Method and system for positioning patients for medical treatment procedures

A system and method for measuring and correcting the position of a patient are disclosed. According to an aspect of the invention, reference coordinates for particular body locations on the patent are determined. At a later treatment session, the relative positioning of the patient's body locations are adjusted to match the relative positioning of the reference coordinates. The entire body of the patient can thereafter be moved as a single unit to match the patient's body location with the absolute location of the reference coordinates.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to medical methods and systems. More
 particularly, the invention relates to a method and system for positioning
 patients that undergo medical treatment procedures.
 2. The Related Art
 The accurate placement and positioning of patients is crucial when
 performing many types of medical treatments. One category of medical
 treatments in which the proper placement and verification of the position
 of patients/patient body parts is of particular importance is in the field
 of radiation therapy.
 Radiation therapy involves medical procedures that selectively expose
 certain areas of a human body, such as cancerous tumors, to high doses of
 radiation. The intent of the radiation therapy is to irradiate the
 targeted biological tissue such that the harmful tissue is destroyed. To
 minimize damage to surrounding body tissues, many conventional treatment
 methods utilize "dose fractionating" to deliver the radiation dosage in a
 planned series of treatment sessions that each delivers only a portion of
 the total planned dosage. Healthy body tissues typically have greater
 capacity to recover from the damage caused by exposed radiation. Spreading
 the delivered radiation over many treatment sessions allows the healthy
 tissue an opportunity to recover from radiation damage, thus reducing the
 amount of permanent damage to healthy tissues while maintaining enough
 radiation exposure to destroy tumoral tissue.
 The efficacy of the radiation treatment depends in large part upon the
 ability to irradiate the exact same position on the body at the various
 radiation sessions. The goal is to place the patient in the same position
 relative to the radiation source at each and every treatment session.
 Inaccuracies in positioning the patient could result in errors in
 radiation dosage and/or treatment locations, leading to unpredictable
 disease relapse or damage to healthy tissues. Maintaining the linear
 accelerator/radiation source in a precise and repeatable position does not
 normally present a problem. The problem arises when attempting to recreate
 the same body position by the patient at every radiation session. In
 conventional medical treatment systems, the accurate placement and
 verification of a repeating treatment location on the human body remains a
 significant problem in implementing dose fractionating treatment plans.
 One approach to controlling patient positioning is to place marks or
 tattoos at specific locations on the patient's skin. Several laser or
 light sources from predetermined locations project beams of light at the
 patient's body. To control the patient positioning, a therapist shifts the
 position of the patient until the marks are aligned with the lines of
 light from the lasers or light sources. A significant drawback to this
 approach is that the accuracy and consistency of the patient positioning
 is heavily dependent upon the skill level of the therapist in manually
 positioning the patient. In addition, with heavier patients, it is
 possible that only the skin of the patient is moved into the proper
 position without moving the body part to be irradiated into the
 appropriate position. Moreover, this approach does not provide an
 efficient way to record and reflect the positioning quality in the
 patient's records.
 Another approach to controlling patient positioning is to utilize an
 immobilization device to maneuver the patient into a particular position.
 An immobilization device physically attaches to the human body to keep the
 patient from moving once proper positioning is achieved. A drawback to
 using an immobilization device is that such devices to not exist for all
 body parts. Immobilization devices are generally effective only for
 positioning the head and neck of a patient. Moreover, in many known
 immobilization devices, a patient can still move to a significant degree
 within the confines of the immobilization device. In addition, these
 devices can be extremely uncomfortable for the patient.
 Positioning a patient with video cameras has been applied to stereotactic
 radiosurgery of the brain where the patient's skull is positioned as a
 rigid object. However, there are drawbacks to applying known positioning
 techniques using video cameras to non-rigid portions of the body. For
 example, such implementations have been incapable of effectively guiding
 the therapist through the steps to be taken for achieving proper posture
 and position. Therefore, therapist confusion occurs resulting in a lengthy
 positioning process.
 Thus, there is a need for a system and method to address the
 above-described problems of the related art. There is a need for a method
 and system that can accurately and consistently control the position of a
 patient for medical treatment.
 SUMMARY OF THE INVENTION
 The present invention provides a method and system for measuring the
 position of a patient, as well as a method and system for positioning a
 patient for medical treatment procedures.
 According to an aspect of the invention, reference coordinates for
 particular body locations on the patient are determined. At a later
 treatment session, the relative positioning of the patient's body
 locations are adjusted to match the relative positioning of the reference
 coordinates. The entire body of the patient can thereafter be moved as a
 single unit to match the absolute position of the reference coordinates.
 Further details of aspects, objects, and advantages of the invention are
 described below in the detailed description, drawings, and claims.

DETAILED DESCRIPTION OF EMBODIMENT(S)
 Process Overview
 FIG. 1 is a process flowchart showing the high level actions performed in
 an embodiment of the present invention. The method of the present
 invention begins with the measurement and storage of reference coordinates
 for various body locations on a patient's body (102). Prior to delivery of
 radiation treatment at each treatment session, the patient should be
 positioned such that the actual coordinates of these locations on the
 patient's body match the reference coordinates. Assuming the radiation
 source is placed in its original position, this will ensure that the
 patient consistently and repeatably receives treatment in the same
 location at every session.
 When undergoing treatment at the various treatment sessions, the patient is
 preferably placed upon a treatment table, treatment couch, or other type
 of patient support structure. For purposes of explanation, the term
 "treatment table" shall be used in this document to refer any support
 structure that can be used to support a patient. Initially, the patient is
 likely be in an incorrect position relative to the reference coordinates.
 Consistent with the invention, the initial positioning action involves the
 correction of the patient's "posture" (104). Posture refers the position
 of one or more of a patient's body parts relative to that patient's other
 body parts. In process action 104, the measured body locations on the
 patient's body are repositioned such that the relative positioning of the
 patient's body parts match the relative positioning of the reference
 coordinates. To achieve this goal, the positioning system provides
 specific instructions to correct the relative positioning of the patient's
 body parts.
 Once the patient's posture has been corrected, the measured body locations
 should be in the correct positions with respect to the rest of the
 patient's body. However, the absolute coordinates of the patient's body
 positions may be incorrect. If the patient is in the correct posture, then
 all of the patient's body locations will be misaligned by the exact same
 offset parameter values when compared to the reference coordinates. Thus,
 the patient's entire body can be shifted as a single unit to place the
 patient in the correct absolute coordinates that match the reference
 coordinates (106).
 The combination of the actions to correct and validate relative position
 errors (104) and absolute position errors (106) greatly accelerates the
 time needed for patient setup prior to a treatment session. Moreover, the
 present invention allows more precise recordation and verification of
 patient positioning in real-time during the treatment sessions.
 In an alternate embodiment, after the relative and absolute positioning
 actions have been performed, an additional positioning action can be
 performed with respect to the patient's internal organs. This additional
 positioning action is performed to adjust the internal target volume
 instead of the patient's external features, and can be used to precisely
 position the patient's internal organs for treatment. Each of the above
 process actions will be explained in more detail below.
 Method and System of the Invention
 In an embodiment, the body locations chosen for measuring reference
 coordinates fall within two general categories. First, one or more body
 location(s) can be chosen for measurement in the general vicinity of the
 body structure/organ undergoing treatment. Second, other body location(s)
 can be chosen if movement of those other body locations affect the
 orientation or location of the body structure/organ undergoing treatment.
 For example, a patient undergoing treatment in the breast area will
 preferably have one or more reference positions measured in the chest
 area, and possibly in other areas of the body that can affect the position
 of the breast area, such as the upper arm and neck areas. A patient
 undergoing treatment in the prostate area will preferably have one or more
 reference positions measured in the hip area, and possibly in other areas
 of the body that can affect the movement of the prostate area, such as the
 upper leg areas.
 Shown in FIG. 2 are the upper torso and upper arms of a patient 202 resting
 on a treatment table 204. For the purposes of illustration, a plurality of
 body locations 206, 208, 210, and 212 has been marked on patient 202. Each
 of the body positions 206, 208, 210, and 212 are assumed to correspond to
 body locations that affect the orientation/location of a body organ
 undergoing treatment Prior to the initial treatment session, each of these
 body locations are measured to provide reference coordinates in Cartesian
 space. In an embodiment, the isocenter of the system is presumed to be
 located at coordinate (0, 0, 0). All measured body positions are given
 coordinates relative to the isocenter. For purposes of explanation, the
 orientation of the x, y, and z-axes are as shown in FIG. 1. Other
 orientation methodologies can be employed within the scope of the
 invention. For example, in an alternate embodiment, the orientation of the
 x, y, and z-axes can be defined with reference to the axial movement or
 rotation of a gantry holding the radiation source or linear accelerator.
 Table 1 provides examples of reference coordinates that may be measured for
 body locations 206, 208, 210, and 212:
 TABLE 1
 Body Location Reference Coordinate
 Body location 206 (-18, 10, -5)
 Body location 208 (-20, -5, -10)
 Body location 210 (0, 10, 2)
 Body location 212 (-10, 0, 4)
 Referring to FIGS. 3a, 3b, 3c, and 3d, these figures illustrate the
 positioning process of the present invention. The patient 202 and marked
 body locations 206, 208, 210, and 212 of FIG. 2 has been duplicated in
 FIG. 3a. Thus, FIG. 3a represents the reference position for patient 202.
 It is assumed that the reference coordinates for body locations 206, 208,
 210, and 212 in FIG. 3a are as set forth in Table 1.
 FIG. 3b illustrates an example of an initial position and posture of the
 patient 202 prior to the beginning of a treatment session. Note that in
 FIG. 3b, the patient's posture is different than the reference posture of
 the patient 202 as shown in FIG. 3a. In other words, the relative
 positions between body locations 206b, 208b, 210b, and 212b shown in FIG.
 3b are different than the relative positions between their corresponding
 reference body locations 206, 208, 210, and 212 in FIG. 3a. In particular,
 the patient's arm and shoulder (body locations 206b and 208b) are in the
 wrong position relative to the rest of the patient's body. Furthermore,
 the absolute position and/or orientation of the patient's body are
 incorrect. Specifically, the patient 202 in FIG. 3b has rotationally
 deviated from the proper reference position shown in FIG. 3a.
 To correct the patient's posture, specific measured body locations on the
 patient are repositioned such that they are in the correct positions
 relative to other measured body locations.
 The reference coordinate for body location 206 in FIG. 3a is (-18, 10, 5)
 and the reference coordinate for body location 208 is (-20, -5, -10). This
 means that the offset between the reference coordinate for body location
 206 and reference coordinate for body location 208 can be expressed with
 respect to each of the Cartesian axes as follows:
 x-axis: -2, y-axis: -15, z-axis: -15
 In an embodiment, the x-axis offset between the two body locations is
 calculated by subtracting the x-coordinate for body location 206 from the
 x-coordinate for body location 208. Similarly, the y-axis offset between
 the two body locations is calculated by subtracting the y-coordinate for
 body location 206 from the y-coordinate for body location 208. The z-axis
 offset between the two body locations is calculated by subtracting the
 z-coordinate for body location 206 from the z-coordinate for body location
 208.
 To correct relative positioning errors between body location 206b and 208b,
 the relative offset between these two body locations should be manipulated
 to match the offset measured for their corresponding reference
 coordinates. For example, if patient 202 in FIG. 3b has been incorrectly
 positioned such that body location 206b is moved to coordinate (0, 0, 0),
 then body location 208b can be moved to coordinate (-2, -15, -15) to
 re-establish the same relative positioning between these two body
 locations as was established for their corresponding reference
 coordinates. The same relative positioning can be obtained by moving body
 location 206b instead of body location 208b, so long as the offset between
 the two body locations is corrected to the reference offset.
 Alternatively, both body locations can be moved during this process, as
 long as the end result is that the relative offset between these two body
 locations matches the relative offset of the corresponding reference
 coordinates.
 Similarly, the offset between the reference coordinate for body location
 210 (0, 10, 2) and reference coordinate for body location 212 (-10, 0, 4)
 can be expressed as:
 x axis: -10, y axis: -10, z axis: -2
 Thus, if body location 210b in FIG. 3b is moved to coordinate (0, 0, 0),
 then body location 212b can be moved to coordinate (-10, -10, -2) to
 maintain the same relative positioning that was measured and established
 between their corresponding reference coordinates.
 The offset between the reference coordinate for body location 208 (-20, -5,
 -10) and body location 210 (0, 10, 2) can be expressed as:
 x axis: 20, y axis: 15, z axis: 12
 If the body location 208b in FIG. 3b is moved to coordinate (0, 0, 0), then
 body location 210b can be moved to coordinates (20, 15, 12) to maintain
 the same relative positioning that was measured and established between
 their corresponding reference coordinates.
 In an embodiment, the positioning system provides specific instructions to
 correct the positioning of the patient's body parts. There are a number of
 techniques to provide a positioning plan or recipe to correct the
 patient's posture. In one approach, the collection of marked body
 locations is mathematically translated as a rigid object to overlap with
 the collection of reference positions. In other words, the location data
 for the entire collection of marked body locations is mathematically
 overlaid atop the location data for the collection of reference positions.
 Thereafter, the location data for the entire collection of marked body
 locations is logically shifted as a single unit to attempt to find a best
 overall match between the collection of marked body locations and the
 collection of reference positions. This can be done by either a center of
 mass translation, or by matching as many marked body locations to
 reference locations as possible. The remaining residual error between the
 position of each unmatched marked body location and its corresponding
 reference location define the posture error associates with that body
 location. In this way, the system can provide specific instructions that
 precisely guide on how to move each body part in order to correct posture
 errors to an accepted level. These specific instructions can be used by a
 therapist to make the posture corrections.
 To illustrate, consider the collection of marked body locations 206b, 208b,
 210b, and 212b in FIG. 3b. If the entire collection of marked body
 location in FIG. 3b is mathematically translated as a rigid object to
 overlap with the collection of reference locations in FIG. 3a, then the
 point of best match could be where body location 210b lines up with
 reference location 210 and body location 212b lines up with reference
 location 212. The alignment errors between the relative locations of body
 locations 206b and 208b and reference locations 206 and 208 would then
 define the posture errors.
 In a display image, the positioning system can present visual instructions
 to correct the posture errors. Instruction arrow 213 in FIG. 3b is an
 example of a visual instruction that can be presented to a therapist to
 provide specific pictorial guidance to correct posture errors. The
 instruction arrow 213 can be overlaid or ghosted over the image of the
 patient to facilitate the correction of the posture error. FIG. 3e depicts
 a magnified view of the instruction arrow 213 from FIG. 3b. As indicated,
 instruction arrow 213 provides specific guidance as to the direction of
 correction, as well as the magnitude of correction required.
 In an alternate embodiment, posture correction is accomplished by selecting
 a single measured body location as the primary reference point. Each other
 measured body location is positioned such that its position relative to
 the primary reference point matches the relative offset established and
 measured between the same two points for the reference coordinates.
 Alternatively, posture correction can be accomplished in a chained
 approach, in which a first measured body location is re-positioned
 relative to a second measured body location. Once the first and second
 measured body locations have been properly positioned relative to each
 other, a third measured body location is positioned relative to the second
 measured body location. Once the second and third measured body locations
 have been properly positioned relative to each other, a fourth measured
 body location is positioned relative to the third measured body location.
 This continues until all the remaining measured body locations have been
 properly positioned.
 FIG. 3c depicts patient 202 once the posture errors of FIG. 3b have been
 corrected. It can be appreciated that once the posture of the patient has
 been corrected, the relative offsets for all the measured body locations
 will match the relative offsets measured and established for their
 corresponding reference coordinates. Thus, the relative positioning
 between body locations 206c, 208c, 210c, and 212c in FIG. 3c match the
 relative positioning between the reference coordinates for body locations
 206, 208, 210, and 212 in FIG. 3a. In particular, the upper arms and
 shoulder of the patient 202 (body locations 206c and 208c) have been
 repositioned to the same position relative to the rest of the body as was
 measured and established for the reference coordinates.
 Once posture correction and verification has been performed, the absolute
 position of the patient's body must be corrected and/or verified. Even if
 posture errors have been corrected, it is possible that the coordinates of
 the measured body locations may not match the absolute coordinates of the
 reference coordinates. In the example of FIG. 3c, although the patient's
 posture has been corrected, the absolute position of the patient 202 is
 not correct since the patient's body is rotationally shifted from the
 reference position shown in FIG. 3a. However, since posture corrections
 have already been performed, every part of the patient's body should be
 misaligned by the exact same offset parameters. Thus, rather than having
 to maneuver individual parts of the patient to establish proper
 positioning, the patient's entire body can be maneuvered as a unit into
 the proper position to match the absolute position of the reference
 coordinates.
 In the example of FIG. 3c, the patient's entire body is rotated in the
 directed indicated by arrow 302. In an embodiment, this movement of the
 patient's entire body is effected by moving the position of the support
 structure that the patient is resting on, e.g., the treatment table. FIG.
 3d depicts patient 202 once the patient's entire body has been
 repositioned. In FIG. 3d, the absolute coordinates of body locations 206d,
 208d, 210d, and 212d have been positioned match their corresponding
 reference coordinates.
 According to an embodiment, an additional positioning action can be
 performed with respect to the internal treatment area of the patient. In
 certain circumstances, the internal body structure or organ to be treated
 may shift from a previous location/position, even though the external body
 locations have been placed to match the reference coordinates.
 A first method to perform internal positioning measurements and corrections
 comprises the use of an imaging system, such as an x-ray imaging system,
 that can detect specific internal landmarks within the patient's body.
 Such landmarks can include particular bony structures near the organ to be
 treated. The imaging system is used to measure reference locations for the
 bony structures chosen as landmarks. At a following treatment session, the
 imaging system is used to measure the positions of the same landmark
 structures within the patient. The patient undergoes fine body adjustments
 until the landmark structures are positioned at the previously measured
 reference locations.
 A second method to perform internal positioning measurements and
 corrections comprises the use of an imaging system that can directly image
 the organ or body structure undergoing treatment, such as ultrasound or
 MRI imaging systems. The imaging system measures a reference position and
 orientation for the organ or body structure undergoing treatment. For
 example, if an ultrasound imaging system is used, then the system can
 detect the position of an organ or body structure with respect to the
 ultrasound probe. The exact position of the ultrasound probe is measured
 with respect to the rest of the system. One or more markers (described in
 more detail below) can be affixed to the ultrasound probe to determine its
 position. Based upon the position of the ultrasound probe, the position of
 the internal organ or body structure can be calculated relative to the
 rest of the system. At a following treatment session, the patient
 undergoes fine body adjustments to place the organ or body structure into
 the original reference position and orientation.
 FIG. 4 depicts a patient measuring and positioning system 400 according to
 an embodiment of the present invention. Shown in FIG. 4 is a patient 402
 supported by a treatment table 404. A plurality of markers 406 (preferably
 retro-reflective or reflective markers) are affixed to a plurality of body
 locations on the patient 402. A method to select particular body locations
 upon which to affix retro-reflective markers 406 has been described above.
 In an embodiment, each retro-reflective marker 406 is comprised of
 reflective material that can reflect light, whether in the visible or
 invisible wavelengths. Two or more cameras 408 are positioned to detect
 and receive light that reflect from markers 406. The output of cameras 408
 are sent to a computer 410 or other type of processing unit having the
 capability to receive video images. According to a particular embodiment,
 computer 410 comprises an Intel Pentium-based processor running Microsoft
 Windows NT and includes a video frame grabber card that is a multi-channel
 device, having a separate channel for each camera 408 utilized in the
 system.
 In operation, one or more illumination sources (which are infrared sources
 in the preferred embodiment) project light at the patient 402 on treatment
 table 404. The generated light is reflected from retro-reflective markers
 406, thereby indicating the position of these retro-reflective markers
 406. The cameras 408, which are directed at patient 402, capture and
 detect the reflected light from the retro-reflective markers 406. Each
 camera 408 generates video images that show the position of the
 retro-reflective markers 406 within its video frame. The generated video
 images are sent to computer 410 for further processing.
 Computer 410 receives video images from cameras 408. In general, the
 received video images from each camera 408 only show the position of the
 retro-reflective markers 406 relative to the particular camera 408 that
 produces the image. The two sets of images received from each camera 408
 are used by computer 410 to triangulate the absolute coordinates of each
 retro-reflective marker 406. At the first treatment session, the reference
 coordinates for each retro-reflective marker 406 are measured and stored
 at the computer 410. Thereafter, at each additional treatment session, the
 system 400 can measure in real-time the absolute coordinates of each
 retro-reflective marker 406 and compare the real-time coordinates of the
 markers 406 to the stored reference coordinates. Although only two cameras
 408 are shown in FIG. 4, any number of cameras 408 may be employed in
 system 400 to receive video images of the patient 402 and retro-reflective
 markers 406. Information regarding the location and orientation of each
 camera 408 is provided to computer 410 to facilitate the triangulation
 computations.
 A possible inefficiency in tracking the retro-reflective markers 406 is
 that the markers may appear anywhere on the video frame, and all of the
 image elements of the video frame may have to be examined to determine the
 location of the retro-reflective markers 406. Thus, in an embodiment, the
 initial determination of locations for the retro-reflective markers 406
 involves an examination of all of the image elements in the video frame.
 If the video frame comprise 640 by 480 image elements, then all 307200
 (640*480) image elements are initially examined to find the location of
 the markers 406.
 For real-time tracking of the retro-reflective markers 406, examining every
 image element for every video frame to determine the location of the
 markers 406 in real-time could consume a significant amount of system
 resources. Thus, in an embodiment, the real-time tracking of the
 retro-reflective markers 406 can be facilitated by processing a small
 region of the video frame, referred to herein as "tracking gate", that is
 placed based on estimation of the locations of the already-identified
 markers 406 in the video frame. The previously determined location of a
 marker 406 is used to define an initial search range (i.e., the tracking
 gate) for that same marker in real-time. The tracking gate is a relatively
 small portion of the video frame that is centered at the previous location
 of the marker 406. The tracking gate is expanded only if it does not
 contain the new location of the marker 406. As an example, consider the
 situation when the previously determined location of a particular marker
 is image element (50, 50) in a video frame. If the tracking gate is
 limited to a 50 by 50 area of the video frame, then the tracking gate for
 this example would comprise the image elements bound within the area
 defined by the coordinates (25, 50), (75, 50), (50, 25), and (50, 75). The
 other portions of the video frame are searched only if the marker 406 is
 not found within this tracking gate.
 At each treatment session, the retro-reflective markers 406 are affixed to
 the same pre-determined positions on the body of patient 402. At the
 beginning of each treatment session, the coordinates for each
 retro-reflective marker 406 are triangulated by computer 410. If the
 relative or absolute positioning of the retro-reflective markers 406 are
 incorrect, then the computer produces specific information that can be
 used to correct these errors. This real-time feedback regarding the
 position of the retro-reflective markers 406 can be provided throughout
 the treatment sessions so that any position errors that arise during
 treatment can be detected and corrected.
 In an embodiment, posture correction is performed manually by a therapist
 or attendant. Computer 410 provides instructions on a video display device
 412 regarding the specific movement of the measured body location that can
 be used to establish the proper relative positioning of the patient's
 body. For example, video display device 412 may display information
 regarding the exact x, y, and z-axis adjustments needed to particular body
 locations to correct posture errors. Alternatively, the treatment table
 404 may comprise body manipulation fixtures that are controlled by
 computer 410. In this embodiment, the computer-controlled fixtures are
 moved by the computer 410 to automatically adjust the posture of patient
 402 to establish the proper relative positioning of the various body
 locations.
 Once the proper posture is established, the patient 402 is shifted into the
 proper absolute position. In an embodiment, the treatment table 404
 comprises a moveable structure that can be positionally and rotationally
 adjusted. The entire body of the patient 402 is shifted as a single unit
 by the treatment table 404 to establish the proper absolute coordinates to
 exactly match the reference coordinates. In the preferred embodiment, the
 movement of treatment table 404 is controlled by the computer 410. Once
 posture adjustments are made, the computer 410 directs the movement of the
 treatment table 404 to bring patient 402 into the proper absolute
 position. In an alternate embodiment, the treatment table 404 is manually
 maneuvered by an attendant or therapist to place the patient 402 in the
 correct absolute position. A video display device 412 provides information
 regarding the proper movement of the treatment table 404 to effect the
 position correction. Fine adjustments to the patient's position to correct
 position errors of internal organs or body structures can also be
 performed by moving the treatment table 404. In an embodiment, the
 movement is controlled by computer 410.
 Referring to FIG. 5, an embodiment of system 400 provides a video image 502
 of the patient (or particular body locations on the patient) on a video
 display device 412. The video image is provided to assist a therapist or
 attendant in positioning the patient. Within the video image 502, the
 reference positions of the retro-reflective markers are displayed as
 illuminated dots 504. The real-time image and position 508 of the
 retro-reflective markers are also displayed on the video display device
 412 as illuminated dots 508 (shown in FIG. 5 as "x" marks) which are
 preferably of a different shape or color than the image of the reference
 coordinates 504. The real-time image 510 of the patient may also be
 displayed. In an embodiment, the reference image 506 of the patient can
 also displayed on the video display device 412. The reference image 506
 can be switched in to replace the real-time image 510 of the patient so
 that only a single image of the patient is displayed at any point in time,
 although both the reference and real-time positions of the markers are
 simultaneously displayed. Alternatively, the reference image 506 can be
 overlaid or superimposed over the real-time image 510 of the patient, so
 that both sets of images of the patient are displayed at the same time.
 The real-time nature of the image display allows any movement of the
 markers or particular body locations on the patient to be immediately
 displayed on the video display device 412. Thus, the image on the video
 display device 412 can be used to assist the correction of posture and
 absolute position errors. For example, the simultaneous display of both
 the reference and real-time positions for the markers allows a therapist
 to visually determine adjustments needed to line up the two sets of marker
 images to correct position errors. Moreover, the display of the reference
 and real-time images of the patient allows a therapist to visually
 determine adjustments needed to correct posture errors.
 FIGS. 6a and 6b depict an embodiment of a camera 408 that can used in the
 present invention. Camera 408 is a charge-couple device ("CCD") camera
 having one or more photoelectric cathodes and one or more CCD devices. A
 CCD device is a semiconductor device that can store charge in local areas,
 and upon appropriate control signals, transfers that charge to a readout
 point. When light photons from the scene to be images are focussed on the
 photoelectric cathodes, electrons are liberated in proportion to light
 intensity received at the camera. The electrons are captured in charge
 buckets located within the CCD device. The distribution of captured
 electrons in the charge buckets represents the image received at the
 camera. The CCD transfers these electrons to an analog-to-digital
 converter. The output of the analog-to-digital converter is sent to
 computer 410 to process the video image and to calculate the positions of
 the retro-reflective markers 406. According to an embodiment of the
 invention, camera 408 is a monochrome CCD camera having RS-170 output and
 640.times.480 pixel resolution. Alternatively, camera 408 can comprise a
 CCD camera having CCIR output and 756.times.567 pixel resolution.
 In a particular embodiment of the invention, an infra-red illuminator 602
 ("IR illuminator") is co-located with camera 408. IR illuminator 602
 produces one or more beams of infrared light that is directed in the same
 direction as camera 408. IR illuminator 602 comprises a surface that is
 ringed around the lens 606 of camera body 608. The surface of IR
 illuminator 602 contains a plurality of individual LED elements 604 for
 producing infrared light. The LED elements 604 are arranged in a spiral
 pattern on the IR illuminator 602. Infrared filters that may be part of
 the camera 408 are removed or disabled to increase the camera's
 sensitivity to infrared light.
 The position and orientation of cameras 408 should be calibrated to ensure
 that the absolute coordinates of retro-reflective markers 406 are properly
 calculated. To calibrate the camera 408, a reference target is placed on
 the treatment table in a specified location. The reference target contains
 a set of well-defined target elements at known heights and orientations.
 The data recorded by the cameras 408 for the reference target is used to
 perform precise position and orientation calibrations. More information
 regarding the calibration of multiple cameras in a video/optical imaging
 system can be found in "Close-range Camera Calibration", Photogrammetric
 Engineering 37, 855-866 (1971) and The Handbook of Non-Topographic
 Photogrammetry, 2nd ed., American Society of Photogrammetry and Remote
 Sensing (1989), both of which are hereby incorporated by reference.
 FIGS. 7a and 7b depict an embodiment of a retro-reflective marker 700 that
 can be employed within the present invention. Retro-reflective marker 700
 comprises a raised reflective surface 702 for reflecting light. Raised
 reflective surface 702 preferably comprises an approximate semi-spherical
 shape such that light can be reflected regardless of the input angle of
 the light source. A flat surface 704 surrounds the raised reflective
 surface. The underside of flat surface 704 provides a mounting area to
 attach retro-reflective marker 700 to particular locations on a patient's
 body. According to an embodiment, retro-reflective marker 406 is comprised
 of a retro-reflective material 3M#7610WS available from 3M Corporation. In
 an embodiment, retro-reflective marker 700 has a diameter of approximately
 .5 cm and a height of the highest point of raised reflective surface 702
 of approximately .1 cm.
 FIG. 8 depicts an apparatus 802 that can be employed to manufacture
 retro-reflective markers 700. Apparatus 802 comprises a base portion 804
 having an elastic ring 806 affixed thereto. Elastic ring 806 is attached
 to bottom mold piece 808 having a bulge protruding from its center. A
 control lever 810 can be operated to move top portion 812 along support
 rods 814. Top portion 812 comprises a spring-loaded top mold piece 814.
 Top mold piece 814 is formed with a semi-spherical cavity on its
 underside. In operation, a piece of retro-reflective material is placed on
 bottom mold piece 808. Control lever 810 is operated to move top portion
 812 towards base portion 804. The retro-reflective material is compressed
 and shaped between the bottom mold piece 808 and the top mold piece 814.
 The top mold piece 814 forms the upper exterior of the retro-reflective
 material into a semi-spherical shape.
 In an alternate embodiment, marker 406 comprises a marker block having one
 or more reference locations on its surface. Each reference location on the
 marker block preferably comprises a retro-reflective or reflective
 material that is detectable by an optical imaging apparatus, such as
 camera 408.
 FIG. 9 depicts an embodiment of a marker block 900 having a cylindrical
 shape with multiple reference locations comprised of retro-reflective
 elements 902 located on its surface. Marker block 900 can be formed as a
 rigid block (e.g., from styrofoam). Blocks made in this fashion can be
 reused a plurality of times, even with multiple patients. The
 retro-reflective elements 902 can be formed from the same material used to
 construct retro-reflective markers 406 of FIGS. 7a and 7b. The marker
 block is preferably formed from a material that is light-weight enough not
 to interfere with normal breathing by the patient.
 One advantage to using a marker block such as marker block 900, is that
 with a-priori knowledge of the relative positions of the reference
 locations/retro-reflective elements 902 on the marker block 900, it is
 possible to determine all six degrees of freedom of the marker block from
 a single camera view. In other words, only a single camera is required to
 derive the absolute coordinates of a marker block 900. This results
 because the relative positioning between the retro-reflective elements 902
 on the surface of marker block 900 are known, and the absolute coordinates
 and viewing orientation of the camera 408 are also known. The detected
 image of the marker block 900 by camera 408 indicates the positioning of
 the visible reference locations/retro-reflective elements 902 relative to
 the camera's viewing orientation. Because the actual relative positions
 between the retro-reflective elements 502 are known, the detected relative
 coordinates of the visible retro-reflective elements 902 from the camera
 image can be used to derive the absolute coordinate of the marker block
 900.
 A marker block can be formed into any shape or size, as long as the size,
 spacing, and positioning of the reference locations are configured such
 that a camera or other optical imaging apparatus can view and generate an
 image that accurately shows the positioning of at least two/three or more
 of the reference locations. For example, FIG. 10 depicts an alternate
 marker block 1000 having a hemispherical shape comprised of a plurality of
 retro-reflective elements 1002 attached to its surface.
 The marker block can be formed with shapes to fit particular body parts.
 For example, molds or casts that match to specific locations on the body
 can be employed as marker blocks. Marker blocks shaped to fit certain
 areas of the body facilitate the repeatable placement of the marker blocks
 at particular locations on the patient. Alternatively, the marker blocks
 can be formed to fit certain fixtures that are attached to a patient's
 body. For example, a marker block can be formed within indentations and
 grooves that allow it to be attached to eyeglasses. In yet another
 embodiment, the fixtures are formed with integral marker block(s) having
 reflective or retro-reflective markers on them.
 An alternate embodiment of the marker block comprises only a single
 reference location/reflective element on its surface. This embodiment of
 the marker block is used in place of the retro-reflective marker 406 to
 detect particular locations on a patient's body with an optical imaging
 apparatus.
 In the foregoing specification, the invention has been described with
 reference to specific embodiments thereof. It will, however, be evident
 that various modifications and changes may be made thereto without
 departing from the broader spirit and scope of the invention. For example,
 the coordinates for various body locations on a patient can be obtained by
 generating images of particular landmarks on a patient body, rather than
 using reflective or retro-reflective markers. The specification and
 drawings are, accordingly, to be regarded in an illustrative rather than
 restrictive sense.