movement of the X-ray beam including thermal drift of the fan beam X-ray , and X-ray detector for outputting a line beam two-channel signal a in response to the movement, if the movement is, the two channel signals a difference a-B is produced relationship between B, and B,
Gain means for switching the magnitude of the gain according to the magnitude of the tube voltage and / or the tube current, inputting the signals A and B, and outputting the signals A ′ and B ′ according to the gain;
When the output signals A ′ and B ′ with respect to the signals A and B obtained after the gain switching are A ′ ≠ B ′ due to the movement of the X-ray beam including thermal drift, the above X is set so that A ′ = B ′. Means for controlling the position of the X-ray tube via the tube driving mechanism;
Processing means for performing image reconstruction from the measurement signal of the multi-channel X-ray detector after the position control;
An X-ray CT apparatus comprising the same is disclosed.
[0010]
The present invention further includes an X-ray tube,
A collimator that converts the emitted X-rays from the X-ray tube into a fan beam;
A multi-channel X-ray detector that rotates around the top plate on which the subject is mounted, facing the X-ray tube;
A two-channel configuration that outputs two signals A and B, and two channel signals A and B having a relationship of A = B without movement of the X-ray beam including thermal drift of the fan beam X-ray , and X-ray detector for outputting a line beam two-channel signal a in response to the movement, if the movement is, the two channel signals a difference a-B is produced relationship between B, and B,
Gain means for switching the magnitude of the gain according to the magnitude of the tube voltage and / or the tube current, inputting the signals A and B, and outputting the signals A ′ and B ′ according to the gain;
A collimator driving mechanism;
When A ′ ≠ B ′ due to movement of the X-ray beam including thermal drift for the output signals A ′ and B ′, the collimator position control is performed via the collimator driving mechanism so that A ′ = B ′. Means to do,
Processing means for constructing an image from the measurement signal of the multi-channel X-ray detector after the position control of the collimator;
An X-ray CT apparatus comprising the same is disclosed.
[0011]
According to the present invention, the gain switching by the tube voltage and / or the tube current is such that when the X-ray intensity signal is small and the levels of the signals A and B are small, the gain is increased compared to when the levels of the signals A and B are large. An X-ray CT apparatus intended for switching is disclosed.
Further, according to the present invention, the gain switching by the tube voltage and the tube current is performed so that A ′ = B ′ in a state where no thermal drift occurs, and the X-ray intensity is small and the levels of the signals A and B are low. An X-ray CT apparatus is disclosed in which the gain is switched to be larger when the level of the signals A and B is larger than when the level of the signals A and B is large.
[0013]
Further, according to the present invention, the gain switching by the tube voltage and / or the tube current increases the gain when the X-ray intensity signal is small and the signals A and B are small compared to when the signals A and B are large. An X-ray CT apparatus according to claim 3 or 4 is disclosed.
[0014]
Further, according to the present invention, the gain switching by the tube voltage and the tube current is performed so that A ′ = B ′ in a state where no thermal drift occurs, and the X-ray intensity is small and the levels of the signals A and B are reduced. The X-ray CT apparatus according to claim 3 or 4, wherein when the signal is small, the gain is switched to be larger than when the levels of the signals A and B are large.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 2 is a configuration example of the X-ray CT system of the present invention. In the figure, an opening is formed in the center of the gantry 1, and the subject 5 is inserted and disposed in the opening as the bed 14 is driven. The gantry 1 is provided with an X-ray tube 2, a collimator 3 and an X-ray detector 4, and the X-ray tube 2, the collimator 3 and the X-ray detector 4 are arranged to face each other with the subject 5 interposed therebetween. The X-ray tube 2 emits X-rays under the control of the control device 6. As shown in FIG. 3, the collimator 3 has a thickness (slice width) of a conical X-ray beam generated when an electron beam emitted from the cathode 2A in the X-ray tube 2 collides with the rotating anode 2B. Is collimated to the fan-shaped beam 3A. The fan-shaped X-ray beam 3 </ b> A that has passed through the collimator 3 passes through the subject 5. An arc-shaped multichannel X-ray detector 4 detects X-rays that have passed through the subject 5.
[0016]
The X-ray tube 2 and the X-ray detector 4 rotate around the subject 5 while facing each other. This rotation is performed by the driving device 7 driven by the scanning unit control device 8. The scanning unit control device 8 is driven by an output signal from the image processing device 11.
Further, the output from the X-ray detector 4 is digitized by an A / D converter 10 via an amplifier 9 and input to the image processing device 11.
Then, the image processing apparatus 11 obtains a reconstructed image by image processing. The reconstructed image is displayed on the display 12 or recorded in the storage device 13.
[0017]
FIG. 1 shows an example of addition of correction control (cancellation or reduction) means for intensity distribution shift according to the magnitude of the tube voltage according to the present invention. The X-ray tube 2 has a cathode 2A that generates an electron beam therein and a rotating anode 2B that is a target of the electron beam. The rotating anode 2B receives an electron beam and emits X-rays 2C. The X-ray tube 2 can be finely moved in the direction of the arrow (left and right of the drawing) by a drive mechanism 18 such as a worm gear.
The emitted X-ray 2C from the rotating anode 2B is collimated into a fan beam shape by controlling the width of the slice by the collimator 3 and irradiated onto the subject 5, and this transmitted X-ray is detected by the X-ray detector 4. The X-ray detector 4 is a multi-channel type, and a channel is formed in a direction orthogonal to the paper surface.
[0018]
The collimator 3B takes in a part of the emitted X-ray 2C and collimates it so that the two-channel X-ray detector 4B can detect the shift due to the movement of the fan beam X-ray and the intensity distribution shift due to the magnitude of the tube voltage.
[0019]
The relationship between the fan beam X-ray F in the figure and the sensitive region of the detector 4B is as follows. The hatched areas 42 and 43 are insensitive areas, and the white areas 40 and 41 are sensitive areas. The detector 4B is arranged with respect to the fan beam X-ray F so that the fan beam X-ray F is in a direction orthogonal to an intermediate position (dead line) 45 between the sensitive regions 40 and 41. A normal position where there is no forward / backward movement shift due to thermal drift is the P ₀ position of the fan beam X-ray. At this P ₀ position, the outputs of the sensitive regions 40 and 41 are equal. On the other hand, when moving forward, the fan beam X-ray F moves in the P ₀ → P ₁ direction, and when moving later, the fan beam X-ray F moves in the P ₀ → P ₂ direction.
Accordingly, the outputs A and B at the positions P ₀ , P ₁ and P ₂ are as follows.
In the P ₀ position, A = B
In the P ₁ position A> B
In the P ₂ position A <B
[0020]
The difference (A−B) between the outputs A and B of the X-ray detector 4B described above is obtained, and control is performed so that this difference becomes zero (A = B). This control system is a comparator 15, a control device 16, and a drive mechanism 18 that form a negative feedback system. That is, the position of the X-ray tube 2 is controlled so that the difference (A−B) is taken by the comparator 15 and the difference (A−B) is made zero by the control device 16 and the drive mechanism 18.
However, if there is an intensity distribution shift due to the tube voltage, A = B does not occur even at the P ₀ position. Even at the P ₁ and P ₂ positions, the influence of the intensity distribution shift appears in the slice width direction in the components of A and B, and the positions of the X-ray tube cannot be correctly controlled because they are not the original A and B. This will be described with reference to FIG.
[0021]
It has been confirmed by the present inventors that the X-ray intensity distribution shifts in the slice direction due to the tube voltage. FIG. 5 shows the intensity distribution at this time. The horizontal axis represents the coordinate value in the slice width direction, and the vertical axis represents the X-ray relative intensity. Three examples of tube voltages of 80 KV, 100 KV, and 120 KV are shown. It is clear from this figure that the distribution peak position shifts to a at 80 KV, b at 100 KV, and c at 120 KV, and the distribution shape changes accordingly depending on the magnitude of the tube voltage. When such a shift of the distribution peak position and the shift of the distribution shape occur, the influence appears on the outputs A and B, which are different from the outputs A and B due to the original thermal drift shift.
[0022]
Therefore, in this embodiment, a variable gain (G) amplifier 17 (for example, AGC) is provided that is adjusted so that A = B when the X-ray tube without thermal drift is started up. Then, when the X-ray tube is started up, the gain is set so that A = B is also obtained by the tube voltage at that time. This gain setting is performed by the gain control circuit 19. If the appropriate gain due to the tube voltage is determined in advance, it may be latched and controlled by this signal 19A. If it is not determined in advance, the X-ray tube 2 is set to a position where there is no thermal drift shift, and X-rays are irradiated by the tube voltage used in that state, and gain selection is performed so that A = B. Do.
[0023]
The G setting is performed when the X-ray CT apparatus is started up. However, even if a thermal drift shift occurs during the actual operation of the X-ray tube after this adjustment, the tube voltage intensity distribution shift is corrected. The following negative feedback system works. That is, a closed-loop negative feedback system consisting of comparator 15 → control device 16 → drive mechanism 18 → detector 4B → comparator 15 is constructed, and this negative feedback system is set so that the output Q of the comparator 15 becomes zero. Work. This is realized by inputting the output Q of the comparator 15 to the control device 16 and operating the drive mechanism 18 so that the output Q becomes zero.
[0024]
As a conventional example of the thermal drift canceling control, there is JP-A-4-227238. In this case, the position (including rotation) of the fan beam collimator on the emission side of the X-ray tube is controlled so as to cancel out the thermal drift. As a result, the fan beam X-ray can be input to the multi-channel X-ray detector without causing a positional shift. This is an example replacing the position control of the X-ray tube, but can also be applied to a method of performing such collimator control. In this case, the collimator is controlled by the output of the comparator 15.
[0025]
In the embodiment of FIG. 1, the gain G is changed depending on the magnitude of the tube voltage, and the comparison is performed by the comparator 15 to cancel the beam shift. However, there is a problem that noise is mixed in the input signal to the comparator 15 to reduce the accuracy of comparison. The reason for this and the new embodiment will be described below.
[0026]
The levels of the two outputs A and B of the X-ray detector 4B vary greatly depending on the X-ray intensity. This is shown in FIGS. 7 (a) and 7 (b). FIG. 7A shows a case where the X-ray intensity is low, and FIG. 7B shows a case where the X-ray intensity is high. In FIGS. 7A and 7B, noise N (N ₁ , N ₂ ) is superimposed on the signals A and B. As the noise source, high-frequency noise from various electric circuits, high-frequency noise generated from a photo coupling mechanism or the like, noise due to slight variations in X-rays from the X-ray source, and the like are considered. In any case, noises N ₁ and N ₂ are superimposed on the signals A and B, respectively.
[0027]
If the high-frequency noises N ₁ and N ₂ are superimposed, it is difficult to accurately compare the signals A and B in FIG. 7A where the signal level is low. In FIG. 7B where the signal level is large, even if noises N ₁ and N ₂ are mixed, the levels of the noises N ₁ and N ₂ are relatively small compared to the original A and B. The comparison of size with is correct.
Therefore, in the present invention, as shown in FIG. 7A, when the levels of the signals A and B are small, gain control is performed to increase the levels so that A is A 'and B is B'. I made it. The embodiment is shown in FIG. In contrast to FIG. 1, variable gain amplifier circuits (for example, AGC) 21 and 22 are provided at two output ends of the X-ray detector 4 </ b> B, and gain control is performed by the gain control circuit 20. The gain control circuit 20 receives the set X-ray intensity (value determined by the tube voltage and tube current) as an input, does not change the gain if the set X-ray intensity is large, and if the set X-ray intensity is small. The gain amplifying circuits 20 and 21 are controlled so as to increase the gain. FIG. 8 shows a control example for (A) and (B) of FIG. 8 (a) corresponds to FIG. 7 (a) and shows an example in which the gain is increased, and FIG. 8 (b) corresponds to FIG. 7 (b) and shows an example in which the gain is not changed (therefore, FIG. 6 (b) and FIG. 7 (b) have the same waveform). FIG. 7 (a) is an example, and there are various other levels close to the waveform of FIG. 7 (b), but depending on the magnitude of the X-ray intensity. What is necessary is just to change a gain.
[0028]
The gain amplifying circuits 20 and 21 have a circuit configuration in which the gain control sensitivity is low in the frequency bands of the noises N ₁ and N _{2 and} the gain control sensitivity is high in the lower frequency band (including the DC component). It is desirable. Thus, although the signals A and B can be distinguished from each other as shown in FIG. 9, even when some of the noises indicated by the noises N ₁ and N ₂ overlap each other in level, As shown in FIG. 8A, two distinct waveforms A and B can be obtained.
[0029]
By performing such gain control, the waveform signal as shown in FIG. 8 (a) becomes the inputs A 'and B' to the comparator 15, and even if the noises N ₁ and N ₂ are on A and B, both It is now possible to reliably compare the levels of levels.
[0030]
In the embodiment of FIG. 6, the gain amplifier circuit 22 is provided to be distinguished from the gain amplifier circuit 17, but separate control or superimposed control may be performed by using both as one. Furthermore, in addition to the position control of the X-ray tube according to the embodiment of FIG. 1, it can be applied to an example of collimator position control according to Japanese Patent Laid-Open No. 4-227238.
[0031]
Although the above has been described with respect to the example of X-ray CT measurement, it can be used not only for CT measurement but also for X-ray fluoroscopic imaging using a CT apparatus. Further, the present invention is applicable to applications other than X-ray CT apparatuses, for example, all X-ray apparatuses using an X-ray source. In addition, although the example of the thermal drift shift has been described, it can also be applied to a fine movement shift of a mechanical system.
[0032]
【The invention's effect】
According to the present invention, the X-ray intensity distribution shift due to the magnitude of the tube voltage can be corrected.
Further, according to the present invention, the X-ray fan beam position shift due to the thermal drift shift can be corrected correctly while correcting the X-ray intensity distribution shift.
Furthermore, according to the present invention, it is possible to eliminate an error in comparing the signals A and B due to noise superposition when the X-ray intensity is small.
[Brief description of the drawings]
FIG. 1 is a configuration example diagram of an X-ray CT apparatus of the present invention.
FIG. 2 is an overall configuration diagram of an X-ray CT system.
FIG. 3 is a configuration diagram for obtaining a fan beam X-ray;
FIG. 4 is a diagram showing a sensitive region and an X-ray fan beam shift example of the two-channel X-ray detector of the present invention.
FIG. 5 is a diagram showing an example of an X-ray intensity distribution shift in a fan beam depending on the magnitude of a tube voltage.
FIG. 6 is a diagram showing an X-ray CT apparatus according to another embodiment of the present invention.
7 is a waveform diagram in which noise is superimposed on signals A and B. FIG.
8 is a waveform diagram obtained by amplifying signals A and B. FIG.
FIG. 9 is another waveform diagram in which noise is superimposed on signals A and B;
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Gantry 2 X-ray tube 3 Collimator 4 Multi-channel X-ray detector 5 Subject 2B Rotating anode 2C Fan beam X-ray 3B Collimator 4B 2-channel X-ray detector 15 Comparator 16 Controller 17 Amplifier 18 Drive mechanism 19 Amplification rate control Circuits 40 and 41 Sensitive areas 42 and 43 Insensitive areas P ₀ , P ₁ and P ₂ fan beams

Claims (4)

An X-ray tube;
An X-ray tube drive mechanism;
A collimator that converts the emitted X-rays from the X-ray tube into a fan beam;
A multi-channel X-ray detector that rotates around the top plate on which the subject is mounted, facing the X-ray tube,
A two-channel configuration that outputs two signals A and B, and two channel signals A and B having a relationship of A = B without movement of the X-ray beam including thermal drift of the fan beam X-ray , and X-ray detector for outputting a line beam two-channel signal a in response to the movement, if the movement is, the two channel signals a difference a-B is produced relationship between B, and B,
Gain means for switching the magnitude of the gain according to the magnitude of the tube voltage and / or the tube current, inputting the signals A and B, and outputting the signals A ′ and B ′ according to the gain;
When the output signals A ′ and B ′ with respect to the signals A and B obtained after the gain switching are A ′ ≠ B ′ due to the movement of the X-ray beam including thermal drift, the above X is set so that A ′ = B ′. Means for controlling the position of the X-ray tube via the tube driving mechanism;
Processing means for performing image reconstruction from the measurement signal of the multi-channel X-ray detector after the position control;
X-ray CT apparatus comprising: An X-ray tube;
A collimator that converts the emitted X-rays from the X-ray tube into a fan beam;
A multi-channel X-ray detector that rotates around the top plate on which the subject is mounted, facing the X-ray tube;
A two-channel configuration that outputs two signals A and B, and two channel signals A and B having a relationship of A = B without movement of the X-ray beam including thermal drift of the fan beam X-ray , and X-ray detector for outputting a line beam two-channel signal a in response to the movement, if the movement is, the two channel signals a difference a-B is produced relationship between B, and B,
Gain means for switching the magnitude of the gain according to the magnitude of the tube voltage and / or the tube current, inputting the signals A and B, and outputting the signals A ′ and B ′ according to the gain;
A collimator driving mechanism;
When A ′ ≠ B ′ due to movement of the X-ray beam including thermal drift for the output signals A ′ and B ′, the collimator position control is performed via the collimator driving mechanism so that A ′ = B ′. Means to do,
Processing means for constructing an image from the measurement signal of the multi-channel X-ray detector after the position control of the collimator;
X-ray CT apparatus comprising:   The gain switching by the tube voltage and / or tube current is performed so that the gain is increased when the X-ray intensity signal is small and the levels of the signals A and B are small compared to when the signals A and B are large. The X-ray CT apparatus according to claim 1 or 2.   The gain switching by the tube voltage and the tube current is performed so that A ′ = B ′ in a state where no thermal drift occurs, and when the X-ray intensity is small and the levels of the signals A and B are small, the signal A The X-ray CT apparatus according to claim 1 or 2, wherein the gain is switched to be larger than that when the level of B is large.

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