referring now to the drawings wherein the showings are for the purposes of illustrating the preferred embodiments of the invention only and not for purposes of limiting same , with reference first to fig1 a patient table or support 10 includes a patient supporting surface 12 that is mounted for longitudinal movement relative to a base portion 14 . the base portion 14 includes a motor for raising and lowering the patient support surface 12 and for moving the patient support surface longitudinally . position encoders are also provided for generating electrical signals indicative of the height and longitudinal position of the patient support . an adjustable support structure 17 is mounted to the patient support . a diagnostic imaging apparatus 18 , preferably a volumetric imaging apparatus , is disposed in axial alignment with the patient table such that a patient or subject on the patient support surface 12 can be moved into and through a bore 20 of the volumetric imager . in the illustrated embodiment , the volumetric imager is a ct scanner which includes an x - ray tube mounted for repeated circular travel within a preselected plane . the x - ray tube projects a fan - shaped beam of radiation through a ring 22 of radiation translucent material , through the patient support 12 , through a region of interest of the subject , and to a ring or arc of radiation detectors positioned opposite the x - ray tube . as the x - ray tube rotates within the plane , a series of data lines are generated , which data lines are reconstructed into at least a slice image by a reconstruction processor included in a control console 24 . the control console is typically remotely located in a shielded room adjacent the scan room . more specifically to the preferred embodiment , the patient support 12 moves longitudinally as the x - ray tube is rotating around the subject such that a selected volume of the patient is scanned along a spiral path or a series of slices . the position of the x - ray tube is monitored by a rotational position encoder , and the longitudinal position of the patient support is monitored by a longitudinal position encoder within the table 10 . the reconstruction processor reconstructs a volumetric image representation from the generated data lines . the control console 24 typically includes one or more monitors 26 and various standard operator input devices such as a keyboard , trackball , mouse , or the like . an interventionist control console 28 is supported from overhead on a track atop the ct scanner . a localizing device in the preferred form of a mechanical frameless stereotactic arm assembly 30 is supported from overhead by a carriage 32 movable on an oval track system 34 affixed to the top of the volumetric diagnostic imaging apparatus 18 as generally shown . the carriage is preferably lockable in one or more predetermined fixed locations on the oval track so that a minimally invasive surgical instrument 36 carried thereon can be positioned in monitored positions and orientations by an interventionist in preparation for ( preoperative planning and visualization ) and in carrying out a surgical procedure . preferably , the plurality of instruments 36 may be interchangeably mounted on the arm assembly 30 . with continued reference to fig1 and further reference to fig3 the surgical instrument 36 of the preferred embodiment is a guidance device known as a grid lattice . the grid lattice 36 includes a two - dimensional array of spaced apart through apertures 37 which serve as needle guides . in particular , needles used in brachytherapy are selectively inserted in one or more of the apertures such that the needle is guided thereby . in a preferred embodiment , the grid lattice is approximately 3 cm × 3 cm and carries a regular array of needle guides spaced at 0 . 5 cm intervals . overall , the position and orientation of the grid lattice is determined by the position of a localizing device . in the preferred embodiment of the invention shown , the localizing device is a mechanical arm assembly 30 . the location of the mechanical arm assembly 30 , the relative position of each arm joint , and the position of the carriage 32 on the oval track system 34 in the scanner coordinate system are used to determine the position and orientation of the grid lattice relative to the image volume data set and relative to the patient to be described below . the articulated arm assembly 30 is shown generally in fig2 and includes a plurality of arm segments which are interconnected by pivot members forming joints between the arm segments . in that way , a free end 40 of the arm is selectively movable in multiple orientations as necessary to position the grid lattice 36 into various desired positions over the patient support 12 and relative to the patient . a base member 42 is rigidly connected to the carriage 32 using suitable fasteners , epoxies , or the like . a base joint 44 permits rotation of a primary support member 46 in a direction marked a . similarly , from the immovable base end of the arm , a shoulder joint 48 permits rotation of an upper arm member 50 in a direction marked b , an elbow joint 52 permits rotation of a lower arm member 54 in a direction marked c , a forearm joint 56 permits rotation of a knuckle member 58 in a direction marked d , and , lastly , a wrist joint 60 permits rotation of a wrist member 62 in a direction marked e . at least one position resolver , preferably an optical incremental encoder , is provided at each joint of the mechanical arm assembly 30 to monitor incremental movement and rotation of the arms relative to each other for reasons that will subsequently become apparent . the optical incremental encoders generate feedback pulses indicative of the relative angular and rotational position of the various arm members with respect to each other in a well - known manner . the feedback pulses are carried back to an imaging apparatus control circuit using suitable wires or flexible shielded cables extending through the multiple members of the arm assembly . in that way , the position and orientation of the wrist member 62 with respect to the imaging apparatus reference frame and the volumetric image representation obtained by the imaging apparatus can be determined . the physical dimensions and other parameters of the guide lattice 36 are known a priori . preferably , the guide lattice 36 provides a unique identification signal to the system . the identification signal is used by the imaging apparatus control circuit to index a look up table for retrieving various physical dimensional and other functional parameters corresponding to the guide lattice 36 . in this manner , the physical dimensions and other functional parameters , together with the mechanical interconnection which is measured by the resolvers and / or encoders , provides an accurate indication of the position and orientation of the guide lattice 36 relative to the ct scanner and , hence , relative to the image acquired by the ct scanner imaging device . with reference now to fig4 an instrument coordinate circuit 72 determines the position and trajectory of the guide lattice 36 in instrument space , preferably in a coordinate system of the guide lattice 36 . the instrument coordinate system circuit includes a guidance device identification circuit 74 and a mechanical arm assembly position circuit 76 . the guidance device identification circuit 74 receives the device identification signal from the guide 36 lattice and indexes a look up table 78 to retrieve dimensional and functional information . the mechanical arm assembly position circuit 76 is connected with the incremental resolvers on the mechanical arm assembly 30 to receive signals indicative of changes of position and orientation of the mechanical arm in instrument space . an instrument - planning scanner correlating processor 80 determining the correlation or transform between the guide lattice 36 and the volumetric scanner 18 coordinate systems . the correlation or transform is normally described in terms of three - dimensional offset , angular offset or rotation , and scaling . using analogous mathematics or known mechanical relationships as above , a device to patient table correlating processor 82 determines the correlation or transform between the patient table and the surgical instrument . table resolvers 84 located in the patient table contribute vertical and longitudinal offsets to the correlation between the guide lattice and the patient table when the table is raised or lowered and when the patient support 12 is moved axially . an instrument to patient correlation processor 86 determines a correlation between the guide lattice system and a patient coordinate system . this may be done to provide reassurance to an interventionist by placing the surgical instrument on three or more known reference points on the patient . such points might include readily identifiable anatomical structures such as the tip of the nose , distinctive points of bones , fiducial markers that are placed during the volumetric imaging process , or the like . the equations for patient support movement are as follows : let the distance of the first image ( i . e ., reference image , maximum z value along the patient support ) from the aperture ( z - 0 in scanner coordinates ) along the z - axis be represented by : d max z image from aperture =( p [ z ] max z image pt support − p [ z ] present pt support ). then the equation for the transformations for patient support movements are as follows : localizer tip [ z ]= localizer tip scanner coord [ z ]− d max z image from aperture where p [ z ] is the patient support position in the z axis . a similar equation can be written for the patient support movements along the y axis . thus , if the localizer has a z value in scanner coordinates which is the same as the reference image , then d max z image from aperture = localizer tip scanner coord [ z ] patient restraint mechanisms ( not shown ) can be used in order to prevent gross movements of the patient relative to the patient support . an instrument to volumetric image coordinate system transform processor 88 receives the correlation or transform from the guide lattice 36 to the planning image processor 80 . the instrument to volumetric image processor operates on input position and orientation coordinates in image space to transform them into volumetric image data space and vice versa . knowing the position of the guide lattice in volumetric or planning data space enables the guide position and orientation to be superimposed on the volumetric planning image data . during a medical procedure , the patient is positioned in the volumetric planning scanner and a volumetric image is generated . the volumetric image is stored in a volumetric or planning data memory 90 . the position of the patient table during the generation of the planning data , particularly as the table moves to generate spiral or slice data , is stored in conjunction with the volumetric planning data such that the data is correlated with the patient table coordinate system . the operator control console 24 controls the volume planning image data memory or a video processor 92 such that the selected slices , projection images , surface renderings , or other conventional displays of the data are generated for display on a planning image display 94 . preferably , the planning image display includes corresponding sagittal , coronal , and axial slices . because the planning image display is generated before the surgical procedure , the planning movement of the surgical instrument is preferably displayed in the planning image coordinate system on the interventionist control console 28 . the coordinates and trajectory of the surgical instrument are conveyed by the instrument to planning image transform processor 88 for conversion into the planning image coordinate system . the location and trajectory of the surgical instrument in the planning image coordinate system is communicated to the video processor 92 which superimposes the lattice position and trajectory on the ct data display . more specifically , fig5 shows multiple different views of the position and orientation of the guide lattice superimposed on the volumetric image data so as to provide the operator with an indication of the position of the guide lattice in relation to the anatomy of the patient . in one embodiment , the four views of fig5 a - d are displayed simultaneously on the operator console 26 . as the position of the physical guide lattice is varied , the position of the guide in relation to the image data is updated accordingly . with particular reference to fig5 a and 5c , the image data is displayed in a plane coplanar with the major plane of the guide lattice with the grid 36 superimposed thereon . in fig5 a , a planar portion 202 of the image data is displayed at a reference or zero depth which corresponds to the physical position of the guide lattice . in procedures involving the prostate gland , this is generally a position which is in physical contact with the anatomy of the patient . in fig5 c , a planar portion 206 of the image data is displayed at a depth preferably corresponding the depth of a virtual needle selected by the operator via operator interface 26 . fig5 b and 5d display planar portions 204 , 208 that are orthogonal to the major plane of guide lattice and to each other . for example , if the guide lattice was placed in a transverse axial position , fig5 b would depict a coronal view , with a cross section of the lattice representing a column of “ virtual needles ” displayed in relation to the image data . fig5 d would depict a sagittal view , with a row of “ virtual needles ” displayed in relation to the image data . if the grid is oriented out of the transverse plane , all views would be multiplanar reformatted to represent oblique slices through the volume data set . with continuing reference to fig5 a - d , the operator can select individual apertures on the guide lattice using a device such as a joystick . a virtual needle 200 corresponding to the selected aperture may then be displayed in highlighted form , for example in a different color . the intersection of the views which are orthogonal to the grid ( i . e ., the views of fig5 b and 5d ) is at the center of the virtual needle 200 . the images of fig5 b , 5 c , and 5 d intersect at a point which corresponds to the tip of the virtual needle 200 . the depth of the virtual needle 200 is adjusted through the operator interface . as the depth of the virtual needle is adjusted , its location is shown adjusted in relation to the image data in each of the views of fig5 b , 5 c , and 5 d . the length of the virtual needle in the depth direction is calculated by the system and is displayed on the operator interface or stored . this distance is used by the operator in setting the correct depth for insertion of the actual needle subsequent to completion of the planning process . a plurality of virtual needles may be selected in succession . once the location corresponding to a first aperture ( and thus seed ) has been determined , the process is repeated for additional apertures . as an aid to visualization , it may be desirable to display the selected location for each of the seeds on the images of fig5 b , 5 c , and 5 d until desired location for a plurality of the seeds is determined . in operation , the patient is imaged , and image data corresponding to the anatomy of the patient is generated . the operator then positions the guide lattice to the desired position and orientation in relation to the anatomy . when in the desired position , the guide lattice may be attached to the adjustable support structure 17 . the desired needle depth is determined selecting the desired apertures and adjusting the depth of the virtual needles . in brachytherapy of the prostate gland , for example , bounding structures such as the wall of the rectum may be located and the desired depth adjusted accordingly . brachytherapy needles are inserted through the appropriate apertures according to a desired dose plan . the needles are inserted to the desired depth , and the seeds are deposited accordingly . verification ct slices may then be taken to confirm the dose distribution . preferably , the guide lattice is detached from the arm 30 and attached to the supplemental support 17 prior to the verification scan . optionally , the guide lattice 36 may be detached from the arm 30 after the lattice 36 has been attached to the support structure . the desired needle may then be attached to the arm 30 . the actual position of the needle may then be superimposed on the image data to provide the operator with a real time indication of the position of the needle in relation to the anatomy of the patient . the invention has been described with reference to the preferred embodiment . obviously , modifications and alterations will occur to others upon reading and understanding the preceding description . it is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof .
a tomographic scanner includes an imaging device for obtaining images of the anatomy of a patient . a localizer device such as an articulated arm includes a base portion mounted in a fixed relationship to the imaging device and a free end adapted for selective movement into varied positions near a patient body disposed on the imaging device . a guide device such as grid lattice is attached to the free end of the arm . a position transducer associated with the localizer device generates information as to the position of the localizer and hence the position of the guide device in scanner coordinates . the guide device contains a plurality of needle guides . the position of the guide device is displayed in relation to images of the anatomy of the patient . this information is used by the operator to guide the insertion of brachytherapy seeds in a desired pattern and location in relation to the anatomy of the patient .