JPH0250597B2 - - Google Patents

Description

[Detailed description of the invention] [Technical field of invention] The present invention belongs to the technical field of X-ray CT scanners.

[Technical background of the invention and its problems] A device (HVPS) that generates a high voltage (tube voltage) to be applied to an X-ray tube rectifies and smoothes alternating current voltage supplied from a commercial power source, for example. The DC voltage obtained is switched by a switching means, this switching voltage is stepped up by a transformer, and then rectified and smoothed again to output a tube voltage.

By the way, the tube voltage output from such an HVPS includes a ripple component (tube voltage fluctuation component) that is synchronized with the switching frequency of the switching means, and when such a tube voltage is applied to the X-ray tube, When X-rays are emitted, the X-ray dose fluctuates due to tube voltage fluctuations, causing artifacts in reconstructed images.
In such a case, the image quality deteriorates and it becomes very difficult to obtain a good image.

Therefore, in conventional X-ray CT equipment,
The smoothing capacitor in the output section of the HVPS was designed to have a large capacity, thereby reducing the ripple component included in the tube voltage and thereby preventing the occurrence of artifacts.

However, when the capacity of the smoothing capacitor is increased, the rise and fall times of the tube voltage increase.
The patient will be exposed to unnecessary X-rays. This is because necessary X-ray data (projection data) is required only when the tube voltage is a predetermined value. Furthermore, a large-capacity smoothing capacitor is large and expensive, which causes problems in terms of device size and cost.

Therefore, using a large-capacity smoothing capacitor is not the best way to prevent deterioration in image quality due to artifacts.

[Object of the Invention] The present invention has been made in view of the above circumstances, and provides an X-ray CT scanner that can obtain good reconstructed images even though the tube voltage includes ripple components. With the goal.

[Summary of the Invention] The outline of the present invention for achieving the above-mentioned object is to provide a tube that is rotatably attached around a subject and that is directed toward the subject by applying a tube voltage obtained by switching a DC voltage. It has an X-ray tube capable of emitting X-rays and a grating detection means capable of outputting a rotation angle detection pulse synchronized with the rotation of the X-ray tube, and the projection data of the subject is detected by the rotation angle detection means. Collect in synchronization with the pulse and reconstruct the image
In the line CT scanner, the switching pulse generating means is capable of multiplying the frequency of the grating pulse by an integer and outputting a switching pulse of a desired frequency in synchronization with the rotation angle detection pulse, and this switching pulse generating means The present invention is characterized by comprising a tube voltage generating means for switching the DC voltage and generating a predetermined tube voltage using an output switching pulse.

[Embodiment of the Invention] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is an explanatory diagram showing the configuration of an X-ray CT scanner according to the present invention. Figure 1 shows the subject (patient),
2 is an X-ray tube that rotates around the subject 1 and generates X-rays directed toward the subject 1; 3 is a detector that detects the X-rays that have passed through the subject 1; 4a is a detector that detects the X-rays that have passed through the subject 1; A grating ring 4b rotates around the subject 1 together with the X-ray tube 2, and a grating ring sensor 4b detects the rotation of the grating ring 4a.

Note that, including this grating sensor 4b and the grating ring 4a, the
A grating detection means (rotation angle detection means) 4 is configured to output a gratayure (rotation angle detection) pulse a corresponding to the rotation of the wire tube 2.

Reference numeral 6 denotes a switching pulse generating means (see details) capable of multiplying the frequency of the grating pulse a in synchronization with the grating pulse a outputted from the grating pulse detecting means 4 and outputting a switching pulse c of a desired frequency. (described later),
Reference numeral 13 indicates a tube voltage generating means (details) for generating a predetermined tube voltage by switching a DC voltage obtained by rectifying and smoothing the commercial power supply voltage 12 using a switching pulse outputted from the switching pulse generating means 6. will be described later).

Further, the output of the detector 3 is provided for each channel by a preamplifier 22 including, for example, an operational amplifier 20 and a resistor 21, and a preamplifier 22 disposed after the amplifier, and for example, an operational amplifier 24 and a resistor. 25, the signal is input to a data processing means (not shown) through an integrating capacitor 26 and an integrating means 28 including a switch 27 connected to the integrating capacitor 26 so as to be short-circuited, and is used for image reconstruction.

The opening/closing operation of the switch 27 is controlled by a data collection timing controller 23 which outputs a timing signal b synchronized with the gradient pulse a.

Next, the configuration of the switching pulse generating means 6 will be described in detail.

This switching pulse generating means 6 is, for example,
PLL (Phase Locked Loop), which is a phase locked loop, which detects a gradient pulse a output from the gradient detection means 4 and a frequency division means 1 to be described later.
Phase difference detection means 7 for detecting the phase difference with the output of
, a low-pass filter 8 that stably operates the PLL by passing only a desired signal component using the output of the phase difference detection means 7; an amplification means 9 that amplifies the output of the low-pass filter 8; The circuit includes an oscillation means (VCO) 10 whose frequency is variable according to the output of the VCO 10, and a frequency division means 11 which divides the output of the VCO 10 into 1/N.

Next, the configuration of the tube voltage generating means 13 will be explained in detail.

This tube voltage generating means 13 includes, for example, a transformer 14 that transforms the commercial power supply voltage 13, and this transformer 14.
a rectifying and smoothing means 15 that rectifies and smoothes the voltage (AC voltage) transformed by and outputs a DC voltage; The switching means 17 is configured to include, for example, a transformer, and the switching means 17 switches according to the switching pulse c.
The voltage booster 18 boosts the voltage switched by the booster 18, and the rectifier and smoother 19 rectifies and smoothes the voltage (AC) boosted by the booster 18 to output a tube voltage.

The tube voltage output from the rectifying and smoothing means 19 is applied to the X-ray tube 2 and used to generate X-rays.

Next, the operation of the X-ray CT scanner configured as described above will be explained with reference to FIG. 2, which shows the waveforms of the main parts.

At first, the VCO 10 oscillates at an arbitrary frequency within a predetermined range, but the X-ray tube 2 and the grating ring 4a start rotating, and the grating detector means 4 detects the rotation of the X-ray tube 2. The phase detection means 7 detects the phase detection means 7.
When input to , this gradation pulse a
(This is called "the loop is locked.") That is, VCO
The phase difference between the output of the frequency dividing means 11 which divides the output of 10 into 1/N and the gradient pulse a outputted from the gradient detecting means 4 is detected by the phase detecting means 7, and the phase difference is detected by the phase detecting means 7. hand
The oscillation frequency of VCO10 will change,
Finally, the frequency of the gradation pulse a and the output frequency of the frequency dividing means 11 match, and the frequency of the gradation pulse a is N times the frequency of the VCO.
10 will oscillate.

Therefore, the pulse signal output from this VCO 10 (pulse signal N times the frequency of the gradation pulse a) multiplies the frequency of the gradation pulse a by N times while synchronizing with the gradation pulse a. It becomes what it is.

Note that the low-pass filter 8 is arranged such that the frequency of the gradation pulse a is equal to the frequency of the gradation pulse a.
Slight wobbling due to uneven rotation speed of a, etc.
It acts to prevent the output of the VCO 10 from becoming unstable (instantaneous loss of lock state).

Next, the pulse signal (output of VCO 10) is
As the switching signal c, the switching means 1
7, this switching means 17
switches the DC voltage (output of the rectifying and smoothing means 15) and generates a tube voltage via the boosting means and the rectifying and smoothing means 19.

Therefore, the ripple component contained in this tube voltage has a waveform d in FIG. 2, and is synchronized with the gradient pulse a.

Note that the tube voltage generated by the tube voltage generating means 13 is applied to the X-ray tube 2 and used to generate X-rays.

When the X-ray tube 2 rotates and the subject 1 is exposed to X-rays generated from the X-ray tube 2 in synchronization with this rotation,
The X-rays that have passed through the object 1 are detected by the detector 3, and the detected signals are amplified by the preamplifier 22 for each channel. This preamplification means 2
The output of 2 is integrated by an integrating means 28 which operates in synchronization with the gradient pulse a. That is, the opening/closing operation of the switch 27 is controlled by the data collection timing controller 23 which outputs the timing signal b synchronized with the gradient pulse a, and the timing signal b is synchronized with the gradient pulse a for a predetermined reset time (pulse width of waveform b) from the rising edge of the gradient pulse a. After the switch 27 is closed for a time determined by , and the switch 27 is opened again, integration is performed until the next gradient pulse a is output. Here, integration refers to the operation of accumulating the output of the preamplifier 22 for a predetermined period of time.

Note that the output of the integrating means 28 is subjected to A/D (analog/digital) conversion and then used for image reconstruction.

In this way, the tube voltage is generated by switching in synchronization with the grating pulse a output in response to the rotation of the X-ray tube 2, and by emitting X-rays based on this tube voltage, the tube voltage can be adjusted. Although the ripple component d is included in the image, a good reconstructed image can be obtained. The reason for this will be explained below.

X-rays that have passed through the object 1, that is, projection data, are collected in synchronization with a grating pulse a that is output in accordance with the rotation of the X-ray tube 2. This is because the X-ray exposure to the subject 1 and the integration of the detection signal (projection data) output from the detector 3 are synchronized with the graticule pulse a (that is, the
This is because it is synchronized with the rotation of the wire tube 2). Moreover, the ripple component d included in the tube voltage is synchronized with the gradient pulse a. Therefore, the acquisition timing of the projection data, that is, the sampling point, is located on almost the same phase of the ripple component d included in the tube voltage, as shown in the waveform e in FIG. 2, for example, and the sampling frequency (e)
In contrast, the ripple frequency (d) is approximately an integral multiple (N times) at any position.

Therefore, if the frequency of the ripple component d is f _H and the frequency of the gradient pulse a is f _S , the ripple frequency f _H ′ after sampling is f _H ′=|f _H −N・f _S | It can be expressed as Here, N is the multiplication number of the gradient pulse a in the switching pulse generating means 6, and N·f _S is naturally approximately equal to f _H (the switching means 17 is approximately equal to N·f S).
(This is because it is switching at a frequency f _S. ) Therefore, the ripple frequency f _H ' after sampling becomes almost zero. This means that the projection data after sampling (that is, the output of the integrating means 28) becomes almost unrelated to the ripple component included in the tube voltage.

In this way, the projection data after sampling is
If it is almost unrelated to the ripple component, even if the tube voltage includes a ripple component, no artifacts due to this ripple component will occur.

Therefore, according to the device of this embodiment, the occurrence of artifacts can be avoided even if excessive stability is not required for the output of the tube voltage generating means 13 (in particular, there is no need for a large-capacity smoothing capacitor as in the conventional device). Since this can be prevented, the reconstructed image will be good, and
The entire device can be made smaller and less expensive, and the rise and fall times of the tube voltage can be shortened (because the capacity of the smoothing capacitor of the rectifying and smoothing means 19 can be made smaller), so unnecessary X There is no radiation exposure to the patient.

It goes without saying that the present invention is not limited to the above-mentioned embodiments, and that modifications can be made as appropriate within the scope of the gist of the present invention.

The low-pass filter 8 in the embodiment described above is for preventing the lock state of the PLL from being instantaneously lost due to the fluctuation of the gradient pulse a output from the gradient pulse detection means 4. The low-pass filter 8 may be omitted if it is stable or if fluctuation is not a problem.

Furthermore, in the embodiment described above, the switching pulse generating means 6 is configured by a PLL, but the point is that the frequency of the gradation pulse is multiplied in synchronization with the gradation pulse, and a switching pulse of a desired frequency can be output. It is not limited to PLL.

Furthermore, in the embodiment, the commercial power supply voltage 12 is transformed by the transformer 14, this transformed voltage is rectified and smoothed by the rectifying and smoothing means 15 to obtain a DC voltage, and this DC voltage is switched by the switching 17. However, the transformer 14 is not necessarily required to obtain the desired DC voltage, and the DC voltage may be supplied from a storage battery or the like.

[Effects of the Invention] According to the present invention described above, by synchronizing the ripple component included in the tube voltage with the rotation of the X-ray tube (graticular pulse), it is possible to eliminate the ripple component included in the tube voltage. Nevertheless, it is possible to provide an X-ray CT scanner that can obtain good reconstructed images without artifacts caused by ripple components.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing the configuration of an X-ray CT scanner according to the present invention, and FIG. 2 is a waveform diagram showing waveforms of main parts of the X-ray CT scanner shown in FIG. 1. 1...Subject (patient), 2...X-ray tube, 3...
Detector, 4...Gratiqueur detection means, 6...
...Switching pulse generating means, 13...Tube voltage generating means.

Claims (1)

[Claims] 1. An X-ray tube that is rotatably mounted around the subject and capable of emitting X-rays toward the subject by applying a tube voltage obtained by switching a DC voltage; a rotation angle detection means capable of outputting a rotation angle detection pulse synchronized with the rotation, the X-ray X-ray is configured to collect projection data of the subject in synchronization with the rotation angle detection pulse and reconstruct an image.
In a CT scanner, a switching pulse generating means capable of multiplying the frequency of the rotation angle detection pulse by an integer and outputting a switching pulse of a desired frequency in synchronization with the rotation angle detection pulse; An X-ray CT scanner comprising: tube voltage generation means for switching the DC voltage and generating a predetermined tube voltage using an output switching pulse.