JP5485592B2 - X-ray CT apparatus and high voltage generator for X-ray tube - Google Patents

Description

  The present invention relates to an X-ray CT apparatus, and more particularly to a multi-energy X-ray CT apparatus (Multi Energy Computed Tomography: MECT) that irradiates X-ray beams having different energies during scanning and obtains respective X-ray transmission data.

  The multi-energy X-ray CT apparatus utilizes a phenomenon in which the X-ray attenuation rate of each tissue in the body changes according to the energy of the X-ray beam, and irradiates a plurality of X-ray beams having different energy to A plurality of different tomographic images are taken. By subtracting the plurality of images and obtaining an enhanced image in which the difference is emphasized, tissues that are imaged with similar pixel values with a single energy are imaged so as to be distinguishable from each other.

  In order to obtain such tomographic images having different image characteristics, when the same subject is imaged with two types of X-rays, conventionally, imaging is performed with a low tube voltage such as 80 kV, and then switched to a high tube voltage such as 140 kV. I was shooting. In this case, the following problems occur. Since the tube voltage is switched to perform two shootings, there is a time difference between the first shooting and the second shooting. If the position and orientation of the subject change between the first imaging and the second imaging, a positional deviation of the subject occurs between the two tomographic images. When such misalignment occurs, artifacts are included in the enhanced image other than true differences due to differences in image characteristics between tomographic images, and appropriate observation and analysis of the enhanced image cannot be performed.

  On the other hand, in a multi-energy type X-ray CT apparatus, there is a method of irradiating X-ray beams of different energy by switching the imaging tube voltage at high speed during scanning (for example, Patent Document 1). In the method disclosed in Patent Document 1 for a luggage inspection scanner, a waveform generator that generates a periodic time-varying waveform is connected to a high-voltage DC power source that is an X-ray tube power source. The output is changed at a predetermined rate of change in response to the waveform generated by the waveform device. Thereby, the tube voltage is switched at a rate of change higher than the rotation period of the scanner, for example, 200 to 800 Hz. Thereby, the technique disclosed in Patent Document 1 aims to improve the scanning speed per unit time in the package inspection.

JP-A-10-73544

  In an X-ray CT apparatus for living tissue, it is desirable to switch the tube voltage at a higher speed. For example, in order to obtain a clear image at a site with intense body movement such as the heart, it is conceivable to switch the tube voltage for each view. In the X-ray CT apparatus, for example, image data is captured at a view rate such as 1800 view / s, and an image is reconstructed each time. In this case, in order to switch the tube voltage for each view, it is necessary to switch every 0.556 ms.

  However, in the combination of the DC power source and the waveform generator as described in Patent Document 1, there is a limit to further speeding up the switching. This is because a rectangular waveform output usually requires a rise time of about several ms to reach a predetermined output value, and the tube voltage cannot be switched at a speed corresponding to the view rate, 0.556 ms in the above example. is there. In order to improve the response characteristics of the tube voltage, the conventional X-ray CT apparatus detects the tube voltage and performs feedback control to increase the speed. However, the stray capacitance existing in the output portion of the high voltage generator is not improved. Due to the influence, tube voltage rise time and fall time of 1 ms or less cannot be realized. Further, as the demand for high resolution is increasing, the rotation speed and view rate of the scanner are increasing, and the realization of a high voltage generator capable of switching the tube voltage at a high speed is an issue.

  The present invention provides a multi-energy X-ray CT apparatus capable of high-speed tube voltage switching for each view, and thereby a multi-energy X-ray capable of obtaining a clear image even for a moving inspection object. An object is to provide a CT apparatus.

In order to solve the above problems, an X-ray CT apparatus of the present invention connects a plurality of DC voltage generators in parallel with each other between an AC power source and an X-ray tube, and each DC voltage generator and X-ray tube. in between, it respectively, provided with a switching means. In a preferred embodiment, one of the switching means can be replaced with a diode, and the diode is inserted between the DC voltage device having the lowest output voltage and the X-ray tube among the plurality of DC voltage generators.

  Specifically, an X-ray CT apparatus of the present invention includes an X-ray source including an X-ray tube and a power supply device thereof, and X-ray detection that detects X-rays generated from the X-ray source and transmitted through an inspection target. An image of the inspection object using data relating to transmitted X-rays detected by the X-ray detector, and a scanner rotating unit having a rotating plate in which the X-ray source and the X-ray detector are arranged to face each other. An image processing unit to be reconfigured, and a control unit for controlling the X-ray source, the X-ray detector, the scanner rotation unit, and the image processing unit. A plurality of DC voltage generators that are connected between the X-ray tube and generate different voltages, and are connected in series between each DC voltage generator and the X-ray tube, and operate according to a control signal from the control unit. Switching means.

  Alternatively, the power supply device of the X-ray tube is connected between a common AC power supply and the X-ray tube, and a plurality of DC voltage generators that generate different DC voltages from each other, and the plurality of DC voltage generators A diode connected in series between one and the X-ray tube, and a DC voltage generator other than the one DC voltage generator and the X-ray tube connected in series, and control from the control unit Switching means operated by a signal.

  According to the X-ray CT apparatus of the present invention, the X-ray tube power supply device includes a plurality of DC voltage generators and switching means for switching them at high speed, so that the tube voltage can be switched at high speed. An X-ray CT apparatus capable of obtaining a sharper enhanced image can be provided.

The figure which shows the whole outline | summary of the X-ray CT apparatus by 1st embodiment. The block diagram of the DC voltage generator of the X-ray CT apparatus by 1st embodiment Circuit block diagram showing an example of switch circuit The figure which shows the whole X-ray CT apparatus outline | summary by 2nd embodiment. Block diagram of the DC voltage generator of the X-ray CT apparatus according to the second embodiment

  Embodiments of the X-ray CT apparatus of the present invention will be described below.

<First embodiment>
FIG. 1 is a diagram showing an overall outline of an X-ray CT apparatus according to the first embodiment. This X-ray CT apparatus mainly includes a high-voltage generator 2 that converts AC power supplied from an AC power source 1 into a DC high voltage, and a DC high voltage output from the high-voltage generator 2 to which X-rays are applied. The X-ray tube 3 that radiates, the X-ray detector 5 disposed opposite to the X-ray tube 3 with the subject 4 interposed therebetween, and the X-ray transmission data of the subject detected by the X-ray detector 5 An image processing device 6 that generates a CT image using the system, a system controller 7 that controls the entire system, a display device 8 that displays the CT image and various information, and an operation console 9 are provided.

  Although not shown, a collimator that variably sets the X-ray irradiation field, a rotating frame (scanner rotating unit) that rotates by mounting the X-ray tube 3 and the X-ray detecting unit 5 are provided. The X-ray detection unit 5 includes a detector array 51 in which a number of X-ray detection elements that convert X-rays into electrical signals are arranged in a one-dimensional or two-dimensional direction, and an amplifier 52 that amplifies signals detected by the X-ray detection elements. It consists of.

  The X-ray CT apparatus of the present embodiment is a dual energy type apparatus that can irradiate two types of X-rays having different energies. For this reason, the high voltage generator 2 applies two different DC voltages to the X-ray tube 3. It is characterized in that it can be applied.

  Specifically, the high voltage generator 2 includes an AC / DC converter circuit 21 that converts AC power supplied from the commercial power source 1 into DC power, and first and second converters that convert this DC power into a higher DC voltage. Two DC / DC converter circuits 22a and 22b, and switching means for switching the outputs of the first and second DC / DC converter circuits 22a and 22b. The first and second DC / DC converter circuits 22a and 22b output different direct current voltages, and these outputs are switched by switching means and connected to the X-ray tube. In the present embodiment, the output of the first DC / DC converter circuit 22a is larger than the output of the second DC / DC converter circuit 22b, and as the switching means, the first DC / DC converter circuit 22a having a large output A switching circuit 23 is connected between the X-ray tube 3 and a diode 24 is connected between the second DC / DC converter circuit 22 b having a small output and the X-ray tube 3.

  In addition, the high voltage generator 2 is provided with a high voltage control circuit 27 that controls the DC / DC converter circuits 22 a and 22 b and the switching circuit 23 in response to a command from the system controller 7. The high voltage control circuit 27 sends a tube voltage setting signal that determines the outputs to the two DC / DC converter circuits 22a and 22b. For example, a high voltage output of −140 kV is set for the first DC / DC converter circuit 22a, and a high voltage output of −80 kV is set for the second DC / DC converter circuit 22b. Further, the high voltage control circuit 27 sends a switching signal having a predetermined frequency determined by the scanning condition to the switching circuit 23, and switches the switching circuit 23 on and off at the frequency. For example, when the irradiation energy is switched for each view, the frequency of the switching signal is 1800 Hz for a view rate of 1800 views / s.

  For example, as shown in FIG. 2, the DC / DC converter circuits 22a and 22b are connected to an inverter circuit 221 that converts AC power having a frequency higher than the frequency of the AC power supply 1, and the AC power is insulated to generate a voltage with a high turn ratio. A high-voltage transformer 222 that boosts the voltage and a high-voltage rectifier 223 that rectifies the high-frequency AC voltage into direct current are provided. The inverter circuit 221 is a circuit in which a semiconductor switching element such as an IGBT (Insulated Gate Bipolar Transistor) is configured as a full bridge, and the output can be controlled by adjusting the conduction time, frequency, and mutual phase relationship. . Therefore, the tube voltage setting signal sent by the high voltage control circuit 27 may be a PWM signal or a frequency signal that drives the inverter circuit 221.

  The switching circuit 23 connected between the first DC / DC converter circuit 22a and the X-ray tube 3 is a switching circuit using a semiconductor switching element such as a MOSFET (Metal Oxide Silicon Field Effect Transistor), and has a high voltage. Switching operation is performed by a control signal (tube voltage switching signal) of the control circuit 27. Since the tube voltage switching signal has a frequency corresponding to the view rate, that is, a frequency of several kHz as described above, the switching circuit 23 is configured to operate at a sufficient speed in response to this frequency. Further, the current flowing through the switching circuit 23 is equal to the tube current, which is 1 A or less, but the voltage at both ends is very high, and a high voltage (for example, −140 kV) generated in the first DC / DC converter circuit 22a, 2 has a circuit configuration and structure having high withstand voltage and noise resistance because the voltage is a difference voltage (in this case, Δ60 kV) from the low voltage (for example, −80 kV) generated in the DC / DC converter circuit 22b. .

  An example of the switching circuit 23 that satisfies such requirements is shown in FIG. This switching circuit 23 is a cascade combination of semiconductor elements (Q1 to Qn in the figure) capable of high-speed switching operation such as MOSFETs, and includes a capacitor C (C1 to Cn) and a resistor Rg (Rg1 to Rgn). ) And a hysteresis element (here, a Zener diode) 231 is connected between the gate of the switching element (MOSFET) Qi (i = 2 to n) and the cathode of the preceding switching element Qi-1. Thus, the respective elements Q1 to Qn are sequentially turned on. Further, Zener diodes ZD1 to ZDn for preventing overvoltage are connected between the gate and the cathode of the switching element.

  In such a configuration, the voltage that can be applied to the switching circuit 23 is such that the breakdown voltage × number (n) of the semiconductor elements can be applied between the terminals (+, −) of the switching circuit 23, and the high voltage is switched at high speed. It becomes possible. Further, when the high voltage is switched at high speed, noise is generated, and the switching circuit or the like is liable to malfunction. However, in this switching circuit 23, by providing a Zener diode 231 in series with the gates of the switching elements Q1 to Qn, noise resistance is increased. I try to increase the nature.

  As the switching circuit 23, a circuit that can operate at a frequency of several kHz and has a high withstand voltage can be employed in addition to the circuit illustrated in FIG. For example, the technique described in Japanese Patent Laid-Open No. 8-212948 can be applied.

Next, the operation of the X-ray CT apparatus of the present embodiment will be described.
The operator uses the operation console 9 to set the first and second tube voltages (for example, −140 kV and −80 kV) and tube current, the scan speed (rotation speed of the scanner rotating unit), the X-ray collimation condition, the scan Scan conditions such as a start position and an end position, a moving speed or step feed pitch of a couch top (not shown) on which a subject is placed, a type of reconstruction filter function, and a field size (FOV) are set.

  This scan condition is input to the system controller 7, where a control signal is generated based on the scan condition and transmitted to each device or circuit. To the high voltage control circuit 27, the first and second tube voltage setting values and a tube voltage switching signal for periodically switching between the first tube voltage and the second tube voltage are sent as control signals.

  The first tube voltage set value is transmitted to the inverter circuit 221 inside the first DC / DC converter circuit 22a, and the second tube voltage set value is sent to the inverter circuit 221 inside the second DC / DC converter circuit 22b. Is transmitted. Based on the set tube voltage value, the frequency and pulse width of each inverter circuit 221 are adjusted, and the first DC / DC converter circuit 22a receives the first tube voltage (−140 kV) and the second DC / DC. A second tube voltage (−80 kV) is output from the converter circuit 22b.

  The switching frequency of the tube voltage switching signal is determined by the view rate from the number of views per rotation and the scanning speed, and is determined by the view rate. When the switching signal of the frequency determined in this way is input as the gate signal of the first switching element Q1 of the switching circuit 23, the switching circuit 23 turns on and off the first DC / DC converter circuit 22a at the set frequency. To do. Thus, when the first DC / DC converter circuit 22a is on, the first tube voltage is applied to the X-ray tube 3, and when the first DC / DC converter circuit 22a is off, the X-ray tube 3 is applied to the X-ray tube 3. A second tube voltage is applied. What is important here is that the first tube voltage generated by the first DC / DC converter circuit 22a is higher than the second tube voltage generated by the second DC / DC converter circuit 22b. It is set. Since the diode 24 is inserted between the second DC / DC converter circuit 22b and the X-ray tube 3, the period during which the first tube voltage is applied to the X-ray tube 3, that is, the switching circuit 23 is turned on. During the period, the second DC / DC converter circuit 22b does not affect the output.

  In synchronization with the tube voltage switching signal being sent to the high voltage control circuit 27, the system controller 7 sends a scan start signal to a scanner rotation unit (not shown). As a result, when the scanner rotation unit rotates and the application of the tube voltage to the X-ray tube 3 starts, the tube voltage is set to a cycle in which the first tube voltage (−140 kV) and the second tube voltage (−80 kV) are set. The X-rays having two different energies are exposed to the subject 4.

  X-rays transmitted through the subject 4 are detected by an X-ray detector 51, and thereafter, processing is performed in the same manner as in a conventional dual energy X-ray CT apparatus. That is, the X-ray detection signal detected by the X-ray detector 51 is amplified by the preamplifier 52 and transmitted to the image processing device 6. The system controller 7 continues to monitor the presence or absence of the scan end signal along with the collection status of the X-ray detection data, and continues the scan until the scan end signal is input. When X-ray detection data for all views in the entire scan range are detected and transmitted to the image processing device 6, a data collection end signal is sent from the image processing device 6 to the system controller 7, thereby scanning from the system controller 7. An end signal is generated. This scan end signal is output to the high voltage generator 2, scanner control device, bed control device, and the like. And operation | movement of each control apparatus is stopped and CT scan is complete | finished.

  In the image processing apparatus 6, this X-ray detection data is divided into data at the time of X-ray irradiation of the first tube voltage and data at the time of X-ray irradiation of the second tube voltage, and each data is subjected to various corrections. Processing is performed to generate projection data. An image reconstruction calculation is performed using these projection data to generate reconstructed image data. Further, a difference image (emphasized image) between two pieces of reconstructed image data having different tube voltage conditions is generated. Note that the reconstruction of the CT image is not started after the scan is completed, but the image reconstruction can be started as soon as the X-ray detection data necessary for the image reconstruction is obtained even during the scan. A CT image obtained by performing image display control on the reconstructed image data is displayed on the image display device 8 together with accompanying information input from the operation console 9 by the operator.

  According to the present embodiment, the first and second tube voltages can be switched at a high switching frequency of several kHz and can be stably operated. By irradiating X-rays of two energies, a clear image can be obtained.

In the present embodiment, the converter circuit that outputs the first tube voltage is connected to the X-ray tube via the switching circuit, and the converter circuit that outputs the second tube voltage smaller than that is connected to the X-ray via the diode. By connecting to the tube, the two outputs can be switched by a single switching circuit and a single control signal, and one output can be reliably prevented from affecting the other output. .
In the present embodiment, the case where the tube voltage switching frequency is set to a frequency corresponding to the view rate has been described. However, the switching may be performed in units of views. For example, according to the characteristics of the examination target region, a plurality of views are displayed. The tube voltage may be switched every time.
In the example shown in the figure, the high voltage control circuit 27 is provided separately from the system controller 7. However, the system controller 7 can have the function of the high voltage control circuit 27.

  Furthermore, in this embodiment, a dual energy type device is illustrated as a typical example of a multi-energy type X-ray CT device, and a high voltage generation device including two DC voltage generation devices (DC / DC converter circuits) is described. However, the high voltage generator may include three or more DC voltage generators. In that case, it is desirable that the converter circuit having the minimum output is connected to the X-ray tube by a diode, and the other converter circuits are connected to the X-ray tube by a series connection body of the diode and the switching circuit, or only the switching circuit.

<Second Embodiment>
FIG. 4 is a block diagram showing a schematic configuration of part of the X-ray CT apparatus according to the second embodiment of the present invention. Also in the X-ray CT apparatus of the present embodiment, the power supply device (high voltage generator) of the X-ray tube includes a plurality of DC voltage generators. The multi-energy X-ray CT apparatus that obtains the CT image and the enhanced image is the same as in the first embodiment, but the internal configuration of the high-voltage generator is different from that in the first embodiment. Therefore, in FIG. 4, the part which overlaps with 1st embodiment is abbreviate | omitted, and only the alternating current power supply 1, the high voltage generator 2, and the X-ray tube 3 are extracted.

  In the first embodiment, the first DC / DC converter circuit 22a and the second DC / DC converter circuit 22b are provided after the AC / DC converter circuit 21 connected to the AC power source 1. On the other hand, in the present embodiment, the inverter circuit 25 is provided in the subsequent stage of the AC / DC converter circuit 21, and the first AC / DC converter circuit 26a and the second AC / DC converter circuit 26b are provided in the subsequent stage. doing. The inverter circuit 25 converts AC power having a frequency higher than the frequency of the AC power supply 1, and the first AC / DC converter circuit 26 a and the second AC / DC converter circuit 26 b have different first tube voltages. The second tube voltage is output. The configurations of the switching means 23 and the diode 24 for switching these outputs are the same as in the first embodiment.

  FIG. 5 shows a configuration example of the first AC / DC converter circuit 26a and the second AC / DC converter circuit 26b. The AC / DC converter circuit 26 includes a high-voltage transformer 262 that insulates AC power and boosts the voltage at a high turns ratio, and a high-voltage rectifier 263 that rectifies the high-frequency AC voltage to DC. For example, when the first tube voltage is −140 kV and the second tube voltage is −80 kV, the turn ratio N of the high-voltage transformer 262 of the first AC / DC converter circuit 26a is set as the second AC / DC. The turn ratio of the high voltage transformer 262 in the converter circuit 26b is N × 80 kV / 140 kV. Thus, even if the inverter circuit 25 is the same by setting the turns ratio of the high voltage transformer 262 of the first AC / DC converter circuit 26a and the second AC / DC converter circuit 26b, A first tube voltage and a second tube voltage can be generated.

  Also in the present embodiment, the tube voltage and the switching frequency of the switching means 23 are controlled by the control signal generated from the high voltage control circuit 27 in response to a command from the system controller 7 as in the first embodiment. is there. However, in the first embodiment, the tube voltage setting signals are respectively sent to the two DC / DC converter circuits 22 and the tube voltage is individually set. In the second embodiment, the tube voltage setting is performed. By sending a signal to the inverter circuit 25, the magnitude of the tube voltage (the output ratio is constant) of the two AC / DC converter circuits 26 can be controlled.

  Since the operation of each element other than the high voltage generator 2 and its control is substantially the same as that of the first embodiment, the description thereof is omitted.

According to the present embodiment, as in the first embodiment, since the operation can be performed even when the switching cycle of the first and second tube voltages is as high as several kHz, the heartbeat can be performed even at a site where body motion is intense such as the heart. It is possible to obtain a clear image by irradiating X-rays of two energies in synchronism with each other.
Further, as compared with the first embodiment having two inverter circuits 221, a single inverter circuit can be provided, and the size can be further reduced.

  According to the present invention, in multi-energy X-ray CT, it is possible to perform imaging while switching a plurality of tube voltages at a switching speed corresponding to a view rate that is an image data capturing speed. Thereby, when an emphasized image is obtained by calculation between images having different energies, a clear image that is not affected by the movement of the subject can be obtained.

DESCRIPTION OF SYMBOLS 1 ... AC power source, 2 ... High voltage generator, 3 ... X-ray tube, 5 ... X-ray detector, 6 ... Image processing device, 7 ... System controller, 8 * ..Image display device, 9 ... operation console, 21 ... AC / DC converter, 22a, 22b ... DC / DC converter circuit, 23 ... switching circuit, 24 diode, 25 ... inverter circuit , 26a, 26b... AC / DC converter circuit, 27... High voltage control circuit.

Claims (8)

An X-ray source including an X-ray tube and a power supply device thereof, an X-ray detector that detects X-rays generated from the X-ray source and transmitted through an inspection object, the X-ray source, and the X-ray detector; A scanner rotating unit having a rotating plate opposed to each other, an image processing unit for reconstructing the image to be inspected using data relating to transmitted X-rays detected by the X-ray detector, the X-ray source, X In an X-ray CT apparatus comprising a line detector, a scanner rotation unit, and a control unit that controls an image processing unit,
The X-ray tube power supply device is connected between a common AC power supply and the X-ray tube, and generates a plurality of DC voltage generators, and one of the plurality of DC voltage generators. A diode connected in series with the X-ray tube, the anode side of which is connected to the cathode of the X-ray tube and the cathode side of which is connected to the one DC voltage generator; and the one DC voltage An X-ray CT apparatus comprising switching means connected in series between a DC voltage generator other than the generator and the X-ray tube, and operated by a control signal from the control unit. The X-ray CT apparatus according to claim 1 ,
The plurality of DC voltage generators are DC / DC converter circuits including an inverter, a transformer connected to the inverter, and a rectifier connected to the transformer and converting an AC voltage into a DC voltage,
The X-ray CT apparatus, wherein the control unit controls the output of the inverter according to imaging conditions. The X-ray CT apparatus according to claim 1 ,
The plurality of DC voltage generators are AC / DC converter circuits including a transformer and a rectifier connected to the transformer and converting an AC voltage into a DC voltage,
The power supply device includes an inverter between the AC power supply and the DC voltage generator,
The X-ray CT apparatus, wherein the control unit controls the output of the inverter according to imaging conditions. The X-ray CT apparatus according to any one of claims 1 to 3 ,
The control unit controls the X-ray source and the scanner rotation unit, measures a number of views during one rotation of the scanner rotation unit, and turns on / off the switching unit for each measurement of one view. An X-ray CT apparatus, wherein a control signal for switching is sent to the switching means. The X-ray CT apparatus according to any one of claims 1 to 4,
  The X-ray CT apparatus, wherein the switching means switches any one of the DC voltage generators other than the one DC voltage generator at a frequency determined by a view rate. A high voltage generator for an X-ray tube,
A plurality of DC voltage generators that are connected to a common AC power source and connected to the X-ray tube and generate different voltages, and a series connection between one of the plurality of DC voltage generators and the X-ray tube A diode having an anode connected to the cathode of the X-ray tube and a cathode connected to the one DC voltage generator; and a DC voltage generator other than the one DC voltage generator; A high voltage generator for an X-ray tube comprising switching means connected in series with the X-ray tube. A high voltage generator for an X-ray tube according to claim 6,
Wherein one of the DC voltage generator, high voltage generator for an X-ray tube, wherein the generation voltage of the plurality of DC voltage generating device is the lowest DC voltage generator. A high-voltage generator for an X-ray tube according to claim 6 or 7,
  The high voltage generator for an X-ray tube, wherein the switching means switches any DC voltage generator other than the one DC voltage generator at a frequency determined by a view rate.

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