X-ray or gamma ray systems or devices – Source – Electron tube
Reexamination Certificate
1999-09-21
2001-03-27
Kim, Robert (Department: 2876)
X-ray or gamma ray systems or devices
Source
Electron tube
C378S004000, C378S137000, C378S131000
Reexamination Certificate
active
06208711
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to scanning electron beam systems for X-ray production in a computed tomography X-ray transmission system, and more particularly to reliably controlling uniformity of the beam space-charge density, preferably by extracting positive ions.
BACKGROUND OF THE INVENTION
Scanning electron beam computed tomography systems are described generally in U.S. Pat. No. 4,352,021 to Boyd, et al. (1982). The theory and implementation of devices to help control the electron beam in such systems is described in detail in U.S. patents to Rand et al. U.S. Pat. Nos. 4,521,900 (1985), 4,521,901 (1985), 4,625,150, (1986), 4,644,168 (1987), 5,193,105 (1993), and 5,289,519 (1994). Applicants refer to and incorporate herein by reference each said patent to Boyd et al. and to Rand et al.
As shown in
FIGS. 1 and 2
and as described in detail in U.S. Pat. No. 4,521,900 to Rand et al., a generalized computed tomography X-ray transmission scanning system
8
includes a vacuum housing chamber
10
wherein an electron beam
12
is generated. The electron beam is caused to scan at least one circular target
14
located within front lower portion of chamber
16
.
Upon striking the target, the electron beam emits a moving fan-like beam of X-rays
18
. The X-rays pass at least partially through a region of a subject
20
(e.g., a patient or other object) and register upon a region of a detector array
22
located diametrically opposite. The detector array outputs data to a computer processing system (indicated by arrows
24
) that processes and records the data. The computer processing system then reconstructs or produces an image of a slice of the subject on a video monitor
26
. As indicated by the second arrow
24
, the computer processing system also controls the scanning system and its production of the electron beam. By repeating the scanning process after the patient has been moved laterally along chamber Z-axis
28
, a series of X-ray images representing axial “slices” of the patient's body is produced.
Referring to
FIG. 2
, more specifically, electron gun
32
is disposed within extreme upstream end
34
of chamber (or “drift tube”)
10
, which chamber may be as long as about 3.8 m in some prior art configurations. A sufficiently long drift tube permits the electron beam to expand and become more uniform, and can promote electron beam space-charge homogenization by evening out electron distribution.
In response to high voltage excitation (e.g., 130 kV) the electron gun produces electron beam
12
. The high voltage electron gun potential accelerates the electron beam downstream along a first straight line path defining the chamber Z-axis. A beam optical system
38
typically includes a focus coil
40
and dipole and quadrupole coils
42
, and is mounted downstream on chamber
10
. Coils
40
and
42
respectively magnetically focus and shape and scan the beam
12
typically about 210° in a scan path across the arc-like ring target
14
.
Although vacuum pump
36
evacuates chamber
10
, residual gases inevitably remain that produce positive ions in the presence of the electron beam. Gases may also be introduced into the chamber for the purpose of producing positive ions, since the ions are beneficial in the downstream chamber region.
The electrons are negatively charged and the resultant space-charge causes the electron beam to diverge or expand in the upstream chamber region between the electron gun and the focus and deflection coils. This upstream region expansion is beneficial because the beam width at the target varies approximately inversely with the beam diameter at the focus and deflection coils. However the positive ions that are created can detrimentally neutralize the space-charge, preventing electron beam divergence in the chamber upstream region. Unless counteracted, this can increase beam width at the target, resulting in a defocused X-ray image. Neutralization can also result in the electron beam becoming unstable and even collapsing completely.
In the chamber region downstream from the focus and deflection coils, a converging electron beam is desired. Here the beam preferably is neutralized by positive ions produced by the electrons from residual gas in the chamber, or from a gas purposely introduced into the chamber. The neutralization eliminates electron self-repulsion, while the beam's attractive magnetic field converges and self-focuses the beam. As a result, the beam can self-focus sharply upon the target to produce a sharp X-ray image. Elements of the beam optical system fine tune the converged beam to produce a sharp X-ray image.
Ideally, the electron beam would exhibit uniform current density, diverging upstream and converging to sharply self-focus downstream. An electron beam with a uniform electron distribution can act as its own perfect lens: self-diverging in the upstream chamber region and self-converging in the downstream chamber region to focus sharply on the target. A uniform space-charge density is desired because any optical aberrations due to the electron beam self-forces would then be eliminated. In addition to degradation from ions, the electron beam space-charge density may not be perfectly uniform due to imperfections in the electron gun and in the beam optics system.
Understandably, achieving a perfectly uniform space-charge density is difficult. For example, electron gun imperfections cause the electron beam to have a non-uniform space-charge density in a plane perpendicular to the Z-axis
28
. Housing discontinuities
37
create gaps that prevent conventional ion clearing devices from subjecting the electron beam to an electric field in the upstream region. In compact systems, drift distance between the electron gun and the beam optics is relatively short, e.g., 40 cm or so, and the electron beam has insufficient time for its space-charge density to become sufficiently homogeneous.
Various specific ion controlling electrode assembly configurations were described in above-referenced U.S. patents to Rand et al. For example,
FIG. 3
is a simplified depiction of ion controlling electrode assembly
44
, based upon U.S. Pat. No. 5,289,519. This electrode assembly improves space-charge density and promotes sharp focusing of a high resolution X-ray image produced by system
8
. Assembly
44
included a multi-sided rotatable field ion clearing electrode
46
(“RICE”), a washer-like positive ion electrode
48
(“PIE”), first and second multi-sided ion clearing electrodes
50
,
50
′ (“ICE”), and a periodic axial field ion controlling electrode
52
(“PICE”), although not all of these elements were necessarily required. PICE
52
comprises a series of washer-like disks with alternate disks being connected to a common power source. The various RICE, PIE, ICE and PICE elements comprising assembly
44
preferably were stainless steel and were mounted within chamber
10
using insulated standoffs.
Electrode assembly
44
was mounted between electron gun
32
and beam optical assembly
38
within housing
10
, with electron beam
12
passing axially through assembly
44
along Z-axis
28
. Ideally Z-axis
28
is coaxial with electron beam
12
upstream from the beam optics assembly
38
, and with both the longitudinal chamber axis and the axis of symmetry for electrode assembly
44
and beam optics assembly
38
.
Typically elements comprising assembly
44
were coupled to one of several various sources of potential, e.g., Va, Vb, Vc, Vd, Ve, Vf, Vg. Typical values for these potentials were Va=0 V, Vb=−0.25 kV, Vc=−0.75 kV, Vd=−1 kV, Ve=−0.75 kV, Vf=−0.25 kV, and Vg=2 kV, In practice, the maximum potential Vd (which is −1 kV in the above example) could be between perhaps −0.8 kV and about −2 kV, with Vd, Vb, Vc, Ve, and Vf being scaled down or up proportionally.
These various potentials create transverse uniform electric fields to which the electron beam is subjected. Electric field non-uniformities, which
Garewal Khem
Heilman James F.
Rand Roy E.
Flehr Hohbach Test Albritton & Herbert LLP
Hobden Pamela R.
Imatron Inc.
Kim Robert
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