Tomograph for producing transverse layer images

In an illustrated embodiment, the fan shaped beam of penetrating radiation has a central ray directed offcenter relative to a row of detectors e.g. by a distance corresponding to one-fourth the detector separation. In this way, for each projection with a given central ray angle, further points can be derived by interpolation using the measurements from other projections. With a given number of detector elements, the number of points per projection can be doubled in this way.

BACKGROUND OF THE INVENTION 
The invention relates to a tomographic apparatus for producing transverse 
layer images of a radiography subject, with a patient couch, with a 
radiation measuring arrangement comprising a radiation source producing a 
fan-shaped beam of rays which penetrates the radiography subject and whose 
cross sectional extent perpendicular to the layer plane is equal to the 
layer thickness and the beam transverse extent being of such a magnitude 
in the layer plane that the whole radiography subject is penetrated, and a 
radiation receiver consisting of a row of detector elements, which detects 
the radiation intensity behind the subject, with a rotating device for the 
measuring arrangement and with a computer for the transformation of the 
signals supplied by the radiation receiver into a tomographic layer image. 
A tomographic apparatus of this kind is described for example in the German 
Offenlengungsschrift 25 53 187. In this type of tomographic apparatus the 
scanning of a patient is carried out by rotating the measuring arrangement 
through an angle of, for example, 360.degree.. During this rotation the 
radiation receiver can be periodically interrogated at equal intervals of 
time. The number of measured values per scanning process is therefore 
given by the number of the interrogation processes of the radiation 
receiver; i.e., by the number of the projections and the number of 
detector elements in the radiation receiver. 
In order to achieve a good image quality it is necessary, on the one hand, 
to select a sufficiently high number of projections, but on the other hand 
also to provide a sufficiently high number of detector elements in the 
radiation receiver. It is not possible however for the number of detector 
elements in the radiation receiver to be increased indefinitely. 
SUMMARY OF THE INVENTION 
The underlying object of the invention is to produce a tomographic 
apparatus of the initially cited type with which, with a relatively low 
number of detector elements in the radiation receiver, a relatively large 
amount of data is available for the image calculation per projection. 
This object is achieved according to the invention in that the radiation 
receiver is arranged so that its axis of symmetry and the axis of symmetry 
of the x-ray beam are offset at a distance from each other in the layer 
plane. In the tomographic apparatus according to the invention, it is 
possible to calculate for a specific projection, by means of 
interpolation, data for the image calculation which originate from other 
projections. In this way, without increasing the number of detector 
elements, a substantial improvement of the image is achieved in comparison 
to the case where the axis of symmetry of the radiation receiver coincides 
with the axis of symmetry of the x-ray beam. A particularly advantageous 
development consists in selecting the distance of offset so that it 
corresponds to a quarter of the detector element separation. 
The invention is hereafter described in more detail with reference to an 
exemplary embodiment represented in the accompanying sheets of drawings; 
and other objects, features and advantages will be apparent from this 
detailed disclosure and from the appended claims.

DETAILED DESCRIPTION 
The tomographic apparatus shown in FIG. 1 has a radiation measuring 
arrangement which consists of an x-ray tube 1 and a radiation receiver 2. 
By means of a schematically represented rotational drive device 3, the 
radiation measuring arrangement 1, 2 can be rotated about a longitudinal 
axis 4a. The x-ray tube 1 emits a fan-shaped x-ray beam 4 which totally 
penetrates in a transverse layer a patient 6 lying on a couch 5. It can be 
seen from the side view in FIG. 1B that the cross-section of the x-ray 
beam 4 perpendicular to the penetrated layer is equal to the layer 
thickness. The x-ray tube 1 is supplied by an x-ray generator 7 with high 
voltage. The output signals of the radiation receiver 2 are processed by a 
measured value converter 8 which calculates therefrom an image in the form 
of a matrix of image point data. This image is reproduced on a display 
unit 9. The radiation receiver 2 consists of a row of detector elements. 
The number of detector elements is selected in accordance with the desired 
image definition and is over 100 on the order of magnitude. 
In order to produce a transverse layer image, the measuring arrangement 1, 
2 is rotated by means of the rotary drive device 3 through 360.degree. 
around the patient 6. At predetermined positions, e.g. at each degree of 
angle, the output signals of the detector elements of the radiation 
receiver 2 are thereby transmitted to the computer of measured value 
converter 8. 
In order to illustrate the fundamental mode of operation, FIG. 2 represents 
the focus 10 of the x-ray tube 1 and the fan-shaped x-ray beam 4. In order 
to illustrate the principle, it is sufficient if, according to FIG. 2, 
only four detector elements 11, 12, 13, 14 are shown by way of example in 
the radiation receiver 2. A collimator element 15 to 18 lies before each 
detector element 11 to 14. 
FIG. 3 again shows the focus 10 of the x-ray tube 1 and the central 
radiation path or axis 24 associated with four detector elements having a 
central axis 23. Thus four measured value points 19 to 22 are represented 
corresponding to the four detector elements taken as a basis. If it is 
conceived that a perpendicular line is drawn from the coordinate origin Z 
representing the center of rotation of the central ray of the x-ray beam 
and that the measured value is plotted at the foot of the perpendicular, 
then the four measured values lie on a circle T which is drawn through 
origin Z and the position of the focus 10 which is under consideration. 
FIG. 4 shows the representation according to FIG. 3 for three different 
positions of the focus 10; namely, the positions 10a to 10c, thus three 
different projections. In accordance with this, three different positions 
are obtained for the measured value points, designated by 19a to 22a, 19b 
to 22b and 19c to 22c lying on the respective circles Ta, Tb, and Tc. It 
can be seen from FIG. 4 that all the measured value points lie on two 
measured value circles K1 and K2 and are distributed unequally over the 
plane shown (the socalled Radon plane) which corresponds to the layer 
plane of the irradiated object. This unequal distribution is one of the 
reasons for artifacts in the image calculated by the computer of converter 
8 in the case of the known x-ray tomographic apparatus. 
According to FIG. 2 in the known x-ray tomographic apparatus the axis of 
symmetry (designated 23) of the radiation receiver 2 coincides with the 
axis of symmetry (designated 24) of the x-ray beam 4. Because of this fact 
there results the distribution represented in FIG. 4, of the measured 
value points 19a to 22c on two measured value circles K1 and K2. 
FIG. 5 (on sheet one of the drawings) now shows a measuring arrangement in 
the case of which the radiation receiver 2 is disposed so that its axis of 
symmetry 23a and the axis of symmetry 24a of the x-ray beam 4 are offset 
at a distance from each other in the layer plane (and as measured at the 
detector) which corresponds to a quarter of the center to center detector 
element distance a. FIG. 6 again represents the distribution of the 
measured value points in the Radon plane. The measured value points 25a to 
28a are obtained for the focus position 10a, the measured value points 25b 
to 28b for the focus position 10b and the measured value points 25c to 28c 
for the focus position 10c. It can be seen from FIG. 6 that in the case of 
the shown mutual displacement of the two axes of symmetry 23a and 24a, all 
the measured value points are on four measured value circles 29 to 32. 
FIG. 7 now shows that by increasing the projections a calculation of 
intermediate data is possible. In addition to the already mentioned 
projections 10a to 10c with the measured value points 25a to 28c there are 
also three projections 10d to 10f and corresponding measured value points 
25d to 28f which also lie on the measured value circles 29 to 32. 
FIG. 7 shows that for the projection 10e, for example, the data of a data 
point 33 can be calculated; namely, from measured value data which is 
derived not from this projection but from other projections; and 
specifically by carrying out interpolation from the measured values of the 
measured value points 25a and 25d. Measured value data is therefore 
utilized from the projections 10a and 10d in order to calculate, by 
interpolation, the data of the data point 33 for the projection 10e. In 
the same way, the data of a data point lying between the data 26e and 27e 
can be calculated by interpolation for the projection 10e; namely, from 
the data of the measured value points 26a and 26d. This data point is 
designated by 34. Furthermore, it is possible to asscertain the data of a 
data point 35 for the projection 10e from the data of the measured value 
points 27a and 27f. Finally, it is also possible, for the projection 10e, 
to calculate from the data of the measured value points 28a and 28f the 
data of a data point 36. If the data of the projection 10e is examined, it 
can be seen that, through the corresponding focus, eight data points are 
coordinated thereto on the circle, of which four are derived from true 
measured values and four have been obtained by interpolation from measured 
values which are derived from other projections. In a similar way an 
intermediate value calculation by interpolation is also possible for other 
projections. By doubling the number of projections during a rotation of 
the measuring arrangement 1, 2, a doubling of the number of data points 
per projection is achieved when a displacement of the axes of symmetry 23a 
and 24a according to FIG. 5 is effected. This doubling of the number of 
data points corresponds to a doubling of the number of detector elements 
in the radiation receiver 2. Thus the possibility for a substantially more 
exact image calculation is created. 
In order to illustrate the conception behind the invention, four detectors 
only are employed in the radiation receiver 2 in connection with the FIGS. 
2 and 5. In practice, however, even in the case of the teaching of the 
invention; i.e. in the case of a mutual displacement of the axes of 
symmetry 23a and 24a according to FIG. 5, the number of detector elements 
in the radiation receiver 2 is over 100 on the order to magnitude. The 
detector elements may be semiconductor detectors, for example. 
In FIG. 5, the measuring system may comprise a rotary frame indicated at 37 
having a center of rotation or axis 38. The rotary frame mounts source 10 
and the associated source collimator so that the central ray of the fan 
shaped beam 4 coincides with the axis of symmetry 24a of the beam. The 
rotary frame mounts the detector array 2 as diagrammatically illustrated 
in FIG. 5 so that the central ray at 24a intersects the central detector 
element 12 at a distance equal to one-fourth the detector separation a 
from the center of the detector array represented by its intersection with 
the axis of symmetry 23a. 
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.