More specifically, the invention relates to apparatus for planar beam radiography using a planar radiation beam produced of ionizing radiation emanating from a line-like radiation source, and to methods of aligning a detector with respect to a line- like radiation source.

DESCRIPTION OF RELATED ART AND BACKGROUND OF THE INVENTION Gaseous-based ionizing radiation detectors, in general, are very attractive at low photon energies since they are cheap to manufacture, and since they can employ gas multiplication to strongly amplify the signal amplitudes. A particular kind of gaseous detector is the one, in which electrons released by interactions between photons and gas atoms can be extracted in a direction essentially perpendicular to the incident radiation.

Such detector is typically a line detector provided with an elongated narrow radiation entrance slit, which collimates incident radiation from a point-like radiation source into a planar beam of radiation. A two-dimensional image is then typically obtained through scanning. Optionally, a further radiation beam-limiting device, parallel with the detector radiation entrance slit, is arranged between the radiation source and the detector.

One problem encountered when using such detectors is the difficulties obtained in aligning the detector, and possibly the radiation beam-limiting device, with respect to the radiation

source. Such alignment has to be very accurate and precise and any misalignment results inevitably in the need of realignment.

Typically the detector and/or the radiation beam-limiting device have/has to be aligned prior to each single measurement, which is both costly and time-consuming. Further, particular alignment means, such as e. g. optical alignment devices, may have to be provided.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide an apparatus for planar beam radiography, which can be easier and faster aligned than prior art apparatus.

It is in this respect a particular object of the invention to provide such apparatus for planar beam radiography that provides for faster and thus cheaper measurements.

A further object of the present invention is to provide such apparatus for planar beam radiography, which is effective, accurate, reliable, and which can be implemented in a simple and cost effective way.

Still a further object of the invention is to provide a method of aligning an ionizing radiation detector with respect to a radiation source, which is easier and faster than prior art alignment techniques.

These objects among others are attained by apparatus and methods as claimed in the appended claims.

Further characteristics of the invention and advantages thereof will be evident from the following detailed description of preferred embodiments of the invention, which are shown in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will become more fully understood from the detailed description of embodiments of the present invention given hereinbelow and the accompanying Figs. 1-4, which are given by way of illustration only, and thus are not limitative of the invention.

Fig. 1 illustrates schematically, in a cross-sectional side view with detector parts cut-away, an apparatus for planar beam radiography according to a first embodiment of the present invention.

Fig. 2 illustrates schematically, in a cross-sectional top view with detector parts cut-away, the apparatus of Fig. 1.

Fig. 3 illustrates schematically, in a cross-sectional side view with detector parts cut-away, an apparatus for planar beam radiography according to a second embodiment of the present invention.

Fig. 4 illustrates schematically, in a cross-sectional top view with detector parts cut-away, the apparatus of Fig. 3.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS With reference to Figs. 1 and 2, which schematically illustrate, in cross-sectional side and top views, respectively, with detector parts cut-away, an apparatus for planar beam radiography, a first embodiment of the present invention will be described.

The apparatus for planar beam radiography comprises a radiation detector 9 including a substantially planar cathode 23 and anode 25, respectively, between which a voltage is capable of being applied, and an ionizable substance 27 arranged between cathode 23 and anode 25. The electrodes 23 and 25 and the ionizable substance 27 are typically confined in a casing 17. Further, a

radiation beam-limiting device 20 having an elongated radiation transparent slit 21 extending essentially parallel with the electrodes 23 and 25 such that radiation can enter sideways between and in parallel with the electrodes. The device 20 can for instance be a thin collimator or similar, the purpose of which being to limit the radiation beam and thus the radiation exposed to the detector, and not necessarily to collimate the beam completely. A divergent planar beam into the detector apparatus is really not a problem. The divergent rays have a high probability of being passed through the electrode surfaces and will then not be absorbed in the ionizable substance 27 and will thus not influence the signals detected. Further, the device 20 may improve the spatial resolution of the signals detected as will be explained in detail below.

The ionizable substance 27 can be a gas or gas mixture comprising for example 90% krypton and 10% carbon dioxide or for example 80% xenon and 20% carbon dioxide. The gas can be under pressure, preferably in a range 1-20 atm. Thus, casing 17 is preferably a gas tight casing provided with a radiation entrance window of a radiation transparent material. Alternatively, ionizable substance 27 is a semiconducting material such as e. g. silicon or a higher Z semiconducting material.

Anode 25 includes a plurality of elongated conductive pads 26 arranged side by side, which is best seen in Fig. 2. Pads 26 are preferably arranged electrically insulated from each other on a dielectric substrate. Anode 25 constitutes also a read-out arrangement of the detector and thus conductive pads 26 constitute read-out elements for one-dimensional mapping of electrons drifted or accelerated towards the anode 25.

Alternatively, a separate read-out arrangement is provided or a separate read-out arrangement may be arranged in vicinity of anode 25, in vicinity of cathode 23, or elsewhere. Typically, such read-out arrangement is separated from the electrode by a dielectric layer, or similar.

It shall be appreciated that the read-out elements are arranged in a way to compensate for the divergence of any incoming radiation. Thus, the read-out elements may be arranged in a fan- like configuration, wherein each of the elements is aiming at the radiation source of the incoming radiation.

Further, the read-out arrangement is connected to a signal- processing device (not illustrated) for necessary and/or desired post-processing of collected signal data. Preferably, the read- out elements 26 are then separately connected to the signal processing circuit by means of individual signal conduits. A signal display unit (neither illustrated) is provided for displaying the processed signal data.

In operation, a sheet of radiation is entered into the ionizable substance through slit 21 of radiation beam-limiting device 20, said radiation ionizing the ionizable substance 27. A voltage is applied between cathode 23 and anode 25 resulting in electrical field, which causes electrons released from ionization (through primary and secondary reactions) to drift towards the anode 25.

Correspondingly produced positive charge carriers (i. e. ions or holes) are drifted towards the cathode 23. The electrons induce electric pulses in the anode or read-out elements 26, which are individually detected as each read-out element has its individual signal conduit to the signal processor. The signal processing electronics processes then the pulses; it possibly shapes the pulses, and integrates or counts the pulses from each readout element. Correspondingly, the positive charge carriers induce pulses that may alternatively, or additionally, be detected.

It shall be appreciated that the electric field between the electrodes may be high enough to cause electron multiplication, i. e. avalanche amplification, of the released electrons to thereby provide for strong signals having large signal-to-noise ratios. Any kind of avalanche electrode arrangement may

optionally be provided between the electrodes 23,25 to facilitate or provide for the electron multiplication functionality.

By providing a one-dimensional array of read-out elements 26 a radiation detector is obtained, wherein charges carriers derivable mainly from ionization by transversely separated portions of the incident radiation sheet are separately detectable. Hereby, the detector provides for one-dimensional imaging.

In order to more easily align the detector 9 the apparatus for planar beam radiography comprises a line-like ionizing radiation source 13, preferably an X-ray source, oriented such that it extends essentially perpendicular to the extension of radiation entrance slit 21 of detector 9. The divergent radiation beam 11 emanating from radiation source 13 makes it possible to produce a sheet-shaped radiation beam 22, divergent or collimated, by means of the radiation entrance slit 21 in a plane orthogonal to the direction of extension of line-like radiation source 13.

Thus, the sheet-shaped radiation beam 22 can be entered into the detector 9 to interact with the ionizable substance 27 therein.

Further, an object 15 to be imaged is arranged in the radiation path of radiation beam 11 between radiation source 13 and detector 9. Object 15 is preferably not a living organism, but other material under investigation, as the object is exposed to considerable radiation dose. Alternatively, the radiation source and the detector are arranged such that radiation beam 11 as reflected off object 15 is impinging onto the detector 9 and a portion thereof is entered through the entrances slit (not illustrated).

By such provisions (i. e. mutual orientation) the alignment of the detector 9 with respect to the radiation source is considerably facilitated. This is in Fig. 1 schematically

indicated by means of possible movement of detector 9 as defined by arrow 29 substantially perpendicular to central axis 19, while the detector still"sees"the radiation source and is thus maintained in alignment with the radiation source. The object is here preferably assumed to be larger or much larger than the dimension of the radiation source. Otherwise the object puts restrictions on the alignment tolerances allowed.

If the arrangement of Fig. 1 is to be used for imaging of a living organism, or a portion thereof, e. g. a patient body part, the radiation source 13 has a maximum length allowed such that the radiation dose exposed to the organism does not exceed a predetermined level, e. g. a limit value allowed.

The line-like radiation source 13 has a length preferably of at least 0.1 mm, more preferably of at least 1 mm, even more preferably of at least 10 mm, and most preferably of about 50 mm. The width of the line-like radiation source 13 may vary from e. g. about 0.05 mm to 2 mm, and is typically about 0.1 mm.

However, it shall be appreciated that the present invention is not restricted to these given focal spot sizes.

The elongated radiation slit entrance 21 has a height, i. e. a dimension in the direction of arrow 29, preferably of 0.01-5 mm, more preferably of 0.02-1 mm, even more preferably of 0.02 -0.3 mm, and most preferably of about 0.05 mm, and a thickness, i. e. a dimension in the direction of the radiation 22 entering detector 9, preferably of 0.01-5 mm, more preferably of 0.05- 1 mm, and most preferably of 0.05-0.3 mm.

The length of radiation slit entrance 21, i. e. a dimension in a direction perpendicular to the direction of arrow 29 and to the direction of the radiation 22 entering detector 9, may range within a tremendously large interval depending on the application in which the arrangement is to be employed.

Typically, however the length of the slit entrance 21 is much

larger than the height thereof. The length of the slit shall be long enough to cover the detector width, or at least a substantial portion thereof.

Note that the function of the radiation slit entrance 21 is not primarily to collimate the beam completely, but to form a planar sheet of radiation, which may in fact be divergent, i. e. the sheet becomes thicker and thicker with propagation distance, and to thereby restrict the radiation exposed to the detector.

Further, the radiation slit entrance 21 may improve the spatial resolution obtained.

Consider for instance an ionization detector, which is long (i. e. dimension along the radiation beam) and wide (i. e. dimension perpendicular to the radiation beam and to arrows 29 of Fig. 3) compared to its inter-electrode distance (i. e. distance between the cathode 23 and the anode 25 in the direction of the arrows 29). Exemplary measures may include a detector length and a detector width of about 50 mm and an inter-electrode distance of about 0.5 mm. In such a detector the active or sensitive volume, where actually the electrons, which contribute to a major extent to the signals detected, are released is given by the height of the radiation slit entrance 21; by the inter-electrode distance; or by the detector geometry and voltages applied.

Two cases may be distinguished: 1. Detector using no avalanche amplification 2. Detector using avalanche amplification In the first case the spatial resolution in the direction of arrows 29 is given by the inter-electrode distance if the height of the radiation slit entrance 21 is larger than the inter- electrode distance and if not it is given by the height of the radiation slit entrance 21.

In the second case the spatial resolution in the direction of arrows 29 is given by detector geometry and voltages applied if the height of the radiation slit entrance 21 is larger than the thickness of the sensitive volume and if not it is given by the height of the radiation slit entrance 21. The thickness of the sensitive volume depends on the geometry of the detector and the voltages applied. Due to the strong exponential amplification achieved in an avalanche detector electrons released close to the cathode will be amplified much more than those released closer to the anode since the former travel a much longer distance in the amplification volume and will thus contribute much more to the signals. For an inter-electrode gap of 0.5 mm a typical thickness of the sensitive volume is about 100 micrometers (located adjacent to the cathode).

It shall be noted that the divergent rays passing radiation slit entrance 21 are to a great extent discriminated by the electrodes and thus in principle the signals detected are originating essentially from rays parallel with the electrode surfaces. This holds true particularly for long detectors where rays are absorbed also at a considerable distance from the radiation slit entrance 21.

Accordingly, by using a detector apparatus with a radiation slit entrance 21 having a height smaller or much smaller than the thickness of the sensitive volume a detector apparatus having an improved or greatly improved spatial resolution is obtained.

Further, a two-dimensional image is typically obtained through scanning. The scanning is preferably made in a pivoting movement around any axis, but preferably an axis passing through the X- ray source or its vicinity. The scanning can also be transversal in the direction of arrow 29. Measurements performed at different positions provide each information of a particular splice through object 15 and thus a two-dimensional image may be reconstructed without the need of moving the radiation source 13

or the object 15 to be imaged. As radiation source 13 possibly has an intensity and a wavelength spectrum that vary along its extension, different techniques to compensate for this may be applied.

It shall be appreciated that a plurality of the inventive detector 9 may be stacked, side-by-side of each other. By such provision multi-line scans can be performed, which reduces the overall scanning distance, as well as the scanning time. Further reference in this respect is made to our co-pending Swedish patent application 0000388-9 entitled Detector and method for detection of ionizing radiation and filed on February 08,2000, said application being hereby incorporated by reference.

With reference to Figs. 3 and 4, which schematically illustrate, in cross-sectional side and top views, respectively, with detector parts cut-away, an apparatus for planar beam radiography, a second embodiment of the present invention will be described. This second embodiment differs from the previous embodiment as regards the following features.

The apparatus of Figs. 3 and 4 further comprises a second radiation beam-limiting device 31 having an elongated radiation transparent slit 32 extending essentially parallel with the entrance slit 21 of detector 9. Radiation beam-limiting device 31 is arranged between radiation source 13 and object 15 to be imaged such that radiation beam 11 from radiation source 13 is firstly limited by the radiation beam-limiting device 31. The radiation beam passing through radiation beam-limiting device 31, denoted 11'in Figs. 3 and 4, is transmitted through object 15 to be imaged. Radiation slit entrance 21 of detector 9 is operating as a second beam limiter to further limit radiation beam and the planar radiation beam entered into detector 9 is as in previous embodiment denoted by 22.

The dimensions of radiation beam-limiting device slit 32 are preferably similar to those of detector radiation entrance slit 21.

The alignment of detector 9 and radiation beam-limiting device 31 with respect to each other and with respect to radiation source is slightly more complicated than the alignment necessitated in the Figs. 1 and 2 embodiment. Either one of detector 9 or radiation beam-limiting device 31 is firstly aligned with respect to the radiation source (alignment movements of detector 9 and radiation beam-limiting device 31, respectively, are indicated by arrows 29 and 33). This alignment is easy as it is sufficient to align the detector or the radiation beam-limiting device 31 so as to obtain a planar radiation beam into the detector or through the radiation beam- limiting device. The ease of such alignment is directly dependent on the extension of radiation source (if not a very small object to be imaged puts restrictions in this respect).

Subsequently thereto, the other one of the detector or the radiation beam-limiting device 31 is aligned, and this alignment has to be performed more accurately.

In this second embodiment a large amount of the radiation 11 emanating from radiation source 13 is shielded from reaching object 15, and thus this embodiment is suitable for diagnostical purposes as the patient dose will be lower.

Nevertheless, the object 15 is exposed to a higher dose than if a point-like radiation source is used. Thus, it is adequate to state that the alignment is easier the longer the radiation source, whereas the radiation exposed to object 15 is higher the longer the radiation source is. Thus an optimal length of radiation source 13 exists for each application.

How to select an appropriate radiation source is thus a crucial task; one way is to use a maximum radiation source length that fulfils a given maximum exposed radiation dose allowed.

In a general case a first cost function related to the ease of alignment may be determined, wherein a long radiation line-like radiation source provides for a lower cost than a short line- like radiation source. A second cost function related to the radiation dose exposed to object 15 is correspondingly determined, wherein a long line-like radiation source provides for a higher cost than a short line-like radiation source. To find an optimal radiation source given the cost functions, the sum of these cost functions is calculated, whereafter the length of line-like radiation source 13 is selected so as to minimize the sum of the cost functions.

It shall be appreciated that the beam shapes as indicated in the Figures are strongly simplified for illustrative purposes.

The divergence of a radiation beam is in a general case complicated to calculated as is well recognized by the man skilled in the art.

It will be obvious that the invention may be varied in a plurality of ways. Such variations are not to be regarded as a departure from the scope of the invention. All such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the appended claims.