Patent Number: 052415783
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 2, presented is a block diagram of a portable x-ray apparatus incorporating the optical alignment system of the present invention. The system includes x-ray machine 20 and grid cassette 33, including anti-scatter grid 21, which is located between x-ray machine 20 and x-ray film cassette 24 which holds x-ray film 22. Attached to collimator housing 26 of x-ray machine 20 is light projector 27 which projects light beam 30 which forms a line or spot of light on the surface of grid cassette 33. Included in or attached to grid cassette 33 is reflector element 28. Reflector element 28 and light projector 27 are located substantially equidistant from central x-ray beam 23. Further, opaque line 29 (shown in more detail with reference to FIG. 12) is applied to the transparent front surface of collimator housing 26, and casts a shadow 31 on the surface of grid cassette 33 within the collimation field projected by the collimation light within collimation housing 26. As explained below in more detail, light projector 27, in combination with reflector element 28, ensures proper angulation alignment of grid cassette 33 relative to the x-ray source within portable x-ray machine 20. Further, when shadow 31 is coincident with light line or spot 40 projected from light projector 27 upon the surface of grid cassette 33, grid cassette 33 is located at the proper focal distance from the x-ray source within portable x-ray machine 20. Further, adjustment of grid cassette 33 so that light beam 30 impinges on grid cassette 33 at a predetermined location (for example, the middle of reflector element 28), ensures centering of x-ray machine 20 with respect to grid cassette 33. In addition, and explained in more detail below, grid cassette 33 is configured to hold film cassette 24 in either a vertical or horizontal position, without changing the orientation of grid cassette 33 relative to light projector 27 and portable x-ray machine 20. Referring now to FIGS. 3 and 4, disclosed is an embodiment of grid cassette 33 in accordance with the present invention. In operation, x-rays from portable x-ray machine 20 impinge upon grid cassette 33 from the backside in FIG. 3, and from the bottom side in FIG. 4. Within grid cassette 33 is included anti-scatter grid 21 which, in this embodiment, is a 6:1 focused linear grid. Grid cassette 33 is configured with bars 34 and 36, which define substantially rectangular apertures 35 and 37 sized to fit the short dimension (14 inch (35.6 cm)) of film cassette 24. This permits grid cassette 33 to accommodate film cassette 24 in either a vertical position (24') or in a horizontal position (24"). This allows grid cassette 33 to maintain a constant position relative to portable x-ray machine 20 (see also FIG. 1), while permitting film cassette 24 to be configured in either a normal horizontal position, or in a vertical position to accommodate broad heavyset patients. As mentioned above, when using a linear grid, alignment is critical across the grid lines, and is less critical along the direction of the grid lines. The grid lines of grid 21 within grid cassette 33 runs substantially parallel to center line 41 which, in turn, is substantially parallel to the longitudinal direction of the lead slats in anti-scatter grid 21 contained within grid cassette 33. Grid cassette 33 also includes two pairs 38, 39 of radiopaque markers which permit an assessment of the accuracy of x-ray beam alignment. Pairs 38 and 39 of radiopaque markers are preferably made of tantalum, but can be of any radiopaque material. The preferred dimensions for each marker of pairs 38 and 39 are 1.times.3.times.0.7 mm, but the markers can be of any dimension or shape. Pairs 38 and 39 of radiopaque markers are discussed below in more detail with respect to FIGS. 8A and 8B and 9A and 9B. Also included in grid cassette 33 are reflector elements 28, shown in more detail in FIGS. 5 and 6. Reflector elements 28 are located a distance d from center line 41 of grid cassette 33. In accordance with the present invention, distance d is substantially equal to the distance between the line or spot generated by light projector 27 and central x-ray beam 23 (see FIG. 2) and in the embodiment shown in FIG. 3, d, is preferably 6.5 inches (16.5 cm). The construction of reflector elements 28 is presented below in more detail with reference to FIGS. 5 and 6. Finally, grid cassette 33 includes hand hold 42 to facilitate the handling of grid cassette 33, and the positioning of grid cassette 33 behind a patient. Although two reflector elements 28 are included in grid cassette 33 of FIG. 3, only one is used at any one time for alignment purposes, and the other is included to accommodate other orientations between grid cassette 33 and x-ray machine 20. Referring now to FIGS. 5 and 6, the details of reflector elements 28 are presented. The view of FIG. 5 is from the backside of grid cassette 33 shown in FIG. 3 (i.e., from the same side as portable x-ray machine 20 (see FIG. 2)), and FIG. 6 is taken through section 6--6 of FIG. 5. Reflector element 28 is substantially rectangular in shape, and in the preferred embodiment is 1.5 wide.times.3 long.times.1 inch deep (3.8.times.7.6.times.2.5 cm). Located in the bottom of reflector element 28 is a reflecting surface, such as mirror 43, and covering a top of reflector element 28 is cover 44 which is preferably substantially transparent, and which is preferably made from Lexan brand plastic material. Cover 44 supports imaging surface 45 which is preferably a porous layer which allows light to pass therethrough while allowing a light image to form thereon. An exemplary material is a thin elastic vinyl material, such as rear projection screen material available from the Edmond Scientific Company. Alternatively, cover 44 and imaging surface 45 can be combined into a single unit. For example, the surface of cover 44 could be frosted, or cover 44 could be translucent. As mentioned above, light projector 27 projects light line or spot 40 onto the surface of grid cassette 33 where reflector elements 28 are located. The projection of light line or spot 40 onto reflector elements 28 facilitates alignment of grid cassette 33 relative to portable x-ray machine 20, as presented more clearly with reference to FIGS. 7A and 7B. In FIG. 7A, light beam 30 projected from light projector 27 forms incident light line or spot 40 on imaging surface 45 of reflector element 28. Light beam 30 continues through cover 44 and is reflected by mirror 43 and back through cover 44 to produce reflected light line or spot 40' on imaging surface 45. The existence of two light images 40, 40' on imaging surface 45 of reflector element 28 is indicative of misalignment between grid cassette 33 and portable x-ray machine 20. The magnitude of misalignment is indicated by the amount of separation of images 40, 40' appearing on imaging surface 45 of reflector element 28. When grid cassette 33 and portable x-ray machine 20 are correctly aligned, as illustrated by the representation of FIG. 7B, light beam 30 is reflected back upon itself when it strikes mirror 43 thus rendering images 40 and 40' substantially collinear. Use of a light line for alignment is more clearly illustrated below with reference to FIGS. 13A and 13B, and use of a light spot for alignment is more clearly illustrated below with reference to FIGS. 14A, B and C. Referring now to FIGS. 8A and 8B, the positioning of radiopaque marker pairs 38 and 39 is presented in more detail. Radiopaque marker pairs 38 and 39 are positioned within front surface 46 and rear surface 47 of grid cassette 33 so that shadows of the individual markers of radiopaque marker pairs 38 and 39 will appear on a developed x-ray film that was exposed while being held in grid cassette 33. The individual radiopaque markers of each pair of markers 38 and 39 are positioned in front surface 46 and in rear surface 47 of grid cassette 33 so that when the x-ray beam is properly aligned and centered, the resultant shadows on the radiograph will be substantially superimposed as illustrated in FIG. 9A. Misalignment or decentering of the x-ray beam relative to grid cassette 33 will result in misregistration of the images of marker pairs 38 and 39, as illustrated in FIG. 9B. Such misalignment can be quantified with a calibrated magnifying loupe, and alignment errors can be determined in both vertical and horizontal dimensions within 1.degree.. The horizontal axes of the graphs of FIGS. 15 and 16, described in more detail below, were determined in this manner. Although two pairs 38, 39 of radiopaque markers are preferred, it will be understood that only one pair is necessary to accomplish the purposes of the present invention. Use of two pairs of markers increases the chances that at least one of the pairs will appear on a radiograph. In addition, if both pairs are visible, the distance between the pairs on the radiograph relative to the distance between the pairs on the grid is indicative of the focal distance at which the radiograph was exposed. Referring now to FIG. 10, presented in more detail is the configuration of portable x-ray machine 20, collimator housing 26 and light projector 27 in accordance with the present invention. In FIG. 10, light projector 27 is shown mounted to face plate 48 of collimator housing 26 of a conventional portable x-ray machine, for example, a type AMX 4, portable x-ray machine available from the General Electric Company. The present invention, of course, could be used with other types of portable x-ray machines, and light projector 27 could be integrated within collimator housing 26, rather than mounted to face plate 48. As described above with reference to FIG. 2, light projector 27 projects light beam 30. Light beam 30 is fan-shaped when projecting a line, and is substantially circular or elliptical in cross-section when projecting a spot, and is projected in a plane which is substantially parallel to face plate 48 and central x-ray beam 23. Light projector 27 is positioned relative to central x-ray beam 23 so that light beam 30 is projected at a distance, d, from central x-ray beam 23. This distance, d, is the same as distance, d, shown in FIG. 3 between center line 41 of grid cassette 33 and light line or spot 40 projected on the surface of grid cassette 33. Light projector 27 is rotatable slightly about axis 49 in order to project light beam 30 to one edge, or the other, of grid cassette 33. In operation, a technician will rotate light projector 27 about axis 49, until light beam 30 strikes the surface of grid cassette 33 to form light line or spot 40 upon reflector element 28, as shown in FIG. 3. Light projector 27 is powered through power cable 51 from the collimation light circuit within collimator housing 26 and is activated whenever the collimation light within collimator housing 26 is activated. Alternatively, light projector 27 could be powered from a battery pack to simplify retrofitting existing x-ray machines. FIG. 11 presents a view through section 11--11 of light projector 27 in FIG. 10. Light projector 27 includes housing 52 with front cover plate 53 and rear cover plate 54. Housing 52 and plates 53 and 54 can be machined, cast or molded from aluminum, or plastic, or other suitable material. Within housing 52 is mounted solid state laser 56 which is powered, as described above, through power cable 51. Solid state laser 56 is preferably a LAS-200-670-5 type laser available from the Laser Max Company of Rochester, N.Y., which emits laser light beam 57 at 670 nanometers, with a power of 4.75 milliwatts. Laser light beam 57 is substantially elliptical in cross-section, with dimensions of approximately 0.8.times.3.3 millimeters. Behind front cover plate 53 is mounted lens mount 58 upon which is mounted cylinder lens 59. Cylinder lens 59 converts laser light beam 57 into fan-shaped light beam 30 which is projected from light projector 27 through optical window 61 mounted in front cover plate 53. Fan-shaped light beam 30 lies in a plane which is substantially perpendicular to the plane of the page of FIG. 11. Alternatively, when projecting a spot, lens mount 58 and cylinder lens 59 can be eliminated, allowing light beam 57 to pass directly through optical window 61. Although in the preferred embodiment, a solid state laser light source is used, it will be understood by those of skill in this art that other types of light sources, including non-laser light sources, could be used, without departing from the spirit and scope of the invention, as long as light projector 27 projects a line or spot of light onto reflector elements 28 mounted in or on grid cassette 33 (see also FIG. 2). Light projector 27 is pivotally mounted to pivot plate 61 by pivot pin 62 to allow rotation of light projector 27 about axis 49 (also see FIG. 10). Pivot plate 61 is connected to a first end of spacer 63, the second end of which (not shown) is mounted to face plate 48 of collimator 26 (see FIG. 10). Referring now to FIG. 12, disclosed is the transparent cover plate 64 of collimator housing 26, in accordance with the present invention. Cover plate 64 includes standard cross-hairs 66, 67, the shadows of which are projected on a patient during the alignment of portable x-ray machine 20 (see also FIG. 2). The intersection of cross-hairs 67 and 68 is substantially coincident with central x-ray beam 23. In accordance with the present invention, added to transparent cover plate 64 is opaque focus line 29 which, as described above, with reference to FIG. 2, casts a shadow onto the surface of grid cassette 33 holding anti-scatter grid 21. During alignment of portable x-ray machine 20 in accordance with the present invention, a technician will position portable x-ray machine 20 a distance from grid cassette 33 until the shadow 31 of opaque line 29 appearing on the surface of grid 33 coincides with light line or spot 40 produced by light beam 30. The focal distance depends upon the focus distance of the grid being used, and in the preferred embodiment the focal distance is 48 inches (122 cm). In addition, to ensure proper centering of grid cassette 33 and central x-ray beam 23, grid cassette 33 is moved until light line or spot 40 strikes a predetermined portion of grid cassette 33, for example, the center of reflector element 28. Referring now to FIGS. 13A and 13B, presented is an alternative embodiment of reflector element 28, in accordance with the present invention. In FIGS. 13A and 13B, rather than being integrally formed with grid cassette 33 as shown in FIG. 3, reflector element 28 is mounted temporarily to grid cassette 33 by use of bracket 69 and magnets (not shown). In the embodiment shown in FIGS. 13A and 13B, reflector element 28 is temporarily mounted to a corner of grid cassette 33, and during the alignment procedure, light line 40 is projected onto reflector element 28. Reflector element 28 shown in FIGS. 13A and 13B is a substantially rectangular-shaped box with a light reflecting surface (not shown) located in a bottom thereof. Imaging surface 45, supported by a transparent plastic plate, facilitates the viewing of light line 40. In FIG. 13A, the single light line 40 indicates accurate alignment between grid cassette 33 and portable x-ray machine 20, whereas the existence of two light lines 40 and 40' shown in FIG. 13B is indicative of misalignment between grid cassette 33 and portable x-ray machine 20 (see also FIG. 2). The embodiments described that use light line 40 are for the purpose of aligning a linear grid which requires alignment in only one dimension (i.e., across the grid lines). Two dimensional grids, for example cross-hatched grids or pinhole grids, require alignment in two dimensions. In other words, referring again to FIG. 2, when using a two-dimensional grid, central x-ray beam 23 should be substantially perpendicular to the plane of grid cassette 33. In accordance with the present invention, to align central x-ray beam 23 and grid cassette 33 in two dimensions, a spot of light is projected by light projector 27. Alignment of grid cassette 33 using a light spot is illustrated in FIGS. 14A, B and C. In FIGS. 14A, B and C, reflector element 28 is the same as that used for linear grid alignment described earlier, and includes a light reflecting surface (not shown in FIGS. 14A, B and C) and an imaging surface 45. Reflector element 28 in FIGS. 14A, B and C is shown temporarily attached to grid cassette 33 with bracket 69. However, reflector element 28 may be integrally formed with grid cassette 33 as shown, for example, in FIGS. 3, 4, 5 and 6. In FIG. 14A, separation between incident light spot 40 and reflected light spot 40' indicates misalignment about the y-axis (drawn on grid cassette 33) between grid cassette 33 and central x-ray beam 23 (see also FIG. 2). In FIG. 14B, separation between incident light spot 40 and reflected light spot 40" indicates misalignment about the x-axis. The single light spot 40 shown in FIG. 14C indicates accurate alignment about both x- and y-axes. In other words, in FIG. 14C, central x-ray beam 23 is substantially perpendicular to the surface of grid cassette 33. Although light spots 40, 40', 40" are elliptical as shown in FIGS. 14A, B and C, it will be understood that light spots of any shape could be used without departing from the scope of the invention. To demonstrate the advantages of the alignment system of the present invention, 400 consecutive conventional portable chest x-ray examinations were identified and reviewed. Of these, 161 had been performed with images of tantalum markers (for example, like those shown in FIGS. 9A and 9B). With the images of the alignment markers obscured, three experienced chest radiologists reviewed the radiographs, and scored each case for overall subjective image quality, using a ten point scale where ten indicated the highest subjective quality, and one the lowest. The results are shown in the graph of FIG. 15. The angulation error plotted as the horizontal axis in FIG. 15 was calculated for each radiograph from the relative positions of the images of the radiopaque markers. As can be seen with reference to FIG. 15, all cases with the highest quality ratings also had accurate grid alignment. Several cases which were accurately aligned received a low quality rating, which was believed to be due to overexposure. However, no case with a large grid angulation error achieved a high quality rating, regardless of exposure. In a separate experiment, grid alignment was determined retrospectively by use of the radiopaque markers for a random series of 200 portable chest radiographs obtained with conventional manual alignment techniques. The percentage of the total cases plotted versus angulation error in heavy cross-hatched bars 72 of the bar chart of FIG. 16. In several cases, excellent manual alignment was obtained, which was believed to be due to intensive technologist training, augmented by feedback provided by the radiopaque alignment markers. Also in FIG. 16 is the range of angulation alignment error for 50 consecutive portable chest radiographs obtained using the optical alignment system of the present invention, plotted with bars 73 having light cross-hatching. As can be seen with reference to FIG. 16, the range of angulation alignment error using the optical grid alignment system of the present invention, is markedly reduced. Major angulation alignment errors, which are known to significantly degrade radiograph quality, are essentially eliminated with practice of the present invention. Practice of the present invention also allows use of a wide latitude x-ray film with a 6:1 anti-scatter grid and x-ray beam energy of 90 KV or lower, while providing consistently high radiograph image quality in portable radiography. It has been found that use of the optical grid alignment system of the present invention does not interfere with the normal operation of the portable x-ray equipment, while providing focus distance and grid alignment information which can be easily assimilated. In addition, because accurate grid alignment is consistent and predictable with use of the present invention, the problem of incorrect exposure of the radiograph can be reduced. Though primarily developed for portable chest radiography, the optical alignment system of the present invention is adaptable to other types of bedside x-ray examination, including non-standard views such as lateral and decubitus projections, where accurate grid alignment is particularly important due to the large amount of scatter generated. Although the present invention has been described with reference to selected preferred embodiments, it will be understood by those of ordinary school in this art that deletions, additions, or modifications can be made to the disclosed preferred embodiments, without departing from the spirit and scope of the present invention.