TECHNICAL FIELD OF THE INVENTION The present invention relates to a scamied-slot x-ray imaging system, having a first collimator and a second collimator arranged in a first distance and a second distance, respectively, from a radiation source and each provided with a slot and a detector located under the second collimator slot, said slot of said second collimator being wider than the said slot of said first collimator and said detector under the second slot is wider than the first collimator slot and the second collimator slot.

BACKGROUND OF THE INVENTION The common systems for x-ray imaging consist of an x-ray source and an area detector placed behind the object to register the image. The main drawback with this set-up is its sensitivity to background noise in form of Compton scattered radiation. Existing methods to remove this background noise are inefficient and also remove a fraction of the primary x-rays that contain the image information. This result in a dose increases exceeding a factor 2 or more.

One way around this problem is a scanned-slot set up. A pre-collimator slot before the object shapes the x-ray beam to match the active detector area. The slot is moved mechanically to image the entire object. It is also possible to have the object moving with respect to the slot, this is however usually more inconvenient because the object is usually heavier than the mechanics for the slot. Since only a narrow fan-beam is crossing the object at any single time and the area of the secondary collimator is small relative to the area of the captured image, the amount of Compton scattered x-rays is minimized. Another advantage with the scanned-slot approach is that the required detector area is much smaller, this cuts cost and also enables the use of more expensive and efficient detector materials if desired.

A drawback with the scanned-slot geometry is that only a small fraction of the x-rays from the source is actually used to form the image. As a result, the time for image acquisition is extended and the x- ray tube need to be turned on for a longer period of time. A way of mitigating this problem and achieve a practical system is to use a multi slot collimator with different detector arrays under each slot. This however makes the image acquisition non-trivial since the information from the different detectors has to be sewn together into one image without any visible artifacts such as boarder lines between areas where different detectors were used.

One of the most important constraints for medical x-ray imaging systems is to avoid any exposure of the patient to x-rays in areas where there is no active detector to register the x-rays. This would only lead to an unnecessary dose increase. In a multi slot set-up alignment is crucial since the detectors need to cover the full area under each slot.

International patent application no. WO 82/01124 describes an apparatus including a planar, proximity type x-ray image intensifier for detecting a fan beam of x-rays and for producing an intensified output visible light image on an output display screen which is sensed by a scannable, linear array of solid state diode detectors. In a first embodiment, a pair of side by side arrays are utilized to eliminate the effects of flare in the display screen. One of the linear arrays looks at the line signal on the output screen and the second linear array looks at a location on the output screen which is adjacent and parallel to the line signal. A net signal is derived by subtracting the signals from adjacent elements of the two parallel arrays so that signal flare in the image intensifier tube is removed. In a second embodiment, display screen flare is eliminated by covering the vacuum side of the display screen with metal having a thickness sufficient to dissipate one third of the kinetic energy of photo-electrons passing through it.

US 4,649,559 discloses a large area, digital radiography apparatus in which a prescatter and a postscatter collimator are moved simultaneously with an x-ray image intensifier tube whose output display is scanned by a stationary scanning camera to produce a digitized x-ray image over a large cross-sectional area of the patient.

It is important to have the detectors covering the whole x-ray-imaging object in the direction orthogonal to the scan without any gaps in between detectors. For semiconductor detectors this is an engineering challenge since there is always a dead-area close to the edge at the detector. This is caused by mechanical damage when cutting the detectors on the wafer, and usually a guard-ring has to be placed between the edge and the active detector area to sink leak current emanating from the mechanical damages. Ideally none of this dead area should be exposed to the diagnostic x-rays.

SUMMERY OF THE INVENTION One object of the present invention is to provide a set-up for multi-slot medical x-ray imaging, which greatly simplifies the alignment and also presents a method for tiling different semiconductor detectors to cover the whole slot without introducing any dead area in between detectors.

Another object of the present invention is to allow for a misalignment with respect to the central symmetry line with less than a safety factor so that no primary radiation is lost in the post collimator.

These objects are obtained through arranging the initially mentioned slot of said second collimator with a width not less than a safety margin and the product of the width of the slot of said first collimator and said second distance divided with the said first distance for allowing a misalignment with respect to a central symmetry line of said slots.

Furthermore, the system can comprise plurality first and second collimators and detectors arranged side by side to enable a multi slot scan.

In a preferred embodiment said detector is a semiconductor detector and it can be oriented such that an edge of faces said incident x-rays. However, the detector can be a film-screen combination, a CCD coupled to a scintillator through optical fibre bundles, or a gas detector.

If the detector is a gaseous detector, it can have a drift field to drift the electrons released through interactions with the x-rays to the edge of the detector where the signal is amplified and registered.

The invention also concerns, in a scanned-slot x-ray imaging system, having a first collimator and a second collimator arranged in a first distance and a second distance, respectively, from a radiation source and each provided with a slot and a detector located under the second collimator slot, said slot of said second collimator being wider than the said slot of said first collimator and said detector under the second slot is wider than the first collimator slot and the second collimator slot, a method for allowing a misalignment with respect to a central symmetry line of said slots. The method comprises arranging said slot of said second collimator such that the width of it is not less than a safety margin and the product of the width of the slot of said first collimator and said second distance divided with the said first distance. Moreover, the collimators can be so arranged that a dead area on said detector is not exposed to said x-ray.

BRIEF DESCRIPTION OF THE DRAWINGS In the following, the invention will be described with reference to non-limiting drawings, illustrating a preferred embodiment, in which Fig. 1 is a schematic cross-sectional view of an embodiment according to the invention, Fig. 2 is the embodiment according to fig. 1, provided with distance signs, and Fig. 3 is a schematic top view of a system with a plurality of first collimator slots.

DESCRIPTION OF A PREFERRED EMBODIMENT A preferred embodiment of a set-up for scanned-slot x-ray imaging is displayed in Figure 1.

It comprises a first collimator 102 provided with a first slot 102a, and a second collimator 104 provided with a second slot 104a. The collimators are spaced apart to provide a space in which an object 103, to be examined, is positioned. Beneath the second collimator 104 a detector 106 is located. A source 100 of X-rays 101 is also provided.

The x-rays 101 incident on the set-up is shaped by the first collimator 102 to hit the detector 106. The purpose of the second collimator 104 is to absorb Compton scattered x rays from the object 103.

Ideally the collimators 102 and 104 and the detector 106 should be symmetrical with respect to the centerline 105. If the slots are equal in width and also the detector bas this width any misalignment in terms of deviations from the symmetry line 105 for one of the slots or the detector will result in a loss in efficiency. To avoid this problem, the second collimator slot 104 is slightly wider compared to the first collimator slot 102. Moreover, the width of the detector 106 is larger than the collimator slot 102 but also larger than the collimator 104. All this is indicated slightly exaggerated in Fig. 1. By means of this set-up the system is insensitive to small misalignments with respect to the symmetry line 105 and manufacturing cost can be decreased and reliability improved.

Fig. 2 shows the principle of the invention. It is assumed that the distance between the source 100, first collimator 102 and the second collimator 104 is a and b, respectively, the width of the slot of first collimator 102 x and the width of the slot of the second collimator 104 y. Taking into the account the magnification due to the divergent x-ray beam and the principle of the similar triangles, then <BR> <BR> <BR> <BR> <BR> cz b x y xb<BR> <BR> <BR> <BR> jcZ?? What is needed is a wider second collimator such that y + 2p = y'> y, i. e xb/a +2p > y, where p is a safety margin and y'is the desired width. Therefore, it is possible to allow for a misalignment with respect to the central symmetry line with less than p and still not loose any primary radiation in the second collimator 2. The same reasoning is applicable to the width of the detector.

The factor p depends on the stability of the actual beam and corresponds to the probability of the <BR> <BR> <BR> <BR> misalignment. The range of p may be between 0-200/m. The distance p should be chosen such that any increase in radiation dose due to misalignment should be less than about 5% of the total radiation dose given to the patient. The probability for misalignment has to be assessed through repetitive measurements under realistic operating conditions for the x-ray imaging set-up. The los factor for primary radiation may be 1%.

Moreover, the dead area 107 caused by mechanical damage when cutting the detectors on the wafer, and usually provide with a guard-ring placed between the edge and the active detector area to sink leak current emanating from the mechanical damages is so covered by the collimator 104 so that it is

not exposed to the x-rays.

The collimators are preferably made from efficient absorbers as for example W, Cu or Fe. The detector could be a silicon strip detector, a CCD (Charge Coupled Device) camera coupled to a scintillating screen or a gaseous avalanche detector such as for example a parallel plate chamber. In the case of the CCD camera coupled to the scintillating screen this coupling could be provided through for example optical fibre bundles.

In case of a silicon strip detectors the wafers can be made at least 500, um thick without problems and the signals are registered by standard state of the art electronics. When the detector is a semiconductor detector it can advantageously be oriented edge-on to the incident x-rays. With edge- on is meant that the x-rays incite one edge of the of the detector, which also can be tilted slightly.

Another option would be to provide a detector in the form of a film screen combination.

A gas-detector with the gas volume oriented edge-on can be made to any desired thickness by introducing a drift volume where the electrons created through interaction with the gas molecules can be collected through an electric drift field and drifted towards the edge of the detector where avalanche multiplication can take place and the signal registered by state of the art electronics.

In Fig. 3, a top view of a system with a plurality of first collimator slots is displayed. Each of the lines 201 indicates one slot; i. e. a hole cut in the metal with a width equivalent to the desired width of the x-ray beam after passing the collimator. As shown there is a plurality of collimators in two dimensions. Figs. 1 and 2 correspond to a cross-section along line A-A in Fig. 3 for any of the slots 201 indicated in Fig. 3.

The invention is not limited the shown embodiments but can be varied in a number of ways without departing from the scope of the appended claims and the arrangement and the method can be implemented in various ways depending on application, functional units, needs and requirements etc.