Patent Number: 060977876
Section: summary

FIELD OF THE INVENTION The invention relates to a radiation emitting device, and more particularly to a system and method for calculating scatter radiation of the radiation emitting device. BACKGROUND OF THE INVENTION Radiation emitting devices are generally known and used, for instance, as radiation therapy devices for the treatment of patients. A radiation therapy device usually includes a gantry which can be swiveled around a horizontal axis of rotation in the course of therapeutic treatment. A linear accelerator is typically located in the gantry for generating a high-energy radiation beam for therapy. This high-energy radiation beam can be an electron radiation or photon (x-ray) beam. During treatment, this radiation beam is typically trained on a zone of a patient lying in the isocenter of the gantry rotation. In order to provide a proper dose of radiation to a patient, a dose chamber may be used. A dose chamber accumulates dose deliveries from the radiation beam. When the dose of the radiation beam reaches a given number of counts, then the radiation beam may be turned off. The unit with which the dose chamber counts is a "monitor unit". Determining how many monitor units to set the dose chamber so that the patient receives a proper dose is typically termed as dosimetry. Once a dose for a patient is determined, this dose typically needs to be translated into monitor units. There may be several factors in translating the dose into monitor units, such as attenuation through the patient, accounting for curvature of patient surface, and accounting of scattered radiation inside the patient. In determining a dose to a patient, a hypothetical plane, often referred to as a calculation plane, a patient plane, or an isocentric plane, directly above the patient may be used in determining the distribution of radiation intensity over the patient. The unit of measurement for radiation intensity is fluence, which is the number of photons per area per time. This calculation plane over the patient may be divided into squares, herein referred to as calculation squares. In determining the fluence over the calculation plane, only one calculation square above the immediate target is typically calculated for the fluence due to the complication of calculating fluence over all of the squares in the calculation plane. A problem with calculating the fluence in only one calculation square is that the approximation for the remaining calculation squares may be inaccurate. In particular, in the field of intensity modulation, this type of approximation for fluence of the calculation plane may be wholly inadequate. Intensity modulation typically improves the ratio of radiation dose to critical structures versus dose to target. Improving this ratio is highly desirable since it is assumed that non-target areas are receiving radiation. A common goal is to maximize the radiation dose to a target, such as the tumor, while minimizing the radiation dose to healthy tissue. Another method for calculating the fluence over the calculation plane attempts to calculate the fluence over each calculation square by using ray tracings through a thin aperture. A potential problem with this conventional calculation is that the volume of ray tracing calculations are typically substantial and a substantial amount of processing power is required. Additionally, a radiation aperture, such as a collimator, typically has enough of a thickness to effect the calculations. Accordingly, calculating with the assumption that the aperture is very thin may result in errors. It would be desirable to have a method for calculating the fluence over the calculation plane which is fast, efficient, and accurate. The present invention addresses such a need. SUMMARY OF THE INVENTION The present invention relates to a fast and accurate method for calculating fluence of a calculation plane over a patient. According to an embodiment of the present invention, only a subset of the collimator leaves are analyzed for the fluence calculation, thus reducing the number of calculations required. Additionally, pre-integrated values of scatter strips, associated with each point of the calculation plane, may be referenced in a lookup table. The use of these pre-integrated values allows the avoidance of adding the fluence contribution of each square on the scattering plane. Rather, pre-calculated values of a subset of the scattering plane (scatter strip) may be referenced and combined, thus reducing the number of calculations required for a final scatter contribution to a point on the calculation plane. Further, the thickness of the collimator leaves is considered in the fluence calculation, thus providing a more accurate model for the scatter contributions of points on the scattering plane. According to an embodiment of the present invention, for each square (herein referred to as a point) on the calculation plane, a subset of collimator leaves which may affect fluence calculation is determined. In addition, scatter strips in the scattering plane associated with the analyzed point on the calculation plane is determined. For every line that can be traced from the calculation point to each scatter strip, it is determined which leaves intersect the traced line on the bottom of the leaf and which leaves intersect the traced line on the top of the leaf, thus taking into consideration the thickness of the leaves within the determined subset of the leaves. Pre-integrated values of the scatter strips may then be referenced in a lookup table to assist in the performance of the fluence calculation over each calculation point in the calculation plane. A method according to an embodiment of the present invention for calculating scatter radiation is presented. The method comprises providing a scattering plane, wherein the scattering plane is divided into a plurality of sections. A scatter strip associated with the scattering plane is determined, wherein the scatter strip contains at least two of the plurality of sections. A fluence value associated with the scatter strip is also determined. A system according to an embodiment of the present invention for calculating scatter radiation is also presented. The system comprises a processor configured to provide a scattering plane, wherein the scattering plane is divided into a plurality of sections. The processor is also configured to determine a scatter strip associated with the scattering plane, wherein the scatter strip contains at least two of the plurality of sections. The processor is further configured to determine a fluence value associated with the scatter strip. A memory is coupled with the processor, wherein the memory is configured to provide the processor with instructions. Another method according to an embodiment for calculating scatter radiation is also provided. The method comprises determining a scatter strip associated with a scattering plane; determining a subset of collimator leaves; and calculating fluence, wherein the fluence calculation is related to the scatter strip and the subset of collimator leaves. Another system according to an embodiment of the present invention for calculating scatter radiation is also presented. The system comprises a processor configured to determine a scatter strip associated with a scattering plane, determine a subset of collimator leaves and calculate fluence, wherein the fluence calculation is related to the scatter strip and the subset of collimator leaves. The system also includes a memory coupled to the processor, the memory being configured to provide the processor with instructions. In another aspect of the invention, a method according to an embodiment of the present invention for calculating scatter radiation is presented. The method comprises providing a collimator leaf position and determining a subset of collimator leaves. The method also calculates fluence, wherein the fluence calculation is related to the subset of collimator leaves. A system according to an embodiment of the present invention for calculating scatter radiation is also presented. The system comprises a memory configured to provide a collimator leaf position and a processor coupled to the memory. The processor is configured to determine a subset of collimator leaves, and is also configured to calculate fluence, wherein the fluence calculation is related to the subset of collimator leaves. Another method according to an embodiment of the present invention for calculating scatter radiation is presented. The method comprises determining whether a ray traced from a calculation point to a portion of a scattering plane intersects a first portion of a collimator leaf or a second portion of the collimator leaf. The method also calculates fluence, wherein the fluence calculation is related to a first intersection, if the ray intersects the first portion; and wherein the fluence calculation is related to a second intersection, if the ray intersects the second portion. Yet another system according to an embodiment of the present invention for calculating scatter radiation is presented. The system comprises a processor configured to determine whether a ray traced from a calculation point to a portion of a scattering plane intersects a first portion of a collimator leaf or a second portion of the collimator leaf; the processor also being configured to calculate fluence, wherein the fluence calculation is related to a first intersection, if the ray intersects the first portion; and wherein the fluence calculation is related to a second intersection, if the ray intersects the second portion. A memory is coupled with the processor, the memory being configured to provide the processor with instructions.