GB1564309A

GB1564309A - Apparatus for obtaining an x-ray image of a slice plane ofan object

Info

Publication number: GB1564309A
Application number: GB35644/76A
Authority: GB
Original assignee: Jeol Ltd; Nihon Denshi KK
Current assignee: Jeol Ltd
Priority date: 1975-09-11
Filing date: 1976-08-26
Publication date: 1980-04-10
Also published as: NL7609599A

Description

(54) APPARATUS FOR OBTAINING AN X-RAY IMAGE OF A SLICE PLANE OF AN OBJECT (71) We, NIHON DENSHI KABUSHIKI KAISHA, a Japanese Company, of 1418, Nakagami - cho, Akishima - shi, Tokyo, Japan, do hereby declare the invention, for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following state ment : - This invention relates to an apparatus for observing the three dimensional structure of an object by means of X-ray ir- radiation.

A method for examining an object by X-ray radiation known as computerised tomography was recently developed with a view to obtaining a greater detail of information than hitherto possible with the conventional X-ray projection image. In this method, the X-ray source is orbited about an axis of an object and X-rays from the source irradiate and pass through a cross-sectional slice plane portion of said object. At the same time, the X-rays transmitted through the slice plane are detected at each irradiation path. By so doing, each matrix area on the slice plane is irradiated many times by an X-ray beam in many different directions and the X-ray absorption coefficient at each matrix area is calculated by the computer from the results of said transmitted X-ray detection and the position data of the irradiating X-ray path. The X-ray image of the slice plane is then obtained by displaying the X-ray absorption coefficients of the respective matrix areas two dimensionally.

This method, however, has certain disadvantages in that the apparatus embodying said method is very complicated and cumbersome; moreover, in order to obtain information on one X-ray image in one slice plane, a relatively long measuring time is required.

The present invention seeks to provide an improved apparatus for computerised tomography.

According to one aspect of the invention there is provided an apparatus for obtaining a two dimensional image of the X-ray adsorption distribution on a cross-sectional plane of an object comprising: a) an electron beam source for generat in an electron beam, b) focusing means for focusing said electron beam on a target to produce X-rays, c) a guide plate, complete with a pin hole or pin holes, for directing the X ray micro-beam, resultant upon target impingement, onto the object, d) scanning means for scanning said electron beam over the surface of the target so as to cause the X-ray micro beam correspondingly to scan a cross sectional plane of the object, e) rotating means for rotating source a), means b), plate c) and means d) around the object, f) a detecting means for detecting and measuring the intensity of the X-rays passed through the object, g) a memory for memorising the output of said detecting means f) along with the corresponding output signals from the scanning means d) and the rotating means e), h) a calculating means for calculating the absorption value at each micro matrix area on said cross-sectional plane from the data memorised in the memory g), and i) a display means for displaying said respective calculated absorption values two-dimensionally.

According to another aspect of the invention there is provided an apparatus for obtaining a two-dimensional image of the X-ray absorption distribution on a cross sectional plane of an object comprising: a) an electron beam source for generat ing an electron beam, b) an annular or partially annular target, the central axis of which coincides with the axis of the central beam generated by the electron beam source a), c) a focusing means for focusing said electron beam on the target b) to pro duce Xays, d) a rotating means for rotating said electron beam around the target b), e) a scanning means for scanning said electron beam over said target b), f) a beam guide having a pin hole or pin holes for directing the X-ray micro-beam, resultant upon target im pingement, onto the object, g) a detecting means for detecting and measuring the intensity of the X-ray passed through the object, h) a memory for memorising the output of said detecting means g) along with the corresponding output signals from the rotating means d) and the scan ning means e), i) a calculating means for calculating the absorption value at each micro matrix area on said cross-sectional plane from the data memorised in the memory h), and j) a display means for displaying said respective calculated absorption values two-dimensionally.

According to another aspect of the invention there is provided an apparatus for obtaining a two-dimensional image of the X-ray absorption distribution on a crosssectional plane of an object comprising: a) an electron beam source for generat ing an electron beam, b) an annular or partially annular tar get, the central axis of which coincides with the axis of the central beam generated by the electron beam source a), c) a focusing means for focusing said electron beam on the target b) to pro duce X-rays, d) a rotating means for rotating the electron beam around the target b), e) a scanning means for scanning said electron beam over the target b), f) a stigmator for changing the cross sectional shape of the electron beam in synchronism with the rotating means g) a beam guide having a pin hole or pin holes for directing the X-ray micro-beam resultant upon target im pingement, on the object, h) a detecting means for detecting and measuring the intensity of the X-rays passed through the object, i) a memory for memorising the output of said detecting means h) along with the corresponding output signals from the rotating means d) and the scan ning means e), j) a calculating means for calculating the absorption value at each micro matrix area on said cross-sectional plane from the data memorised in the memory i), and k) a display means for displaying said respective calculated absorption values two dimensionally.

The invention will further be described with reference to the accompanying drawings, of which: Figure 1 is a schematic drawing showing one embodiment of this invention; Figures 2, 3 and 4 are drawings for explaining the embodiment shown in Figure 1; Figure 5 is a schematic drawing showing another embodiment of this invention Figure 6 is a drawing for explaining the embodiment shown in Figure 5; Figure 7 shows the beam guide plate of the embodiment shown in Figures 1 and 5 in detail; Figure 8 is a drawing for explaining the signal processing procedure used in the embodiments of Figures 1 and 5; Figures 9 and 10 are block schematic drawings showing further embodiments of this invention; Figure 11 is a drawing for explaining the embodiment shown in Figure 10 Figure 12 shows the essential part of yet another embodiment according to this invention; and Figure 13 is a drawing for explaining the embodiment shown in Figure 12.

Referring to Figure 1, there is shown the vacuum column 1 of an X-ray gener atingapparatus, said column containing an electron beam source 2 for generating an electron beam 3, a target 4, a condenser lens 5 for focusing the electron beam 3 on said target 4, deflecting means 6X, 7X, 6Y and 7Y, and a beam guide plate 8 having a pin hole P. An X-ray detector 9 is provided which consists essentially of a large scintillator, a photomultiplier and a collimator for preventing the detection of scattered X-rays, said X-ray detector being arranged opposite to the beam guide plate 8 of the vacuum column 1.

In use, an object body 10 is positioned between the beam guide plate 8 and the X-ray detector 9.

The physical arrangement of the apparatus is shown in Figure 2; there being a rotatable supporting member 11 on which the vacuum column 1 and the X-ray detector 9 are mounted, said supporting member 11 serving to rotate the column 1 and the detector 9 around the axis 12 of the object body 10 reclining on a table 14. A driving means 13 is supported on a base 15, the driving means 13 being interlocked with the rotatable supporting member 11 so as to rotate the column 1 and the detector 9. Part of the output of an amplifier 16 (Figure 1) is applied to an image display means 17 via a switching circuit 18 while the remaining part of the output of said amplifier is applied to a memory circuit 19 via an A-D converter 20. The outputs of a scanning signal generator 21 are applied to the deflecting means 6X, 7X, 6Y and 7Y, the memory circuit 19 and the image display means 17 via switching circuit 18. A calculating circuit 22 calculates the X-ray absorption coefficient of each matrix area of the ir- radiated cross-sectional slice plane 23 of the object body 10 from signals read out from the memory circuit 19. The output of the calculating circuit 22 is applied to the image display means 17 via a D A converter 24 and the switching circuit 18. 25 is a computer control circuit for controlling the calculating circuit 22, memory circuit 19, driving means 13, switching circuit 18, and scanning signal generator 21.

When the computer control circuit 25 applies signals to the various circuits in order to obtain a computerised tomograph X-ray image of the slice plane 23 of the object body 10, the scanning signal generator 21 applies signals to deflecting means 6X, 7X, 6Y and 7Y so as to sweep the electron beam over the target 4 in a direction E perpendicular to the line connecting pin hole P and axis 12, as shown in Figure 3, which shows a sectional view of the embodiment of Figure 1. Consequently, the X-ray micro-beam 26 taken off through pin hole P irradiates and scans the entire slice plane 23. The computer control circuit 25 controls the driving means 13 so as to move the vacuum column 1 in steps in an orbit around the axis 12, for example from positions 1' and 9' to positions 1" and 9" after completing an X-ray microbeam scan at each step. In this way, the driving means 13 rotates the vacuum column 1 and the detector 9 several degrees at a time around the axis 12 over a range of more than 100 . During the above rotary sequence, the output signals from the X-ray detector 9 are fed into the memory circuit 19 via the amplifier 16 and the A-D converter 20 together with the output signals from the scanning signal generator 21 and those from the driving means 13. These memorised signals are read out to the calculating circuit 22 and used for calculating the X-ray absorption coefficient of each matrix area on the slice plane 23. The calculated signals are then fed into the image display means 17 via the D-A converter 24 and switching circuit 18 so that an X-ray image of the slice plane 23 of the object body 10 is displayed thereon.

In another mode of operation, when the computer control circuit 25 applies control signals to the various circuits in order to obtain an X-ray projection image, the vacuum column 1 and X-ray detector 9 remain stationary at a certain position and the scanning signal generator 21 generates a scanning signal which scans the electron beam 3 over the target 4 two-dimensionally. Consequently, the X-ray micro-beam 26 taken off through pin hole P of the beam guide plate 8 also irradiates a certain scanning area of the object body 10 two-dimensionally as shown in Figure 4.

At the same time, the transmitted X-rays are detected by the X-ray detector 9. In this case, if the window of the X-ray detector 9 is wide enough (in the direction of axis 12) to receive all the X-rays transmitted through said scanning area of the object body 10, it is not necessary to shift the X-ray detector 9. However, if the window of the detector 9 is not wide enough, it becomes necessary to use a shifting means (not shown) to shift said detector 9 in the direction of the axis 12, for example, from position 9a to position 9b (see Figure 4), depending on the value of the scanning signal generator 21 output signal, said shifting means being attached to the rotatable supporting member 11. The output signal of the detector 9 is applied to the image display means 17 via amplifier 16 and switching circuit 18.

Moreover, since the scanning signal from the scanning signal generator 21 is also applied to the image display means 17 via switching circuit 18, said image display means is synchronised with the X-ray micro-beam scan. Consequently, an X-ray projection image is displayed on the image display means 17, said display image being used for determining the position of the cross-sectional slice plane 23 intended for observation.

Figure 5 shows another embodiment of this invention capable of producing an X-ray image of a cross-sectional slice plane 27 inclined with respect to the crosssectional slice plane 23 perpendicular to the axis 12 of the object 10. This is achieved by incorporating shifting means 28 and 29 which serve to shift the X-ray detector 9 and vacuum column 1 parallel with the axis 12 by controlling said shifting means 28 and 29 through a control means 30 which is in turn controlled by the output signal from the computer control cir cuit 25.

The movements of the above shifting means satisfy the relationship shown in Figure 6 corresponding to the movement of driving means 13 which is also controlled by computer control circuit 25 via a rotation control circuit 31. In Figure 6, the abscissa indicates the amount of shift z, the co-ordinate indicates the rotating angle 56; the solid line d corresponds to the position of the pin hole P and the broken line b' corresponds to the position of the X-ray detector 9. The computer control circuit 25 also controls the scanning signal generator 21 and a deflection signal generating circuit 32 connected to a deflecting means 33 which is used for varying the angle 8 formed by the X-ray micro beam 26 and the line 34 perpendicular to the beam guide plate 8. Namely, the computer control crcuit 25 controls angle g and the position of pin hole P so that the X-ray micro beam 26 always irradiates the inclined cross-sectional slice plane 27 regardless of the rotating angle cF When the computer control circuit 25 applies control signals to the various circuits in order to irradiate the inclined cross-sectional plane as mentioned above, the transmitted X-rays are detected by the X-ray detector 9 and memorised by the memory circuit 19 via the A-D converter 20 together with the output signals from the scanning signal generator 21 and the rotation control circuit 31. The memorised data is then read out by the calculating circuit 22 and the X-ray absorption coefficient of each matrix area on the inclined cross-sectional slice plane 27 is obtained therefrom. After which, the calculated results from the calculating circuit 22 are fed into the image display means 17 via D-A converter 24 in order to display an X-ray image of the inclined cross-sectional slice plane of the object.

Incidentally, even if angle 0 is zero, the electron beam is deflected by the deflecting means 33 away from the axis 12 as shown in Figure 5. The reason for this is to increase the X-ray micro beam intensity by increasing the take-off angle a of the X-ray micro beam from the target 4 and the incident angle P of the electron beam.

As mentioned above, it is possible to dispense with the shifting means 28 by utilising an X-ray detector having a large window. Moreover, if the X-ray detector is equipped with an annular window, it is not necessary to rotate the X-ray detector 9.

Figure 7 shows the beam guide plate 8 of the embodiments according to Figures 1 and 5 in detail. In the figure, a thin film filter 35 is made of aluminium, copper, or any other element with a suitable atomic number, is arranged in the pin hole P of the beam guide plate 8. By so doing, the low energy of the white X-ray component, which- forms part of the X-rays 36a and 36b, is reduced. The reduction effect of the filter is more constant than would be the case if the filter is arranged at 35b. This is because at 35b, the X-ray micro beam would pass through the filter with a greater spread and the X-rays would therefore be subjected to inconsistencies due to the slight variations in the thickness of the filter caused by processing imperfections.

An X-ray image on the inclined slice plane can be obtained without inclining the X-ray micro beam. Figure 8 is a drawing showing one method for obtaining the above X-ray image on the inclined slice plane. In the Figure, 37a, 37b, . . . 37d show slice planes of the object 10, said slice planes, which are perpendicular to the axis of said object 10, being irradiated by the X-ray micro beam in consecutive order. This is carried out with the apparatus shown in Figure 5 by using the shifting means 28 and 29. The X-ray coefficient at each matrix area on each slice plane is calculated as explained in the embodiments shown in Figures 1 and 5, and is once memorised in the memory circuit 19. Moreover, the supposed inclined slice plane 38 is designated by the computer control circuit 25 which reads out the calculated values relating to the cross lines 38a, 38b, . . . 38d which cross said slice planes 37a, 37b . . . 37d and said supposed slice plane 38. After which, the read-out values are applied to the image display means 17 via the D-A converter 24, in order to display an X-ray image of the supposed inclined slice plane 38.

Inicidentally, if an organism, for example particularly a human pulmonary, renal, vascular, or cerebral, organ etc., is measured as an object by means of the above embodiment, the X-ray image of the sectional slice plane will be partially blurred because, at present, the measuring time is longer than the variation periods of the various organs constituting the body.

In order to compensate for this, the embodiment shown in Figure 9 incorporates a monitor 39 which serves to detect said variation periods as electric signals by monitoring the electric currents, sounds or movements generated by a specific part of the body. The output signals of the monitor 39 are memorised by the memory circuit 19 together with the output signals of the scanning signal generator 21, rotation control means 31 and X-ray detector 9. From the memorised data, only information appertaining to a specific condition of the selected organ is supplied to the calculating circuit 22 by the com puter control circuit 25. By so doing, the image display means 17 is able to display still X-ray images of the cross-sectional slice plane at any one of the various conditions of the body by designating the read-out condition in the computed control circuit 25.

Further, if an X-ray image on the slice plane at only one specific body condition is required, e.g. with lungs expanded, it is desirable to supply the output signal of the monitor 39 directly to the computer control circuit 25 as shown by the broken line in Figure 9. By so doing, the computer control circuit 25 operates the Xray generating apparatus and its shifting means according to the result of analysis of the monitoring signals, thereby subjecting the body to only the amount of X-ray irradiation necessary for actual measurement.

Figure 10 shows the embodiment according to this invention which incorporates an annular target 40 located at one end of a funnel-shaped vacuum column 41. The electron beam 3 generated by the electron beam source 2 is focused by the condenser lens 5 and deflected by deflecting means 42 and 43, so that a fine diameter beam irradiates the surface of the annular target 40. The deflecting means 42 and 43 are supplied with signals from the deflecting signal generator 44 so as to change the beam irradiating position continuously or in steps along the periphery of the target.

The deflecting signal generator 44 is in turn controlled by the output signals of the scanning signal generator 21 and the rotating control circuit 31. The beam guide plate 45 is attached to an annular supporting member 46 which in turn is rotated about its centre axis coaxially with the axis 12 of the body 10. The position of the beam guide plate 45 is changed circumferentially by means of a motor 47 which rotates the annular supporting member 46.

Movement of plate 45 corresponds with the rotation of the electron beam 3, because the motor 47 is controlled by the rotation control circuit 31 via a motor control circuit 48. Thus, the X-ray micro-beam 26, which passes through an annular filter 49 and the pin hole of the beam guide plate 45, is directed towards the centre axis 12 at right angles and is then detected by the X-ray detcetor 9 attached to the supporting member 46. Accordingly, the deflecting signal generator 44 and the deflecting means 42 and 43 respectively work as the driving means 13 and the scanning means 6X and 7X shown in Figure 1. Moreover, in this embodiment, the measuring time for obtaining one X-ray image is shorter than that of the aforementioned embodiments, since the driving means 13 for rotating X-ray generating column 1 is not incorporated. 50 and 51 represent a stigmator and stigmator power supply respectively, the latter being controlled by the rotation control circuit 31. The stigmator 50 is used to compensate for distortion of the cross-sectional shape of the electron beam at the target surface caused by the deflecting means 42 and 43.

It is possible to irradiate the annular target 40 by using only a single stage deflecting means and its power supply. However, it is better to incorporate a two or more stage deflecting means, as in the case of the embodiment, shown in Figure 10, as this allows the funnel-shaped portion of the apparatus to be made more commodious for the reclining object body without enlarging the apparatus as a whole.

Further, in the two or more stage deflecting means, it is preferable to use an electrostatic deflecting means for the second or successive stages rather than a magnetic deflecting means as a large current power supply is needed in the case of the magnetic deflecting means in the second and subsequent stages, whereas in the case of the electrostatic deflecting means comprising two ring-shaped coaxial deflecting plates 43a and 43b as shown in Figure 10, the electron beam can be deflected by a comparatively low power voltage supply.

Additionally, with an electrostatic deflecting means, the cross-sectional shape of the electron beam at the target surface can be changed at will as shown in Figure 11. To be more explicit, since the crosssectional shapes 55a, 55b and 55c on the target 40 are elliptical and their longitudinal axes are always orientated radially with respect to the axis 12, a strong intensity X-ray micro-beam having a circular cross-sectional and a comparatively low electron beam intensity per target unit area is taken off through the pin hole 45p. The stigmator 50 and its power supply 51 in Figure 10 provided the means for changing the cross-sectional shape of the electron beam.

Figure 12 shows the essential part of a modified embodiment of Figure 10. In this embodiment in which a fixed annular detector 52 is incorporated, the radiant direction of the X-ray 53 for irradiating the object body 10 is slightly inclined with respect to the plane 54. A detector 52 is suitably located so that the X-ray passed through the object body 10 are detected without being intercepted.

In this embodiment, if the rotating member 46 is rotated at the same angular speed as the angular deflection speed of the electron beam 3, the X-ray micro-beam will pass through only one of the pin holes of the beam guide plate 56. Even if the rotating member 46 is held fixed, the Xray micro-beam is still able to irradiate the object body at various angles. In this case, the measuring time is much shorter than in the case of the previously described embodiments, since in this embodiment, all the mechanical rotating means become superfluous; that is to say, are not used.

The relation between the electron beam deflection pattern on the target and the pin holes 57a etc. is schematically shown in Figure 13 which represents a cross-sectional view of Figure 12 through AA'. In Figure 13, the solid lines 58a, 58b, . . . etc.

on the generally circular arc 59 represent the electron beam irradiating lines (micro areas). Accordingly, the X-ray micro-beam scans the object body 10 at each pin hole; viz., 57a, 57b, . . . etc. However, if for one reason or another, it is impossible to increase the number of pin holes sufficiently, the X-ray data for forming an X-ray image of a cross-sectional plane of the object body will be insufficient. That is unless, for example, the beam guide plate is rotated at a speed lower than and in synchronism with (as a sub-multiple of) the electron beam deflection. By so doing, while the actual number of pin holes remains the same, the effective number increases. Accordingly, adequate data for forming the above X-ray image can be be obtained.

Additionally, it is possible to use a partially annular target, filter and detector instead of the fully annular target 40, filter 49 and detcetors 9 and 52.

WHAT WE CLAIM IS: 1. An apparatus for obtaining a twodimensional image of the X-ray absorption distribution on a cross-sectional plane of an object comprising: a) an electron beam source for generat ing an electron beam, b) focusing means for focusing said electron beam on a target to produce X-rays, c) a guide plate, complete with a pin hole or pinholes, for directing the X ray micro-beam, resultant upon target impingement, onto the object, d) scanning means for scanning said electron beam over the surface of the target so as to cause the X-ray micro beam correspondingly to scan a cross sectional plane of the object, e) rotating means for rotating source a), means b), plate c) and means d) around the object, f) a detecting means for detecting and measuring the intensity of the X-rays passed through the object, g) a memory for memorising the output of said detecting means f) along with the corresponding output signals from the scanning means d) and the rotat ing means e), h) a calculating means for calculating the absorption value at each micro matrix area on said cross-sectional plane from the data memorised in the memory g), and i) a display means for displaying said respective calculated absorption values two-dimensionally.

2. An apparatus as claimed in Claim 1, wherein said scanning means d) scans the electron beam two-dimensionally over the surface of the target, the display means is synchronised with the scanning means d), and the output of said detecting means is used as a brightness modulation signal in order to display an X-ray projection image of the object.

3. An apparatus as claimed in either of the preceding claims, wherein said source a), means b) and plate c) are shifted parallel with the rotational axis of the rotating means e) so that the X-ray micro-beam always scans a cross-sectional plane not perpedicular to the rotating axis.

4. An apparatus as claimed in any of the preceding claims wherein the electron beam generated by the electron beam source is deflected away from the rotating axis of said rotating means so as to increase the take-off angle of the X-ray micro-beam.

5. An apparatus as claimed in any of the preceding claims, wherein a filter is centrally arranged in the guide plate pin hole.

6. An apparatus as claimed in any of the preceding claims, wherein the scanning means d) is controlled by a signal from a signal generator which monitors change in the object.

7. An apparatus as claimed in claim 6 wherein the calculating means h) incorporates a sampling means for sampling the data memorised in the memory g) according to the output of the signal generator which monitors changes in the object.

8. An apparatus for obtaining a twodimensional image of the X-ray absorption distribution on a cross-sectional plane of an object comprising: a) an electron beam source for generat ing an electron beam, b) an annular or partially annular target, the central axis of which coincides with the axis of the central beam generated by the electron beam source a), c) a focusing means for focusing said electron beam on the target b) to pro duce X-rays, d) a rotating means for rotating said electron beam around the target b),

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. of the beam guide plate 56. Even if the rotating member 46 is held fixed, the Xray micro-beam is still able to irradiate the object body at various angles. In this case, the measuring time is much shorter than in the case of the previously described embodiments, since in this embodiment, all the mechanical rotating means become superfluous; that is to say, are not used. The relation between the electron beam deflection pattern on the target and the pin holes 57a etc. is schematically shown in Figure 13 which represents a cross-sectional view of Figure 12 through AA'. In Figure 13, the solid lines 58a, 58b, . . . etc. on the generally circular arc 59 represent the electron beam irradiating lines (micro areas). Accordingly, the X-ray micro-beam scans the object body 10 at each pin hole; viz., 57a, 57b, . . . etc. However, if for one reason or another, it is impossible to increase the number of pin holes sufficiently, the X-ray data for forming an X-ray image of a cross-sectional plane of the object body will be insufficient. That is unless, for example, the beam guide plate is rotated at a speed lower than and in synchronism with (as a sub-multiple of) the electron beam deflection. By so doing, while the actual number of pin holes remains the same, the effective number increases. Accordingly, adequate data for forming the above X-ray image can be be obtained. Additionally, it is possible to use a partially annular target, filter and detector instead of the fully annular target 40, filter 49 and detcetors 9 and 52. WHAT WE CLAIM IS:

1. An apparatus for obtaining a twodimensional image of the X-ray absorption distribution on a cross-sectional plane of an object comprising: a) an electron beam source for generat ing an electron beam, b) focusing means for focusing said electron beam on a target to produce X-rays, c) a guide plate, complete with a pin hole or pinholes, for directing the X ray micro-beam, resultant upon target impingement, onto the object, d) scanning means for scanning said electron beam over the surface of the target so as to cause the X-ray micro beam correspondingly to scan a cross sectional plane of the object, e) rotating means for rotating source a), means b), plate c) and means d) around the object, f) a detecting means for detecting and measuring the intensity of the X-rays passed through the object, g) a memory for memorising the output of said detecting means f) along with the corresponding output signals from the scanning means d) and the rotat ing means e), h) a calculating means for calculating the absorption value at each micro matrix area on said cross-sectional plane from the data memorised in the memory g), and i) a display means for displaying said respective calculated absorption values two-dimensionally.

e) a scanning means for scanning said electron beam over said target b), f) a beam guide having a pin hole or pin holes for directing the X-ray micro-beam, resultant upon target impingement, on the object, g) a detecting means for detecting and measuring the intensity of the X-ray passed through the object, h) a memory for memorising the output of said detecting means g) along with the corresponding output signals from the rotating means d) and the scan ning means e), i) a calculating means for calculating the absorption value at each micro matrix area on said cross-sectional plane from the data memorised in the memory h), and j) a display means for displaying said respective calculated absorption values two-dimensionally.

9. An apparatus as claimed in Claim 8, wherein said rotating means d) incorporates plural deflecting stages.

10. An apparatus as claimed in Claim 9, wherein one of said plural deflecting stages comprises electrostatic deflecting plates.

11. An apparatus as claimed in any of Claims 8 to 10, wherein said beam guide f) is rotated by a mechanical rotating means in synchronism with said rotating means d).

12. An apparatus as claimed in any of Claims 8 to 10, wherein said beam guide f) is fixed.

13. An apparatus as claimed in any of Claims 8 to 12, wherein the shape of said detecting means g) is annular or partially annular and is fixed.

14. An apparatus for obtaining a twodimensional image of the X-ray absorption distribution on a cross-sectional plane of an object comprising: a) an electron beam source for generat ing an electron beam, b) an annular or partially annular target, the central axis of which coincides with the axis of the central beam generated by the electron beam source a), c) a focusing means for focusing said said electron beam on the target b) to produce X-rays, d) a rotating means for rotating the electron beam around the target b), e) a scanning means for scanning said electron beam over the target b), f) a stigmator for changing the cross sectional shape of the electron beam in synchronism with the rotating means d), g) a beam guide having a pin hole or pin holes for directing the X-ray micro-beam resultant upon target im pingement, on the object, h) a detecting means for detecting and measuring the intensity of the X-rays passed through the object, i) a memory for memorising the output of said detecting means h) along with the corresponding output signals from the rotating means d) and the scanning means e), j) a calculating means for calculating the absorption value at each micro matrix area on said cross-sectional plane from the data memorised in the memory i), and k) a display means for displaying said respective calculated adsorption values two-dimensionally.

15. Apparatus substantially as hereinbefore described with reference to Figures 1 to 4, 7 and 8 or Figures 5, 6, 7 and 8 or Figure 9, or Figures 10 and 11 or Figures 12 and 13 of the accompanying drawings.