Patent Number: 
Section: description

In x-ray imaging the number of detected x-rays determines the image acquisition time yielding an acceptable image quality. In a scanned slot set up it is thus possible to adjust the required image acquisition time by changing the width of the slots and thus the number of x-ray hitting the object and the detector. Note that the detector needs to be wider than the largest slot used in order to detect all incident x-rays. In scanned slot x-ray imaging the spatial resolution in the dimension orthogonal to the slot is determined by the slot width. By varying the slot width it is possible to adjust the spatial resolution to comply with the diagnostic requirements for the moment. For the invention essential parts of an x-ray imaging apparatus 10 according to known techniques are illustrated in FIG. 1. Other parts present in the apparatus, obvious for a skilled person, but not important for the invention are not shown for simplicity reasons. The simplified x-ray imaging apparatus comprises a radiation source 11, a collimator 12 and a detector assembly 13. An object 14 to be examined is located between the collimator 12 and the detector assembly 13. The collimator is made of an x-ray blocking material and is arranged to expose a determined part of the detector for x-rays through slots 15. Two different types of collimators are illustrated in FIGS. 2a and 2b. The collimator 22a of FIG. 2a is provided with slots 25a arranged in two rows and displaced relative each other along the longitudinal axis of the collimator. The collimator 22b of FIG. 2b is provided with an oblong slot 25b, which can be divided into smaller slots through partition walls 26b. The form and arrangement of the slots are described in more detail in prior art as disclosed above. The collimator may comprise a 30 line slots or 30 plus 30 half lines. The slots and corresponding detectors may in some cases also be cut with an angle different from 90 degrees with respect to the scanning motion. FIG. 3 illustrates a schematic collimator-detector assembly, in which a collimator 32 having slots 35 is arranged to expose a predetermined part of the detector 33 to the x-ray radiation (indicated with arrows). The surface section of the detector 33 exposed to the radiation is indicated with a thicker line. According to the invention, the objective of the invention is obtained by varying the exposed surface of the detector to the x-rays, it is the projection of the slot(s) on the detector. An embodiment according to the invention is illustrated in FIGS. 4, 5 and 6. According to the embodiment of FIG. 4, a collimator 42 comprises at least two relative each other displaceable parts 42a and 42b, which provide slots 45 with variable width. The parts are arranged in different planes. A first position of the collimator sections providing a maximal slot width is indicated with dashed lines. A minimal slot width (=0 mm) is obtained when no slot are placed in front of each other. It is also possible to arrange both parts laterally displaceable relative each other. FIG. 5 is a top view of a collimator 52 comprising a first part 52a and a second part 52b arranged movable relative each other, e.g. through insertion of one part into the other one. Hence, the width of each slot 55 is variable. One position of the collimator parts providing a wide slot width is illustrated with dashed lines and a second position, in which the slot width is narrow is illustrated with solid line. The displacement of the collimator parts may be achieved by means of a step-motor (not shown) or the like by providing one or both of the parts with, e.g. teeth/wheel, belt or the like. The step motor may be controlled by means of a computer unit, e.g. with respect to the objects"" density and/or thickness. In FIG. 6, the collimator 52, comprises two substantially similar collimators, a first (stationary) part 52a with a fixed slot 55a width, and a second movable part 52b having a slots 55b similar to the first part, arranged to be displaced on one side of the first part to cover the slots 55a and change the slot-width. By arranging collimators on top of each other one can adjust the slot width for all slots by one single movement of collimators relative to each other in the dimension orthogonal to the slots. Although, the examples show collimators with slots displaced relative a longitudinal axis of the collimator, it is however possible to use the same technique for collimators having slots along a longitudinal axis of the collimator. The mechanical displacement of the second part can be accomplished using a step-motor (not shown) or the like, e.g. by providing the second part with teeth/wheel, belt or the like, or piezoelectric actuators. The motor/actuator may be controlled by means of a computer unit, e.g., with respect to the objects"" density and/or thickness. Moreover, the invention also allows providing collimators with high precession. Slots are very small recesses in a carrier, each slot having a width of for example 50 xcexcm, which is difficult to produce depending on the manufacturing process and material. However, it is possible to produce collimators with 150 xcexcm slots and arrange them according to the provisions of the invention to achieve smaller slot widths. It is especially useful in case of complex slot configurations. Thus, the invention provides means for producing high precession collimators. FIG. 7 is a further embodiment. The collimator 72 is arranged rotatable along a longitudinal axis 77 (anywhere along the short side of the substantially rectangular shaped collimator). According to this embodiment, the variation of exposed area is achieved by rotating the collimator 72 so that the slot 75 is positioned in an angle xcex1, then if assuming the width of the slot is b and the width of a section exposed through the slot is xcex1, then the variation of xcex1: xcex94xcex1 is obtained through xcex94xcex1=b. cos xcex94xcex1 (for xcex1=0, a=b). Consequently, the width of the section c, exposed to the radiation, on the tilted detector 73, tilted in an angle xcex2 is: c=xcex1/cos xcex2 and accordingly the variation of c: xcex94c=b. cos xcex94xcex1/cos xcex2. A first position of the collimator 72 is shown with dashed line and a second rotated position with the solid line. The mechanical rotation can be accomplished using a step-motor (not shown) or the like through providing the second part with, e.g. teeth/wheel, belt or the like or piezoelectric actuators (not shown). The motor/actuator may be controlled by means of a computer unit, e.g. with respect to the objects"" density and/or thickness. It is also possible to provide both the detector and the collimator rotatable. Consequently, the collimator can be both rotatable and comprise of parts for varying the slots. The detector may be any of detectors mentioned in the background part of the present specification and do not need to be tilted as shown in the various embodiments. The invention is not limited to the shown embodiments and 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. In one embodiment it is possible to rotate the tilted detector to change the tilting angle with respect to the slot. Instead of detectors it is also possible to use a film, known per se, in which case additional collimators should be arranged after the object to be examined. It is also possible to stack more than two collimator parts.