Patent Number: 
Section: description

The present invention relates to a multiple layer multileaf collimator with improved resolution. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiment will be readily apparent to those skilled in the art and the generic principles herein may be applied to other embodiments. Thus, the present invention is not intended to be limited to the embodiment shown but is to be accorded the widest scope consistent with the principles and features described herein. FIG. 1 illustrates a radiation treatment device 2 which utilizes a multiple layer multileaf collimator 4 in accordance with the present invention along with a treatment processing unit 100. The radiation treatment device 2 comprises a gantry 6 which can be swiveled around a horizontal axis of rotation 8 in the course of therapeutic treatment. Collimator 4 is fastened to a projection of gantry 6. To generate the high-powered radiation required for the therapy, a linear accelerator is located in gantry 6. The axis of the radiation bundle emitted from the linear accelerator and gantry 6 is designated 10. Electron, photon, or any other detectable radiation can be used for the therapy. During the treatment, the radiation beam is trained on a zone 12 of an object 13, for example, a patient who is to be treated, and who lies at the isocenter of the gantry rotation. The rotational axis 8 of the gantry 6, the rotational axis 14 of a treatment table 16, and the beam axis 10 all preferably intersect in the isocenter. The multiple layer multileaf collimator 4 in accordance with the present invention suitably comprises two pairs of opposing elongated radiation blocking leaves, the pairs in separate planes and offset at a desired angle relative to one another. As mentioned above, a prior art approach to multiple multileaf collimator design is described in U.S. Pat. No. 5,591,983 (hereinafter Yao). FIG. 2a illustrates a side view of a prior art multiple layer collimator 4xe2x80x2 as presented in Yao. Two identical layers, an upper layer 20 and a lower layer 22, of pairs of opposed leaves are shown. The top layer 20 comprises a middle section having a plurality of relatively narrow leaves 24 positioned in a side-by-side relationship, which is flanked on its left side by a relatively wide trimmer leaf 26 and on its right side by a relatively wide end leaf 28. The construction of bottom layer 22 is a mirror image of layer 20 and therefor common reference numbers are used for leaves 26 and 28. However, since in the middle section of layers 20 and 22, the narrow leaves are physically overlapping, the narrow leaves of layer 22 are referred by reference 30. It should also be noted that although wide and narrow leaves are shown in this embodiment, the leaves can all be of substantially the same widths or can be different widths and they could still be utilized within the spirit and scope of the present invention. As shown in a top view of collimator 4xe2x80x2 in FIG. 2b, top layer 20 being shown in solid lines and bottom layer 22 being shown in dashed lines, frame 32 supports each of the leaves 24 and 28 of the top layer 20 and each of leaves 30 and 28 of lower layer 22 in a paired opposed relationship, so that they are independently movable in their longitudinal dimension into and out of beam axis 10 (the Y direction shown in FIG. 2b). In general, the maximum size field is a rectangle of dimension Wxc3x97L. For the illustration of FIG. 2b, the leaves 24 and 30 are shown in various positions to create shape 34. The operation of the leaves of layer 20 and layer 22 for creating a treatment field is as is conventional in prior art single layer multileaf collimator arrangements, which are well understood by those skilled in the art. FIG. 3 illustrates a portion of the top view of FIG. 2b in greater detail. As functionally shown therein, frame 32 includes a plurality of motors 40 mounted thereon which are used in a conventional manner to individually position the leaves, e.g, 24, and 30, of the collimator 4xe2x80x2 into and out of the radiation beam for controllably defining the treatment field. One example of drive means (not shown) suitable for this is an individually driven worm gear for individually engaging a toothed track or floating nut mounted on each leaf. Details of one such prior art leaf driving means are provided in U.S. Pat. No. 5,160,847, issued to Leavitt, et al., on Nov. 3, 1992. While providing a reduction in leakage through the arrangement of one layer over another and a shift of the blades in a lateral direction to cover gaps between blades in a lower layer, the prior art multiple layer multileaf collimator lacks preferred resolution in isolating the treatment area and avoiding undue exposure of healthy tissue or organs to the radiation therapy. The present invention improves resolution in a multiple layer multileaf collimator by providing a multiple layer multileaf collimator 4 comprising two layers of opposing pairs of elongated radiation blocking leaves, where the layers are not linked and can be rotated to within a desired angle, e.g., approximately between 0xc2x0 and 90xc2x0, of one another. In a preferred embodiment, a top layer is capably positioned substantially between about 0xc2x0 and 90xc2x0 relative to a bottom layer. FIG. 4 illustrates a three-dimensional view of two multileaf layers, 22 and 50, in accordance with a preferred embodiment of the present invention. For convenience of illustration, layer 22 is described as being provided in a manner equivalent to layer 22 of the prior art shown in FIG. 2a. As shown in the example configuration of FIG. 4, multileaf layer 50 is positioned to be substantially perpendicular to multileaf layer 22. Layer 50 suitably comprises a multileaf collimator that functions similarly to multileaf layer 22 as described above, however, the positioning of the multileaf layer 50 to a desired angle through a conventional motor/control mechanism (not shown) allows greater resolution of coverage for targets than typical sold block arrangements or even the parallel multiple multileaf arrangement of Yao. Further, it should be appreciated that while in the arrangement of Yao differing widths are used for the end and trimmer leafs as compared with the middle section leaves, this serves as one example of a suitable leaf set. Of course, the present invention is capably achieved with other leaf arrangements, including those having leaves of substantially equivalent widths. FIG. 5 illustrates more particularly coverage capabilities for a multiple layer multileaf collimator arrangement in accordance with the present invention. In FIG. 5, a partial top view of one-half of each multileaf layer 22 and 50 is shown, where a top multileaf layer 50 is positioned at a preferred angle of about 90xc2x0 (i.e., substantially perpendicular) relative to a bottom multileaf layer 22. As further shown in FIG. 5, the individual leaves of each layer, 30 in layer 22 and 24xe2x80x2 in layer 50, are separately movable via motors 40 attached to frame means 32 and 32xe2x80x2, respectively, operating as described above. The placement of multileaf layer 50 at an angular offset, e.g., of about 90xc2x0, allows the individual leaves 24xe2x80x2 to be moved in close proximity to and in a close correspondence with a target 60, e.g., a tumor, from multiple directions rather than being limited to merely equivalent directions as in the layer arrangement of Yao. Thus, the resolution of coverage for targets is improved, as well as gaining the advantage that because there is crossing over between the top and bottom layer, the leakage areas between leaves are protected. It should be appreciated that the multiple multileaf arrangement in accordance with the present invention is suitable for use with any type of multileaf design type or beam defining system, as is known and/or in use in the art. For example, the present invention is applicable in single focus and double focus multileaf collimators. An example of a single focus multileaf collimator is provided in the aforementioned U.S. Pat. No. 5,166,531, while an example of a double focus multileaf collimator is provided in U.S. Pat. No. 4,463,263 issued Jul. 31, 1984 to Brahme. Thus, the present invention is suitable with multileaf collimator designs that employ blades/leaves of varying thickness, size, depth, etc. Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.