Methods for positioning an internal portion of a body relative to an extracorporeal referent

I claim:

1. A method of positioning a small internal portion of a body at a predetermined point relative to an extracorporeal referent, said body portion being detectable by penetrating radiation and said body being initially so disposed that the body portion is generally adjacent said predetermined point, said method comprising

(a) viewing said body with penetrating radiation along two convergent viewing axes intersecting at said predetermined point for producing two images of the interior of the body as viewed by said penetrating radiation respectively along said two viewing axes, each of said images including a portion representing said body portion;

(b) determining, from said two images, two-dimensional coordinates of the projections of the imaged body portion by the pentrating radiation on two notional planes respectively perpendicular to the viewing axes of the images, thereby to establish two sets of two-dimensional body portion coordinates, one for each image;

(c) from said two sets of two-dimensional coordinates, deriving for each image spatial coordinates of the imaged body portion in a coordinate system having as one axis the viewing axis of the image, thereby to establish two sets of three-dimensional body portion coordinates, one for each image;

(d) transforming said two sets of three-dimensional coordinates into a single set of three-dimensional coordinates in a common coordinate system in which the coordinates of said predetermined point are known, said single set of three-dimensional coordinates representing the location of said body portion in said last-mentioned system, thereby to determine the magnitude and direction of movement of said body required to position said body portion at said predetermined point; and

(e) displacing said body with a resultant motion having said magnitude and direction.

2. A method according to claim 1, wherein the viewing step is performed by directing penetrating radiation, from two sources of radiation disposed below the body, respectively along each of said two viewing axes through the body; receiving penetrating radiation from said sources, after passage of the radiation through the body, with two radiation detectors disposed above the body and respectively aligned with the two viewing axes, each of said detectors producing an output convertible into a two-dimensional image of the body volume through which radiation received by that detector has passed, in an image plane perpendicular to the viewing axis with which that detector is aligned; and converting the output of each detector into such two-dimensional image.

3. A method according to claim 2, wherein the position of the imaged body portion on each said image relative to a referent representing a coordinate system in the notional plane perpendicular to the viewing axis of that image differs from the position of the body portion projection on the last-mentioned notional plane relative to the last-mentioned coordinate system by ascertainable factors, and wherein the coordinate-determining step comprises

(i) identifying, on each said image, a portion thereof representing said body portion;

(ii) measuring the position of the identified image portion on each said image relative to said last-mentioned referent, thereby to obtain apparent two-dimensional coordinates for the identified image portion on each said image;

(iii) ascertaining said factors for each said image; and

(iv) modifying the apparent two-dimensional coordinates by the ascertained factors for each said image to establish said two sets of two-dimensional coordinates.

4. A method according to claim 3, wherein the two-dimensional coordinates of the body portion projection on each said notional plane differ, from the spatial coordinates of the body portion geometrically projected in a plane parallel to the notional plane and containing said predetermined point, by a parallax deviation dependant, for each image, on the distance between source and detector along the viewing axis of the image; and wherein the deriving step includes measuring the last-mentioned distance for each image and deriving said two set of three-dimensional coordinates from these measurements and from the two sets of two-dimensional coordinates, preestablished measurements of the distances between the radiation sources and the predetermined point, and a preestablished measurement of the angle between the two viewing axes.

5. A method according to claim 4; wherein each said image is produced on a cathode ray tube; wherein said factors include vignetting and electronic magnification; wherein the factor-ascertaining step includes ascertaining calibration values, for each image, to compensate for vignetting and electronic magnification; and wherein the modifying step includes applying said calibration values to said apparent coordinates.

6. A method according to claim 5, wherein said two sets of three-dimensional coordinates are established, respectively, with reference to two three-dimensional rectangular coordinate systems each having said predeterined point as origin, an x axis coincident with one of said viewing axes, a y axis coplanar with both said viewing axes, and a z axis coincident with the z axis of the other of said two last-mentioned three-dimensional coordinate systems, said viewing axes lying in a common plane oblique to the horizontal; and wherein the transforming step comprises

(i) rotating both of the last-mentioned coordinate systems about their coincident z axes until their y axes coincide to constitute a common coordinate system;

(ii) rotating the last-mentioned common coordinate system about the y axis thereof until the x and y axes lie in a horizontal plane, constituting a common coordinate system with a vertical z-axis having therein two sets of three-dimensional coordinates;

(iii) determining the distance between the spatial locations respectively represented by the two sets of three-dimensional coordinates; and

(iv) upon ascertaining that said distance is less than a predetermined value, determining the three-dimensional coordinates, in said last-mentioned common coordinate system, of the midpoint between said spatial locations, thereby to establish said single set of three-dimensional coordinates.

7. A method according to claim 1, wherein said two sets of three-dimensional coordinates are established, respectively, with reference to two three-dimensional rectangular coordinate systems each having said predetermined point as origin, an x axis coincident with one of said viewing axes, a y axis coplanar with both said viewing axes, and a z axis coincident with the z axis of the other of said two last-mentioned three-dimensional coordinate systems; and wherein the transforming step comprises rotating both of the last-mentioned coordinate systems about their coincident z axes until their y axes coincide to constitute a common coordinate system.

8. A method according to claim 7, wherein the two viewing axes lie in a common plane oblique to the horizontal, and wherein the transforming step further includes rotating the last-mentioned common coordinate system about the y axis thereof until the x and y axes lie in a horizontal plane.

9. A method according to claim 1, wherein the transforming step comprises

(i) transforming said two sets of three-dimensional coordinates into a common coordinate system to provide therein two sets of three-dimensional coordinates;

(ii) determining the distance between the spatial locations respectively represented by the two seets of three-dimensional coordinates;

(iii) upon ascertaining that said distance is less than a predetermined value, determining the three-dimensional coordinates, in said last-mentioned common coordinate system, of the midpoint between said spatial locations, thereby to establish said single set of three-dimensional coordinates.

Description Submit all comments and votes

BACKGROUND OF THE INVENTION

This invention relates to methods for positioning a small internal portion of a body at a predetermined point relative to an extracorporeal referent. In an important aspect, the invention is concerned with positioning a small internal body portion detectable by penetrating radiation, such as a concretion within the body of a living human patient, at a predetermined point relative to an external energy source or the like, e.g. for treatment of the body portion with focused or directional wave energy from the source.

One specific application of the invention, which will be particularly considered herein for purposes of illustration, is in the treatment of kidney stones by extracorporeal shock wave lithotripsy (ESWL). Apparatus and procedures for ESWL treatments are described, for example, in U.S. Pat. No. 3,942,531, No. 4,530,358, and No. 4,539,989, and in Ch. Chaussy et al., Extracorporeal Shock Wave Lithotripsy (Basel: Karger, 1982), the disclosures of all of which are herein incorporated by this reference.

Typical ESWL apparatus includes a water-filled shock wave focusing chamber in the form of a partial ellipsoid, truncated orthogonally to the major axis, mounted on (and opening into) a patient-receiving water tank, and so arranged that the first focus of the ellipsoid is within the focusing chamber while the second focus of the ellipsoid is outside the focusing chamber but within the tank. Means such as an arc discharge device are provided for generating high energy shock waves at the first focus of the ellipsoid. The patient to be treated is suspended in the tank on a cradle movably carried by a gantry. With the patient so positioned that the kidney stone to be treated is located at the second focus of the ellipsoid, reflected shock waves generated in the focusing chamber (and propagating through the chamber and tank water and the patient's body) cause the stone to disintegrate. Thereby, in at least many instances, the patient is ultimately able to pass the stone fragments without surgical intervention.

Precise positioning of the stone at the ellipsoid second focus within the tank is critically important, both to achieve the desired result of stone disintegration by the shock waves and also to minimize the possibility of detrimental effects of the shock waves on other portions of the patient's body. To this end, in current practice, two X-ray image intensifiers (herein sometimes also termed fluoroscopes) are mounted above the tank, pointing downwardly toward the patient position, and are oriented at an angle to each other such that their respective viewing axes converge and intersect at the second focus of the ellipsoid. Each fluoroscope is movable along its viewing axis. The two viewing axes define a plane, oblique to the horizontal, which intersects the patient's body transversely. Two X-ray sources (X-ray tubes) are disposed below the tank, in positions for respectively directing radiation upwardly through the patient to the two fluoroscopes along the aforementioned viewing axes. The outputs of the fluoroscopes are respectively displayed on monitors each having a cathode ray tube provided with crosshairs representing rectangular-coordinate axes perpendicular to (and intersecting at) the associated fluoroscope viewing axis. Thus, an operator viewing these monitors can ascertain the positional relation of a kidney stone (which is imaged by X-radiation distinguishably from surrounding body tissue) to the viewing axes of the two fluoroscopes. Owing to the above-described orientation of the fluoroscopes, the kidney stone is properly positioned at the ellipsoid second focus when, but only when, its image is centered in the crosshairs of both monitors.

Heretofore, patient positioning in ESWL apparatus of the described type has been an essentially manual procedure, in which the operator, observing the monitor screens (after the patient has been lowered into the tank on the gantry cradle), manipulates the gantry controls to move the gantry and patient until the kidney stone image is centered on both monitors. This procedure can be relatively difficult and time-consuming; in particular, it involves undesirably long exposure of the patient to X-radiation, since the kidney stone image must be continuously viewed on the monitors as the patient is moved. Moreover, in some circumstances, the gantry cradle may drag across and damage the focusing chamber while the operator is moving the gantry in accordance with visual observation of the monitor.

SUMMARY OF THE INVENTION

The present invention contemplates the provision of a new and improved method of positioning a small internal portion of a body at a predetermined point relative to an extracorporeal referent, the body portion being detectable by penetrating radiation and the body being initially so disposed that the body portion is generally adjacent the predetermined point. Stated broadly, the method of the invention comprises the steps of viewing the body with penetrating radiation along two convergent viewing axes intersecting at the predetermined point for producing two images of the interior of the body as viewed by the penetrating radiation respectively along the two viewing axes, each of the images including a portion representing the aforesaid body portion; determining, from the two images, two-dimensional coordinates of the projections of the imaged body portion by the penetrating radiation on two notional planes respectively perpendicular to the viewing axes of the images, thereby to establish two sets of two-dimensional body portion coordinates, one for each image; from the two sets of two-dimensional coordinates, deriving for each image spatial coordinates of the imaged body portion in a coordinate system having as one axis the viewing axis of the image, thereby to establish two sets of three-dimensional body porton coordinates, one for each image; transforming these two sets of three-dimensional coordinates into a single set of three-dimensional coordinates in a common coordinate system in which the coordinates of the predetermined point are known, this single set of three-dimensional coordinates representing the location of the body portion in the last-mentioned system, thereby to determine the magnitude and direction of movement of the body required to position the body portion at the predetermined point; and displacing the body with a resultant motion having that magnitude and direction.

In exemplary and currently preferred embodiments of the invention, the viewing step is performed by directing penetrating radiation, from two sources of radiation disposed below the body, respectively along each of the two viewing axes through the body; receiving penetrating radiation from the sources, after passage of the radiation through the body, with two radiation detectors disposed above the body and respectively aligned with the two viewing axes, each of the detectors producing an output convertible into a two-dimensional image of the body volume through which radiation received by that detector has passed, in an image plane perpendicular to the viewing axis with which the detector is aligned; and converting the output of each detector into such two-dimensional image. It will be appreciated, of course, that "above" and "below" are used herein only as relative terms, since the absolute orientation of the sources and detectors is immaterial to the invention as long as the sources and the detectors are opposed with respect to the body. By way of specific example, the radiation sources may be X-ray tubes, and the detectors may be image intensifiers (fluoroscopes) having outputs imaged on cathode ray tubes, as in known ESWL apparatus of the type described above.

A significant advantage of the present method for medical applications is that it enables the body to be positioned using information derived from two concurrently produced initial images, necessitating only relatively brief irradiation of the patient while the requisite coordinates are obtained, rather than (as in prior practice) involving continuing exposure of the patient to radiation during the actual positioning of the body by an operator watching images of the moving body. The reduction in radiation exposure thus afforded by the invention may exceed 50%. At the same time, since the magnitude and direction of body motion are determined in this method from initial image information rather than by continuing observation of the changing position of the moving body portion, it is essential that the initial position coordinates of the body portion relative to the predetermined point be determined with fairly high accuracy. Since the imaged body portion is typically off center (displaced from the viewing axis) in one or both images, and since the observed or apparent coordinates of off-center portions of such images differ from the true spatial coordinates of the imaged object or body portion owing to various factors, it is necessary to apply compensating modifications or corrections to the apparent coordinates in order to obtain useful approximations of the true spatial coordinates. Important particular features of the invention reside in the inclusion, in the method broadly set forth above, of steps or procedures for modifying the apparent image coordinates of the body portion to compensate for these factors, which include parallax magnification and (with cathode ray tube images) electronic magnification and so-called vignetting.

Compensation for vignetting and electronic magnification is accomplished in the step of determining the two-dimensional coordinates, by modifying the observed position of the body portion in each image in accordance with data obtained by calibration. Parallax magnification is dependent on the source-to-detector spacing, and in the case of detectors movable along their respective viewing axes (e.g., to accommodate bodies of different sizes, as in known ESWL apparatus), the correction for parallax magnification (accomplished in the step of deriving two sets of three-dimensional coordinates) involves determining the location of each detector along its viewing axis when each image is produced.

Stated in terms of rectangular coordinate viewing systems, for each image there is posited a fluoroscope coordinate system in which the x axis is the viewing axis along which the image is taken. The displayed image is a projection in the aforementioned notional plane, viz. the plane perpendicular to the viewing axis at the input face of the detector which produces the image. Thus, only the apparent y and z coordinates of the body portion are directly measurable on the displayed image. The viewing axes of the two images, intersecting at the predetermined point, define a plane (herein termed the viewing plane) which also contains the y axes of the two fluoroscope systems; and the z axes of the two fluoroscope coordinate systems coincide as a common perpendicular to the viewing plane. With this geometry, the x coordinates of the body portion in the two fluoroscope coordinate systems can be readily derived from the corrected y and z coordinates determined from the image data, as taken together with the measured positions of the fluoroscopes along the respective viewing axes and preestablished measurements of system geometry. Thus, for each image, there is established a three-dimensional set of coordinates for the body portion.

Transformation of these two sets of three-dimensional coordinates into a single set of body portion coordinates can be accomplished, in accordance with the invention, by rotating the two major coordinate systems about their common z axis, until their y axes coincide in the viewing plane, their x axes then also coinciding in the same plane in a line bisecting the angle formed between the two viewing axes. The origin of this single or combined coordinate system is still the predetermined point to which the body portion is to be moved. Conveniently, a further rotation may be effected to conform to the machine coordinates of the apparatus in which the body portion is to be positioned. For instance, in an illustrative ESWL apparatus, the viewing plane is oblique to the horizontal, while the machine coordinate system to which the patient-carrying gantry is referred has a vertical z axis and horizontal x and y axes; the x axis of the aforementioned combined coordinate system in the viewing plane lies in the xz plane of the machine coordinate system, and the y axis of the combined viewing plane coordinate system coincides with the y axis of the machine coordinate system, so that rotation of the combined viewing plane coordinate system about its y axis brings the latter system into coincidence with the machine coordinate system.

Frequently, the two three-dimensional sets of body portion coordinates respectively derived from the two images do not precisely coincide when they are transformed into a common coordinate system, e.g. as a result of imprecise measurement and/or compensation for distorting factors. That is to say, the spatial locations respectively identified by these two sets of coordinates may be somewhat spaced apart in the common coordinate system. When this occurs, it is necessary first to determine whether the two locations thus identified both in fact represent the same object, viz. the body portion to be positioned. In accordance with the invention, the distance betwen the two locations is compared with a predetermined value; if that distance is less than or equal to the predetermined value, then the coordinates of the midpoint between the two locations are derived from the coordinates of the two locations, and that midpoint is taken as the situs of the body portion.

Once the coordinates of the body portion in the final single coordinate system have been established (either by coincidence of the two sets of coordinates respectively derived from the two images, or by calculation of midpoint coordinates as just described), these coordinates and the coordinates of the predetermined point define a displacement vector representing the magnitude and direction of motion of the body needed to bring the body portion to the predetermined point. With this information, the body can be moved either linearly or (as a further advantage of the invention, if direct linear displacement would cause interfering contact with and possible damage to adjacent apparatus elements) by a roundabout path, in either case with a resultant motion having the magnitude and direction represented by the displacement vector.

The invention may be further understood by reference to exemplary apparatus for use in the practice of the foregoing method in a system for treating a small internal portion of a body with shock waves or the like from an external energy source, wherein the body portion is to be positioned at a predetermined point relative to the energy source, the system including means for moving the body and two detectors (for receiving penetrating radiation after passage through the body) having viewing axes intersecting at the predetermined point, each detector being movable along its viewing axis and each producing a cathode ray tube image. Such apparatus includes, in combination, means including a stylus for sensing the position of an operator-selected image portion on a cathode ray tube screen and producing a first output signal representing the position on the screen of the selected image portion; means respectively associated with the two detectors for sensing the positions of the detectors along their viewing axes and producing second and third output signals respectively representing those positions; and means for combining the first, second and third output signals to produce an information output representing the magnitude and direction of body displacement needed to bring the body portion to the predetermined point.

Further features and advantages of the invention will be apparent from the detailed description hereinbelow set forth, together with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is highly simplified schematic sectional side elevational view of an example of ESWL apparatus with which the method of the present invention may be practiced;

FIG. 2 is a view taken along the line 2--2 of FIG. 1, also showing diagrammatically a particular example of apparatus for practice of the invention, as incorporated in the ESWL apparatus of FIG. 1;

FIG. 3 is a diagram showing a three-dimensional rectangular coordinate system (x.sub.o, y.sub.o, Z.sub.o) for the apparatus of FIG. 1 and representing features of apparatus geometry pertinent to the practice of the present method in an illustrative embodiment;

FIG. 4 is a diagram showing the position of an exemplary point (c, d, e) relative to a three-dimensional rectangular coordinate system for one fluoroscope of the apparatus of FIG. 1;

FIG. 5 is an elevational view of a calibrating instrument or eisenkugel employed in a calibrating procedure in the practice of the present method;

FIG. 6 is a perspective view of a calibrating grid structure employed in a calibrating procedure in the practice of the present method;

FIG. 7 is a diagram in explanation of the parallax correction included in the aforementioned embodiment of the method of the invention;

FIG. 8 is a diagram, in the viewing plane, showing the position of an exemplary point relative to the x and y coordinate axes of both fluoroscopes of the FIG. 1 apparatus, and also illustrating the display of that point on the fluoroscope monitors;

FIG. 9 is a diagram of the coordinate system of FIG. 3 in explanation of further features of the aforementioned embodiment of the present method; and

FIG. 10 is a flow chart representing various data acquisition and processing steps included in that embodiment of the method.

DETAILED DESCRIPTION

The invention will be described with reference to its use in ESWL apparatus of known type, having features generally set forth in one or more of the above-cited patents and/or in the above-cited Chaussy et al. publication. An example of such apparatus, arranged for treatment of a kidney stone in a living human patient P, is illustrated schematically at 10 in FIGS. 1 and 2. As there shown, such apparatus includes a water tank 11, adapted to be filled with water 12 and dimensioned to receive an adult patient P in a generally recumbent position. The patient is supported on a cradle 14 carried by an overhead gantry 16, which is adapted to impart to the cradle (and thus to the patient) all modes of motion, viz. lateral, longitudinal, and vertical, necessary to position the patient precisely at a desired location within the tank for treatment. Once positioned, the patient is held securely and stably in the selected location by the cradle and gantry. Means (not shown) such as a motor are provided for moving the gantry in response to manipulation of a control system (also not shown) by an operator; the gantry position at any given time, with reference to the machine coordinate system described below, is sensed and displayed by an instrumentality 18 (XYZ display).

A hollow truncated ellipsoidal shock wave focusing chamber 20 is mounted in the floor of the tank 11 so as to open upwardly at an angle of about 14.28.degree. from the vertical into the interior of the tank; i.e., although for convenience of illustration the chamber is shown in FIG. 1 as if its axis (containing points F.sub.1 and F.sub.2) were vertical, in fact the latter axis in the view of FIG. 1 should be at 14.28.degree. to the vertical. This chamber 20 defines part of the surface of an ellipsoid of revolution having a first focus F.sub.1 within the chamber and a second focus F.sub.2 above the chamber but within the tank. The ellipsoid is configured to locate F.sub.2 sufficiently above the tank floor so that an internal portion of a patient's body can be positioned to coincide with F.sub.2. Associated with the chamber 20 is an arc discharge device 22 having a spark gap positioned at focus F.sub.1 within the chamber. In operation, with the interior of the chamber 20 and the tank 11 filled with water, a spark discharge at the gap generates a high energy shock wave in the water at F.sub.1. Direct and reflected components of this shock wave propagate upwardly through the tank, with a focus or intensity maximum at F.sub.2. If a human patient having a concretion such as a kidney stone within his or her body is suspended in the tank water by the cradle and gantry, with the kidney stone positioned at F.sub.2 and the surrounding region of the body immersed in the tank water, a succession of such spark-generated shock waves will cause the kidney stone to disintegrate without damaging other body portions. For safe and effective treatment, however, careful positioning of the kidney stone at F.sub.2 is highly important; and since the kidney stone is a small internal portion of the body, it is located and positioned with the aid of X-ray (fluoroscopic) examination of the patient after the patient's body has been initially so disposed that the kidney stone-containing region of the body is generally adjacent F.sub.2.

More particularly, the ESWL apparatus is provided with a pair of X-ray sources (X-ray tubes) 24, 25 disposed beneath the tank 11 so as to direct X radiation upwardly through the tank along oblique paths respectively having axes x.sub.n and x.sub.s intersecting at F.sub.2. A pair of detectors or fluoroscopes 26, 27 are mounted above the tank, respectively in alignment with the axes x.sub.n and x.sub.s, which are hereinafter referred to as viewing axes. Each fluoroscope is adapted to receive X radiation from its associated X-ray source after the radiation has passed through the tank and patient P, and to produce an output that is convertible into a two-dimensional cathode ray tube (CRT) image of the body volume traversed by the radiation received by the fluorosocope. In the form shown, each fluoroscope comprises an image intensifier 28 or 29 and a television camera 30 or 31 which transmits the fluoroscope output for display on the screen 32 or 33 of the cathode ray tube of an associated television monitor 34 or 35. Thus, two images are displayed, one for each fluoroscope, on the screens of the two monitors respectively connected to the two fluoroscopes.

Each of the fluoroscopes 26 and 27 is mounted (by means shown as a track 38 in FIG. 1) for movement in the directions indicated by arrow 40 in FIG. 1, viz. along the viewing axis x.sub.n or x.sub.s with which it is aligned, being provided with a worm gear drive (not shown) to effect such movement. This axially directed movement, through a restricted range of positions, permits the fluoroscopes to be brought close to the patient's body while accommodating patients of differing girth and/or permitting unobstructed movement of the patient's body into the proper position. The fluoroscopes are also pivotally movable out of the space above the tank to facilitate introduction of a patient to the tank.

The CRT screen 32 of monitor 34 (connected to fluoroscope 26) is shown as provided with a referent 42 comprising cross-hairs representing rectangular coordinate axes y.sub.n and z.sub.n,s which are perpendicular to each other and to the viewing axis x.sub.n of the fluoroscope 26. Similarly, the CRT screen 33 of monitor 35 (connected to fluoroscope 27) is provided with a referent 43 comprising cross-hairs representing rectangular coordinate axes y.sub.s and z.sub.n,s which are perpendicular to each other and to the viewing axis x.sub.s of the fluoroscope 27. Thus, the body image produced on each screen is a two-dimensional projection in a plane perpendicular to the viewing axis of the associated fluoroscope, the viewing axis being centered on the screen at the intersection of the cross-hairs. A concretion such as a kidney stone included in the imaged body region is discernible in each image as a spot (46n or 46s) visually distinguishable from surrounding body tissue, because the concretion is less transmissive to X radiation than is the surrounding tissue. Since the two fluoroscope viewing axes x.sub.n and x.sub.s converge and intersect at F.sub.2, a kidney stone is properly positioned at F.sub.2 if but only if the spots 46n and 46s representing it on the two CRT images are both centered on the cross hairs of the respective CRT screens. If either or both kidney stone image spots are away from the cross-hair intersections, the kidney stone is not yet located at F.sub.2 and the body must be moved before shock wave treatment can begin.

The apparatus as thus far described is, as already indicated, generally known or conventional for ESWL treatment. In currently used positioning procedures in such apparatus, the operator initially positions the patient in the tank so that the kidney stone location within the patient's body is generally adjacent the location of F.sub.2 ; then, while continuously or repetitively irradiating the patient from both X-ray sources and observing the location of spots 46n and 46s on the two monitor screens, the operator manipulates the gantry controls to move the patient until the spots 46n and 46s both coincide with the cross-hair intersections on the respective screens. The present invention, now to be described, enables the initial position of a kidney stone relative to F.sub.2 to be derived from two single initial static images (respectively displayed on the two monitor screens), with accuracy sufficient to determine the resultant direction and magnitude of body displacement that will bring the stone to F.sub.2. This direction and magnitude of motion may then be imparted to the body, either directly or circuitously (e.g., when necessary to avoid possibly damaging contact of the cradle with the focusing chamber), by operation of the gantry controls without further or continuing irradiation of the patient.

Features of the geometry of the above-described ESWL apparatus 10 pertinent to an understanding of the present invention are illustrated in FIGS. 3 and 4. In FIG. 3, the machine coordinate system of the ESWL apparatus 10 is the three-dimensional rectangular coordinate system having horizontal axes x.sub.o and y.sub.o and vertical axis z.sub.o with an origin at F.sub.2, the x.sub.o axis being oriented lengthwise of the patient position in the tank 11. The positions of the two fluoroscope viewing axes x.sub.n and x.sub.s in the machine coordinate system are defined by the origin F.sub.2 (at which the two viewing axes intersect) and, respectively, by the coordinates i, j, -k, and i, -j, -k; thus the two viewing axes diverge symmetrically (at equal but opposite angles) from the x.sub.o -z.sub.o plane. The viewing plane, viz. the plane in which viewing axes x.sub.n and x.sub.s both lie, is oblique to the horizontal (x.sub.o -y.sub.o plane) and is oriented to intersect a patient transversely. In the viewing plane, the angle a between axes x.sub.n and x.sub.s is bisected by the x.sub.o -z.sub.o plane, which intersects the viewing plane at line x.sub.T. This line x.sub.T, then, lies both in the x.sub.o -z.sub.o plane and in the viewing plane, and also passes through F.sub.2, forming equal but opposite angles a/2 with the two viewing axes.

For each of the two fluoroscopes 26 and 27, there is defined a three-dimensional rectangular coordinate system for which the fluoroscope viewing axis x.sub.n or x.sub.s is the x axis. The origins of both fluoroscope coordinate systems are at F.sub.2, and they share a common z axis (designated z.sub.n,s) lying in the x.sub.o -z.sub.o plane. Consequently, the y axis of each fluoroscope coordinate system (designated y.sub.n for fluoroscope 26, and y.sub.s for fluoroscope 27) lies in the viewing plane. The coordinate system of fluoroscope 26 is represented in FIG. 4, which illustrates the spatial coordinates c, d, e of an object (e.g., a kidney stone) in that system.

It will be seen that the y.sub.n and z.sub.n,s axes define a plane containing F.sub.2 and perpendicular to the viewing axis x.sub.n of the fluoroscope 26. The image plane of the monitor screen 32 (FIG. 2) may be considered to be parallel to this plane, with referent 42 representing the y.sub.n and z.sub.n,s axes therein. In the absence of imaging distortions, a kidney stone image spot (46n) on screen 42 would represent a geometric projection of the imaged kidney stone on the y.sub.n -z.sub.n,s plane, and the y and z spatial coordinates (d and e, in FIG. 4) of the kidney stone in the fluoroscope 26 coordinate system would be the same as the measured coordinates of spot 46n relative to referent 42. Similarly, the y.sub.s and z.sub.n,s axes define a plane containing F.sub.2 and perpendicular to the viewing axis x.sub.s of fluoroscope 27; the image plane of monitor screen 33 may be considered to be parallel to the latter plane, with referent 43 representing axes y.sub.s and z.sub.n,s, and the measured coordinates of an image spot 46s on screen 33 relative to referent 43 would equal the spatial y and z coordinates of the imaged kidney stone in the fluoroscope 27 coordinate system, except for imaging distortions. As hereinafter further explained, however, the measured positions of spots 46n and 46s relative to the respective referents 42 and 43 differ significantly (because of imaging distortions) from the spatial y, z coordinates of the object they represent in the respective fluoroscope coordinate systems.

For the practice of the method of the invention in the present embodiment, it is necessary to pre-establish certain fixed dimensions of system geometry, viz. the angle a between the viewing axes x.sub.n and x.sub.s and the angle between the x.sub.n -x.sub.s viewing plane and the horizontal x.sub.o -y.sub.o plane of the machine coordinate system. These dimensions may be established by preliminary calibration operations, e.g. utilizing an eisenkugel 48 (FIG. 5). The eisenkugel is a steel rod 50, e.g. 2 mm. in diameter and one meter long, having a steel ball 52 (e.g. 4 mm. in diameter) at one end and a cylindrical enlargement 54 (e.g. about 7 mm. in diameter, and 5 cm. long) at the other end; the enlargement 54 friction fits into a hole (not shown) on the gantry 16, from which the rod 50 and ball 52 then depend vertically so as to enable the ball 54 to be positioned at F.sub.2.

It is also necessary to establish calibration values for correcting imaging distortions. A calibrating grid structure 56 (FIG. 6) may be used for this purpose. The grid structure is formed by stringing fine-guage copper wires 58 through holes spaced 1 mm. apart in a square plastic case 60, and pulling the wire tight to define a grid pattern of 1 cm..times.1 cm. squares. The grid is completely enclosed in the plastic case, which has a small square 62 marked in one corner for indicating the orientation of the grid when viewed through a fluoroscope, and a lateral handle 64 for attachment to a flexible manipulating arm (not shown).

An exemplary embodiment of apparatus with which the present method may be performed is illustrated diagrammatically in FIG. 2. This apparatus comprises, in combination with the gantry, X-ray sources, fluoroscopes, and CRT monitors already described, means including a stylus 66 and associated digitizer 68 for sensing the position of an operator-selected image portion (e.g., 46n or 46s) on either monitor screen and producing a first digital output signal representing the position on the screen of the selected image portion; means including a sensor 70 and associated digitizer 72 for sensing the position of the fluoroscope 26 along its viewing axis x.sub.n and producing a second digital output signal repre