Note: Descriptions are shown in the official language in which they were submitted.
CA 02425075 2003-04-03
WO 02/29700 PCT/USO1/42506
INTRA-OPERATIVE IMAGE-GUIDED NEUROSURGERY WITH
AUGMENTED REALITY VISUALIZATION
Reference is hereby made to Provisional Patent Application No. 60/238,253
entitled INTRA-
OPERATIVE-MR GUIDED NEURO~URGERY WITH AUGMENTED REALITY
VISUALIZATIOIvT. filed October 10, 2000 in the names of Wendt et al.; and to
Provisional
Patent Application No. 60/279,931 entitled METI~OD AND APPARATUS FOR
AUGMENTED REALITY VISUALIZATION, filed March 29, 2001 in the name of Saner,
whereof the disclosures are hereby herein incorporated by reference.
The present invention relates to the field of image-Guided surgery, and more
particularly to
MR-guided neurosurgery wherein imaging scans, such as magnetic resonance (MR)
scans,
are taken intra-operatively or inter-operatively.
In the practice of neurosurgery, an operating surgeon is generally required to
look back and
forth between the patient and a monitor displaying patient anatomical
information for
guidance in the operation. In this manner, a for n of "mental mapping" occurs
of the image
information observed on the monitor and the brain.
Typically, in the case of surgery of a brain tumor, 3-dimensional (3D) volume
images taken
with MR (magnetic resonance) and CT (computed tomography) scamers are used for
diagnosis and for surgical plamzing.
After opening of the skull (craniotomy), the brain, being non-rigid in its
physical the
brain will typically further defoum. This brain shift makes the pre-operative
3D imaging data
fit the actual brain geometry less and less accurately so that it is
significantly out of
con-espondence with what is confronting the surgeon during the operation.
Flowever. there are tumors that look like and are textured like normal healthy
brain matter so
that they are visually indistinguishable. Such tumors can be distinguished
only by MR data
and reliable resection is generally only possible with MR data that are
updated during the
course of the surgery. The term "intra-opera ive" MR imaging usually refers to
MR scans
that are being taken while the actual surgery is ongoing, whereas the tern
"inter-operative"
MR imaging is used when the surgical procedure is halted for the acquisition
of the scan and
resumed afterwards.
CA 02425075 2003-04-03
WO 02/29700 PCT/USO1/42506
Equipment has been developed by various companies for providing intra/inter -
operative MR
imaging capabilities in the operating room. For example. General Electric has
built an MR
scanner with a double-doughnut-shaped magnet, where the surgeon has access to
the patient
inside the scanner.
U.S. Patent No. 5,740,802 entitled COMPUTER GRAPHIC AND LIVE VIDEO SYSTEM
FOR ENHANCING VISUALIZATION OF BODY STRUCTURES DURING SURGERY.
assigned to General Electric Company, issued April 21, 1998 in the names
ofNafis et al., is
directed to an interactive surgery planning and display system which mixes
live video of
extem~al surfaces of the patient with interactive computer generated models of
internal
anatomy obtained from medical diagnostic imaging data of the patient. The
computer images
and the live video are coordinated and displayed to a surgeon in real-time
during surgery
allowing the surgeon to view internal and extem~al structures and the relation
between them
simultaneously, and adjust his surgery accordingly. In an alternative
embodiment, a nomnah
anatomical model is also displayed as a guide in reconstructive surgery.
Another
embodiment employs tlu-ee-dimensional viewing.
Work relating to ultrasound imaging is disclosed by Andrei State, Mark A.
Livingston,
Gentaro Hirota, William F. Garrett, Mary C. Whitton, Henry Fuchs. and Etta D.
Pisano,
"Technologies for Augmented Reality Systems: realizing Ultrasound-Guided
Needle
Biopsies, "Proceed. of SIGGRAPH.(New Orleans, LA, August 4-9, 1996), in
Computer.
Graphics Proceedings, Amoral Conference Series 1996, ACM SIGGRAPH, 439-446.
For inter-operative imaging, Siemens has built a combination of MR scarcer and
operating
table where the operating table with the patient can be inserted into the
scanner for MR image
capture (imaging position) and be withdrawn into a position where the patient
is accessible to
the operating team, that is, into the operating position.
In the case of the Siemens equipment, the MR data are displayed on a computer
monitor. A
specialized neuroradiologist evaluates the images and discusses them with the
neurosurgeon.
The neurosurgeon has to understand the relevant image infomnation and mentally
map it onto
the patient's brain. While such equipment provides a useful modality, this
type of mental
mapping is difficult and subjective and carrot preserve the complete accuracy
of the
information.
An object of the present invention is to generate an augmented view of the
patient from the
surgeon's own dynamic viewpoint and display the view to the surgeon.
2
CA 02425075 2003-04-03
WO 02/29700 PCT/USO1/42506
The use of Augmented Reality visualization for medical applications has been
proposed as
early as 1992; see, for example, M. Bajura, H. Fucks. and R. Ohbuchi. "Merging
Virtual
Objects with the Real World: Seeing Ultrasound lmagery within the Patient."
Proceedings of
SIGGRAPH X32 (Chicago, IL, July 26-31, 1992). In Computer Graphics 26, #2
(July 1992):
203-210.
As herein used, the "augmented view" generally comprises the "real" view
overlaid with
additional "virtual' graphics. The real view is provided as video images. The
virtual graphics
is derived from a 3D volume imaging system. Hence. the virtual graphics also
corresponds to
real anatomical structures; however, views of these structures are available
only as computer
graphics renderings.
The real view of the external structures and the virtual view of the internal
structures are
blended with an appropriate degree of transparency, which may vary over the
field of view.
Registration between real and virtual views makes all structures in the
augmented view
appear in the correct location with respect to each other.
In accordance with an aspect of the invention, the MR data revealing inten~al
anatomic
-sta-uctu-r-es_aue_show-n-in=situ.-oueil.ai.d_on th.e_surgeon'-s ~i.e_w_of
tlae_patient._ With this
Augmented Reality type of visualization, the derived image of the inteu~al
anatomical
structure is directly presented in the surgeon's workspace in a registered
fashion.
In accordance with an aspect of the invention, the surgeon wears a head-
mounted display and
=cari=ex~aiiiin~ tliewspatial relationship between the anatomical structures
from varying positions
in a natural way.
n a~T' ccordance wifh an aspec~of~he mW o' n, tl~e~ is- practically elimmate~
fo'r the
surgeon to look back and forth vefweeW oonitor and patient, and to mentally
map the image
infom~ation to the real brain. As a consequence, the surgeon can better focus
on the surgical
task at hand and perform the operation more precisely and confidently.
The invention will be more fully understood from the following detailed
description of
preferred embodiments, in conjunction with the Drawings, in which
Figure 1 shows a system block diagram in accordance with the invention;
Figure 2 shows a flow diagram in accordance with the invention:
Figure 3 shows a headmounted display as may be used in an embodiment of the
invention;
Figure 4 shows a frame in accordance with the invention:
CA 02425075 2003-04-03
WO 02/29700 PCT/USO1/42506
Figure 5 show a boom-mounted see-tlu-ough display in accordance with the
invention;
Figure 6 shows a robotic amn in accordance with the invention:
Figure 7 shows a 3D camera calibration object as may be used in an embodiment
of the
invention; and
Figure 8 shows an MR calibration object as may be used in an embodiment of the
invention.
Ball-shaped MR markers and doughnut shaped MR marl<ers are shown
In accordance with the principles of the present invention, the MR infom~ation
is utilized in
an effective and optimal n Lamer. In an exemplary embodiment, the surgeon
wears a stereo
video-see-through head-mounted display. A pair of video can eras attached to
the head-
mounted display captures a stereoscopic view of the real scene. The video
images are
blended together with the computer images of the internal anatomical
structures and
displayed on the head-n counted stereo display in real time. To the surgeon,
the internal
structures appear directly superimposed on and in the patient's brain. The
surgeon is free to
move his or her head around to view the spatial relationship of the structures
from varying
positions, whereupon a computer provides the precise. objective 3D
registration between the
coiizpufer iniages-of the internal stouctures and the video in gages of the
real brain. This in situ
or "augmented reality" visualization gives the surgeon intuitively based,
direct, and precise
access to the image infom~ation in regard to the surgical task of removing the
patient's tumor
without Hurting vital regions.
In an alternate embodiment, the stereoscopic video-see-tlu-ough display may
not be head-
mounted but be attached to an articulated mechanical arm that is, e.g.,
suspended from the
ceiling (reference to "videoscope"~i~ovisioiial fili~2g)(include is2 claims).
For our purpose, a
video-see-through display is understood as a display with a video camera
attacluoent,
whereby the video camera looks into substantially the same direction as the
user who views
the display: A-stereoscopic video-see-through display combines a stereoscopic
display, e.g. a
pair of miniature displays, and a stereoscopic camera system, e.g. a pair of
cameras.
Figure 1 shows the building blocks of an exemplary system in accordance with
the invention.
A 3D imaging apparatus 2. in the present example an MR scanner, is used to
capture 3D
volume data of the patient. The volume data contain infounation about internal
structures of
the patient. .A video-see-through head-mounted display 4 gives the surgeon a
dynamic
viewpoint. It comprises a pair of video cameras 6 to capture a stereoscopic
view of the scene
4
CA 02425075 2003-04-03
WO 02/29700 PCT/USO1/42506
(external structures) and a pair of displays 8 to display the augmented view
in a stereoscopic
way.
A tracking device or apparatus 10 measures position and orientation (pose) of
the pair of
cameras with respect to the coordinate system in which the 3D data are
described.
The computer 12 comprises a set of networked computers. One of the computer
tasks is to
process., with possible user interaction. the volume data and provide one or
more graphical
representations of the imaged structures: volume representations andlor
surface
representations (based on segmentation of the volume data). In this context,
we understand
the teen graphical representation to mean a data set that is in a "graphical"
format (e.g.
VRML fomnat), ready to be eff ciently visualized respectively rendered into an
image. The
user can selectively enhance structures. color or annotate them, pick out
relevant ones;
include graphical objects as guides for the surgical procedure and so forth.
This pre-
processing can be done "off line", in preparation of the actual image
guidance.
Another computer task is to render, in real time. the augmented stereo view to
provide the
image guidance for the surgeon. For that purpose. the computer receives the
video images
-a~ad-the_cam.era_po-s-e-ialfonl~ation.-and._mal~-es-use_of tlae-pre=pr-
ocessed_3D. data, i.e. the stored.
graphical representation If the video images are not already in digital form,
the computer
digitizes them. Views of the 3D data are rendered according to the camera pose
and blended
with the-cowesponding-video images. The augmented images are then output to
the stereo
display._
An optional recording means 14 allows one to record the augmented view for
documentation
and. training.. The-recording-means can-be a digital storage device, or it can
be a video
--recorder, if necessary, combined with a scan convertor.
A general user interface 16 allows one to control the system in general, and
in particular to
interactively select the 3D data and pre-process them.
A realtime user interface l 8 allows the user to control the system during its
realtime
operation, i.e. during the realtime display of the augmented view. It allows
the user to
interactively change the augmented view, e.g. invoke an optical or digital
zoom, switch
between different degrees of transparency for the blending of real and virtual
graphics, show
or turn off different graphical structures. A possible hands-free embodiment
would be a
voice controlled user interface.
CA 02425075 2003-04-03
WO 02/29700 PCT/USO1/42506
An optional remote user interface 20 allows an additional user to see and
interact with the
augmented view during the system's realtime operation as described later in
this document.
For registration. a C011~117011 frame of reference is defned. that is. a
common coordinate
system, to be able io relate the 3D data and the 2D video images, with the
respective pose and
pre-determined internal parameters of the video cameras, to this common
coordinate system.
The common coordinate system is most canveniently one in regard to which the
patient's
head does not.move. The patient's head is fixed in a clamp during surgery and
intemnittent
3D imaging. Markers ugidly attached to this head clamp can serve as landmarks
to define
and locate the common coordinate system.
Figure 4 shows as an example a photo of a head clamp 4-2 with an attached
frame of markers
4-4. The individual markers are retro-reflective discs 4-6, made from 3M's
Scotchlite 8710
Silver Transfer Film. A preferred embodiment of the marker set is in form of a
bridge as seen
in the photo. See Figure 7.
The markers should be visible in the volume data or should have at least a
known geometric
relationship to other markers that are visible in the volume data. If
necessary, tlvs
i-elatioi~sliip can Ue detei-iiiii~e~ iiz aii i~iitial~ calibWatioii step.-
TheWthe~volume data can Me
measured with regard to the common coordinate system, or the volume data can
be
transformed into this common coordinate system.
The calibration procedures follow in more detail. For correct registration
between graphics
and=patient.-the system-needs to be-calibrated. O.ne needs to detenmine the
transformation that
maps the medical data onto the patient, and one needs to deteunine the inters
gal parameters
and=relative poses of~tl~e video cameias~slfow~the'ii~appiipg-cou-ectly in the
augmented
view. _ _ _ . . .
Camera-calibration and camera-patient transfom~ation. Fig. 7 shows a photo of
an
example of a calibration object that has been used for the calibration of a
camera triplet
consisting of a stereo pair of video cameras and an attached tracker camera.
The markers 7-2
are retro-reflective discs. The 3D coordinates of the markers were measured
with a
commercial Optotrak~ system. Then one can measure the 2D coordinates of the
markers in
the images, and calibrate the cameras based on 3D-2D point con-espondences for
example
-with-Tsai=-s-algoritlun as-described in Roger Y. Tsai,"A versatile Camera
Calibration
Technique for High-Accuracy 3D Machine Vision Metrology Using Off the-Shelf TV
Cameras and Lenses", IEEE Journal of Robotics and Automation, Vol. RA-3, No.
4, August
6
CA 02425075 2003-04-03
WO 02/29700 PCT/USO1/42506
1987, pages 323-344. For realtime tracking, one rigidly attaches a set of
markers with known
3D coordinates to the patient (respectively a head clamp) defining the patient
coordinate
system. For more detailed infonna ion, refer to F. Saner et al., "Augmented
Workspace:
Designing an AR Testbed," IEEE and ACM lnt. Symp. On Augmented Reality - ISAR
2000
(Munich. Germany. October 5-6, 2000), pages 47-53.
MR data - patient transformation for the example of the Siemens inter-
operative MR
imaging an-an~ement. The patient's bed can be placed in the magnet's fringe
field for the
surgical procedure or swiveled into the; n Magnet for MR scarring. The bed
with the head
clamp, and therefore also the patient's head, are reproducibly positioned in
the magnet with a
specified accuracy of ~lnmn. One can pre-deternine the transformation between
the MR
volume set and the head clamp with a phantom and then re-apply the sane
transformation
when mapping the MR data to the patient's head, with the head-clamp still in
the same
position.
Fig. 8 shows au example for a phantom that can be used for pre-detenniung the
transformation. It consists of two sets of narkers visible in the MR data set
and a set of
optical markers visible to the tracker camera. Oa~e type of MR markers is ball-
shaped 8-2 and
can, e.g.' be obtained from Brainlab, Inc. The other type of MR marlcers 8-4
is doughnut-
shaped, e.g. Multi-Modality Radiographics Markers from IZI Medical Products,
Inc. In
principle, only a single set of at least three MR markers is necessary. The
disc-shaped retro-
reflective optical markers 8-6 can be punched out from 3M's Scotchlite 8710
Silver Transfer
Fihn. Oi~e-tracks-the optical markers, and ~ with the lcnowledge of the
phantom's geometry -
detennines the 3D locations of the MR markers in the patient coordinate
system. One also
determines the 3D locations of the MR markers in the MR data set, and
calculates the.
transfon nation between the two coordinate systems based on the 3D-3D point
correspondences.
The pose position and orientation) of the video cameras is then measuoed in
reference to the
common coordinate system. This is the task of the tracking means. In a
preferred
implementation, optical tracking is used due to its superior accuracy. A
prefen-ed
implementation of optical tracking comprises rigidly attaching an additional
video camera to
the stereo pair of video cameras that provide the stereo view of the scene.
This tracker video
-camera poin s iris substantially the sane direction as the ofher~wo video
can9eras. When the
surgeon looks at the patient. the tracker video camera can see the
aforementioned markers
that locate the common coordinate system, and from the 2D locations of the
markers in the
7
CA 02425075 2003-04-03
WO 02/29700 PCT/USO1/42506
tracker camera's image one can calculate the tracker camera's pose. As the
video cameras
are rigidly attached to each other, the poses of the other two cameras can be
calculated from
the tracker camera's pose, the relative camera poses having been determined in
a prior
calibration step. Such camera calibration is preferably based on 3D-2D point
correspondences and is described, for example, in Roger Y. Tsai, "A versatile
Can sera
Calibration Technique for High-Accuracy 3D Machine Vision Metrology Using Off
the-
Shelf TV Cameras and Lenses", IEEE .Tournal of Robotics and Automation, Vol.
RA-3, No.
4, August 198 7, pages 323-344.
Figure 2 shows a flow diagram of the system when it operates in real-time
mode, i.e. when it
is displaying the augmented view in real time. The computing means 2-2
receives input from
tracking systems; which are here separated into tracker camera (understood to
be a head-
mounted tracker camera) 2-4 and external tracking systems 2-6. The computing
means
perform pose calculations 2-8, based on this input and prior calibration data.
The computing
means also receives as input the real-time video of the scene cameras 2-l 0
and has available
the stored data for the 3D graphics 2-12. In its graphics subsystem 2-14, the
computing
means renders graphics and video into a composite augmented view, according to
the pose
information. Via the user interface 2-16, the user can select between
different augmentation
modes (e.g. the user can vary the transparency of the virtual structures or
select a digital
zoom for the rendering process). The display 2-18 displays the rendered
augmented view to
the user.
To allow for a comfortable and relaxed posture of the surgeon during the use
of the system.
the two video cameras that provide the stereo view of the scene point downward
at an angle,
whereby the surgeon can work on the patient without having to bend the head
down into an
uncomfortable position. See the pending patent application Ser. No. entitled
AUGMENTED REALITY VISUALIZATION DEV1CE, filed September 17, 2001, Express
Mail Label No. EL727968622US, in the names of Saner and Bani-Ha hemi, Attorney
Docket
No. 2001 P 14757US.
Figure 3 shows a photo of a stereoscopic video-see-tlv-ough head-mounted
display. It
includes the stereoscopic display 3-2 and a pair of downward tilted video
cameras 3-4 for
capturing the scene (scene cameras). Furthermore, it includes a tracker camera
3-6 and an
infrared illuminator in form of a ring of infrared LEDs 3-8.
8
CA 02425075 2003-04-03
WO 02/29700 PCT/USO1/42506
In another embodiment, the augmented view is recorded for documentation and/or
for
subseguent use in applications such as trammg.
It is contemplated that the augmented view can be provided for pre-operative
plamiing for
surgery.
In another embodiment, interactive annotation of the augmented view is
provided to permit
communication between a user of the head-mounted display and an observer or
associate who
watches the augmented view on a monitor, stereo monitor, or another head-
mounted display
so that the,a.ugmented view provided to the surgeon can be shared; for
example, it can
observed by neuroradiologist. The neuroradiologist can then point out, such as
by way of an
interface to the computer (mouse, 3D mouse, Trackball. etc.) certain features
to the surgeon
by adding extra graphics to the augmented view or highlighting existing
graphics that is being
displayed as part of the augmented view.
Figure 5 shoves a diagram of a boom-mounted video-see-tlv-ough display. The
video-see-
tlu-ough display comprises a display and a video camera, respectively a stereo
display and a
stereo pair of video cameras. In the example. the video-see-tlu-ough display
52 is suspended
from a-ceiling 50 by a b00111 54. For tracking. tracking means 56 are attached
to the video-
see-through display, more specifically to the video cameras as it is their
pose that needs to be
determined for rendering a col7~ectly registered augmented view. Tracking
means can include
a tracking camera=that-works in-conjunction w,=itlu active or passive optical
markers that are
placed in the scene. Altel~l~atively, traelcing means can include passive or
active optical
markers that work in conjunction with an exten~al tracker camera. Also,
different kind of
tracking systems can be employed such as magnetic tracking, inertial tracking,
ultrasonic
-tracking, etc. Mechanical tracking is possible by fitting the joints of the
boom with encoders.
However, optical tracking is prefen-ed because of its accuracy.
Figure 6 shows elements of a system that employs a robotic arm 62, attached to
a ceiling 60.
The system includes a video camera respectively a stereo pair of video cameras
64. On a
remote display and control station 66, the user sees an augmented 'video and
controls the
robot. The robot includes tools, e.g. a drill, that the user can position and
activate remotely.
Tracking means 68 enable the system to render an accurately augmented video
view and to
position the instruments colTectly. Embodiments of the tracking means are the
same as in the
description of Figure 5.
9
CA 02425075 2003-04-03
WO 02/29700 PCT/USO1/42506
In an embodiment exhibiting remote use capability, a robot carries scene
cameras. The
tracking camera may then no longer be required as robot arts can be
mechanically tracked.
However, in order to establish the relationship between the robot and patient
coordinate
systems, the tracking camera can still be useful.
The user, sited in a ren rote location. can move the robot "head" around by
remote control to
gain appropriate views, Ioolc at the augmented views on a head-mounted display
or other
stereo viewing display or external monitor, preferably in stereo. to diagnose
and consult. The
remote user may also be able to perform actual surgery via remote control of
the robot, with
or without help of personnel present at the patient site.
In another embodiment in accordance with the invention, a video-see-tlu-ough
head-mounted
display has downward looking scene camera/cameras. The scene cameras are video
cameras
that provide a view of the scene, mono or stereo, allowing a comfortable work
position. The
downward angle of the camera /cameras is such that - in the preferred work
posture - the head
does not have to be tilted up or dawn to any substantial degree.
In another embodiment in accordance with the invention, a video-see-through
display
_comprises_an.inte.grated tracker camera whereby the tracker camera is forward
looking or is
looking into substantially the same direction as the scene cameras, traclcing
landmarks that
are positioned on or around the object of interest. The tracker camera can
have a larger field
of view than the scene cameras, and can work in limited wavelength range (for
example, the
infrared wavelength range). See the afore-mentioned pending patent application
Ser. No.
entitled AUGMENTED REALITY VISUALIZATION DEVICE, filed
September 17, 2001, Express Mail Label No. EL727968622US, in the names of
Sauer and
BaW =I-Iashemi, -Attorney Docket No.- 2001P14757US, hereby incorporated herein
by
reference.
In accordance with another embodiment of the invention wherein retroreflective
markers are
used, a light source for illumination is placed close to or around the tracker
camera lens. The
wavelength of the light source is adapted to the wavelength range for which
the tracker
camera is sensitive. Alternatively, active markers, for example small
lightsources such as
LEDs can be utilized as marl<ers.
Tracking systems with large cameras that work with retroreflective markers or
active markers
are commercially available.
CA 02425075 2003-04-03
WO 02/29700 PCT/USO1/42506
In accordance with another embodiment of the invention, a video-see-through
display
includes a digital zoom feature. The user can zoom in to see a magnified
augmented view;
interacting with the computer by voice or other interface. or telling an
assistant to interact
with the computer via keyboard or mouse or other interface.
It will be apparent that the present inventions provide certain useful
characteristics and
features in comparison with prior systems. For example, in reference to the
system disclosed
in the more-mentioned U.S. Patent No. 5.740.802. video cameras are attached to
head-
mounted display in accordance with the present invention, thereby exhibiting a
dynamic
viewpoint, in contrast with prior systems which provide a viewpoint,
implicitly static or
quasi-static. «~lllCll 1S Otlly "substa.ntially" the same as the surgeon's
viewpoint.
In contrast with a system which merely displays a live video of external
surfaces of a patient
and an augmented view to allow a surgeon to locate internal structures
relative to visible
external surfaces, the present invention malces it unnecessary for the surgeon
to look at an
augmented view, then determine the relative positions of external and internal
structures and
thereafter orient himself based on the external structures. drawil~g upon his
memory of the
relative position of the internal structures.
The use of a "video-see-through" head mounted display in accordance with the
present
invention provides an augmented view in a more direct and intuitive way
without the need for
-the-user to look vaclc-and~forth between-monitor and patient. This~also
results in better spatial
perceptionbecau-se_of_kinetic (parallax) depth cues and_there is no need for
the physician to
orient himself with respect to surface landmarks. since he is directly guided
by the augmented
view.
In such a prior art system mixing is performed in the video domain wherein the
graphics is
convened into video format and then mixed with the live video such that the
mixer
an -angement creates a composite image with a movable window which is in a
region in the
composite image that shows predominantly the video image or the computer
image. In
contrast, an embodiment in accordance with the present invention does not
require a movable
window; however, such a movable window may be helpful in certain kinds of
augmented
views. In accordance with a principle of the present invention, a composite
image is created
in the computer graphics domain whereby the live video is converted into a
digital
representation in the computer and therein blended together with the graphics.
11
CA 02425075 2003-04-03
WO 02/29700 PCT/USO1/42506
Furthemuore, in such a prior art system, internal structures are segmented and
visualized as
surface models; in accordance with the present invention, 3D images can be
shown in surface
or in volume representations.
The present invention has been described by way of exemplary embodiments. It
will be
understood by one of sleill in the art to which it pertains that various
changes, substitutions
and the like n gay be made without departing from the spirit of the invention.
Such changes
are contemplated to be within the scope of the clams following.
12
