X-ray apparatus for computed tomography scanner

In an X-ray apparatus for a computed tomography scanner, a plurality of combined X-ray tube segments are arranged round the central axis of the apparatus on which a subject to be examined is disposed. The X-ray tube segments each include a large number of electron beam generating means to produce an electron beam, a strip-like target having a surface against which the electron beam strikes and extending along a circular arc round the central axis, and an envelope containing these components. Along the surfaces of the X-ray tube segments facing the central axis lie a large number of X-ray detector elements to detect doses of fan-shaped X-ray beams emitted from the target and transmitted through the to-be-examined subject. The electron beam generating means successively produce electron beams, and fan-shaped X-ray beams are applied one after another to the subject from around the same.

This invention relates to an X-ray apparatus, more specifically to an X-ray 
apparatus for a computed tomography scanner, capable of reconstructing 
x-ray tomographic images of a subject or body to be examined. 
Appearing in U.S. Pat. No. 3,983,398, for example, a computed tomography 
scanner is generally known as an apparatus to reconstruct a 
cross-sectional image of a subject or body to be examined by detecting by 
means of an X-ray detector a transmission value of X-ray beams transmitted 
through a region of interest of the body and processing the value by means 
of a computer. In such computed tomography scanner, the X-ray beams need 
be radiated toward the to-be-examined body from various directions. 
Heretofore, there has been employed a means for rotating an X-ray tube 
round the body to be examined in order to radiate the X-ray beams from 
around the body. The problem with the computed tomography scanner using 
such rotating means lies in that scanning for the reconstruction of a 
tomographic image requires more than several seconds, which makes it 
difficult to photograph a region of interest of a subject that moves 
incessantly; such as the heart of a human body, missing the possibility of 
reproducing a clear tomographic image. An X-ray apparatus overcoming this 
problem is disclosed in Laid-Open Publication No. 50,186/77 published on 
Apr. 21, 1977 with the respect to Japanese Patent Application No. 
12,436/76 to which priority is claimed on the basis of British Patent 
Applications Nos. 42,869/75, 3,614/76, 8,646/76 and 30,984/76. The X-ray 
apparatus appearing in this Laid-Open Publication is provided with a 
doughnut-shaped X-ray tube having an elongate target, and has its electron 
beam emitting point moved in a mechanical or electrical manner, a subject 
to be examined being disposed in the central region of the apparatus. 
Since the position of the electron beam emitting point faced to the target 
is changed in turn and an X-ray beam generating point on the target shifts 
its position correspondingly, so X-ray beams may be radiated from around 
the body or subject to be examined. 
According to the X-ray apparatus disclosed in the aforementioned 
Publication, the to-be-examined body can securely be scanned at a high 
speed, and the tomographic image may be reconstructed in a shorter time. 
However, the subject to be examined is relatively bulky, so that the X-ray 
tube must be large in size, requiring, for example, a doughnut-shaped 
envelope with a diameter of 2 m. It is very difficult to manufacture such 
type of X-ray tube, and besides it is not easy to incorporate or remove 
such a large tube into or from a computed tomography scanner. Moreover, 
only a partial defect in the X-ray tube would require that the whole body 
of the X-ray tube be replaced by a new one. 
The object of this invention is to provide an X-ray apparatus for a 
computed tomography scanner that is easy to manufacture, assemble and 
replace, and is capable of scanning a subject to be examined at a high 
speed. 
According to this invention, there is provided an X-ray apparatus for a 
computed tomography scanner, having a central axis thereof, comprising two 
or more X-ray tube segments combinedly arranged along a circular arc 
around the central axis, each X-ray tube segment including at least one 
electron beam generating means to produce an electron beam, a target 
having a surface for emitting a X-ray beam from one of the focal points 
struck by the electron beam toward the central axis, said target being 
extended substantially along a circular arc around the central axis, and 
an airtight envelope containing the electron beam generating means and the 
target, and an X-ray detecting means composed of a plurality of detector 
elements for detecting the X-ray beam emitted from the X-ray tube segment 
and transmitted through a central region including the central axis.

FIG. 1 is a schematic view of a computed tomography scanner with an X-ray 
apparatus 2 according to this invention, in which a subject or body 5 to 
be examined is located in a central position 4 of the X-ray apparatus 2 
which is doughnut-shaped as illustrated. The X-ray apparatus 2 is composed 
of several, e.g. six, X-ray tube segments 6 which are combined into a 
doughnut-shaped entire body. As described later in detail, each of these 
segments 6 is formed of a plurality of electron beam generating means 9, 
an anode target 12 to produce X-rays from a focal point to which an 
electron beam is applied, and a vacuum envelope 14 containing these 
components. Each of the electron beam generating means 9 is formed of an 
elongate cathode 8 to produce electron beams and a control grid 10 
disposed correspondingly to the cathode 8 and allowing the electron beams 
from the cathode 8 to be emitted. A collimator (not shown in FIG. 1) is 
attached to a side of the X-ray tube segment 6 facing the subject 5. An 
X-ray beam passed through the collimator is fan-shaped and is exposed to 
the subject 5. Further, the X-ray apparatus 2 is provided with an X-ray 
detector unit 16 consisting of a plurality of reference X-ray detectors 
for directly measuring a dose of X-rays emitted from the target 12 and a 
plurality of main X-ray detectors for measuring a dose of X-rays 
transmitted through the subject 5, the X-ray detector unit 16 being 
arranged along the inner periphery of the X-ray apparatus 2. 
The reference and main X-ray detectors of the X-ray detector unit 16 are 
connected to a CPU (central processing unit) 22 through amplifiers 18 and 
20, respectively, and detected signals are supplied to the CPU 22. The 
cathode 8 of each X-ray tube segment 6 is connected to an electric power 
source 24 which supplies the cathode 8 with electric power. The power 
source 24 is connected to the CPU 22, which controls the power supply from 
the power source 24 to the cathode 8. A controller 26, which controls the 
control grids 10 of all the X-ray tube segments 6, is connected to the CPU 
22. The controller 26 controls the respective control grids 10 so as to 
on-voltage temporarily, the control grids 10 being successively kept at 
cut-off voltage, and then restore them to cut-off voltage again. The CPU 
22 is connected further to a cathode ray tube (CRT) 28. 
A cross-sectional image of the body 5 to be examined by the above-mentioned 
computed tomography apparatus is obtained as follows. Prior to 
photographing, the cathodes 8 of the X-ray tube segments 6 are supplied 
with electric power from the electric power source 24 in response to a 
command from the CPU 22, and are all heated simultaneously. In this case, 
the targets 12 are at the ground potential, the cathodes 8 are maintained 
at a high negative potential, and the control grids 10 are held at a 
negative cut-off potential as compared with the cathodes 8 in course of 
preparation for the photographing. Then, in the photographing, the 
controller 26 is operated in accordance with a command from the CPU 22 to 
release the cut-off state of the control grids 10 successively from the 
one located at the end of the first X-ray tube segment 6, and thereafter 
to restore the control grids 10 severally to the cut-off state. Thus, an 
electron beam is emitted from each cathode 8 and hits against the anode 
target 12, and a X-ray beam is radiated from a corresponding point of the 
target 12 struck by the electron beam. When all the control grids 10 of 
the first X-ray tube segment 6 have been driven from one end to the other, 
the control grids 10 of the second X-ray tube segment 6 adjacent to the 
first one are driven in the same manner. Thus, fan-shaped X-ray beams are 
emitted successively toward the to-be-examined body 5. Doses of X-rays at 
various X-ray emitting points on the target 12 are detected by the 
reference X-ray detectors, and applied as electric signals to the CPU 22 
via the amplifier 18. These electric signals are used for electrical 
correction of unevenness, if any, in doses of X-rays at the several X-ray 
emitting points. A dose of X-rays transmitted through the body 5 is 
detected by a number of main X-ray detectors arranged on the opposite side 
of the body 5 to the target 12 from which the X-ray beam is radiated, and 
is supplied as an electric signal to the CPU 22 through the amplifier 20. 
Thus, practical electron beam scanning is achieved by the electrical 
control, X-rays are successively emitted at regular intervals to cover the 
whole periphery of the body 5, and a dose of X-rays and an X-ray signal 
are detected with every such emission. The detected doses and signals are 
arithmetically processed by the CPU 22, whereby an X-ray tomographic image 
can be reconstructed on the CRT 28, for example. 
For the photographing of cross-sectional images of the trunk of a human 
body, for example, the doughnut-shaped X-ray apparatus 2 formed of a 
plurality of X-ray tube segments 6 may have outside and inside diameters 
of 1.8 m and 1.5 m, respectively. If the electron beam generating means 9 
of each X-ray tube segment 6 has 50 control grids arranged at regular 
intervals, X-rays are successively emitted from 300 (50.times.6) positions 
on the target 12 for the entire periphery of the X-ray apparatus 2. 
Accordingly, 300 reference X-ray detectors and 500 to 1,000 main X-ray 
detectors are arranged at regular intervals over the whole periphery of 
the apparatus 2. If the bias voltage of each of the 300 control grids 10 
is controlled in 200 .mu.sec, photographing of one cross-sectional image 
may be accomplished in 60 msec (200 .mu.sec.times.300). It is to be 
understood that these figures are given as an example and that the time 
required for the photographing may be further reduced. 
If the anode voltage and the anode current between the anode target and the 
cathode are 100 kV and 1A (pulse width 100 .mu.sec) respectively, the 
amount of heat applied to the target of one X-ray tube segment is 
approximately 500 J, which may be fully radiated by directly 
liquid-cooling a metal base to which the anode target is fixed. 
Referring now to the drawings of FIGS. 2 to 8, there will be described in 
detail the construction of the X-ray apparatus 2. As already mentioned, 
each X-ray tube segment 6 is one of e.g. six radially divided equal parts 
of the doughnut-shaped X-ray apparatus 2. The vacuum envelope 14 of the 
segment 6 is formed of material highly permeable to X-rays, such as glass 
and ceramics. The opening end of the envelope 14 is sealed airtightly with 
an anode base metal 32 with high heat conductivity (e.g. copper) by means 
of a sealing metal 30 such as Kovar. Inside the other end of the vacuum 
envelope 14, the electron beam generating means 9 composed of the cathodes 
8 and control grids 10 and held by a supporting member 34, and a 
reduced-diameter neck portion 36 formed integrally with the main body of 
the envelope 14 and protruding therefrom and an exhaust pipe 38 are formed 
at the other end. The neck portion 36 is airtightly penetrated by a pair 
of cathode lead wires 40 and a plurality of control grid lead wires 42. As 
shown in detail in FIGS. 4 and 5, the electron beam generating means 9 
each include a focusing electrode 46 with an elongate focusing slot 44 for 
focusing electron beams emitted from the cathode 8, the cathode 8 formed 
of a spiral tungsten filament stretched inside the electrode 46 and 
elongated radially with respect to the central axis 4, and the control 
grid 10 formed of a plurality of wires so arranged as to control the 
emission of electron beams from the cathode 8. These electron beam 
generating means 9, 50 in number, are arranged at regular intervals with 
insulators 48 such as ceramics between in each X-ray tube segment, facing 
the arcuate anode target 12. The control grid 10 is welded directly to the 
focusing electrode 46 for the same electric potential, and is connected to 
its corresponding control grid lead wire 42. The grid 10 and the focusing 
electrode 46 may, however, be electrically insulated so as to have 
different potentials. Each cathode 8 is insulated from its corresponding 
focusing electrode 46 by means of an insulating pipe 50 penetrating the 
focusing electrode 46, and is parallel-connected with the lead wires 40 by 
means of supporting rods 52, with both sides supported respectively by the 
rods 52. As shown in FIG. 6, the cathode 8 is disposed correspondingly to 
each of the control grids 10, and a control voltage may be supplied 
independently to each control grid 10. All the cathodes 8 are connected in 
parallel with a power source 24, and ignited at the same time. 
The X-ray emitting target 12 made of tungsten or some other heavy metal is 
embedded in a surface of the anode base metal 32 facing the cathode 8, the 
plane of the target having a tilt relative to the central axis 4. An 
electron beam emitted from the cathode 8 is projected substantially in 
parallel with the central axis 4 to form an electron beam impact region or 
X-ray actual focal point 53 on the target 12. The actual focal point is a 
rectangular form elongated radially with respect to the central axis 4. 
However, an effective focal point obtained by viewing the rectangular 
actual focal point through the collimator is apparently small, and such 
small effective focal point may be regarded as a source of an X-ray beam 
55 to be applied to the to-be-examined subject 5. At each side of the 
target surface or inner surface of the base metal 32 is provided a flange 
54 to protect the joint portion between the sealing metal 30 and the 
envelope 14 against electron impact, while arcuate radiating fins 56 are 
formed on the outer surface of the base metal 32. Respectively at both 
ends of the outer surface of the base metal 32 are flanges 58 protruding 
outward. A liquid-cooling jacket 62 is screwed to the flanges 58 through a 
packing 60 of rubber or the like. The liquid-cooling jacket 62, having an 
annular shape, is singly fitted on all of the six X-ray tube segments 
after they are put together. The liquid-cooling jacket 62 is fitted with 
pipes 64 and 66 for introducing and discharging cooling water into and 
from a space between the liquid-cooling jacket and the outer surface of 
the base metal 32. Each two adjacent X-ray tube segments are coupled 
liquid-tightly to each other with a rubber packing 68 interposed between 
their respective base metals. Thus, a circulating path for circulating the 
coolant throughout the whole circumference is defined along the outer 
surfaces of the base metals of all the X-ray tube segments. In order to 
improve the cooling efficiency, the coolant is circulated in a direction 
opposite to the electron beam scanning direction. The pipes 64 and 66, 
therefore, are attached to positions corresponding to the respective 
positions of the cathodes for the first and last emissions of the X-ray 
beam 55 in one operation of the X-ray apparatus 2. That is, referring to 
the hour hand of a clock, if the X-ray beam 55 is allowed first to be 
emitted from a position corresponding to nearly 1:00, and the X-ray beam 
emitting position is shifted in the clockwise direction for scanning, then 
the coolant is circulated in the opposite direction to the clockwise 
direction. To attain this, the introduction pipe 64 and the discharge pipe 
66 are attached respectively to positions just ahead of and behind the 
position corresponding to 1:00, and a partition is provided in close 
vicinity to the position of 1:00 inside the jacket. By doing this, the 
position subject to the electron beam impact and generation of heat may be 
cooled always by the lowest-temperature portion of the coolant. 
A collimator 70, which is formed of an X-ray screening metal plate such as 
lead, is provided with a slit 72 for the passage of the X-ray beam 55. The 
X-ray beam 55 passed through the slit 72 is rectified into a fan-shaped 
beam. As shown in FIGS. 2, 3 and 7, a large number of reference X-ray 
detectors 74 with their X-ray input surfaces directed toward the target 
12, as well as a number of main X-ray detectors 76 with their X-ray input 
surfaces directed toward the central axis 4, are arranged in the vicinity 
of the slit 72. The reference X-ray detectors 74, which tend directly to 
detect X-rays from the target 12 to provide reference signals, are 
arranged correspondingly to the X-ray focal points 53 formed on the target 
by electron beams from their corresponding cathodes 8. 50 reference X-ray 
detectors are provided for each X-ray tube segment 6. Each reference X-ray 
detector 74 is connected with a lead wire 78 through which the signals are 
taken out. The main X-ray detectors 76, 150 in number, are arranged at 
regular intervals along the inner periphery of each X-ray tube segment 6, 
tending to receive X-rays emitted from the target 12 of the X-ray tube 
segment 6 facing thereto and transmitted through the subject 5. Thus, a 
total of 900 main X-ray detectors are disposed over the whole inner 
periphery of the X-ray apparatus 2. As shown in FIGS. 2 and 8, the 
reference and main X-ray detectors 74 and 76 are isolated by the X-ray 
screening metal plate consituting the collimator 70. The collimator 70 and 
the X-ray detector unit 16 including the reference and main X-ray 
detectors 74 and 76 are constructed in a body, and fixed to the flange 54 
on the inner side of the base metal 32 of each X-ray tube segment 6 for 
accurate location. Naturally, instead of being fixed to the X-ray tube 
segment 6, these components may be disposed separately. In this invention, 
the group of these detector elements are defined as the X-ray detector 
unit 16 even in a case where both or one of the reference and main 
detectors 74 and 76 are arranged in the vicinity of the target 12. 
The X-ray tube segments 6 are put together into a doughnut-shaped body by 
suitable fixing means. These X-ray tube segments 6, which may be housed in 
a suitable casing filled with insulating oil, are independently removably 
fixed, in any case. Accordingly, if any one of the X-ray tube segments 6 
is found defective, it may be replaced singly. Although the joint between 
each two adjacent X-ray tube segments 6 may form a discontinuous region 
where the X-ray emitting points or focal points 53 are missed, such 
discontinuity can be limited to a distance corresponding at most to one 
focal point, hardly exerting any bad influence upon the quality of 
resultant X-ray tomographic images. 
The X-ray apparatus 2 of this invention as described above may be produced 
in commercial quantity since the maximum length of each X-ray tube segment 
can be limited to approximately 80 cm. Moreover, the individual X-ray tube 
segments can easily be attached and detached, so that the assembly and 
handling of the apparatus may be facilitated and any defective X-ray tube 
segment may be replaced independently. Furthermore, by fixing the 
collimator and the X-ray detector unit separately to the X-ray tube 
segment, they may be located accurately, and their positions may 
substantially be prevented from being shifted. 
Referring now to FIGS. 9 to 17, there will be described a modification of 
the X-ray apparatus of this invention. 
In an example of FIG. 9, reference X-ray detectors 74-1 and 74-2 and main 
X-ray detectors 76-1 and 76-2 are closely arranged on both sides of an 
X-ray beam path or the slit 72, respectively. With thus arranged two types 
of X-ray detectors, two close cross-sectional images may be reproduced by 
one scanning operation. Two close fan-shaped X-ray beams may be radiated 
simultaneously toward the to-be-examined body 5 respectively through two 
slits provided for the collimator so that X-ray signals for the two 
cross-sectional images can be obtained severally by means of the X-ray 
detector unit 16. 
In another example of the electron beam generating means 9 as shown in FIG. 
10, an elongate flat cathode 82 as shown in FIG. 11 and an elongate 
focusing electrode 84 with a plurality of focusing slots 44 bored therein 
extend inside the vacuum envelope of one X-ray tube segment from one end 
thereof to the other, the cathode 82 and the focusing electrode 84 being 
connected at the same potential. Inside each focusing slot 44 is a control 
grid 88 fixed on an insulator 86 and connected to its corresponding lead 
wire. The flat cathode 82 may be of a directly heated type such as pure 
tungsten and thorium-turngstem, or of an indirectly heated type. This 
electron beam generating means 9 is easier to assemble because the 
electrodes except the control grid 88 are formed in a block. 
The X-ray tube segment 6 according to an alternative embodiment as shown in 
FIGS. 12 to 14 is so constructed that each individual electron beam 
generating means 9 can be replaced independently, and that the direction 
of electron beam can be adjusted minutely. Further, the X-ray tube segment 
6 has its vacuum envelope 14 separable. That is, the vacuum envelope 14 
formed of ceramics or other insulating material has two openings, one of 
which, as already described, is fitted with the base metal 32. In this 
embodiment, the other opening of the envelope 14 is airtightly fitted with 
a cover portion 90 by means of flanges 92 and 94 that are formed on the 
envelope 14 and the cover portion 90 respectively. The cover portion 90 is 
fixed by an insulating base plate 96 by means of a metal fitting 98. The 
insulating base plate 96 is provided with connecting terminal pins 100 and 
102 for the cathode and control grid, while the lead wires 40 and 42 are 
provided for the cover portion 90. The control grid terminal pin 102 is 
connected to the lead wire 42 by means of a strip line 104. On the 
insulating base plate 96 are 50 electron beam generating means 9 removably 
fixed at regular intervals. Namely, the terminal pins 100 and 102, as 
shown in FIG. 12, are provided with insertion holes in which the cathode 
supporting rods 52 and lead wires 106 for the control grid and focusing 
electrode are inserted, respectively. Metal fittings 108 are screwed on 
screws 110 rotatably attached to the base plate 96, and are fixed securely 
by fixing nuts 112. The screws 110 are provided for the four corners of 
each electron beam generating means 9. The inclination of each electron 
beam generating means 9 relative to the base plate 96 may freely be 
regulated by adjusting the screws 110. The flanges 92 and 94 fixed to part 
of the vacuum envelope are fixed tight by means of screws 116 through 
vacuum ring gaskets 114. The anode base metal 32 is provided with an 
exhaust path 118, which is fitted with an exhaust pipe 120. A vacuum pump 
122 maintains a vacuum inside the vacuum envelope. If the focal points of 
X-ray beams are subject to unevenness, the screws 116 are unfastened to 
remove the cover portion 90 after stopping the vacuum pump, and the X-ray 
beam focal points are located to accurate positions by finely adjusting 
the screws 110. Further, in the X-ray tube segment 6 of this embodiment, 
if a cathode suffers disconnection or defective electron emission, it may 
independently be replace for the resumption of operation. Moreover, after 
the positions of the X-ray focal points are located properly, the flanges 
92 and 94 of the vacuum envelope 14 and the cover portion 90 may be 
airtightly welded together. 
In an embodiment as shown in FIGS. 15 to 17, each X-ray tube segment 6 is 
so constructed that X-ray beams are emitted from a number of focal points 
on the target 12 to scan the to-be-examined subject 5 by deflection 
scanning or the target 12 with a single electron beam. The X-ray tube 
segment 6 includes the inclined anode base metal 32 and the elongate 
target 12. The vacuum envelope 14 has a curved large-diameter portion 124 
to hold the target 12 and a small-diameter neck portion 126 which contains 
an electron gun 128 to produce electron beams. An electromagnetic 
deflecting coil 130 for electron beam deflection scanning control is 
located around the outer periphery of the neck portion 126 ahead of the 
electron gun 128. The electron gun 128 has a control grid (not shown in 
FIG. 15). The deflection scanning may be performed by using an 
electrostatic deflecting device instead of the electromagnetic deflecting 
coil 130. As shown in FIG. 17, each of the reference X-ray detectors 74 
attached to the collimator 70 is provided with an X-ray window 132 to 
admit only X-rays emitted from an electron beam impact position 
corresponding to the relevant detector 74. The use of the X-ray windows 
132 makes it possible to detect accurately the focal point form which an 
X-ray beam is emitted and a dose of X-rays at that point. The electron 
beams may be produced pulsatively or continuously to cause X-rays to be 
emitted from the targets 12 over the whole circumference of the X-ray 
apparatus 2. 
In the apparatus of this invention, the X-ray tube segments need not be 
arranged over the whole circumference of a circle round the central axis 
4. However, to obtain a good tomography image, these segments may be 
preferably combined and arranged so as to cover an angular range of 
180.degree. or more round the central axis 4, as shown in FIG. 18. In this 
case, however, the main X-ray detectors 76 out of the X-ray detector unit 
16 need be disposed also on the opposite side to the X-ray tube segments. 
Moreover, as shown in FIG. 19, the shape of each X-ray tube segment 6 may 
resemble that of a side of a polygon so that a substantially 
doughtnut-shaped X-ray apparatus may be formed. In this case, the target 
12, electron beam generating means 9 and vacuum envelope 14 need not have 
arcuate outward shapes, so that they may have rectilinear outlines, 
additionally facilitating the manufacture. However, the X-ray detectors 74 
and 76 are to be arranged in a loop, and deviations in dose of rays and in 
angle attributable to the differences between the distances between the 
variedly located focal points and the central axis 4 should be corrected 
by means of the CPU. 
As described above, the X-ray apparatus of this invention provides high 
economical efficiency, as well as facility for manufacture, assembly and 
handling, capable of sharply reducing the time required for the 
photographing of a cross-sectional image.