X-ray or gamma ray systems or devices – Source – Electron tube
Reexamination Certificate
1999-05-06
2001-01-30
Bruce, David V. (Department: 2876)
X-ray or gamma ray systems or devices
Source
Electron tube
C378S136000, C378S119000
Reexamination Certificate
active
06181771
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an X-ray source of the type having an electron emitter for the production of an electron beam, an anode on which the electron beam strikes in an X-ray focal spot, and a magnet system that produces a dipole field and a quadrupole field superimposed thereon, for the deflection and focusing of the electron beam onto the anode.
2. Description of the Prior Art
In an X-ray source known from U.S. Pat. No. 5,883,936, fashioned as a rotating bulb source, a magnet system is provided for the deflection and focusing of the electron beam, which emanates from the electron emitter and has an initially circular cross-section. For the production of an X-ray focal spot that is substantially circular, seen opposite the primary direction of propagation of the X-rays emanating from the X-ray focal spot, the quadrupole field is selected such that it modifies the cross-section of the electron beam emitted by the cathode, which initially has a circular cross-section. This modification occurs in such a way that the X-ray focal spot arising at the edge of the anode is elongated in the radial direction due to the anode edge being beveled relative to the primary direction of radiation of the X-ray radiation, relative to the width of the electron beam measured in the tangential direction (length-to-width ratio). This has in turn the result that, as seen in the direction opposite the primary direction of propagation of the X-rays emanating from the X-ray focal spot, the extension of the electron beam in the radial direction corresponds to the extension of the electron beam in the tangential direction. The X-ray focal spot thus has substantially circular shape, with the electron density of the electron beam shortly before the X-ray focal spot being higher than immediately adjacent to the cathode. The Gaussian distribution of the electrons over the cross-section is, however, maintained.
Expensive measures would be necessary in order to enable an adjustment of the size of the X-ray focal spot in such a rotating bulb source, e.g. by means of a switching the largest elongation.
The size of the focal spot could be adjusted in a known manner by means of an adjustable focusing voltage applied to a focus cup that surrounds the electron emitter. An electron emitter with a variable emission surface alternatively could be used that could be constructed as a flat or spiral emitter with several emission surfaces, in particular concentrically arranged, which can be activated individually or together, corresponding to the desired size of the X-ray focal spot. This would have the advantage that the type of drive would be maximally compatible with existing generators. However, disadvantages would include higher manufacturing costs and reduced flexibility. In addition, narrow tolerances in the cathode manufacturing would have to be taken into account.
In addition, it is disadvantageous that neither of the two possibilities offers advantageous conditions for an optimization of the intensity distribution of the X-rays emanating from the X-ray focal spot in the sense of a rectangular curve of the intensity of the X-rays in the radial direction of the X-ray focal spot.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an X-ray source of the type described wherein several different sizes of the X-ray focal spot are possible at low cost
According to the invention, this object is achieved in an X-ray source having an electron emitter for the production of an electron beam, having an anode on which the electron beam strikes in an X-ray focal spot, and a magnet system that produces a dipole field and a quadrupole field that is superimposed on this dipole field, for the deflection and focusing of the electron beam, and an arrangement that operates together with the magnet system for the adjustment of the size of the X-ray focal spot. This arrangement, in order to set a desired size of the X-ray focal spot, adjusts the quadrupole field so that the X-ray focal spot has a width corresponding to the desired size of the X-ray focal spot, and supplies to the magnet system a wobble signal that influences the dipole field. This wobble signal effects a periodic displacement of the electron beam in a direction transverse to the extension of the width of the X-ray focal spot over a distance such that the effective length—resulting from the deflection and measured in the direction of the deflection—of the X-ray focal spot is dimensioned to achieve a particular ratio of the effective length to the width of the X-ray focal spot.
Thus in the case of the inventive X-ray source the width of the X-ray focal spot can be adjusted by influencing the quadrupole field, and then, if the cross-section of the electron beam, with respect to its ratio of length to width at the strike point of the electron beam on the anode does not correspond to the desired ratio of length to width of the X-ray focal spot, a dipole field is influenced by a wobble signal so that the electron beam is periodically deflected over such a distance and in such a direction that an X-ray focal spot results with an effective length that produces the desired ratio of length to width of the X-ray focal spot.
In the invention, X-ray focal spots of different size thus can be produced at low cost, since the only additional expenses is that required for components to produce the wobble signal that influences the dipole field.
In a preferred embodiment of the invention, the particular ratio of effective length to width of the X-ray focal spot is adjustable. Arbitrarily small ratios of effective length to width of the X-ray focal spot thus are not possible, since for each width of the X-ray focal spot there is a minimum length, since the cross-section of the electron beam can be influenced by the quadrupole field only in such a way that, together with the width of the cross-section of the electron beam, the length of the cross-section of the electron beam is also modified. As the width of the cross-section of the electron beam increases the length of the cross-section of the electron beam decreases.
Preferably, according to a variant of the invention a particular ratio of effective length to width of the X-ray focal spot is produced so that this ratio, as seen opposite to the primary direction of propagation of the X-ray beam emanating from the anode, is equal to one, since then a high image quality can be achieved, This ratio of effective length to width of the X-ray focal spot of one is of particular importance when the electron emitter produces an electron beam with substantially circular cross-section, since then the X-ray focal spot, as seen opposite the primary direction of propagation of the X-rays, has a circular shape that enables a further improved image quality.
If the X-ray source has a rotating anode with an anode edge that is beveled relative to the primary direction of propagation of the X-rays emanating from the anode, the X-ray focal spot being located on this edge, then according to a variant of the invention the width of the X-ray focal spot extends in the tangential direction of the anode and the resulting length of the X-ray focal spot extends in the radial direction of the anode. Advantageous imaging conditions then exist.
In X-ray sources fashioned as rotating bulb sources, the invention can be realized with a particularly low expense, since then a magnet system that produces a dipole field with a superimposed quadrupole field is present anyway.
According to a variant of the invention, the wobble signal exhibits a chronological curve such that the intensity distribution of the X-ray radiation emanating from the X-ray focal spot has, in the direction of the deflection of the electron beam, a predetermined shape that deviates from a Gaussian distribution and is preferably rectangular. An intensity distribution of the X-rays that is approximately rectangular in the direction of the resulting length of the X-ray focal spot can be realized if the chronological curve of
Hell Erich
Mattern Detlef
Schardt Peter
Bruce David V.
Hobden Pamela R
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
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