Abstract:
The invention presents tools to improve a 3-D image aided biopsy or treatment procedure for prostate gland by providing additional functionality and additional visual cues on an output image of the prostate, which may be generated substantially in real-time. The tools include i) the identification of various parts of prostate to classify as per regular classification in pathological reports, ii) Computing and displaying the insertion depth of needle with respect to a selected target point during the procedure, iii) Computing and displaying the distance from needle tip to prostate surface following a procedure and, iv) Calibration for misalignment of a 2-D imaging transducer when used under tracked motion for a procedure.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority under 35 U.S.C. 119 to U.S. Provisional Application No. 61/113,479, entitled: “Improved System and Method for Prostate Biopsy,” filed on Nov. 11, 2008, the contents of which is incorporated herein as if set forth in full. 
     
    
     BACKGROUND 
       [0002]    When cancer is suspected on an organ such as a breast, prostate, kidney or liver, a common diagnostic tool is to take biopsies of the organ tissue. A cancer may only be present in a very small portion of the organ in the early stages of its growth. However, this generally represents the stage when it is most desirable to detect the cancer. Cancer may also be uncorrelated and therefore may be present in a very small portion of the organ but at multiple places. Therefore the decision of where in the organ to take a biopsy will have a significant impact on successful early detection of a cancer as well as increasing the confidence of a negative result. 
         [0003]    Maximum confidence in a biopsy outcome is currently only possible if the organ has been sampled at a multitude of locations. This technique is known as saturation biopsy and the number of core sites can reach 90 or more. Such saturation biopsy often causes considerable discomfort to the patient. Further, smaller organs such a prostate, which may only be 4 cm in diameter, may be damaged by sampling at numerous biopsy sites required for a saturation biopsy. However, as the number of biopsy sites decreases, so does the probability of an accurate result. 
         [0004]    Management of biopsy targets is critical to a successful procedure. The operator must be able to plan each biopsy core site location, be able to accurately navigate the needle to each site to take the biopsy and finally to record the actual location of the biopsy site for future reference. This becomes overwhelming when a large number of sites are used. When a patient returns for multiple repeat visits the amount of data and complexity is increased proportionally. 
       SUMMARY OF THE INVENTION 
       [0005]    The presented inventions pertain to improving workflow and proving additional information to a physician for an image guided procedure on prostate gland. The tools and methods described mostly pertain to application to cancer diagnosis (image guided biopsy) and treatment (image guided dose delivery). Prostate cancer is one of the most common causes of cancer among men and the diagnosis includes PSA level test, DRE test and biopsy. So far, prostate biopsy has been found to be only conclusive way to confirm presence of cancer in the gland. Typically, a biopsy is performed under guidance of a 2-D ultrasound image either trans-rectally or trans-perineally. A transrectal ultrasound transducer is often used for such a procedure. Some ultrasound machines allow viewing of more than one plane at a time to assist needle insertion during biopsy. Recently, 3-D image guidance has been applied to prostate biopsy such that the prostate can be sampled in a desired fashion. Such approaches require acquisition of 3-D image, which may be reconstructed from a sequence of uniformly spaced 2-D frames acquired from the ultrasound transducer. The uniform spacing may be either linear or rotational. These approaches require a tracking device to measure the location and orientation of ultrasound transducer during the procedure so that the correspondence between the live video from ultrasound machine with respect to the acquired 3-D image is known. 
         [0006]    Similar developments have been made in field of prostate treatment using image guidance, where a 3-D image of prostate gland may be acquired prior to a procedure. The treatment plan is then made from the 3-D image, which may be acquired using an imaging technique other than ultrasound. For example, a treatment plan may be based on MRI or CT image acquired earlier. The invention applies to a number of treatment procedures and tools described provide workflow enhancements in a general sense. The prostate boundaries may further be extracted from the 3-D image for performing dose computations. In many procedures where the plan was based on 3-D image, the imaging modality during the procedure is still 2-D ultrasound. The correspondence of field of view (2-D ultrasound frame) with the 3-D image is either fixed by design or computed using a tracking device such that there is a clear method for reaching the desired target locations as per the 3-D image-based plan. 
         [0000]    Aspects of the presented invention aim to make the following workflow enhancements: 
         [0007]    1. Dividing prostate into regions 
         [0008]    2. Distance measurements during a procedure
       a. Distance of planned point from prostate boundaries   b. Automatic plan based on uniform sampling of prostate   c. Distance measurement from prostate boundaries on live 2-D ultrasound image before needle insertion   d. Distance measurements from prostate boundaries after a needle insertion       
 
         [0013]    3. Alignment correction for hardware misalignments. 
         [0014]    The inventions provide systems and methods for 3D image guided biopsy, where the urethra can be delineated in a 3-D image of prostate. The urethra may be manually delineated using a series of control points along it using multiple views of the prostate gland. Alternatively, it may be fully automatically or semi-automatically segmented. 
         [0015]    In another arrangement, the systems and methods of the inventions, in various aspects, may also contain a subsystem for automatic or semi-automatic segmentation of prostate. Using this information, the base and apex of the prostate can be identified as intersection of the urethra with the prostate surface. After identification of base and apex, the system can divide the prostate into a number of zones—Left/Right/Left Lateral/Right Lateral, Base/Mid/Apex. 
         [0016]    The systems and methods of the inventions may, in various aspects, provide a method for identification of various zones that can be displayed overlaid on the prostate 3-D image using a color scheme such that one color corresponds to one zone. Each zone may be selectable and turned on one at a time. 
         [0017]    The systems and methods of the inventions may, in various aspects, provide a method for identification of the zones and usage of zonal information to plan a biopsy or treatment procedure for prostate gland by selecting a zone and placing point in it. 
         [0018]    The systems and methods of the inventions may, in various aspects, may provide a system for computing prostate zones for 3-D transrectal ultrasound guided biopsy such that the zonal information may be used to automatically place a point at centroid of each such zone, thus creating a template for targeted biopsy based on the partitioning of a patient&#39;s prostate and customized to the shape of that particular prostate. 
         [0019]    The systems and methods of the inventions may, in various aspects, provide methods for identification and reporting the zone for a sampled core following a procedure and recording of actual location of a sampled or treated site. The definition of zones can be adjusted based on how a physician might collect samples from the prostate. 
         [0020]    The systems and methods of the inventions may, in various aspects, may also provide visual aids to the user for needle insertion for a biopsy or treatment procedure such as brachytherapy by displaying needle insertion depth. The visual aids are provided to the user for needle insertion for a procedure on prostate for reaching a pre-planned target point corresponding to either a biopsy sampling point or a point for dose delivery under a treatment procedure (such as brachytherapy, cryotherapy, thermal ablation). The target point may be planned on a 3-D image acquired earlier and the needle may be guided to the target point under navigation through a tracking device. The visual aids include displaying the target point planned in 3-D overlaid on the live view by applying the transforming from 3-D frame of reference to the 2-D live video. The overlay is displayed only when the target point is computed to be within a small distance (say, 5 mm) from the 2-D frame corresponding to the live video. 
         [0021]    The systems and methods of the inventions may, in various aspects, provide visual aids to the user for needle insertion for a procedure on prostate for reaching a pre-planned target point corresponding to either a biopsy sampling point or a point for dose delivery under a treatment procedure (such as brachytherapy, Cryotherapy, thermal ablation). The target point may be planned on a 3-D image acquired earlier and the needle may be guided to the target point under navigation through a tracking device. For a given target point and needle type, the system displays how deep the needle should be inserted before firing such that after firing; the planned site lies at the center of the needle core (or bead location for brachytherapy). It may be desirable to know this information before firing, since there are cases when a physician may not want to overshoot the needle and damage neighboring organ by placing a radioactive seed there or piercing it. 
         [0022]    The systems and methods of the inventions may, in various aspects, provide visual aids to the user for needle insertion for a procedure on prostate for reaching a pre-planned target point corresponding to either a biopsy sampling point or a point for dose delivery under a treatment procedure (such as brachytherapy, cryotherapy, thermal ablation). It may not always be possible for a user to plan placement of radioactive seeds or plan biopsy targets in advance such that needle avoids certain anatomical structures such as organs (bladder, urethra) or nerve bundles. Additional visual cues are provided in this system to facilitate a procedure based on just prostate shape. The system displays a ruler representing the distance from the prostate surface at the further end from the needle entry point along the needle trajectory. The system shall, at any time, display a ruler from the distal surface (anterior for a transrectal procedure, for example) such that the user always knows how deep the user is inserting the needle relative to the prostate boundaries along the needle trajectory. 
         [0023]    The methods are provided for computing and displaying the distance of the needle tip with respect to the prostate surface following a procedure and recording of actual location of a sampled or treated site. Likewise, the system can compute the distance of the actual core taken from the prostate surface. 
         [0024]    The systems and methods of the inventions may, in various aspects, also include a subsystem for computing distance from prostate boundaries to facilitate planning. In this subsystem, the system computes and displays the distance of a planned location for sampling a site or placement of radioactive seeds or applicator. Additionally, the system and methods may compute and display iso-surfaces at user-selected distances from prostate surface so as to assist the user in planning and during the procedure. There may be a number of iso-surfaces at user-selected distances from prostate surface to assist the user in planning and during the procedure. For some biopsy procedures, user may want to sample only points within a certain distance from the boundaries of the prostate and a colored iso-surface inside the surface representing the prostate boundaries allows them to do this. In addition, the systems and methods may compute and display the iso-surfaces overlaid on the grayscale image such that a combination of intensity information and boundary information can be utilized in planning a procedure. The computation of distances in 3-D makes it easier for user to interpret 2-D image slices typically seen during planning. 
         [0025]    The systems and methods of the inventions may, in various aspects, also have a subsystem to compute a plan automatically using a user-selected uniform spacing for a procedure, based on prostate boundaries. This makes it easier for procedures where user has to plan uniformly over the prostate for either a biopsy or treatment procedure. 
         [0026]    The systems and methods of the inventions, in various aspects, may also have a subsystem to compute a plan automatically using a user-selected uniform spacing for a procedure, based on prostate boundaries. This makes it easier for procedures where user has to plan uniformly over the prostate for either a biopsy or treatment procedure. In addition, the automatic loading of such a plan also takes into consideration the regions where placing of points is undesirable by avoiding locations close to structures such as urethra and nerve bundles, if such an information is available. 
         [0027]    The systems and methods of the inventions may, in various aspects, also contain a subsystem for correcting any image artifacts caused by misalignment of an ultrasound transducer with respect to the tracking device. The tracking device assumes a known frame of reference with respect to a transducer and as the transducer is manipulated, the tracking device computes the correspondence of the live video frame with the frame of reference of 3-D image. Any misalignment causes artifacts in a reconstructed 3-D image as well as in finding the correspondence between a live 2-D video frame and the 3-D frame of reference. The subsystem for correction image artifacts caused by misalignment of an ultrasound transducer may include applying a known (previously computed or measured) transformation to the transducer frame of reference throughout the procedure. This keeps the correspondence from transducer to the 3-D frame of reference updated throughout the procedure. The setup for computing the transformation may include an ultrasound phantom with landmarks (or beads) placed at known relative locations. The known relative locations in the phantom are used for alignment of the tracking device for the selected transducer. The setup for computing the transformation may also include alignment software which makes it easy for user to scan the ultrasound phantom and computes the transformation automatically after the user follows the directions for scanning the ultrasound phantom. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0028]      FIG. 1  illustrates a prostate imaging system. 
           [0029]      FIG. 2  illustrates an output with a display device of the prostate imaging system. 
           [0030]      FIG. 3  represents a typical zonal division of prostate used by many urologist as a map for 12-core biopsy. 
           [0031]      FIG. 4  represents urethra orientation in sagittal and transverse views of the prostate. 
           [0032]      FIG. 5  shows automatic selection of points from urethra delineation for finding mid-sagittal plane. 
           [0033]      FIG. 6  shows the computation of normal to the mid-sagittal plane based on automatically selected points along urethra. 
           [0034]      FIG. 7  represents various measurements needed to identify the zone a planned site is placed in. 
           [0035]      FIG. 8  represents an example of iso-surfaces that may be used for planning a biopsy based on the distance from prostate boundaries. 
           [0036]      FIG. 9  shows a virtual grid overlaid on the prostate for aid in planning or biopsy. 
           [0037]      FIG. 10  shows various distance measurements obtained from prostate boundary estimation overlaid on a live ultrasound image. 
           [0038]      FIG. 11  represents a ruler overlaid on the live ultrasound image based on distance from distal end of prostate boundary along the needle trajectory. 
           [0039]      FIG. 12  shows the measurements overlaid on 3-D prostate model after a biopsy. 
           [0040]      FIG. 13  shows the reconstruction of 3-D image from live 2-D ultrasound video from an ultrasound machine as the probe is rotated under tracking. 
           [0041]      FIG. 14  shows effect of a misalignment in axis of rotation on the reconstructed image. 
           [0042]      FIG. 15  represents the ultrasound video for detecting misalignment. 
           [0043]      FIG. 16  represents a phantom that could be created for correction of misalignment 
       
    
    
     DETAILED DESCRIPTION 
       [0044]    Reference will now be made to the accompanying drawings, which assist in illustrating the various pertinent features of the various novel aspects of the present disclosure. Although the invention is described primarily with respect to an ultrasound imaging embodiment, the invention is applicable to a broad range of imaging modalities and biopsy techniques, including MRI, CT, and PET, which are applicable to organs and/or internal body parts of humans and animals. In this regard, the following description is presented for purposes of illustration and description. Furthermore, the description is not intended to limit the invention to the form disclosed herein. Consequently, variations and modifications commensurate with the following teachings, and skill and knowledge of the relevant art, are within the scope of the present invention. 
         [0045]    Initially, an exemplary embodiment of the invention will be described in relation to performing prostate biopsy using transrectal ultrasound (TRUS) guidance. As shown in  FIG. 1 . The ultrasound probe  10  has a biopsy needle assembly  12  attached to its shaft inserted into the rectum from the patient&#39;s anus. The probe  10  is an end-fire transducer that has a scanning area of a fan shape emanating from the front end of the probe (shown as a dotted outline). The probe handle is held by a robotic arm (not shown) that has a set of position sensors  14 . These position sensors  14  are connected to the computer  20  of the imaging system  30  via an analog to digital converter. Hence, the computer  20  has real-time information of the location and orientation of the probe  10  in reference to a unified Cartesian (x, y, z) coordinate system. 
         [0046]    With the dimensions of the probe  10  and needle assembly  12  taken into the calculations, the 3D position of the needle tip and its orientation is known. The ultrasound probe  10  sends signal to the ultrasound system  30 , which may be connected to the same computer (e.g., via a video image grabber) as the output of the position sensors  14 . In the present embodiment, this computer is integrated into the imaging system  30 . The computer  20  therefore has real-time 2D and/or 3D images of the scanning area in memory  22 . The image coordinate system and the robotic arm coordinate system are unified by a transformation. Using the acquired 2D images, a prostate surface  50  (e.g., 3D model of the organ) and biopsy needle  52  are simulated and displayed on a display screen  40  with their coordinates displayed in real-time as best shown in  FIG. 2 . A biopsy needle may also be modeled on the display, which has a coordinate system so the doctor has the knowledge of the exact locations of the needle and the prostate. 
         [0047]    The computer system runs application software and computer programs which can be used to control the system components, provide user interface, and provide the features of the imaging system. The software may be originally provided on computer-readable media, such as compact disks (CDs), magnetic tape, or other mass storage medium. Alternatively, the software may be downloaded from electronic links such as a host or vendor website. The software is installed onto the computer system hard drive and/or electronic memory, and is accessed and controlled by the computer&#39;s operating system. Software updates are also electronically available on mass storage media or downloadable from the host or vendor website. The software, as provided on the computer-readable media or downloaded from electronic links, represents a computer program product usable with a programmable computer processor having computer-readable program code embodied therein. The software contains one or more programming modules, subroutines, computer links, and compilations of executable code, which perform the functions of the imaging system. The user interacts with the software via keyboard, mouse, voice recognition, and other user-interface devices (e.g., user I/O devices) connected to the computer system. 
         [0048]    Once the biopsy sites are selected, biopsy sample collection is preformed such that extracted tissues may be provided for pathological tests. As the position of the biopsy needle may be know in relation to the ultrasound image, the locations within the prostate from which biopsy samples are extracted may be saved into the ultrasound image. Once all biopsy locations are saved into the image, the composite image including the information associated with the biopsy locations may be stored for future use. 
         [0049]    Biopsies are typically performed with a thin, 18-gauge needle mounted on a spring-loaded gun connected to the ultrasound probe, forcing the needle to stay in the imaging plane so that it is always visible in the ultrasound image. Such needles typically take biopsy cores that are approximately 19 mm long and 1.8 mm in diameter. However, different sized cores may be taken with different sized needles and are within the scope of the present invention. Each core is separately identified as to its location, so that the pathologist can report the extent and grade of cancer or other cells of interest. Further, each core sample is marked to identify the end closest to the needle gun, or the end farthest from the needle gun. It is, therefore, desirable to know exactly where the initial sample was obtained in order to target more relevant tissue if a repeat biopsy is performed. 
         [0000]    Dividing Prostate into Regions 
         [0050]    A large number of custom biopsy plans used by physicians depend upon their interpretation of different prostate regions and many pathology reports report findings as such.  FIG. 3  Error! Reference source not found. gives an example of regions in prostate, which may be used to perform a biopsy. The figure shows a transverse view such that the top of image represents base of the prostate and the bottom represents the apex of the prostate. The image shows 12 zones: left-lateral-base, left-lateral-mid, left-lateral-apex, left-base, left-mid, left-apex, right-base, right-mid, right-apex, right-lateral-base, right-lateral-mid and right-lateral-apex. The image is representative of a biopsy plan. Different types of plans may be included provided that they are in some relation to the shape and orientation of the prostate. The zone identification subsystem in presented inventions not only computes the zones based on a urethra delineation or segmentation, but also can compute and report the zones for sampled biopsy cores. The definition of zones may be adjustable as per user&#39;s preferences, for example if the user wishes to only distinguish between left and right, the system can be adjusted by the user accordingly. As a result, the zone identification subsystem is very useful to do a planning as well as after the procedure such that the true region is reported in the pathology report. 
         [0051]    The urethra passes through center of the prostate and may be used as the main structure for identifying the zones of prostate. The urethra lies in roughly mid-sagittal plane and divides the prostate into left and right regions when looked in a coronal (front to back) direction. Therefore, the first step is to determine the plane containing urethra center line. There are two ways in which it can be done:
       1. At the beginning of the procedure, the user aligns the TRUS probe to the mid sagittal plane such that urethra can be seen along the slice. This image may be captured and a deformable model may be fitted on it to automatically detect the urethra, or it may be detected semi-automatically by user clicking on the base and apex points.   2. The user finds urethra in reconstructed 3-D ultrasound image by browsing through 3-D scan using multi-planar view and finding the slice where the urethra can be seen. The user then clicks on a number of points along the urethra and the information is stored in 3-D world coordinate systems.
 
Once the information about central line of urethra is available, it is easy to establish the mid-sagittal plane. The mid-sagittal plane can be defined by three non-collinear points along the urethra. The three points may be computed automatically for selection of most robust set of points. This may be done by selecting points P 0  and P 2  as the first and last points of the urethra center line and selecting point P 1  such that the triangle P 0 P 1 P 2  contains largest area. Let these points be P 0 , P 1  and P 2  (see  FIG. 4 , which shows the sagittal and transverse views of the prostate along with the urethra), where P 0  represents center of base, P 1  represents a point between base and apex, but not equal to either and P 2  represents Apex, respectively. Then the following vectors can be defined:
       
 
         [0000]    
       
      
       P 
       10 
       =P 
       0 
       −P 
       1  
      
     
         [0000]    
       
      
       P 
       12 
       =P 
       2 
       −P 
       1  
      
     
         [0000]    Then, the normal to the plane in the right direction can be computed as: 
         [0000]        n =( P   12   ×P   10 )/(∥ P   12   ∥∥P   10 ∥) 
         [0000]    (see  FIGS. 5 and 6 ). In the top of  FIG. 5 , three points P 0 , P 1  and P 2  defined along the urethra such that they are not collinear. In the bottom of  FIG. 5 , vectors P 10  and P 12  can be defined between the points.  FIG. 6  illustrates that normal to the mid-sagittal plane can be found using cross product of the two vectors lying in the mid sagittal plane. The combination (P 1 , n) completely defines the mid-sagittal plane. Likewise, a combination (P 1 ′, n 1 ) and (P 2 ′, n 2 ) can be defined to represent planes that divide the line joining P 0  and P 2  such that: 
         [0000]        n   1   =n   2 =( P   2   −P   0 )/∥ P   2   −P   0   ∥, P   1   ′=P   0 +( P   2   −P   0 )/3 and  P   2   ′=P   0 +2*( P   2   −P   0 )/3. 
         [0000]    Then, plane (P 1 ′, n 1 ) divides the prostate between base and mid regions of prostate and plane (P 2 ′, n 2 ) divides the prostate between mid and apex region of the prostate along the line from base to apex. 
         [0054]    The division between left and left-lateral may be done by dividing left part of the surface into half along the direction of the normal to the mid-sagittal plane. Likewise, a division between right and right-lateral planes may be made. Using same methodology, it is easy to distinguish between anterior and posterior regions of prostate, if desired, using the plane (P 1 , (n×n 1 ). 
         [0000]    The division of prostate between different zones is beneficial in not only planning, but also in relating pathological findings to a region. With quality of life after prostate cancer treatment being a major concern, it is extremely important to know what region the cancer lies in so that focal therapy can be delivered only to the malignant part of the gland. The first step towards finding the cancer is to locate the cores taken during biopsy such that the pathology found can be localized to a region. In order to do that, it is of interest to label a sampled core as one of the zones identified before sending it for pathological studies. The presented invention does it automatically such that during planning, it computes which region the planned site lies in and the zone corresponding to the sampled core is also automatically computed. This is done by first computing the 3-D location of the sampled core (or planned site) and then doing the checks for the point being on one side or another for each plane mentioned. This way, a point can be independently categorized to lie in one of the zones. Example below illustrates how to compute whether a point lies on left side of prostate or right side. The same is extended to all the planes dividing prostate into regions. 
         [0055]    Given a planned site S i , the following needs to be implemented to check whether the point lies on right side or left side of the plane: 
         [0000]    Find closest point S i ′ on the plane from S i  (see  FIG. 7 , which illustrates that given a biopsy site S i , the projection of site to the plane S i ′ can be computed and the direction can be determined), using the following relation: 
         [0000]        S   i   ′=S   i   −S   i   ·n /( n·n )* n.          1. Compute vector n i ′=(S i −S i′ )/|S i −S i′ ∥. This represents the normal from the plane to the biopsy site.   2. Compute dot product n i ·n i ′. If the value returned is 1, then the biopsy site is on right side of the urethra and if the value is −1, then the biopsy site S i  is on left side of the urethra.
 
If the urethra is not available, an approximation may be made for dividing a prostate into various zones. For example, if the orientation of prostate image is known, the prostate may be divided into left and right about halfway in sagittal (left to right) direction, and this may be treated as the mid-sagittal plane.
         
       Distance Measurements During a Procedure 
       [0058]    The invention contains subsystems to compute various distances during a procedure to aid in making clinical decisions. The following subsystems are discussed in more details:
       1. Distance of planned point from prostate boundaries   2. Automatic plan based on uniform sampling of prostate   3. Distance measurement from prostate boundaries on live 2-D ultrasound image before needle insertion   4. Distance measurements from prostate boundaries after a needle insertion.
 
Distance of Planned Point from Prostate Boundaries
       
 
         [0063]    It is often desirable for some procedures to know the distances from the boundaries of prostate in 3-D such that a user can select a point accordingly for performing a procedure. For this purpose, the system uses computed prostate boundaries to compute iso-surfaces at user-selected distances and overlays them onto grayscale ultrasound (or any other modality image for planning) image such that for a selected point, the user knows the distances in true 3-D. Iso-surfaces represent the surfaces computed such that each surface is at a user-defined distance from the prostate boundaries. The displays of grayscale volumes typically include three orthogonal. 2-D slices and it becomes hard for users to know the distances. The user may select more than one iso-surfaces to show at different distances from the boundaries.  FIG. 8  shows one such example where iso-surfaces computed at user-selected distances such that the user can use the distance information to plan a procedure. 
       Automatic Plan Based on Uniform Sampling of Prostate 
       [0064]    The invention contains a subsystem that overlays a grid with user-selected grid spacing over the prostate image for a trans-rectal or trans-perineal procedure. The subsystem computes the grid with lines spaced at the distance specified by the user and overlays it over the prostate grayscale image along the direction of view, as shown in  FIG. 9 . As shown, a virtual grid (or template) overlaid onto a) transrectal view of prostate and b) transperineal view of prostate. The grid may have a user-adjustable spacing and may automatically place planned sites on all grid elements lying inside the prostate that avoid certain regions. The user may then, either manually select sites based on this place for a plan, or automatically load a system generated plan customized to the prostate. The custom plan may be computed such that the user-specified regions such as urethra and neighboring organs and nerve bundles may be avoided during plan generation. This helps avoid accidental placement of needle for either biopsy or placing a bead at these sensitive locations during a procedure.  FIG. 9  shows the plans for trans-rectal and trans-perineal procedures. The images above show the plan (small white circles at intersection of grid lines) inside the prostate that avoids urethra. In such a procedure, the entire prostate is uniformly sampled at a user defined spacing to find and localize malignancies inside the prostate. 
         [0000]    Distance Measurement from Prostate Boundaries on Live 2-D Ultrasound Image before Needle Insertion 
         [0065]    The invention contains a subsystem for providing visual feedback to the user for reaching a planned target for biopsy or dose delivery procedure. The system is calibrated to different needle types used for these procedures. For a given needle type, the system computes the throw distance and displays to the user how deep the needle should be inserted such that the selected target lies at center of the needle core. This ensures that the planned target site is sampled properly. The planned site is displayed only when the distance of the planned site from the current field of view of live ultrasound view is a preset small value (say, 5 mm). In addition to showing how deep the needle should be inserted before firing, the system also shows how deep the needle will be penetrated following firing. All the computations are done in 3-D frame of reference of the acquired image. 
         [0066]    For a biopsy procedure, the user shall reach the target point as such: first, using the 3-D image guidance, align the needle guide with the target site. Then, as shown in  FIG. 10 , the target point is displayed as a circle and two lines appear on top and bottom of the live ultrasound video. The figure shows various distances in live image. Distance D 0  represents the distance of the point of firing from the top of the prostate as seen in the live view. Distance D 1  represents the distance from the top surface of prostate to the distal point of the core. Point A represents the depth of needle insertion before firing so that the center of the needle core contains the planned target site at its center. Point B represents the depth of needle penetration if it is fired after insertion up to point A. The bottom line displays how deep the needle should be inserted while the top line displays how deep the needle will go after firing. The user has to insert the needle up to the bottom line and fire the needle gun to collect the sample. 
         [0067]    It may not always be possible that the target site is selected such that the top line does not go beyond prostate and into the bladder or unwanted regions. In such cases, the system displays the top line to be beyond prostate boundaries and user may not follow the protocol of inserting the needle up to the bottom line. For these cases, a ruler is displayed on the live view as shown in  FIG. 11 . The ruler shows the distance from the prostate boundary along the needle guide. For a given needle guide, as the user inserts the needle along the needle trajectory, the system computes the needle trajectory in 3-D, finds intersection of the trajectory with the pre-computed prostate boundaries in 3-D and converts the intersection point back to frame of reference of 2-D image. The distance can be shown in cm (or mm) from the point of intersection of the needle guide with the segmented surface. The system then drops down a ruler from this point of intersection to give the user the distance measurement from the distal end of prostate such that the user can always see how far the needle tip is from the prostate boundaries and can thus avoid overshooting. 
         [0000]    Distance Measurements from Prostate Boundaries after a Needle Insertion 
         [0068]    In the presented invention, needle is segmented from the video captured during a sample extraction and the needle tip and trajectory are identified. Using tracking information, the 3-D location and trajectory of the needle can be identified in frame of reference of the 3-D image by applying transformation to the needle tip and entry points. The prostate surface is internally represented as a set of triangles connected together to form a closed surface. The intersection point of surface along the line of needle trajectory is computed by the system. The Euclidean distance can be directly computed from the needle tip to the intersection point on the surface. 
         [0069]    The line from the tip of needle to the surface is displayed as a dotted yellow line for visual verification. The distance is displayed in mm along the line as shown in  FIG. 12 , which shows the display of needle tip distance from the surface of prostate along the needle trajectory. The figure shows the display for the sampled biopsy site ( 1 ), the distance was computed to be 11.0 mm and the dotted line represents the trajectory up to the point of intersection. The following process is used for computing distance from top:
       1. Compute needle trajectory by transforming two points on needle guide into 3-D frame of reference.   2. For each triangle in prostate surface:
           a. Find intersection of line with the triangle plane   b. Find whether intersection point lies inside the triangle or outside   c. If intersection lies outside, move to next triangle   d. If intersection found, find whether intersection is along the needle trajectory in positive direction. This can be tested by computing dot product of triangle normal with the needle trajectory. The sign of dot product determines whether triangle is facing in to or facing away from the needle.   e. If intersection lies inside and towards distal end of the needle, compute perpendicular distance.   
               
 
         [0077]    Alternatively, if the intersection of the surface is already computed with the current plane, then the calculation can be performed in 2-D frame of reference of live image.  FIG. 13  shows the distances with reference to the prostate surface. The live image has 2-dimensions, and tracking information is used to compute the transformation matrix T VW  which transforms video coordinate system into software defined image coordinate system. 
       Alignment Correction 
       [0078]    When performing a procedure (biopsy or treatment) using tracked instruments, the tracking device assumes a known correspondence with the transducer during the procedure. The hardware is designed such that it holds the ultrasound transducer in a known geometry relative to the tracker. However, due to tolerances in mechanical system, there may be situations when the transducer is slightly misaligned relative to the tracking device. Such a misalignment can cause serious problems in not just image reconstruction but also during the procedure where the correspondence between live 2-D ultrasound video and the tracking device is incorrect.  FIG. 13  shows a reconstruction method where the video output from ultrasound machine is reconstructed into a 3-D volume by application of a transformation from video frame of reference to the 3-D image frame of reference. The tracking information is used to dynamically generate a transformation matrix T VW  that transforms the live video into the 3-D image frame of reference.  FIG. 14  shows an example where the transducer is not at the correct geometry with respect to the tracking device, causing reconstruction errors. A misalignment between assumed axis of rotation and actual axis of rotation (left panel) for a rotation-based image acquisition scheme may result in incorrect image reconstruction (right panel). The presented invention contains a subsystem that corrects for this misalignment by computing a correction matrix and always pre-multiplying the live image coordinates with this transformation matrix. 
         [0079]    In ultrasound image, if a point P 1V  can be identified in 2-D frame of reference, then with the probe rotated about its axis till it is in field of view on other side (half rotation), then the axis of rotation in the plane containing the pivot point P 0V , point P 1V  and the point of intersection of normal from P 0V  to the imaging axis can be directly computed ( FIG. 15 , Point P 1  in video frame of reference at the beginning). The process needs to be repeated four times for different points at different known locations so that the rotation matrix can be uniquely found. The rotation matrix R alignment  can be computed as such: 
       Fixing the Axis to One Line in Live 2-D Ultrasound Image: 
       [0080]    The first step to computing the axis of rotation is to fix an axis that can be robustly and reproducibly computed. For this, a phantom is constructed with three collinear points along a vertical line. When viewed from probe at home position, these points will show as a straight line, or the probe is maneuvered till these points show up on screen at the same time. The line passing through these points will then be computed and stored in memory as ground truth locations and the center of imaging axis in this probe orientation is fixed as the rotational axis. 
       Computing Rotation Matrix: 
       [0081]    The phantom also contains additional points (at least 4 more points) at various depths and distances. Care must be taken in design to ensure that the points are linearly independent, i.e., not more than 2 points are collinear and not more than 3 points are co-planar. The locations of these points are known in advance relative to the three collinear points as discussed in (1) above. These locations represent real world coordinate locations (x i ,y i ,z i ) for i=0, 1, 2, 3 relative to the origin at probe tip center with probe oriented as in step (1). The probe is rotated to face each point, one at a time in the upper half of the probe and the position of the point converted into world coordinate system is noted for each point. This provides a set of four points (x′ i , y′ i , i=0, 1, 2, 3 related to their real world coordinates as follows: 
         [0000]    
       
         
           
             
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         [0000]    Four such equations can be combined together to find R uniquely. In order to make R less sensitive to the error in accurate placement of the points in live ultrasound video and in machining the phantom, more than four points may be used and R may be computed as a least square fit solution as follows: 
         [0000]      R=YX −1    
         [0082]    Where, 
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         [0000]    Once R is computed, the video to world transformation matrix is always post-multiplied or concatenated over the R matrix. 
         [0083]    The actual relative location of all the beads may be stored in a alignment correction file on the system. The software records the perceived 3-D locations of the beads and finds the rotation matrix that aligns them and stores in a calibration file queried by the software for that probe. The transformation matrix may be computed using a phantom box as shown in  FIG. 16 , where point P 1  in video frame of reference at the beginning. The ultrasound phantom may be constructed from Agar and may contain a number of beads in known non-planar locations. The ultrasound transducer mounted on the tracking device is used to scan the beads in a sequential order. The computed transformed locations are compared with the known locations of the beads in the phantom and the system automatically computes the correction factor. 
         [0084]    The foregoing description of the present invention has been presented for purposes of illustration and description. Furthermore, the description is not intended to limit the invention to the form disclosed herein. Consequently, variations and modifications commensurate with the above teachings, and skill and knowledge of the relevant art, are within the scope of the present invention. The embodiments described hereinabove are further intended to explain best modes known of practicing the invention and to enable others skilled in the art to utilize the invention in such, or other embodiments and with various modifications required by the particular application(s) or use(s) of the present invention. It is intended that the appended claims be construed to include alternative embodiments to the extent permitted by the prior art.