Method and apparatus for scanning X-ray tomography

I claim:

1. In a method of producing a tomographic image of a subject that includes the steps of generating X-rays at a moving origin point by directing a charged particle beam to a target surface, deflecting said charged particle beam to travel said origin point through a predetermined raster scan at said surface, detecting variations of X-ray intensity during the course of said raster scan at a plurality of spaced apart detection points situated at the opposite side of said subject from said origin point, generating a first sequence of data values that is indicative of variations of X-ray intensity at a first of said detection points at successive times during the course of said raster scan and generating at least a second sequence of data values that is indicative of variations of X-ray intensity at a second of said detection points at successive times during the course of the same raster scan, the improvement comprising:

combining successive individual data values of said first sequence that are generated by X-rays from successive particular locations in said raster scan with at least individual data values of said second sequence that are generated by X-rays from predetermined successive different locations in the same raster scan in order to produce a composite sequence of data values, and

producing an image corresponding to at least a portion of said raster scan which depicts variations of the magnitude of successive data values of said composite sequence.

2. The method of claim 1 including the step of combining each data value of said first sequence with the data value of said second sequence that is generated a predetermined constant time interval later in the course of the same raster scan.

3. The method of claim 1 including the step of combining successive data values of said first sequence that are generated by X-rays from successive particular locations along said raster scan with successive data values of said second sequence that are generated by X-rays from another series of locations in a preselected fixed direction by a preselected fixed distance.

4. The method of claim 3 including the step of preselecting said fixed distance in order to produce a tomographic image of a specific selected plane within said subject.

5. The method of claim 1 wherein said X-ray origin point is traveled along a plurality of parallel raster scan lines, including the step of producing said sequence of composite data values by combining each first sequence data value that is generated by X-rays from a location along a particular scan line with at least the second sequence data value that is generated by X-rays from the corresponding location along another scan line.

6. The method of claim 1 wherein said X-ray origin point is traveled along a plurality of parallel raster scan lines, including the step of producing said sequence of composite data values by combining each first sequence data value that is generated by X-rays from a location along a particular scan line with at least the second sequence data value that is generated by X-rays from another location along the same scan line situated a fixed distance away along the scan line.

7. The method of claim 1 including the further steps of generating additional sequences of data values each of which is indicative of variations of X-ray intensity at a separate additional one of said detection points during the course of said raster scan, and producing said composite sequence of data values by combining individual data values of each of said additional sequences with said individual data values of said first and second sequences, the individual data values of said first and second and additional sequences that are combined to produce each composite data value being ones that are generated by X-rays at different locations in said raster scan which are selected to provide a tomographic image of a specific selected plane within said subject.

8. The method of claim 1 including the further steps of producing a first non-tomographic image of said raster scan which displays variations of the magnitude of data values of said first sequence during the course of said raster scan, producing at least a second non-tomographic image of said raster scan which displays variations of the magnitude of data values of said second sequence during the course of said raster scan, and producing said tomographic image by combining said non-tomographic images with one thereof being positionally shifted relative to the other in order to superimpose successive data values of said first sequence with successive data values of said second sequence that originated at different times in the course of the raster scan.

9. The method of claim 8 wherein said combining of said non-tomographic images includes preparing photographic negatives of each of said non-tomographic images and printing each of said negatives onto a single area of photographic print paper or the like.

10. The method of claim 1 wherein said composite sequence of data values is descriptive of a first planar area within said subject including the further steps of producing a plurality of additional composite sequences of data values each of which is descriptive of a separate one of a series of additional spaced planar areas within said subject that are parallel to said first planar area, and combining portions of each of said composite sequences of data values to produce a constructed composite sequence of data values that is descriptive of a differently oriented area within said subject.

11. The method of claim 1 including digitized and storing each value of said first sequence of data values together with an address which identifies the location of said X-ray origin point in said raster scan at the time that the value was generated, digitizing and storing each successive value of said second sequence of data values together with an address which identifies a location in said raster scan that is spaced a predetermined distance from the location of said origin point at the time that the value was generated, and producing said composite sequence of data values by adding each of a series of data values of said first sequence and the data value of said second sequence which has the same storage address.

12. The method of claim 1 wherein said image is produced by traveling a light origin point in a raster pattern corresponding to said raster scan at said target surface of said X-ray source and utilizing said composite sequence of data values to modulate a characteristic of the light emitted from said light origin point during the course of said raster pattern.

13. A method of producing a tomographic image of an area within a subject comprising the steps of:

traveling an X-ray origin point through a raster scan which includes a plurality of parallel scan lines,

detecting X-ray intensity during the course of said raster scan at a plurality of spaced apart detection points situated at the opposite side of said subject from said X-ray origin point and producing a plurality of signal sequences each of which defines a successive series of values that differ from each other in accordance with variations of X-ray intensity at an associated one of said detection points at successive times during the course of the same raster scan,

combining signal values from each of said sequences that originate at predetermined different times in the course of said raster scan to produce a sequence of tomographic image signal values, and

producing said tomographic image by traveling a light origin point through a raster scan similar to said raster scan of said X-ray origin point while modulating a characteristic of the light emitted at said light origin point in accordance with said sequence of tomographic image signal values.

14. In tomographic imaging apparatus having an X-ray source wherein a charged particle beam is directed to a target surface to generate X-rays at an X-ray origin point at said surface and which has beam deflection means for moving said origin point through a raster scan at said surface, a plurality of X-ray detectors which are spaced apart from said source and positioned to detect X-rays at separate detection points that are spaced apart from each other, said detectors including a first detector which transmits a first sequence of data values that is indicative of variations of X-ray intensity at a first of said detection points at successive times during the course of said raster scan and at least a second detector which transmits a second sequence of data values that is indicative of variations of X-ray intensity at a second of said detection points at successive times during the course of the same raster scan, the improvement comprising:

means for combining individual data values from said first detector that are generated by X-rays from successive particular locations in said raster scan with individual data values of at least said second sequence that are generated by X-rays from predetermined successive different locations in the same raster scan in order to produce a composite sequence of data values, and

means for displaying an image corresponding to at least a portion of said raster scan which depicts variations of the magnitude of successive data values of said composite sequence.

15. Apparatus of claim 14 wherein said means for combining individual data values combines each of a series of data values from said first sequence with the data values of said second sequence that was generated a predetermined constant time interval later in the course of said raster scan.

16. The apparatus of claim 14 further including:

first data storage means for storing data values of said first sequence in an arrangement which identifies each value with the location of said X-ray origin point at the time the was generated,

second data storage means for storing data values of said second sequence in an arrangement which identifies each value with a location that is spaced a predetermined distance from the location of said X-ray origin point at the time the value was generated, and

wherein said means for combining individual data values adds each of a series of stored data values from said first data storage means to the stored data value from said second data storage means that is identified with the same location.

17. The apparatus of claim 14 wherein said detectors include still additional detectors which transmit additional sequences of data values each indicative of variations of X-ray intensity at a different one of said detection points during the course of said raster scan, further including means for combining an additional data value from each of said additional sequences with the combined data values of said first and second sequences, said additional data values being values which were originated by X-rays from predetermined different locations in said raster scan.

18. The apparatus of claim 14 further including:

first and second digital data storages of the form which store a plurality of digital values at individual successive addresses,

a first analog to digital signal converter connected between said first detector and said first digital data storage and a second analog to digital signal converter connected between said second detector and said second digital data storage,

means for changing the storage addresses of data values in at least one of said digital data storages,

wherein said means for combining data values includes a third digital data sotrage and a digital adder having input buses connected to said first and second digital data storages and having an output bus connected to said third digital data storage.

19. The apparatus of claim 18 wherein said means for displaying an image includes a display device of the form in which a light origin point is traveled through a raster scan and which has means for modulating a characteristic of the light emitted from said light origin point and further includes a digital to analog signal converter connected between said third digital data storage and said means for modulating a characteristic of the light.

20. The apparatus of claim 18 further including address generating means for generating a series of successive address location signals during the course of said raster scan at said X-ray source, a second digital adder having first and second data inputs and having a data output connected to said one of said digital data storages, said first data input of said second digital adder being coupled to said address generating means, and a switch register of the form which stores and transmits a predetermined digital value, said switch register being connected to said second data input of said second digital adder.

21. The apparatus of claim 18 wherein said means for changing storage addresses enables selective displacing of addresses in either of said first and second digital data storages in either direction along two orthogonal coordinate axes.

22. The apparatus of claim 14 wherein said detectors include a plurality of detectors in addition to said first and second detectors each of which transmits an additional sequence of data values that is indicative of variations of X-ray intensity at a separate one of said detection points at successive times during the course of said raster scan, further including:

a plurality of data storages of the form which store successive data values at successive address locations,

means for transmitting each of said first, second and additional sequences of data values to a separate one of said data storages, and

wherein said means for combining data values shifts the addresses of data values in at least all but one of said data storages and then adds the data values stored at the same address in all of said data storages to produce said composite sequence of data values.

23. The apparatus of claim 14 further including means for storing a plurality of said composite sequences of data values each of which is descriptive of a separate one of a series of spaced parallel planar areas within said subject, and means for joining portions of successive ones of said plurality of composite sequences to produce an altered composite sequence that is descriptive of a differently oriented area.

24. The apparatus of claim 14 wherein said raster scan includes a plurality of parallel scan lines and wherein said means for combining individual data values produces successive data increments of said composite sequence by summing successive data values of at least said first and second sequences that originate from corresponding locations along two spaced apart ones of said raster scan lines.

25. The apparatus of claim 24 further including means for generating a plurality of said sequences of composite data values each of which is descriptive of a separate one of a series of parallel spaced apart planes within said subject, data storage means for storing the successive lines of data of each of said sequences of composite data values, and readout means for sequentially transmitting a line of data from each of said plurality of sequences to said display means to produce a tomographic image of an area that is angled relative to said parallel spaced apart planes.

26. The apparatus of claim 14 wherein said means for combining individual data values includes a digital computer having said particular locations and said predetermined different locations stored therein.

27. Apparatus for producing a tomographic image of a predetermined plane within a subject comprising:

an X-ray source having means for traveling an X-ray origin point through a raster scan which includes a plurality of sequential parallel scan lines,

a plurality of X-ray detectors spaced apart from said source and being positioned to detect X-rays at detection points which are spaced apart in a direction parallel to the plane of said raster scan, each of said detectors having means for generating a signal during the course of each raster scan line in accordance with variations of the detected X-ray intensity at successive times during the course of the raster scan line,

means for digitizing said detector signals to produce a plurality of primary sequences of data values,

means for adding individual data values from each of said primary sequences that originate at predetermined different times during the course of said raster scan to produce a sequence of composite data values,

a display device having means for displaying an image corresponding to at least a portion of said raster scan by traveling a light origin point along a plurality of sequential parallel scan lines, and

means for modulating said light origin point during said travel thereof along said parallel scan lines in accordance with variations of the successive data values of said composite sequence of data values.

Description Submit all comments and votes

TECHNICAL FIELD

This invention relates to the production of X-ray images and more particularly to tomographic methods and apparatus for generating sectional radiographic images of the interior of a subject.

BACKGROUND OF THE INVENTION

X-ray images produced by non-tomographic techniques are often difficult to interpret and may fail to provide needed information about a medical patient or an inanimate object that is being examined for structural flaws. Data originating from a specific internal region of particular interest may be obscured by overlapping or superimposed imaging of other regions that are forward from or behind the region of interest.

The more recently developed tomographic X-ray imaging techniques are not subject to the above discussed disadvantage. Computer aided tomography can generate a cross sectional depiction of a single plane that is essentially free of data arising from other planes within the subject. Variations of X-ray absorbency between different areas of the imaged plane are made apparent without ambiguity as to location and with much greater clarity than is usually realizable with older techniques.

Most prior tomographic X-ray installations require a bulky, elaborate and costly mechanical scanning system. Installations of this kind have an X-ray source which directs a narrow X-ray beam to a detector at the opposite side of the subject. The source and detector are jointly translated relative to the subject, or the subject itself may be translated, so that the X-ray beam cuts across a plane within the subject that is to be imaged. A single translation of this kind cannot provide a meaningful tomographic or sectional image. The location of points within the plane where a change of X-ray absorbency was detected can be determined with respect to one coordinate but not with respect to the orthogonal coordinate. Consequently it is necessary to turn the source and detector angularly relative to the subject and repeat the translation. The location of the points in both coordinates than becomes determinable by data processing operations comparable to triangulation.

As a practical matter it is usually necessary, in such installations, to perform a large number of translations of the source and detector alternated with a large number of angular repositionings of such components in order to generate an image of desirable resolution and clarity. The mechanisms which enable the source and detector or the subject to be traveled through this repetive combination of linear and angular motions accounts for a considerable part of the bulk, complexity and cost of such installations. The mechanical positioning and scanning structure becomes even more complex if sectional images of more than one plane or of oblique planes are to be generated from a single scanning sequence.

Disadvantages of scanning X-ray installations of the above discussed kind are not limited to size, complexity and cost. An undesirably long period of time is required to perform the mechanical scanning operations. This limits productivity and prolongs the radiation exposure of the subject. The effects of scattered X-rays decrease resolution in a tomographic image and long exposure times aggravate such image degradation.

The problems discussed above are alleviated to a considerable extent by another form of tomographic X-ray scanning system described in my prior U.S. Pat. No. 4,144,457. In the method and apparatus described in that prior patent, the X-ray source has an electron beam which is electrostatically or magnetically deflected to establish a moving X-ray origin point at a broad target plate. Thus the translation portion of the scanning operation is accomplished electronically without necessarily requiring physical movement of the source and detector or the subject for those portions of the scanning operation. Angular motion of the source and detector or the subject continues to be necessary between electronic translations but the mechanism for the purpose can be relatively compact and simple as only simple rotational motion is needed.

The apparatus of my above identified prior patent can be mechanically simpler, more compact and less costly than the wholly mechanical scanning systems which have been hereinbefore discussed. Electronic scanning can be conducted more rapidly than mechanical scanning thereby increasing productivity, decreasing radiation exposure of the subject and with a reduction of image degradation from scattered X-rays.

The above discussed advantages of electronic scanning would become even more pronounced if it were possible to generate a tomographic image without necessarily relying on any relative movement of the source and detector or the subject. Heretofore it has appeared that it is not possible to extract the data that is needed for generating a tomographic iamge in the absence of physical repositionings of the source and detector or subject during the course of scanning operations.

The present invention is directed to overcoming one or more of the problems discussed above.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a method of producing a tomographic image of a subject that includes the steps of generating X-rays at a moving origin point by directing a charged particle beam to a target surface, deflecting the charged particle beam to travel the origin point through a predetermined raster scan at the surface, detecting X-ray intensity during the course of the raster scan at a plurality of spaced apart detection points situated at the opposite side of the subject from the origin point, generating a first sequence of data values that is indicative of variations of X-ray intensity at a first detection point at successive times during the course of the raster scan and generating at least a second sequence of data values that is indicative of variations of X-ray intensity at a second detection point at successive times during the course of the same successive raster scan. Individual data values of the first sequence that are generated by X-rays from successive particular locations in the raster scan are combined with at least individual data values of the second sequence that are generated by X-rays from predetermined successive different locations in the same raster scan in order to produce a composite sequence of data values. An image corresponding to at least a portion of the raster scan is produced which depicts variations of the magitude of successive data values of the composite sequence.

In another aspect, the invention provides tomographic imaging apparatus having an X-ray source in which a charged particle beam is directed to a target surface to generate X-rays at an origin point at the surface and which has beam deflection means for moving the origin point through a raster scan at the surface. The apparatus includes a pluraliuty of X-ray detectors which are spaced apart from the X-ray source and positioned to detect X-rays at separate detection points. The detectors include a first detector that transmits a first sequence of data values indicative of variations of X-ray intensity at a first of the detection points at successive times during the course of the raster scan and at least a second detector which transmits a second sequence of data values that is indicative of variations of X-ray intensity at a second of the detection points at successive times during the course of the same raster scan. The apparatus further includes means for combining individual data values from the first detector that are generated by X-rays from successive particular locations in the raster scan with at least individual data values of the second sequence that are generated by X-rays from predetermined successive different locations in the same raster scan in order to produce a composite sequence of data values. The apparatus still further includes means for displaying an image corresponding to at least a portion of the raster scan which depicts variations of the magnitude of successive data values of the composite sequence.

The invention does not inherently require motion of mechanical components of the X-ray source and detectors or movement of the subject itself in order to generate tomographic image data although some repositioning of components relative to the subject may be desirable for other reasons in some operations. Data needed for constructing a tomographic image can be obtained more rapidly than is possible with mechanical scanning systems which rely wholly or in part on actual movement of the X-ray source and detectors relative to the subject. Consequently, more scanning operations can be accomplished in a given time, radiation exposure of the subject may be reduced and less image degradation from scattered X-rays is present. The invention enables tomographic imaging of planes that are essentially perpendicular to the direction of X-ray travel through the subject and, in some embodiments, enables such imaging of planar or non-planar sections having other orientations. Radiology facilities embodying the invention may be compact and substantially less costly than older tomographic imaging installations in which scanning is accomplished in whole or in part by motion of heavy components.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts tomographic X-ray imaging apparatus in accordance with a first embodiment of the invention, certain components being shown in perspective and certain other components being shown in schematic circuit form.

FIG. 2 is a diagram illustrating steps which may be used to produce a tomographic image on film during the practice of some embodiments of the invention.

FIG. 3 is a view of a tomographic image on film as produced by the steps shown in FIG. 2.

FIG. 4 is a more diagrammatic view of certain components of the apparatus of FIG. 1 that facilitates understanding of the principles of operation.

FIG. 5 is a diagram of selected X-ray paths in the apparatus of the preceding figures which further facilitates understanding of the principles of operation.

FIG. 6 is a chart showing image offsets used in systems having a plurality of X-ray detectors.

FIG. 7 is a perspective view of additional elements which may be used to reduce radiation exposure of the of the subject and to reduce scattered X-ray effects.

FIG. 8 is a digital circuit diagram depicting one form of data processing system which may be used to generate tomographic images from the output of X-ray detectors that receive X-rays transmitted through a subject as shown in FIG. 1.

FIG. 9 is a digital circuit diagram illustrating additional components which may be combined with the circuit of FIG. 8 when a larger number of X-ray detectors are used in the scanning operations.

FIG. 10 is a digital circuit diagram illustrating still additional components which may be combined with the circuit of FIG. 8 to obtain tomographic images of planes or curved sections having different orientations than the images which are produced in the absence of the additional components.

FIG. 11 is a circuit diagram showing another digital data processing system for producing tomographic image data from the output of the X-ray detectors.

FIG. 12 is a diagram depicting data storage conditions in frame buffers of the system of FIG. 11.

FIG. 13 is a computer program flow chart applicable to the data processing system of FIG. 11.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring initially to FIG. 1 of the drawings, the primary components of tomographic imaging apparatus 11 in accordance with a first embodiment of the invention include an X-ray source 12 and a plurality of X-ray detectors 13 which perform the scanning operations and data processing means 14 for generating image data from the detector output signals.

X-ray source 12 may be of the known form, as described in my prior U.S. Pat. No. 3,949,229 for example, having an electron gun 16 which directs an electron beam 17 to a broad target plate 18 to establish an X-ray origin point 19 at the target plate. A deflection yoke 21, which may be of either the magnetic or electrostatic type, provides for deflection of electron beam 17 to sweep the X-ray origin point 19 through a raster scan 22 at target plate 18. Raster scan 22 in this example is of the rectilinear type in which the origin point 19 is traveled in the X coordinate direction along a series of spaced apart substantially parallel scan lines 23a to 23g although it is possible to use other raster scan patterns. To produce the raster scan 22, an X-sweep frequency generator 24 applies a repetitive ramp signal voltage to the X terminal 26 of yoke 21 while a Y-sweep frequency generator 27 applies a lower frequency ramp signal voltage to the Y terminal 28 of the yoke.

For clarity of illustration and to provide for an explanation of the principles of operation which will be hereinafter set forth, the raster scan 22 is shown in FIG. 1 and in subsequent figures with fewer scan lines 23 than are typically present and with greater scan line spacing than is normally the case. Raster scan parameters of the kind found in commercial cathode ray tube devices, such as oscilloscopes or television picture tubes are usually preferably but are not essential in all cases depending on the degree of resolution which is needed in the tomographic images. In general, resolution is increased by increasing the number of scan lines 23 per unit area of the target plate 18.

Two detectors 13a and 13b are used in this particular embodiment of the invention although it is advantageous to use a larger number in many cases as will hereinafter be discussed. The detectors 13 are of the scintillation and photomuliplier tube type in this example but may alternately be of any of the other known forms of X-ray detector that produce electrical output signal voltages in response to detection of X-rays at a small point-like X-ray sensitive region of the detector. The detection points D1 and D2 of detectors 13a and 13b respectively are spaced apart in a direction parallel to the plane of the raster scan 22, the detection points being spaced apart in the Y coordinate direction of the raster scan in this example although the spacing may also be in the X coordinate direction in some cases. Detection points D1 and D2 are equidistant from the plane of raster scan 22 and preferably equidistant from the center of the raster scan although compensating adjustments can be made in the data processing operations in circumstances where these preferred geometrical relationships are not met.

The output signals from detectors 13a and 13b may be a series of distinct voltage pulses each indicative of an individual X-ray in instances where the rate of detection of X-rays at points D1 and D2 is low. More commonly such output signals are continuous voltages which vary in magnitude as the rate of detection of X-rays at points D1 and D2 varies during the course of a scanning operation. In either case, the output voltages in effect provide sequences of data values indicative of variations of X-ray flux intensity which occur at detection points D1 and D2 during the course of a raster scan. Production of a tomographic or sectional image of a selected plane 31a within a subject 32 then includes the step of combining successive data values produced by one of the detectors 13 with successive data values that are produced by the other detector a predetermiend interval later during the course of the raster scan to produce a sequence of composite data values. An image corresponding to at least a portion of the raster scan 22 is then generated in which successive points along the raster scan exhibit variations corresponding to variations between successive data values of the composite sequence. Such an image is a tomographic depiction of the selected plane 31a as a result of effects which will be hereinafter described.

Data processing means 14 for performing the above described steps may take a variety of forms including both digital and analog systems. An analog system will be described initially as the principles of operation of the invention are more easily understood when considered in that context.

In particular, the data processing means 14 of FIG. 1 includes first and second display devices 33a and 33b, such as cathode ray tube oscilloscopes for example, of the type that generate visible images at screens 34 in response to horizontal or X-axis sweep frequency signals, vertical or Y-axis sweep frequency signals and Z-axis or intensity signals. Display devices 33a, 33b may also be television receiver sets if the input signals are processes through a video scan converter.

The first display device 33a receives the same X and Y sweep frequency signals, from generators 24 and 27, that are applied to the scanning X-ray source 12. The output signal of the first detector 13a are applied to the Z or intensity signal terminal of first display device 33a through a preamplifier 36a at the detector and a primary amplifier 37a. Thus as the X-ray origin point 19 at source 12 is swept through a raster scan 22, an image corresponding to the raster scan is generated at the screen 34 of first display device 33a in which the brightness of successive points along the imaged raster scan varies in accordance with variations of X-ray intensity at detection point D1 as the X-ray origin point passes along corresponding successive points in the course of the raster scan.

The second display device 33b enables the step of combining successive data values produced by one of the detectors 13a with successive data values produced by the other detector 13b a predetermined interval later during the course of the raster scan 22 as previously described. In particular, the image generated by second display device 33b during the course of the raster scan 22 is shifted a predetemined distance along one coordinate axis, the Y axis in this example, relative to the image being concurrently generated by the first display device 33a. Superimposing the positionally shifted image of second display device 33b with the unshifted image of first display device 33a, as will hereinafter be discussed in more detail, is then in effect a combining of data values of the type discussed above.

In order to shift the image at second display device 33b, the X sweep frequency signal from generator 24 is applied to the second display device in the same direct manner that it is applied to the first display device 33a but the Y sweep frequency signal from generator 27 is modified before being applied to the second display device. In particular, the Y sweep frequency from generator 27 is applied to the second display device 33b through one input of a summing amplifier 38. The other input of the summing amplifier 38 is provided with a selected D.C. voltage from the adjustable tap of a potentiometer 39 which is connected across a direct current voltage source 41.

Summing amplifier 38 adds the selected fixed voltage to the Y sweep frequency voltage causing the position of the image at the screen 34 of second display device 33b to be shifted upward relative to the position of the image at the screen of the first display device 33a. The distance that the image is shifted is a function of the magnitude of the D.C. voltage that is added to the Y sweep frequency voltage and determines the particular plane 31a that will be depicted in the tomographic image.

Output signals from detector 13b are transmitted to the Z or intensity signal terminal of second display device 33b through a second preamplifier 36b and primary amplifier 37b. Consequently, the shifted image exhibits brightness variations indicative of variations of X-ray intensity at a detection point D2 during the course of the raster scan 22 but a given point in the raster scan, such as point C is imaged at a higher location at the screen 34 of display device 33b than it is at the screen of display device 33a.

While the image at one of the display devices 33b is shifted upwardly in this example, tomographic images may also be produced by shifting the image at either display device 33 in either direction relative to the image at the other display device by adding a D.C. voltage of appropriate polarity to the Y sweep frequency signal that is supplied to the device at which the image is to be shifted. While the image at one of the display devices 33 is shifted in the Y coordinate direction in this example, it is also possible to shift the image in the X coodinate direction at either display device by adding a D.C. voltage from a potentiometer 39 to the X sweep frequency voltage at the device at which the image shift is desired although this requires that the detection points D1 and D2 be spaced apart in the X-coordinate direction of the raster scan 22.

The desired tomographic image of the selected plane 31a is produced by superimposing the images at display devices 33a and 33b in register with each other as they appear on the screens 34 of the devices. Optical devices which combine two separate images for direct viewing may be used or, as in this example, a permanent tomographic image may be produced by photographing the images at the two screens 34 at one half of the normal exposure. The camera or cameras 42 are identically positioned at the two screens 34, normally in centered relationship to the screens. To facilitate registering of the two images during the printing operation, indicia marks 43 may be provided in identical positions on the two screens 34.

Referring now to FIG. 2, the developed negatives 44a and 44b obtained from the photographing operation will each have one half of the normal contrast. One of the negatives 44a is then disposed against unexposed print film 46 and is contact printed using a light source 47 and conventional printing techniques. Negative 44a is then removed and the other negative 44b is identically positioned against the print film 46 and is similarly contact printed on the once already exposed film 46. Developing o