EP1259940A2

EP1259940A2 - Method and system for selecting and displaying a portion of an image of a body

Info

Publication number: EP1259940A2
Application number: EP01908094A
Authority: EP
Inventors: Eyal Zadicario; Roni Yagel; David Freundlich; Yoav Modan
Original assignee: Insightec-Image Guided Treatment Ltd
Current assignee: Insightec-Image Guided Treatment Ltd
Priority date: 2000-02-29
Filing date: 2001-02-27
Publication date: 2002-11-27
Also published as: WO2001065490A2; WO2001065490A3; AU2001235951A1

Abstract

A method and system for displaying an image volume of a body such as the body of a surgical patient. The system of the present invention includes a memory for storing the image volume, a monitor for displaying the image volume, a pointer for selecting portions of the image volume to display with reference to the body, a mechanism for determining the position and orientation of the pointer, and a processor for overall control. The image volume is registered with the body. A portion, such as a slice, of the image volume is defined by a viewpoint that is in turn defined by positioning and orienting the pointer relative to the body. For example, the viewpoint may be defined by a target point on the body and by the orientation of the pointer. The tip of the pointer is pointed at the target point, or is placed in contact with the target point, and the pointer is pivoted. A slice of the image volume is displayed in accordance with this viewpoint. Registration of the image volume with the body is facilitated by using the pointer to sample the coordinates of points on the surface of the body. Alternatively, a suitably equipped pointer is used to sample the coordinates of points within the body, for example by ultrasound echolocation. Preferably, the pointer also is used for overall processor control.

Description

METHOD AND SYSTEM FOR SELECTING AND DISPLAYING A PORTION OF AN IMAGE OF A BODY

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to interactive displays and, more particularly, to a

method and system for displaying a portion of a previously obtained image volume of

the interior of a body with reference to the body itself. Following the discovery of X-rays at the end of the Nineteenth Century, images of the interiors of patients have been used by physicians as diagnostic tools. In particular, such images have been used to guide surgical procedures. Before the development of digital imaging modalities such as CAT scans and MRI scans, surgeons typically brought x-ray photographs of patients into the operating room, hung the photographs on light boxes and used the photographs as guides in surgery. More recently, it has become possible to acquire 3D digital image volumes of patients. Slices of these image volumes commonly are used to guide surgical procedures, but in a manner derived from the old days of X-ray film: many successive slices of the image volume, taken in a predetermined orientation relative to the image volume, are posted on a chart in the operating room, and the surgeon performs surgery on the patient with

reference to these thumbnail images.

It is known to navigate a medical instrument, such as a catheter, an endoscope

or a biopsy needle, through the body of a patient, with reference to a 3D digital image

volume. For example. Acker et al., in US 5,558,091, attach a fiducial marker to the

body of a patient and acquire a 3D digital image volume of a portion of the patient

together with the fiducial marker. The digital image volume is registered to the

patient with reference to the fiducial marker. The medical instrument is provided with a magnetic field sensor that is used to measure the position and orientation of the

medical instrument as the medical instrument is navigated through the body of the

patient. The fiducial marker includes a similar magnetic field sensor so that an icon representing the medical instrument can be displayed, superposed on a display of the

3D digital image, to represent the true position and orientation of the medical

instrument within the body of the patient. In particular, a slice of the 3D digital image may be extracted for display from the 3D digital image in any convenient (vertical, horizontal, oblique) orientation relative to the (typically parallelipipedal) 3D data volume.

In many surgical procedures, notably orthopedic procedures such as total hip replacement and total knee replacement, in which the surgeon must cut through the patient's bones at a precise angle, it would be useful to have a similarly flexible method of displaying slices of 3D digital image volumes, rather than forcing the surgeon to visualize the surgical target in three dimensions based on slices of the digital image volume taken in a predetermined, fixed orientation. There is thus a widely recognized need for, and it would be highly advantageous to have, a method and system for interactively selecting and displaying slices of a 3D digital image volume of the body of a patient with reference to the patient's body itself.

SUMMARY OF THE INVENTION

According to the present invention there is provided a method of displaying an

image volume of a body having a surface, including the steps of: (a) registering the

image volume with the body; (b) selecting a viewpoint with reference to the body; and

(c) displaying a first portion of the image volume with reference to the viewpoint. According to the present invention there is provided a system for selecting a

portion of an image volume of a body and for displaying the selected portion,

including: (a) a pointer for selecting a viewpoint with reference to the body; (b) a

memory for storing the image volume; (c) a processor for retrieving the portion of the image volume from the memory, with reference to the viewpoint; and (d) a monitor

for displaying the retrieved portion.

According to the present invention there is provided a six-degree-of-freedom

input device, including: (a) a handle; (b) a stylus, operationally connected to the handle; and (c) a mechanism for providing signals representative of a location and an orientation of an element of the device selected from the group consisting of the

handle and the stylus.

The primary application of the present invention is to the interactive display of slices of a 3D image volume of the body of a surgical patient, and the present invention is described herein with reference to this primary application. Nevertheless, it is to be understood that the scope of the present invention includes the interactive display of any body, animate or inanimate, for which a 3D image volume has been acquired.

As defined herein, a "viewpoint" is an abstract geometric entity, in the space occupied by and surrounding the body, that defines the portion of the image volume

that is to be displayed. The use in this manner of the term "viewpoint" is patterned after the use of this term to describe the relationship of a camera to an object that the

camera is photographing. In the case of a camera, the viewpoint may include as many

as seven coordinates: the three spatial coordinates of the camera relative to the object;

pitch, yaw and roll coordinates of the camera; and a magnification coordinate. The "viewpoints" of the present invention generally include a target point that may be

inside the body, on the surface of the body or outside the body. The "viewpoints" of

the present invention may also include orientations with respect to the body and planes

that intersect the body.

The method of the present invention begins by registering the image volume

with the body of the patient. According to one preferred embodiment of the present invention, four or more fiducial points are matched to corresponding reference points in the image volume. According to another preferred embodiment of the present

invention, coordinates of a feature of the body are sampled and matched to a representation of that feature in the image volume. The feature may be the surface of the body, or alternatively a feature internal to the body. In the case of the internal feature being the surface of a skeletal bone, the coordinates of the surface of the skeletal bone are sampled using an echolocation procedure such as ultrasound.

The image volume having been registered to the body of the patient, a viewpoint, for defining the portion of the image volume to be displayed, is selected. The process of viewpoint selection includes two phases. In the first phase of viewpoint selection, the viewpoint style is selected. The term "viewpoint style" is used herein to refer to the qualitative aspects of the viewpoint. These qualitative aspects include, but are not necessarily limited to, the type of coordinates included in

the viewpoint. In the second phase of viewpoint selection, the quantitative aspects of

the viewpoint, including specific values of the coordinates, are selected.

One preferred viewpoint of the present invention includes a target point on the

body surface adjacent to the portion of the body that is included in the image volume,

and a display orientation. One or more slices of the image volume are displayed with reference to the target point and the display orientation. Preferably, the selections of

the target point and of the display orientation are effected using a hand-held pointer

that includes a handle to which is rigidly attached a stylus. Most preferably, the stylus is perpendicular to the handle. The tip of the stylus is pointed at the target point and

the handle is pivoted to indicate the display orientation. Alternatively, the tip of the

stylus is placed in contact with the target point and the stylus is pivoted about the

target point, using the handle, to indicate the display orientation. This hand-held pointer constitutes an invention in its own right: a six-degree-of freedom input/control device. The image volume is stored in a memory, and slices of the image volume are retrieved from the memory and displayed on a monitor, under the control of a processor. The pointer includes a mechanism for providing signals representative, both of the location of the tip of the stylus, and of the spatial orientation of the pointer itself. These signals are sent to the processor, which retrieves and displays the slices of the image volume in accordance with these signals. A preferred electromagnetic

embodiment of this mechanism includes coils rigidly mounted in the handle of the pointer. A preferred optical embodiment of this mechanism includes light emitting

diodes rigidly mounted on the handle of the pointer.

The display orientation of the slices of the image volume usually coincides

with the spatial orientation of the stylus, with the planes of the slices being

perpendicular to the long axis of the stylus; but this is not obligatory. To allow for the pointer of the present invention having a different spatial orientation than the display

orientation, the spatial orientation of the pointer is referred to herein as the "pivot

orientation". Successively deeper or shallower slices of the image volume, relative to the direction in which the pointer points, are displayed on the monitor at a fixed

display orientation. Alternatively, a single depth is selected, the pointer is pivoted

about the target point, and successive slices at that depth but at different orientations

are displayed on the monitor.

Further preferred embodiments of the present invention include other

viewpoint styles, also defined with reference to the body using the hand-held pointer.

According to one preferred embodiment of the present invention, the pointer functions as a virtual camera, with the tip of the stylus defining the three spatial coordinates of the virtual camera and with the pivot orientation of the pointer defining the pitch and

yaw coordinates. According to a second preferred embodiment of the present invention, the stylus defines an axis that runs through the tip of the stylus, and pointing the stylus at the body defines, as a viewpoint, a point along the axis within the body. The display consists of three mutually perpendicular planes of the image volume whose common intersection point is the viewpoint. According to a third preferred embodiment of the present invention, both the stylus and the handle define respective axes. The display again consists of three mutually perpendicular slices of the image volume: a first slice in a plane that includes the stylus axis and that is perpendicular to the body axis, a second slice in a plane parallel to the plane defined

by the two axes, and a third slice that is perpendicular to the first two slices.

Most preferably, the pointer functions in two modes, a control mode and a viewpoint definition mode. In control mode, the pointer is used to control the

processor with reference to a cursor displayed on the monitor, much in the manner of

a conventional computer mouse. For this purpose, the pointer sends control signals to

the processor. These control signals include, inter alia, signals that define the desired viewpoint style. In viewpoint definition mode, signals indicative of the spatial

position and orientation of the pointer are transmitted to the processor, as discussed

above in the context of the first preferred viewpoint style, to complete the selection of

the desired viewpoint. In both modes, the signals are sent from the pointer to the

processor via a wireless datalink.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, with reference to

the accompanying drawings, wherein: FIG. 1 is an illustration of a system of the present invention;

FIG. 2 illustrates the selection of slices of the image volume for display;

FIG. 3 shows two preferred embodiments of the pointer of FIG. 1;

FIGS 4 and 5 illustrate two other methods of selecting slices of the image

volume for display.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is of a method and system for interactively displaying

portions of an image volume of a body. Specifically, the present invention can be used to display slices of an image volume of the body of a patient immediately prior to

surgery or during surgery.

The principles and operation of interactive image display according to the

present invention may be better understood with reference to the drawings and the

accompanying description. Referring now to the drawings, Figure 1 illustrates a system of the present

invention, used to select and display slices of a 3D image volume 14 stored in a

memory 18. Image volume 14 is a three dimensional parallelipipedal array of voxels

corresponding to a portion of a body 10 of a surgical patient. The volume of space

imaged in image volume 14 is shown as a dashed box 14' in Figure 1. Note that part

of box 14' extends past body 10, so that part of the surface 12 of body 10 is included

in the imaged volume. A processor 16 retrieves selected slices of image volume 14

from memory 18 and displays these slices on a monitor 20.

The slices to be displayed are selected using a pointer 26. One end of pointer

26 is a pointed tip 28 which is pointed at a target point 30 on surface 12 of body 10, or

which, alternatively, is placed in contact with target point 30. Figure 2 is an enlargement of a portion of Figure 1 that shows how pointer 26 is used to select slices

of image volume 14 for display. In Figures 2A and 2B, pointer 26 is shown with tip

28 in contact with target point 30. In Figure 2C, pointer 26 is shown pointing at target point 30. Pointer 26 defines an axis 34 that intersects tip 28. Pointer 26 is pivoted

about tip 28 to define the orientation of the slices of image volume 14 that are

displayed on monitor 20. Specifically, the slices that are displayed on monitor 20 are

perpendicular to axis 34. Three such slices, 40a, 40b and 40c, are indicated in Figures

2A and 2C by dashed lines. As these slices generally are not parallel to the planes of

the voxel array that constitute image volume 14, processor 16 computes the pixels of

slices 40 by interpolating the voxels of image volume 14. The depth, along the

extension of axis 34 into body 10, of the slice that is displayed, is controlled by two

buttons 36 and 38: pushing button 36 moves the display depth to a deeper slice and

pushing button 38 moves the display depth to a shallower slice. Alternatively, pivoting pointer 26 about target point 30, as shown in Figure 2B, changes the

orientation (the tilt) of the displayed slice at constant depth. In Figure 2B, when

pointer 26 is oriented at angle α_d from vertical, slice 40d is displayed, and when

pointer 26 is at angle α_e from vertical, slice 40e is displayed. As discussed below,

signals representative of the spatial orientation of pointer 26, and so of the orientations

of slices 40, as well as signals generated using buttons 36 and 38, are relayed to

processor 16 by a datalink that is represented symbolically in Figure 1 by an arrow 32.

Most preferably, datalink 32 is a wireless datalink.

The lateral positioning of the displayed slice on the screen of monitor 20 is

determined by target point 30. Specifically, the point on the slice, at which the

extension of axis 34 into body 10 intersects the slice, is positioned in the center of the screen. Rotating pointer 26 about axis 34 rotates the displayed slice about the center point on the screen.

It should be noted that the selection, using pointer 26, of target point 30 and of the display orientation of the slice that is displayed on the screen of monitor 20 are concurrent, and are described separately above only for clarity of exposition.

Processor 16 immediately alters both the lateral positioning and display of the slice

displayed on the screen of monitor 20 in response to the positioning and orientation of

pointer 26, as pointer 26 is translated and pivoted.

As noted above, the orientation of pointer 26 is referred to herein as the "pivot orientation". A suitable mechanism is provided to indicate the pivot orientation of

pointer 26 to processor 16. One such mechanism, an electromagnetic mechanism

similar to the mechanism taught by Ben-Haim et al. in WO 96/05768, which

publication is incorporated herein by reference for all purposes as if fully set forth herein, is partially depicted in Figure 1. A transmitter 22 drives AC currents in three

transmitting antennas 24 to generate low-frequency electromagnetic fields. These

fields induce the flow of corresponding AC currents in three mutually perpendicular

sensing coils (not shown) in pointer 26. Signals representative of these currents are

relayed to processor 16 by datalink 32. Processor 16 infers, from these signals, the

location of tip 28 (hence, the location of target point 30) and the pivot orientation of

pointer 26. Preferably, the location of tip 28 is expressed in Cartesian coordinates and

the pivot orientation of pointer 26 is expressed as Euler angles, in a suitable

coordinate frame. In the present context, the Euler angles are considered to be examples of "coordinates" of the pivot orientation of pointer 26.

Figures 3A and 3B are side and top views, respectively, of a preferred

embodiment of pointer 26. This embodiment of pointer 26 includes a lozenge-shaped handle 50 that defines a handle axis 35 and from which emerges a stylus 48 that

includes tip 28 and that defines axis 34. To distinguish axis 34 from axis 35 in the

context of this embodiment of pointer 26, axis 34 is referred to herein as the "stylus

axis". Stylus 48 is rigidly attached to handle 50, with stylus axis 34 substantially

perpendicular to handle axis 35. Handle 50 includes buttons 36 and 38 as described

above, and three other manual input interfaces: a trackball 46 and two more buttons 42

and 44. Buttons 42 and 44 are used to toggle pointer 26 between viewpoint definition

mode, in which the position and orientation of pointer 26, along with buttons 36 and

38, define the viewpoint, and control mode, in which trackball 46 and buttons 36 and

38 are used to control the display on monitor 20 in the manner of a conventional

mouse. (It should be noted that, as described below, there are alternative preferred

viewpoint styles in which trackball 46 participates in the definition of the viewpoint.) In control mode, trackball 46 is used to move a cursor on the screen of monitor

20 to point at graphical user interface tools such as icons, sliders and menu selections

on the screen of monitor 20, and buttons 42 and 44 are used to select and/or activate

these graphical user interface tools, for the purpose of controlling processor 16 and

modifying the appearance of the displayed portion of image volume 14. For example,

the magnification of the displayed slices may be adjusted using a slider, as may the default 45-degree orientations of the three slices of the "3D textured plane" display.

Appropriate circuitry is included in handle 50 to sense the forces applied to buttons

36, 38, 42 and 44 and to trackball 46 and to relay signals representative of these forces

to processor 16 via datalink 32. When pointer 26 is toggled from viewpoint definition mode to control mode, the viewpoint coordinates of the displayed portion of image volume 14 are frozen until pointer 26 is toggled back to viewpoint definition mode.

As an alternative to trackball 46, a force sensor such as the TrackPoint™ sensor used in IBM laptop computers may be used. Such a force sensor is sensitive to

forces in the ±x (right-left), ±y (forward-back) and -z (down) directions, and so

replaces both trackball 46 and one of buttons 36 and 38. The remaining button 36 or

38 is used to toggle the force sensor between interpreting a force in the z-direction as a

-z force and interpreting a force in the z-direction as a +z force.

As a further subalternative, the force sensor is mounted on a rotary dialer knob such as the dialer knob used in the ABA2020 VoicePod™ record/playback device

produced by Altec Lansing of Milford PA. Twisting the force sensor clockwise

represents a +z force. Twisting the force sensor counterclockwise represents a -z

force. Under this subalternative, buttons 36 and 38 are not needed. In Figure 3B, handle 50 is partially cut away to show three mutually

perpendicular sensor coils 52 that are used to sense the fields generated by antennas

24. Coils 52 are rigidly attached to handle 50, so that a simple rigid translation and

rigid rotation of the Cartesian coordinates and the Euler angles defined by coils 52

transforms these coordinates and angles to the Cartesian coordinates of tip 28 and the

Euler angles of stylus 48. Alternatively, a sufficiently miniaturized magnetic field

sensor is embedded inside stylus 48 or is rigidly attached to stylus 48, near tip 28.

The location of sensor coils 52 in handle 50 is illustrative, rather than

limitative. Appropriate field sensors may be positioned anywhere on or in pointer 26. For example, the NOG A™ Cardiac Mechanics and Hemodynamic Mapping System, produced by Biosense Webster (a subsidiary of Johnson and Johnson of New Brunswick NJ), includes such a sensor, mounted within a catheter, that is small

enough to be mounted similarly in stylus 48 near tip 28.

Figure 3C is a front view of an alternative preferred embodiment of pointer 26.

Instead of coils 52, this embodiment of pointer 26 includes four infrared light emitting

diodes 54, for use in conjunction with an optical tracking system, such as the Optotrak system available from Northern Digital Inc. of Toronto, Ontario, Canada.

Several methods may be used to register image volume 14 to body 10. In one

registration method, a number of fiducial points on surface 12 are matched to

corresponding reference points in image volume 14. For this purpose, tip 28 is placed in contact with the fiducial points and the coordinates of the fiducial points are

measured. At least four such fiducial points are needed, with the accuracy of the

match being improved by using more than four fiducial points. Anatomical features

of surface 12 may be used as fiducial points. Alternatively, small blocks of a material that has a high contrast in the imaging modality used to create image volume 14 are

attached to surface 12 prior to the acquisition of image volume 14. For example, if

image volume 14 is a CAT scan, then the material of the blocks is radio-opaque.

These blocks then serve as the fiducial points, and their representations in image

volume 14 serve as the reference points.

A second registration method is based on fitting a mathematical surface to a

portion of a representation, in image volume 14, of a feature of body 10. The most

convenient feature of body 10 to use for this purpose is surface 12. A set of points,

typically between 40 and 60 points, on surface 12 are digitized by touching tip 28 to

these points and measuring the coordinates of these points. Then image volume 14 is

registered to body 10 by applying to image volume 14 the rigid translation and the rigid rotation that leads to the best fit, in a least squares sense, of the mathematical

surface to the digitized points. Because the points on surface 12 that are to be sampled for this purpose are not known a priori, the portion of the representation of

surface 12 in image volume 14 to which the mathematical surface is fitted generally is

more extensive than the portion of surface 12 that is actually sampled by tip 28.

Some parts of body 10, such as a leg, are too symmetrical for the second

registration method to obtain a unique match of the sampled points of surface 12 to the mathematical surface. In such a case, an interior feature such as the surface of a

skeletal bone is used for registration. The mathematical surface is fitted to a portion

of the representation of the surface of the bone in image volume 14. Coordinates of

points on the surface of the real bone are sampled by an echolocation method such as

ultrasound. Preferably, for this purpose, and as illustrated in Figure 3B, distal end 58

of stylus 48 includes a piezoelectric sensor 56 that serves as both an ultrasound transmitter and an ultrasound receiver. Sensor 56 is connected to appropriate

ultrasound transmission and reception circuitry (not shown) in handle 50 in the

conventional manner. As before, image volume 14 is registered to body 10 by

applying to image volume 14 the rigid translation and the rigid rotation that leads to

the best fit, in a least squares sense, of the mathematical surface to the sampled points.

As noted above, the viewpoint style described above that is based on surface

target point 30 is only one of many possible viewpoint styles, each of which defines a different display mode. A second viewpoint style is defined by using the preferred

embodiment of pointer 26, as illustrated in Figure 3, as a virtual camera. Stylus 48 is

pointed at body 10, and the displayed image is provided in accordance with what

would be photographed by a camera pointed similarly at body 10. The spatial

coordinates of the virtual camera are the spatial coordinates of tip 28. Pitch is defined as rotation in the planes of axes 34 and 35. Yaw is defined as rotation about body axis

35. Roll is defined as rotation about stylus axis 34. In one preferred display mode, surfaces of internal organs of patient 10, as represented in image volume 14, are contoured and shaded by methods that are well-known in the art, and are displayed as a three-dimensional rendition of these organs as these organs would be photographed

by a camera having a position and orientation similar to that of pointer 26. In another

preferred display mode, used to display a CT image volume 14, a projection of image

volume 14 parallel to stylus axis 35 is displayed, to simulate the output of an x-ray

camera having a position and orientation similar to that of pointer 26.

A third viewpoint style is defined as illustrated in Figure 4. Here, a target

point 30' is defined to be at a distance d from tip 28 along stylus axis 34. Pointer 26 is

held with target point 30' inside imaged volume 14'. The display consists of three orthogonal planes 60, 62 and 64, of the voxels of image volume 14, that all intersect at

target point 30'.

A fourth viewpoint style is defined as illustrated in Figure 5. This viewpoint

style includes three viewpoint planes 66, 68 and 70. Viewpoint plane 66 is

perpendicular to handle axis 35 and includes stylus axis 34. Viewpoint plane 68 is the

plane defined by axes 34 and 35. Viewpoint plane 70 is perpendicular to viewpoint

planes 66 and 68 and includes handle axis 35. When stylus 26 is pointed at body 10,

three slices 72. 74 and 76 of imaged volume 14' are defined with the help of

viewpoint planes 66, 68 and 70. Image slice 72 is the intersection of viewpoint plane

66 with imaged volume 14'. Image slice 74 is parallel to viewpoint plane 68. Image

slice 76 is parallel to viewpoint plane 70. The lateral position of image slice 74 within imaged volume 14' is controlled using trackball 46. The depth of image slice 76

within imaged volume 14' is controlled using buttons 34 and 36. For clarity, imaged

volume 14' is not shown explicitly in Figure 5, and image slices 72, 74 and 76 are

shown truncated at surface 12 of body 10.

As noted above, the description of the mechanism, for sensing the position and

orientation of pointer 26, as being located in handle 50, is illustrative rather than

limitative. In the variant of pointer 26 that uses a field sensor mounted in stylus 48,

that sensor defines the axes that define the viewpoint styles. In such a pointer 26,

stylus 48 need not be rigidly attached to handle 50.

While the invention has been described with respect to a limited number of

embodiments, it will be appreciated that many variations, modifications and other

applications of the invention may be made.

Claims

WHAT IS CLAIMED IS:

1. A method of displaying an image volume of a body having a surface,

comprising the steps of:

(a) registering the image volume with the body;

(b) selecting a viewpoint with reference to the body; and

(c) displaying a first portion of the image volume with reference to said

viewpoint.

2. The method of claim 1, wherein said viewpoint includes: (i) a target point; and

(ii) a first display orientation,

3. The method of claim 2, wherein said target point is on the surface of the body.

4. The method of claim 3, further comprising the steps of:

(d) providing a pointer having a tip, said selecting of said target point then

being effected by positioning said tip in contact with said target point.

5. The method of claim 4, wherein said selecting of said first display

orientation is effected by pivoting said pointer about said target point while said tip is

in contact with said target point.

6. The method of claim 5, further comprising the step of:

(f) determining a pivot orientation of said pointer relative to the body, said

first display orientation then being determined by said pivot

orientation.

7. The method of claim 6, wherein said first display orientation is substantially identical to said pivot orientation.

8. The method of claim 3, further comprising the step of:

(d) providing a pointer, said selecting of said target point then being effected by pointing said pointer at said target point.

9. The method of claim 8, wherein said selecting of said first display orientation is effected by pivoting said pointer.

10. The method of claim 9, further comprising the step of:

(e) determining a pivot orientation of said pointer relative to the body, said first display orientation then being determined by said pivot

orientation.

11. The method of claim 10, wherein said first display orientation is

substantially identical to said pivot orientation.

12. The method of claim 3, further comprising the step of:

(d) displaying a second portion of the image volume with reference to said

target point and said display orientation.

13. The method of claim 3, further comprising the steps of:

(d) replacing said first display orientation of said viewpoint with a second display orientation; and

(e) displaying a second portion of the image volume with reference to said target point and said second display orientation.

14. The method of claim 2, wherein said target point is outside the body, the method further comprising the step of:

(d) providing a pointer having a tip, said selecting of said viewpoint then being effected by positioning said tip at said target point and by pivoting said pointer in accordance with said first display orientation.

15. The method of claim 1, wherein said viewpoint includes a target point

within the body, the method further comprising the step of:

(d) providing a pointer that defines an axis and that has and a tip on said axis, said selecting of said viewpoint then being effected by pointing

said pointer at the body so that said target point is on said axis at a

predetermined distance from said tip.

16. The method of claim 1 , wherein said viewpoint includes a first plane

that intersects the body, the method further comprising the step of:

(d) providing a pointer that includes a stylus and a handle, said stylus

defining a stylus axis, said handle defining a handle axis, said selecting of said viewpoint then being effected by pointing said pointer at the

body so that said stylus axis intersects the body, said first plane then being perpendicular to said handle axis and including said stylus axis.

17. The method of claim 16, wherein said viewpoint further includes a

second plane that includes both said axes.

18. The method of claim 17, wherein said stylus includes a tip on said stylus axis, and wherein said viewpoint further includes a third plane that is perpendicular to said first and second planes and that intersects said stylus axis at a fixed distance from said tip along said stylus axis.

19. The method of claim 1, wherein said registering of the image volume with the body is effected by steps including:

(i) providing the image volume with at least four reference points;

(ii) selecting a like number of fiducial points on the surface of the body;

and (iii) matching each said reference point to a respective said fiducial point.

20. The method of claim 1 , wherein said registering of the image with the

body is effected by steps including:

(i) obtaining, from the image volume, a representation of at least a first

part of a feature of the body; (ii) sampling coordinates of at least a second part of said feature of the

body; and

(iii) matching said sampled coordinates to said representation.

21. The method of claim 20, wherein said feature is the surface of the body.

22. The method of claim 20, wherein said feature is an interior feature of the body.

23. The method of claim 22, wherein said sampling of said coordinates is effected by steps including echolocation of said interior feature.

24. The method of claim 1, further comprising the steps of:

(d) providing a plurality of viewpoint styles; and

(e) selecting one of said viewpoint styles; said viewpoint then being selected in accordance with said selected viewpoint style.

25. A system for selecting a portion of an image volume of a body and for

displaying the selected portion, comprising: (a) a pointer for selecting a viewpoint with reference to the body;

(b) a memory for storing the image volume;

(c) a processor for retrieving the portion of the image volume from the

memory, with reference to said viewpoint; and

(d) a monitor for displaying the retrieved portion.

26. The system of claim 25, wherein said pointer includes a mechanism for providing, to said processor, signals representative of said viewpoint.

27. The system of claim 26, wherein said mechanism is an optical mechanism.

28. The system of claim 26, wherein said mechanism is an electromagnetic mechanism.

29. The system of claim 26, wherein said mechanism includes a datalink to

said processor.

30. The system of claim 29, wherein said datalink is wireless.

31. The system of claim 25, wherein said viewpoint includes a target point,

and wherein said pointer includes:

(i) a handle; and

(ii) a stylus, operationally connected to said handle, for pointing at said

target point to select said target point.

32. The system of claim 31, wherein said stylus includes a tip for placing

adjacent to said target point to select said target point.

33. The system of claim 31, wherein said stylus is rigidly attached to said

handle.

34. The system of claim 33, wherein said stylus is substantially

perpendicular to said handle.

35. The system of claim 31, wherein said stylus includes a piezoelectric sensor at a distal end thereof.

36. The system of claim 25, wherein said pointer includes a mechanism for providing, to said processor, signals for controlling said processor.

37. The system of claim 36, wherein said mechanism includes a datalink to

said processor.

38. The system of claim 37, wherein said datalink is wireless.

39. The system of claim 36, wherein said processor is operative to interpret

at least one of said signals as a signal for selecting a viewpoint style, said processor

then retrieving said portion of the image volume from the memory with reference to

said selected viewpoint style.

40. A six-degree-of-freedom input device, comprising:

(a) a handle;

(b) a stylus, operationally connected to said handle; and

(c) a mechanism for providing signals representative of a location and an orientation of an element of the device selected from the group

consisting of said handle and said stylus.

41. The device of claim 40, wherein said stylus is rigidly attached to said handle.

42. The device of claim 41, wherein said stylus is substantially perpendicular to said handle.

43. The device of claim 40, wherein said mechanism is electromagnetic.

44. The device of claim 43, wherein said mechanism includes a plurality of coils rigidly mounted in said element.

45. The device of claim 40, wherein said mechanism is optical.

46. The device of claim 45, wherein said mechanism includes a plurality of

light emitting diodes rigidly mounted on said element.

47. The device of claim 40, wherein said stylus includes a piezoelectric

sensor at a distal end thereof.