Patent Number: 051270307
Section: summary

DESCRIPTION 1. Technical Field The invention relates to tomographic imaging using penetrating radiant energy and more particularly to production of tomographic images which do not require use of image reconstruction techniques such as used in conventional computed tomography. 2. Cross-Reference to Related Application The present invention is an improvement of the invention described in Annis application Ser. No. 888,019 for Tomographic Imaging, the disclosure of which is incorporated herein by this reference. 3. Background Art An improved form of tomographic imaging is described in the cross-referenced application. As used herein, the term "tomography" or "tomographic imaging" represents imaging a selected slice of an object where the slice may or may not be planar; the term is analogous to laminography or planigraphy. The apparatus described in the- copending application includes a source of penetrating radiant energy to illuminate the object to be imaged, a sweep arrangement to form a pencil beam and to sweep the pencil beam over a line in space to form a sweep plane, some apparatus to support the object to be imaged so that the sweep plane intersects the selected slice of the object to be imaged, a radiation detector to detect energy scattered by the object, and a collimator. In the cross-referenced application the collimator is described as a line collimator because it is constructed to focus on or in the immediate vicinity of a line in space. By arranging the sweep plane to intersect the selected slice in or along the focal line of the collimator, an essential characteristic of tomographic imaging is satisfied. That essential characteristic is some way to localize or focus in on energy scattered by a single elementary volume of the object being imaged in preference to radiation scattered by other portions of the object. Since the illumination travels a linear path which is arranged to intersect the object, there are many elementary volumes of the object along that linear path which may scatter energy. If all or a significant portion of that scattered energy were detected, there would be no way to determine which of the energy was scattered by one elementary volume in preference to others. The line collimator performs a part of this localizing process by focusing on a line which can be considered the locus of a plurality of elementary volumes of the object. The focal line of the collimator is arranged so that it lies within the selected slice. In this fashion, energy scattered by elementary volumes of the object which do not lie within the selected slice is filtered. Because the pencil beam exists only at one path within the sweep plane, at any instant of time, the completion of the localizing function is effected. More particularly, while the collimator can accept energy scattered by any elementary volume lying along the preferential line, the pencil beam illuminates only a single elementary volume lying along the preferential line at any instant in time. An image of all the elementary volumes lying along the preferential line is formed as the pencil beam sweeps along the preferential line. At different instants in time, different ones of the elementary volumes along this preferential line are illuminated and energy scattered by the different elementary volumes lying along the preferential line are detected one after the other. The scattered energy which passes the collimator is detected and processed, sampled, digitized and stored. Thus as the pencil beam sweeps the line in space a line image is created in the digital memory storing the processed signals. A tomographic image of the entire slice is created by providing relative movement between the object and the imaging apparatus so the preferential line coincides with different linear segments of the selected slice as the relative motion displaces the selected slice relative to the imaging apparatus. In the copending application the sweep plane coincides with a plane of symmetry of the collimator, and the collimator is located between the source of illuminating radiation and the object being imaged. This arrangement works well (i.e. it is sensitive) for features in the selected slice which lie generally parallel to the sweep plane. However, the arrangement has a number of disadvantages.- As described in the copending application, it is important to collect as much of the scattered energy as possible. However, the parameters of the collimator determine the slice thickness and this requires that the channels of the collimator (defined between adjacent pairs of vanes) near the plane of symmetry be relatively narrow. The thin channels reduce the scattered flux which is accepted. This reduction can be understood by considering the need for thin channels and the practical limits on the thickness of the plates between which the channels are defined. The need for thin channels and the practical limits on plate thickness results in a minimum ratio of open area of the collimator's face to the blocked area of the face. Furthermore, the need to split the collimator so as to allow the sweep plane to pass through the collimator also reduces the effective solid angle subtended by the collimator at the selected slice. The present invention arranges the sweep plane so it does not coincide with a plane of symmetry of the collimator. In accordance with the present invention the slice thickness is defined not by the dimensions of the collimator, but by the illumination or pencil beam itself. This eliminates the constraint on the spacing of the vanes, for example it allows the spacing of the vanes which define the collimator to be made equal. Furthermore, depending on the orientation of the sweep plane relative to the plane of symmetry of the collimator, the solid angle subtended by the collimator can be significantly increased even as compared to the apparatus described in the copending application. For example in one embodiment of the invention the sweep plane is arranged to be substantially perpendicular to a plane of symmetry of the collimator. In another embodiment of the invention the sweep, plane makes an angle other than 90.degree. with a plane of symmetry of the collimator. The present invention has an additional advantage over the arrangement shown in the copending application in that x-ray photons which are "doubly scattered" (scattered twice) will not be detected with as high an efficiency as they would be in the arrangement of the copending application for a collimator with the same number of sheets or veins. Double scattered x-ray photons may pass through the collimator and enter the detector even though they do not suffer their first or second scatter along the focal line (the preferential line). The only requirements are that the second scatter take place within the acceptance wedge of the collimator and that the final angle of the second scatter be parallel to the extended line of the collimator plate (vein or sheet) which intersects the point of the second scatter. Since the x-ray beam in accordance with the present invention does not intersect the acceptance wedge of the collimator except at the focal line or preferential line (see FIG. 2 or 3), the first scatter, in order to generate a detected photon, must position the photon into the acceptance wedge in order for the photon to pass through the collimator after the second scatter. More particularly, the imaging apparatus in accordance with the invention is arranged to produce a tomographic image of a selected slice and includes a source of penetrating radiation and a sweep means for forming energy from the source into a pencil beam and for repeatedly sweeping the pencil beam over a line in space. The motion of the pencil beam defines a sweep plane which defines, at least in part, the size and location of the selected slice. There is means for supporting an object to be examined so the pencil beam and sweep plane intersect the object. Further means are provided for preferentially detecting radiation scattered, at any instant, by one of a group of selected volume elements in the slice, the second means subtending a large solid angle relative to the selected volume element, the second means including: radiation detector means developing at any instant in time a single signal reflecting radiation impinging on the radiation detector means, and a collimator located between the object and the radiation detector means, the collimator including: a plurality of radiation transmitting channels collectively establishing a field of view which intersects the sweep plane in a linear segment which is a locus of said selected volume elements so that the collimator passes radiation scattered by different elementary volume elements of the object lying along the linear segment as the sweep illuminates the different volume elements and where the collimator has an axis of symmetry which does not coincide with the sweep plane. Generally there is the desire to maximize the scattered energy which is detected without compromising the localizing function. This is accomplished by maximizing (or at least increasing) the solid angle subtended by the detector or collimator. Preferably the solid angle is within range of 0.05.pi. to 2.pi. steradian, or larger; a typical angle is .pi./2 steradian. Depending on the application, there may be one or more collimators, each associated with an element of the radiation detector. More particularly, it is a particular advantage of the invention that, at any instant, any radiation which is scattered, and which passes a suitable collimator (one which has a field of view intercepting the sweep plane in a bounded line along which the pencil beam is swept) was generated by the same elementary volume element of the object being imaged. Accordingly, wherever such scattered energy is detected, it can be used in forming the pixel representing that particular elementary volume. In some embodiments of the invention, the sweep plane is perpendicular to a plane of symmetry of the collimator, and the collimator is located only on one side of the object being illuminated. This limitation necessarily limits the solid angle subtended by the collimator to no more than 2.pi. steradians. However, by placing a second collimator (and associated radiation detector) on the other side of the object, this limitation on the solid angle is removed and the solid angle subtended by the collimators together can approach 4.pi.. The same is true for cylindrical bodies where the selected slice lies in the cylinder wall. With cylindrical bodies, although the "second" collimator could be placed on the other side of the body, that would mean that energy scattered by the selected slice might have to travel through another cylindrical wall section before reaching the collimator/detector. It would be preferable, given a sufficient radius of curvature of the cylindrical body, to locate the "second" collimator and detector, "inside" the cylindrical object. The use of multiple collimator/detectors is not limited to embodiments in which the sweep plane lies perpendicular to a plane of symmetry of the collimator(s).