X-Ray tube for producing a flat wide-angle fan-shaped beam of X-rays

A rotating-anode X-ray tube for producing a flat wide-angle fan-shaped beam with a substantially uniform distribution of energy comprises a cylindrical anode and a cathode axially or peripherally offset from the target area or focus bombarded by the electrons so that the axis of the fan-shaped beam emitted by that area can extend radially to the cylindrical anode surface. An arcuate shield closely paralleling this cylindrical surface is apertured at its center in front of the focus and intercepts stray electrons which would be liable to bombard the anode at points outside the target area so as to give rise to extra-focal radiation. Such a tube is useful in apparatus designed for axial transverse tomography.

FIELD OF THE INVENTION 
My present invention relates to a rotating-anode X-ray tube for producing 
in cooperation with collimating means such as a slit diaphragm, a flat 
wide-angle fan-shaped beam with a substantially uniform distribution of 
the radiating energy in a plane and in all directions within its angle of 
divergance. A beam source comprising a tube of this type is more 
particularly intended for a transverse-axial-tomography apparatus, also 
termed a tomodensimeter, having a row of X-ray detectors juxtaposed in the 
plane of the fan-shaped beam constituting the section plane so as to be 
capable of measuring the absorption of the object simultaneously in 
several directions. 
BACKGROUND OF THE INVENTION 
Tomodensimeters according to the state of the art employ conventional X-ray 
tubes with fixed or rotating anodes which generally comprise a linear 
cathode surrounded by an electron concentrator and producing an electron 
beam of rectangular section parallel to the axis of the anode. Generally, 
the anode surface is beveled or frustoconical so that its generatrices are 
inclined, on the one hand, relative to the electron beam bombarding it 
and, on the other hand, relative to the useful beam of X-rays obtained by 
collimation (by means of a diaphragm) of the radiation emitted by the 
bombarded surface portion, termed the focus or target area. It has been 
found that the distribution of the energy of the emitted radiation as a 
function of its angle relative to the surface normal at the target area is 
not uniform and that in the anode-cathode plane, defined by the axes of 
the anode and the cathodic filament, this distribution of radiated energy 
varies considerably, with a maximum of emission in the direction of the 
aforementioned surface normal. 
Another drawback of the use of these beveled anodes for producing a 
wide-angle fan-shaped radial beam for irradiating a row of detectors, is 
that the projection of the real focus or target area on the rectangular 
face of each of these detectors i.e. the virtual focus, undergoes a 
distortion which increases with the mean angular deviation from the 
surface normal so that the detectors located at the ends of the row see 
only a small part of the virtual focus and consequently receive only a 
small part of the radiated energy. 
In order to compensate for these defects, certain electronic means have 
been developed. In the present state of the art, X-ray tubes are also 
employed which comprise a rotating anode whose frustoconical or beveled 
surface is bombarded by an electron beam of elongated section (practically 
filiform), oriented radially relative to the axis of rotation of the anode 
and forming on the frustoconical surface of the latter an elongate thermal 
focus coinciding with a generatrix of the conical surface. The radiation 
emitted by this focus is collimated in such manner as to select the rays 
emitted about the tangent to the frustoconical surface in the region of 
the thermal focus so as to obtain a fan-shaped radiation with an energy 
distribution which is more uniform than with conventional substantially 
pin-point sources employing the same type of tube. As this uniformity is 
still insufficient, owing to the variation of the angle at which the rays 
emerge, the use of a wedge-shaped attentuator for compensating for this 
defect has been proposed. 
Further, in these X-ray tubes, when the anode is bombarded by the electron 
beam, a certain number of secondary electrons are emitted from the thermal 
focus and are reaccelerated in the anode-cathode space, being thus liable 
to bombard the anode at points outside the target area and to produce an 
X-ray radiation, termed an extra-focal radiation, which has an adverse 
effect on the quality of the desired flat fan-shaped X-ray beam. 
OBJECTS OF THE INVENTION 
An object of the present invention is to provide an X-ray tube for 
producing a planar wide-angle fan-shaped beam having an energy 
distribution which is substantially uniform throughout its width without 
requiring the aforementioned attenuation-type compensating means. 
Another object of my invention is to provide a tube of this type whose 
virtual focus has no notable deformation in any direction within the 
useful fan shape, so that the projection of the thermal focus or target 
area on the input faces of the detectors is practically without 
deformation and retains its elongated rectangular shape irrespective of 
the mean angular position of the part of the beam striking them. 
It is also an object of my invention to provide means in, the X-ray tube 
for reducing the extra-focal radiation and thereby still further improving 
the quality of the emitted beam. 
SUMMARY OF THE INVENTION 
In accordance with my present invention, an X-ray tube for a 
radiodiagnostic apparatus, having a rotating anode with an X-ray-emissive 
cylindrical surface centered on an axis in a vacuum-tight envelope, has 
its cathode offset from a radial line normal to the target area toward 
which a beam of electrons is emitted by the cathode, the usual collimating 
means for converting emitted X-ray radiation into a planar fan-shaped beam 
being a slitted diaphragm disposed in a plane transverse to the anode axis 
which includes that radial line. Thus, the beam shaped by the collimating 
means spreads within this radial plane into the desired fan shape. 
According to another important feature of my invention, the anode is at 
least partially enshrouded by shield means closely paralleling its 
cylindrical surface in the vicinity of the target area for intercepting 
stray secondary electrons, the shield means having an aperture in line 
with the diaphragm slit confronting the target area for giving passage to 
the electron beam emitted by the cathode and to the X-ray radiation 
emitted from the target area. 
In conformity with conventional practice, the cathode may be located in a 
tubular neck forming an extension of the envelope. This neck, pursuant to 
my invention, preferably has a centerline which includes an acute angle 
with the radial line normal to the target area. As more particularly 
described hereinafter, this centerline my lie either in the transverse 
plane containing the collimating means or in an axial plane of the anode 
including the aforementioned radial line. In either case, the electron 
beam initially approaches that radial line and is then deflected through 
the shield aperture onto the target area.

In all these Figures like references designate like elements. 
SPECIFIC DESCRIPTION 
FIG. 1 shows a first embodiment of a X-ray tube according to the invention 
in axial section. In this Figure, the X-ray tube comprises a glass 
envelope 1 having a generally cylindrical shape whose ends are connected 
in an ultra-vacuum-tight manner (by means of discs 3 and 4 of a 
conventional alloy of a metal having a coefficient of thermal expansion 
close to that of the glass) to corresponding ends of a hollow metal shaft 
2 which permits the circulation of a cooling fluid in the direction of the 
arrows. 
Journaled on the hollow shaft 2 by means of ball bearings 6 and 7 is a 
tubular metal shaft 5 to which there are fixed a cylindrical copper rotor 
8 disposed in a rotating field, produced by a stator (not shown) which is 
fitted in a conventional manner on the envelope 1, and a rotating anode 10 
which has a cylindrical surface whose generatrices are parallel to its 
axis of rotation as is known per se. 
The rotating anode 10 has a cylindrical body 11 of an electrically 
conductive material (of a metal such as, for example, copper or 
molybdenum, or of graphite) at least the surface of which, bombarded by a 
beam of electrons, is covered with a layer 12 of a material which emits 
X-rays, such as tungsten. 
It is also possible to make the entire body 11 of this X-ray-emitting 
metal. 
In conventional cylindrical-rotating-anode X-ray tubes, the cathode 
filament is disposed in front of the cylindrical surface in such manner as 
to emit an electron beam perpendicular to the surface and consequently to 
the axis of rotation of a anode. An arrangement of this type has the same 
drawbacks as the tube having a frustoconical anode, since the useful beam 
of X-rays includes an angle close to 90.degree. with the normal to the 
target area, that is to say, the useful beam has a small angular deviation 
from the plane tangent to the focus (of the order of 6.degree. to 
10.degree.) and consequently a highly non-uniform energy distribution. 
In the X-ray tube according to the invention, the cathode 20, comprising a 
filament 22 and an element 21 for concentrating the electrons, is 
laterally offset from the anode 10 so that the space in front of the 
thermal focus is left free and the axis of the X-ray beam may be 
substantially normal to the target area and consequently perpendicular to 
the axis of rotation of the anode. This arrangement, best seen in FIG. 2, 
provides a planar, wide-angle, fan-shaped beam 17 of vertex angle 
.alpha.&gt;60.degree. with a substantially uniform distribution of the 
radiated energy and with rectangular virtual foci throughout the fan 
shape. The beam 17 is bisected by a plane z-z' including the anode axis. 
For this purpose, the cathode 20 is disposed in a projecting tubular neck 9 
having an end which is closed in a sealed manner and through which extend 
sealed and conductive leads 23 which are embedded in the end of the neck 
and serve to support the filament 22 and the cup-shaped electron 
concentrator 21 and to supply them with operating current. The two leads 
23 supporting the ends of the filament 22 extend through the end of the 
cup-shaped concentrating element 21 and are surrounded by insulating 
sleeves 24 (see FIG. 3) so as to permit the application to the element 21 
of a biasing voltage which is negative relative to the potential of the 
filament 22. 
I may construct the envelope 1 from a metal which is either substantially 
transparent to the useful X-rays or provided on the part thereof facing 
the target area with a window (not shown) of an X-ray-transparent material 
such as glass or a ceramic which is sealed, for example by brazing, to the 
metal envelope 1. 
In this embodiment (FIG. 1) the focal track 12 is disposed in a recess 
constituted by an annular groove 13, bounded by two projecting flanges or 
collars 14, whereby the extra-focal radiation may be markedly reduced, the 
emitting layer 12 covering the bottom of the groove 13 of the generally 
spool-shaped anode 10. 
FIG. 2 shows a modification, in axial section, of the embodiment of FIG. 1. 
Here, the anode no longer has the shape of a spool but is perfectly 
cylindrical and provided with a suppressor of extra focal radiation 
according to another feature of the invention which is much more effective 
than the edge beads 14 flanking the conventional recess 13 of FIG. 1. 
The radiation suppressor 15 is an arcuate shield which is centered on the 
axis of revolution of the rotating anode 10 and closely parallels the 
cylindrical surface of this anode. The center of the shield is apertured 
at 27 so as to leave a free passage for the incident electron beam 16 and 
for the beam of radiating energy emitted from the focus. 
The shield 15 comprises two layers A and B. The outer layer A is made of 
light material, such as graphite or titanium, and serves to absorb by a 
retarding action on its outer face the secondary electrons which are 
released by the impact of the main beam on the target area and which, when 
reaccelerated, might bombard the anode at points outside that area so as 
to produce an extra-focal radiation. 
The inner layer B consists of a material of high atomic number, such as 
tungsten, so as to absorb the X-radiation emitted at points of the anode 
other than the focus. 
The thickness of layer A depends on the maximum operating voltage of the 
tube and should be so chosen that the residual X-radiation produced by the 
bombardment of the secondary electrons on this layer is negligible. 
The thickness of layer B depends on the extra-focal radiating energy to be 
absorbed and therefore also on the maximum operating voltage of the tube. 
In order to obtain optimum efficiency, this shield has its inner layer B 
located very close to the cylindrical surface of the anode, for example a 
few tenths of a millimeter therefrom. 
The sole source of X-rays is therefor limited to a surface area of the 
anode having a width corresponding to that of the aperture 27 of the 
shield 15 and a length equaling at most the breadth of the cylindrical 
anode. This source produces a fan-shaped X-radiation 17. 
In the embodiment of FIG. 1 the first embodiment of an X-ray axis of the 
neck 9, and consequently the axis of the beam of electrons bombarding the 
anode track 12, is located in the radial plane of the anode containing the 
beam of X-rays emitted by the track. The neck axis constituting the 
centerline of the electron source 21, 22 is skew to the anode axis so that 
a free space remains in front of the focus enabling the emplacement of a 
diaphragm 30 having a rectangular slit 31 which is also skew to the anode 
axis and lies as close as possible to the X-ray-emitting focus. In this 
first embodiment, the coiled filament 22 is oriented parallel to the axis 
of the anode 10 so that the elongated (quasi-linear) rectangular focus on 
the focal track 12 substantially coincides which a generatrix of its 
cylindrical surface. 
The orientation of the axis of the neck 9 is shown in FIG. 1 as being 
substantially perpendicular to a generatrix of the surface of the anode 
10, no supplementary electrode or magnetic coil for deflecting or focusing 
the electron beam being provided. 
However, this orientation of the neck 9 at right angles to the axis of the 
X-ray beam, while allowing the diaphragm 30 to approach the anode to the 
maximum extent, is not necessarily the most advantageous from the point of 
view of the fineness of the linear focus since the electric field acting 
on the electrons moving between the cathode 20 and the anode 10 does not 
eliminate the effect of the Gaussian dispersion of the energies of the 
electrons leaving the filament, which is manifested by a broadening of the 
focus. 
Thus, I may orient that it is possible to employ in this embodiment the 
axis of the neck 9 at an acute or obtuse angle with respect to the axis of 
the X-ray beam which is normal to the target area and, in the latter case, 
may utilize conventional means (not shown) for deflecting and 
concentrating the electron beam, as known in electron optics, whereby the 
electrons approaching the plane z-z' are caused to impinge substantially 
perpendicularly upon the focal track 12 by way of shield aperture 27. 
The axis of the neck 9 is shown oriented in FIG. 2 in conformity with the 
embodiment of FIG. 1, i.e. skew to the anode axis, but includes an acute 
angle with the axis of the X-ray beam which is normal to the target area 
so that the aperture 27 of the shield 15 giving access to the bombarding 
electron beam could be very narrow in order to limit as far as possible 
the extra-focal radiation. 
FIG. 3 shows an axial sectional view of a second embodiment of an X-ray 
tube according to the invention in which the cathode 20 is offset from the 
anode 10 in a direction parallel to the axis of rotation thereof indicated 
at x-x'. Thus, the axis of neck 9 is inclined relative to the radial plane 
containing the fan-shaped X-ray beam; the diaphragm serving to shape the 
beam has not been illustrated in this Figure. 
The electron beam 16 emitted by the axially offset cathode is inclined at 
an acute angle to the axis of rotation x-x' of the anode 10 in a plane 
defined by this axis and the normal to the target area forming part of the 
cylindrical surface 12. In order to obtain a quasi-linear focus coinciding 
with a generatrix of the anode surface 12, the centerline of the filament 
22 and of the cavity of the concentrating cup 21 containing that filament 
is located in the plane of offset and so oriented as to pass substantially 
through the center of the focus. This plane of offset, as will be apparent 
from the drawing, is defined by the anode axis x-x' and the surface normal 
of the target area. 
In FIG. 3, the rotating anode is supported by a rotor 18 centered on the 
axis x-x' and supported by a metal disc 26, the vacuum-tight connection of 
the latter with the rotor being ensured by a thin metallic rotating sleeve 
19. The rotor 18 is located in a rotating field produced by a stator 25 
having the same potential as the anode. 
The shield 15, provided with the two layers A and B, is integral with the 
metal disc 26 supporting the rotor along with the anode and is maintained 
at the same potential as anode 10. As in the embodiment of FIG. 2, the 
electron beam 16 is deflected away from the axis of neck 9 in order to 
pass through the shield aperture to the target area of track 12. 
Besides reducing the extra-focal radiation, shield 15 may have the function 
of absorbing the thermal radiation from the anode. In this case, the 
surface of the shield facing the anode is extended to cover the entire 
cylindrical surface of the anode and also the two circular end faces 
thereof. The shield consequently has the shape of a hollow cylinder 
enshrouding the anode, its cylindrical and circular surfaces being 
respectively parallel to the cylindrical and circular surfaces of the 
anode. 
The heat is then carried off by means of a cooling fluid circulating in the 
shield. This fluid may be for example water or oil, depending on the 
operating potential of the anode (ground or positive high voltage). 
The X-ray tubes according to the present invention may be used in 
transverse-axial-tomography apparatus comprising a row of numerous 
radiation detectors all of which are irradiated simultaneously by a 
wide-angle fan-shaped beam. 
The anode 10 may be driven in rotation by any known means other than those 
described hereinbefore.