X-ray or gamma ray systems or devices – Specific application – Absorption
Reexamination Certificate
2001-08-09
2003-09-09
Dunn, Drew A. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Absorption
C378S004000, C378S062000, C378S147000
Reexamination Certificate
active
06618467
ABSTRACT:
BACKGROUND OF THE INVENTION
In external radiation therapy, converging beams of radiation are used to irradiate cancerous tissue within a patient.
Tomotherapy is a form of external radiation therapy in which the radiation source is placed on a gantry rotating in a single plane about an axis through the patient. The patient may be translated across the plane during the rotation to impart a relative helical motion between the patient and a point on the gantry. During this rotation, the radiation beam is modulated by a multi-leaf collimator (MLC) or other modulating device which divides the radiation beam into independently controllable rays. By controlling the intensity of each ray as a function of gantry angle, radiation dose may be precisely placed in arbitrary cross-sectional regions within the body. Methods of constructing and of operating such tomotherapy equipment are described in U.S. Pat. No. 5,317,616 issued May 31, 1994 entitled Method and Apparatus for Radiation Therapy, and U.S. Pat. No. 5,548,627 issued Aug. 20, 1996 entitled Radiation Therapy System With Constrained Rotational Freedom as assigned to the assignees of the present application and hereby incorporated by reference.
Tomotherapy's ability to precisely place a radiation dose makes it important to be able to accurately image the treatment region and precisely locate the patient during treatment. U.S. Pat. No. 5,724,400 issued Mar. 3, 1998 entitled Radiation Therapy System With Constrained Rotational Freedom also assigned to the assignees of the present invention and hereby incorporated by reference describes a combination computed tomography (CT) machine and radiation tomotherapy machine that can provide both imaging and accurate registration of the patient.
In CT, a planar beam of kilovolt energy x-rays are projected through the patient at a variety of gantry angles much like that done with tomotherapy but without modulation of the beam rays by an MLC. Bodily structures attenuate the rays and this attenuation is detected by a detector. A cross-sectional image may be reconstructed from “projections” of attenuation data of rays at each gantry angle over a range of gantry angles that define a “tomographic projection set”. The tomographic projections set typically includes rays spanning the entire width of the patient over at least 180° of gantry angles. Reconstructing an image from less than a tomographic projection set can cause severe image artifacts obscuring essential body structure.
While normally CT uses kilovoltage x-rays having much lower energy than those used in radiation therapy, it is known that images can be constructed using the same megavoltage x-rays used in radiation therapy. In this way the need for a separate kilovoltage x-ray source is avoided. Megavoltage images have the further advantage of better representing the actual absorption of the body structures of radiation at the megavoltage lever thus making those images superior for treatment planning and dose verification. U.S. Pat. No. 5,673,300 issued Sep. 30, 1997 entitled Method of Registering A Radiation Treatment Plan To A Patient teaches methods of using a detector with megavoltage x-rays to verify patient location and for dose evaluation. These patents are also incorporated by reference.
Nevertheless, the use of megavoltage x-rays for the generation of a CT image (MVCT) has some disadvantages. First, the requirement that a complete tomographic projection set be obtained normally requires a pre-scanning of the patient before radiation therapy. This pre-scanning process increases the total time of the treatment and entails the possibility of patient movement between the scan and therapy. Pre-scanning typically forgoes the ability to use the MVCT images for real-time correction or verifications. Finally, high contrast MVCT over the required tomographic projection set significantly increases the dose to the patient.
BRIEF SUMMARY OF THE INVENTION
The present invention eliminates or reduces the amount of additional time required to take an MVCT scan and/or minimizes the additional dose required by MVCT by collecting at least a portion of the CT data during the radiation therapy. The problem of image artifacts resulting from the inevitably incomplete projections provided by the modulated radiation therapy radiation, is overcome by supplementing this “high flux” radiation therapy data with low flux data obtained separately either in a pre-scan or during the radiation therapy. A tomographic projections set is formed of mixed low and high flux data.
Using low flux data minimizes the extra radiation to the patient required for MVCT whereas the high noise inherent in using low flux data is substantially corrected by the high flux data obtained from the therapy. Alternatively in certain situations, the low flux data alone may be used.
Specifically then, the present invention provides a combination radiation therapy and tomographic imaging machine having a megavoltage radiation source directing a radiation beam formed of rays directed generally along an axis. A gantry holds the radiation source to rotate the angle of the axis about a treatment volume and a radiation detector is positioned on the gantry across the treatment volume opposite the radiation source to provide data of a projection. A modulator is positioned between the radiation source and the treatment volume to modulate the flux of the rays.
An electronic computer communicating with the modulator controls the modulator to direct selected high flux rays at selected axis angles at the patient within the treatment volume according to a radiation treatment plan. The selected rays and angles comprise less than the tomographic projection set and thus produce a high flux, incomplete tomographic projection set. Low flux rays are also directed at the patient within the treatment volume to acquire low flux data. The low flux data is combined with the high flux incomplete tomographic projection set to produce an augmented but complete tomographic projection set which may be reconstructed into an image.
Thus, it is one object of the invention to employ radiation used in radiation therapy for CT imaging even though it is seriously incomplete for tomographic purposes. The low flux data augments the incomplete high flux radiation therapy data to provide a tomographic image without overly increasing the total dose to the patient.
The low flux data that is collected may be a complete tomographic projection set or may be an incomplete tomographic projection set.
Thus it is another object of the invention to provide great flexibility in acquisition of the low flux data.
In one embodiment, the modulator is a multi-leaf collimator having multiple leaves that may be opened or closed to pass or occlude rays and the low flux data are obtained from leakage through the leaves when they are closed.
Thus it is another object of the invention to provide a simple method of producing low flux data in a radiation therapy machine.
The leaves may be open for a proportion of time during increments of axis angle according to the radiation treatment plan to produce the high flux incomplete tomographic projection set and the electronic computer may execute the stored program to position the proportion of time in the open state after the beginning of each angular increment so that the low flux data may be collected at the beginning of each angular increment.
Thus it is another object of the invention to provide a ready means of obtaining a complete tomographic projections set of low flux data at regular intervals during the radiation therapy itself.
Alternatively, the low flux rays may be obtained by opening the leaves to pass rays not required by the radiation treatment plan.
Thus it is another object of the invention to provide a means of obtaining a complete tomographic projections set of low flux data.
The electronic computer may execute the stored program to center the proportion of time the leaves are in the open state within each angular increment and the low flux data may be collected at the center of each an
Mackie Thomas R.
Olivera Gustavo H.
Ruchala Kenneth J.
Dunn Drew A.
Quarles & Brady LLP
Thomas Courtney
Wisconsin Alumni Research Foundation
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