CIRCUIT FOR CONTROLLING X-RAY EXPOSURE WITH MONOPOLAR OR BIPOLAR POWER SUPPLY BY MEANS OF A GRID CURRENT

Abstract

 A circuit that aims to control the exposure of X-rays by means of an intermediate or feedback current which is directly proportional to the grid current (IG), which in turn controls the current of the anode of the tube and which is ultimately responsible for the emission of the X-ray photons, comprising an anode (A), a cathode (K) connected to ground, as well as a grid (G) connected to a grid current controller circuit (IG) comprising a DC-DC converter (CONV) or "buck converter" to which a power supply voltage (Vsup) is connected, the output of which is connected to an inverter (INV) that is in turn connected to a transformer (TR), where the output of this transformer (TR) is connected to a rectifier (RECT) whose outputs are connected to the grid (G) and to ground. In addition, in said controller circuit there is a control circuit (CONT).

Description

OBJECT OF THE INVENTION

[0001] The object of the present invention, as indicated by its title, is a circuit for controlling X-ray exposure by means of the grid current in a cold cathode x-ray tube.

[0002] The present invention is characterised by the special design and configuration of each and every one of the elements that are part of the control circuit, so that it is possible to control the X-ray exposure by means of an intermediate or "feedback" current, which is directly proportional to a grid current, which in turn controls the current of the tube anode and which is ultimately responsible for the emission of the X-ray photons.

[0003] Thanks to the features of the circuit a more precise control is achieved, with less voltage ripple between the anode and the cathode and therefore a greater control of the emitted dose, where the elements that are part of the circuit do not have to withstand high voltages.

[0004] Therefore, the present invention lies within the scope of X-ray apparatuses and specifically cold-cathode apparatuses.

BACKGROUND OF THE INVENTION

[0005] It is known in the art that the energy of X-ray radiation depends on the kV applied, while the amount of radiation depends on the current as well as the exposure time. One of the ways to control such radiation is by controlling the the temperature of the filament, which is known as thermionic emission control.

[0006] Currently, the cold cathode X-ray tubes are being used where the control of electron emission is carried out by a cold cathode composed by a series of carbon nanotubes, where a grid has been interposed between the cathode and the anode.

[0007] Figure 1 shows a schematic diagram of a cold cathode x-ray apparatus comprising an anode (A), a cathode (K) formed by a series of carbon nanotubes and arranged opposite the anode (A), and between which a grid (G) has been interposed, there being a first loop formed by the anode (A), the cathode (K) and a voltage-controlled power supply which can be monopolar or bipolar. The configuration with the bipolar source (figure 1a) has the middle part of the source connected to ground to reduce by half the stress in the insulations of the source itself and of the x-ray tube with respect to ground. There is also a second loop formed by the grid (G), the cathode (K) and a current-controlled power supply. Both loops share a common section, so that a current IA flows through the first loop, and a current IG flows through the second loop, while a current IA+IG flows through the common section.

[0008] Figure 2 shows a graph indicating that the anode current (IA) is linear with the grid current (IG), while figure 3 shows that the relationship between the anode current (IA) and the grid current (IG) with respect to the grid voltage (VG) is exponential and therefore controlling the anode current (IA) through the grid voltage is very complicated.

[0009] Figure 4 shows a diagram of a circuit for controlling the anode current (IA) through the voltage, where the anode is grounded through a power supply (VAG), the grid (G) is directly grounded, and the cathode (K) is grounded through a power supply (Vsup) and a MOSFET switch, in which drops a voltage (Vcont) and a signal of the current demanded by the cathode is controlled.

[0010] In the control scheme shown in figure 4 it is fulfilled that:

[0011] This control scheme carries out a linear control of the cathode current (IK) but in a very complex way. In addition, it requires a MOSFET capable of working at several thousand volts, , where the voltage drop between source and drain is very large.

[0012] Another major drawback of this type of control is that the voltage between the anode and the cathode of the X-ray tube depends on the grid voltage (which varies with the aging of the X-ray tube), which directly impacts the energy of the emitted radiation.

[0013] Therefore, the object of the present invention is to develop a circuit for controlling X-ray exposure by means of the grid current in an isolated and independent manner, which establishes a constant voltage VAK between the anode and the cathode that is independent of the grid control, developing a controller circuit such as that described below, the essence of which is contained in the first claim.

DESCRIPTION OF THE INVENTION

[0014] The object of the present invention is essentially contained in the independent claim and the different embodiments are contained in the dependent claims.

[0015] The objective of the circuit is to control the exposure of X-rays by means of the IFB current, which is directly proportional to the grid current (IG), which in turn controls the tube anode current and is ultimately responsible for the emission of the X-ray photons.

[0016] The circuit for controlling X-ray exposure may be configured for a monopolar or bipolar x-ray tube power supply. In the monopolar configuration, the circuit that controls X-ray exposure comprises an anode that is connected to a power supply which is grounded through a shunt which measures anode current , further comprises a cathode which is also connected to ground, and in addition comprises a grid that has a grid current controller circuit in which a power supply voltage is connected with a DC-DC converter or "buck converter", whose output is connected with an inverter whose output is connected with a transformer, where the output of said transformer is connected to a rectifier whose outputs are connected to the grid and to ground.

[0017] In the bipolar power supply configuration of the X-ray tube, the X-ray exposure control circuit comprises an anode that is connected to a power supply which is grounded through a shunt that measures the anode current, further comprises a cathode that is connected to a power supply also connected to ground, and also comprises a grid provided with a grid current control circuit in which a power supply voltage is connected to a DC-DC converter or "buck converter", whose output is connected to an inverter, the output of which is connected to a transformer, and the output of said transformer is connected to a rectifier whose outputs are connected to the grid and to the cathode.

[0018] Furthermore, in said controller circuit there is a control that receives a signal from a feedback current (I_FB), a signal from the demand current and a preload and an exposure signal and has as outputs a first signal to the DC-DC converter and a second signal to the inverter.

[0019] In a preferred but non-limiting embodiment the DC-DC converter is formed by a capacitor in parallel with a first switch; a first diode and a coil and a return diode in parallel with the assembly formed by the first switch and the coil.

[0020] The inverter is formed by a bridge comprising a second switch, a third switch, a fourth switch and a fifth switch, wherein the second switch and the fifth switch are in series forming a first branch, while the third switch and the fourth switch are in series forming a second branch, both branches being in parallel with each other.

[0021] Each branch is connected in its middle with a transformer whose output is connected with a rectifier bridge where the grid current is obtained, while the feedback current signal (I_FB) is obtained from the lower end of both branches of the inverter and the DC-DC converter.

[0022] Thanks to the described control circuit, control of the grid is achieved by means of the current instead of by means of the voltage, since the control of the grid voltage is very complex to perform. In addition, control of the grid by means of the voltage applied requires oversizing the elements used, such as the MOSFET switch, which must be able to support up to 15 kV if necessary. Moreover, the voltage drop between the source and the drain is very large. This voltage varies with the aging state of the X-ray tube, which varies the total voltage applied between the anode and the cathode and has a direct influence on the X-ray exposure dose, because as that voltage (VAK) varies, the energy of the emitted photons and therefore the total emitted dose varies.

[0023] All these drawbacks are overcome by the control circuit by means of the grid current which in turn controls the anode current that is ultimately responsible for the emission of X-ray photons.

[0024] Unless indicated otherwise, all the technical and scientific elements used in this specification have the meaning usually understood by a person skilled in the art to which this invention belongs. In the practice of this invention, methods and materials similar or equivalent to those described in the specification may be used.

[0025] In the description and claims, the word "comprises" and its variants do not intend to exclude other technical characteristics, additives, components or steps. For persons skilled in the art, other objects, advantages and characteristics of the invention will be partly inferred from the description and partly from the practice of the invention.

EXPLANATION OF THE FIGURES

[0026] In order to complement the description being made herein, and with the object of aiding the better understanding of the characteristics of the invention, in accordance with a preferred practical embodiment thereof, said description is accompanied, as an integral part thereof, by a set of drawings where, in an illustrative and non-limiting manner, the following has been represented:

Figure 1 shows a schematic diagram of a cold cathode x-ray apparatus with monopolar power supply of the x-ray tube.

Figure 1a shows the same X-ray apparatus, in this case with bipolar power supply.

Figure 2 shows a graph indicating that the anode current (IA) is linear with the grid current (IG).

Figure 3 shows the relationship between the anode current (IA) and the grid current (IG) with respect to the grid voltage (VG).

Figure 4 shows a schematic diagram of a circuit of the prior art for controlling the anode current (IA) through the cathode current (IK), which in turn affects the voltage between anode and cathode (VAK) (and therefore also affects the energy of the emitted radiation).

Figure 5 shows the circuit for controlling X-ray exposure by means of the grid current (IG) with monopolar power supply of the x-ray tube.

Figure 5a shows the same control circuit, but with bipolar power supply of the x-ray tube.

A possible embodiment of such a circuit is shown in figure 6.

Figure 7 shows the work cycles of a first working mode or preload and exposure mode.

Figure 8 shows the working cycles of a second working mode or direct exposure mode.

Figure 9 shows a possible embodiment of a current control circuit.

PREFERRED EMBODIMENT OF THE INVENTION

[0027] In view of the figures, a preferred embodiment of the proposed invention is described below.

[0028] Figures 1 to 4 correspond to explanations of the prior art of x-ray exposure control.

[0029] Figure 5 shows the x-ray exposure control circuit with monopolar power supply of the x-ray tube, where the anode (A) is connected to a power supply (VAK) that is connected to ground through a shunt to measure the current of the anode IA, while the cathode (K) is also connected to ground; on the other hand, the grid (G) is connected to a grid current (IG) control circuit in which the power supply (Vsup) is connected to a DC-DC converter (CONV), known as a "buck converter", which in turn is connected to an inverter (INV) whose output is connected to a transformer (TR), and the output of this transformer (TR) is connected to a rectifier (RECT) whose outputs are connected to the grid (G) and to ground.

[0030] Figure 5a shows the x-ray exposure control circuit with bipolar supply of the x-ray tube, where the anode (A) is connected to a power supply (VAK/2) that is grounded through a shunt to measure the anode current (IA), while the cathode (K) is connected to another power supply (VAK/2) that is also grounded. Furthermore, the grid (G) connected to a grid current (IG) control circuit in which the supply voltage (Vsup) is connected to a DC-DC converter (CONV), known as a "buck converter", in turn is connected to an inverter (Inv) whose output is connected to a transformer (TR), and the output of this transformer (TR) is connected to a rectifier (RECT) whose outputs are connected to the grid (G) and to the cathode (K).

[0031] In addition, in said controller circuit there is a control circuit (CONT) that receives a signal from a current (I_FB), a signal of the demanded current (I_dem) and a preload signal (PRE) and an exposure signal (EXP) and has as outputs a first signal (S1) towards the DC-DC converter and a second signal (S2) towards the inverter (Inv). The circuit can have 3 states: OFF, Preload and Exposure, and therefore two control signals are needed, a preload signal (PRE) and an exposure signal (EXP).

[0032] Figure 6 shows a non-limiting preferred embodiment of the above control circuit elements, wherein the DC-DC converter is formed by a capacitor (C) in parallel with a first switch (Q1), a diode (D1) and a coil (L) and a return diode (D2) in parallel with the assembly formed by the first switch (Q1) and the coil (L).

[0033] The inverter is formed by a bridge comprising a second switch (Q2), a third switch (Q3), a fourth switch (Q4) and a fifth switch (Q5), where the second switch (Q2) and the fifth switch (Q5) are in series forming a first branch, while the third switch (Q3) and the fourth switch (Q4) are in series forming a second branch, the two branches being in parallel with each other.

[0034] Each branch is connected in its middle with a transformer (TR1) whose output is connected to a rectifier bridge from where the grid current (IG) is obtained, while the current signal (I_FB) is obtained from the lower end of both branches of the inverter and the DC-DC converter.

[0035] Figure 7 shows a first working mode of the circuit for controlling X-ray exposure, which corresponds to a preload mode.

[0036] This figure shows different cycles (C1, C2, C3, C4, C5, C6, C7 and C8) and the state of the different switches (Q1, Q2, Q3, Q4, Q5), as well as the current demand value (I_dem) and the current (I_FB).

[0037] In the C1 cycle, all 5 transistors (Q1, Q2, Q3, Q4 and Q5) are on. We see how the current (I_FB) rises linearly, limited by the value of the inductance (L).

[0038] In the C2 cycle, everything remains the same, because the current (IFB) has not yet reached the demand value (IDEM).

[0039] In the third cycle C3, eventually the current (I_FB) reaches the value (I_dem) and (Q1) stops conducting, causing the current (I_FB) to begin to fall. In the next cycle, (Q1) is switched on again and will only be switched off when (I_FB) reaches the value (I_dem) again. This Preload state could be maintained indefinitely, with the 4 inverter bridge transistors (Q2, Q3, Q4 and Q5) on and the transistor (Q1) keeping the current (I_FB) in the inductance L.

[0040] At a certain point, the System Controller decides to start the exposure by means of the signal (EXP), transferring the current stored in (L) to the grid circuit. To initiate this new state, the inverter bridge transistors (Q2, Q3, Q4 and Q5) open the short-circuit and start operating as Inverter, i.e. as a DC to AC converter. To do this (Q2) and (Q4) are turned on, turning off (Q3) and (Q5). In the next cycle (Q2) and (Q4) are turned off and (Q3) and (Q5) are turned on, keeping this alternating cycle indefinitely as long as the X-ray exposure remains active. In each and every cycle and whatever the Preload or Exposure state, the transistor (Q1) continues to keep the current (I_FB) at the value (I_DEM).

[0041] When the System Controller decides to terminate the X-ray exposure by means of the signal (EXP), the 5 transistors (Q1, Q2, Q3, Q4 and Q5) are switched off instantly and simultaneously. At that moment the current stops circulating towards the grid, instantly turning off the X-ray exposure. The energy stored in the inductance (L) is transferred to the power supply (Vsup) through the diodes (D1) and (D2).

[0042] Another way to terminate the X-ray exposure, is to turn on the 4 transistors of the inverter bridge (Q2, Q3, Q4 and Q5) while maintaining the control of the current (I_FB) with (Q1), so that it will enter Preload mode, being able to perform a new exposure at any time and without any delay.

[0043] Figure 8 shows a second mode of operation of the circuit for controlling X-ray exposure, which corresponds to a direct mode.

[0044] The objective of the direct mode is to start the X-ray exposure instantly, without waiting for the grid to have the optimal current. In this mode, the exposure time will be slightly longer than in Preload Mode.

[0045] As can be seen in Figure 8, the transistor (Q1) is in current controller mode, i.e. it will turn off when (I_FB) reaches the required value (I_DEM). In turn, the transistors (Q2, Q3, Q4 and Q5) begin to work in inverter mode, transferring all the current to the grid from the initial moment.

[0046] In cycle C1, (Q1) conducts 100% because I_FB has not yet reached the value (I_DEM). Then (Q2) and (Q4) turn on, while (Q3) and (Q5) remain off. In the next cycle, C2, (Q1) continues to conduct permanently because the value (I_dem) is still not reached while the transistors (Q2) and (Q4) are turned off and (Q3) and (Q5) are turned on to invert the polarity of the magnetic field of the core of (TR1), to prevent it from saturation.

[0047] The same cycles are repeated until eventually (I_FB) reaches the value (I_dem) and Q1 begins to regulate the current.

[0048] As can be seen in figure 8, in direct mode it takes longer than in Preload Mode to reach the value (I_dem). This is because in Direct Mode we begin to transfer the energy of the inductance (L) from the first moment, while in Preload Mode we accumulate that energy until reaching the optimal value, discharging it abruptly at the time of the exposure of X-rays.

[0049] As in Preload Mode, when the System Controller decides to terminate the X-ray exposure the 5 transistors (Q1, Q2, Q3, Q4 and Q5) are switched off instantly and simultaneously. At that moment, the current stops circulating towards the grid, instantly turning off the X-ray exposure. The energy stored in the inductance (L) is transferred to the power supply (V_sup) through the diodes (D1) and (D2).

[0050] Also as in the Preload Mode, the X-ray exposure can be cut off by turning on the 4 inverter bridge transistors (Q2, Q3, Q4 and Q5) while maintaining the control of the current (I_FB) with (Q1), with which it would enter the preload mode, being able to carry out a new exposure at any time and without any delay.

[0051] Finally, figure 9 shows the control circuit for the current (I_FB) which, as can be seen, comprises a flip-flop (FF1), preferably of type D triggered by a positive flank that will activate its output (Q) to turn on (Q1) when the inputs (AND1) of the gate are both "1", that is, when it is requested to start preloading (PRE) and the signal (Clock) is received.

[0052] When the current (I_FB) reaches the value (I_dem) in the comparator (COMP1), it will cause its output to be "0", restarting the output Q of (FF1) and turning off the transistor (Q1), until it is switched on again in the next clock cycle (Clock).

[0053] It is noteworthy that the grid current (IG) ripple, although it is directly proportional to the anode current, has no influence on the dose control of the X-ray exposure, since this depends on the amount of photons that are generated in the anode of the tube, i.e. the integral over the exposure time of the anode current, internationally known as mAs. On the contrary, the ripple of the VA-K voltage is very important, because it generates photons of different energy, which has a very important direct impact on the emitted dose.

[0054] Having sufficiently described the nature of the present invention, in addition to the manner in which to put it into practice, it is hereby stated that, in its essence, it may be put into practice in other embodiments that differ in detail from that indicated by way of example, and to which the protection equally applies, provided that its main principle is not altered, changed or modified.

Claims

1. X-ray exposure control circuit by means of a grid current, characterized in that it comprises an anode (A) that is connected to a power supply (VAK) ó (VAK/2) which in turn is connected to ground, also comprises a cathode (K) that is also connected to ground, and on the other hand it has a grid (G) connected to a grid current control circuit (IG) that comprises a DC-DC converter (CONV) or "buck converter" to which a power supply voltage (Vsup) is connected, the output of the DC-DC converter (CONV) is connected to an inverter (INV) whose output is connected to a transformer (TR), and the output of this transformer (TR) is connected to a rectifier (RECT) whose outputs are connected to the grid (G) and to ground; in addition, in said control circuit there is a control circuit (CONT) that receives a current signal (I_FB), a demanded current signal (I_DEM) and a preload signal (PRE) as well as an exposure signal (EXP) and has as outputs a first signal (S1) to the DC-DC converter and a second signal (S2) to the inverter (INV).

2. X-ray exposure controller circuit by means of a grid current, according to claim 1, characterized in that the DC-DC converter is formed by a capacitor (C) in parallel with a first switch (Q1), a diode (D1) and a coil (L) and a return diode (D2) in parallel with the assembly formed by the first switch (Q1) and the coil (L).

3. X-ray exposure control circuit by means of a grid current, according to claim 1 or 2, characterized in that the inverter (INV) is formed by a bridge comprising a second switch (Q2), a third switch (Q3), a fourth switch (Q4) and a fifth switch (Q5), where the second switch (Q2) and the fifth switch (Q5) are in series forming a first branch, while the third switch (Q3) and the fourth switch (Q4) are in series forming a second branch, the two branches being in parallel with each other and where each branch is connected at its midpoint with a transformer (TR1) and the output is connected to a rectifier bridge from where the grid current (IG) is obtained, while the current signal (I_FB) is obtained from the lower end of both branches of the inverter and the DC-DC converter.

4. X-ray exposure controller circuit by means of a grid current, according to claim 3, characterized in that the controller circuit operates in preload mode in which in the first cycle the 5 transistors (Q1, Q2, Q3, Q4 and Q5) are switched on and the current (I_FB) rises limited by the inductance (L).

5. X-ray exposure controller circuit by means of a grid current, according to claim 3, characterized in that the controller circuit operates in direct mode in which the X-ray exposure starts instantly, without waiting for the grid to have the optimal intensity.

6. X-ray exposure control circuit by means of a grid current, according to any of the preceding claims, characterized in that the Intensity control circuit (I_FB) comprises a flip-flop (FF1), preferably of type D, triggered by a positive flank that will activate its output (Q) to turn on (Q1) when the inputs (AND1) of the gate are both "1", that is, when it is requested to start preload (PRE) and the (Clock) signal is received; it also comprises a comparator (COMP1) that has as inputs the demanded current (I_dem) and the current (I_FB) inverted so that when the intensity (I_FB) reaches the value (I_dem) in the comparator (COMP1), it will cause its output to be "0", restarting the output Q of (FF1) and turning off the transistor (Q1), until it is turned on again in the next clock cycle (Clock).

7. X-ray exposure control circuit by means of a grid current, according to any of the previous claims, characterized in that the power supply is a monopolar power supply from a power supply (VAK) where the anode (A) is connected to a power supply (VAK) that is grounded through a shunt to measure the anode current IA, while the cathode (K) is also grounded.

8. X-ray exposure control circuit by means of a grid current, according to any of claims 1-6, characterized in that the power supply is a bipolar power supply where the anode (A) is connected to the power supply (VAK/2) that is grounded through a shunt to measure the anode current (IA), while the cathode (K) is connected to another power supply (VAK/2) that is also grounded

Drawing