Method of and device for forming an image of a layer of a three-dimensional object

The invention relates to a method of and a device for forming images of a layer of a three-dimensional object. In order to obtain high-quality images, the object is irradiated by means of a necessarily large number of radiation sources which are arranged in different mutually parallel planes or which are arranged to be displaceable (for example, rotatable) with respect to the object. Alternatively, the object with the record carrier (X-ray film) may be arranged to be displaceable with respect to the radiation sources. In the decoding step (superposition and summing of the shadow images) use is made of the two dimensional distribution of all radiation sources used during coding (irradiation) of the object.

BACKGROUND OF THE INVENTION 
The invention relates to a method of forming images of a layer of a 
three-dimensional object. The object is irradiated by a group of radiation 
sources (of a multiple radiation source) which are arranged in one plane. 
The irradiated object casts shadow images which are recorded by a 
radiation-sensitive layer which is situated in a recording plane. An image 
of a layer of the object is obtained from the shadow images by 
superposition and summing. 
A method of this kind is already known from German Offenlegungsschrift No. 
25 14 988. According to this method, the object is irradiated from a large 
number of radiation sources, for example, from 20 to 30 different 
directions (by means of a corresponding number of radiation sources), in 
order to record shadow images which are situated in different planes and 
to obtain adequate information concerning the three-dimensional object for 
medical diagnostic radiology. Such a large number of radiation sources, 
for example X-ray tubes having stationary anodes, however, cannot be 
accomodated within an arbitrarily small space because of the size of the 
radiation sources. For example, in order to prevent high-voltage 
flashovers in X-ray tubes, it is not possible to arrange the X-ray tubes 
in the closest packing which is physically possible. 
Consequently, the recording angle of the radiation source distribution 
(i.e. the angle at which the central rays, of the radiation beams which 
are emitted by the radiation sources which are situated furthest from each 
other, intersect each other when there is a fixed distance between the 
plane of the radiation sources and the object) assumes comparatively high 
values. In tomography there are special examination methods (zonography) 
by means of which images can be formed of thicker object layers. However, 
this requires very small recording angles of, for example, 10.degree.. 
Such small recording angles, however, cannot be obtained by means of known 
multiple radiation sources in view of the large number of radiation 
sources, their predetermined plot distribution and the required minimum 
distance therebetween. Abstaining from the use of the radiation sources 
which are furthest apart is not possible either, because information 
concerning the three-dimensional object is lost, so that the quality of 
the image to be realized is reduced. In order to achieve a high quality 
imager of a layer, it is necessary to irradiate the object with an as 
large as possible number of radiation sources, even when the recording 
angle is large. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a method of forming an image of 
a layer of an object where the object can be irradiated at a very small as 
well as a very large recording angle from the number of radiation source 
positions, which is required for forming high-quality images of layers of 
an object. 
The method according to the invention, where the shadow images are recorded 
in recording planes which are situated at different distances from the 
object which are parallel to the radiation source plane, and which are 
subsequently scaled, is characterized in that the three-dimensional object 
is further irradiated by radiation sources which are situated in at least 
one further radiation source plane which is parallel to the first 
radiation source plane. The group of radiation sources situated in a 
radiation source plane is consecutively switched on and off. The shadow 
images of each group of radiation sources are recorded on 
radiation-sensitive layers which are situated at different distances from 
the object, the ratio of the distances of the various radiation source 
planes and of the associated recording planes from the object being 
constant. An image of a layer of the object is the obtained by a 
superposition of the scaled shadow images, produced by all radiation 
sources used for the irradiations. 
In a further method according to the invention, the object is irradiated at 
least twice by a number of radiation sources which are situated in one 
plane. Each of the radiation sources moves relative to the object after 
each irradiation and assumes a different position in planes which are 
parallel with respect to each other. The position of the 
radiation-sensitive layer in the recording plane remains unchanged with 
respect to the object . An image of a layer of the object is then obtained 
by super-position of the shadow images, produced by all radiation sources 
used. 
When the number of individual radiation sources situated in a radiation 
source plane of a multiple radiation source is reduced by not using 
individual radiation sources of the distribution which are situated far 
from each other, the recording angle can be substantially reduced for a 
given distance between radiation source plane and object. If the total 
number of individual radiation sources is smaller than the number of 
radiation sources required for obtaining high-quality images, any 
resultant information loss concerning the three-dimensional object can be 
avoided by additional irradiations of the object by means of sources which 
are arranged in two or more mutually parallel radiation source planes. All 
radiation sources are then stopped down so that they irradiate 
substantially a common object volume. 
The number of separate radiation sources, or the number of directions from 
which the object is irradiated, can also be increased by moving the 
radiation sources and the object with respect to each other. For example, 
the multiple radiation source can be rotated with respect to the 
stationary object around a central axis which extends perpendicular to the 
radiation source plane and through the object. The individual radiation 
beams intersect each other in the object and the central axis, so that 
almost the same object volume is irradiated in any position of the 
radiation sources after a rotation. The position of a record carrier for 
recording the shadow images then remains unchanged with respect to the 
object. 
However, it is alternately possible to displace the object with respect to 
the stationary multiple radiation source in a plane parallel to the 
radiation source plane, for example in a straight line, while the record 
carrier (radiation-sensitive layer) is moved in the same way parallel with 
respect to the plane of the radiation source, so that the position thereof 
again remains unchanged with respect to the object. During displacement of 
the object it is only necessary to ensure that the object is irradiated in 
all positions by all radiation beams. This can be realized, for example, 
by displacing a diaphragm plate parallel to the plane of the radiation 
source. This plate is provided with apertures such that all radiation 
beams of the multiple radiation source are always stopped down to cover 
the object.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows a device for forming shadow images. The device comprises a 
multiple radiation source 1 in which several separate X-ray tubes 2a, 2b, 
3a, 3b and 3c for (example X-ray tubes having stationary anodes) are 
arranged in two mutually parallel radiation source planes. X-ray beams 2 
and 3, generated by the X-ray tubes, pass through apertures in diaphragm 6 
to irradiate an object 8 arranged on an object table 7. 
The multiple radiation source 1 is, for example, an oil-filled X-ray tube 
tank in which the separate X-ray tubes 2a, 2b and 3a-3c are mounted 
without a further protective housing. The angle between the radiation 
beams 3, or the central rays thereof, emitted by the X-ray tubes 3a and 3c 
represents the recording angle .gamma. of the device, which is chosen to 
be comparatively large in this case for the sake of clarity. 
The radiation beams 2 and 3 generated by the X-ray tubes arranged in the 
planes 4 and 5 of the radiation source are aligned so that they irradiate 
a common area of the object 8. The central rays of the radiation beams 2 
and 3 intersect each other in a single point through which the central 
axis 9 of the multiple radiation source 1 also passes. The central axis 
extends perpendicular to the planes 4 and 5 of the radiation source. The 
point of intersection 30 of the central rays is situated within the object 
plane 12 of the object 8 shown. At the area of the point of intersection 
30, rays pass through the same section of the object 8 which is situated 
parallel to the planes 4 and 5 of the radiation source. 
The distribution of the X-ray tubes in the separate radiation source planes 
is preferably nonredundant. The distribution obtained by the projection of 
all X-ray tubes in the direction of the central rays of the radiation 
beams 2 and 3 on the radiation source planes 4 and 6 preferably is also 
nonredundant, so that images of a layer of the object which contain 
particularly few artefacts can be made. 
The groups of X-ray tubes 2a, 2b, and 3a-3c are also connected to a 
generator and a control unit (not shown) by means of which the groups of 
X-ray tubes can be consecutively switched on and off. The multiple 
radiation sources, however, may also consist of a single vacuum vessel in 
which separate cathodes are situated opposite a common anode or opposite 
separate anodes. Different focusses can then be realized by the 
simultaneous activation of the cathodes. 
T 
The shadow images thus obtained are consecutively recorded on a common 
record carrier (radiation-sensitive layer), for example, a film. To this 
end, the firm or a film carrier is situated underneath the object 8 in a 
recording plane 10 which extends parallel to the radiation source plane 5. 
The film or film carrier is arranged to be displaceable in the direction 
of the central axis 9. In order to enable simple decoding of both groups 
of shadow images by means of superposition, for example, after the first 
irradiation by means of the separate X-ray tubes 3a-3c, the film is 
displaced from the recording planes 10 to the recording plane 11. After 
the move, the second irradiation with the X-ray tubes 2a-2b is performed. 
During the separate irradiations, the ratio (A/C or B/D) of the distance 
between the radiation source plane (4 or 5) and the object plane 12 with 
respect to the distance between the recording plane 10 or 11 and the 
object plane 12 is constant. Obviously, the groups of shadow images can 
also be recorded on separate X-ray films which are consecutively 
positioned in the recording planes 10 and 11 in a corresponding manner. 
FIG. 2 is a schematic plan view of a multiple radiation source 13 which 
comprises four X-ray tubes 14 which are situated in one radiation source 
plane. The number of positions from which an object is irradiated is 
doubled by rotation of the multiple radiation source 13 around the central 
axis 90, (extending perpendicular to the radiation source plane) through a 
predetermined angle .alpha.. After rotation through the angle .alpha., the 
four X-ray tubes occupy the positions 14'. For each angular position of 
the multiple radiation source 1, all X-ray tubes 14 are simultaneously 
flashed. The various groups of shadow images can then be recorded each 
time on separate films or on a common film. The X-ray tubes 14 are 
preferably arranged so that when the multiple radiation source 13 is 
rotated through the angle .alpha., the radiation source distribution 
composed of the rotated and the original nonredundant radiation source 
distribution is again nonredundant. Obviously, the X-ray tubes can also be 
arranged in other suitable radiation source distributions in the relevant 
radiation source planes. 
The multiple radiation source can also be moved along arbitrary paths 
situated within the radiation source plane, for example, elliptical paths. 
The movement may be, for example, continuous with all radiation sources 
being flashed at given points on their paths. The corresponding groups of 
shadow images can then be recorded on a separate film. 
For the irradiation of an object from a large number of different 
directions, the object can be rotated instead of rotating the radiation 
sources or the multiple radiation source. To this end, the object table is 
arranged to be rotatable, the axis of rotation coinciding with the central 
axis 9 of the multiple radiation source 13. The groups of shadow images 
obtained in various positions of the object with respect to the multiple 
radiation source 13 are then recorded, for example, on a separate film 
which is situated in accordance with the orientation of the object table 
or the object. In medical diagnostic radiology, a recording technique of 
this kind is not always possible, but it is so in the field of material 
testing where mainly nonliving and hence immobile objects are dealt with. 
Obviously, the multiple radiation source shown in FIG. 1 with different 
radiation source planes can also be rotated around its central axis 9 in 
order to increase the number of radiation source positions. The radiation 
source distributions resulting from the rotation preferably are again 
nonredundant. 
FIG. 3 shows a device for the recording of shadow images where an object 16 
is displaced parallel to the radiation source plane 17 and, for example, 
in a straight line (arrow 18). By means of this device, the object 16 can 
be irradiated in a first recording step by means of the X-ray tubes 19 and 
20. The radiation beams 21 and 22 of the X-ray tubes 19 and 20 are stopped 
down (to produce radiation beams 21a and 22a) by means of a diaphragm 
plate 23, which has apertures, such that the beams irradiate only the 
object 16 to be examined. The corresponding shadow images A are recorded 
on a film 24 (postion a) or on another suitable record carrier. The shadow 
images A on the film 24 are shown again on the line b for the sake of 
clarity. 
During the subsequent recording step, the object 16 is displaced parallel 
to the radiation source plane 17 in a straight line, for example, over the 
object width. (The new location of the object 16' is denoted by broken 
lines.) This can be simply realized by displacement of the patient 
examination table 25 on which the object 16 is arranged. The film 24 is 
displaced simultneously with the examination table 25, i.e. in the same 
direction as the object 16 and over the same distance. To this end, the 
film 24 or the cassette accomodating the film, for example, is 
mechanically coupled to the examination table. As a result of translatory 
displacement of the diaphragm plate 23 (the new position is denoted by the 
broken line 23') parallel to the radiation source plane, the radiation 
beams 21 and 22 are then stopped down (to produce beams 21b, and 22b) so 
that they irradiate only the object 16' in its new position. 
The shadow images B thus generated are recorded on the same film 24 (on 
line c the shadow images A and B associated with the different positions 
of the object 16 are denoted by broken lines). When the film 24 is 
returned to its original position, the object 16 then being in its 
starting position again, it will be seen that the film 24 contains shadow 
images which have been obtained by means of the X-ray tubes 19 and 20 
(images A on line d) and by means of two "ficticious" X-ray tubes 19' and 
20' (images B on line d). The number of X-ray tubes is thus apparently 
doubled, which is particularly advantageous when the object is irradiated 
at small recording angles. The number of X-ray tube positions can be 
further increased by displacing and irradiating the object several times. 
The described method is suitable, for example, for examination of parts of 
the human vertebral column. To this end, the X-ray tubes can be arranged 
in a line, the longitudinal direction of the examination table 25 on which 
the body is positioned then extending tranverse to the connecting line of 
the X-ray tubes. For the recording relevant groups of shadow images, the 
examination table with the X-ray film is then displaced each time 
tranverse to the longitudinal direction of the examination table 25 and 
parallel to the radiation source plane over the width of the vertebral 
column. 
For the formation of images of a layer of the three-dimensional object, the 
shadow images can be decoded according to German Offenlegungsschrift No. 
25 14 988. The decoding (superposition and summing of the shadow images) 
is preferably performed by means of lenses, the positions of which 
correspond to the positions of all X-ray tubes used for the recording of 
the shadow images. For the decoding of the shadow images obtained in FIG. 
3, therefore, lenses should also be used in the positions which correspond 
to the positions of the "ficticious" X-ray tubes 19' and 20'.