GB1602521A

GB1602521A - Arrangement for producing an image of a body section using gamma or x-radiation

Info

Publication number: GB1602521A
Application number: GB11670/78A
Authority: GB
Original assignee: Philips Gloeilampenfabrieken NV
Current assignee: Koninklijke Philips NV
Priority date: 1977-03-28
Filing date: 1978-03-23
Publication date: 1981-11-11
Also published as: JPS53120294A; BR7801844A; AU513395B2; AU3466278A; SE7803371L; FR2386055B1; DE2713581A1; DE2713581C2; CA1101133A; AU4272278A; AU526138B2; IT1093720B; FR2386055A1; IT7821630A0; ES468229A1; BE865386A; NL7803229A

Description

(54) AN ARRANGEMENT FOR PRODUCING AN IMAGE OF A BODY SECTION USING GAMMA OR X-RADIATION (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 an arrangement for the production of an image of a section of a body including at least one gamma or X-radiation source for generating a primary beam limited to a small cross-section and directing said beam through the body, a detector array which is disposed outside the primary beam and which receives a part of the scattered radiation produced in the body by the primary beam, and a diaphragm device between the body and the detector array.

The magazine "Phys. med. Bio." 1959 (4), pages 159 to 166 describes an arrangement in which a gamma or X-ray beam of small cross-section is passed through the body under examination. The scattered radiation then produced is measured by a scintillator crystal in conjunction with a photomultiplier, which is arranged behind a collimator, which is focussed at a point in the primary radiation beam, so that only the scattered radiation originating from this point, which is dependent on the object density in said point, is received.

This arrangement enables the density of the object in a planar section to be measured when first of all the object (or the collimator with the photomultiplier) is moved in the direction of the radiation beam, so that the scintillator with the photomultiplier scans the density distribution in the radiation beam. Subsequently, the body and radiation source are shifted relative to each other, so that the radiation is passed through a different cross-sectional region of the object, the described scanning operation being repeated. This arrangement demands much time for measuring the intensity distribution in a planar section.

The magazine 'Phys. med. Biol" 1974, Vol. 19, No. 6, pages 808 and further also describes an arrangement in which thescattered radiation produced along the primary beam path is measured by a crystal which supplies an output signal which is dependent on the energy of the incident radiation. As is known, the energy of the scattered radiation incident on the crystal, will depend on the angle between the scattered radiation direction and the direction of the primary radiation (compton effect). Any arbitrary amplitude of the crystal detector, which corresponds to a specific energy, consequently also corresponds to a specific angle or to a specific point along the path of the primary beam.

By means of a pulse height analyser and a suitable computer it is thus possible to reconstruct the density distribution along the path of the primary beam in a single measurement. This arrangement is therefore highly complex. It has a limited spatial resolution and requires the use of a radiation source which generates monoenergetic radiation, i.e. for example an isotope; an X-ray tube cannot be employed as the radiation source.

Finally, it is known to employ a gamma camera for the detection of scattered radiation, spatial allocation being obtained by means of a collimator having a corresponding number of apertures which are directed at different points along the primary beam.

The spatial resolution of such a collimater is very low and the complete arrangement is comparatively insensitive, so that relatively high doses would be necessary for an examination.

It is an object of the invention to provide an arrangement for the production of an image of a section which can be non-planar of a body, having an improved spatial resolution and a satisfactory sensitivity.

According to the invention there is provided an arrangement for the production of an image of a section of a body, including at least one gamma or X-radiation source for generating a primary beam limited to a small cross-section and directing said beam through the body, a detector array which is disposed outside the primary beam and which receives a part of the scattered radiation produced in the body by the primary beam, and a diaphragm device between the body and the detector array, wherein the diaphragm device is provided with an elongate aperture whose longitudinal dimension extends in a direction which is substantially perpendicular to a plane containing the primary beam, and the detector array comprises a plurality of adjacent detectors arranged in such a way beyond said aperture with respect to said beam, that the radiation scattered by each successive elemental region of the body located along the path of the primary beam in the region under examination and passing through the elongate aperture is respectively limited by said diaphragm device so as to be directly incident on a corresponding said detector.

The elongate aperture in the diaphragm device located between the detectors and the body under examination, ensures that each detector can only receive scattered radiation produced in a specific elemental section of the primary beam, in such a way that each detector corresponds to a different section and all detectors together cover the part of the primary beam which passes through the region under examination.

In accordance with an embodiment of the invention the detectors are strip-shaped. the longitudinal dimension of a detector and the longitudinal dimension of the elongate aperture being respectively co-planar. As a result of this the sensitivity of the arrangement is increased substantially, without loss of spatial resolution, because the spatial resolution only depends on the dimensions of the aperture and of the detectors in a direction parallel to the primary beam.

In the extreme case each detector could concentrically surround the primary beam, yielding the additional advantage that the detector output signal would become substantially independent of the position of the primary beam relative to the body being examined. In accordance with a further embodiment of the invention the lastmentioned effect can also be obtained in that a plurality of groups of detectors are each provided with a slot diaphragm and that the output signals of the detectors of different groups which are disposed in the same plane normal to the primary beam are superimposed on each other.

In the body under examination the primary beam is attenuated both by scattering and by absorption, so that - for the same density of the tissue through which the primary beam is passed - the intensity of the scattered radiation on the side which faces the radiation source will be higher than on the side which is remote from the radiation source. In accordance with a further embodiment of the invention this can be avoided in that two radiators are arranged on either side of the region to be examined in such a way that their limited beams coincide.

A different further embodiment of the invention is characterized by a detector which is disposed in the primary beam for measuring the intensity of the beam which has been attenuated by the body and which has been limited and directed by the facing radiation source. The output signal of the detector which is disposed in the primary beam can be employed for correcting the output signals of the detectors used for measuring the scattered radiation.

An embodiment of the invention will now be described by way of example, with reference to the accompanying drawings of which Figure 1 schematically shows a perspective view of an embodiment of the invention, Figure 2 schematically shows a vertical axial section through the arrangement which is shown in perspective in Figure 1, and Figure 3 shows a schematic arrangement which enables the output signals obtained by means of the detectors to be converted directly into a visual image.

The body 1 to be examined is located on a table top 2 and a horizontal primary beam 3 is passed through it, which beam is produced by two X-ray tubes 4a and 4b arranged on either side of the body 1 and is limited by a diaphragm device 5a and Sb respectively (Figure 2). The dimensions of the limited primary beam determine the resolution of the arrangement; the resolution will increase as the beam cross-section is decreased.

The voltage applied to the X-ray tube during an examination is approximately 350 kV. Thus, the radiation dose administered to the patient is minimized, while the attenuation of the primary beam as a result of (photo) absorption is low in comparison with the attenuation by (Compton) scattering.

The scattered radiation produced in the region of the body through which the primary beam 3 is passed, reaches a detector group D and D' which, respectively comprise a plurality of detectors d1, d2, d3, and d1', d2', d3' which are arranged in line adjacent one another parallel to the primary beam) via the elongate, preferably adjustable apertures 7 and 7' of a slot diaphragm 6 and 6' respectively disposed underneath and above the body under examination as shown in Figure 1. The surface area of each detector, which serves as a radiation sensing and measuring surface, has the shape of an oblong rectangle, whose long sides are disposed in a plane parallel to the primary beam which is at right angles to an axial plane containing the primary beam. The rectangular aperture 7 or 7' of the respective diaphragm 6 or 6' has a corresponding shape whose dimensions in both directions are smaller as the ratio of the distance of the diaphragm from the primary beam to the distance from the detectors to the primary beam.

The detectors may for example be chambers filled with a pressurized highly radiation-absorbing inert gas (xenon) in which two parallel electrodes are arranged, which carry off the charge carriers which have been generated by ionization caused by the scattered radiation. Such detectors are for example described in German Offenlegungsschrift 26 24 448.

Owing to the slot diaphragm an unambiguous relationship is obtained between a point on the primary beam and a detector within the detector group D or D'. As is shown in Figure 2, the cone of scattered radiation which is produced in the body from a point 8 on the primary beam and which is limited by the respective slot diaphragm 6 or 6', is incident on the respective detector di or dj' and accordingly each point within the limits of the primary beam represented by the dashed lines 10 corresponds to a detector in each of the two detector groups, both detectors being disposed on the same plane normal to the primary beam. The output signals of the detectors dl, d2, d3 and d1,, d2', d3, respectively are a measure of the density of the region of the body 1 under examination through which the primary beam 3 is passed.

The output signals supplied by the detectors will tend to increase accordingly as the width of the slot apertures 7 and 7' in the diaphragms 6 and 6' respectively are increased. On the other hand the spatial resolution will improve accordingly as the width of the slot is decreased. A satisfactory compromise is obtained when the width s of the slot apertures 7 and 7' (i.e. the dimension in a plane which contains the primary beam) satisfies the equation s = wb/(a + b), where w represents the width of a detector (i.e. its dimension in a direction parallel to the primary beam), a is the distance from the detector group D or d' to the slot diaphragm 6 and 6' respectively, and b is the distance from the slit diaphragm 6 and 6' respectively to the primary beam 3. The spatial resolution is then substantially in conformity with the expression wb/a.

As previously stated, it suffices in principle to employ only one radiation source for generating a primary beam, but in the case of generation by two radiation sources.

which are arranged as shown, a more uniform intensity distribution along the direction of the primary beam can be obtained. Similarly, it would also suffice to use a single detector group for measuring the scattered radiation produced in the primary beam between the limits 10, 10', however, when two detector groups are employed an improved signaI-to-noise ratio can be obtained and moreover the sum of the output signals of a detector pair (for example d3, d31) will tend to be less dependent on the position of the primary beam in the object than in the case where only one detector group is used.

It is not absolutely necessary that the connecting lines between the centres of the slot and the centre of the detectors should form right angles with the primary beam, as is shown in Figure 2. The detector groups and associated slots may rather be shifted in a horizontal direction in comparison with the arrangement shown in Figure 2. Nor is it necessary that the plane which is formed by the detector arrays D and D' respectively should extend parallel to the primary beam 3. - It is merely important that the dimension s of the slot in the plane containing the one primary beam should be small and that the detectors should be arranged so that they can measure the scattered radiation which is produced by the primary beam in the region under- examination 10 - 10' and which passes through the slot.

As previously stated, each detector group provides information on the scattering properties of the object (i. e. in particular the average electron density) along the line which is defined by the position of the primary beam in the object. The individual detector groups provide information on individual elemental sections (cells) along these lines.

By arranging a relative displacement between an assembly comprising the primary beam source the detectors and the slot diaphragm, and the body under examination, the density distribution along a different line 13 or 14 (represented by an interrupted line) in the body can be measured, and from a plurality of such measurements the density distribution in a planar section or in a different region of the body can be determined. Between two successive measurements, the output signals of the detectors should either be stored or applied to a display device.

Each successive relative displacement between the primary beam and the body 1 under examination, can be provided by displacing the table top 2 through approximately 2 mm, i.e. a travel corresponding to the width of the primary beam and to the spatial resolution, in a direction at right angles to the primary beam. Forms of patient examination table, whose table top can be moved through a defined path by means of a motor drive, are generally known in X-ray technology, in particular tomography, and will therefore not be shown in further detail. After the next beam path, which has thus been set up has been measured and the measurement values stored or applied to a display device, the table top is once more displaced by the same amount in the same direction and the measurement repeated.

It is alternatively possible to move the table top in one direction with constant speed and to sample the signals from the detectors at regular time intervals. The speed of movement should then substantially correspond to the quotient of the width of the primary beam and the time necessary to obtain each measurement. By comparison with the stepwise movement, this continuous movement has the advantage that accelerations of the body, which may give rise to blurring and thus to an unsharp image representation, can be avoided.

The relative movement between the primary beam and the body under examination can be performed in any desired direction at right angles to the primary beam. For example (when the force of gravity is directed normal to the plane of the drawing), the table top 2 may be disposed parallel to the plane of drawing and may be moved perpendicularly thereto.

Figure 3 shows schematically a simple embodiment of an image generating arrangement. For each detector pair (for example, d1 and dl' or d3 and d3') there is provided a sample-and-hold circuit S1, S2, S3 which stores the sum of the two output signals or the geometric mean or the root mean square of output signals of a detector pair at the end of a measurement period. The stored signals are consecutively applied to the Wehnelt cylinder 111 of an image storage tube 110, as is schematically indicated by the selector switch 112 which consecutively connects the outputs of the sample-and-hold circuits to the Wehnelt cylinder 111. A deflection generator 113, which feeds the horizontal deflection coil 114, supplies a deflection signal which changes stepwise in synchronism with the switching operation, so that a line is displayed on the image storage tube 11 when the density distribution of the next line is being measured. The cross-section of the electron beam of the image storage tube 11 should then correspond to the dimensions of a cell on the target of the image-storage tube.

Before the next line begins the current which flows through the vertical deflection coil 115 of the image storage tube is also varied by one step by the vertical deflection generator 116, in such a way that the electron beam is moved in a vertical direction by the width of one cell. Thus, the density distribution measured in the object 1 under examination, is stored on the target in the image storage tube 11. Subsequently, it can be read out and displayed on a display tube.

Instead of an image storage tube which has a very limited gray scale, it is possible to make direct use of a display tube, in which the density distribution is written timesequentially. By means of a photographic camera, whose shutter is open during the time needed for writing all the lines, a (photographic image of the density distribution in the scanned planar section of the body can be provided.

In this respect it is to be noted that apparatus for computed tomography is known (see for example German Offenlegunsschrift 19 41 433), which also enables a reconstruction of the density distribution in a planar section of a body to be provided. In such apparatus, however, it is not the scattered radiation which is measured, but the intensity of the primary beam beyond the object. Such apparatus requires the use of an expensive computer, which calculates the density distribution from the detector output signals, because a detector output signal does not represent the density at a specific point in the planar section, but the integral of the density along a straight line in said plane and crossing the body section. In an arrangement in accordance with the present invention such a computer is not necessary.

The dose which is administered to a patient during an examination with an arrangement embodying the invention and with the known arrangement, will be of approximately the same order of magnitude. In computed tomograph apparatus, the primary beam is measured directly, whereas in embodiments of the present invention only a small portion of the emergent scattered radiation is measured, so that - in order to obtain equal detector output signals - the intensity of the primary beam would have to be increased substantially in the case of embodiments of the present invention, but this is compensated for because in the case of such embodiments the body must be exposed only once whereas with a computed tomography arrangement the planar section has to be exposed to radiation approximately 180 times from different directions, and because the present invention enables the use of substantially harder radiation, which is scarcely attenuated by photoabsorption.

An advantage of the known arrangement is that it enables a quantitative representation of the density distribution in the planar slice to be obtained, while in the case of embodiments of the invention - if no additional correction steps are taken - only one qualitative representation is possible, which however in most cases is satisfactory. The departure from the correct values of the detector output signals, has causes which depend on the geometry of the measuring arrangement and causes which may be attributed to the different attenuation of the primary radiation and the of scattered radiation in the object, and to the fact that scattered radiation on its way to the detector may be further scattered several times under certain conditions, so that the assumed relationship between the point from which the scattered radiation originates and the detector which receives the measuring signal is not fulfilled for all the detected radiation.

However, it is possible to correct the measurement values obtained with an arrangement embodying the invention by means of a digital computer, which enables the geometric factors to be corrected by suitable weighting of the detector output signals which is dependent on the geometry of the arrangement but not on the body under examination. Measurement errors as a result of attenuation of the primary and the scattered radiation in the body can also be corrected if allowance is made for the attenuation of the radiation along the line path followed by the primary beam and the scattered radiation respectively.

If it is for example assumed that first of all the line path of the planar body-section is scanned from which scattered radiation originates which reaches the detector array without attenuation by interposed tissue, the scattered radiation originating from the first cell (elemental section) of said line will not yet be subject to any attenuation and may therefore be used directly as a measure of the density in this cell. The primary beam which reaches the second cell of said line will have been is attenuated by the amount which has been converted into scattered radiation in the first cell, and because this amount is known from the measurement of the first cell, allowance can be made for this by accordingly increasing the output signal of the detector associated with the second cell in comparison with the output signal of the detector associated with the first cell.

For the third cell of said line allowance should then be made for the attenuation by the two first cells etc. - It is true that for the first cell of the next line the primary beam will again not be attenuated, but the scattered radiation from said cell will be attenuated by the cells of the preceding line disposed between the diaphragm slot and said cell. As the attenuation of radiation through these cells has already been determined during the preceding measurement, the measurement value corresponding to the first cell of the second line can be corrected accordingly. With the output signal of the detector which measures the scattered radiation obtained from the second cell of the second line, allowance must then be made for both the attenuation of the primary beam by the adjacent first cell and the attenuation of the scattered radiation by the cells of the line situated adjacent thereto.

If the attenuation of the primary beam along one line path has been assessed correctly by means of this correction, the attenuation factor for the primary radiation which has been measured indirectly by measurement of the scattered radiation, should be in conformity with the attenuation factor which is obtained when the intensity of the primary beam before entrance into the body (for example known by means of a measurement) is compared with its intensity measured by a detector after passage through the body. In the event of a discrepancy the attenuation values determined for the individual cells should be changed accordingly. Such a detector for measuring the primary beam which has been attenuated by the body is also necessary in the case of an arrangement with two radiation sources. This detector is designated 12 in Figure 2 and has a bore through which the beam produced by the radiation source 4A, 5a is passed without giving rise to an output signal from the detector 12, while the beam which has been generated and limited by the radiation source 4b, Sb will be incident on the sensitive surface of the detector, because this beam will inevitably have been widened during its passage through the body.

Errors in the measurement result resulting from multiple scattering, can be avoided in the case of radiation sources producing mainly monoenergetic radiation (radio isotopes), in that each detector only measures that part of the incident radiation whose wavelength has the value to be anticipated from the wavelength of the primary beam and the given scatter angle. This can be effected in known manner by employing (crystal) detectors followed by suitable amplitude discriminators.

In the case of an X-ray source errors resulting from multiple scattering can be reduced by subtracting the average value of output signals provided by additional detectors, not shown, from the detector output signals, said additional detectors being arranged in such a way that they cannot receive the scattered radiation produced in the primary beam 3 between the limits 10, 10' but only radiation scattered by other regions of the body as a result of multiple scattering.

WHAT WE CLAIM IS: 1. An arrangement for the production of an image of a section of a body, including at least one gamma or X-radiation source for generating a primary beam limited to a small cross-section and directing said beam through the body, a detector array which is disposed outside the primary beam and which receives a part of the scattered radiation produced in the body by the primary beam, and a diaphragm device between the body and the detector array, wherein the diaphragm device is provided with an elongate aperture whose longitudinal dimension extends in a direction which is substantially perpendicular to q plane containing the primary beam, and he detector array comprises a plurality of adjacent detectors arranged in such a way beyond said aperture with respect to said beam, that the radiation scattered by each successive elemental region of the body located along the path of the primary beam in the region under examination and passing through the elongate aperture is respectively limited by said diaphragm device so as to be directly incident on a corresponding said detector 2. An arrangement as claimed in clain 1. wherein the detectors are strip-shaped.

the longitudinal dimension of each detector and the longitudinal dimension of the elongate aperture being co-planar.

3. An arrangement as claimed in claim 1 or 2, wherein a plurality of groups of detectors are provided each associated with a corresponding elongate aperture in a diaphragm, and the output signals from corresponding detectors in the different groups, which detectors are disposed in the same plane at right angles to the primary beam, are superimposed on one another.

4. An arrangement as claimed in any one of claims 1 to 3* wherein two radiation sources are arranged one on either side of the region under examination in such a way that their limited beams coincide.

5. An arrangement as claimed in any one of claims 1 to 4, including a further detector which is disposed in the path of the primary beam for measuring the intensity of the beam limited and directed by the facing radiation source, after it has been attenuated by the body.

6. An arrangement for producing an image of a body section substantially as herein described with reference to the accompanying drawings.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. output signals provided by additional detectors, not shown, from the detector output signals, said additional detectors being arranged in such a way that they cannot receive the scattered radiation produced in the primary beam 3 between the limits 10, 10' but only radiation scattered by other regions of the body as a result of multiple scattering. WHAT WE CLAIM IS:

1. An arrangement for the production of an image of a section of a body, including at least one gamma or X-radiation source for generating a primary beam limited to a small cross-section and directing said beam through the body, a detector array which is disposed outside the primary beam and which receives a part of the scattered radiation produced in the body by the primary beam, and a diaphragm device between the body and the detector array, wherein the diaphragm device is provided with an elongate aperture whose longitudinal dimension extends in a direction which is substantially perpendicular to q plane containing the primary beam, and he detector array comprises a plurality of adjacent detectors arranged in such a way beyond said aperture with respect to said beam, that the radiation scattered by each successive elemental region of the body located along the path of the primary beam in the region under examination and passing through the elongate aperture is respectively limited by said diaphragm device so as to be directly incident on a corresponding said detector

2. An arrangement as claimed in clain 1. wherein the detectors are strip-shaped.