X-ray or gamma ray systems or devices – Source support – Shielding
Reexamination Certificate
2001-01-16
2002-03-05
Church, Craig E. (Department: 2882)
X-ray or gamma ray systems or devices
Source support
Shielding
Reexamination Certificate
active
06352363
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to magnetically shielding x-ray sources, and in particular to magnetically shielded x-ray sources, methods of magnetically shielding x-ray sources, and to a magnetic surgical system with a magnetically shielding x-ray source.
Recently magnetic surgery techniques have been developed in which one or more permanent magnets or electromagnets is used to magnetically navigate medical devices and substances in an operating region inside the patient's body. To monitor the procedure it is desirable to at least periodically if not continuously image the operating region. A widely used method of imaging is x-ray fluoroscopy, however the strong magnetic fields generated by the magnets can interfere with the operation of the x-ray sources. The increasing use of fluoroscopic imaging in the vicinity of significant magnetic fields such as generated by magnetic resonance imaging (MRI) devices and magnetic surgery systems (MSS) has resulted in a need for the protection of the tubes which provide the x-ray beam as well as the image intensifiers on the screens which receive the imaged beam. Conventional shielding in medical situations most often uses mu-metal or a combination of mu-metal and low-carbon steel formed sheets. These are not very useful in shielding of larger magnetic fields in congested regions near x-ray or fluoroscopic equipment.
Two elements in the typical x-ray generating tube are vulnerable to magnetic fields significantly stronger than the Earth's field. The electron beam which impacts on the anode to create the x-rays is, near its origin, of very low energy, and therefore soft to bending by a magnetic field. Such bending can shift an image, twist the image, or change its contrast and brightness. The beam can also be defocused and cause a completely washed out image. Experience shows that commonly designed x-ray tubes show effects of magnetic fields in the region of 50 Gauss, or so, depending on direction of the field.
A second element of magnetic vulnerability occurs in tubes with rotating metal anodes. These anodes can have eddy currents which cause a drag that slows the anode rotation. The magnetic field levels at which this effect is significant are more variable, depending on field direction and variation in time. Experience has shown that slowly varying fields of 50 Gauss or so do not result in significant effect on the anode rotation.
Prior attempts to shield the x-rays using housing formed from sheet metal have generally been unsatisfactory because of the difficulty and expense of fabricating a shield that closely conforms to the x-ray tube yet does not interfere with the operation of the x-ray tube. A powerful x-ray generating tube has several electrical leads as well as coolant tubes connected to it. The leads, and other features of the design, cause the design of a magnetic shield for the tube to be a matter totally different from the design of magnetic shields commonly in use in the past. Such common shields are used for computer monitors and for sensitive equipment.
It is known that field penetration of a shield through holes leads to “leakage” to the interior. (See Classical Electrodynamics, 2nd Ed., J. D. Jackson, Wiley and Sons, pages 201-204 and 408 to 411, the latter to be evaluated in the limit of very low frequencies). A larger aperture leads to deeper field penetration. Common magnetic shield design for monitors and delicate apparatus uses layered permeable material, sometimes containing “mu-metal” either of several grades or in conjunction with low-carbon steel. The high permeability mu-metal is vulnerable to relatively small fields, say of the order of one Gauss, because it draws so much flux into its layer that it saturates. In the protection of an x-ray generating tube, such high permeability material is not necessary, or even desirable. This is because the fields in question, even inside the shield, are at a level at which mu-metal would saturate, at least in layers of commercially feasible thickness.
Another effect is the concentration of field caused by sharp curves in a shield surface, resulting in concentration of flux causing a local high field, and/or saturation of the shield.
A lesser known effect is the geometrical effect of “flux directing” by the shape of the shield. In this effect there is a dependence on the size and distance of the source field relative to the shield. A relatively close source field can saturate the front of a shield before achieving a high field at the rear. If the same source field at the location of the center of the shield were caused by a physically large source, this front-rear discrimination would not occur. In the relatively close case, the shape of the shield can be important, and the location of holes should be at the rear (away from the source).
In the regime of shielding concerned here, layering of any permeable material is ineffective. This is because the upper boundary of field within a layer is no more than 25 Tesla due to saturation, and any feasible layer will saturate well before it can remove enough magnetic flux to prevent saturation in the next layer. For an ideal enclosed shield the net effect is that n layers of thickness t will have virtually the same interior field as a single layer of thickness n times t.
SUMMARY OF THE INVENTION
The present invention relates to a shielded x-ray source, a method of shielding an x-ray source, and a magnetic surgical system with shielded x-ray source.
Generally, the shielded x-ray source of the present invention has a cast shield of an iron based material substantially enclosing and closely conforming to the x-ray tube to shield the x-ray tube imaging beam from interference from magnetic fields. The shield is preferably made of cast iron, but could also be made of cast steel. The shield is preferably at least ¼ inch thick. Because the shield is cast, it can be inexpensively made to closely conform to the external shape of the x-ray tube. There is preferably less than ¼ inch gap between the x-ray tube and the shield, and more preferably nor more than {fraction (1/16)} inch gap between the x-ray tube and the shield. The shield is preferably cast in two or more pieces, which are assembled around the x-ray tube and secured together. Such a field can be more efficient than others and therefore significantly lighter for mounted on c-arms and other apparatus.
Moreover, it will have a smaller magnetic moment and less disturbing force on it than less efficient shields.
Generally, the method of the present invention comprises providing a shield cast from an iron-based material in a shape having a cavity to receive and closely conform to the x-ray tube, and installing the cast shield around the x-ray tube. The shield is preferably cast iron, but could also be made of cast steel. The shield is preferably at least ¼ inch thick. Because the shield is cast, it can be inexpensively made to closely conform to the external shape of the x-ray tube. There is preferably less than ¼ inch gap between the x-ray tube and the shield, and more preferably nor more than {fraction (1/16)} inch gap between the x-ray tube and the shield. The shield is preferably cast in two or more pieces, which are assembled around the x-ray tube and secured together.
Generally, the magnetic surgical system comprising at least one magnetic for magnetically navigating a medical device in an operating region in a patient's body, and an imaging apparatus including at least one x-ray tube for imaging the operating region, the improvement including a cast shield of an iron-based material substantially enclosing and closely conforming to the at least one x-ray tube. The shield is preferably cast iron, but could also be made of cast steel. The shield is preferably at least ¼ inch thick. Because the shield is cast, it can be inexpensively made to closely conform to the external shape of the x-ray tube. There is preferably less than ¼ inch gap between the x-ray tube and the shield, and more preferably nor more than
Munger Torrey
Werp Peter
Church Craig E.
Harness Dickey & Pierce PLC
Stereotaxis, Inc.
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