X-ray or gamma ray systems or devices – Source – Electron tube
Reexamination Certificate
2002-11-27
2004-10-26
Glick, Edward J. (Department: 2882)
X-ray or gamma ray systems or devices
Source
Electron tube
C378S119000
Reexamination Certificate
active
06810110
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to an x-ray tube operating in a magnetic field. More particularly, it relates to an x-ray tube having sensors, internal magnets, and/or active compensation means operatively coupled thereto or integrated therein for aligning the cathode-anode axis of the x-ray tube with a magnetic field.
BACKGROUND ART
Magnetic resonance imaging (MRI) and x-ray fluoroscopic imaging are important imaging tools. For example, in medical imaging they are routinely used for diagnosing diseases and for image-guided interventional procedures. Each method provides its own advantages: MRI provides excellent soft tissue contrast, three-dimensional visualization, physiological information, and the ability to image in any scan plane, while x-ray imaging offers much higher spatial and temporal resolution in a projection format, useful for visualization and placement of guidewires, catheters, stents, and other medical devices. Combining the two imaging systems therefore offers significant benefits over using each system alone. Currently, several approaches are used for combining the systems. In one, an x-ray fluoroscope is located in a room adjacent to the MRI system. In another, the x-ray and MRI systems are in the same room, but the patient must be moved out of the magnetic field to be imaged by the x-ray system. Moving the patient is undesirable, because it is time consuming, possibly dangerous, and can render the images inconsistent. Therefore, one wants to minimize the distance between the two systems, and perhaps overlap them. This will place critical components of the x-ray system within a high magnetic field.
The ideal system is one in which x-ray imaging and magnetic resonance imaging can be performed in the same location, eliminating the need to move the patient. Before a combined MRI and x-ray system can be constructed, however, the individual systems must be modified to ensure that the high magnetic field of the MRI system does not affect the x-ray system, and that the x-ray system does not disturb the operation of the MRI system. For example, conventional x-ray fluoroscopy detectors are image intensifiers, which are exceedingly sensitive to magnetic fields and therefore cannot be used near, let alone inside, an MRI system. However, flat panel x-ray detectors that are relatively immune to magnetic field effects are now available.
A major obstacle to combining MRI and x-ray systems is the x-ray source, which consists of an x-ray tube and its housing. X-rays are generated using an x-ray tube, in which electrons are accelerated from a heated cathode to an anode by a very high potential (e.g., 50 to 150 kV). Interactions between the high energy electrons of the beam and atoms of the anode target material cause deceleration of the electrons and production of x-ray photons.
FIG. 1
is a schematic diagram of a fixed anode x-ray tube
10
of the prior art. The tube
10
is evacuated and contains a tungsten filament cathode
12
and a more massive anode
14
, typically a copper block
16
with a metal target
18
plated on or embedded in the copper surface. The target
18
is most often tungsten, but other metals can be used, such as molybdenum, rhodium, silver, iron, or cobalt. Separate circuits are used to heat the filament
12
and to accelerate the electrons to the target
18
. The accelerating potential determines the spectrum of wavelengths (or photon energies) of the emitted x-rays. A high voltage is connected between the cathode
12
and anode
18
to provide the accelerating potential. Typically, the anode and cathode voltages are maintained at plus and minus half of the total accelerating voltage, respectively. X-rays generated at the target
18
exit the tube
10
through an x-ray transparent window
20
and are directed toward the object being imaged.
When an x-ray tube is operated within or near an MRI system, it experiences the static magnetic field B
o
, as illustrated schematically in FIG.
2
. The magnetic field at the location of the x-ray tube exerts a force on moving electrons and may deflect or defocus the electron beam. The force on an electron is proportional to the cross-product of the velocity of the electron and the magnetic field; that is, only the velocity component that is perpendicular to the magnetic field is perturbed. This will alter the direction of the electron motion, thereby making the direction of the deflecting force time-dependent. In the example of
FIG. 2
, the electrons are emitted from the cathode with some initial velocity and are accelerated toward the anode by the electric field E. The macroscopic result of the time-dependent force is to produce a deflection away from what would be observed without B
o
present, with a deflection in the direction of B
o
, and an additional deflection of the beam v
⊥drift
in a direction perpendicular to both B
o
and the electric field E. Because the ideal electron velocity is in the direction of the target, as is due to the acceleration caused by the electric field, unless the magnetic field is parallel to the electric field, it will deflect the electrons away from the center of the target, possibly causing them to miss the target entirely. Thus the effect of the static magnetic field of the MRI system on the x-ray tube can be highly undesirable and may damage the tube if it is operated under non-ideal conditions, or it may lower the x-ray intensity to a level that is unacceptable. In the combined system, it is not desirable—indeed it may be impossible—to turn off the static magnetic field before acquiring x-ray images, and so the effect of the magnetic field on the x-ray tube must be addressed.
A number of combined magnetic resonance imaging and x-ray imaging systems are disclosed in the prior art. U.S. Pat. No. 5,713,357, issued to Meulenbrugge et al., discloses a combined system that minimizes or eliminates the distance an object being imaged must be displaced between individual systems. In one embodiment, the object is displaced a small distance along a track between adjacent MRI and x-ray imaging systems with non-coincident fields of view. In another embodiment, the object is not moved and the fields of view of the two systems are coincident, but the x-ray imaging system is moved out of the MRI field of view during MR image acquisition. During x-ray imaging, the x-ray source is either out of range of the static magnetic field, passively shielded from the magnetic field, or positioned so that the electron beam is parallel to the magnetic field. In this alignment, the electron beam should not be deflected by the magnetic field. This technique, however, limits the system in that the x-ray tube must remain fixed at a certain orientation and/or distance with respect to the static magnet. Moreover, Meulenbrugge et al.'s invention does not teach or suggest how to control/maintain the alignment of the electric and magnetic fields in the x-ray tube.
U.S. Pat. No. 6,151,384, issued to Reed et al, describes an x-ray tube that uses a curved magnetic field to steer electrons toward the anode. This tube cannot be used for imaging since it has multiple, relatively large anode targets that produce x-rays. Further, since Reed et al's magnetic field is curved, their method will not work well when the source is placed in an environment with a strong external field, such as in the neighborhood of an MRI system.
U.S. Pat. No. 5,818,901, issued to Shulz, discloses a combined system with simultaneous MR and x-ray imaging and coincident fields of view. A solid state x-ray detector containing amorphous hydrated silicon, which is not affected by the magnetic field, is used in place of an image intensifier. The x-ray source is positioned far enough from the MR apparatus that the influence of the magnetic field on the x-ray source is slight. Additionally, the influence is reduced further by surrounding the source with a cladding material that shields the source from the magnetic field. The goal of the cladding or shielding is to reduce the magnetic field at the location of th
Pelc Norbert J.
Wen Zhifei
Glick Edward J.
Lumen Intellectual Property Services Inc.
The Board of Trustees of the Leland Stanford Junior University
Thomas Courtney
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