GB1603593A

GB1603593A - Device for computed tomography

Info

Publication number: GB1603593A
Application number: GB21386/78A
Authority: GB
Original assignee: Philips Gloeilampenfabrieken NV
Current assignee: Koninklijke Philips NV
Priority date: 1977-05-26
Filing date: 1978-05-23
Publication date: 1981-11-25
Also published as: BR7803296A; ES470148A1; NL7705788A; FR2391698A1; JPS53147493A; DE2822089A1; BE867413A; SE7805837L; CA1107407A; FR2391698B1; IT1095910B; JPS6251622B2; IT7823714A0

Description

(54) DEVICE FOR COMPUTED TOMOGRAPHY (71) We, N.V. PHILIPS' GLOEILAM PENFABRIEKEN, a limited liability Company, organised and established under the laws of the Kingdom of the Netherlands, of Emmasingel 29, Eindhoven, the Netherlands, 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 statement: The invention relates to a device for computed tomography, comprising an X-ray source for generating a flat, fan-shaped beam of X-rays, and an X-ray detector comprising a plurality of detector elements which are arranged along an arc of a circle, said X-ray source and X-ray detector being rotatable about a common axis of rotation which is directed at right angles to the plane of the fan-shaped beam.

A device of this kind is particularly suitable for X-ray diagnosis. During such an examination, a part of the body of a patient is irradiated by the flat, fan-shaped beam from different directions. Locally transmitted radiation is measured and, using the measurement data thus obtained, a computer calculates the density distribution of the part of the body of the patient in the irradiated plane, the result being displayed, for example. on a television monitor.

A device of the described kind is known from Netherlands Patent Application No.

7,503,520 laid open to public inspection.

This specification describes a device in which use is made of a flat, fan-shaped beam of X-rays which completely encloses the part of the body to be examined in at least one direction. In order to obtain an adequate number of measurement data, the X-ray source and the X-ray detector rotate together about the patient who is arranged in the vicinity of a common axis of rotation.

The device comprises means to compensate for the effect exerted on the measurement signals by differences in the sensitivity of the various detector elements.

The X-ray source and the X-ray detector of the known device rotate together about the axis of rotation at a uniform speed during examination. The output signals of the detector elements are integrated over a short period of time in order to obtain reliable measurements, the X-ray source and the X-ray detector rotating through a small angle of, for example 1" during said period of time. Subsequently, the logarithms of the intensity measurements are determined. During examination, each detector element thus supplies a set of logarithms of the intensities of X-radiation transmitted by the patient in different directions.

During the examination, this data is stored in a first electronic memory and, after completion of an examination, the data originating from the different detector elements, is sorted into sets of logarithmic values relating to the intensity of Xradiation transmitted along sets of parallel paths oriented in corresponding directions.

The sorted sets are stored in a second electronic memory. Subsequently, the local density present at each defined representational picture point relating to the irradiated plane of the part of the body under examination, is calculated by means of a reconstruction technique, which means that the measurement data relating to all those measurement paths from the sorted sets which intersect the relevent representational picture point are summed.

In the known device, the occurrence of disturbing ring-like interference patterns in the display of the calculated density distribution, referred to as ring artefacts, is suppressed by compensation of the differences in sensitivity of the detector elements.

To achieve this, a fast movement of the X-ray source is separately superposed on the uniform, common rotary movement of X-ray source and X-ray detector, so that each measurement by each detector element is repeated by a neighbouring detector element during examination. On the basis of the measurements thus obtained, the detector elements are compared with each other and differences in sensitivity are compensated for; for this purpose, an additional electronic processing network is included.

The present invention has for an object to provide a device for computed tomography in which on the one hand the tendency for differences in the sensitivity of detector elements to become manifest by the occurrence of ring artefacts can be reduced, whereas on the other hand each detection element can provide, during an examination, a set of measurements of the intensity of the X-radiation transmitted in parallel directions. so that the sorting of the measurements after termination of the examination can he dispensed with.

According to the invention there is provided a device for computed tomography including an X-ray source for generating a flat, fan-shaped beam of X-radiation, said X-ray source having in operation a comparatively small X-ray emissive area which is substantially stationary relative to the body of the X-ray source, an X-ray detector comprising a plurality of detection elements arranged along an arc of a circle so as to receive radiation from said fan-shaped beam after passing through a body under examination, each said detector element having a comparatively small radiation-sensitive area, first and second rotatable supporting members each pivotally supported by stationary supporting means so as to be rotatable about a common axis directed at right angles to the plane of said fan-shaped beam, said X-ray source being fixedly mounted on said first rotatable supporting member so as to direct said fan-shaped beam generally towards said common axis, and said X-ray detector being fixedly mounted on said second rotatable supporting member so that the distance between the radiation-sensitive surface of each detector element and said common axis is substantially the same as the distance between the X-ray emissive area of the X-ray source and said common axis, and means for rotating said first and said second rotatable supporting members about said common axis at substantially the same angular speed but in opposite rotational directions. During operation, the output signals of the detection elements are integrated over a short period of time in order to achieve adequate measurement accuracy, the X-ray source and the X-ray detector being rotated through a small angle of, for example, 1" during said period of time.

Because the X-ray source and the X-ray detector move in opposite directions, the part of the body to be examined is scanned by the X-ray source and a given detector element in mutually substantially parallel, consecutive paths. This means that each calculation of the density in each picture point of the irradiated plane includes a measurement by each detector element. As a result, interference patterns in the display of the calculated density distribution, caused by differences in the sensitivity of the detector elements, can be substantially reduced. The said ring artefacts can also thus be reduced.

In a preferred embodiment of the device for computed tomography, the X-ray detector comprises a closed circular array of detector elements. As a result, sets of measurements of the intensity of the Xradiation transmitted in parallel directions can be determined for all directions in the plane of examination.

One embodiment of the invention will now be described by way of example, with reference to the accompanying diagrammatic drawings, of which: Figure 1 is a diagrammatic longitudinal sectional view of a device for computed tomography embodying the invention; Figure 2 is a diagrammatic cross-sectional view of the device shown in Figure 1, taken along the line II - II; Figure 3 shows a circuit diagram of a device for computed tomography embodying the invention; Figure 4 diagrammatically shows the position of the X-ray source and a detector element with respect to each other at successive instants; Figure 5 diagrammatically shows an X-ray detector which is particularly suitable for use as a detector element, and Figure 6 illustrates a scanning operation which can be performed by the device shown in the Figures 1 and 2.

Figures 1 and 2 are a diagrammatic longitudinal sectional view and a diagrammatic cross-sectional view, respectively, of the same device for computed tomography in which a patient 1, resting on patient table 2, is irradiated by a flat, fan-shaped beam of X-rays 3. The beam of X-rays 3 has an angular spread, referred to hereinafter as the fan angle a, of, for example, 30 in the plane of the drawing of Figure 2. and is comparatively flat in the direction perpendicularly thereto, its thickness being approximately 10 mm. The fan angle is sufficiently large for the beam 3 to span the entire patient 1 in the fan direction. The X-ray beam is generated by an X-ray tube 4, comprising a rotary anode (not shown).

Because the emissive surface of the rotary anode (the actual X-ray source) is comparatively small, its length and its width being approximately 2 mm, the X-ray source may be considered effectively as a point source.

The radiation transmitted by the patient is measured by an X-ray detector 5 which comprises a series of, for example, 400 detector elements 6 which are arranged on a circle. The detector elements 6, to be described hereinafter and to be considered effectively to be points because they are of comparatively small dimensions, comprise, for example, a scintillation crystal and a light detector. The X-ray tube 4 is mounted on a rotatable supporting member comprising a ring 7 which is journalled on wheels 10 which are secured to stationary supporting means comprising a housing 9. The ring 7 is rotatable, by means of a drive motor 12, about an axis 14 which extends at right angles to the plane of the fan-shaped beam 3. The actual X-ray source - the emissive region on the surface of the rotary anode - is rotated about the axis 14 so as to follow a circular path 15. The X-ray detector 5 is connected to a further rotatable supporting member comprising a ring 8 which is journalled on wheels 11 connected to the housing 9 and which is rotatable about the axis 14 by means of a drive motor 13. The ring 7 and the ring 8 are rotated in opposite directions during an examination. The detector elements 6 are displaced along a circular path, the radius of which is substantially equal to the radius of the circular path 15 along which the actual X-ray source is displaced; the radii are, for example, 90 cm and 85 cm, respectively. The processing of the measurement data obtained from the detector element 6, will now be described hereinafter with reference to Figure 3.

Figure 3 shows diagrammatically a device of the described kind, with a patient 20, an X-ray source 21 and an X-ray detector 22 which comprises a series of detector elements 23 which are arranged on a circle.

The X-ray source 21 and the detector elements 23 are rotated about the central axis (14 in Figure 1 but not shown in Figure 3) in opposite directions during an examination, so as to follow the same circular path 24 to a first approximation. All the detector elements are connected, even though this is shown for only three elements, to a corresponding integration circuit 25 in which the measurement signals of the detector elements 23 are integrated over a short period of tine of, for example, 10 ms in order to achieve adequate measurement accuracy.

Because the X-ray source 21 and the X-ray detector 22 move in opposite directions, the part of the body 20 under examination, is scanned in mutually substantially parallel, consecutive paths as shown in Figure 4, in which the position of the X-ray source 21 and one detector element 23 is indicated with respect to each other at consecutive instants ta, tb, tc, ... th. The X-ray source 21 and the X-ray detector 23 to a first approximation follow the same circular path 24 during the examination, so that the distance between the X-ray source and a detector element will change during the examination, with the result that the intensities measured by the detector elements will change during the examination, independently of the local absorption of the patient. These changes are corrected in the circuits 26. After correction, the logarithms of the measurement signals of the detection elements are formed by logarithmic amplifiers 27 and the resultant signals are stored in a memory 28.

Thus, during an examination each detector element 23 produces a set of logarithms of the intensities of X-radiation transmitted in substantially parallel directions through the patient. After completion of the examination, the computer 29 calculates, by means of a reconstruction technique, the distribution of local density in the irradiated part of the body 20; this is displayed, for example, on a television monitor 30.

Figure 5 shows diagrammatically an X-ray detector 31 which is particularly suitable for use as a detector element in the described device for computer tomography. The detector 31 comprises a cylindrical scintillator 32, having a diameter of, for example, 5 mm and a length of, for example, 20 mm, and a light detector 33 which is axially coupled thereto. The two beams of X-rays 34 and 35, diagrammatically shown and incident on the detector from different directions, are detected in the same manner due to the cylindrical-symmetry thereof. This property renders the detector particularly suitable for said application, because the direction of the X-radiation incident on a detector element continuously changes during the examination. This change is independent of the local absorption of the patient and, therefore, it should not affect influence the measurement signal.

A form of scanning operation which can be performed with the described device, will now be described with reference to Figure 6.

Therein, the circle 41 denotes the path to be followed by a point X-ray source 42 and an X-ray detector 43 comprising a series of detector elements 44. The position of the X-ray source 42 is shown at three instants, i.e. at the beginning of the examination t = 0, at the end of the examination t = T, and at an instant therebetween. The position of the detector 43 is shown at t = 0. During the examination, the source moves with a uniform angular velocity co about the common central axis through M, and the total angular displacement of the source and of the detector is o.T. The fan angle a of the X-ray beam is determined by a diagrammatically shown diaphragm 45 which moves with the source so that the centre of the beam of X-rays is always directed towards the centre M of the circle 41. During the examination, the detector elements produce sets of measurements of the intensity of X-radiation transmitted by the patient in mutually parallel directions, the direction being situated between the diagrammatically shown X-rays 46 and 47. These two rays enclose the examination angle p. In the triangle determined by the source positions at the instants t = 0 and t = T and the intersection of the rays 46 and 47, it can be seen that o T = a + p. Because radiation is measured only in mutually parallel directions situated between the extreme rays 46 and 47, the X-ray beam is shielded, except for the ray 46 at the instant t = 0, and except for the ray 47 at the instant t = T, by a further diaphragm 48 which moves with the source. As a result, the radiation dose to which the patient is exposed is substantially reduced. The diaphragm performs only a translatory movement, from left to right in Figure 6, so that the aperture angle (3is equal to 0, as can be readily seen. During the examination, the detector 43 follows the circular path 41 in the indicated direction at an angular velocity to. During the time T, the detector element which measures the ray 47 at the instant t = T, completes the angle co T. In order to enable the examination to be performed, a detector is required whose detection angle 0 equals .T+ .T + A, for which A = 360 - w. T - 2(180-a - A) = w.T - 2a (as can be seen in Figure 6), so that H = 2 (co.T - a) = 2 p.

The foregoing illustrates that, in order to perform an examination scan through PI = 1800 using a fan angle a = 60 , the X-ray source and the detector must be rotated through an angle co.T = 240 in opposite directions, a detector having a detection angle 0 = 360" then being required. A moving diaphragm then preferably has an aperture angle = 1800. It is to be noted that, if the detection angle n = 360 , the examination angle is increased to = 360" by using each detector element 43 twice, o).T then being 420".

For examination angles p of 1800 and more, the X-ray tube 4 and the X-ray detector 5 of Figures 1 and 2 cannot move in the same circular path in practice, but they can move in circular paths whose radii only differ by a few centimeters. During an examination, each detection element then supplies - for the same angular speed veloc ity of both the source and the detector - a set of measurements of the intensity of the radiation transmitted by the patient in a number of directions which are not exactly mutually parallel. This non-parallelity is so small for a fan angle of approximately 30 and a length of the radii of the circular paths of approximately 100 cm, that the effect thereof on the calculation of local densities in the irradiated plane, is negligibly small.

Correction, if desired, is possible by making the X-ray source, moving in the smaller circular path, rotate slightly faster than the X-ray detector.

For a fan angle a = 30 , only an angle of 60 of the detection elements 44 of Figure 6 is being used at any instant, so that the examination can be accelerated by using, for example, three or five X-ray sources which are arranged on the circle 41 according to a regular triangle or pentagon.

WHAT WE CLAIM IS: 1. A device for computed tomography including an X-ray source for generating a flat, fan-shaped beam of X-radiation, said X-ray source having in operation a comparatively small X-ray emissive area which is substantially stationary relative to the body of the X-ray source, an X-ray detector comprising a plurality of detection elements arranged along an arc of a circle so as to receive radiation from said fan-shaped beam after passing through a body under examination, each said detector element having a comparatively small radiation-sensitive area, first and second rotatable supporting members each pivotally supported by stationary supporting means so as to be rotatable about a common axis directed at right angles to the plane of said fan-shaped beam, said X-ray source being fixedly mounted on said first rotatable supporting member so as to direct said fan-shaped beam generally towards said common axis, and said X-ray detector being fixedly mounted on said second rotatable supporting member so that the distance between the radiation-sensitive surface of each detector element and said common axis is substantially the same as the distance between the X-ray emissive area of the X-ray source and said common axis, and means for rotating said first and said second rotatable supporting members about said common axis at substantially the same angular speed but in opposite rotational directions.

2. A device for computed tomography as claimed in Claim 1, wherein said X-ray detector comprises a closed circular array of detector elements.

3. A device for computed tomography as claimed in Claim 1 or Claim 2, including a plurality of said X-ray sources mounted on said first rotatable supporting member in a circular arc about said common axis.

4. A device for computed tomography as claimed in any one of the preceding Claims, including an electrical circuit for correcting the output signal provided by each detector element so as to compensate for the variation in the distance between that detector element and the or a corresponding said X-ray source during said rota

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. X-rays is always directed towards the centre M of the circle 41. During the examination, the detector elements produce sets of measurements of the intensity of X-radiation transmitted by the patient in mutually parallel directions, the direction being situated between the diagrammatically shown X-rays 46 and 47. These two rays enclose the examination angle p. In the triangle determined by the source positions at the instants t = 0 and t = T and the intersection of the rays 46 and 47, it can be seen that o T = a + p. Because radiation is measured only in mutually parallel directions situated between the extreme rays 46 and 47, the X-ray beam is shielded, except for the ray 46 at the instant t = 0, and except for the ray 47 at the instant t = T, by a further diaphragm 48 which moves with the source. As a result, the radiation dose to which the patient is exposed is substantially reduced. The diaphragm performs only a translatory movement, from left to right in Figure 6, so that the aperture angle (3is equal to 0, as can be readily seen. During the examination, the detector 43 follows the circular path 41 in the indicated direction at an angular velocity to. During the time T, the detector element which measures the ray 47 at the instant t = T, completes the angle co T. In order to enable the examination to be performed, a detector is required whose detection angle 0 equals .T+ .T + A, for which A = 360 - w. T - 2(180-a - A) = w.T - 2a (as can be seen in Figure 6), so that H = 2 (co.T - a) = 2 p. The foregoing illustrates that, in order to perform an examination scan through PI = 1800 using a fan angle a = 60 , the X-ray source and the detector must be rotated through an angle co.T = 240 in opposite directions, a detector having a detection angle 0 = 360" then being required. A moving diaphragm then preferably has an aperture angle ss = 1800. It is to be noted that, if the detection angle n = 360 , the examination angle is increased to = 360" by using each detector element 43 twice, o).T then being 420". For examination angles p of 1800 and more, the X-ray tube 4 and the X-ray detector 5 of Figures 1 and 2 cannot move in the same circular path in practice, but they can move in circular paths whose radii only differ by a few centimeters. During an examination, each detection element then supplies - for the same angular speed veloc ity of both the source and the detector - a set of measurements of the intensity of the radiation transmitted by the patient in a number of directions which are not exactly mutually parallel. This non-parallelity is so small for a fan angle of approximately 30 and a length of the radii of the circular paths of approximately 100 cm, that the effect thereof on the calculation of local densities in the irradiated plane, is negligibly small. Correction, if desired, is possible by making the X-ray source, moving in the smaller circular path, rotate slightly faster than the X-ray detector. For a fan angle a = 30 , only an angle of 60 of the detection elements 44 of Figure 6 is being used at any instant, so that the examination can be accelerated by using, for example, three or five X-ray sources which are arranged on the circle 41 according to a regular triangle or pentagon. WHAT WE CLAIM IS:

1. A device for computed tomography including an X-ray source for generating a flat, fan-shaped beam of X-radiation, said X-ray source having in operation a comparatively small X-ray emissive area which is substantially stationary relative to the body of the X-ray source, an X-ray detector comprising a plurality of detection elements arranged along an arc of a circle so as to receive radiation from said fan-shaped beam after passing through a body under examination, each said detector element having a comparatively small radiation-sensitive area, first and second rotatable supporting members each pivotally supported by stationary supporting means so as to be rotatable about a common axis directed at right angles to the plane of said fan-shaped beam, said X-ray source being fixedly mounted on said first rotatable supporting member so as to direct said fan-shaped beam generally towards said common axis, and said X-ray detector being fixedly mounted on said second rotatable supporting member so that the distance between the radiation-sensitive surface of each detector element and said common axis is substantially the same as the distance between the X-ray emissive area of the X-ray source and said common axis, and means for rotating said first and said second rotatable supporting members about said common axis at substantially the same angular speed but in opposite rotational directions.

tion of said first and said second rotatable supporting members about said common axis.

5. A device for computed tomography substantially as herein described with refer ence to the accompanying drawings.