Tomographic apparatus for the production of transverse layer images

In an exemplary embodiment, a radiation measuring arrangement has two x-ray tubes of different radiation energy which generate radiation beams penetrating the radiography subject, and also has a radiation receiver which determines the radiation intensity behind the subject. The radiography subject is irradiated from different directions. A computer determines, for every image point, the attenuation coefficient for every radiation energy and determines therefrom the mean ordinal number and the density. The radiation receiver is designed as a stationary detector ring. For scanning only the x-ray tubes are rotated about the radiography subject. They are maintained at such a distance from one another that no detector element is simultaneously impinged upon by both radiation beams.

BACKGROUND OF THE INVENTION 
The invention relates to a tomograph for the production of transverse layer 
images of a radiography subject, comprising an x-ray measuring arrangement 
which contains two x-ray sources of different radiation energy which 
generate radiation beams penetrating the radiography subject, the 
cross-sectional extent of said radiation beams perpendicular to the layer 
plane being substantially equal to the layer thickness, and also 
containing a radiation receiver which determines the radiation intensity 
behind the subject, as well as comprising a rotating device for 
irradiation of the radiography subject from different directions, and 
comprising a computer for the determination of the attenuation 
coefficients of every image point in an image point-matrix disposed in the 
examined layer, wherein the computer is so designed that it determines, 
for every image point, the attenuation coefficient for every radiation 
energy, and determines therefrom the median ordinate value and the 
density. 
A computer tomograph of this type is known in which, for scanning of the 
subject, there is present, for every radiation source, one radiation 
receiver which is fixedly connected with the respective radiation source 
(U.S. Pat. No. 3,971,948). For scanning of the subject, there alternately 
takes place a common lateral displacement of both radiation sources and 
both radiation receivers and a rotation of these structural units. In the 
case of the known tomograph, it is disadvantageous that the technical 
outlay for the scanning of the radiography subject is very great. 
SUMMARY OF THE INVENTION 
The object underlying the invention resides in designing a tomographic 
apparatus of the type initially cited such that the outlay is reduced in 
relation to the state of the art. 
In accordance with the invention, this object is achieved in that the 
radiation receiver, as a stationary ring, is comprised of a plurality of 
detector elements, and that, for scanning, only the radiation sources are 
rotated about the radiography subject, and that the radiation sources are 
maintained at such a distance from one another that no detector element is 
simultaneously impinged upon by both radiation beams. In the case of the 
inventive tomograph, for scanning the radiography subject, only the two 
radiation sources are rotated about the radiography subject. The radiation 
receiver, in the form of a detector ring, is stationary. Every radiation 
source can thus emit a fan-shaped radiation beam whose marginal rays form 
the marginal rays of the measuring field, so that a lateral displacement 
for the purpose of scanning the entire layer of which an image is to be 
formed is not necessary. 
The invention shall be explained in greater detail in the following on the 
basis of an exemplary embodiment illustrated in the accompanying drawing 
sheet; and other objects, features and advantages will be apparent from 
this detailed disclosure and from the appended claims.

DETAILED DESCRIPTION 
In FIG. 1, two x-ray tubes 1 and 2 are illustrated which are fed by an 
x-ray high voltage generator 3. The patient 4 is encompassed by a 
radiation receiver 5 which is comprised of a plurality of detector 
elements for x-radiation. The radiation receiver 5 receives 
simultaneously, respectively, two ray fans 7 and 8 traversing (or 
penetrating) the measuring field 6 and hence traversing a cross-section of 
the patient 4, the marginal rays 7a, 7b; 8a, 8b of said ray fans 7 and 8 
forming the marginal rays of the measuring field 6. The thickness of the 
ray fans 7 and 8, perpendicularly to the layer plane, is equal to the 
layer thickness. Every detector element of the radiation receiver 5 is 
connected to a computer 9 which calculates an image of the irradiated 
cross-section of the patient 4 from the output signals of the detector 
elements of the radiation receiver 5 obtained during rotation of the x-ray 
tubes 1 and 2 about the patient 4; namely, about an axis 4a. The computer 
9 effects the reproduction of said image on a display unit 10. The image 
formation (or production) proceeds in that the computer determines the 
attenuation coefficients of image points in an image point-matrix disposed 
in the examined layer of the radiography subject 5. 
The x-ray tubes 1 and 2 are supplied with different high voltages and 
therefore deliver different radiation energies. The computer 9 calculates 
the attenuation coefficient for every radiation energy and every image 
point, so that there are present, for every image point, two attenuation 
coefficients from which the median ordinate value of the material in the 
image point and the density can be calculated. The entire scanning is 
terminated when both x-ray tubes 1, 2 have rotated through 360.degree. 
about the patient 4. 
The radiation receiver 5, which is designed as a detector ring, is 
stationary. By means of a rod 11, the x-ray tubes 1, 2 are maintained at a 
fixed distance from one another which is so selected that no detector 
element of the radiation receiver is simultaneously struck (or impinged 
upon) by the two ray fans 7, 8 or by parts thereof. It is therefore 
possible, with only one detector system, to simultaneously pick up 
(detect) two data-sets--required for the reconstruction of two 
images--with two different effective energies. The two data sets are, 
indeed, obtained with one scan operation; however, always two different 
regions of the radiation receiver 5 are irradiated by the two ray fans 7, 
8, so that every detector element is not simultaneously, but successively, 
struck (or impinged upon) by both ray fans 7, 8. 
The chronological sequence of the measured values recorded by a detector 
element is illustrated in an example by FIG. 2. One obtains two 
chronologically separate signals which can be separated prior to the image 
reconstruction. For example for counterclockwise rotation of sources 1, 2, 
a given detector might generate successive outputs for successive 
rotational positions one angular degree apart. After a succession of 
outputs as indicated at 7-1 for successive angular positions of the 
rotating sources (due to radiation from source 1), there might be a time 
interval representing several angular degrees of rotation where neither 
source was directed toward the detector, after which outputs as indicated 
at 8-1 would result for successive angular positions of the rotating 
sources (due to radiation from source 2). 
It will be apparent that many modifications and variations may be effected 
without departing from the scope of the novel concepts and teachings of 
the present invention.