Abstract:
A system for analyzing radiological images using three-dimensional (3D) stereo pairs comprises capturing 3D image data; storing the 3D image data; segmenting the 3D image data; creating a model from the segmented 3D image data; creating a first 3D volumetric monocular-view image for the current model position; rotating the model a prescribed amount and creating a second 3D volumetric monocular-view image for the rotated position; creating the 3D stereo pair using the first and second 3D volumetric monocular-view images; and viewing the 3D stereo pair on a 3D stereo viewer.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates in general to medical images and in particular to viewing of three-dimensional (3D) stereo pairs.  
       BACKGROUND OF THE INVENTION  
       [0002]     It is desirable to provide medical professionals with a system for viewing true three-dimensional (3D) stereo images captured using 3D radiographic modalities. Examples of medical radiographic modalities commonly used to capture 3D medical images are: CT-scanners, MR-scanners, PET-scanners, and cone-beam CT scanners.  
         [0003]     The scanner energy source(s) and imaging detector(s) are located at specific geometric positions with respect to the 3D object to be scanned. The positions depend on the object being scanned, the physics of the energy source and imaging detector, and the structures in the scanned 3D object to be viewed in the images. The scanner captures 3D image data of the object being scanned by taking a time-sequence of images while moving the energy source(s) and imaging detector(s) through a prescribed motion sequence (e.g. a helical path for CT-scanners) of known positions around the object. Alternately, the object can be moved while the energy source(s) and imaging detector(s) remain stationary.  
         [0004]     Image data captured in the previously described method is mathematically transformed (e.g. Radon transforms) from a helical scan (i.e. polar coordinate system) image into the more familiar 3D Cartesian coordinate system. For example, in medicine, 3D CT-scan, MR-scan, and PET-scan data are typically viewed on a piece of radiographic film or high-quality 2D computer monitor as two-dimensional (2D) slices. These 2D slices are represented in one or more of the three orthogonal Cartesian coordinate system views referred to in medicine as the axial (i.e. as viewed along the body&#39;s major axis), coronal (i.e. as viewed from the front/back), and sagittal (i.e. as viewed from the side) views. Each of these axial, coronal, sagittal views represents a viewer perspective along one of the three Cartesian coordinate system axes defined with respect to the scanner&#39;s geometry. Alternately for specialized viewing applications, the user can define an “oblique view” axis to reorient the Cartesian coordinate system views to one different to those provided by the traditional scanner-referenced Cartesian coordinate system.  
         [0005]     Image processing is usually performed to digitally adjust the radiographic image appearance to improve the ability of the radiologist or clinician to see the areas of interest in the image. This processing is dependent on many factors including the study being performed, the body part being imaged, patient characteristics (e.g. weight, age, etc.), clinician preferences, and so forth. Examples of this image processing known in the art include adjustments to the image sharpness, contrast, brightness, and density-specific image detail.  
         [0006]     In addition to looking at the 2D axial, coronal, and sagittal slices of an object, it is often desirable to visualize a 3D volumetric rendering of the object to get a better understanding of the positioning of the object&#39;s features in 3-space. This is especially useful for clinicians that are using these radiographic images to prepare for clinical procedures such as surgery, interventional radiology, and radiation oncology procedures. The increasing availability of hardware-accelerated 3D computer graphics engines for rendering computer-generated 3D models makes it advantageous to construct a 3D model from patient medical images captured with the previously described 3D medical radiographic image capture modalities.  
         [0007]     It is well known to create a 3D model from this information. A high-level description of the 3D model creation process includes segmenting the image into regions and representing the regions spatially using mathematical models. References known in the prior art include the following.  
         [0008]     U.S. Patent Application Publication No. 2003/0113003 A1 (Cline et al.) describes a method and system for segmentation of medical images.  
         [0009]     U.S. Pat. No. 5,319,551 (Sekiguchi et al.) describes a region extracting method and 3D display method.  
         [0010]     U.S. Pat. No. 6,373,918 (Wiemker et al.) describes a method for the detection of contours in an X-Ray image.  
         [0011]     U.S. Pat. No. 5,796,862 (Pawlicki et al.) describes an apparatus and method for identification of tissue regions in digital mammographic images.  
         [0012]     U.S. Pat. No. 5,268,967 (Jang et al.) describes a method for automatic foreground and background detection in digital radiographic images.  
         [0013]     U.S. Patent Application Publication No. 2005/0018893 A1 (Wang et al.) describes a method for segmenting a radiographic image into diagnostically relevant and diagnostically irrelevant regions.  
         [0014]     U.S. Pat. No. 6,542,628 (Muller et al.) describes a method for detection of elements of interest in a digital radiographic image.  
         [0015]     U.S. Pat. No. 6,108,005 (Starks et al.) describes a method for converting two-dimensional images to 3D images by forming at least two images from one source image where at least one image has been modified relative to the source image such that the images have a different spatial appearance.  
         [0016]     U.S. Pat. No. 6,515,659 (Kaye et al.) describes an image processing method for converting two-dimensional images into 3D images by using a variety of image processing tools that allow a user to apply any number or combination of image pixel repositioning depth contouring effects or algorithms to create 3D images.  
         [0017]     Despite the ability to create 3D medical image models, the image displays commonly used for medical image viewing are generally based on 2D display technology (e.g. paper, radiographic film, computer monitors and projection systems). This 2D display media is limited to displaying pixels in a single plane, with the same planar image being viewed by both eyes of the human observer.  
         [0018]     It is well known that fine artists and, more recently, graphic artists have developed techniques for creating the illusion of 3D depth when displaying images on 2D display media. These techniques include: forced perspective, shape-from-shading, relative size of commonly known objects, rendering detail, occlusion and relative motion. These techniques work by creating an optical illusion, triggering several psychophysical cues in the human eye-brain system that are responsible for creating the human viewer&#39;s experience of depth perception in the viewed scene. However, these artistic techniques for displaying 3D volumetrically rendered images on a single, unaltered planar 2D display media device can not produce binocular disparity in the human eye-brain system. Binocular disparity is one of the dominant psychophysical cues necessary to achieve true stereo depth perception in humans.  
         [0019]     It is well known in the art that stereo imaging applications allow for viewing of 3D images in true stereo depth perception using specialized stereo pair image viewing equipment to produce binocular disparity. In his paper  The Limits of Human Vision , Sun Microsystems, Michael F. Deering describes “a model of the perception limits of the human visual system.” 
         [0020]     The idea of utilizing two-dimensional images to create an illusion of three dimensionality, by using image horizontal parallax to present slightly different left and right images to the viewer&#39;s left and right eyes, respectively, (i.e. a stereo pair) seems to date back at least to the 16th century, when hand-drawn stereograms appeared. In the 19th century, photographic stereograms of exotic locations and other topics of interest were widely produced and sold, along with various hand-held devices for viewing them. More decently, the ViewMaster® popularized stereo depth perception using a handheld viewer that enabled the observer to view stereo pair images recorded on transparency film.  
         [0021]     U.S. Pat. No. 6,515,659 (Kaye et al.) describes an image processing method and system for converting two-dimensional images into realistic reproductions, or recreations of three-dimensional images.  
         [0022]     McReynolds and Blythe,  Advanced Graphics Programming Techniques Using OpenGL , SIGGRAPH, 1998, describes a method for computing stereo viewing transforms from a graphical model of the 3D object where the left eye view is computed based on transforming from the viewer position (the viewer position is nominally equidistant between the left-eye and right-eye viewing positions with the left- and right-eye viewing angles converging to a point on the surface of the object) to the left-eye view, applying viewing operation to get to viewer position, applying modeling operations, then changing buffers and repeating this sequence of operations to compute the right eye view.  
         [0023]     Batchelor,  Quasi - stereoscopic Solar X - ray Image Pair , NASA; nssdc.gsfc.nasa.gov/solar/stereo_images.htm, describes a method for computing a quasi-stereoscopic image pairs of the Sun. “The image pair represents a step towards better investigation of the physics of solar activity by obtaining more 3D information about the coronal structures. In the time between the images (about 14 hours) the Sun&#39;s rotation provides a horizontal parallax via its rotation. The images have been registered and placed so that the viewer can train the left eye at the left image and right eye at the right image, obtaining a quasi- stereoscopic view, as if one had eyes separated by one tenth the distance from Earth to the Sun. Much of the Sun&#39;s coronal structure was stable during this time, so depth can be perceived.” 
         [0024]     Wikipedia (http://en.wikipedia.org/wiki/Stereoscopy) summarizes many of the current 3D stereo devices used to produce binocular stereoscopic vision in humans from digital, film, and paper image sources. These include: autostereo viewers, head-mounted microdisplays, lenticular/barrier displays, shutter glasses, colored lens glasses, linearly polarized lens glasses, and circularly polarized lens glasses.  
         [0025]     Unfortunately, it is not uncommon for many of these 3D stereo devices to induce eye fatigue and/or motion sickness in users. The cause for these negative physical side effects in users can be explained largely by inconsistencies between the induced binocular disparity and the cues of accommodation (i.e. the muscle tension needed to change the focal length of the eye lens to focus at a particular depth) and convergence (i.e. the muscle tension need to rotate each eye to converge at the point of interest on the surface of the object being viewed).  
         [0026]     Technical advances in 3D stereo image viewer design have reduced the magnitude and frequency of occurrence of these negative side effects to the point where they can be used without placing undue stress on medical personnel.  
         [0027]     U.S. Pat. No. 6,871,956 (Cobb et al.) and U.S. Patent Application Publication No. 2005/0057788 A1 (Cobb et al.) describe an autostereoscopic optical apparatus for viewing a stereoscopic virtual image comprised of a left image to be viewed by an observer at a left viewing pupil and a right image to be viewed by an observer at a right viewing pupil.  
         [0028]     Technology and engineering developments have enabled the potential size and cost of these 3D stereo medical image viewers to be reduced to a level where they are practical to deploy for medical image viewing.  
         [0029]     U.S. patent application Ser. No. 10/961,966, filed Oct. 8, 2005, entitled “Align Stereoscopic Display” by Cobb et al. describes a method and apparatus for an alignment system consisting of a viewer apparatus for assessing optical path alignment of a stereoscopic imaging system. The apparatus having a left reflective surface for diverting light from a left viewing pupil toward a beam combiner and a right reflective surface for diverting light from a right viewing pupil toward the beam combiner. The beam combiner directs the diverted light from left and right viewing pupils to form a combined alignment viewing pupil, allowing visual assessment of optical path alignment.  
         [0030]     U.S. patent application Ser. No. 11/156,119, filed Jun. 17, 2005, entitled “Stereoscopic Viewing Apparatus” by Cobb et al. describes an optical apparatus for building a small, boom-mountable stereoscopic viewing has a first optical channel with a first display generating a first image and a first viewing lens assembly producing a virtual image, with at least one optical component of the first viewing lens assembly truncated. A second optical channel has a second display generating a second image and a second viewing lens assembly producing a virtual image, with at least one optical component of the second viewing lens assembly truncated along a second side. A reflective folding surface is disposed between the second display and second viewing lens assembly to fold a substantial portion of the light within the second optical channel. An edge portion of the reflective folding surface blocks a portion of the light in the first optical channel. The first side of the first viewing assembly is disposed adjacent the second side of the second viewing lens assembly.  
         [0031]     The benefits of viewing 3D stereo medical images are becoming well known. True 3D stereo medical image viewing systems can provide enhanced spatial visualization of anatomical features with respect to surrounding features and tissues. Although radiologists are trained to visualize the “slice images” in 3D in their mind&#39;s eye, other professionals who normally work in 3D (e.g. surgeons, etc.) cannot as easily visualize in 3D. This offers the potential for improve the speed of diagnosis, reduce inaccurate interpretations and provide improved collaboration with clinicians who normally perform their work tasks using both their eyes to view natural scenes in 3D (e.g. surgeons).  
         [0032]     However, with the ever-increasing resolution (number of slices) of radiology 3D medical image capture modalities, it takes diagnostic radiologists longer, using traditional methods, to review all the “slice” images in each individual radiographic study. This increased resolution makes it harder for radiologists to visualize where structures are with respect to features in adjacent slices. These trends may offer diagnostic radiologists the opportunity to benefit from true 3D stereo medical image viewing as well.  
         [0033]     An article in Aunt Minnie entitled 3 D: Rendering a New Era , May 2, 2005 states “(Three-dimensional) ultrasound provides more accurate diagnoses for a variety of obstetrical and gynecological conditions, helping physicians make diagnoses that are difficult or impossible using 2D imaging,” said longtime obstetrical ultrasound researcher Dr. Dolores Pretorius of the University of California, San Diego. “(Three-dimensional) ultrasound is valuable in diagnosing and managing a variety of uterine abnormalities.” Compared with MRI, 3D ultrasound has the same capabilities but is faster and less expensive, Pretorius said. Also in that article was this note about accuracy using 3D. “We also use 3D for lung nodules, because it measures more accurately, where the radiologist might get different measurements each time,” Klym said. Dr. Bob Klym is the lead 3D practitioner at Margaret Pardee.  
         [0034]     Most recently deployed medical imaging systems capable of capturing and storing 3D medical image data (i.e. Picture Archive and Communication Systems (PACS)) currently have the capability to render a monocular (i.e. non-stereo without horizontal image disparity between left and right eye images) volumetric image view from 3D medical image data and displaying this volumetric rendering on a 2D CRT or LCD monitor. Upgrading these existing systems to compute true stereo image pairs using practices known in the industry would result in one or both of doubling the graphics engine throughput and/or significant software/firmware changes to accommodate the second stereo viewing data stream. Both of these upgrades would add considerable upgrade expense and time for institutions to upgrade their current PACS systems to provide true 3D stereo image viewing for their radiologists and clinicians.  
         [0035]     The goal of this invention is to provide a method for leveraging the existing monocular volumetric rendering using the existing 3D graphics engine available in most current PACS medical imaging systems to enable true 3D stereo viewing of medical images with binocular disparity. Another goal of this invention is to enable true 3D stereo viewing without the need to purchase significantly more graphics engine hardware. By using this invention to reduce the cost, time and complexity required to enable existing PACS medical imaging systems to provide true 3D stereoscopic viewing for clinicians, the benefits of this technology can be more rapidly deployed to benefit clinicians and their patients.  
       SUMMARY OF THE INVENTION  
       [0036]     Briefly, according to one aspect of the present invention a system for analyzing radiological images using 3D stereo pairs comprises capturing, storing, and segmenting the 3D image data. A model is created from the segmented 3D image data. A first 3D volumetric monocular-view image for a current model position is created. The model is rotated a prescribed amount and creates a second 3D volumetric monocular-view image for the rotated position. The 3D stereo pair is created using the first and second 3D volumetric monocular-view images. The 3D stereo pair is viewed on a 3D stereo viewer.  
         [0037]     This invention provides a way to produce 3D stereo depth perception from stereo pair images of medical images, with significantly reduced the computational load and provide the potential for adapting an aftermarket true stereo viewer to existing systems providing a single sequence of volumetric rendered monocular views (e.g. the ability to view a volumetric reconstruction of an object on a 2D display device such as a CRT, LCD monitor or television screen). To do this, the computational load is reduced to the order of one rendered 3D volumetric monocular image view per viewing position instead of computing two independent views (i.e. one for each eye view in the stereo viewing system) as has been done in the prior art.  
         [0038]     The invention and its objects and advantages will become more apparent in the detailed description of the preferred embodiment presented below. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0039]      FIG. 1  is a schematic of a prior art stereo pair calculation from 3D image model.  
         [0040]      FIG. 2  is a schematic view showing calculation of 3D stereo pairs according to the present invention.  
         [0041]      FIG. 3  is a geometric representation of prior art calculations shown in  FIG. 1 .  
         [0042]      FIG. 4  is a geometric representation of calculations according to the present invention shown  FIG. 2 .  
         [0043]      FIG. 5  is a more detailed view of section A shown in  FIG. 4 .  
         [0044]      FIG. 6  is a schematic view showing the monocular view according to the prior art.  
         [0045]      FIG. 7  is a superimposed binocular view of the prior art with the present invention.  
         [0046]      FIG. 8  shows the micro-stepping methodology of the present invention.  
         [0047]      FIG. 9  is a schematic view showing calculation of 3D stereo pairs according to the present invention with the addition of a graphics engine output switch and rotation direction control. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0048]      FIG. 1  is a schematic of a prior art stereo pair calculation from 3D image model using 3D stereo pairs and is shown as background for this invention. Many of these components are also used in  FIG. 2  and are explained in the context of the present invention. Of particular distinction is the presence of two (2) 3D graphics engines  14  shown in  FIG. 1  as prior art. This invention, as  25  described in  FIG. 2 , uses a single 3D graphics engine  14  with the addition of the 3D model rotation calculator  16  and delay frame buffer  44  not used in the  FIG. 1  prior art.  
         [0049]      FIG. 2  shows the system of this invention for analyzing medical images  9  using 3D stereo pairs. Medical image data  9  is captured by scanning object  10  using scanner  11  which is capable of producing 3D image data. This medical image data  9  is stored in data storage  8 .  
         [0050]     Image segmentation  41  is performed on the medical image data  9  resulting in labeled regions of medical image data  9  that belong to the same or similar features of object  10 . Image segmentation  41  is based on known medical image segmentation rules as described in the prior art. Examples include threshold-based segmentation algorithms using pixel intensity criteria up through complex image morphology rules including edge finding and region growing. Image segmentation  41  results for medical image data  9  are stored in data storage  8 .  
         [0051]     High performance 3D graphics engines now widely available from companies such as ATI Technologies, Inc. (www.ati.com) and nVidia Corporation (www.nvidia.com). for use in computers supporting image processing, advanced gaming and medical Picture Archive and Communication Systems (PACS). To improve system performance and take advantage of these high performance graphics engines, a 3D model  42  of object  10  is constructed. The 3D modeling process  40  uses the image segmentation  41  and medical image data  9  to produce the 3D model  42 . The 3D model  42  is stored in data storage  8 .  
         [0052]     Viewer perspective  25 , reference Paul Bourke,  Calculating Stereo Pairs ; http://astronomy.swin.edu.au/˜pbourke, defines the position and orientation of the viewer with respect to the 3D model  42  of object  10 . Viewer perspective  25  is traditionally specified using the 3-degrees of freedom specifying the viewer&#39;s position in 3-space (e.g. X, Y, and Z coordinates in a Cartesian coordinate system) and the 3-degrees of freedom specifying the viewer&#39;s orientation (i.e. direction of view) from that position in 3-space.  FIG. 4  further shows viewer perspective  25  defined from the viewer perspective reference  5  viewing the 3D model  42  of object  10  along viewer perspective line  20  at the fusion distance  12  from the 3D model  42  of object  10 .  
         [0053]     Returning the  FIG. 2 , in some applications the viewer perspective  25  may be static with respect to the 3D model  42  of object  10 , in which case no viewer perspective control device  24  is required. Generally, the user desires control over the 6-degrees of freedom that define the viewer perspective  25  with respect to the 3D model  42  using a viewer perspective control device  24 . Alternately, the 3D model  42  can be repositioned with respect to the viewer perspective  25  using a viewer perspective control device  24 .  
         [0054]     Viewer perspective control device  24  examples include joysticks, data gloves, and traditional 2D devices such as a computer mouse and keyboard. The viewer perspective control device  24  controls the 3-degrees of freedom specifying the viewer&#39;s position in 3-space (e.g. X, Y, and Z coordinates in a Cartesian coordinate system) and the 3-degrees of freedom specifying the viewer&#39;s orientation (i.e. direction of view) from that position in 3-space, which combine to specify the viewer perspective  25  in 3-space. Viewer perspective control device  24  controls position and orientation directly or indirectly via other parameters such as velocity or acceleration. For example, flight simulators use a joystick with thrust and rudder controls as the preferred viewer perspective control device  24  to control the plane model&#39;s position (i.e. altitude above the ground (Z) and it&#39;s projected X and Y position on the earth&#39;s surface) and the plane&#39;s orientation (i.e. roll, pitch, and yaw) in 3-space.  
         [0055]     Viewer head-eye model  46  describes the properties and parameters of the viewing subsystem. The eye model portion of the viewer head-eye model  46  describes viewer first eye  1  and viewer second eye  2  including their physical characteristics and capabilities. These models are well-known in the art and contain parameters such as, but not limited to, field of view, resolution, lens focal length, focus capability, light sensitivity by wavelength and signal-to-noise ratio as is required to predict the response of the first viewer eye  1  and second viewer eye  2  to “viewing” the 3D model  42  of object  10 . Michael F. Deering, in his paper  The Limits of Human Vision , Sun Microsystems) describes “a model of the perception limits of the human visual system.” The head model portion of viewer head-eye model  46  describes the physical location, orientation, and interaction of one or more viewer eyes with respect to the other viewer eyes as well as with respect to the system&#39;s viewer perspective reference. In the present invention, the viewer head-eye model  46  describes the properties of and relationship between viewer first eye  1 , viewer second eye  2 , and viewer perspective reference  5 .  
         [0056]     Eye perspective calculator  23  uses the viewer head-eye model  46 , viewer perspective  25  and 3D model  42  of object  10  in  FIG. 2  to compute the first eye perspective approximation line  37  and first eye field-of-view for viewer first eye  1  and the second eye perspective approximation line  38  and second eye field-of-view for viewer second eye  2  shown in  FIG. 4 .  
         [0057]     The first eye perspective approximation line  37 , first eye field-of-view, the second eye perspective approximation line  38 , second eye field-of-view, fusion distance  12 , interocular distance  28 , distance R  7 , viewer perspective reference  5 , viewer perspective line  20 , axis of rotation 3, direction of rotation  33 , microstep increment angle  43 , angle theta  36  and 3D model  42  of object  10  are used to control the 3D model rotation calculator  16 , 3D graphics engine  14 , and delay frame buffer  44  in  FIG. 2  to maintain the viewing geometry of this invention detailed in  FIG. 4 . 3D graphics engine  14  renders a 3D volumetric monocular image view  45  (e.g. V 1 ) for the viewer first eye  1  viewing along the first eye perspective approximation line  37  for each microstep increment angle  43 .  
         [0058]     In  FIG. 2 , the 3D model rotation calculator  16  uses results from eye perspective calculator  23  and the 3D model  42  of object  10  to calculate the microstep increment angle  43 , shown in  FIG. 8 . Microstep increment angle  43  is applied to 3D model  42  by the 3D graphics engine  14  to produce a 3D volumetric monocular image view  45  (e.g. V 1 ) for each microstep increment angle  43 , thus forming the sequence of volumetric rendered monocular views  47  of 3D model  42 , as shown in  FIG. 2  and schematically in  FIG. 8 .  
         [0059]     In  FIG. 2 , the first eye frame buffer  13  and the delay frame buffer  44  receive the sequence of volumetric rendered monocular views  47  from the 3D graphics engine  14 . The first eye frame buffer  13  stores each individual 3D volumetric monocular image view  45  contained in the sequence of volumetric rendered monocular views  47 , while that view is transmitted to the 3D stereo viewer  4  for viewing by the viewer first eye  1  through first eyepiece  48 . After a period of time, the first eye frame buffer  13  is updated with the next individual 3D volumetric monocular image view  45  from the sequence of volumetric rendered monocular views  47 .  
         [0060]     Delay frame buffer  44  is implemented as a queue capable of storing one or more individual 3D volumetric monocular image view  45  (i.e. “frames”) and is used to create a time delay before transmitting each individual 3D volumetric monocular image view  45  in the sequence of volumetric rendered monocular views  47  to the second eye frame buffer  53  relative to that same individual 3D volumetric monocular image view  45  being transmitted to the first eye frame buffer  13 . The delay duration of the delay frame buffer  44  is computed by eye perspective calculator  23  to maintain the viewing geometry of this invention as detailed in  FIG. 4 .  
         [0061]     Summarizing, the 3D model rotation calculator  16 , 3D graphics engine  14 , and delay frame buffer  44  are controlled such that the same single sequence of volumetric rendered monocular views  47  are viewed sequentially, but delayed in time, through the second eyepiece  49 , with respect to the same sequence of volumetric rendered monocular views  47  being viewed through the first eyepiece  48 .  
         [0062]     The second eye frame buffer  53  stores each individual 3D volumetric monocular image view  45  contained in the sequence of volumetric rendered monocular views  47 , appropriately delayed by delay frame buffer  44 , while that individual 3D volumetric monocular image view  45  is transmitted to the 3D stereo viewer  4  for viewing by viewer second eye  2  through second eyepiece  49 . After a period of time, the second eye frame buffer  53  is updated with the next individual 3D volumetric monocular image view  45  from the sequence of volumetric rendered monocular views  47  retrieved from delay frame buffer  44 .  
         [0063]     Concluding, the previously described components are controlled to maintain the angle theta  36  between the first eye perspective approximation line  37  of the viewer first eye  1  and the second eye approximation line  38  of the viewer second eye  2  viewing the 3D model  42  of object  10  through the first eyepiece  48  and the second eyepiece  49 , respectively, of the 3D stereo viewer  4 . This is done using a single sequence of volumetric rendered monocular views  47  viewed with appropriate delay, as previously described, by viewer first eye  1  and viewer second eye  2 .  
         [0064]     For human viewing, it is preferable to simultaneously update first eye frame buffer  13  and second eye frame buffer  53 . The update frame rate will depend on the desired effect and processing speed of the components used to construct this invention, especially the 3D graphics engine  14 .  
         [0065]     Under certain circumstances, it may be desirable to stop rotation of the 3D model  42  of object  10  as viewed using the 3D stereo viewer  4 . This provides the opportunity to study the 3D model  42  of the object  10  in detail without the distraction of a moving 3D model  42 . To maintain stereo perception when the 3D model rotation calculator  16  stops rotating the 3D model  42 , the delayed relationship between the individual views from the sequence of rendered monocular views in the first eye frame buffer  13 , as viewed through first eyepiece  48 , and the second eye frame buffer  53 , as viewed through second eyepiece  49 , must be maintained.  
         [0066]     This is accomplished by simultaneously freezing both the individual view currently stored in the first eye frame buffer  13  and the individual view currently stored in the second eye buffer  53 . Freezing these respective views from the sequence of volumetric rendered monocular views  47 , with the view in the second eye buffer  53  delayed by the delay frame buffer  44 , can be accomplished in several ways. One approach is to inhibit both first eye frame buffer  13  and second eye frame buffer  53  from accepting new inputs while maintaining their current output to the first eyepiece  48  and second eyepiece  49 , respectively. Alternately, both the output of 3D graphics engine  14  and delay frame buffer  44  could be frozen while the first eye frame buffer  13  and second eye frame buffer  53  continue to operate.  
         [0067]     As described in  FIG. 4 , this maintains the angle theta  36  between the first eye perspective approximation line  37  used by the viewer first eye  1  and the second eye approximation line  38  used by the viewer second eye  2  to view the 3D model  42  of object  10  such that stereo perception is maintained when looking at the still view through the first eyepiece  48  and second eyepiece  49  of the 3D stereo viewer  4 .  
         [0068]      FIG. 3  shows the geometry of a stereo image viewing system in the prior art described by McReynolds and Blythe and offered as reference for explaining the nature of this invention. The viewer first eye  1  and viewer second eye  2  are separated by interocular distance  28 . Viewer perspective reference  5  is located equidistant between and in the same vertical and horizontal planes as the viewer first eye  1  and viewer second eye  2 . From John Wattie,  Stereoscopic Vision: Elementary Binocular Physiology ; nzpoto.tripod.com/sterea/3dvision.htm, the average human eye separation (i.e. interocular distance  28 ) is approximately 65 mm; the eyes are normally approximately equally spaced from the nose bridge (i.e. viewer perspective reference  5 ) with the average displacement of each eye from the nose bridge is then one-half the human interocular distance  28  or (0.5*I)=0.5*65 mm=32.5 mm.  
         [0069]     Stereo fusion is the process by which the eye-brain creates the illusion of a single scene with relative depth perception. In humans, only a portion of each eye&#39;s field of view, called Pamum&#39;s Fusional Area, located around the eye&#39;s fovea can effectively fuse stereo images. With normal stereo viewing, the left and right eye fovea viewpoints converge at the convergence point  26 , on the object  10  surface, increasing the potential that stereo fusion will occur in the region of the viewer&#39;s focus.  
         [0070]     The first eye perspective view axis  17  is defined to be the direction of gaze fixation from the viewer first eye  1  to the convergence point  26  on the surface of 3D model  42  of object  10 . The first eye infinite-viewing-distance line  21  is parallel to the viewer perspective line  20  and represents the direction of gaze fixation from the viewer first eye  1  to a virtual object located at an infinite distance from the viewer first eye  1 .  
         [0071]     Similarly, the second eye perspective view axis  18  is defined to be the direction of gaze fixation from the viewer second eye  2  to the convergence point  26 . Also similarly, the second eye infinite-viewing-distance line  22  is parallel to the viewer perspective line  20  and represents the direction of gaze fixation from the viewer second eye  2  to a virtual object located at an infinite distance from the viewer second eye  2 .  
         [0072]     The first eye perspective view axis  17  and second eye perspective view axis  18  intersect at the convergence point  26  located on the surface of 3D model  42  of object  10  at fusion distance  12  from the viewer perspective reference  5  as measured along the viewer perspective line  20 . The viewer perspective  25  is defined from the viewer perspective reference  5  viewing the 3D model  42  of object  10  along viewer perspective line  20  to the convergence point  26  on the surface of 3D model  42  of object  10  and is located fusion distance  12  from the 3D model  42  of object  10 .  
         [0073]     From geometry, the first eye infinite-viewing-distance line  21 , second eye infinite-viewing-distance line  22  and viewer perspective line  20  are all parallel to each other and serve as reference lines for describing this system. Angle alpha  27  is the angle formed by the viewer first eye  1 , first eye perspective view axis  17 , convergence point  26 , second eye perspective view axis  18  and viewer second eye  2 . The viewer perspective line  20  bisects angle alpha  27 . The angle formed by the first eye infinite-viewing-distance line  21 , the viewer first eye  1 , and the first eye perspective view axis  17  is congruent with the angle formed by the second eye infinite-viewing-distance line  22 , the viewer second eye  2 , and the second eye perspective view axis  18 ; these angles have measurement equal to angle (alpha/2)  39 .  
         [0074]     To achieve convergence on the object surface, the first eye perspective view axis  17  is therefore depressed from the first eye infinite-viewing-distance line  21  toward the viewer perspective line  20  by an angle (alpha/2)  39 . Similarly, the second eye perspective view axis  18  is depressed from the second eye perspective infinite-viewing-distance line  22  toward the viewer perspective line  20  by an angle (alpha/2)  39 .  
         [0000]     Using trigonometry:
 
angle (alpha/2)=tan −1  [( I/ 2)/ F] 
        where: I is the interocular distance  28      F is the fusion distance  12      Solving for angle alpha  27 , we have:
 
angle alpha=2*tan −1  [( I/ 2)/ F] 
       
 
         [0078]      FIG. 4  shows the geometry of the stereo image viewing system that is the subject of this invention. To achieve computational simplicity, a goal of this invention, the system geometry shown in  FIG. 4  is constructed to approximate the geometry of the prior art system described in  FIG. 3 . Under many practical viewing situations found in 3D stereo medical image viewing applications, this approximation enables a single graphics engine, present in most medical Picture Archiving and Communication Systems (PACS), to drive a true 3D stereo viewer  4  from the same sequence of volumetric rendered monocular views  47  used to drive the traditional 2D medical diagnostic monitor.  
         [0079]     The viewer first eye  1  and viewer second eye  2  are separated by interocular distance  28 . Viewer perspective reference  5  is located equidistant between and in the same vertical and horizontal planes as the viewer first eye  1  and viewer second eye  2 . The first eye infinite-viewing-distance line  21  is parallel to the viewer perspective line  20  and represents the direction of gaze fixation from the viewer first eye  1  to a virtual object located at an infinite distance from the viewer first eye  1 . Similarly, the second eye infinite-viewing-distance line  22  is parallel to the viewer perspective line  20  and represents the direction of gaze fixation from the viewer second eye  2  to a virtual object located at an infinite distance from the viewer second eye  2 . The viewer perspective  25  is defined from the viewer perspective reference  5  viewing the 3D model  42  of object  10  along viewer perspective line  20  to the convergence point  26  defined in  FIG. 3  on the surface of 3D model  42  of object  10  and is located fusion distance  12  from the 3D model  42  of object  10 .  
         [0080]     The present invention differs from the prior art and achieves is computational simplicity and efficiency by not using the geometry defined by the first eye perspective view axis  17  intersecting with the second eye perspective view axis  18  at the convergence point  26  on the surface of 3D model  42  of object  10  as shown in the prior art in  FIG. 3 . Instead, the present invention defines the first eye perspective approximation line  37  to be the direction of gaze fixation from the viewer first eye  1  to the axis of rotation  3  of the 3D model  42  of object  10 . Similarly, the second eye perspective approximation line  38  is defined to be the direction of gaze fixation from the viewer second eye  2  to the axis of rotation  3  of the 3D model  42  of object  10 . Therefore, the first eye perspective approximation line  37  and the second eye perspective approximation line  38  intersect at the point defined to be the axis of rotation  3  of the 3D model  42  of object  10 .  
         [0081]     The axis of rotation  3  of the 3D model  42  of object  10  is defined to be perpendicular to the plane defined by the first eye perspective approximation line  37  and the second eye perspective approximation line  38 . This enables rotation of 3D model  42  of object  10  around the axis of rotation  3  in direction of rotation  33  to produce horizontal binocular disparity in the images being simultaneously viewed by the viewer first eye  1  and the viewer second eye  2  using the 3D stereo viewer  4  as described in this invention. Distance R  7  is the projected linear distance along viewer perspective line  20 , from the axis of rotation  3  defined in  FIG. 4  to the convergence point  26  on the surface of the 3D model  42  of object  10  as described in  FIG. 3  and shown for reference in  FIG. 4 .  
         [0082]     Note that the axis of rotation  3  does not need to pass through the center of the 3D model  42  of object  10  for the invention to operate properly. However, for many objects, placement of the axis of rotation  3  though the center of 3D model  42  of object  10  may yield preferred results.  
         [0083]     Note also that while the axis of rotation  3  is generally implemented collinear with viewer perspective line  20  as viewed in  FIG. 4 , this is also not a limitation of the invention. Defining the axis of rotation  3  non-collinear with viewer perspective line  20 , will still provide stereo perception, with the 3D model  42  of object  10  appearing off to one side when viewed on the 3D stereo viewer  4 . The symmetry of the system geometry described in  FIG. 4  is slightly distorted when the axis of rotation  3  is not collinear with viewer perspective line  20 , but the invention still provides a reasonable approximation to the prior art system shown in  FIG. 3 .  
         [0084]     In practice, this variation is minimized by the user desire to see as much of the 3D model  42  of object  10  as possible. In practical use, the user tends to align the area of the 3D model  42  of object  10  being studied so that the area of interest is being imaged onto each of their eyes&#39; retinas at or near the eye&#39;s fovea. Panum&#39;s fusional area is the limited area on the retina where retinal differences can be fused and interpreted as a 3D stereo rather than double vision. Since Panum&#39;s fusional area of the human retina roughly corresponds to the location of the human eye fovea, the user will naturally tend to position the 3D model  42  of the object  10  close to collinear with the viewer perspective line  20 , enabling this invention to provide desirable 3D stereo viewing results.  
         [0085]     While the axis of rotation  3  of the 3D model  42  of object  10  is ideally defined to be perpendicular to the plane defined by the first eye perspective approximation line  37  and the second eye perspective approximation line  38 , this assumption can also be relaxed. Even for the ideal (i.e. perpendicular) orientation of the axis of rotation  3 , rotation around it produces a small amount of undesirable vertical misalignment as well as the larger desired horizontal parallax. As the axis of rotation  3  moves away from the ideal perpendicular orientation, the amount of vertical misalignment induced is increased relative to the desired horizontal parallax (a dominant source of human stereoscopic vision) as the 3D model  42  of object  10  is rotated around the axis of rotation  3 . As long as the undesirable vertical misalignment is kept relatively small, the viewer&#39;s brain is still able to successfully fuse the two separate images viewed by the viewer first eye  1  and viewer second eye  2  in the 3D stereo viewer  4  into a single stereoscopic image of the 3D model  42  of object  10 . According to John Wattie,  Stereoscopic Vision: Elementary Binocular Physiology , “The brain is tolerant of small differences between the two eyes. Even small magnification differences and small angles of tilt are handled without double vision.” 
         [0086]     Further note that this invention will also allow the 3D model  42  of object  10  to be pre-oriented with respect to the geometric system defined in  FIG. 4 , prior to the definition of the axis of rotation  3  using the viewer perspective control device  24 . The user may desire to do this to improve the view of key features of the 3D model  42  of object  10  based on user viewing preference and area of interest. Examples of this pre-orientation include but are not limited to, tilting the 3D model  42  toward the viewer perspective reference  5 , rotating the 3D model  42  around the viewer perspective line  20 , and rotating the 3D model  42  around it&#39;s vertical axis, or any combination of these pre-orientation operations. Once the 3D model  42  of object  10  is pre-oriented, the axis of rotation  3  is defined to satisfy the geometry of the invention described in  FIG. 4 . The pre-oriented 3D model  42  of object  10  is then rotated around the axis of rotation  3  defined relative to the pre-oriented 3D model  42  of object  10 .  
         [0087]     In  FIG. 4  using geometry, the first eye infinite-viewing-distance line  21 , second eye infinite-viewing-distance line  22  and viewer perspective line  20  are all parallel to each other and serve as reference lines for describing this invention. Angle theta  36  is the angle formed by the viewer first eye  1 , the first eye perspective approximation line  37 , axis of rotation  3  of 3D model  42  of object  10 , the second eye perspective approximation line  38  and viewer second eye  2 . The viewer perspective line  20  bisects angle theta  36 . The angle formed by the first eye infinite-viewing-distance line  21 , the viewer first eye  1  and the first eye perspective approximation line  37  is congruent with the angle formed by the second eye infinite-viewing-distance line  22 , the viewer second eye  2  and the second eye perspective approximation line  38 ; these angles have measurement equal to angle (theta/2)  35 .  
         [0088]     To intersect at the object axis of rotation  3 , the first eye perspective approximation line  37  is depressed from the first eye infinite-viewing-distance line  21  toward the viewer perspective line  20  by angle (theta/2)  35 , where angle theta  36  is the angle formed between the first eye perspective approximation line  37  and the second eye perspective approximation line  38  as previously described. Similarly, the second eye perspective approximation line  38  is depressed from the second eye infinite-viewing-distance line  22  toward the viewer perspective line  20  by angle (theta/2)  35 .  
         [0000]     Using trigonometry:
 
angle (theta/2)=tan −1  [( I/ 2)/( F+R )]
        where: I is the interocular distance  28  
            F is the fusion distance  12  
 
 R is distance R  7  defined as the projected linear distance along viewer perspective line  20 , from the axis of rotation  3  defined in  FIG. 4  to the convergence point  26  on the surface of the 3D model  42  of object  10  as described in  FIG. 3 . 
 
 Solving for angle theta  36 , we have:
 
angle theta=2*tan −1  [( I/ 2)/( F+R )]
   
               
 
         [0091]     From this equation, as distance R  7  gets small compared with the fusion distance  12  and approaches zero, angle theta  36  approaches being equal to angle alpha  27  as shown below:  
                 lim     R   -&gt;   0       ⁢     (   theta   )       =       ⁢       lim     R   -&gt;   0       ⁢     {     2   *       tan     -   1       ⁡     [       (     I   /   2     )     /     (     F   +   R     )       ]         }                   =       ⁢     2   *       tan     -   1       ⁡     [       (     I   /   2     )     /   F     ]                     =       ⁢     angle   ⁢           ⁢   alpha               
 
 For R&lt;&lt;F, angle theta  36  is a very good approximation of angle alpha  27 . 
 
         [0092]     Bourke describes a well-known criterion for natural appearing stereo in humans as being met when the ratio of fusion distance  12  to interocular distance  28  is on the order of 30:1. At ratios greater than  30 : 1 , human stereo perception begins to decrease; human stereoscopic vision with the unaided eye becomes virtually non-existent beyond approximately 200 meters (ratio of approximately 3000:1). Ratios less than 30:1, especially ratios of 20:1 or less, give an increasingly exaggerated stereo sensation compared with normal unaided human eye viewing. This exaggerated stereo effect is generally referred to as hyper-stereo. Increasing this ratio results in reduced perception of stereo depth perceived by the viewer in the stereo image when compared to typical human experience in viewing natural scenes.  
         [0093]     Substituting for the fusion distance  12  with thirty times the interocular distance  28  (F=30*I) in the previous equation for angle theta  36 :
 
Angle theta=2*tan −1  [( I/ 2)/(30 *I+R )]=2*tan −1  [( I/( 60 *I+ 2R)]
 
 Estimating the magnitude of angle theta  36  under these conditions in this equation, it is clear that angle theta  36  is largest when R=0.  
                 lim     R   -&gt;   0       ⁢     (   theta   )       =       ⁢     2   *       tan     -   1       [     (     I   /     (     60   *   I     )       ]                     =       ⁢     2   *       tan     -   1       [     (     I   /   60     ]                     =       ⁢     1.9   ⁢           ⁢   degrees   ⁢           ⁢     (       where   ⁢           ⁢   F     =     30   *   I       )                 
 
 It can be seen from inspection of this equation that as distance R  7  increases, angle theta  36  decreases. Under the natural appearing stereo assumptions described by Bourke that lead to natural appearing stereo in humans:
 
Angle theta&lt;=1.9 degrees
 
         [0094]      FIG. 5  is a more detailed view of Section A shown in  FIG. 4 , providing an enlarged view of the object  10  and the geometry of the invention. The axis of rotation  3  of 3D model  42  of object  10  with direction of rotation  33  is defined as in  FIG. 4 . The first eye perspective approximation line  37  intersects the surface of the 3D model  42  of object  10  at the first eye view surface intersection point  30 . Similarly, the second eye perspective approximation line  38  intersects the surface of the 3D model  42  of object  10  at the second eye view surface intersection point  31 . The distance between the first eye view surface intersection point  30  and the second eye view surface intersection point  31 , measured perpendicular to the viewer perspective line  20 , is the horizontal parallax error  32 . Horizontal parallax error  32  is introduced by the geometry of this invention, specifically the assumption that first eye perspective approximation line  37  and second eye perspective approximation line  38  intersect at the axis of rotation  3  of 3D model  42  of object  10  as shown in  FIG. 4  instead of intersecting at the convergence point  26  as shown in  FIG. 3 . For the case where the viewer perspective line  20  passes through the convergence point  26  and the axis of rotation  3 , it bisects angle theta  36  into angle (theta/2)  35 . The horizontal parallax error  32  is represented mathematically as:
 horizontal parallax error=2 *R *sin(theta/2) 
 where: R is distance R  7  defined as the projected linear distance along viewer perspective line  20 , from the axis of rotation  3  defined in  FIG. 4  to the convergence point  26  on the surface of the 3D model  42  of object  10  as described in  FIG. 3 . 
 
         [0095]     From previous calculations when the criterion described by Bourke for natural appearing stereo in humans is met, angle theta &lt;=1.9 degrees, therefore:
 
horizontal parallax error &lt;=2 *R  sin(1.9/2)
 
&lt;=2 *R *(0.0166)
 
horizontal parallax error &lt;=0.0332 *R (less than 3.5% of  R )
 
         [0096]     Again according to Wattie “the brain is tolerant of small differences between the two eyes. Even small magnification differences and small angles of tilt are handled, without double vision.” 
         [0097]     There are situations in medical image viewing when the previous assumptions on the interocular distance  28 , fusion distance  12 , and distance R  7  are satisfied. Therefore, it has been mathematically demonstrated that, when building a medical imaging system for viewing 3D stereo images, it is feasible to use the approximations of this invention to yield suitable 3D stereo viewing performance. Namely, that the first eye perspective approximation line  37  can be used to approximate the first eye perspective view axis  17  and second eye perspective approximation line  38  can be used to approximate the second eye perspective view axis  18  and that the first eye perspective approximation line  37  and second eye perspective approximation line  38  intersect at the axis of rotation  3  instead of at the convergence point  26  and that the 3D model  42  of object  10  is rotated around the axis of rotation  3  in the direction of rotation  33 . This geometry is used to generate the sequence of volumetric rendered monocular views described in  FIG. 2  and  FIG. 4  and further explained in  FIG. 6 .  
         [0098]      FIG. 6  shows a schematic of the geometry of a system for creating a 3D volumetric monocular image view  45  of 3D model  42  of object  10  for display on a non-stereo viewing system as known in the prior art. For example, currently available medical imaging systems are capable of displaying volumetrically rendered 3D medical image data on standard 2D radiographic diagnostic monitors as is done by the Kodak CareStream Picture Archiving and Communication System (PACS). To enable comparison with the current invention, the fusion distance  12 , 3D model  42  of object  10 , convergence point  26 , axis of rotation  3 , direction of rotation  33 , viewer perspective line  20  and distance R  7  are labeled and defined as before.  
         [0099]     As described by Bourke, “binocular disparity is considered the dominant depth cue in most people.” Current systems creating 3D volumetric monocular image view  45  do not enable the viewer to perceive true stereo depth. These systems are incapable of creating binocular disparity since the identical 3D volumetric monocular image view  45  of 3D model  42  of object  10  seen by viewer first eye  1  is also simultaneously being as seen by viewer second eye  2 , usually on a 2D flat-panel LCD monitor. To create binocular disparity, 3D volumetric monocular image view  45  of 3D model  42  of object  10  seen by viewer first eye  1  must be different from the 3D volumetric monocular image view  45  seen by the viewer second eye  2 .  
         [0100]     Despite the inability to create binocular disparity, systems that create a single 3D volumetric monocular image view  45  at a time do generate other weaker human-perceivable depth cues in the image by using well-known artistic techniques also summarized by Bourke. Occlusion and relative motion are commonly used by current medical systems capable of rendering a 3D volumetric monocular image view  45  systems. These 3D model  42  of object  10  can be rotated until the axis along which it is desired to determine depth information is aligned with the plane of the 2D viewing device, i.e. the dimension the viewer wishes to see is displayed across the face of the 2D viewing device. Depth information is visualized as the viewer is looking perpendicular to the dimensions they wish to measure.  
         [0101]      FIG. 7  shows a schematic representation of the 3D volumetric monocular image view  45  system from  FIG. 6  superimposed with the key components of the current invention described in  FIG. 5 . A circle is used to represent the 3D model  42  of object  10 . As previously defined, 3D model  42  of object  10  is rotated around the axis of rotation  3  in the direction of rotation  33 . The axis of rotation  3  is shown perpendicular to the plane formed by the first eye perspective approximation line  37  and the second eye perspective approximation line  38  as previously defined in  FIG. 4 . Angle theta  36  is the angle between the first eye perspective approximation line  37  and the second eye perspective approximation line  38 . The first eye perspective approximation line  37  intersects the surface of the 3D model  42  of object  10  at the first eye view surface intersection point  30 . The second eye perspective approximation line  38  intersects the surface of the 3D model  42  of object  10  at the second eye view surface intersection point  31 .  
         [0102]     3D volumetric monocular image view  45  is defined from the viewer perspective reference  5  at fusion distance  12  from the convergence point  26  defined by the intersection of the viewer perspective line  20  and the surface of 3D model  42  of object  10 . Distance R  7  is the distance from the axis of rotation  3  to the convergence point  26  at the intersection of the viewer perspective line  20  and the surface of 3D model  42  of object  10 .  
         [0103]     Control the rotation speed of the 3D model  42  of object  10  in the direction of rotation  33  around axis of rotation  3  such that the angle swept out in a given time period it is equal to angle (theta/2)  35 . Further define a vector originating at the axis of rotation  3  and passing through first eye view surface intersection point  30  at initial time and rotating with the 3D model  42  of object  10 . At the end of the first time period, the vector is passing through convergence point  26 . At the end of the second time period, the vector is passing through second eye view surface intersection point  31 . Vectors extending from the axis of rotation  3  through a given point on the surface of the 3D model  42  of object  10  and moving in the direction of rotation  33  around axis of rotation  3 .  
         [0104]     To further explain the geometry of the invention described in  FIG. 7 , consider the analogy of a lighthouse. The lighthouse beacon originates at the center of the light tower and projects into the night. In the analogy, the viewer first eye  1 , viewer perspective reference  5  and viewer second eye  2  can be represented by three observation points along the gunwale of a ship traveling parallel to the lighthouse shoreline. As the beacon rotates, it&#39;s light will sequentially illuminate the observation positions on the ship corresponding to the viewer first eye  1 , viewer perspective reference  5  and viewer second eye  2 . The viewer first eye  1  will be illuminated when the lighthouse beacon direction corresponds to the first eye perspective approximation line  37 . The viewer perspective reference  5  will be illuminated when the lighthouse beacon direction corresponds to viewer perspective line  20 . The viewer second eye  2  will be illuminated when the lighthouse beacon direction corresponds to the second eye perspective approximation line  38 .  
         [0105]     Taking the lighthouse analogy further, assume the lighthouse has two beacons, a first beacon and a second beacon in the same plane with respect to each other and moving in the direction of rotation  33  around the axis of rotation  3 , separated from each other by angle theta  36 . As the dual lighthouse beacons rotate, there will exist an instant in time when the first beacon is passing through the first eye view surface intersection point  30  and illuminates the first observer representing the viewer first eye  1  while at the same instant, the second lighthouse beacon passed through the second eye view surface intersection point  31  and illuminates the second observer representing the viewer second eye  2 .  
         [0106]     Generalizing the previous lighthouse analogy, the lighthouse may have multiple beacons, with each beacon located at an angle theta  36  from its previous and subsequent beacon. This corresponds to a sequence of volumetric rendered monocular views  47  rendered by 3D graphics engine  14  of the 3D model  42  of object  10 , where each 3D volumetric monocular image view  45  is separated by angle theta  36  from its previous and subsequent 3D volumetric monocular image view  45  while the 3D model  42  of object  10  is rotated in the direction of rotation  33  around axis of rotation  3 .  
         [0107]      FIG. 8  describes the further invention of microstepping the rotation of 3D model  42  of object  10  at microstep increment angle  43 , such that microstep increment angle  43  is less than angle theta  36 , in the direction of rotation  33  around axis of rotation  3 . Microstepping creates a sequence of volumetric rendered monocular views  47  rendered by 3D graphics engine  14  of the 3D model  42  of object  10  such that a 3D volumetric monocular image view  45  is created for each microstep increment angle  43 . Since the microstep increment angle  43  is less than angle theta  36 , the sequence of volumetric rendered monocular views  47  rendered using the microstep increment angle  43  will contain more 3D volumetric monocular image view  45  for a complete revolution of the 3D model  42  of object  10  than the sequence of volumetric rendered monocular views  47  rendered using an angle theta  36  increment.  
         [0108]     Having more “in-between” 3D volumetric monocular image view  45  in the sequence of volumetric rendered monocular views  47  using microstep increment angle  43  enhances the perceived smoothness of 3D model  42  of object  10  rotation around the axis of rotation  3 . Using the microstep increment angle, each 3D volumetric monocular image view  45  represents a smaller change from the previous and subsequent 3D volumetric monocular image view  45  in the sequence of volumetric rendered monocular views  47 .  
         [0109]     In the system of this invention, using the microstep increment angle to control the rotation of the 3D model  42  of object  10  performs a function similar to an animated motion picture “in-betweener.” “In-betweeners” create additional animated motion picture frames between key animation frames drawn by more experienced master animators, improving the animated motion smoothness and perceived quality.  
         [0110]     When using the microstep increment angle to control the rotation of the 3D model  42  of object  10  around the axis of rotation  3 , an angle theta  36  must be maintained between the 3D volumetric monocular image view  45  representing the view of 3D model  42  of object  10  along the first eye perspective approximation line  37  and the 3D volumetric monocular image view  45  representing the view of 3D model  42  of object  10  along the second eye perspective approximation line  38  to provide natural stereo depth perception when viewing 3D model  42  of object  10  using 3D stereo viewer  4 .  
         [0111]     Selecting the microstep increment angle  43  such that it evenly divides into the angle theta  36  has the added benefit of allowing an exact number of “in-between frames” to be created between the “key frames.” This is not required by the current invention to operate, but may improve display results.  
         [0112]      FIG. 9  shows the addition to the present invention needed when it is desirable to reverse the direction of rotation  33  of the 3D model  42  of object  10  being viewed using the 3D stereo viewer  4 . In situations when only a portion of the 3D model  42  of object  10  contains the region of interest to be viewed, it is not efficient to continue to rotate the 3D model  42  of object  10  in complete (i.e. 360 degree) rotations in the current direction of rotation  33  around the axis of rotation  3 . There are several alternatives for the user to control the current invention in cases of limited desired viewing area. The user can stop rotation of the 3D model  42  of object  10 , as previously described, thus maintaining a still stereo image as viewed using the 3D stereo viewer  4 .  
         [0113]     Alternately, the user can limit the range of rotation in the direction of rotation  33  around axis of rotation  3  so that only the portion of 3D model  42  of object showing the region of interest is rotated into view. Once the rotation is complete, the 3D model  42  of object  10  is reset to the initial position and the rotation cycle is repeated.  
         [0114]     Another alternative enabled by the addition of graphics engine output switch  54  and rotation direction control  55  in  FIG. 9 . Graphics engine output switch  54  control the output of 3D graphics engine  14  to either: 
        drive the input to first eye frame buffer  13  directly with the delay frame buffer  44  and 3D model rotation calculator  16  working as previously described. The input to second eye frame buffer  53  is processed through the delay frame buffer  44  as shown in  FIG. 2 . (or)     reverse the direction of delay frame buffer  44 , using graphics engine output switch  54  to switch the output of 3D graphics engine  14  to drive the input to second eye frame buffer  53  directly as well as and the other side of the delay frame buffer  44  directly. Input to first eye frame buffer  13  will be delayed by the “reversed” delay frame buffer  44  as shown in  FIG. 9 .        
 
         [0117]     This approach has the benefit of having the 3D model  42  of object  10  appear to oscillate, rotating back and forth through the region of interest.  
         [0118]     The present invention will be directed in particular to elements forming part of, or in cooperation more directly with the apparatus in accordance with the present invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art.  
         [0119]     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention.  
       PARTS LIST  
       [0000]    
       
           1  viewer first eye  
           2  viewer second eye  
           3  axis of rotation  
           4  3D stereo viewer  
           5  viewer perspective reference  
           7  distance R  
           8  data storage  
           9  medical image data  
           10  3D object  
           11  scanner  
           12  fusion distance  
           13  first eye frame buffer  
           14  graphics engine  
           15  first eye perspective  
           16  3D model rotation calculator  
           17  first eye perspective view axis  
           18  second eye perspective view axis  
           20  viewer perspective line  
           21  first eye infinite-viewing-distance line  
           22  second eye infinite-viewing-distance line  
           23  eye perspective calculator  
           24  viewer perspective control device  
           25  viewer perspective  
           26  convergence point  
           27  angle alpha  
           28  interocular distance  
           30  first eye view surface intersection point  
           31  second eye view surface intersection point  
           32  horizontal parallax error  
           33  direction of rotation  
           35  angle (theta/2)  
           36  angle theta  
           37  first eye perspective approximation line  
           38  second eye perspective approximation line  
           39  angle (alpha/2)  
           40  3D modeling process  
           41  image segmentation  
           42  3D model  
           43  microstep increment angle  
           44  delay frame buffer  
           45  3D volumetric monocular image view  
           46  viewer head-eye model  
           47  sequence of volumetric rendered monocular views  
           48  first eyepiece  
           49  second eyepiece  
           53  second eye frame buffer  
           54  graphics engine output switch  
           55  rotation direction control