Method and apparatus for performing single-point projection imaging

A method and apparatus for performing single-point projection imaging uses an X-ray source and a line scan camera present at a set distance therefrom and provided with a digital detector. The X-ray source, as well as the line scan camera, are adapted to rotate around an object placed between the X-ray source and the line scan camera. The X-ray source's focal spot is aligned at a desired position and the object is then imaged by scanning it with a beam emanating from the X-ray source, which beam is received by the detector of the line scan camera. The scanning motion is effected in such a way that the focal spot remains essentially stationary during the imaging process.

BACKGROUND OF THE INVENTION

The present invention relates to a method for performing single-point projection imaging by using an X-ray apparatus, comprising an X-ray source and a line scan camera present at a set distance therefrom and provided with a digital detector, the X-ray source as well as the line scan camera being adapted to rotate around an object to be placed between the X-ray source and the line scan camera.

Traditionally, such single-point projection imaging operations have been performed, as illustrated schematically inFIG. 1, by using an area detector4and a point-like focus1, from which emanates an X-ray beam through an object2for a projection3on the detector. This type of single-point projection imaging produces a projection image, in which an object becomes recorded over its entire depth on a detector.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a solution, which enables this type of single-point projection imaging to be performed by using a small area detector to avoid the use of a large and expensive area detector. Another object is to provide a solution, which enables single-point projection imaging to be performed from a wide range of directions with a single apparatus without a necessity of realigning the object between separate imaging procedures. Yet another object is to provide a solution, which is implementable even with presently available X-ray equipment provided only with necessary reprogramming.

In view of fulfilling these objects, the inventive method comprises performing the alignment of the focal spot of an X-ray source at a desired position and then imaging an object by scanning it with a beam emanating from the X-ray source, which is received by the detector of a line scan camera, in which method the scanning motion is effected in such a way that the focal spot remains essentially stationary during the imaging process.

The inventive apparatus for implementing single-point projection imaging comprises an X-ray source and a line scan camera present at a set distance therefrom, the X-ray source as well as the line scan camera being adapted to rotate around an object to be placed therebetween, means for aligning the X-ray source's focal spot at a desired position, and means for effecting a scanning motion necessary for imaging the object in such a way that the focal spot remains essentially stationary during the imaging process. The alignment of the X-ray source's focal spot at a desired position is effected, e.g. by placing the object to be exposed between the X-ray source and the line scan camera by means of appropriate guides and supports at a set distance from the X-ray source's focal spot.

The apparatus comprises preferably a frame element, on which is mounted pivotably about a rotation axis an element housing an X-ray source and a line scan camera, said rotation axis being adapted for displacement relative to the frame element during a scanning motion, such that the centre of rotation is essentially in line with the focus, whereby the focus remains essentially stationary during a scanning motion. The rotation axis is preferably adapted for displacement along a linear path while the element housing the X-ray source and the line scan camera rotates to perform a scanning motion. Displacement of the rotation axis during a scanning motion can also be adapted to occur along a curving or meandering path, whereby a more accurate result is achieved regarding the immobility of the focus than by a combination of linear motion and rotary motion.

DETAILED DESCRIPTION OF THE INVENTION

A panoramic X-ray apparatus10shown inFIG. 2comprises a vertical member11, from which extends a horizontal member13having suspended therefrom, pivotably about a rotation axis6, a C-frame12, having one14of its upright arms provided with an X-ray source with its focus indicated by reference numeral1and having a primary collimator in the vicinity thereof, indicated by reference numeral6. A second upright arm15is provided with a line scan camera, including a detector7, preferably a CCD detector.

The panoramic imaging apparatus and its operation is known as such for a skilled person. In panoramic imaging, an object to be imaged is placed between the arm members of the C-frame12by means of appropriate guides and supports, whereafter the X-ray source is activated and the C-frame12is rotated around the rotation axis6, the primary collimator5having its aperture chosen in such a way that a beam7in a substantially vertical plane is focused on the detector7, which is fitted behind the substantially vertical slit of a line scan camera mounted on the arm member15of the C-frame12and from which visual information is conveyed further, e.g. to a microprocessor. The operation of a line scan camera is described e.g. in U.S. patent Application 20030161438 A1 and the operation of a detector e.g. in U.S. Pat. No. 5,528,645, and, thus, those are not described in further detail in this specification.

According toFIG. 3, the rotation axis6is adapted for displacement in a slot8along a linear path.

If imaging is performed by pivoting the X-ray source about the rotation axis6for imaging an object by means of a vertically directed beam21, the vertical ratio of magnification remains the same as without a scanning motion but, on the other hand, the horizontal ratio of magnification changes, because the centre of rotation coinciding with the rotation axis6becomes an effective focal spot instead of the X-ray source's focal spot1.FIG. 4illustrates a situation, in which the rotation axis6remains stationary, whereby, as the X-ray source is rotating around a centre of rotation defined by the rotation axis6, the X-ray beams21travel at each angular position through the centre of rotation, whereby the effective focal spot for radiation arriving at an image plane (detector)18by way of an object17will be constituted by the centre of rotation. The focus1moves relative to the object17during the rotary motion. Thus, the horizontal ratio of magnification, which is defined by the ratio of a distance between the focus and the image path (SID=source-Image distance) to a distance between the focus and the object (SOD=source-object distance), is equal to the ratio of a distance between a centre of rotation defined by the rotation axis6and the imaging plane17to a distance between the centre of rotation and the image path18.

According to the invention, the rotation axis6is adapted for displacement preferably by a linear movement transversely to the beam21.FIG. 5illustrates a situation, in which the rotation axis has been displaced during a rotary motion by a linear movement from a position6ato a position6b, whereby the centre of rotation and hence the effective focal spot shifts to essentially coincide with the X-ray source's focus1, said focus1remaining substantially stationary during the imaging process. In this case, both horizontal and vertical magnification is equal at each point between focus and detector and, hence, the ratio of magnification (SID/SOD) will be equal because the focus remains effectively stationary.

The inventive solution enables the use of presently available panoramic equipment for single-point projection imaging by providing the same with necessary programming modifications, which allow the execution of a scanning motion in such a way that the focus1remains essentially stationary during the imaging process unlike in panoramic tomography, in which the objective is to provide a slice image by means of a continuous steady movement of the effective focus.