Medical radiographic apparatus

Abstract

In a computer-assisted tomography (CAT) apparatus, in which a fan-shaped X-ray distribution is rotated about the patients body, it is possible to use a ring of stationary detectors disposed to receive the radiation after passage through the body. In this arrangement the ring of detectors is of smaller radius than the locus of motion of the radiation source. It would appear that this would cause the detectors to obscure the body from the radiation. An arrangement disclosed moves out of the radiation beam those detectors which would present an obstruction so that only those required to collect radiation lie in the beam.

Description

There is shown in schematic form in FIG. 1 a scanning arrangement which is an alternative to the fixed detector systems discussed hereinbefore. This arrangement performs a different scanning motion but provides absorption data in a form similar to that of the fixed detector systems so that the processing can be substantially similar to that which may be used with those systems.

The apparatus includes an X-ray tube 1 which is a source of a fan-shaped distribution 2 of X-rays. The fan-shaped distribution is, in this example, substantially co-planar with the plane of the Figure. Tube 1 is orbited about an axis 3, perpendicular to said plane, which intersects an aperture in which the body 4 of a patient may be situated for examination. The locus of rotation of the point of origin of the X-rays is shown by the chain dotted circle 5.

Disposed at a constant radius from axis 3 there is a ring 6 of detector devices of which a few are indicated at 7. These detectors may be scintillator crystals cooperating with photomultipliers or, as in this example, with photodiodes.

Although in this example the detectors extend over a complete 360.degree. ring, it is not necessary that this be so. It is, however, desirable that they extend over at least 180.degree..

The X-ray fan 2 shown in FIG. 1 is of 60.degree. extent and irradiates slightly less than half of the total number of detectors. In the course of rotation the fan 2 irradiates progressively different detectors so that for a source at 1a the fan 2a of radiation irradiates detectors over the span between the long dashed arrows. Thus between 1 and 1a the radiation leaves .[.detector.]. .Iadd.detectors .Iaddend.9 and irradiates instead detectors 10.

In this example irradiation continues over a full 360.degree. rotation of source 1. At a subsequent source position 1b the fan 2b of radiation irradiates detectors between the short dashed arrows. Clearly some of these detectors would intercept the radiation of earlier source positions, such as 1a, if the detector ring and locus 5 were coplanar. It is to avoid this problem that other arrangements situate the source locus inside the radius of the detector ring 6.

The present invention places the source locus outside that ring, thus reducing the extent of detector ring and associated collimators required. However it ensures that any detectors which would otherwise pass between the source and the patient's body are displaced so as to allow the radiation 2 to pass unobstructed. Many different arrangements can be adopted to ensure that the detectors only intercept the radiation as required. The preferred arrangement of this example uses a detector ring which does not have any angular motion relative to the body about axis 3 but which is subject to a nutation to obtain the desired relationship.

It should be noted that not only does the possible reduced size of the detector ring and collimators allow reduced size and complexity for those and other parts of the apparatus, for example the scanning gantry, but also it allows more efficient use of the x-ray photons and consequent reduced power and cooling requirements for the X-ray tube. This latter improvement results from a reduced source to image distance between source and detectors obtained by putting the detector ring at smaller radius than a source which is substantially as close to the patient as is normally acceptable.

The arrangement can be more clearly seen from FIG. 2 which is a side elevation of FIG. 1. At position 1 the source, which is moving out of the page as shown by the arrow head, projects the radiation along 2 to intercept the body, not shown, and, for a beam in the plane of the paper, a detector 11. The 180.degree. opposing detector 12 is displaced to the right to allow the radiation to pass. As the source is orbited, the detector ring is subjected to the nutating motion indicated by the horizontal arrows. This ensures that, when the source is at the 180.degree. opposing position 1c, the radiation at 2c, still in the same plane, intercepts detector 12 at 12c but is not obstructed by detector 11 at 11c.

Clearly the nutating motion and offset position of the detector ring 6 means that the detectors lie on an apparently curved path as viewed from the source 1. To avoid problems from this the detectors should be of sufficient width to intercept the entire thickness of the fan of radiation over its entire extent.

FIG. 3 shows the effect of this, as viewed from the source at 1a. In this Figure the line of incidence of the radiation is shown at 13. It can be seen that the radiation if fully intercepted by detectors across the entire fan, including the newly irradiated detectors 10. However detectors 9, which would not fully intercept the fan, are not irradiated. This arrangement does mean that the region of the detector which is irradiated varies between the center and opposite edges. For this reason it is preferable to use detectors which have a uniform response over the entire receiving surface or to predetermine corrections for differences in sensitivity.

FIG. 1 also shows that the radiation is of sufficient extent for some detectors to receive radiation which has not passed through the patient's body 4. This means that, in the course of the orbit, all of the detectors will receive some radiation which has not passed through body 4 and which, therefore, provides a calibration signal for correcting differences between the detector sensitivities. It will also be apparent that irradiation need not be confined to one source provided the detector arrangement does not allow any detectors to block the radiation between a source and the body 4.

The practical arrangement for implementing this example of the invention is shown in FIGS. 4 and 5, which respectively show front and side elevations of the entire apparatus, and subsequent FIGS. 6-8. FIGS. 4-8 show the basic equipment required to implement the invention but omit many details familiar to CAT machines and which will be well known to those skilled in the art.

The apparatus includes a main frame 14 together with an outer cover 15. These may be mounted directly on the floor but in the example shown are mounted on a rocker 16 which cooperates with rollers 17 and is driven, via gear teeth 18, by motor 19 to provide a tilting facility of axis 3.

Frame 14 and cover 15 have an aperture 20, to which axis 3 is central, to admit the patient, not shown.

The X-ray tube 1 is supported on a member 21 which is rotatable about axis 3 on bearings 22. The member 21 is driven, via a belt 23 and gearbox 24, by a motor 25 mounted on the main frame. Power supplies and cooling oil to the X-ray tube 1 are carried by cables 26, which are of sufficient extent and have a suitable cabling handling arrangement to allow the tube to revolve through a working 360.degree. plus, say, 180.degree. to come to working angular velocity and, say, 180.degree. to stop.

The detector ring 6 is mounted to encircle aperture 20, in a fixed angular relation with main frame 14 but capable of the nutation described hereinbefore. This arrangement can be seen in more detail in FIG. 6, which shows only the relevant features.

As can be seen from FIG. 6, member 21, which is not shown in full, includes a ring 27 having two bearings, in the form of ball bearings 28 and 29. Bearing 28 is disposed parallel to the plane of the radiation and is an additional bearing between member 21 and main frame 14. Bearing 29 is, however, inclined to that plane, to bring its axis to 3', and supports the detector ring 6.

The position of bearing 28 is determined by fixed member 14 and the relative positions of bearings 28 and 29 is determined by their relative positions, as shown, on ring 27. Thus in operation, as ring 27 is rotated with member 21, bearings 29 cause the ring 6 and individual detectors, such as 11 and 12, to execute the nutating motions shown.

Each detector is connected by a looped wire 30 to a terminal 31 mounted on main frame 14. From the terminals 31, cables supply the detector outputs to processing circuits not shown.

It will be apparent that the rotation of member 21 and ring 27 will tend to cause detector ring 6 to rotate although it would be restrained by the wires 30. It is not, however desired that ring 6 has any rotational motion and for this reason it is also connected to main frame 14 by a series of anti-rotation webs 32 which are not flexible in the direction of rotation but are flexible perpendicular to that direction. The anti-rotation webs, one of which can be seen in the detail drawing of FIG. 7, allow the desired nutating motion of ring 6 but prevent rotation. Suitable material for these webs is spring steel strip.

Further details of the ball bearing 29 can be seen in FIG. 8.

Bearing 29 positions ring 6 so that the detectors fully intercept the planar fan 2, of radiation, over the full extent, as shown in FIG. 1 and 2, but do not obscure the radiation prior to the patient's body. To ensure that the radiation is sufficiently confined to a plane to allow this, a fan shaped collimator 33 is provided at the tube 1. Further collimators, not shown, may be provided at the detectors to exclude radiation scattered in the patient's body. These collimators must, however, admit radiation over the full range of the source motion for which a detector is irradiated. This may be arranged by having collimators moving with the source 1.

It is desirable to monitor the progress of the source scan so as to properly organise the data provided by the detectors. This may be provided by a transparent ring carrying graticule markings, mounted on member 21, and a stationary light source and photocell detecting passage of the markings. However the progressive irradiation of different detectors by the advancing radiation indicates scan progress directly.

As mentioned hereinbefore the data provided by the detectors may be processed in known manner to provide the desired representation of absorption of the radiation. This may be as described in U.S. Pat. No. 3,778,614 or by the convolution process described and claimed in U.S. Pat. No. 3,924,129. The processing is indicated in FIG. 9. The data from the detectors are taken as they are derived to amplifiers 34. An individual amplifier is, in principle, required for each detector. However in practice, because the detectors are not all irradiated at the same time, some multiplexing of outputs is possible, with consequent saving of equipment. The data signals are then integrated in integrators for a period which represents one beam of radiation as received by that detector, taking into account the source motion in that time. The required timing signals are provided by the source position indicators as described hereinbefore. The data signals are then subject to analogue to digital conversion is converters 36 and are converted into logarithmic form in converters 37 to provide signals suitable for reconstruction processing in circuits 38.

As mentioned hereinbefore circuits 38 may process the data as described in U.S. Pat. No. 3,778,614 so as described in U.S. Pat. No. 3,924,129. In the latter case the convolution processing may be of a form requiring data for sets of parallel beams of radiation in which case the data must be presorted into the correct sequence.

Alternatively the convolution processing used may be appropriate to data for sets of beams in a fan-shaped distribution so that no presorting is needed. Of course any other processing suitable for CAT equipment may be used as desired.

The processed data are finally displayed on suitable equipment such as a T.V. monitor or line printer or stored for future use, on equipment indicated generally at 39.

It has been mentioned that other arrangements may be adopted to provide the desired detector motion. For example the detector ring may be mounted on a fixed ring by a series of resilient members. These members can be arranged so that they urge the detector ring into the plane of the radiation but so that any part of the detector ring can be pushed out of the radiation by deforming the resilient members. The source can then be provided with a cam arrangement so that, as it progresses, it pushes out of the radiation that part of the detector ring having detectors which would obscure the body. The resilient members will tend to restore to position that part of the ring opposing the source.

FIG. 10 shows in simplified form an arrangement in which the detector ring 6 is mounted on a fixed ring 40 by resilient members, some of which are shown at 41, in the form of springs in flexible cases to limit outward travel. The springs are arranged to be still in compression when their expansion is so limited; at which point the detectors lie in the path of the radiation.

The detector ring 6 is formed with a peripheral cam ring 42 which engages with a cam follower 43 fixed to X-ray tube 1. As tube 1 rotates about axis 3 the cam follower 43 engages ring 42 are forces the ring 6 out of the X-ray beam against the action of local springs 41.

Other alternative arrangements falling within the scope of the invention may readily be devised.

Claims

1. Medical radiographic apparatus, for investigating a cross-sectional slice of a patient's body, including at least one x-ray tube and means for supplying electrical energy to the at least one x-ray tube, locating means for locating the at least one x-ray tube in relation to the patient's body so that a fan-shaped distribution of x-radiation is directed towards said slice, scanning means for rotating said at least one x-ray tube around the patient's body to direct the radiation towards said slice from a plurality of different directions, detector means comprising a plurality of detector devices, sensitive to said radiation, disposed along an arcuate path surrounding, or partly surrounding, the patient's body, the detector devices being substantially immobile in the direction of rotation of said at least one-x-ray tube, means locating said detector devices such that said arcuate path is closer to the patient's body than the at least one x-ray tube throughout the rotational movement of said at least one x-ray tube, and means for moving detector devices for the time being disposed at the same side of the patient's body as the at least one x-ray tube so that they do not interrupt the detection of said radiation by detector devices for the time being disposed at the opposite side of the patient's body to said at least one x-ray tube.

2. Apparatus according to claim 1 in which the detector devices are distributed over 360.degree. about the patient's body.

3. Apparatus according to claim 1 including a plurality of anti-rotation means, which are compliant in a direction perpendicular to the direction of rotation of said at least one x-ray tube but not compliant in a direction tangential thereto, connecting the detector ring member with stationary parts of the apparatus to maintain the detector devices substantially immobile in said direction of rotation.

4. Apparatus according to claim 3 in which the antirotation means comprise flexible metal strips.

5. Apparatus according to claim 1 including a detector ring member, on which the detector devices are mounted to intercept the radiation after passage through the patient's body, and in which the means for moving detector devices is arranged to move said detector ring member.

6. Apparatus according to claim 5 including a source ring member, disposed with its axis coincident with the axis of rotation of the at least one x-ray tube and linked to said at least one x-ray tube to rotate therewith and in which the means for moving detector devices comprise a guiding means mounted thereon to maintain the detector ring member with its axis inclined to the axis of rotation of the at least one x-ray tube.

7. Apparatus according to claim 5 including resilient means disposed to urge the detector ring member into the path of the x-radiation and in which the means for moving detector devices comprise a cam member, linked to the at least one x-ray tube, arranged to deflect the detector ring member against said resilient means.

8. Apparatus according to claim 7 in which the resilient means comprise a plurality of spring means.

9. A medical diagnostic x-ray machine for producing an x-ray picture of the anatomy of a slice which extends along a planar section through the patient including:

at least one x-ray tube producing a fan-shaped distribution of x-radiation directed at the patient and passing through the slice and means for supplying electrical energy to said at least one x-ray tube;

means for orbiting the at least one x-ray tube about the patient to cause the origin of the radiation to move along a source orbit which surrounds the slice and is coplanar with the slice;

detectors of x-radiation which face the slice and are arranged in a detector ring which surrounds the slice and is within the source orbit; and

means for nutating the detector ring in synchronism with the orbiting of the at least one x-ray tube, and therefore with the motion of the origin along the source orbit, to prevent interception of the x-radiation from the origin by detectors which are between the at least one x-ray tube and the slice, but to cause interception of the x-radiation from the origin by detectors facing the side of the slice which is away from the at least one x-ray tube.

10. A medical diagnostic x-ray machine as in claim 9 including a collimator secured relative to the x-ray tube to orbit, together with the x-ray tube, about the patient, said collimator facilitating the containment of said x-radiation in a fan-shaped distribution, said collimator being outside the detector ring.

11. Medical radiographic apparatus for investigating a cross-sectional slice of a patient's body, the apparatus including: at least one x-ray tube generating a fan-shaped distribution of x-radiation and means for supplying electrical energy to said at least one x-ray tube; locating means for locating the at least one x-ray tube, in relation to the patient's body, so that the said radiation is directed towards said slice; scanning means for orbiting said at least one x-ray tube about the patient's body so as to direct x-radiation towards said slice from a plurality of different directions; detector means comprising a detector ring member, of radius less than the radius of the orbit of the at least one tube, and a plurality of detector devices, sensitive to the radiation, mounted on the detector ring member to intercept the radiation after passage through the slice; deflecting means, linked to said at least one x-ray tube, arranged to deflect the detector ring member during the orbit of the at least one x-ray tube to prevent interruption of the radiation thereby prior to passage through the slice, and means arranged to prevent motion of said detector ring member in the direction of orbit of said at least one x-ray tube.

12. Medical radiographic apparatus for investigating a cross-sectional slice of a patient's body, the apparatus including: at least one x-ray tube generating a substantially planar fan-shaped distribution of x-radiation and means for supplying electrical energy to the at least one x-ray tube; locating means for locating the x-ray tube in relation to the patient's body so that the x-radiation is directed towards said slice; scanning means for orbiting the at least one x-ray tube about an axis perpendicular to the slice so as to direct the x-radiation towards the slice from a plurality of directions; detector means comprising a plurality of detector devices disposed around the patient's body in a detector ring, of radius less than the orbital radius of the at least one tube, about an axis inclined to the orbital axis of the at least one x-ray tube; deflecting means, linked to the x-ray tube, for nutating the axis of the detector ring about said orbital axis to cause the detector devices to intercept the radiation only after it has passed through the body; and means for substantially preventing rotation of the detector devices about the nutating axis.

13. A medical diagnostic x-ray machine for producing an x-ray picture of a slice which extends along a section through a patient comprising:

an x-ray tube and means for supplying electrical energy to the x-ray tube, and an x-ray collimator secured to the x-ray tube, said x-ray tube and x-ray collimator together producing a fan-shaped distribution of x-radiation which is directed at the patient and passes through the slice.

means for rotating both the x-ray tube and the x-ray collimator, as a unit, around the slice to thereby cause the fan-shaped distribution of x-radiation to pass through the slice along a number of different means directions,

an array of detectors of x-radiation which surrounds the slice, with the individual detectors facing the slice, said array being within the orbits inscribed by the x-ray tube and x-ray collimator in rotating around the slice, and

means for nutating the detector array in synchronism with the rotation of the x-ray tube and the x-ray collimator around the patient to prevent interception of the x-radiation by detectors which are between the x-ray tube and the slice but to cause interception of the x-radiation by detectors facing the side of the slice which is away from the x-ray tube..Iadd.

14. An X-ray diagnostic apparatus for producing transverse layer images of a radiography subject with an X-ray measuring arrangement comprising an X-ray source which produces a fan-shaped X-ray beam penetrating the radiography subject, the cross-sectional extent of the beam, perpendicular to the layer plane, being equal to the layer thickness and, in the layer plane, being of such a magnitude that the entire layer is penetrated with radiation, and comprising also a radiation receiver which detects the intensity of radiation emanating from the layer, said radiation receiver being constructed as a circular ring into which the radiography subject may be inserted and comprising a series of detectors, with means for changing the direction of the axis of symmetry of the X-ray beam to effect a scanning operation, and with a measured value converter for the transformation of the signals supplied by the radiation receiver during a scanning operation into a layer image, characterized in that the X-ray source is arranged outside the radiation receiver, in that the radiation receiver has swiveling mounting means, and guide means acting on said radiation receiver for swiveling that particular part of the circular ring forming the radiation receiver, which is required to detect the X-radiation issuing from the radiography subject, into the X-ray beam..Iaddend..Iadd.15. An X-ray diagnostic apparatus for producing transverse layer images of a radiography subject with an X-ray measuring arrangement comprising an X-ray source which produces a fan-shaped X-ray beam penetrating the radiography subject, the cross-sectional extent of the beam, perpendicular to the layer plane, being equal to the layer thickness and, in the layer plane, being of such a magnitude that the entire layer is penetrated with radiation, and comprising also a radiation receiver which detects the intensity of radiation emanating from the layer, said radiation receiver being constructed as a circular ring into which the radiography subject may be inserted and comprising a series of detectors, with means for shifting the instantaneous origin point of the X-ray beam and for simultaneously changing the direction of the axis of symmetry of the X-ray beam to effect a scanning operation, and with a measured value converter for the transformation of the signals supplied by the radiation receiver during a scanning operation into a layer image, characterized in that the X-ray source provides an instantaneous origin point for the X-ray beam for each direction of the axis of symmetry thereof which is outside of the circular ring forming said radiation receiver, the radiation receiver having mounting means accommodating shifting movement of the circular ring forming said radiation receiver such that for each direction of the axis of symmetry of the X-ray beam the part of the circular ring which is adjacent the instantaneous origin point for such X-ray beam is laterally offset from such X-ray beam, and guide means for positioning said circular ring forming said radiation receiver such that for each instantaneous origin point the adjacent part of the circular ring is laterally offset from the X-ray beam while the part of the circular ring required to detect X-radiation issuing from the radiography subject is disposed in the path of the X-ray beam from such instantaneous origin point..Iaddend..Iadd.16. X-ray diagnostic apparatus for the examination of a transverse layer of a radiography subject, comprising

(a) X-ray source means for producing respective fan-shaped X-ray beams penetrating the radiography subject from respective different directions, with origin points of the respective fan-shaped beams being equidistant from a central axis, the cross-sectional extent of the beams, perpendicular to the layer plane, being equal to the layer thickness, and the lateral extent of the beams in the layer plane corresponding to the lateral extent of the layer to be examined so that the entire layer is penetrated with radiation upon production of each beam,

(b) patient support means for locating a radiography subject such that the fan-shaped X-ray beams are directed from the X-ray source means toward the layer to be examined from the respective different directions, and such that the central axis intersects the layer to be examined,

(c) scanning means for effecting irradiation of the layer to be examined from respective different directions to produce respective scans thereof and operable for effecting a rotational displacement of the direction from which the layer is scanned by sequentially producing fan-shaped X-ray beams from the X-ray source means which are directed toward the layer from successive angularly offset positions about said central axis, and operable during a scanning operation for producing a sufficient number of scans of the layer to be examined so as to enable production of an image thereof,

(d) detector means comprising a series of detectors, sensitive to said radiation, disposed at successive positions about the patient support means and arranged such that radiation of each of the fan-shaped beams will impinge on a plurality of successive detectors of said series without requiring rotational movement of said detectors in the direction of rotational displacement of said scanning means during a scanning operation of said scanning means,

(e) said detectors of said series being substantially immobile in the direction of rotational displacement of the scanning means throughout a scanning operation of said scanning means,

(f) means for locating said detectors so that the distance of each detector from said central axis is less than the distance of the origin points of the fan-shaped beams from the central axis during a scanning operation of said scanning means, and

(g) means for moving potentially beam obstructing detectors of said series for the time being disposed at the same side of the central axis as an origin point of a fan-shaped beam to be produced so that such detectors do not obstruct the detection of radiation from such origin point by detectors of said series disposed on the side of the central axis opposite to that of such origin point..Iaddend..Iadd.17. X-ray diagnostic apparatus according to claim 16 with said detector means comprising a circular radiation receiver having said series of detectors disposed at successive positions therealong, said means for moving potentially beam obstructing detectors of said series being arranged for moving said

circular radiation receiver as a unit..Iaddend..Iadd.18. X-ray diagnostic apparatus according to claim 17, with said means for moving comprising a ring rotatable about said central axis during a scanning operation and coupled with said radiation receiver to maintain the axis of the radiation receiver inclined to said central axis such that potentially beam obstructing detectors of said series are offset from each sequentially produced fan-shaped X-ray beam..Iaddend..Iadd.19. X-ray diagnostic apparatus for the examination of a transverse layer of a radiography subject, comprising

(a) X-ray source means for producing respective fan-shaped X-ray beams penetrating the radiography subject from respective different directions, with origin points of the respective fan-shaped beams being equidistant from a central axis, the cross-sectional extent of the beams, perpendicular to the layer plane, being equal to the layer thickness, and the lateral extent of the beams in the layer plane corresponding to the lateral extent of the layer to be examined so that the entire layer is penetrated with radiation upon production of each beam,

(b) patient support means for locating a radiography subject such that the fan-shaped X-ray beams are directed from the X-ray source means toward the layer to be examined from the respective different directions, and such that the central axis intersects the layer to be examined,

(c) scanning means for effecting irradiation of the layer to be examined from respective different directions to produce respective scans thereof and operable for effecting a rotational displacement of the direction from which the layer is scanned by sequentially producing fan-shaped X-ray beams from the X-ray source means which are directed toward the layer from successive angularly offset positions about said central axis, and operable during a scanning operation for producing a sufficient number of scans of the layer to be examined so as to enable production of an image thereof,

(d) detector means comprising a series of detectors, sensitive to said radiation, disposed at successive positions so as to completely encircle the patient support means and arranged such that radiation of each of the fan-shaped beams will impinge on a plurality of successive detectors of said series without requiring rotational movement of said detectors in the direction of rotational displacement of the scanning means during a scanning operation of said scanning means,

(e) said detectors of said series being substantially immobile in the direction of rotational displacement of the scanning means throughout a scanning operation of said scanning means,

(f) means for locating said detectors so that the distance of each detector from said central axis is less than the distance of the origin points of the fan-shaped beams from the central axis during a scanning operation of said scanning means, said detectors of said series including with respect to each origin point, an adjacent series of detectors located on the same side of the central axis as such origin point, and a remote series of detectors located on the opposite side of the central axis from such origin point, and

(g) means constructed so as to laterally offset, with respect to each origin point, the adjacent series of detectors relative to the fan-shaped X-ray beam therefrom, so that the fan-shaped beam from each origin point impinges on the radiography subject without obstruction by such adjacent series of detectors, and so as to position the remote series of detectors with respect to each origin point so that the fan-shaped beam from each origin point impinges on such remote series of detectors..Iaddend..Iadd.20. An X-ray diagnostic apparatus for producing transverse layer images of a radiography subject with an X-ray measuring arrangement comprising an X-ray source which produces a fan-shaped beam penetrating the radiography subject, the cross-sectional extent of the beam, perpendicular to the layer plane, being equal to the layer thickness and, in the layer plane, being of such a magnitude that the entire layer is penetrated with radiation, and comprising also a radiation receiver which detects the intensity of radiation emanating from the layer, said radiation receiver being constructed as a circular ring into which the radiography subject may be inserted and comprising a series of detectors, with means for changing the direction of the axis of symmetry of the X-ray beam to effect a scanning operation, and with a measured value converter for the transformation of the signals supplied by the radiation receiver during a scanning operation into a layer image, characterized in that the X-ray source is arranged outside the radiation receiver, in that the radiation receiver has mounting means permitting the radiation receiver to incline in any direction, and guide means acting on said radiation receiver for swiveling that particular part of the circular ring forming the radiation receiver, which is required to detect the X-radiation issuing from the radiography subject, into the X-ray beam..Iaddend.