Abstract:
In order to increase the accuracy and speed of catheter reconstruction in surgical procedures such as an HDR prostate implant procedure, an automatic tracking system is provided preferably using an electromagnetic tracking device. The system uses a transmitter with a sensor used for catheter position. Due to substantial interference in the electromagnetic field from the surgical table, implant stepper/stabilizer etc, a calibration algorithm using a scattered data interpolation scheme is implemented to correct tracking location errors. The invention includes methods and systems used to carry out the methods.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to methods and systems usable in human and animal surgical procedures. For example, the invention is applicable in the field of human brachytherapy treatment procedures. 
       BACKGROUND OF THE INVENTION 
       [0002]    In typical brachytherapy surgical procedures, a physician inserts a number of hollow catheters into a target structure within the human body. The number and location of the catheters is determined by a treatment plan, prescribed by a physician based on imaging studies usually done prior to treatment and many other factors. Often, a grid-like guide template structure is used as a guide for catheter insertion having insertion passages arranged in an orthogonal grid pattern. After inserting a number of such catheters at the prescribed loading position and depth, radioisotope sources are either placed permanently in the tissue as “seeds” (low dose rate or LDR brachytherapy), or are loaded into the catheters and are moved robotically inside the catheter to expose tissue surrounding the catheter to a desired radiation dose and then removed (high dose rate “HDR” brachytherapy). The radiation exposure dose is intended to cause radiotoxicity and destroy targeted human tissue, for example cancerous tumors or other structures. One application of this technique is in the area of human prostate brachytherapy. Among other applications, these techniques are also useful for human esophageal brachytherapy. 
         [0003]    In human prostrate brachytherapy, many catheters are placed at desired positions using a locating template, positioned on the patient&#39;s perineum. However, due to structural characteristics of the catheters, their tips, and density variations in the human tissue, the insertion paths and final positions of the catheters cannot be assumed to be along straight lines extending from the template. Since the actual position of the catheters is critical to provide desired dose application, the radiologist needs confirmation of the catheter placements. This is presently done through ultrasonic imaging procedures. Unfortunately, the ultrasonic procedure used for human prostrate brachytherapy does not provide a clear image of catheter placement. There are numerous artifacts in the image reconstruction and, moreover, there are fundamental limits in the use of a rectally inserted ultrasonic probe during catheter placement procedures. For a real-time ultrasound guided HDR prostate implant procedure, catheter reconstruction has always been challenging and time consuming. This is due in part to many factors including high speckle noise, inter-needle interference, artifacts from calcifications, hyper-echoic tissues, and coil markers for external beam treatment. Furthermore, the catheters are always not straight. They are often curved either inadvertently, or intentionally to reduce normal tissue dose and increase conformity, making the reconstruction of catheter geometry even more difficult. 
         [0004]    In view of the foregoing, there is a need for a detection system which provides higher accuracy and a reduction in evaluation time for verifying catheter placement for procedures such as LDR or HDR brachytherapy. 
         [0005]    This invention describes a novel system to perform real-time catheter tracking. This system will significantly improve catheter reconstruction speed and accuracy while increasing operator confidence in precise dose delivery. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1(   a ) is a schematic diagram of an electromagnetic tracking system in accordance with one embodiment of the present invention. 
           [0007]      FIG. 1(   b ) is a pictorial view of an electromagnetic tracking system in accordance with one embodiment of the present invention. 
           [0008]      FIG. 2  is a screenshot of a graphical user interface (GUI) in accordance with an embodiment of the present invention. 
           [0009]      FIGS. 3(   a )- 3 ( f ) are graphical views of catheter tracking results produced by an embodiment of the present invention before calibration;  FIGS. 3(   a ),  3 ( c ), and  3 ( e ), and after calibration;  FIGS. 3(   b ),  3 ( d ), and  3 ( f ).  FIGS. 3(   a ) and  3 ( b ) are x-y plots,  FIGS. 3(   c ) and  3 ( d ) are x-z plots, and  FIGS. 3(   e ) and  3 ( f ) are y-z plots. 
           [0010]      FIG. 4(   a ) is a graphical view of tracking results of catheter placement produced by an embodiment of the present invention. 
           [0011]      FIG. 4(   b ) is a graphical view of tracking results of catheter placement produced using CT-based catheter reconstruction. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    In accordance with this invention, an electromagnetic tracking system  10  is employed. The tracking system  10  as shown in  FIG. 1(   a ) utilizes a transmitter unit  12 , preferably one using so-called passive magnetic DC technology (e.g. products available from Ascension Technology Corporation including their “3D Guidance driveBAY”, or “3D Guidance trakSTAR” systems). It is also possible to other tracking systems  10  in accordance with this invention, including those using passive magnetic AC technology. Tracking system  10  include the transmitter  12  mentioned previously, along with one or more miniature sensors  14  which are small enough in size to be inserted into brachytherapy catheters  22  (catheters  22  may also be referred to as “needles”), shown in  FIG. 1(   b ). The system  10  allows the relative position between the transmitter  12  and sensor  14  to be detected and displayed. Catheters  22  have a distal end  28 , proximal end  30 , and a hollow lumen  32  therebetween. 
         [0013]    Systems utilizing passive magnetic DC (or AC) technology like system  10  are inherently influenced by surrounding structures of magnetic materials. In the particular applications considered here, a patient on a surgical couch or operating table  26  during a brachytherapy catheter placement procedure has numerous metallic structures near the surgical site, including the table, surgical tools, and the brachytherapy catheter placement system. These metallic structures are sources of interference. It is therefore necessary in accordance with this invention to correct measured position values using the aforementioned passive magnetic DC (or AC) technology systems to actual positions. For other electromagnetic systems for example using radio frequency or other location systems, it is expected that structures of the surgical site will also be sources of measurement interference requiring correction, thereby also requiring correction. 
         [0014]    Both the transmitter  12  and the sensor  14  are connected to control box  16  controlled by a computer  34  through USB cable  18 . An exemplary transmitter  12  has a range of 36 cm and is placed on a supporting bracket  20 , as shown in  FIG. 1(   b ), that can be positioned close to the surgical site and the catheters  22 . An exemplary sensor  14  has a diameter of 0.9 mm and can be inserted into 16-gauge needles or catheter lumens  32 .  FIG. 1(   b ) further shows an ultrasonic probe attached to a stepper unit to move forward and backward for imaging the prostate as part of HDR brachytherapy treatment. That figure further shows a three-dimensional grid like phantom structure  38  used to demonstrate the present invention, and provide system calibration. Structure  38  has grid plates  40  and  42  having apertures for receiving catheters  22  and positioning them in desired orientations. 
         [0015]      FIG. 2  shows the graphical user interface (GUI) image  24  of the program used to control the system  10 . The tracking process in accordance with this invention is conducted in the following steps: 1) after finishing insertion of a plurality of catheters  22  into the patient at the surgical site, sensor  14  is inserted into the proximal end  30  of one catheter  22 , and driven to the distal end  28 ; 2) click the “Start Tracking” button on the GUI and then retract the sensor  14  out of the catheter  22 ; 3) once the sensor  14  is out of the catheter  22 , click the “Stop Tracking” button on the GUI. During the above process, transmitter  12  and sensor  14  are activated to provide tracking. The tracking data corresponds to the catheter  22  will be saved to the plan; 4) go to the next catheter  22  and repeat the previous steps for all catheters; 5) apply calibration (described below) to the tracking result (the calibration can also be applied during the tracking process); 6) export the tracking results (RT plan) to the treatment planning system for planning. Since the sensor  14  is physically constrained to move along the catheter lumen  32 , detecting its path also describes the shape and position of the inserted catheters  22 . Calibration could also be conducted during insertion of sensor  14 , i.e. “Start Tracking” could be done during sensor  14  insertion rather than during retraction as mentioned above. Moreover, tracking could be done in both directions if desired. 
         [0016]    Calibration is accomplished using a calibration algorithm involving a scattered data interpolation scheme. The QA phantom structure  38  with known catheter positions (shown in  FIG. 1(   b )) is used for calculating calibration profiles.  FIGS. 3(   a )- 3 ( f ) shows orthogonal views of the tracking results for the  10  catheters  22  displayed in the right panel of  FIG. 2  using phantom  38 . The reconstruction results before correction ( FIGS. 3(   a ),  3 ( c ), and  3 ( e )) and after correction ( FIGS. 3(   b ),  3 ( d ), and  3 ( f )) are shown. As shown in  FIGS. 3(   a ),  3 ( c ), and  3 ( e ), the system&#39;s accuracy degrades as the sensor-transmitter distance increases. In one experiment using the present invention tracking at distances of 140 mm to 280 mm was conducted. However, after calibration, the error can be minimized as shown in  FIGS. 3(   b ),  3 ( d ), and  3 ( f ). Once the actual positions of the catheters  22  are known, treatment plan modification can be made to provide desired dosing. Once the calibration factors for a particular surgical arrangement are developed using the phantom structure  38 , the assumption is made that patient-to-patient differences are small as related to the calibration. The calibration factors determined as described above are used to modify detected positions of catheters positioned in a patient to more closely determine actual catheter placement. 
         [0017]    As mentioned previously, calibration is needed due to the influences of surrounding magnetic structures and other sources of interference. Even without such interference however, calibration will be needed since outputs are affected by the position of transmitter  12  relative to catheters  22 . Accordingly, it is necessary that the relationship between the position of transmitter  12  and the catheters  22  is reproduced between establishing the correction process using the phantom structure  38  and during surgical procedures. 
         [0018]    As a reproducibility study for the present invention, the calibration profiles were tested under various equipment arrangements. While the profiles are sensitive to the relative position between the transmitter  12  and the operating table  26 , reasonable position variations of the stepper, ultrasound machine, and leg stirrups (sources of transmitter-sensor tracking errors) introduce &lt;1 mm error. 
         [0019]    To further validate the system  10 , straight catheters  22  in the QA phantom structure  38  were bended and tracked with the system as shown in  FIG. 4(   a ). To verify the corrected catheter positions, the phantom  38  was then scanned with CT (computed tomography) and the catheters  22  were reconstructed in the Oncentra® Brachy, as shown in  FIG. 4(   b ). The CT scanned positions are used as a baseline of actual catheter positions. It should be noted that CT scanning of catheter placements is not preferred for patient use due to cost, complexity, and patient radiation dose exposure, but is used here to validate the inventive approach. In an experiment for demonstrating the present invention, average tracking accuracies after calibration were found to be 0.4±0.3 mm; and 2.4±1.7 mm without calibration. The max standard deviation was 0.9 mm in the test range for the reproducibility test. Thus, the calibration steps used in this invention significantly improved catheter position determination. The total tracking time for ten catheters  22  was less than four minutes and the reconstruction result matches CT data within 2.0 mm. 
         [0020]    Compared to conventional ultrasound based real-time catheter reconstruction method in the HDR prostate implant; the system  10  of this invention can reduce the error from &gt;3 mm to &lt;1.5 mm, and shorten the procedure time from 15-60 minutes to &lt;4 minutes. Furthermore, this technique can also be used for other HDR implants. 
         [0021]    While the present invention has been described in terms of certain preferred embodiments, it will be understood that the invention is not limited to the disclosed embodiments, as those having skill in the art may make various modifications without departing from the scope of the following claims.