X-ray or gamma ray systems or devices – Accessory – Alignment
Reexamination Certificate
1999-07-26
2001-07-17
Kim, Robert H. (Department: 2882)
X-ray or gamma ray systems or devices
Accessory
Alignment
C378S204000, C378S163000, C378S164000
Reexamination Certificate
active
06260999
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to imaging systems, typically used for radiation treatment. In particular, the present invention relates to imaging systems for linear accelerators (linacs) which may be used in radiation therapy.
BACKGROUND OF THE INVENTION
The use of a linear accelerator in radiation therapy is generally known. Such linear accelerators are typically used for treating patients with x-rays or electron beams. Such x-rays are created when high energy electrons are decelerated in a target material such as tungsten. Alternatively, the electrons themselves may be used directly for treatment.
The major modules in a linear accelerator typically include a movable gantry with a treatment head, a stand, a control console and a treatment couch. The stand is typically anchored firmly to the floor and the gantry typically rotates on bearings in the stand. The operational accelerator structure, housed in the gantry, typically rotates about a horizontal axis fixed by the stand for treatment of a patient lying on the treatment couch.
In the radiation therapy treatment of a patient, geometric accuracy is a very important factor to the success of the treatment. The goal is commonly to hit a specific target, such as a tumor, and miss critical regions of the patient's body, such as the spine. Properly positioning the patient may be a critical issue in avoiding damage to tissue and critical organs. Typically, within reason, the more accurate the x-ray delivery to the exact target, the higher the dose a patient may receive.
An electronic portal image may be captured for the purpose of determining whether the target on the patient is within the treatment beam and whether critical regions of the patient are missed. Typically, people are responsible for taking these images and determining if the patient is positioned correctly. If film is used for the image, then the film must typically be developed and placed next to a reference image to compare the two images. The reference image is typically an x-ray image, which has been marked up by the patient's doctor. The two images are typically compared to ensure that the area which is actually being treated is the same area that the patient's doctor has marked up in the reference image. This comparison is typically a visual comparison. A technician may visually compare the two images and try to match visual landmarks between the two images. A potential problem with this visual comparison is human error in the comparison between the two images. The person making the comparison is commonly looking for very small errors, on the order of millimeters, which are normally very difficult to visually compare.
Another issue which may compound the problem is that high energy x-ray is commonly used. Accordingly, most of the x-ray goes through the body of the patient and a bony landmark is typically needed to give the person making the comparison an indication of the image reference. This visual comparison between a vague patient positioning image and a reference image may be substantially inaccurate.
Electronic portal imaging systems may produce an image without the use of film, however, a person still needs to visually compare the resulting image with a reference image. Some measuring tools may be used on the electronic portal imaging, however, the comparison is still substantially a manual process.
Although there are known algorithms for comparing two images electronically, there is typically no way of ensuring that the two images may be compared with the same frame of reference to ensure a proper match. The frame of reference of the portal imaging device is typically unknown due to mechanical errors. The gantry of the portal imaging device typically rotates around the patient. When the gantry is rotated, there is commonly a mechanical sag to the gantry which may shift the frame of reference of the image. Additionally, the detector housing of the imaging device is typically retractable into the gantry and the detector housing may not be exactly in the same position every time it is extended. Although the mechanical sag may be fairly slight, a millimeter or half a millimeter may still make a difference in patient positioning. Accordingly, the image may be offset compared to the reference image.
Once an image has been compared to the reference image, a multi-leaf collimator may be used to direct the treatment beam onto a selected area of the patient. However, without a well defined point of reference, the collimator may direct the treatment beam slightly off target.
It would be desirable to accurately identify a frame of reference so that an electronic comparison of the patient positioning image and the reference image may be performed to provide a precise and accurate comparison. It would also be desirable to accurately identify a point of reference for the multi-leaf collimator so that the collimator may be properly calibrated for the patient. The present invention addresses such needs.
For further background information on the construction and operation of a typical radiation therapy device, a brochure entitled “A Primer On Theory And Operation Of Linear Accelerators In Radiation Therapy”, U.S. Department of Commerce, National Technical Information Service, December 1981, may be referenced.
SUMMARY OF THE INVENTION
The present invention relates to a system and method for finding an isocenter of an image. Once the isocenter of the image has been identified, every other point in the image may be referenced with respect to the isocenter. Accordingly, finding the isocenter may facilitate matching a frame of reference for the image with a reference image. Once a frame of reference has been identified for both images, known algorithms may be used to electronically compare the two images for an accurate and precise measurement. Additionally, finding the isocenter may also assist in calibrating a radiation therapy device, such as a collimator.
According to an embodiment of the present invention, an isocenter of an image may be found by using a multi-leaf collimator. According to this embodiment, a center leaf is projected into the center of the x-ray field with all other leaves retracted. An image is acquired. A line through the center of the center leaf is identified. This line is also a line through isocenter. The multi-leaf collimator is then rotated and a second image is acquired. A second line going through the center of the center leaf is also identified. Again this second line is also a line which goes through isocenter. Accordingly, the intersection of the two lines is isocenter.
Another method according to an embodiment of the present invention for finding isocenter uses an accessory. The accessory according to an embodiment of the present invention includes a metal marker in the center of the accessory with metal markers interspersed away from the center at a predetermined interval. An image may be acquired through the accessory, with the patient on the table. The resulting image may be analyzed to identify the position of the metal markers to determine isocenter. Once isocenter is determined, the image maybe electronically compared with a reference image.
A method according to an embodiment of the present invention for locating an isocenter of an image is presented. The method comprises projecting a center leaf of a collimator; acquiring an image through the collimator; and identifying a line traversing through the center leaf, wherein the line also traverses through an isocenter of the image.
A system according to an embodiment of the present invention for locating an isocenter of an image is also presented. The system comprises a collimator including a center leaf; and an image capturing device configured to capture an image of the collimator, wherein a line through the center leaf is identified as traversing an isocenter of the image.
In another aspect of the invention, a method according to another embodiment of the present invention for locating an isocenter of an image is also presented. The method compris
Hernandez-Guerra Francisco M.
Wofford Mark
Ho Allen C.
Kim Robert H.
Siemens Medical Systems Inc.
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