THERMALLY MODULATED FIELD EMISSION CATHODE AND BEAM CURRENT MEASUREMENT TECHNIQUE CROSS REFERENCES TO RELATED APPLICATIONS [0001] This application claims benefit of U. S. Provisional Application Serial No.

60/440,454, entitled"X-ray Source Having A Thermally Modulated Field Emission Cathode"and filed on January 16,2003. This application further claims benefit of U. S.

Provisional Application Serial No. 60,440, 483, entitled"Beam Current Measurement Technique"and filed on January 16,2003.

FIELD OF THE INVENTION [0002] The present invention relates to x-ray brachytherapy systems and more particularly to an x-ray source having a thermally modulated field emission cathode, and to a technique for measuring beam current in the x-ray source, for use in x-ray brachytherapy systems.

BACKGROUND [0003] X-ray brachytherapy refers to x-ray radiation treatment in which an x-ray source is located close to or within the area receiving treatment. An x-ray brachytherapy system, typically utilizes a miniaturized low power radiation source that can be inserted into, and activated from within, a patient's body. The miniaturized, insertable probe is capable of generating x-ray radiation local to the target tissue, so that radiation need not pass through the patient's skin, bone, or other tissue prior to reaching the target tissue.

The insertable probe emits low power x-rays from a nominal"point"source located within or adjacent to the desired region to be affected. In x-ray brachytherapy, therefore, x-rays can be applied to treat a predefined tissue volume without significantly affecting the tissue adjacent to the treated volume. Also, x-rays may be produced in predefined dose geometries disposed about a predetermined location. Finally, x-ray brachytherapy allows the operator to control over time the dosage of the delivered x-ray radiation.

[0004] An x-ray brachytherapy systems typically include a tubular probe enclosing an electron source for emitting electrons, and an x-ray emitting target. Means are provided for establishing an accelerating electric field that acts to accelerate the electrons emitted from the electron source toward the target, which emits x-rays in response to incident electrons from the electron source. Typically, the sizes of the x-ray tubes range from about 1 to about 5 mm. It is known in the art to use resistively heated thermionic cathodes, laser heated thermionic cathodes, and photocathodes, as a source for electrons.

In a resistively heated thermionic cathode, a metal filament is used that emits electrons when resistively heated to over 1000 degrees Celsius. The requirement of a high operating temperature frequently cause failure of the metal filament. A more efficient x- ray source having a laser-heated thermionic cathode is disclosed in U. S. Patent No.

6,480, 568 (hereinafter the"'568 patent") (commonly owned by the assignees of the present application and hereby incorporated by reference in its entirety).

[0005] Field emission cathodes are an'attractive alternative source for electrons in miniature x-ray tubes. In a field emission cathode, electrons are emitted at room temperature, under the influence of a strong electric field. Field emission is a quantum mechanical effect, in which the electrons tunnel through a potential barrier. Field emission cathodes are simple, physically small devices that require no external power for use.

[0006] The output current from a field emission cathode is voltage controllable. The usual mode of operation of a field emission cathode is to use a control electrode adjacent to the field emitter, to control the electric field near the field emitter and thus control the emitted current. Depending on the spacing of the control electrode, voltages in a 200V- 1000V range are needed to control the current adequately.

[0007] The design of hyperminiature x-ray tubes dictates the use of the simplest possible structures, as there are severe physical and assembly limitations on the design. The field emission cathode, while having very attractive operating characteristics, ordinarily dictates the use of at least a three-terminal x-ray tube. Physically assembling the control structure is very challenging when trying to design x-ray tubes in the 1-3 mm diameter range. In addition, for x-ray tubes designed to operate at the end of a catheter or cable, the cable must now be either triaxial or have two isolated conductors internal to the cable at high voltage.

[0008] Miniaturized x-ray tubes designed for radiotherapy must have a very stable x-ray output, in order to be applied successfully for treatment and therapeutic purposes.

Because an actual in-vitro measurement of the x-ray output of miniature x-ray tubes is impractical, the usual method of ensuring stable x-ray output is to monitor the voltage and current of the x-ray tube for stability. The assumption is that if voltage and current are steady, and all of the electrons propagate to the target, the x-ray output will be stable.

Unfortunately, x-ray tubes with isolating elements that make accurate beam current measurement possible are too complicated to fabricate in the necessary size.

[0009] Beam current measurement in a diode-configured x-ray tube is complicated by the presence of several error current sources, including field emission currents and leakage currents. For example, unwanted field emission from the surface of the x-ray tube can cause an electron current that does not reach the target, and thus does contribute to x-ray output. The presence of weakly conduction films on the insulator surfaces to control electric fields and stray currents causes a time and temperature variable dc current. Both of these sources of current error are ordinarily indistinguishable from beam current in diode configuration x-ray tubes.

[0010] For these reasons, it is desirable to provide a simpler control mechanism for field emission cathodes, for use in miniature x-ray tubes. Furthermore, there is a need for a more accurate and reliable method for measuring beam current in miniaturized x-ray tubes.

SUMMARY OF THE INVENTION [0011] The present invention features an alternative control system for field emission cathodes for use in a miniaturized x-ray source. This control system greatly simplifies the structures for building the miniaturized x-ray source. The present invention also features a methodology for differentiating the x-ray generating beam current from error currents such as field emission currents from solid surfaces, and stray currents resulting from the presence of weakly conducting films on the insulator surfaces.

[0012] In one embodiment, the present invention features a carbon nanotube FE (field emission) cathode that is heated with laser light propagated down a fiber optic cable.

When such a carbon nanotube FE cathode is used in an x-ray tube, a two-terminal x-ray tube is obtained that allows independent control of tube voltage and tube current.

Control of tube voltage can typically be achieved using a control electrode, whereas the tube current emitted by the field emission cathode can be controlled by thermal modulation.

[0013] In one embodiment, the present invention features a method for accurately measuring x-ray generating beam current in an x-ray tube of the type including a cathode for generating an electron beam upon activation of a laser, and an x-ray target for emitting x-rays in response to accelerated electrons from said cathode impinging thereupon. The cathode may be a thermionic cathode which thermionically emits electrons when heated to a sufficient temperature by the laser. Alternatively, the cathode may be a field emission cathode (preferably a carbon nanotube field emission cathode), which emits electrons through field emission, and is heated by laser light so that field emission is thermally controlled. The method includes activating the cathode and adjusting the intensity of the laser, until a desired dose of output x-ray radiation is attained, and then measuring the total x-ray tube current. The cathode is then turned off, so as to turn off the electron beam. The total x-ray tube current is measured again. The cathode is turned on again, immediately thereafter. Finally, the difference between the two values of the total x-ray tube current is taken to obtain the desired beam current.

[0014] The present invention also features a beam current measurement system for accurately measuring x-ray generating beam current in an x-ray source of the type including an optically activated cathode for generating an electron beam upon activation of an optical source, and an x-ray target for emitting x-rays in response to accelerated electrons from said cathode impinging thereupon. The system includes an on-off switch for activating and de-activating the cathode, a current measuring device for measuring the total x-ray tube current, and a processor. The on-off switch initially activates the cathode. The system also includes an x-ray detector for measuring the dose of output x- ray radiation from the x-ray source, and a light intensity controller for adjusting the intensity of the light from the optical source, in response to the measured dose of output x-ray radiation. After the initial activation of the cathode, the light intensity controller adjusts the intensity of the light until a desired dose of output x-ray radiation is attained.

The current measuring device then measures the total current from the x-ray tube, to establish a first value of total x-ray tube current. The switch then de-activates the cathode, so as to turn off the electron beam. The current measuring device then I establishes a second value of the total x-ray tube current by measuring once again the total current from the x-ray tube. The switch then re-activates the cathode. The processor then calculates the desired beam current by computing the difference between the first value of the total tube current and the second value of the total tube current.

BRIEF DESCRIPTION OF THE DRAWINGS [0015] FIG. 1 illustrates the emission characteristics of a field emission cathode, as a function of the spacing between the FE cathode and the control electrode, and of the voltage difference between the FE cathode and the control electrode.

[0016] FIG. 2 is a diagrammatic view of one embodiment of an x-ray source constructed in accordance with the present invention, and including a laser heated, carbon nanotube field-emission cathode.

[0017] FIG. 3 illustrates in flow-chart form the steps for measuring beam current, using a method in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION [0018] The present invention features a miniaturized x-ray source including a carbon nanotube field emission cathode that can be thermally modulated by laser light that is propagated down a fiber optic cable. In this way, the x-ray tube current, as well as x-ray tube voltage, can be controlled independently. The present invention also features a method for accurately measuring the x-ray generating beam currents in miniaturized x- ray tubes, and for differentiating these beam currents from error currents.

[0019] FIG. 1 illustrates the emission characteristics of a field emission cathode, as a function of the spacing between the FE cathode and the control electrode, and of the voltage difference between the FE cathode and the control electrode. As shown in FIG.

1, the emission of the FE (field emission) cathode depends on the spacing between the FE cathode and the control electrode and also on the voltage difference between the FE cathode and the control electrode.

[0020] In addition, a temperature parameter can also be introduced, because there is a temperature related term in the field emission equation. A family of curves similar to FIG. 1 can thus be generated with temperature, instead of voltage, as the control function. By heating the FE cathode to control its emission, i. e. by thermally modulating the FE cathode, a two-terminal x-ray tube can be created that allows independent control of tube voltage and tube current.

[0021] Ordinarily, heating the FE cathode to control its emission would result in a more complicated system. However, the characteristics of carbon nanotube FE cathodes lend themselves to a simpler method of thermal control. Carbon nanotubes are extremely thin, hollow cylinders made of carbon atoms. A carbon nanotube may be viewed as a hexagonal network of carbon atoms that has been rolled up to form a cylinder just one nanometer across, and tens of microns in length. Carbon nanotubes have an extremely high temperature tolerance, and are an exceptionally good adsorber of light. Therefore, carbon nanotubes can be heated very efficiently in the vacuum environment of the x-ray tube. The fundamental cylindrical structure (composed of carbon atoms) is referred to as a"single-wall carbon nanotube. "Carbon nanotube FE cathodes may be fabricated, by way of example, by coating a layer of carbon nanotube bundles on a flat metal or silicon disc by electrophoretic deposition, although other methods of fabrication are also within the scope of the present invention.

[0022] In the present invention, a carbon nanotube FE cathode is placed at the end of an optical delivery structure, and light propagated down the optical delivery structure is used to heat the cathode. FIG. 2 illustrates one embodiment of an x-ray source 100 constructed in accordance with the present invention, and including an optically heated carbon nanotube field-emission cathode. In overview, the x-ray source 100 includes a flexible optical delivery structure 110, an optical source 120, and an x-ray generator assembly 130. The optical delivery structure has a proximal end and a distal end, and is adapted to transmit optical radiation incident on the proximal end to the distal end.

Preferably, the optical delivery structure is a fiber optic cable 110; however, other embodiments may use other types of optical delivery structures, for example a lens. The x-ray generator assembly 130 is coupled to the distal end of the fiber optic cable 110.

The optical source 120 is preferably a laser 120; however, other types of optical sources, e. g. LEDs (light-emitting diodes) may be used in other embodiments of the invention.

[0023] The x-ray generator assembly 130 includes a carbon nanotube FE cathode 160, and an anode 170, and means (not shown) for generating an electric field between the field emission cathode 160 and the anode 170. By way of example, the means for generating an electric field may include a high-voltage generator or power supply, and a conducting cable for connecting the high-voltage generator, the cathode, and the anode.

The electric field generating means may include multiple conductive regions, on the outside of the structure 110, or may include one conductor extending axially along the structure 110 and one conductor extending on the outside of the structure 110. The carbon nanotube FE cathode 160 emits electrons in response to the applied electric field.

The anode 170 is preferably spaced apart from and opposite the cathode 160, and includes at least one x-ray emissive material adapted to emit x-rays in response to incident accelerated electrons emitted from the cathode 160. The power supply includes means for controlling the voltage between the cathode and the anode, i. e. the x-ray tube voltage. Typically, a control electrode is used to control the x-ray tube voltage.

[0024] In one embodiment, the anode 170 may be a small beryllium (Be) substrate coated on the side exposed to the incident electron beam with a thin film or layer of a high-Z, x-ray emissive element, such as tungsten (W), uranium (U) or gold (Au). Other embodiments of the invention may include other types of x-ray emissive anodes.

[0025] As seen from FIG. 2, the carbon nanotube FE cathode 160 is deposited at the distal end of the fiber optic cable 110, which is preferably a fused silica fiber optic cable.

The fused silica fiber optic cable 110 is adapted to transmit a beam of light, generated by the laser 120 and incident on a proximal end of the optical fiber to the distal end of the optical fiber. The fiber optic cable 110 is further adapted to direct the beam of light, that has been transmitted therethrough, to impinge upon the FE cathode 160, so as to heat the cathode to a desired temperature. The fused silica is a very poor conductor of heat, so the cathode 160 can be heated very efficiently in the vacuum environment of the x-ray tube. Beam current can be supplied to the cathode by additional nanotubes or a very thin film of refractory metal such as tungsten down the side of the fiber. In a preferred embodiment, the entire cathode structure is the size of the end of the fiber optic cable, typically 150 microns.

[0026] The x-ray source 100 further includes means (not shown) for controlling the power level of the light generated by the laser 120. In other words, the laser source 120 is a variable output laser source, and includes a controller for controlling the output level of laser light to a desired level. The design of the x-ray source 100 is such that an electric field is created at the FE cathode 160 that is slightly below the room temperature threshold of current emission for the cathode 160. When beam current is needed for x- ray generation, laser light is used to heat the FE cathode to the temperature needed for the desired beam current. The power level of the light from the laser 120 is controllable so as to heat the field emission cathode to a temperature sufficient to generate a desired level of field emission current. In this way, the x-ray source 100 of the present invention enables the user to independently control both the tube voltage, and the tube current. As discussed earlier in paragraph [23], a control electrode is used to control the x-ray tube voltage, whereas the x-ray tube current is controlled by regulating the output level of laser light that heats the field emission cathode. A closed-loop system can be used to control the laser, resulting in an accurate, stable beam current.

[0027] A beam current measurement technique is used in the present invention to measure the x-ray beam current. This technique accurately differentiates the x-ray beam current from error currents. In general, the total x-ray tube current is obtained by adding: 1) the desired x-ray beam current; and 2) the sum of all the error currents, from all sources. There are a number of sources of error currents, which are hard to distinguish from the desired beam currents. Two major sources of error currents include: 1) field emission currents; and 2) surface leakage or sheet currents.

[0028] Field emission currents are caused when electrons are emitted from the surface of a solid (for example, the inner surface of the x-ray tube), when the local electric field at the surface is high enough. The electrons are extracted by quantum tunneling through the surface potential barrier. The emitted current depends directly on the local electric field at the emitting surface, E, and on the workfunction of the solid surface. Typically, the dependence of the emitted current on the local electric field and the workfunction is exponential-like. Sheet or surface leakage currents are the currents on the conductive walls of activated x-ray tubes. As mentioned earlier, the presence of semi-conducting or weakly conducting films on the insulator surfaces to control electric fields and stray currents may cause a time and temperature variable dc current.

[0029] In general, the error currents are relatively stable in time, and can be considered dc currents for times of approximately one second. This fact, coupled with the ability to turn the field emission or laser heated thermionic cathodes on and off in milliseconds, permits a measurement technique that separates the error currents from the beam current.

[0030] The beam current measurement method of the present invention can be used with a low power, miniaturized x-ray source having a carbon nanotube field emission cathode, as described in paragraphs [22] - [26] above. The method of the present invention can also be used with x-ray sources having cathodes other than field emission cathodes, for example laser-heated thermionic cathodes, as described in the'568 patent. The'568 patent discloses a miniature therapeutic radiation source that uses a laser-heated thermionic cathode that provides a reduced-power, increased efficiency electron source for the x-ray source. As described in the'568 patent, an x-ray source of the type disclosed in the'568 patent includes an optical source (e. g. a laser), an optical delivery structure (e. g. a fiber optic cable), an x-ray target element, and a thermionic cathode.

The optical fiber directs onto the cathode a beam of laser radiation, which heats the cathode so as to cause thermionic emission of electrons. X-rays are generated in response to incident accelerated electrons that impinge upon the target.

[0031] FIG. 3 illustrates in flow-chart form the steps for measuring beam current, using a method, in accordance with one embodiment of the present invention, that allows the x- ray beam current to be reliably separated from error currents. The technique of the present invention is analogous to the well known"auto zero"process used in analog signal processing. As seen from FIG. 3, a value of the x-ray beam current is initially established, after which the tube is allowed to equilibrate. In order to establish an initial value of the x-ray beam current, the power supply is turned on, and the laser is activated.

As light from the laser heats the cathode, thermionic emission occurs if the cathode is a thermionic cathode, and field emission occurs if the cathode is a field emission cathode.

[0032] The cathode current is adjusted to the desired x-ray beam current level, by adjusting the laser intensity until the desired x-ray output level is reached. The desired x-ray output level may be dictated by the needs of the radiotherapeutic procedure that is being undertaken. The x-ray tube is then allowed to equilibrate.

[0033] The total x-ray tube current is then measured, after which the cathode is turned off. The total x-ray tube current is measured once again. The cathode is turned on again, as soon as the second current measurement is made. The difference between the first and the second current measurements is the actual x-ray tube beam current.

[0034] The time necessary for this measurement process can be quite short, and if repeated at intervals of ten of seconds, can be inconsequential to the x-ray dose delivery.

Thus, a means of accurately measuring and controlling the x-ray tube beam current can be created.

[0035] The present invention features a beam current measurement system (not illustrated) for accurately measuring x-ray generating beam current in an x-ray source of the type described in paragraphs [22]- [26], or in an x-ray source of the type described in the'568 patent. The beam current measurement system includes an on-off switch for activating and de-activating the cathode, a current measuring device for measuring the total x-ray tube current, and a processor. The system also includes an x-ray detector for measuring the dose of output x-ray radiation from the x-ray source, and a light intensity controller for adjusting the intensity of the light from the optical source, in response to the measured dose of output x-ray radiation.

[0036] The on-off switch initially activates the cathode, so that electrons are emitted.

After this initial activation, the light intensity controller adjusts the intensity of the light until a desired dose of output x-ray radiation is attained. The current measuring device then measures the total current from the x-ray tube, to establish a first value of total x-ray tube current. The switch then de-activates the cathode, so as to turn off the electron beam. The current measuring device then establishes a second value of the total x-ray tube current by measuring once again the total current from the x-ray tube. The switch then re-activates the cathode. The processor then calculates the desired beam current by computing the difference between the first value of the total tube current and the second value of the total tube current.

[0037] While the invention has been particularly shown and described with reference to specific preferred embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.