Patent Number: 053032829
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A radiation imager system 10, such as a medical computed tomography (CT) system, incorporating the device of the present invention is shown in schematic form in FIG. 1. CT system 10 comprises a radiation point source 20 and a radiation detector 30 comprising a plurality of radiation detector panels 40 and a plurality of collimators 50 disposed between radiation source 20, typically an x-ray source, and detector panels 40. Each detector plate comprises a plurality of detector elements (not shown) that convert incident radiation into electrical signals. The detector elements are typically arranged in a one- or two-dimensional array. The radiation detector elements are coupled to a signal processing circuit 60 and thence to an image analysis and display circuit 70. Detector panels 40 are mounted on a curved supporting surface 80 which is positioned at a substantially constant radius from radiation point source 20. This arrangement allows an object or subject 90 to be placed at a position between the radiation source and and the radiation detector for examination. Collimators 50 are positioned over radiation detector panels 40 to allow passage of radiation beams that emanate directly from radiation source 20, through exam subject 90, to radiation detector panels 40, while absorbing substantially all other beams of radiation that strike the collimator. The details of steps in the fabrication, and the resulting structure, of collimators 50 in accordance with this invention are set out below. FIG. 2 is a cross-sectional view of a representative portion of a collimator substrate 110. Substrate 110 comprises photosensitive material, i.e., a material that will react to exposure to light in a manner similar to photoresist, to allow etching of a pattern in the material. Such photosensitive material may lose its photosensitive characteristics after it has been exposed to light and processed. One example of this type of substrate material is the Corning, Inc. product known as Fotoform.RTM. glass. An optically opaque mask 112 is formed by conventional methods on a first surface 110a of collimator substrate 110. The pattern of openings in mask 112 corresponds to the pattern of detector elements in each radiation detector panel 40 (FIG. 1). For example, mask 112 would have a pattern generally mimicking the arrangement, e.g., rows and columns in a two-dimensional array, as well as the cross-sectional shapes of detector elements at the interface between radiation detector panel 40 and collimator 50 (FIG. 1). Alternatively, mask 112 need not be on the surface of the collimator substrate but can be positioned with respect to the substrate in accordance with known photolithographic techniques to provide the desired exposure of the photosensitive material in substrate 110. In any event, the pattern of the mask is selected to expose areas of photosensitive collimator substrate 110 of sufficient size and orientation so that, upon completion of the fabrication of collimator 50, the surface of each radiation detector element for receiving the radiation is exposed to radiation passing along the desired paths from the radiation source. In accordance with the present invention, collimator substrate 110 and mask 112 are exposed to light from light source 114. Light source 114 is preferably a laser, an ultraviolet light source, or the like, and is positioned with respect to collimator substrate 110 so that light beams pass through the openings in mask 112 and strike collimator substrate 110 along paths corresponding to direct paths between radiation point source 20 and radiation detector 30 (FIG. 1). As illustrated in FIG. 2, exemplar pairs of light beams 116a-b, 116c-d, and 116e-f define the boundaries of exposed photosensitive material shown in cross section. The light beams exposing the photosensitive material under each respective opening in mask 112 strike the collimator substrate at slightly different angles, the magnitude and orientation of which depend on the position along the length of the collimator substrate where the light strikes. For example, light beams 116a and 116b strike the collimator substrate at angles which differ in magnitude and orientation (i.e. left or right with respect to a perpendicular between the substrate and the light source) from light beams 116c-d and 116e-f. The light beams falling on photosensitive collimator substrate 110 define a plurality of respective exposed volumes 118 in the photosensitive material under each opening in the mask through which the light beams pass. Each exposed volume 118 has a longitudinal axis at a selected orientation angle corresponding to the angle at which the light beams emanating along a direct path from light source 114 strike the collimator substrate. Thus light beams 116a-b expose a volume that has a selected orientation angle .beta., whereas light beams 116e-f expose a volume having a different selected orientation angle, .differential.. The position of the collimator substrate with respect to light source 114 is selected to correspond with the distance that the collimator substrate will be from the radiation source in the assembled imager. Further, to ensure that the exposed volumes have the correct selected orientation angles required for collimating radiation in the assembled device, the plane of the collimator substrate is oriented at a "planar angle" so that the plane of the substrate has the same orientation with respect to the light source as the radiation detector panel with respect to the radiation source in the assembled device. Collimator substrate 110 is then etched using conventional techniques appropriate for the photosensitive material used in the substrate to remove the exposed volumes 118 of photosensitive material and create a plurality of channels or passages 120 through the substrate, as illustrated in FIG. 3. Each of these channels has a longitudinal axis 122 aligned with the selected orientation angle defined when the photosensitive material was exposed to light source 114 (FIG. 2). Typically the selected orientation angles of the longitudinal axes of the channels range between about 0.degree. and 10.degree.. Each channel has a channel sidewall 124 which is substantially smooth along its length and has a substantially uniform slope formed when the photosensitive material exposed by the light beams in the previous step is removed in the longitudinal axes of the channels range between about 0.degree. and 10.degree.. Each process. The slope of the sidewalls is typically substantially aligned with the selected orientation angle of the channel defined by those sidewalls. The remaining portions of mask 112 may next removed to prepare the collimator substrate for the next step in the process of forming the collimator. A radiation absorbent material layer 130 (FIG. 3) is then applied on collimator substrate 110 so as to cover at least the surfaces of the substrate which will be exposed to incident radiation when assembled in an imager device. The radiation absorbent material at least covers all of the sidewalls defining the channel. The cross-sectional portion of the radiation absorbent material on the sidewalls and the top and bottom of substrate 110 is illustrated in FIG. 3 in cross-hatch, while the radiation absorbent material on the "back" sidewall of the channel is illustrated in alternating cross-hatch and dashed lines. The radiation absorbent material can be applied through known techniques, such as vapor deposition techniques. Radiation absorbent material 130 is selected to absorb radiation of the wavelength distribution emitted by radiation source 20 (FIG. 1) in the imager device. The radiation absorbent material typically has a relatively high atomic number, e.g., greater than about 72, and advantageously comprises tungsten, lead, or gold when the radiation used in the imager device is x-ray. The thickness of the radiation absorbent material layer is selected to provide efficient absorption of the incident radiation and depends on the type of incident radiation and the energy level of the radiation when it strikes the collimator. For example, in a typical CT system using an x-ray point radiation source of about 100 KeV positioned approximately one meter from the detector array, a total thickness in the range of about 30 to 40 mils of tungsten in one or more layers disposed along the path of the radiation will substantially absorb the x-rays emitted by the source. After application of the radiation absorbent material, the cross-sectional area of the opening or void space in the channel is substantially the same as the area for receiving radiation on the detector element which it adjoins so as to allow substantially all radiation rays emanating along direct paths from the radiation source to strike the detector element. When two or more substrates are joined together to form the collimator body, the openings of the channels in the respective surfaces of the collimator substrates are aligned to form continuous channels through the collimator body. The channel sidewalls are advantageously aligned so that the sidewalls of the respective channels in the adjoining substrates are contiguous. Dependent on the energy level and wavelength of the radiation to be collimated, different thicknesses of collimator bodies may be required. Once the necessary thickness has been determined, an appropriate thickness of collimator substrate, or plurality of substrates, can be selected and fabricated in accordance with this invention. For example, the thickness of a collimator for an imager system using x-rays, such as a CT system, may be only about 8 mm, but for an imager using gamma rays, the collimator preferably would be three to five times thicker than that used for x-ray radiation. In the assembled device, collimator body 55 is disposed to adjoin radiation detector panel 40, as illustrated in FIG. 4. Radiation detector elements 42 are positioned along detector panel 40 and typically comprise a scintillator coupled to a photodetector. Collimator body 55 is positioned to allow incident radiation on a direct path between the radiation source and one of the radiation detector elements 42 to pass through the channels in the collimator. Beams of radiation that are not aligned with such a direct path strike the collimator body and are absorbed. The collimator of the present invention is readily used with either a one-dimensional or a two-dimensional array of radiation detector elements. A plan view of a collimator fabricated in accordance with the present invention and showing a representative number of channels 120 appears in FIG. 5. The figure has been marked to show left, right, upper and lower edges solely to provide a reference for ease of discussion, and the selection and positioning of such references is not meant to constitute any limitation on the structure or positioning of the device of the invention. Openings 122 of channels 120 on the opposite surface of collimator body 55 are shown in phantom. In the two-dimensional array the center channel is in substantial vertical alignment with the radiation source, and the opening 122 of the channel on the side of the collimator body opposite the radiation source is aligned with the opening in the surface closest to the radiation source. As the radiation beams spread out as they emanate from the point source, each of openings 122 has a slightly larger cross-sectional area than the respective opening of the channel 120 in the surface of the collimator closest to the radiation source. Openings 122 for channels on the left, right, top, or bottom are slightly offset from being in vertical alignment with their respective openings in the upper surface of the substrate. The direct path from the radiation source to a radiation detector in the upper left hand corner, for example, is offset both to the left and the upper side of the array. The selected orientation angle of the axis of the channel is substantially aligned with this direct path, and the channel thus extends through the collimator body at this angle. The selected orientation angle for each channel is different from any other channel in the collimator. Such a structure, which would be extremely difficult and time consuming to construct with conventional collimator fabrication techniques, is readily produced in accordance with this invention. While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.