JP5129692B2 - X-ray generator and driving method of X-ray tube - Google Patents

Description

  The present invention relates to an X-ray generation apparatus used for a medical X-ray imaging apparatus, and more particularly to an X-ray generation apparatus and an X-ray tube driving method in which an imaging preparation period is shortened.

  Conventionally, a medical X-ray imaging apparatus irradiates a subject with X-rays generated from an X-ray tube, detects X-rays transmitted through the subject with an X-ray detector, and processes the detected image data for diagnosis. I have an image. Alternatively, X-rays transmitted through the subject are directly photographed on the film.

  Generally, the X-ray generator has an X-ray tube having a bipolar structure. The X-ray tube applies a high DC voltage (tube voltage) between the cathode and anode, heats the cathode with a filament current, emits thermoelectrons, and focuses the thermoelectrons emitted from the filament onto the anode target. To emit X-rays. Tungsten is used for the target (anode), and the thermoelectrons emitted from the cathode are accelerated by the anode voltage and collide with the target as a thermionic current.

  In the X-ray generator, a tube voltage control circuit for controlling the tube voltage applied to the anode of the X-ray tube and a tube current control circuit for controlling the X-ray tube current (tube current) by controlling the filament current are provided. Furthermore, a circuit for preheating the filament is provided. The filament preheating circuit is provided to speed up the rise of the tube current. Generally, a preheating current that does not give an excessive load to the filament is allowed to flow for a predetermined time, and then the tube voltage is generally applied. It has been adopted.

  A method of preheating the filament and applying the tube voltage after that is described in Non-Patent Document 1, for example.

  FIG. 9 is a timing chart for explaining a conventional X-ray tube driving method. In FIG. 9, when the X-ray irradiation switch is turned on at timing t0, preheating control of the filament is started, and a current is passed through the filament to accumulate thermoelectrons. At this time, the filament current is controlled to be constant.

  When a preset time elapses, the tube current control circuit starts operating instead of the filament preheating control circuit at timing t1, and at the same time, the tube voltage control circuit also starts operating. As a result, a high voltage is applied to the X-ray tube, the thermoelectrons accumulated in the filament jump out, and an X-ray tube current flows from the cathode to the anode of the X-ray tube.

  The tube current control circuit keeps the filament current flowing by controlling the X-ray tube current so as to be close to the target value, and the tube voltage control circuit also controls the tube voltage so that the voltage becomes the target value. Thus, when the target value is reached (timing t2), X-ray imaging is performed under preset tube voltage and tube current conditions. The X-ray irradiation time is set by the operation unit, and when the set time elapses, all the control circuits are stopped and the operation ends at timing t3.

  FIG. 10 shows waveforms of the X-ray tube filament preheating current, the X-ray tube voltage, and the X-ray tube current. The vertical axis represents current and voltage, and the horizontal axis represents time. In the conventional X-ray tube driving method, a filament current is passed for a long time (for example, 2 seconds) at the time of preheating the filament. The trouble that becomes. For this reason, the filament current during preheating must be kept low.

  In FIG. 10, the shaded area indicates the preheating control period. When the preheating control period ends and the control is switched to the X-ray tube current and the X-ray tube voltage at time t1, the X-ray tube current and the X-ray tube voltage gradually increase. At this time, space charge is formed in the X-ray tube by preheating the filament. In this region, the X-ray tube current increases as the X-ray tube voltage increases. However, if the amount of thermoelectrons in the filament during the initial preheating is insufficient, it takes time for the X-ray tube current to reach the target value as shown in FIG. Time t2 represents the time when the X-ray tube current reaches the target value.

  Accordingly, the preheating control period (t0-t1) and the period (t1-t2) until the X-ray tube current reaches the target value tend to be long, and the tube voltage and tube current set in advance after the switch is turned on. It took more than 3 seconds to irradiate X-rays under the conditions. For this reason, in order to obtain an image suitable for diagnosis, there is a problem that the X-ray imaging time becomes long. In addition, when photographing a moving part such as the heart of a subject, the photographing timing may be delayed and an accurate image may not be taken. In addition, since the X-rays are emitted until the X-ray tube current reaches the target value, the subject may be exposed to extra exposure.

In addition, there is an example in which a ready switch and an X-ray irradiation switch are provided in order to perform imaging with good timing. In this example, the ready switch is turned on to preheat the filament, the ready lamp is turned on when the preheating is completed, and then the X-ray irradiation switch is pressed at an appropriate timing to irradiate X-rays. . However, it takes time until the X-ray is actually irradiated after the ready switch is turned on, and the operation is complicated.
"Radiological equipment studies (1)" published by Corona in 1990

  In the conventional X-ray generator, the X-ray tube filament preheating period and the period until the X-ray tube current reaches the target value are long, so the X-ray imaging time is long, and the moving part such as the heart of the subject. When shooting, there was a problem that the shooting timing was delayed and an accurate image could not be taken. In addition, the subject was exposed to excessive exposure.

  In view of the above circumstances, the present invention provides an X-ray generation apparatus and an X-ray tube driving method capable of shortening a rise time to an X-ray tube current as a target value and quickly performing X-ray imaging. With the goal.

The X-ray generator of the present invention includes a high voltage circuit for applying a tube voltage to an anode of an X-ray tube, an operation switch for X-ray irradiation, and a filament of the X-ray tube in response to the operation of the operation switch. A preheating control circuit that preheats the filament by flowing a filament current set near the maximum rating of the filament current for a predetermined time, accumulates thermoelectrons in the X-ray tube and maintains a space charge limited state, and the X-ray tube starts the control operation of the tube voltage to the space-charge limited state in the preheating period in which they are, by controlling the tube voltage of the X-ray tube to the target voltage value, the tube of the X-ray tube A tube voltage control circuit in which the tube current reaches the target current value before the voltage reaches the target voltage value, and the tube current of the X-ray tube is changed to the target current value by increasing the tube voltage of the X-ray tube; When the preheating system is A tube current control circuit for controlling the filament current instead of a circuit and controlling the tube current to a target current value; a control start time of the tube voltage and the tube voltage and the tube current based on the operation of the operation switch; And a controller that stops supply of the tube voltage and tube current after the tube voltage and tube current of the X-ray tube reach target values and the stop time elapses. Characterize

According to the X-ray tube driving method of the present invention, in response to operation of an operation switch for X-ray irradiation, a filament current set in the vicinity of the maximum rating of the filament current is allowed to flow through the filament of the X-ray tube for a predetermined time. Within the preliminary heating period in which the X-ray tube maintains the space charge limited state based on the operation of the operation switch. The tube voltage control operation is started , the tube voltage of the X-ray tube is controlled to a target voltage value, and the tube current reaches the target current value before the tube voltage of the X-ray tube reaches the target voltage value. When the tube current of the X-ray tube reaches a target current value by increasing the tube voltage of the X-ray tube, the filament current is controlled instead of the preheating to control the tube current to the target current. Control the value to The tube voltage and tube current stop time is set based on the switch operation, and the tube voltage and tube current of the X-ray tube reach the target values and the tube voltage and tube current are The supply is stopped.

  According to the present invention, it is possible to perform X-ray imaging promptly by shortening the time from when an X-ray irradiation operation switch is turned on to when a desired X-ray is output, and there is movement of a subject such as a heart. It is possible to take a picture with good timing when photographing a part. Further, since the time required to reach a desired X-ray tube current can be shortened, the subject can be prevented from being excessively exposed.

  Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a block diagram showing the overall configuration of an X-ray generator 10 according to the present invention. The X-ray generator 10 includes a high voltage circuit 11 and an X-ray tube 12. The high voltage circuit 11 is, for example, a circuit that boosts the DC voltage of the main power supply to the tube voltage within the use range of the X-ray tube. As the main power source, a battery power source or a power source circuit that rectifies and smoothes an AC voltage such as a commercial AC power source is used.

  The X-ray tube 12 includes an anode 13 and a filament (cathode) 14, and a DC high voltage (tube voltage) from the high voltage circuit 11 is applied between the anode 13 and the filament 14. In the following description, the voltage applied to the anode of the X-ray tube is called X-ray tube voltage or tube voltage, and the current flowing through the X-ray tube is called X-ray tube current or tube current.

  In order to supply a filament current to the filament 14, a DC power supply 15, an inverter circuit 16, and a filament transformer 17 are provided. The DC power supply 15 may be a power supply circuit that rectifies and smoothes an AC voltage from a commercial AC power supply to obtain a DC power supply voltage, or a battery power supply.

  The inverter 16 converts a DC power supply voltage from the DC power supply 15 into a high-frequency AC voltage, and includes a semiconductor switching element 161. The output terminal of the inverter circuit 16 is connected to the primary winding of the filament transformer 17 and the secondary winding is connected to the filament 14.

  The X-ray generator 10 further includes an operation unit 18, a tube voltage control circuit 19, a tube current control circuit 20, and a preheating control circuit 21. The operation unit 18 includes a power switch, an operation switch for X-ray irradiation, and the like, and further includes a control unit 181 including a CPU. The operation switch is operated by an operator, and the control unit 181 controls each circuit in response to the operation of the operation switch. The operation unit 18 also sets the tube voltage and tube current of the X-ray tube 12 according to the imaging region of the subject, and instructs the X-ray tube 12 to supply the tube voltage and tube current suitable for imaging for each region. To do.

  The tube voltage control circuit 19 controls the high voltage circuit 11 to control the tube voltage (Vp) applied to the anode of the X-ray tube 12. The tube current control circuit 20 controls the tube current (Ip) of the X-ray tube 12 and includes a detection circuit 22, a comparison circuit 23, and a drive circuit 24. The detection circuit 22 detects the tube current of the X-ray tube 12. The comparison circuit 23 compares the target current value given from the operation unit 18 with the current value detected by the detection circuit 22, and controls the drive circuit 24 according to the comparison result (integral value). The drive circuit 24 drives the switching element 161 of the inverter 16 so that the tube current of the X-ray tube 12 becomes a target current value, and controls the filament current (If).

  The preheating control circuit 21 includes a reference voltage source 25 that generates a reference voltage, a comparison circuit 26, and a drive circuit 27. The preheating control circuit 21 supplies a filament current to the filament 14 during a preheating period (described later), and outputs a constant reference voltage from the reference voltage source 25 based on an instruction from the operation unit 18. A constant signal is given to the drive circuit 27 by comparing the internal voltage of the circuit 26 with the reference voltage.

  The drive circuit 27 controls the filament current by driving the switching element 161 of the inverter 16 so that the current of the filament 14 becomes a predetermined current value during the preheating. The filament current during preheating is set near the maximum rating of the filament current of the X-ray tube 12 and can be arbitrarily set by selecting the reference voltage of the reference voltage source 25. For example, if the maximum rating of the filament current is 3 [A], it is set to a current close to that (for example, about 2.9 [A]), so that space charge can be formed in a short time.

  Next, the operation of the X-ray generator 10 of the present invention will be described with reference to FIG. FIG. 2 is a flowchart showing an operation from the start of X-ray imaging to the end of the X-ray generation operation.

  When the operation switch for X-ray irradiation is turned on in start step S1 of FIG. 2, the preheating control circuit 21 starts preheating control of the filament 14 in step S2. In the next step S3, it is determined whether or not a preset time has elapsed. When the preset time is reached, the process proceeds to step S4. The time set in step S3 is determined by the operation unit 18, and is set within 0.2 seconds, for example.

  Step S4 and subsequent steps are an X-ray irradiation period. In step S4, control of the X-ray tube voltage is started, and the tube voltage control circuit 19 controls the tube voltage. When the tube voltage control circuit 19 starts to operate, a high voltage is applied to the X-ray tube 12. At this time, the X-ray tube current rises as the X-ray tube voltage increases, and reaches the target value determined by the operation unit 18. The X-ray tube current reaches the target current value before the X-ray tube voltage reaches the target voltage value.

  In step S5, it is determined whether or not the X-ray tube current has reached the target current value. When the target current value is reached, the preheating control circuit 21 switches to control by the tube current control circuit 20 in step S6. As a result, the tube current is controlled to be the target current value. Then, the X-ray tube voltage reaches the target value with a slight delay.

  Thus, when control of the X-ray tube voltage is started, X-rays are emitted from the X-ray tube 12. The X-ray irradiation time is set by the operation unit 18. When the set irradiation time has elapsed, the control circuits 19 and 20 immediately stop operating, and the operation ends in step S7.

  FIG. 3 is a timing chart for explaining the X-ray tube driving method of the present invention. In FIG. 3, when the operation switch for X-ray irradiation is turned on at timing t0, the preheating control circuit 21 operates to supply a filament current to the filament 14 and accumulate thermoelectrons. At this time, the filament current is controlled to be constant, and the filament current flows up to the vicinity of the maximum rating of the filament current of the X-ray tube 12. When a predetermined time has passed from the preheating start t0, the tube voltage control circuit 19 starts operating at timing t1. At this time, since the X-ray tube 12 continues the filament preheating state, the X-ray tube current increases as the X-ray tube voltage increases and increases according to the 3/2 power law.

  When the X-ray tube current reaches the target current value at timing t2, the preheating control circuit 21 switches to the control of the tube current control circuit 20, and the X-ray tube voltage reaches the target voltage value with a slight delay. Thereafter, the X-tube current and the X-ray tube voltage are controlled by the tube current control circuit 20 and the tube voltage control circuit 19 so as to maintain target values.

  The X-ray irradiation starts from the time when the X-ray tube voltage control starts (timing t1), and the irradiation period is set by the operation unit 18. When the determined X-ray irradiation period has passed, the control circuit 19.20 immediately stops at timing t3, and all operations are terminated.

  FIG. 4 shows respective waveforms of the preheating current, the X-ray tube current, and the X-ray tube voltage. In FIG. 4, when the operation switch for X-ray irradiation is turned on at time t <b> 0, the preheating control circuit 26 starts operating, and a preheating current flows through the filament 14. At this time, since the filament current set in the vicinity of the maximum rated current of the filament current flows through the X-ray tube 12, the thermal electrons generated from the filament accumulate space charges.

  Next, when a predetermined time t1 has passed from the operation of the operation switch, the tube voltage control circuit 19 starts to operate and raises the tube voltage of the X-ray tube 12. At this time, since the filament current continues to be in the filament preheating state, the X-ray tube current increases according to the 3/2 power law as the X-ray tube voltage increases. For this reason, the X-ray tube current can realize a fast rise.

  The X-ray tube 12 maintains the space charge limited state until the X-ray tube current completely rises, and the X-ray tube current is targeted at a time earlier than the X-ray tube voltage (timing t2) due to the characteristics of the X-ray tube 12. Reach value. When the target current value is reached, the preheating control circuit 21 switches to the control of the tube current control circuit 20, and the X-ray tube voltage reaches the target voltage value with a slight delay.

  Thereafter, the X-ray tube current and the X-ray tube voltage are controlled by the tube current control circuit 20 and the tube voltage control circuit 19 so as to maintain the target values. The current and the X-ray tube voltage are constant, and X-rays are irradiated under preset tube voltage and tube current conditions.

  The shaded area in FIG. 4 indicates the preheating control period. The preheating control period is set to a very short time (for example, within 0.2 seconds) so that the filament current flows to the vicinity of the maximum rating of the filament current, and the X-ray tube current and the X-ray tube voltage are set as targets. The time to reach the value can also be shortened to 1/10 or less compared to the conventional case. Therefore, the preparation period for actual X-ray imaging can be shortened to almost zero. Even if the filament current is allowed to flow near the maximum rating, the period is very short, so that the life of the filament 14 can be prevented from being shortened.

  Next, the operation of the X-ray generator 10 of the present invention described above will be supplementarily described with reference to the characteristic diagrams of FIGS.

  FIG. 5 is a characteristic diagram of the X-ray tube 12. The vertical axis represents the tube current Ip, the horizontal axis represents the tube voltage Vp, and shows the Ip-Vp characteristics when the filament current If is changed. The X-ray tube 12 includes a temperature limiting region A in which the amount of thermoelectrons is determined by the temperature of the cathode and a space charge limiting region B in which the tube current is limited by the tube voltage. The temperature limited region A is in a state where the tube current Ip is hardly influenced by the tube voltage Vp, and the tube current Ip mainly depends on the amount of thermoelectrons generated from the filament.

In the space charge limiting region B, since the thermoelectrons generated from the filament form space charges, the tube current Ip depends only on the tube voltage Vp. The tube current Ip in the space charge limited state (B) is obtained by the 3/2 power law as shown by the equation (1). However, G is a proportionality constant determined by the electrode structure.

  However, the actual operation of the X-ray tube cannot be explained only by the two states described above. Therefore, as a result of measuring the detailed characteristics using a bipolar tube (5Y3GT) having the same characteristics as the X-ray tube, the characteristics shown in FIGS. 6, 7, and 8 were obtained.

  FIG. 6 is a characteristic diagram showing the relationship between the tube current Ip and the tube voltage Vp. The vertical axis represents the tube current Ip, the horizontal axis represents the tube voltage Vp, and shows the Ip-Vp characteristic when the filament current If changes. ing.

  FIG. 7 is a characteristic diagram showing the relationship between the tube current Ip and the filament current If. The vertical axis represents the tube current Ip, the horizontal axis represents the filament current If, and shows the Ip-If characteristic when the tube voltage Vp changes. ing.

  FIG. 8 is a characteristic diagram showing the relationship between the tube voltage Vp and the filament current If. The vertical axis represents the tube voltage Vp, the horizontal axis represents the filament current If, and the Vp-If characteristic when the tube current Ip changes is shown. Show.

  From these figures, it can be seen that there exists a state in which the tube current Ip is affected by both the tube voltage Vp and the filament current If (region C indicated by a dotted line in FIGS. 6 to 8). That is, there is a mixed region C between the temperature limited region A and the space charge limited region B. It has also been confirmed that a general X-ray tube used for X-ray imaging operates in this mixed region C.

  As shown in FIG. 5, the rise characteristic when the tube voltage Vp is applied to the preheated X-ray tube 12 rises while satisfying the equation (1), and is in the space charge limited state (B). Thereby, the tube current Ip can realize an early rise. On the other hand, in the temperature limited region A, the tube current Ip increases as the filament temperature rises, but the rise of the tube current Ip is very gradual.

  Therefore, in order to maintain the space charge limited state (B) until the tube current Ip rises, it is necessary to emit sufficient thermoelectrons and not to enter the temperature limited state (A). Further, in order to form the space charge limited region B simultaneously with the application of the tube voltage Vp, it is necessary to sufficiently preheat the filament.

  The current density Js of the thermoelectrons generated after the preheating time tp is obtained by the equation (2).

However, Ifp is the filament preheating current, and R is the filament resistance [Ω].

  From the equation (2), in order to perform sufficient preheating, it is necessary to increase the filament preheating current Ifp or increase the preheating time tp. However, if the preheating time tp is increased, as described above, X It takes a long time from when the radiation irradiation operation switch is turned on until X-ray imaging is actually performed.

  In view of the above, the present invention first adopts a method of increasing the filament preheating current Ifp. Further, the space charge limited state (B) is maintained by continuing to flow the filament current thereafter. The tube voltage control circuit 19 operates in a state where the space charge limited state is maintained, and a high voltage is applied to the X-ray tube 12 until the target voltage value is reached.

  At this time, the X-ray tube current rises in accordance with the 3/2 power rule, and when the desired target current value is reached, the tube current control circuit 20 operates to control the target current value to be maintained. Thereafter, the X-ray tube voltage also reaches the target voltage value, and thereafter the target voltage value is maintained.

  Thus, in the X-ray generator of the present invention, by driving the X-ray tube while making the best use of the characteristics of the X-ray tube, the rise time of the X-ray tube current to the target current value is greatly shortened and shortened. The X-ray tube current and the X-ray tube voltage can be raised with time.

  Accordingly, the time from when the X-ray irradiation operation switch is turned on until the desired X-ray is output is shortened, so that it is possible to quickly perform imaging when X-ray imaging is desired. In particular, it is possible to take an image with good timing when photographing a moving part such as the heart of the subject or photographing a subject such as an animal that moves rapidly. Further, since the rise of the X-ray tube current is fast and the time required to reach the desired X-ray tube current can be shortened, the subject can be prevented from being excessively exposed.

  The X-ray generator of the present invention can be used with a battery or a commercial AC power supply, and is therefore suitable for a portable X-ray generator. Moreover, since the preheating control circuit 21 is configured to compare the voltage of the reference voltage source 25 and the voltage in the comparison circuit 26, a feedback circuit is not required and the circuit configuration is simplified. Therefore, the circuit board can be miniaturized and the entire apparatus can be miniaturized, so that it can be said that the structure is more suitable for a portable X-ray generator.

  Needless to say, the present invention is not limited to a portable type and can be used for an X-ray imaging apparatus installed in a medical institution or the like. Various modifications can be made without departing from the scope of the claims.

The block diagram which shows one Embodiment of the X-ray generator of this invention. The flowchart explaining operation | movement of the X-ray generator of this invention. The timing chart explaining operation | movement of the X-ray generator of this invention. The current / voltage waveform diagram explaining the operation of the X-ray generator of the present invention. FIG. 3 is a tube current-tube voltage characteristic diagram illustrating characteristics of an X-ray tube. The tube current-tube voltage characteristic diagram which explains supplementarily the characteristic of an X-ray tube. The tube current-filament current characteristic view which supplementarily explains the characteristics of the X-ray tube. The tube voltage-filament current characteristic figure explaining supplementarily the characteristic of an X-ray tube. The timing chart explaining operation | movement of the conventional X-ray generator. FIG. 6 is a current / voltage waveform diagram for explaining the operation of a conventional X-ray generator.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... X-ray generator 11 ... High voltage circuit 12 ... X-ray tube 13 ... Anode 14 ... Filament 15 ... DC power supply 16 ... Inverter circuit 161 ... Switching element 17 ... Filament transformer 18 ... Operation part 181 ... Control part 19 ... Tube voltage Control circuit 20 ... tube current control circuit 21 ... preheating control circuit 22 ... detection circuit 23 ... comparison circuit 24 ... drive circuit 25 ... reference voltage source 26 ... comparison circuit 27 ... drive circuit

Claims (2)

A high voltage circuit for applying a tube voltage to the anode of the X-ray tube;
An operation switch for X-ray irradiation;
In response to the operation of the operation switch, a filament current set near the maximum rating of the filament current is passed through the filament of the X-ray tube for a predetermined time to preheat the filament, and hot electrons are accumulated in the X-ray tube. A preheating control circuit that maintains a space charge limited state;
The control operation of the tube voltage starts the preheating within a period in which the X-ray tube is maintained the space-charge limited state, by controlling the tube voltage of the X-ray tube to the target voltage value, the X-ray A tube voltage control circuit configured such that the tube current reaches the target current value before the tube voltage of the tube reaches the target voltage value;
When the tube current of the X-ray tube reaches a target current value due to an increase in the tube voltage of the X-ray tube, the filament current is controlled instead of the preheating control circuit, and the tube current is set to the target current value. A tube current control circuit to control,
The tube voltage control start time and the tube voltage and tube current stop time are set based on the operation of the operation switch, and the tube voltage and tube current of the X-ray tube reach target values and the stop time is reached. A controller for stopping the supply of the tube voltage and tube current after elapse of
An X-ray generator characterized by comprising: In response to the operation of the operation switch for X-ray irradiation, a filament current set near the maximum rating of the filament current is passed through the filament of the X-ray tube for a predetermined time to preheat the filament, and the thermoelectrons in the X-ray tube The space charge limit state,
Based on the operation of the operation switch, the X-ray tube starts the control operation of the tube voltage to the space-charge limited state in the preheating period in which they are the target of the tube voltage of the X-ray tube by controlling the voltage value, the tube current so as to reach the target current value before the tube voltage of the X-ray tube reaches the target voltage value,
When the tube current of the X-ray tube reaches a target current value due to an increase in the tube voltage of the X-ray tube, the filament current is controlled instead of the preheating to control the tube current to the target current value,
The tube voltage and tube current stop time are set based on the operation of the operation switch, and the tube voltage and tube current after the tube voltage and tube current of the X-ray tube reach target values and the stop time elapses. A method for driving an X-ray tube, characterized in that supply of current is stopped.

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