Note: Descriptions are shown in the official language in which they were submitted.
~5~S47
PHD 79 119
The invention relates to a device for producing
` images of a layer of a three-dimensional object, with radi-
; ation sources arranged in a radiation source plane, for
irradiating the object with beams of penetrating rays from
S different directions, and with several planar recording
layers, arranged parallel to one another, for the purpose
of recording all shadow images thereby generated.
In the German Patent application DE-OS 25 14 988
by U. Tiemens and which was published on October 10, 1976,
10 there has already been shown, using a multiple radiation
source containing X-ray tubes, how to record coded images
on several successive films, the coded images consisting in
each case of shadow images superimposed on one another to
more or lesser degrees. From these coded images it is pos-
15 sible, in a later step, to reconstruct images of layers ofthe ob~ect (see German patent applications DE-OS 27 19 386
and 27 46 035 by H. Weiss et al which were published on
April 5, 1979 and April 19, 1979, respectively).
The images of layers of an object (DE-OS 2514 988)
20 are obtained in principle from shadow images, but the latter
are not present in an isolated but in an overlapped form,
then because of this overlap additional artefacts are trans-
ferred into the reccnstructed image in the process of layer
representation and thus impair the picture quality. The
25 influence of the artefacts becomes greater with increasing
degree of overlap. The artefacts can in fact be removed;
but the method requires multichannel decoding devices for
decoding the overlapping shadow images recorded in each of
the respective planes. By means of separate shadow images
30 in each recording plane, however, i~ is possible to avoid
the above artefacts and thus considerably improve the image
quality. In the case of separate images, however, the
object size is severely limited unless film formats that
are too big are used. Using a beam geometry that is tech-
35 nically feasible (approximately 25 tubes, focus-object
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Pl~ 79 119 2 1~-9-198()
distance FOD = 1200 mm (object-film distance OFA = 500 mm))
and a commercial film format of 40 x 40 cm2 it is only
possible therefore to record objec-ts with a diameter of
approxima-tely 50 mm, separately. In objects of this size,
however, it is very difficult for the doctor to make an
accurate diagnosis because, with this size, orientation
towards adjacent, known and larger object details (for
example vessels filled with contrast medium), which can be
an important aid to a doctor making his diagnosis, is usu-
lO ally impossible because this information is not present inthe images or lies at an unfavourable spot, for example
at the edge of the image. I-t is desirable, therefore~ that
an image first be prepared of a relatively large object
area, which will enable the doctor to orientate himself. An
15 additional exposure, however, means a greater radiation
load on the patient. The patient has also to be subjected
to more contrast medium if this is being used for exposure
purposes. The longer examination period -that results is
also, of course, a disadvantage for both patient and doctor.
It is the objec-t of the present invention, there-
fore, to create a device for producing images of a layer of
an object whereby it is possible to prepare both shadow
images of large objec-t areas in the case of relatively
marked overlap of the shadow images and simultaneously
25 shadow images, which hardly or not overlap, which repre-
sent only a part of said large object area. According to
the invention the device is charac-terized in that in fron-t
of each recording layer is placed a diaphragm arrangement
allocated to it for masking of the beams of rays, -the
30 diaphragm apertures o~ each diaphragm arrangement being so
much the smaller the greater the distance between the
diaphragm arrangement and the radiation sources.
In this way, the shadow images produced with the
multiple radiation source are recorded in a set of films
35 (recording ~ayers~ stacked one above the other in which
film by film, the object area and the associated overlap
of the shadow images become lesser and lesser.
This means tha-t, when a relatively large object
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PHD 79 119 3 16~9-1980
area is :irradia-ted, it is possible to record shadow images
of an object on a first film, these shadow images over-
lapping one another to a large extent but from which it
'; is possible to make an initial diagnosis of the patient.
' 5 At the same time, without the patient or the object having
to be irradiated again, further shadow images -of a smaller
area of the object are produced which shadow images do not
overlap one another or only to a slight extent. From these
shadow images it is then possible to reconstruct artefact-
'' lO free (small) images of a layer of the object. It is im-
portant here that the medically relevant object detail
should still be present in the masked (thus small) object
' area.
Fig. 1 represents schematically an embodiment of
the invention.
It shows a multiple radiation source 1 with three
X-ray tubes 2, 3 and 4. The object 5 is mas~ed with a
diaphragm arrangement 6, for example a diaphragm plate. The
X-ray beams 2a, 3a, 4a can be restricted in such a way
20 that an area of the `object which is i~radiated, which is
as large as possible~ or that the ~ilm formats used are
fully exposed. The diaphragm 6 can be provided with dia-
phragm apertures of different or the same size. Diaphragm
apertures of a different size can be used, for example,
25 if the shadow images are to be exposed with little overlap
and distributed as uniformly as possible over a film. It
is also, of course, possi'ble to arrange underneath the
object, that is to say 'between objec-t and first recording
layer, a further diaphragm for masking the primary beam of
30 rays (not shown).
The shadow images 7a are recorded on a film 8 at
an object-film-distànce OFA1. With this distance and with
this masked object area -the individual shadow images over-
lap considerably. The result of this is reduced image
35 quality o~ the reconstructed image of a layer of the object.
' At the distances OFA2 and OFA3, for example, further films
9 and 10 are simultaneously arranged on which the more
strongly masked shadow images 7b and 7c are recorded wi-th
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PI-ID 79 119 4 16-9-1980
the aid of diaphragms (for example lead diaphragms) 11 and
12 arranged between them. For the sake of clarity the full
paths of the beams are shown only for radiation source 4.
The diaphragm apertures of the diaphragm arrangements 11
and 12 become smaller with increasing object-film distance.
In this example, therefore, the perspective images 7c are
recorded separately at the distance OFA3. The masking can
be done, for example, centrally to the primary beams of
rays 2a, 3a, 4a, but also in any other form. Furthermore,
10 the masking may also be different for the individual
shadow images~ the purpose being always to obtain li-ttle
or no overlap of the shadow images. Without the diaphragm
arrangement 11 and 12 the path of the beam for X-ray source
4 would be as shown with the dashed lines 15.
In this example the object-film distance OFA1 is
selected in such a way that the film format of film 8 is
utilized to the optimum, i.e. right to the edge. Films 8, 9
and 10 are combined, in a kno~in manner, on both sides with
intensifying foils; for the sake of clarity, however, these
20 have not been shown. By suitable adaptation of the inten-
sifier foils (intensification factor) it is possible to
ensure that the varying irnage inforlnation (degree of over-
lap, object-film distance~ is recorded on the d:ifferen-t
filrns 8, 9 and 10 with appro~imately the same photographic
25 densi-ty. The intensification factor is on a r:ising scale
from 8 to 10. In principle, the adap-ta-tion can a:Lso be
achieved by using X-ray films of different sensitivity.
When such a set of films is used, the dose of radiation to `
which -the patient is subjected increases only insubstan-
3U tia]ly.
Filrns 7, 8 and 9 are housed, preferably, in a
cornmon film cassette (simul-taneous cassette) or are simul-
taneously moved into their respective position by means of
a film-changing device. In addition, the individual dia-
35 phragm arrangements 11 or 12 have a thickness decreasingtowards the edge of -the diaphragm apertures, so that the
edges of the shadow images will face out. From tihe shadow
images it is possible~ using known decoding methods (DE-AS
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PHD 79 119 5 16-9-1980
~'
~; 27 19 386 and 27 46 035), to obtain images of layers of the
three-dimensional object by means of scale variation and
correlation. All the shadow images can be evaluated one
after the other with the same decoding device. Rough object
orientation is then possible with the layer images recon-
structed from the considerably overlapping shadow images
7a, whereas detailed and accurate diagnoses can be made
with the layer images reconstructed from shadow images 7b
or 7c that have little or no overlap at all. It is assumed,
10 of course, that the desired object detail was also recorded
in the set of shadow images present. By means of so-called
"survey" exposures, however, that are often made in radio-
logy, it is possible to obtain a rough object orientatlon
for the exposure of the set of shadow images with the
5 multiple radiation source.
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