Abstract:
Portal images are combined with 3D ultrasound to determine adjustments to patient treatment parameters. The images are acquired while the patient is in an initial position, and the images are registered to a treatment coordinate system. The images are combined and outlines of anatomical structures are superimposed on the portal images, resulting in new portal images that incorporate the anatomy extracted from the ultrasound. The enhanced portal images are used to identify modifications to the treatment parameters.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to and the benefit of U.S. provisional patent application Ser. No. 60/951,005, filed Jul. 20, 2007, the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     This invention relates to methods and systems for using imaging to guide radiotherapy treatments. 
     BACKGROUND INFORMATION 
     Radiation therapy relies on devising a treatment plan, which includes the arrangement of therapeutic radiation beams, patient positioning relative to the beams, beam energies, apertures, and doses, and other factors. Typically, the treatment plan is based on a three-dimensional (3D) computed tomography (CT) dataset acquired prior to the first treatment session. Although CT scans provide a good physical map of electron density within the patient, they also have limitations. For example, CT scans are devoid of functional information about tumors and provide poor soft-tissue contrast for some organs. To circumvent these limitations, other secondary images may also be acquired, using modalities such as positron-emission tomography (PET), magnetic resonance imaging (MRI) or ultrasound. PET, for example, gives functional information about tumor metabolism, and MRI and ultrasound give superior soft-tissue contrast for some organs. 
     After the treatment plan is developed, the patient is positioned in the treatment room using external skin markings and radiation is delivered according to the treatment plan. This is typically repeated for a number of sessions, for example, once a day for 30 sessions. During this time period, however, a patient&#39;s internal anatomy may change. For example, it is known that the prostate can change positions significantly depending on rectal and bladder filling. In an attempt to provide more accurate delivery of radiation therapy, image-guided radiotherapy (IGRT) has become more common. Using IGRT, an image is acquired prior to each session and used to correct the treatment plan for anatomical changes. In principle, a completely new treatment plan can be generated prior to each treatment session—a technique known as adaptive radiotherapy (ART). Although effective, ART is not generally undertaken because re-planning is time-consuming and must be validated and approved for each session. Instead, current clinical practice commonly corrects for physiological changes by shifting the patient (using the treatment couch, for example) in order to best align the target anatomy to the planned location. This is accomplished by comparing the target structure position to its position on a reference image acquired during planning. 
     One common technique for implementing IGRT is portal imaging, i.e., using the treatment beam to acquire images with either film or a two-dimensional (2D) electronic portal imaging detector (EPID). Due to the high energy (megavolt range) of the treatment beam, image quality is generally inferior to diagnostic (kilovolt range) x-ray images, and provides little or no soft-tissue contrast. Portal images can be effective, however, for localization of bony anatomy, air pockets, and imaging skin surface. One advantage to using the IGRT approach is that the information is inherently acquired by and related to the treatment beam. To ensure the treatment position is correct relative to bony anatomy, the portal images are compared to digitally reconstructed radiographs (DRRs), which are the reconstruction of a 2D projection radiograph from a given beam direction, calculated from the planning CT dataset. EPIDs have been developed not only for electronic record-keeping, but also to make the acquisition more rapid, and to allow online corrections to patient position prior to each treatment fraction. Further, the introduction of flat-panel detectors has improved image quality of EPIDs such that it is comparable to conventional film-based imaging. 
     Software has been developed to enable rapid displacement calculations using localization images. Typically, 2D structures, called overlays, are extracted from CT contours and displayed on the DRRs on a console. EPID images are acquired from (typically) two angles, such as anterior-posterior and lateral, and the DRR overlays are shown superimposed on the port films. The operator then moves the overlays such that they fit the anatomy as seen on the EPIDs, and the amount of shift is calculated. This allows the therapist to displace the couch to compensate for any discrepancies. 
     Since portal images do not show soft-tissue contrast, one practice facilitating IGRT is to implant, in the treated organ (e.g., prostate), gold seeds that may be identified on the portal images. By comparing these positions to those on the planning CT, shifts can be executed to correct for organ motion. The use of seeds is invasive, however, and the resulting images do not give a complete picture of the organ and surrounding anatomy. 
     Another approach includes placing a conventional or cone-beam CT scanner in the treatment room. These scanners generate 3D images, and can either be of diagnostic quality or can use the high-energy treatment beam to produce the 3D images (referred to as megavoltage CT). These images provide a good geometric image of the patient, are similar in nature to the planning CT, and have some soft-tissue contrast which can be used to perform IGRT. For some sites such as prostate, however, fiducial marker seeds are typically used because the soft-tissue contrast is still unacceptable. 
     Ultrasound has also been used for IGRT, as it provides good soft-tissue contrast. Two or more ultrasound images are referenced to a 3D coordinate system, and are either used individually or are reconstructed to a full 3D image dataset. Patient displacements can then be determined from the ultrasound images. Although they can provide excellent soft-tissue contrast for organs such as the prostate, uterus or breast tumor cavities and do not require fiducial markers, ultrasound images do not give bony anatomy, or a complete anatomical image of the patient. 
     There has been research and development into the use of in-treatment-room MRI and PET for IGRT, but technical hurdles remain before this technology becomes commercially available. 
     SUMMARY OF THE INVENTION 
     The present invention provides systems and methods for improving imaging for IGRT. Embodiments of the invention rely on the integration and use of multiple modalities in the treatment room, registered to a common coordinate system, in order to modify treatment parameters. 
     In one aspect, the invention combines portal images and 3D ultrasound to determine a patient shift to be applied during radiotherapy treatment. A set of baseline radiotherapy treatment parameters are established, and, during therapy, two or more registered images of the area under treatment are taken, with at least some of the images being portal images taken from at least one beam direction, and at least one of the images being a non-x-ray-based 3D image. The images are then registered to a coordinate system associated with a treatment device. The non-x-ray-based 3D image is segmented to form a 3D surface, which is projected onto the plane of each portal image, thereby enhancing the portal images with data from the projected surface. The baseline treatment parameters are then updated based on the registered images. In this fashion, new portal images are produced which incorporate the anatomy extracted from the ultrasound. 
     In another aspect a system for identifying changes to patient treatment parameters during delivery of radiotherapy includes a first and second register and a processor. The first register stores a set of baseline treatment parameters, and the second register stores the images of an anatomical region to be treated. The images are obtained using different imaging modalities, such that at least some of the images are portal images from at least one beam direction, and at least one of the images is a non-x-ray-based 3D image. The processor is configured to register the images to a coordinate system associated with a treatment device, segment the non-x-ray-based 3D image to form a 3D surface, project the surface onto the plane of each portal image, enhance the portal images with data from the projected surface, and determine modifications to the baseline treatment parameters based on the registered images. 
     The present invention is not limited to the above applications, but encompasses the use of more than one image in the treatment room, registering these two or more images, and using anatomy from at least two of the images to modify treatment parameters. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead is generally being placed upon illustrating the principles of the invention. 
         FIG. 1  schematically shows the combination of a 3D ultrasound image and a portal image. 
         FIG. 2  schematically shows the creation of a 2D projection contour onto a portal image, created from a 3D surface of an organ obtained from a 3D ultrasound image. 
         FIG. 3  shows portal images enhanced in accordance with the present invention. 
         FIG. 4  schematically illustrates a system in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In  FIG. 1 , which illustrates an embodiment of the invention, the beam  100  of a linear accelerator (not shown), at a known gantry angle, is used in combination with a portal imager  105  to form a 2D image of the patient. The gantry angle may then be changed, and another image acquired. Typically at least two images are acquired from different directions using the portal imager  105 . The portal imager  105  is preferably an EPID, producing digital images using the treatment beam  100 . The portal images may be stored in a computer. A 3D ultrasound image  110  is also acquired, before or after the portal images but within as close a time frame as possible so that the patient does not move significantly. The portal and ultrasound images may be calibrated to a common coordinate system  115  whose origin coincides with the mechanical isocenter of the linear accelerator. This coordinate system  115  may be identified using perpendicular lasers passing through the origin. Systems and methods for calibrating 3D ultrasound images to such as coordinate system are known in the art. 
     Still referring to  FIG. 1 , relevant anatomy in the 3D ultrasound image is contoured (either manually or using an automatic segmentation algorithm) to form one or more 3D surfaces, each corresponding to a separate anatomical region. For example, the bladder and prostate can be contoured separately, even though they may appear in the same image set. The surface(s) are then projected onto the 2D portal images in the direction of the beam (i.e., along a beam&#39;s eye view). 
     In particular, from a given angle corresponding to a single portal image, a 3D surface can be projected into the portalimage by various methods. Referring to  FIG. 2 , a 3D surface  200  is projected onto the 2D image plane  205  (which coincides with the portal and ultrasound images) to produce a 2D surface projection outline  210 . This can be done, for example, by tracing a ray from the beam source  215  to a given pixel  220  in the image  205 . If the ray passes through the surface  200 , then the pixel  220  is considered within the projection outline  210 , otherwise it is considered outside the outline. This procedure may be repeated for all the pixels (or a reasonable subset consistent with resolution requirements) of the image  205 . The filled pixels then represent the projection  210  of the surface  200 . The outline of the filled pixels can be extracted and the projection contour  210  outlined. 
     In some cases, adjustments to various imaging parameters may be needed to correctly calibrate the 3D surface  200  to the image plane  205 . For example, the scale of the portal image may require modification, the center of the portal image may need to be moved relative to the source  215 , and/or rotations of the image may be needed relative to the room coordinate system  115 . In principle, all images may be scaled such that distances are measured in relation to the isocenter of the linear accelerator, i.e., the origin of the room coordinate system  115 . Accordingly, knowing the pixel size of the portal image is not enough, since the distance between the plane of the portal imaging detector (the medium on which the portal image is recorded) and the origin of the room coordinate system affects the scaling for a particular image. The image calibration parameters can be calculated by detector calibration, image pre-calibration or image self-calibration. 
     For detector calibration, the imaging detector is itself calibrated such that its parameters are known. For example, since the physical pixel size of the detector elements is known, and the image receptor can be calibrated to be at a known physical distance from the beam source, the scaling at isocenter may be readily computed. If the distance from the source changes, the pixels can be scaled to account for the new distance. The center pixel of the detector is also calibrated to be at a known offset from the central axis of the beam—either the offset is permanently fixed or the detector electronics can determine an offset value. The rotation of the detector is accurately fixed such that it is always aligned with the room coordinate system. Even with frequent detector calibration, there are likely to be deviations from ideal and drifts over time, thus requiring further calibration. 
     To assist with calibration of the detector prior to imaging, an object (or “phantom”) of known geometry may be imaged with the detector. In some embodiments, the phantom is a plate having an arrangement of some number (e.g., four) radio-opaque markers at known distances relative to the center of the plate. An image of the plate is acquired using the portal imaging device, and the markers identified on the image. The relationship of the markers on the image relative to their known positions on the phantom can be used to calculate the calibration parameters. In principle, each gantry angle has its own calibration parameters, since, for example, the detector may sag as the gantry is rotated. As a result, the central axis of the beam may not always pass exactly through the same point as the gantry is rotated, and therefore the calibration parameters should be checked periodically to identify any drift. 
     For image self-calibration, the outline of the radiation field is detected and the outline edges are compared to the expected beam aperture, which may be rectangular or, in some instances, an irregular shape extracted from the treatment plan. Comparing the expected shapes to the detected shapes, the calibration parameters can be determined for the image. One advantage of this approach is that the state of the detector is known relative to the room coordinate system, i.e., the detector can be moved in any configuration and an accurate calibration can still be computed. 
     Other approaches can also be used to calibrate the portal images, such as using a graticule (a radio-opaque grid placed in the head of the linear accelerator), which appears in the portal image. Furthermore, instead of using portal images, similar 2D images may acquired using diagnostic-energy x-ray tubes mounted in the treatment room. 
     With the ability to project the surface contour onto a calibrated portal image, each acquired portal image can be enhanced by augmenting it with an image of the soft-tissue anatomy extracted from the 3D ultrasound scan, as shown in the portal images of  FIG. 3 . Image  300  was acquired at an anterio-posterior beam direction, while the image  305  was acquired with a lateral beam direction. The prostate is not visible in either, but the bony anatomy of the pelvis is clearly visible. The prostate surface, as contoured on ultrasound images, is projected as a white line superimposed on the portal images, as indicated at  310  and  315 . 
     The ultrasound-enhanced portal images, once created, can be used to calculate patient shifts or other changes in treatment parameters, thereby permitting treatment delivery to account for changes in anatomy that deviate from the plan. 
     Portal images are typically compared to digitally DRRs. These are simulated projections through the CT dataset, from the planned beam angles (or other more convenient angles), to form 2D images for each beam. By observing differences between the portal images and the DRRs, the treatment couch can be shifted to improve patient alignment with the beam(s). If the portal images have been enhanced using the above methods, the DRR may also be enhanced using ultrasound imaging. If, for example, a 3D ultrasound image is acquired during the planning CT session (e.g., as described in co-pending patent application Ser. No. 10/343,336, which is incorporated in its entirety herein by reference), the contours obtained from the ultrasound image can be projected onto the DRRs, thereby allowing for direct comparison between ultrasound-enhanced portal images and ultrasound-enhanced DRRs. Enhancing a DRR with the ultrasound contour obtained at time of simulation is done in the same fashion as described above with respect to the enhancement of portal images, except typically the DRRs need not be calibrated since their geometric parameters are typically known. 
     While the invention has been described particularly in relation to using both portal images and ultrasound for IGRT, the invention also extends to matching the coordinate systems of any two or more imaging modalities, and using images obtained using these modalities to modify treatment parameters. For example, CT and ultrasound images can be acquired in succession prior to a patient treatment; each is calibrated to the room coordinates of the linear accelerator. The ultrasound-derived anatomical contours may then be superimposed onto the CT image and the treatment parameters modified to better align with anatomy imaged by both modalities. For example, the bladder, rectum and bony anatomy can be identified on the CT, while the prostate can be better identified on the ultrasound. In other instances, one or more organs can be identified on images obtained using both modalities, but some organ edges are better revealed by one modality than by the other. After the anatomy is identified using the multimodality images, beam shapes, angles, energies, patient position, etc. can be modified to account for the observed anatomy, which may differ from the planning anatomy. 
       FIG. 4  schematically depicts a hardware embodiment of the invention realized as a system  400  for modifying treatment parameters based on multimodal images. The system  400  comprises a register  405  and a processor  415 . 
     The register  405 , which may be any suitably organized data storage facility (e.g., partitions in RAM, etc.), receives images from a plurality of imagers, collectively indicated at  420 , which reflect different imaging modalities. Imagers  420  may include one or more of an MRI, CT/PET scanner, ultrasound device, or x-ray device. In some embodiments, the images are stored on a data-storage device separate from the imager (e.g., a database, microfiche, etc.) and sent to the system  400 . The register  405  may receive the images through conventional data ports and may also include circuitry for receiving analog image data and analog-to-digital conversion circuitry for digitizing the image data. 
     The register  405  provides the images to the processor  415 , which implements the functionality of the present invention in hardware or software, or a combination of both on a general-purpose computer. In particular, processor  415  registers the images and creates an enhanced image, which may be displayed on a device  430 . The processor  415  thereupon computes patient shifts or other changes in treatment parameters, which are communicated to the controller  435  of a treatment device such as a linear accelerator. The controller  435 , in turn, causes appropriate adjustments to be made based on the modified treatment parameters. 
     Alternatively or in addition, a user, via an input device  425 , may influence, approve, override or revise the modifications to the treatment parameters based on his or her review of the composite image on device  430 . 
     The programming for processor  415  may be written in any one of a number of high-level languages, such as FORTRAN, PASCAL, C, C++, C#, Java, Tcl, or BASIC. Further, the program can be written in a script, macro, or functionality embedded in commercially available software, such as EXCEL or VISUAL BASIC. Additionally, the software can be implemented in an assembly language directed to a microprocessor resident on a computer. For example, the software can be implemented in Intel 80×86 assembly language if it is configured to run on an IBM PC or PC clone. The software may be embedded on an article of manufacture including, but not limited to, “computer-readable program means” such as a floppy disk, a hard disk, an optical disk, a magnetic tape, a PROM, an EPROM, or CD-ROM. 
     While the invention has been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The scope of the invention is thus indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced.