US3484604A - Axial transverse tomography wherein the angle of spread of radiation is controlled - Google Patents

Description

Dec. 16. 1969 TADAYOSHI MATSUDA ET 3,

AXIAL TRANSVERSE TOMOGRAPHY WHEREIN THE ANGLE OF SPREAD OF RADIATION IS CONTROLLED Filed Oct. 1.7, 1967 2 Sheets-Sheet l 2b 40 6'0 mbolzomofsofao ANGLE OF SCANNING 7 1 M yoaw MAUI 94;, 145w E YUM? INVENTOR$ Dec. 16. 1969 TOADAYOSHI M-ATSUDA AL 3,484,604

AXIAL TRANSVER SE TOMOGRAPHY WHEREIN THE ANGLE Filed 001. I7, 1967 DISTAME THROUGHfiE BODY SPREADING ANGLE OF DIAPHRAGM SPREAD OF RADIATION IS CONTROLLED 2 Sheets-Sheet 2 0 2'0 40 6'0 ab 46042240460180 ANGLE 0F SCANNING Fae] 0 2'0 4 0 60 mbmoMmmo ANGLE QFS'CANMNG United States Patent 3,484,604 AXIAL TRANSVERSE TOMOGRAPHY WHEREIN THE ANGLE 0F SPREAD 0F RADIATION IS CONTROLLED Tadayoshi Matsuda and Shigeo Endo, Toyohashi-shi, Sumio Makino, Yokohama-shi, and Katsumi Nagai, Tokyo, Japan, assignors to Tokyo Shibaura Electric Co., Ltd., Kawasaki-shi, Japan, a corporation of Japan Filed Oct. 17, 1967, Ser. No. 675,972 Int. Cl. G01n 23/00; Htllj J/52 US. Cl. 25061.5 2 Claims ABSTRACT OF THE DISCLOSURE When rotating a source of X-ray radiation and a cassette holder opposing thereto about a centre of rotation included in a cross-section of the patients body, the angle of the useful beam arriving at the film through the body is controlled to follow a predetermined program in accordance with the angle of rotation so that the concentration of the image on the developed film corresponding to the peripheral portion of the body is correlated with that of the image corresponding to the tumour inside the body.

This invention relates to tomography and more particularly to rotatography wherein tomographs are taken with respect to a cross-section intersecting with the axis of the patients body at right angles.

An image corresponding to the tumour part of a patients body obtained by a rotatography on a film and an image corresponding to contour of the body are required to be clear in order to ascertain their relative positions. However, the mean degree of X-ray absorption or the mean degree of attenuation from the beginning of r0- tating motion to the end thereof is lower near the periphery of the body than in the inner portions of the body so that the degree of blackening of the developed film is higher at portions corresponding to the periphery of the body than at portions corresponding to the inside of the body. When the intensity of X-rays is adjusted to clearly indicate on the film the image of the tumour part in the body the concentration of the image of the periphery of the body will approach or exceed the saturation of the film density thus rendering the image non-clear.

Such a difference in the degree of X-ray absorption at various portions on the cross-section during rotating motion is caused by inherent defects of tomography, that is variation in the distance between the X-ray source and the point on the film at which the image is focused as well as the variation in the incident angle of X-rays upon the film surface during rotating motion where said focal point does not coincide with the centre of rotation of the film. Generally, however, from the standpoint of the construction of X-ray photographic apparatus, the actual distance between the X-ray source and the film surface is made large in order to make relatively long the distance between the cross-section and the film surface whereby to enlarge the image photographed on the film. As a consequence, the variation in the distance between any focal point on the film corresponding to the cross-section of the patient and the X-ray source and the variation in the incident angle would be very small for any focal point and thus would not cause in any way uneven degree of X-ray absorption.

The direct cause for the above deficiency is the degree of absorption of X-rays that penetrate through the patients body. Factors that determine the degree of absorbing X-rays by the patients body are the distance through the body and the tissue thereof. However, when pro-ducing an image of a cross-section the tissue does not unevenly affect the film density because it is photographed by rotating motion. It was confirmed by experience and experiment that uneven concentration of the image on the film is caused by the difference in the distance through the patients body. We have actually measured the effect of the difference in the distance of X-rays through the body upon the density formed on the film by utilising a waist phantom. In this measurement it was assumed that the tissue inside the patients body is homogeneous and that the body is a vertical cylinder having the same crosssection throughout the length as that of a body to be photographed by neglecting irregular contours of the body. In order to cause the results of measurement to closely approach the actual conditions of X-ray tomography, in the distance through the body was included the distance through a table on which the patient is supported. As shown in FIG. 1 in a cylindrical body having the same cross-section as that of the body as assumed hereinabove the distance through the body is represented as follows:

During rotating motion, the angle 0 as well as the distance between an X-ray source and a focal point on the film would vary except the case wherein X-rays pass through the center of rotation, but the amount of such variation is negligibly small. As a consequence, in considering the length I it is sufficient to investigate only the relation between the concentration of the image on the film and the length R. Ordinally, the angle of rotating motion from the starting point to the final point of rotating motion is nearly equal to 200, but in measuring the phantom this angle was set to be equal to 180.

Distances of travel of X-rays (including distances through the bed) at various points of inspection I, II, III, IV, V and VI on a cross-section of the phantom as shown in FIG. 2 are plotted in FIG. 3 as a group of curves with reference to the angle of rotating motion. As can be noted from FIG. 3, characteristic curves at the point of inspection of II through V substantially overlap one upon the other so that results of integration from the starting point to the final point of rotating motion are approximately equal. This means that the density of images on developed films are nearly equal. Curve I corresponding to the point of inspection I has smaller ordinates than curves II through V corresponding to points of inspections II through V, respectively, at angles smaller than 50 and larger than which means that distances of travel of X-rays are short. Whereas in a range of angle 4) represented by 50 140, curve I substantially overlaps upon curves II through V so that the integral of the curve I from the starting point to the final point of the rotating motion is smaller than those of curves II through V. This means that the density of the image on the developed film is higher than those images at points of inspection II through V. Further the ordinate of curve IV corresponding to the point of inspection VI is smaller than those of curves II through V at angles larger than 30 and smaller than Thus in this range of angles the distances of travel of X-rays are greatly reduced. On the other hand in ranges of angle smaller than 30 and larger than 150, the curve VI substantially overlaps upon curves II through V. This means that the integral of curve VI in a range of angle of 30 150 the integral of the path of travel of X-rays is smaller than those of curves II through V thus increasing the concentration of the image on the film.

This invention contemplates the coincidence of density of the images on the developed film corresponding to the periphery of the patients body and to the tumour in the body obtained by the rotatography whereby to clearly indicate the relative position of a tumour with reference to the peripheral portion of the body.

In order to attain this object, in accordance with this invention there is provided a method of tomography, wherein when effecting rotating motion of a source of radiation and a cassette opposing thereto about a centre of rotation included in a cross-section of the patients body, the angle of spreading of the radiation flux arriving at the film through the patients body is controlled to follow a predetermined program. Said control of the angle of spreading is effected by means of a diaphragm located between the source of radiation and the body to be examined. The diaphragm may be substituted by a filter which attenuates the X-ray passing therethroughv In the accompanying drawings:

FIG. 1 represents relative positions of a source of radiation and a photographic film with reference to a Waist phantom wherein a cross-section along a line Y-Y' shows a section of a body to be examined;

FIG. 2 shows a schematic cross-section of the Waist phantom representing positions of various points of inspection I through VI included in said cross-section;

FIG. 3 shows a group of characteristic curves showing distances of travel of X-rays at each of said points of inspection I through VI wherein the abscissa represents the angle of rotating motion es of the radiation source and the ordinate the distance I of transmission through the patients body including the table;

FIG. 4 is a diagrammatic representation of a conventional X-ray tomographic apparatus;

FIG. 5 is a diagram to explain diaphragm control of X-rays;

FIG. 6 is a graph to show the relation between the opening of the diaphragm and the angle of rotating motion; and

FIG. 7 is a graph similar to FIG. 3 but diaphragm control is employed.

The radiation photographic apparatus shown in FIG. 4 is designed to be especially suitable for tomography. As shown in FIG. 4 a source of radiations, for example an X-ray tube 12 and a cassette holder 13 (the imaging means) are supported by a supporting beam 15 on the opposite sides of a table 11 supporting a human body 20, said beam being rotatably journalled on a horizontal shaft 14 fixed to a frame 16. At the projection window of the X-ray tube 12 there is provided a suitable diaphragm mechanism, not shown, to control the angle of spread of radiations emanated from the X-ray tube 12. The opening of the diaphragm is controlled by means of a control device (not shown) to follow a predetermined program (which shows the degree of throttling generally predetermined with respect to a particular cross-section) with reference to the angle of rotation of the horizontal shaft 14 or the angle of rotating motion of the X-ray tube 12.

The relation between the operation of the diaphragm control and the rotating motion angle of the X-ray tube 12 and the cassette holder 13 will now be considered with reference to FIG. 5. Points 12a, 12b, 12c and 12d represent respective particular positions of the X-ray tube during rotating motion. The membranes of the diaphragm mechanism 17 at these positions are schematically represented by 17:1 17a 17b 1717 171: 17c and 172 17e and angles of spreading of X-rays defined by centre lines extending between the centre of radiation of the X-ray tube 12 at each of said positions and the centre of the body (a point included in a section to be photographed) and the edges of said membranes are represented by :1 ,8 a {3 a 5 respectively. The angles of spreading or and [3 of the diaphragm 17 are controlled with the increase of rotating motion angle 95, as shown in FIG. 6, as the X-ray tube 12 is rotated. It is to be understood that the curve illustrated in FIG. 6 is to explain the principle of the invention and that the configuration of the actual curve is more complicated.

Characteristic curves shown in FIG. 3 are corrected in the following manner. Thus in the range of rotation angle of less than 50 the angle of opening at is gradually increased as shown in FIG. 5 but gradually decreased in a range greater than 140 as shown by a portion ,8. Thus, by controlling the angle of opening of the diaphragm 17 in correlation with the rotating motion of the X-ray tube and in accordance with a predetermined program to provide corrected characteristic curves I and VI as shown in FIG. 7, the concentration of the image on the photographic film of each of said points of inspection I through VI would be proportional to the integral of the distance of travel of X-rays through the body throughout the angle of rotation or rotating motion of the X-ray tube, whereby the density of respective points of inspection would become substantially equal. For this reason when the position of the tumour is clearly developed the peripheral portion of the body will also be clearly photographed, thus enabling accurate and easy determination of the relative positions thereof.

Although the diaphragm mechanism has been shown to completely intercept X-rays it may be constructed of a material that suitably attenuates X-rays whereby to act as a filter.

What is claimed is:

1. A method of cross-section tomography comprising the steps of:

rotating an X-ray source and imaging means around an axis intersecting and perpendicular to the crosssection of a body under scrutiny, said X-ray source and imaging means being coupled together and mounted on opposite sides of the body under scrutiny; and

controlling the angle of spread of the radiation beam emitted by said X-ray source as a function of the body being examined and of the angular position of said X-ray source and said imaging means during said rotation in accordance with a predetermined program so that the intensity of that part of the image on the developed film which corresponds to the peripheral portions of the cross-section of the body and of that part of the image corresponding to inner parts of the body are substantially the same.

2. A method according to claim 1 wherein the angle of spread of the radiation beam is controlled by a diaphragm assembly, said diaphragm assembly including means for attenuating X-rays, thereby acting as a filter.

References Cited UNITED STATES PATENTS 1,535,359 4/1925 Tousey 250- X 3,248,547 4/1966 Van De Geijn 25 -86 FOREIGN PATENTS 1,081,186 5/1960 Germany.

RALPH G. NILS'ON, Primary Examiner a A. L. BIRCH, Assistant Examiner US. Cl. X.R. 25065, 105

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