Patent Number: 
Section: description

A preferred embodiment of a scanned-slot X-ray imaging setup is displayed in FIG. 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 so that a space is provided in which an object 103 to be examined is positioned. A detector 106 is located beneath the second collimator 104. An X-ray source 100 is also provided. X-rays 101 incident to the setup are shaped by the first collimator 102 so that they hit the detector 106. The second collimator 104 absorbs Compton scattered X-rays from the object 103. Ideally, the collimators 102,104 and the detector 106 are symmetrical with respect to a centerline 105. If the slots are equal in width, and the detector also has this width, any misalignment caused by deviation from the symmetry line 105 for one of the slots or the detector results in a loss in efficiency. To avoid this problem, the second collimator slot 104a is slightly wider in comparison to the first collimator slot 102a. Moreover, the detector 106 width is not only larger than the collimator slot 102a, but also larger than the collimator 104. This arrangement is indicated in slightly exaggerated form in FIG. 1. With this setup, the system is insensitive to small misalignments with respect to the symmetry line 105, decreasing manufacturing cost while improving reliability. FIG. 2 illustrates 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 102x, and the width of the slot of the second collimator 104y. Taking into the account the magnification due to the divergent X-ray beam and the principle of similar triangles, then       a    x    =                    b        y            ⁢              xe2x80x83            ⁢              or            ⁢              xe2x80x83            ⁢              x        a              =                                                      
                        ⁢            y                    b                ⇒        y            =                        x          ⁢                      xe2x80x83                    ⁢          b                a             What is needed is a wider second collimator such that y+2p=yxe2x80x2 greater than y, i.e., xb/a+2p greater than y, where p is a safety margin and yxe2x80x2 is the desired width. Therefore, a misalignment can be allowed with respect to the central symmetry line less than the safety margin p and still not loose any primary radiation in the second collimator 2. The same reasoning is applicable to the detector width. The safety factor p depends on the stability of the actual beam, and corresponds to a probability of misalignment. The range of p may be between about 0 to about 200 xcexcm. 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 loss factor for primary radiation may be about 1%. Moreover, the dead area 107 is due to mechanical damage when cutting the detectors on the wafer. This dead area 107 is usually provided with a guard-ring placed between the edge and the active detector area to sink leak current emanating from the mechanical damage. The dead area is so covered by the collimator 104 that it is not exposed to the X-rays. The collimators 102,104 are preferably made from efficient absorbers, such as W, Cu or Fe. The detector could be a silicon strip detector, a charge coupled device (xe2x80x9cCCDxe2x80x9d) camera coupled to a scintillating screen or a gas avalanche detector such as 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 fiber bundles. In case of silicon strip detectors, the wafers can be made at least about 500 xcexcm thick without problems. The signals are registered by standard state of the art electronics. When the detector is a semiconductor detector, it can be advantageously oriented edge-on to the incident x-rays. By edge-on, it 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 Axe2x80x94A in FIG. 3 for any of the slots 201 indicated in FIG. 3. While there has been disclosed effective and efficient embodiments of the invention using specific terms, it should be well understood that the invention is not limited to such embodiments as there might be changes made in the arrangement, disposition, and form of the parts without departing from the principle of the present invention as comprehended within the scope of the accompanying claims.