DE2822089A1 - ARRANGEMENT FOR COMPUTER TOMOGRAPHY - Google Patents

Description

Arrangement for computed tomography.

The invention relates to an arrangement for computed tomography with an X-ray beam for generating a flat fan-shaped x-ray beam and with a X-ray detector with a sequence of detection elements arranged on an arc of a circle, with X-ray tractors and X-ray detector can be rotated about a common axis of rotation directed perpendicular to the plane of the fan beam.

Such an arrangement is particularly suitable for X-ray diagnostics. During this investigation, a Part of the body of a patient interspersed with the flat fan-shaped X-ray beam from several directions. The Andes Radiation transmitted through different points is measured,

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and the measurement data determined in this way becomes with With the help of a computer, the density distribution of the patient's body part in the irradiated plane is determined and for example shown on a television monitor.

An arrangement of the type mentioned at the beginning is known from NL-OS 7 503 520. In this patent application an arrangement is described in which a flat fan-shaped X-ray beam is used, the to examining body part at least in one direction completely includes. In order to get a sufficient number of measurement data, the X-ray tracer and the X-ray detector during the examination together around the patient who is arranged close to a common axis of rotation. the Arrangement contains means for compensating for the influence of the differences in sensitivity of the various Detection elements on the measurement signals.

In the known arrangement, the X-ray tracer and the X-ray detector rotate during the examination with smooth speed together around the axis of rotation.

The output signals of the detection elements are used for Obtaining reliable measurements over a short period of time " integrated, in which the X-ray tracer and the X-ray detector at an acute angle of, for example, 1 to be turned around. The integrated signals are then logarithmized. Each detection element delivers during the Underlies a set of logarithms of the intensities of the X-rays transmitted in different directions by the patient. These data will be used during the investigation written in a first electronic memory and after

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Examination sequence in sets of several detection elements resulting logarithms of the intensities of X-rays transmitted in parallel directions orderly. The ordered sentences are written in a second electronic memory. Then with a Reconstruction technique of each pixel in the irradiated plane of the examined body part the density calculates what amounts to the fact that, of the ordered sets, the data that contain the pixel in question, can be summed up.

In the known arrangement, the occurrence of disruptive ring-shaped interference patterns in the reproduction of the calculated density distribution, called ring artifacts, is suppressed by compensating for the differences in sensitivity of the detection elements. For this purpose, a rapid movement of the X-ray emitter is superimposed separately on the uniform joint rotary movement of the X-rays and the X-ray detector in such a way that each measurement of each detection element from an adjacent detection element is repeated during the examination. On the basis of the data determined in this way, the detection elements are compared with one another and differences in sensitivity are compensated for, for which purpose an additional electronic processing group is included.

. The invention has for its object to provide an arrangement for computed tomography, in which the one hand, differences in sensitivity of detection dor brine Merite not come in the occurrence of ring artifacts expressed and on the other hand each DotoktJons-

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element a set of measurements during the investigation of the intensity of the X-rays transmitted in parallel directions, so that a rearrangement of the measurement signals can be omitted after the examination.

According to the invention, this object is achieved in that the distance between the radiation input surface of the detection elements from the axis of rotation at least almost equal to the distance of the focal spot of the X-ray source from the Is the axis of rotation and the X-ray tracer and the X-ray detector with at least almost the same angular velocity, but are rotatable in the opposite sense about the axis of rotation. During operation, the output signals are of the detection elements for obtaining sufficient measurement accuracy over a short period of time integrated, during which the X-ray tracer and the X-ray detector at an acute angle of, for example turn. Since the X-ray tracer and the X-ray detector move in opposite directions, the one to be examined becomes Body part from the X-ray tracer and from a detection element in adjacent to one another almost parallel to one another Stripes scanned. This means that in order to determine the density in each pixel, the irradiated Level, a measured value from each detection element is used. This can cause interference patterns in the reproduction of the determined density distribution due to differences in the sensitivity of the doping elements do not occur. the The ring artifacts mentioned are thereby avoided.

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A preferred embodiment of one according to the invention Arrangement for computed tomography is characterized in that the X-ray detector consists of a a circle around the axis of rotation arranged closed sequence of detection elements. This allows for all directions in the plane of investigation, sentences of Intensity measurement values of the X-rays penetrating the body along parallel radiation paths are determined will.

10 The invention is described below and by way of example

explained in more detail with reference to the drawing. It shows

Fig. 1 shows a schematic longitudinal section through an arrangement according to the invention for computed tomography,

FIG. 2 shows a schematic cross section through the arrangement shown in FIG. 1 along the line II-II;

Fig. 3 ei ⁿ schematic diagram of an inventive arrangement for the Coitiputertomographie, Fig. H diagrammatically the mutual position of the X-ray source and a detection element at successive instants,

Fig. 5 diagrammatically shows an X-ray detector which is particularly suitable for use as the detection element, and Fig. 6 is an illustration € ec Abtastmöglichkeiten the arrangement according to Fig. 1 and 2.

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Fig. 1 and. 2 show a schematic longitudinal section and a schematic cross section through the same arrangement for computed tomography, in which a patient 1 lying on an examination table 2 is penetrated by a flat, fan-shaped X-ray beam 3. The X-ray beam 3 forms an opening angle in the plane of the drawing according to FIG. 2, in the further fan angle O *. called, for example 30 and is relatively flat with a thickness of about 10 mm in the direction perpendicular thereto. The fan angle is so large that the beam 3 encompasses the entire patient 1 in the fan direction. The X-ray beam is generated with an X-ray tube k with a rotating anode (not shown). Since the focal point of the rotating anode - the actual radiator - has a length and. a width of approximately 2 mm is relatively small, the x-ray beam can be viewed as point-shaped in terms of radiation technology. Radiation let through by the patient is measured with an X-ray detector 5 which contains a sequence of, for example, 100 detection elements 6 arranged on a circle. The detection elements 6 to be explained in more detail below, with regard to the relatively small dimensions, are to be regarded as point-shaped in terms of radiation technology, for example, contain a scintillation crystal and a light detector. The X-ray tube h is fastened on a ring 7 which is mounted on wheels 10 fastened to the housing 9 and can be rotated with the aid of a drive motor 12 about an axis 1h perpendicular to the plane of the fan beam 3.

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The actual X-ray tracer, the focal spot of the rotating anode X-ray tube, rotates in a circular path If). The X-ray detector 5 is fastened on a ring 8 which is mounted on wheels 11 fastened to the housing 9 and the axis 1h can be rotated with the aid of a drive motor I3. The ring and the ring 8 rotate in opposite directions during the examination. The detection elements 6 rotate in a circular path, the radius of which is almost equal to the radius of the circular path I5 in which the actual X-ray emitter is located. The radii are, for example, 90 and cm. The processing of the measurement data is explained in more detail below with reference to FIG. 3.

3 shows very schematically an arrangement specified in the above description with a patient 20, an X-ray tracer 21 and an X-ray detector 22 which contains a sequence of detection elements 23 arranged in a circle. The X-ray tracer 21 and the detection elements 23 rotate during the examination in opposite directions - as a first approximation - in the same circular path Zh.

The detection elements are, although such is only indicated for three, all with an integration circuit 25 connected, in which the measurement signals of the detection elements to obtain a sufficient measurement accuracy for a can be integrated for a short time, for example 10 ms. Since the X-ray tracer 21 and the X-ray detector 22 move in the opposite sense, the body part 20 to be examined will run almost parallel to one another

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adjoining strips are scanned, as indicated in FIG. h , in which figure the mutual position of the X-ray emitter 22 and a single detection element 25 is indicated at successive times t, t,, t ... t. The X-ray tracer 22 and the X-ray detector Zh traverse the same circular path 2k during the examination as a first approximation, so that the distance between the X-ray emitter and a detection element changes during the examination, whereby the intensities measured by the detection elements change independently of that at the relevant point change the patient's measured absorption during the examination. These changes are corrected in circuits 26. After the correction, the measurement signals of the detection elements are logarithmized in logarithmic amplifiers 27 and the signals processed in this way are written to a memory 28. During the examination, each detection element 23 thus produces a signal sequence which corresponds to the logarithms of the intensities of the x-ray radiation transmitted by the patient in almost parallel directions. After the examination has ended, the computer 29 uses a reconstruction technique to determine the density distribution of the irradiated body part 20 and, for example, on a television monitor 30

dax'geste.llt.
5 schematically shows an X-ray detector 3I which is particularly suitable for use as a detection element in the arrangement described above. The detector 31 contains a cylindrical scintillator '] 2 with a diameter of, for example, 5 »:""

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and a length of for example 20 mm and a light detector 33 axially coupled therewith. The two schematically shown X-rays 34 and 35, which from different Directions reaching the detector are detected in the same way due to their cylindrical symmetry. These Property makes the detector particularly suitable for the aforementioned use, because it is during the examination continuously changes the direction of the X-ray radiation falling on a detection element. These Change is independent of the absorption at the relevant point of the patient and may therefore affect the measurement signal not affect.

The scanning possibilities of the described arrangement are then explained in more detail with reference to FIG. Here the circle 41 shows the path to be traversed by a point X-ray emitter 42 and an X-ray detector 43 with a detection element sequence 44. The position of the X-ray emitter 42 is shown at three points in time, namely at time t = 0, at time t = T and at one point in time between these two times. The emitter moves at a uniform speed UJ during the examination, so the total angle of rotation of the emitter is WT The fan angle 0 £ of the X-ray emitter is determined here by a schematically illustrated collimator, which is moved with the emitter in such a way that the center of the emitter is always on öi e center M of the circle is directed. During the investigation, they deliver

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Detection elements Sequence of measurement signals, the intensity of the in mutually parallel directions from X-rays allowed to pass through the patient. the

directions given by the detection elements between the schematically illustrated x-rays 46 and

47. These two rays close the examination angle φ Gin. In the triangle determined by the emitter positions at times t = O and t = T and by the intersection of rays 46 and 47, it can be seen that VO.T = ^ 1 + φ. Because only radiation is measured in mutually parallel directions, which lie between the outermost rays 46 and 47, the X-ray beam at time t = 0 except for beam 46 and at / time t = T except for beam 47 of that with the Radiator shielded along with moving aperture 48.

This severely limits the radiation dose received by the patient. The diaphragm only performs a translational movement, from left to right in the figure, so that, as can be seen, the opening angle ^ 3 is equal to φ . During the examination, the detector 43 traverses the circular path 41 in the specified direction at an angular velocity IO. In the time T, the detection element, which measures the beam 47 at the time t = T, sets the angle tOT

Zj

return. In order to enable the investigation, a detector is required whose detection angle θ is equal to (**. T + C * , where, as can be seen in FIG. 6, A = 36Ο - WT - 2 (180 - & 2-Λ ) = WT ~ 2 & such that θ = 2 ((> iT - &) = 2 0.

From the above description it can be seen that for

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Performing 2 ^ 0 have to be rotated a study of φ = 18O with a fan angle L * = 6O the Röntgenstrahier and the detector, the opposite sense by an angle CU.T =, wherein a detector having a detection angle θ = 36O is needed. A diaphragm 38 that moves with it preferably has an opening angle ^! = 180. It should be noted here that with a detection angle 0 = 36θ, the examination angle can be increased to φ = 3 ^ 0 by using each detection element 43 twice during the examination, with UJ.T = 420.

In practice, examination angles φ of 180. and above that the X-ray tube h and the X-ray detector 5 in FIGS. 1 and 2 do not move on the same circular path, but only on circular paths whose radii differ from one another by a few centimeters. Each detection element supplies a set of measurement signals during the examination at the same angular velocity of the emitter and the detector, which represents the intensity of the radiation let through by the patient along a number of beam paths which are not parallel to one another. This non-parallelism is at a fan angle of

ο
about 30 and a length of the radii of the circular orbits of

about 100 cm so small that their influence on the calculations of the densities in the irradiated plane is negligibly small. A correction is possible by rotating the X-ray tracer, which moves in the smallest circle, a little faster than the X-ray detector. With a fan angle &> = 30 becomes to each

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Time only an angle of 60 of the detection elements kh in FIG. 6 is used, so that the examination, for example by learning the use of 3 or 5 x-ray beams, which are arranged on a regular triangle or pentagon on the circle 41, can be accelerated .

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Claims (1)

SV Philips ¹ G! N .-, :: v. ■ - ■ '''■' - ■■■ '■' ■■ --'- T ^ PATENT CLAIMS : 1. Arrangement for computed tomography with an X-ray tracer for generating a flat fan-shaped X-ray beam and with an X-ray detector with a sequence of detection elements arranged on an arc of a circle, wherein the X-ray beam and X-ray detector about a common one directed perpendicularly to the plane of the fan beam Rotation axis are rotatable, characterized in that the distance between the radiation input surface of the detection elements from the axis of rotation at least almost equal to the distance of the focal spot of the X-ray source from the Is the axis of rotation and the X-ray tracer and the X-ray detector with at least almost the same angular velocity, but are rotatable in the opposite sense about the axis of rotation. 2. Arrangement according to claim 1, characterized in that the X-ray detector consists of a closed sequence of detection elements arranged on a circle around the axis of rotation.
3. Arrangement according to claim 1 or 2, characterized in that the arrangement contains a number of X-ray tractors set up on an arc of a circle. 4. Arrangement according to one of the preceding claims, characterized in that the arrangement is an electronic one Circuit (26) for the correction of the by changing the distance of the detection element from the X-ray tracer during the rotation about .the axis of rotation conditional Contains fluctuations in the measurement signals of a detection element, 809850 / 0684- 2822039 5. Arrangement according to one of the preceding claims, wherein a first the fan angle (ς ^) determining aperture arrangement (^ · 5) is provided, characterized by that
a second diaphragm arrangement (48) which translates with
the emitter (42) is moved with it, but not rotated about the axis of rotation (m) and its angular position or
Maintains alignment in space. 809850/0684