CA1098627A - Apparatus for making laminar radiograms - Google Patents

Abstract

9- l 1-1977 ABSTRACT:

In tomosynthesis, as is known, a series of individual images taken-in different orientations, are superimposed. By shifting the individual images relative to each other the position of the layer which is sharply re-produced can be changed. However, there may still be unsharpness in said layer, in the case of image distortion during recording of the individual images, for example owing to the curvature of the input screen. This image distortion can hardly be compensated for, because it greatly depends on the projection angle. The invention provides a simple method of compensating for such image distortion. The image intensifier is then arranged and steered so that its optical axis is always parallel to the central ray. The distortion caused by the image intensifier curvature is then independent of the projection angle and can be eliminated in a com-paratively simple manner. Steering the image intensifier in this way gives rise to additional distortion, because the planes, of which subsequently laminar radiograms are made by superposition, and the optical axis of the image inten-sifier are no longer perpendicular to each other. However, this is a simple geometric distortion which can be eliminated in a simple manner.

Description

~ 7 PHD 76-198 The invention rela-tes to apparatus for making laminar radiograms with an image recording device/ which com-prises an image intensifier and a television camera for record-ing individual images in different positions of the X-ray radiator and of the image recordi;ng device, and with a super-position device for the formation of a laminar radiogram by the superposition of the individual images which have been shifted relative to each other by predetermined amounts.
Such apparatus is known (compare for example DT-OS 17 64 414 Siemens AG which was filed on June 1, 1968 and laid open to public inspection on March 12, 1972 and US-PS 3,499,146 by A.G~ Richards and which issued on March 3, 1970). In this appara~us a series of individual images from different directions of projection are superimposed. The superposition image only gives a sharp reproduction of a thin layer of the object. Laminer radiograms of other planes of : the object can be produced, in that the individual images are : shifted relative to each other to an accurateIy defined extent prior to superposition. However, perfectly sharp laminar radiographs are then obtained only if prior to superposition the indivldual images can be recorded and further processed without any geometric image distortion.
The use of an image intensifier, which con-verts the X-ray shadow image into a visible image with in-: creased brightness, and a camera tube which converts the imageon the output screen of the image intensifier into a video signal, gives rise to considerable image distortions. These distortions arlse as a result of distortions which are caused by the electron~optical systems in the image:intensifier and

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~ PHD 76-198 in the camera tube and are independent of the geometric con-ditions during recording, and as a result of distortions which depend on the projection of the object onto the gener-ally spherically curved input screen of the X-ray image inten-sifier. In apparatus of the type mentioned in the preamblethis distortion component greatly depends on the projection angle.
An electronic method for the correction o~
such image distortions is known from DT-OS 24 37 529 to Philips Patentverwaltung GmbH and which was filed on August 3, 1974 and laid open to public inspection on February 12, 197~. According to this method correction voltages are superimposed on the deflection voltages for the camera tube or a storage tube in-cluded in the superposition device, which correction voltages are produced by a computer controlled correction-network. This method is complicated and very difficult to realize, because the parameters of the image distortion to be compensated for change from image to image depending on the projection angle.
It is an object o~ the present invention to -construction an apparatus of the type mentioned in the preamble in such a way that image distortions of the individual images are reduced in a simple manner.
According to the invention this object is achieved in that the~image recording device is arranged and steered so that its optical axis is always parallel to the , . .
~ central ray, that there is provided a correction device which .
reduces distortions arising as a result of the construction of the image recording device, and that during read-in of an ~ individual image the deflection voltages or currents for the ~ television camera or for an image storage tube which serves :

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, P~ID 7 6 -- 1 9 8 for superposition can be modifiecl in such a way, depending on the lalninar angle and the orbit angle, that geometric distortions arising as a result of the slvpe of the input screen of the image intensifier relative to the laminar pl.ane are compensated for.
In known apparatus of the type mentioned in the prearnble the image recording device is steered so that its optical axis always intersects the laminar plane perpen~
dicularly, so that the optical axis and the central ra-y, which connects the focus of the X-ray tube to the centre of th~ ima~e intensifier input screen, make an angle which differs from zero with each other. On this condition only is it achieved that all parts of a lamina of the object are sharply reproduced in the superposition image, whilst more-over assuming that theimage recording device represents a flat proJection plane.
However, in accordance with the invention the image recording device is steered in such a way that its optical axis remains parallel to the central ray'(which can for example be achieved in a simple manner in that the image recording device is secured to a guide rod in a suitable . manner, which rod provides the mechanical coupling between : the ~-ray tube and the image recording device), so that the : laminar plane and the plane of the input screen of the image~: 25 intensifer Logether make an angle with each other which is equal to the laminar angle (which is the angle between the central ray and the normal to the laminar plane).. The in-ventlon is based~'on the recognition that with such.a steering distortions as a result of the construction of the image recording device (distortions as a result of-the electron-~ optical system as a result of the coupling objective between :: ~; : _ 4-~ . .

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the output screen o~ -the image intensi~ier and the television cnmera, owing to the curva-ture o~ -the image intensifier inpul;
screen etc.) in con-tradistinction to the known appara-tus-become independent of the geometrical relations during re-cording and can thus be compensated for in a much simpler manner.
Correction devices by means of which dis-tortions as a result o~ the construction o~ the image re-cording dovice can be compensated for, are already comrner-'cially available. If laminar images are to be derived from the corrected images by superposition, an additional image correction operation is to be performed, by means o~ which the projection plane which is normal to the central ray is rotated in 'the'projection plane which is perpendicular to the normal to the laminar plane. The magnitude of this image correction depends on the laminar angle ~ ? ~lowever, this is a comparatively uncomplicated trape-~ium correction (i e a rectangle in the laminar plane, whose sides respectively ex-' tend parallel and perpendicularly to the projection of the central ray on the laminar plane, is imaged as a trapezium in a plane which is perpendicular to the central ray). More-over, the amount o~ correction is small~ For example, for a projection angle range o~ d = ~ 15 and an image diameter of 1/10 o~ the X-ray'tube ~ocus/image distance the image is to be extended by - . In addition,' a trapezium correotion of only approx. ~ 1. 3% is necessary. This correction can be superimposed on the beàm de~lection of the televis~ion camera :

tube. ~Iowever, it may a]so be superimposed on the beam de-flection of an image storage tube used for image summation ` (superposition) during writ~e~in of the ind:ividual projection images.
The geometric distortions are eliminated in that during scanning of the individual lmage on the target of the television camera or during read.-in o~ the indivldual image in for example an image s~rage tube whi.ch serves for the superposition of the individual images the de:flection voltages or deflection curren-ts are dependent on the laminar angle and the orbit angle (which is the angle which the projection of the central ray on the laminar plane makes with a straight line which is preferably parallel to the line-scanning direction~. The field to be scanned of the television camera is then deformed in the same way as a rectangle in the lam:Lnar plane is deformed owing to the geometric dis-.
tortion. However, the deflection field of for example an image storage tube, in which the video signals corresponding to the individual images are stored for the purpose of .~ 20 - superposition and formation of a laminar radiogram, is to be deformed inversely, i.e. (areas~ which are expanded by ~ the projection should be contracted and vice versa .
In accordance with a further embodiment of the invention the deflection voltages or deflection currents ~ ~ 25 are applied to the deflection units of the television camera ; - via a computing device, which modifies. the x and y deflection voltages or deflection currents of a defleotion generator ' PIID 7~
9~ 977 ~ t7 by u = x. cos ~ ~ y. sin kl= cos ~ - 1 2 a sin ~ , through an additive correction in accordance with the relationship x~=x -~ k1. u . cos~ ~ k2 . u . x ~ k1.k~. u . cos ~ (1) and Y'~y ~ k~ . u sinl~ ~ k2. u. y ~ k~1. k2 . u . sin Y (2) where x', y' are the deflection vol-tages or currents applied to the deflection units, ~ is the orbit angle, ~ the laminar angle, and a a constant which corresponds -to the distance of`
the focus of the X-ray tube to the image-intensifier input screen. Solving the above-mentioned equations w:ith respect to - x and y yields the relationship in accordance wit~ which the ; - respective deflection voltages or currents (x', y') of a deflection generator are to be modified during read-in of the individual images into for example an ima~e stor,age tube, so ; that the corresponding deflection units receive~ such deflection voltages and currents (x, y) that the said geometric d:istortions are eliminated.
The invention will now be described in more detail with reference to the drawing which shows an embodimcnt.
` In the-drawing ~Fig. 1 schematlcally represents an embodiment of the invention, Fig 2 is an example of a circuit included ~: :

, P~ID 7~ S
9~11--1977 between the de~lection generator and the de~lection unit of the television camera, which mocL:i~ies the de~lection voltages or de~lcction currents supplied by the deflec-tion generator in accordance with the equa-tions (1) and (2).
In Fig. 1 an X-ray source is designated 1, whose radiation passes through the body o~ a pa-tient 3 which is positioned on a table top 2. The ray -transmission behind the patient is intercepted by an image in-tensi~ier l~ and converted into a visible image, which is recorded by a -te:Le-vision camera 5. The signals then produced, which each corres-pond to an individual image, are trans~erred into a bu~fer memory 6, ~or example a disc memory, ~rom which they can be extracted and stored in an image storage tube ;7 ~or the ~ormation o~ a superposition image. The relative shi:~t of the individual images, which depends on the position of the laminar plane to be reproduced by superposition, is then computed by a control computer, not shown, which also controls the de~lection units of the irnage storage~ tube ~ accordingly.
The laminar radiogram obtained by superposition can-be read from the target o~ the image storage tube 7 and displayed by ~ an image display unit S. So far the construction o~ the ~ . . I
apparatus shown in Fig.`l~ -is known.
The individual images are taken in di~ferent positions of the system radiator image-recording device (image lntensi~ier 4 and television camera 5). Fig. 1 shows three o~ -these positions, two positions being represented by dashed lines only. The points o~ intersection of central rays 9 determines the position of the plane 10 that can be ' ..

PHD 76~198 reproduced sharply by superposition of the individual images without relative displacement.
In Fig. l it can be seen that the image record-ing device is steered so that its optical axis (which is the line which is perpendicular to the centre of the image-intensifier input screen) always extends parallel to the central ray 9, whilst in known apparatus the image recording device is arranged so in the various positions, that its optical axis is perpendicular to the laminar plane lO. Steer-ing the image-recording device in such a way that its optical axis extends parallel to the central ray is simple, because most tomography equipment comprises a coupling rod which links the X-ray radiator and the image recording device to each other and causes them to be moved in opposite directions.
This coupling rod extends parallel to the central rayl and therefore it need only be linked to the image-recording - device in such a way that it extends parallel to the optical axis of the image recording device~
In this arrangement of the image recording device the image distortion owing to the curvature of the image-intensifier input screen, owing to the electronics, owing to the coupling objective, not shown, between the out-put screen of the image intensifier and the teleuision camera etc. is the same for all positions of the system radiator/

:
~25~ image recording device and is independent of the angle at which the central ray 9 intersects the laminar plane lO.

Therefore, it can be eliminated by commercially~available correction devices tfor example, a pin-cushion distortion device o~ Infraton).

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PHD 7G_198 9~ 1977 These correction devices, which are for example employed ~or correcting the image geometry in high-resolu-tion cathode-ray tubes, derive distorted defléction voltages xx and Yc from the deflection vol-tages or deflectior currents x', y', the distortion of the deflec-tion voltages bein6 adjustable in such a way that the distortion caused by the curvature o~ the irnage-intensifier inpu-t screen tc.
can be elirninated. The apparatus in'accordance w:ith Fig. 1 comprises such a correction dev:ice 11, whose inputs'are 'connected'to'the outputs of a def~ection generator 12 via a circuit 13 whose furlction is to'be explained hereinafter, and whose outputs are connected to the de~'lection unï-ts of the television camera 5.
~s previously stated, additional geometric - dis'tortion is produced as a result-of the slope-of the image intensifiër-`input-screen relative to-the laminar plane. As an example, in the posi-tion of the X-ray radiator and the image recording devi'ce in which the X-ray radiator is located . .
at the left and'the image intensi~ier at the right, the areas of the laminar plàne to the right of the intersection ' with the central ray are imaged smaller than those situated to the le~t of'the intersection, because the'areas situated to the right~of the central ray are situa-ted nearer the image intensifier input~ screen. It can be'demonstrated that a point ; - 25 situated in the laminar plane 10, which point corresponds to the coordinates x, y in the case'of the usual parallel~

~ ' projection on the image-intensi~ier'input screen, is imaged ' ~ in a~ plane~(corresponding to the image-intensifier input .
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screen) which is perpendicular to ths central ra-y 9, as a point ~ith the coordinates x', y' ~the zer.~ point of' the coordi.na-te system x, y and x', y' being situated'on the cen-tral ray 9),'for which as an approximation'the following relat:ionship i.s vali.d:
x' = x -~ kl . u . cos ~ -~ k2 ~ u . x ~ k1 ' k2 . U2 . cos 1~.
and y~ - y -~ kl . u . sin ~ + k2 . u . y ~ k.l . k2 . u . sin where.
u = x . cos ~ ~ y . sin k1 - cos ~( -1 and k2.-.~a sin ~ . .. . ~ ;
' ' Therein ~ is the orbit angle, i.e. the angle enclosed by the '` 15 projection of the central ray 9 on the laminar plane and t~le x-axis, ~ the laminar angle, and a.the distance of the focus o~ the X ray radiator from the image intensifier input--plane.
These geometric distortions can ~e el;iminated,-when'the deflection voltages or de~lection currents in the-:- :
~ . ' x and y direction,'wllich are also des:ignated x and y in this , :; case~ a:re modified into x~ and:y' in aocordance with'the ; equations (1) and (2). The de:~lection voltages or currents of : ' the storage tube 7 and'the display unlt then remaln undis-25~ torted. The quantlties'x' and y' not only depends on x.and y respecti-vely,: but also an ~ and x. This means that~when x~is the hor.iz'ontal deflection voltage or the horizontal deflection current and y the vertical deflection voltage or " : : ~ . :

, ~ ~

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Pl-ID 76~198 9~ l977 .... .

the vertical de:flectlon current, the modif:ied quantity x', in addition to the horizontal frequency comp~nents~ also com-prises components of vertical frequency and that the modif:ied quantity yl~ in addition to components of ver-tical frequency~
also contains horizontal-frequency components, whose magnitude' depends on the orbit angle ~ and the laminar angle ~, - The circuit arrangement 13 rnodifies'the sawtooth-shaped hori~ontal and ver-tical deflection voltages or currents in accordance with the equation (1) i'nto voltages or currents x~ and y~ so that geometric distortion as a result of the slope o.f the image-intensifier inpu-t sc:reen relative to the laminar plane is eliminated. The modified voltages`or'curren-ts ~ and y~ are applied to the previously de-scribed correction device 11, which derives therefrom the deflection voltages or deflection:currents ~c and Yc for'the deflection units of the teIevision camera 5 and'thus also - eliminates~distortion as a result of the.curvature of the ~ image-intensifier input screen-etc.
. Pig. 2 shows an example of the'circ'uit arra~gement 13'for eliminating geometric~distortion. The deflection voltages or the deflection currents ~ and y from the defleotion generator'12 are applied to''an adder stage 25 a~d 35 respectively~ and to a mul-tiplier circult 21 and 31 respectively, in whi¢h`they'are'multipli'ed by cos ~- and sin'~ respectively. l'he output-signals'of the two mul-tiplier : -circuits 21 and 31 are applied'to the inputs of an adder cir-. ~
.... cuit 14, so that the output signal of this circuit,~ which is . ~
. . :. . - . , - . .
applied to the two multiplier circuits 15 and 16, corresponds ~: : to the quantity u in e.quations (1j and (2). In the multiplier , : 30 cir¢uit 16'the'output signal of the adder circuit 14 is . -12-~ ' : , P~ID 76-'l9 ~ 9~ 1977 multipli.ed by the value k1. The ou-tput of the multiplier circui-t 16 is connected to an input o* the multiplier cir-cuit 22 and-32 respectively, in which circuits the output signal o~ the multiplier circuit 'l6 is multiplied'by cos ~ and sinL~ respect:ively.'Hence~ the ou-tput''signal o~
the circuits 22 and 32 corresponds to the second term in equations (1) and ~2) rcspectively. It is applled to a 'further input o~"the adder circuits 25 and 35 respectively ' and''moreover to an adder circuit 23 and 33 respec'tively, which aclds the output signal of 22 or 32 to the i.nput quant.ity x or y-respectively.'The output signal o~ thc circuits 23 and 33'is multipli.ed by -the output signal of the multiplier circuit 15 in a multiplier ci'rcuit 24~
which circuit'15 in its turn multiplies the output signal ' .
o~ the circuit '14 by the factor-k2.'The output signal of the multiplier circuit 24 or 34'then corresponds'to~the'sum o~ the third and ~ourth terms in equations (1) and (2) res-' pectiv~ly. It is applied'to a'further input~o~ the adder - circuit 25'and 35 respectively~ so tl~t at the ou-tpu-t thereof - the respective quantity x' or y' is- availabIe.
,. ' : , , ' Special advantages are obtained when'-the '~ multipl-ier stages 15~ 16~ 21, 22~'31~and 32 are replac'ed .
y so-called mu].tiplying digital-to-analog converters (for' example "Mult~plying D/~-Converters" ~rom Analog Devices, Noruood~'Mass.~ U5A)~ which in respect of their acouracy : , , .
' and bandwidth are highly superior to analog multipliers.~
Th~e correction parameters kl, k29 cos ~ ', and sin~ should 13- ~

PIID 7fi--19~3 2'7 then be made ava:ilahle in digital forM. The parameters k1 and k2 can now readily he calculated by the-control computer~ which is neeclecl anyway in such-appara-tus. They are cons-tant in the'case of a circu~ar blurring'pat-tern.
The values cos 1~' and sinlip vary from'inclividual image'to inclividual image-'at any rate when the system'radiator/
i~age recording~ device is not positioned along a'straight line during recording of the various indiviclual lmages.
Therefore, they'have to be. recaleula-ted for each indiviclual 10 ' image. When the angle ~ is'measured digi-tally'during reoording, the angular functions can be determined by means o~ tables stored in fi~ced-programme semiconductor memories.
The periodicity and the symmetry of the ang~lar fune-tions . enables both angular-functions in all;four quadrants to be raleulated by re~ns are a t~bLe for o ~ qunlran' only.

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Claims (4)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS

1. Apparatus for making laminar radiograms of laminar planes of an object with an image recording device, which comprises an image intensifier and a television camera, for recording individual images in different positions of an X-ray radiator and the image recording device, and with a superposition device for the formation of a laminar radio-gram by the superposition of the individual images which have been shifted relative to each other by predetermined amounts, characterized in that the image recording device is arranged and steered so that its optical axis is always parallel to a central ray emitted by the X-ray radiator, that there is provided a correction device which reduces distortions arising as a result of the construction of the image recording device, and that during read-in of an individual image the deflection voltages or currents (x, y) for the television camera or for an image storage tube which serves for superposition can be modified in such a way, depending on a laminar angle (.alpha.) enclosed by the central ray and a line perpendicular to the laminar plane and an orbit angle (?) enclosed by the projection of the central ray on the laminar plane and an X-axis in said laminar plane, that geometric distortions arising as a result of the slope of the input screen of the image intensifier relative to the laminar plane are compensated for.
.

2. Apparatus as claimed in Claim l, characterized in that the deflection voltages or deflection currents (x,y) are applied to the deflection units of the television camera via a computing device, which modifies the deflect-ion voltages or currents of a deflection generator by u = x . cos ? + y . sin ?
k1 = cos .alpha. - 1 k2 = ? sin .alpha.
through an additive correction in accordance with the relationship x' = x + k1 . U . cos ? + k2 . u . x + k1 . k2 . u2 . cos ?
and y' = y + k1 . u . sin ? + k2 . u . y + k1 . k2 . u2 . sin ?
where x', y' are the deflection voltages or currents applied to the deflection units, ? is the orbit angle, .alpha. the laminar angle, and a a constant which corresponds to the distance of the focus of the X-ray tube to the image-intensifier input screen.

3. Apparatus as claimed in Claim 2, superposition of the individual images being effected in a storage tube, characterized in that the deflection voltages or currents are applied to the storage tube in undistorted form.

4. Apparatus as claimed in Claim 2 or 3, characterized in that the computing device and a correction device are connected after each other.