Patent Number: 052415783
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a grid alignment system for portable radiography. Portable radiography accounts for an increasing proportion of x-ray examinations performed in hospitals. In the University of Chicago hospitals, approximately 50% of all chest radiographs are obtained at the bedside with portable radiographic apparatus. Though the clinical importance of these examinations is beyond question, the image quality is generally inferior to that obtained with fixed radiographic apparatus in an x-ray department. The inferior image quality obtained using portable radiography apparatus is widely recognized, and is a source of concern to radiologists and clinicians. This poor image quality is commonly attributed to intrinsic limitations of portable radiography apparatus, however, it is in fact mainly due to uncontrolled scattered radiation, which fogs the radiograph, reducing contrast and obscuring diagnostic information. Use of an accurately aligned anti-scatter grid can provide consistently high image quality, but precise alignment of the grid relative to the x-ray source is essential for good results. Such precise alignment is difficult to achieve with conventional manual or "eye-ball" techniques. For portable radiography, excellent results can be achieved with a 6:1 or 8:1 anti-scatter grid, provided that the x-ray beam energy is no greater than 90 KV, and provided that the anti-scatter grid is accurately aligned with respect to the x-ray source. Referring to FIG. 1, accurate alignment of portable radiographic machine 20 relative to anti-scatter grid 21 and x-ray film 22 is illustrated in position A, while inaccurate alignment is illustrated in position B. In the case of a conventional lead strip linear anti-scatter grid, alignment is critical only in one dimension, that is, across the grid lines. Moderate angulation error along the direction of the grid lines does not significantly impair image quality. In other words, referring again to FIG. 1, moderate misalignment about a horizontal axis lying within the plane of the page is not so critical, whereas alignment about an axis perpendicular to the plane of the page is critical. Further, with a focused grid, it is also necessary for central x-ray beam 23 of portable x-ray machine 20 to be centered accurately with respect to anti-scatter grid 21. Further, variations in focus distance are important, but mainly affect film density, provided that the portable x-ray machine 20 is properly centered and aligned relative to anti-scatter grid 21. In the case of a two dimensional grid, for example a cross-hatch grid or pinhole grid, alignment is critical in two dimensions. A system that addresses this alignment problem is presented in U.S. Pat. No. 4,752,948, issued Jun. 21, 1988, and assigned to the same assignee as the present application. The disclosure of U.S Pat. No. 4,752,948 is expressly incorporated herein by reference. While adequately addressing the problem of alignment between x-ray beam and anti-scatter grid in a portable x-ray apparatus, this patented device presents a mechanical system which has proven somewhat difficult to retrofit to existing portable x-ray apparatus, or to use with very ill patients who are unable to cooperate. Therefore, a need exists for a simple alignment system which can be easily retrofitted to existing portable x-ray apparatus, and which can be used in the most difficult clinical situations. Commercially available grid cassettes (x-ray film holders incorporating anti-scatter grids) include a 14.times.17 inch (35.6.times.43.2 cm) lead strip grid encased in nylon/plastic material. These grid cassettes are relatively crudely manufactured and are heavy and cumbersome to handle. When the grid lines are oriented vertically, the total transverse dimension of the grid is 14 inches (35.6 cm). Therefore, in heavy-set broad patients, it is necessary to rotate the grid cassette 90.degree. so that the grid lines run transversely, orienting the longer dimension of the grid horizontally. This maneuver, which has been found to be necessary in between 25% and 50% of portable chest radiographs, frequently results in severe misalignment. This is so because the necessary vertical adjustments which must be performed accurately to align the x-ray beam with the grid are more difficult to judge accurately when the grid is transversely oriented In addition, transversely orienting the grid requires the x-ray source and collimator to be rotated, an additional adjustment step. Therefore, it would also be desirable to maintain the anti-scatter grid orientation in a vertical direction for all patients, while permitting the x-ray film to be oriented either vertically or horizontally facilitating the accommodation of heavy-set broad patients. SUMMARY OF THE INVENTION The present invention in large part avoids the above-noted problems of the prior approaches to alignment between a portable x-ray apparatus and an anti-scatter grid by providing a simple grid alignment system, which can be retrofitted to existing equipment. In addition, the invention provides a new grid cassette which facilitates the horizontal or vertical orientation of x-ray film in portable radiography, while permitting the anti-scatter grid to maintain a single orientation. To facilitate accurate alignment and centering of the central x-ray beam with a focused grid in a clinical setting, the present invention employs a light projector, specifically, a laser light projector, and a unique compact reflector element. The projector is mounted in or on the collimator housing of a portable x-ray machine, and can be powered from the collimation light circuit or from a separate battery. The light projector is positioned so that a light line or spot is projected parallel to the central x-ray beam of the x-ray source. An opaque line on the transparent front surface of the collimator housing appears as a dark shadow within the field projected by the collimation light. The coincidence of this dark shadow with the laser light is indicative of placement of the grid cassette at the proper focal distance from the x-ray source. Angulation errors between the x-ray source and grid cassette are indicated by a compact reflector element which either temporarily attaches to or is formed integrally with the grid cassette. The front of the reflector element is covered by an imaging surface, and a reflecting surface is located behind the imaging surface. The incident light line or spot creates an image on the imaging surface, and the light line or spot reflected from the reflecting surface also forms an image on the imaging surface. The amount of separation between the two images on the imaging surface is indicative of the magnitude of angulation alignment error between the grid cassette and the x-ray source. When alignment is accurate, the incident light line or spot and reflected light line or spot are superimposed on the imaging surface. To confirm the accuracy of beam alignment, the present invention contemplates at least one pair of small radiopaque markers which are inserted into the front and rear surfaces of the grid cassette. Each pair of markers is positioned so that when the x-ray beam is perfectly aligned and centered, the images produced by the markers on the x-ray film are substantially superimposed. Misalignment or decentering of the x-ray beam results in misregistration of the markers, and the amount of misalignment can be quantified by assessing the amount of misregistration. The present invention also contemplates a grid cassette which is capable of accommodating either vertically or horizontally oriented film cassettes without requiring reorientation of the grid cassette. This allows accommodation of broad, heavyset patients without the misalignment errors that typically occur when the grid cassette is reoriented. The grid cassette also includes an integrally formed hand hold for easy portability and to facilitate placing the grid cassette behind a patient. These and other features and advantages of the present invention will become apparent to one of skill in this art with reference to the drawings and following detailed description of the preferred embodiments.