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Background Uncorrected patient or prostate motion may impair targeting prostate areas during fusion image-guided procedures. We evaluated if a prototype “tracked Foley catheter” (TFC) could maintain fusion image alignment after simulated organ motion. Methods A pelvic phantom model underwent magnetic resonance imaging (MRI), and the prostate was segmented. The TFC was placed in the phantom. MRI/ultrasound (US) fusion was performed. Four trials were performed varying motion and TFC presence/absence: (1) TFC/no-motion, (2) TFC/motion, (3) no-TFC/no-motion, and (4) no-TFC/motion. To quantify image alignment, screen captures generated Dice similarity coefficient (DSC) and offset distances (ODs) (maximal US-to-MRI distance between edges on fusion images). Three anatomical targets were identified for placement of a needle under fusion guidance. A computed tomography scan was used to measure system error (SE), i.e., the distance from needle tip to intended target. Results The TFC presence improved MRI/US alignment by DSC 0.88, 0.88, 0.74, and 0.61 in trials 1, 2, 3, and 4, respectively. Both OD (trial 2 versus trial 4, 4.85 ± 1.60 versus 25.29 ± 6.50 mm, p < 0.001) and SE (trial 2 versus trial 4, 6.35 ± 1.31 versus 32.16 ± 6.50 mm, p < 0.005) were significantly lower when the TFC was present after artificial motion, and significantly smaller OD when static (trial 1 versus trial 3, 4.29 ± 1.24 versus 6.42 ± 2.29 mm, p < 0.001). Conclusion TFC provided better image alignment with or without simulated motion. This may overcome system limitations, allowing for more accurate fusion image alignment during fusion-guided biopsy, ablation, or robotic prostatectomy.

Prostate cancer is often over treated with standard treatment options which impact the patients’ quality of life. Laser ablation has emerged as a new approach to treat prostate cancer while sparing the healthy tissue around the tumor. Since laser ablation has a small treatment zone with high temperature, it is necessary to use accurate image guidance and treatment planning to enable full ablation of the tumor. Intraoperative temperature monitoring is also desirable to protect critical structures from being damaged in laser ablation. In response to these problems, we developed a navigation platform and integrated it with a clinical MRI scanner and a side firing laser ablation device. The system allows imaging, image guidance, treatment planning and temperature monitoring to be carried out on the same platform. Temperature sensing phantoms were developed to demonstrate the concept of iterative treatment planning and intraoperative temperature monitoring. Retrospective patient studies were also conducted to show the clinical feasibility of the system.

A novel platform was developed that fuses pre-biopsy magnetic resonance imaging with real-time transrectal ultrasound imaging to identify and biopsy lesions suspicious for prostate cancer. The cancer detection rates for the first 101 patients are reported.This prospective, single institution study was approved by the institutional review board. Patients underwent 3.0 T multiparametric magnetic resonance imaging with endorectal coil, which included T2-weighted, spectroscopic, dynamic contrast enhanced and diffusion weighted magnetic resonance imaging sequences. Lesions suspicious for cancer were graded according to the number of sequences suspicious for cancer as low (2 or less), moderate (3) and high (4) suspicion. Patients underwent standard 12-core transrectal ultrasound biopsy and magnetic resonance imaging/ultrasound fusion guided biopsy with electromagnetic tracking of magnetic resonance imaging lesions. Chi-square and within cluster resampling analyses were used to correlate suspicion on magnetic resonance imaging and the incidence of cancer detected on biopsy.Mean patient age was 63 years old. Median prostate specific antigen at biopsy was 5.8 ng/ml and 90.1% of patients had a negative digital rectal examination. Of patients with low, moderate and high suspicion on magnetic resonance imaging 27.9%, 66.7% and 89.5% were diagnosed with cancer, respectively (p0.0001). Magnetic resonance imaging/ultrasound fusion guided biopsy detected more cancer per core than standard 12-core transrectal ultrasound biopsy for all levels of suspicion on magnetic resonance imaging.Prostate cancer localized on magnetic resonance imaging may be targeted using this novel magnetic resonance imaging/ultrasound fusion guided biopsy platform. Further research is needed to determine the role of this platform in cancer detection, active surveillance and focal therapy, and to determine which patients may benefit.

Purpose To show utility, accuracy, and clinical outcomes of electromagnetic tracking and multimodality image fusion for guidance of biopsy and radiofrequency (RF) ablation procedures. Materials and Methods A combination of conventional image guidance (ultrasound[US]/computed tomography [CT]) and a research navigation system was used in 40 patients undergoing biopsy or RF ablation to assist in target localization and needle and electrode placement. The navigation system displays electromagnetically tracked needles and US images relative to a preprocedural CT scan. Additional images (prior positron emission tomography [PET] or magnetic resonance [MR] imaging) can be fused with CT as needed. Needle aiming with and without tracking were compared, the utility of navigation for each procedure was assessed, the system's off-target tracking error for two different registration methods was evaluated, and setup time was recorded. Results The tracking error could be evaluated in 35 of 40 patients. A basic tracking error of 3.8 mm ± 2.3 was shown using skin fiducial markers for registration. The error improved to 2.7 mm ± 1.6 when using prior internal needle positions as additional fiducial markers. Real-time fusion of US with CT and registration with prior PET and MR imaging were successful and provided clinically relevant guidance information, enabling 19 of the 40 procedures. Conclusions The spatial accuracy of the navigation system is sufficient to display clinically relevant image guidance information during biopsy and RF ablation. Breath holding and respiratory gating are effective in minimizing the error associated with tissue motion. In 48% of cases, the navigation system provided information crucial for successful execution of the procedure. Fusion of real-time US with CT or prior diagnostic images may enable procedures that are not feasible with standard, single-modality image guidance.

We determined whether there is a correlation between D'Amico risk stratification and the degree of suspicion of prostate cancer on multiparametric magnetic resonance imaging based on targeted biopsies done with our electromagnetically tracked magnetic resonance imaging/ultrasound fusion platform.A total of 101 patients underwent 3 Tesla multiparametric magnetic resonance imaging of the prostate, consisting of T2, dynamic contrast enhanced, diffusion weighted and spectroscopy images in cases suspicious for or with a diagnosis of prostate cancer. All prostate magnetic resonance imaging lesions were then identified and graded by the number of positive modalities, including low-2 or fewer, moderate-3 and high-4 showing suspicion on multiparametric magnetic resonance imaging. The biopsy protocol included standard 12-core biopsy, followed by real-time magnetic resonance imaging/ultrasound fusion targeted biopsies of the suspicious magnetic resonance lesions. Cases and lesions were stratified by the D'Amico risk stratification.In this screening population 90.1% of men had a negative digital rectal examination. Mean±SD age was 62.7±8.3 years and median prostate specific antigen was 5.8 ng/ml. Of the cases 54.5% were positive for cancer on protocol biopsy. Chi-square analysis revealed a statistically significant correlation between magnetic resonance suspicion and D'Amico risk stratification (p0.0001). Within cluster resampling demonstrated a statistically significant correlation between magnetic resonance suspicion and D'Amico risk stratification for magnetic resonance targeted core biopsies and magnetic resonance lesions (p0.01)Our data support the notion that using multiparametric magnetic resonance prostate imaging one may assess the degree of risk associated with magnetic resonance visible lesions in the prostate.

Several new image-guidance tools and devices are being prototyped, investigated, and compared. These tools are introduced and include prototype software for image registration and fusion, thermal modeling, electromagnetic tracking, semiautomated robotic needle guidance, and multimodality imaging. The integration of treatment planning with computed tomography robot systems or electromagnetic needle-tip tracking allows for seamless, iterative, “see-and-treat,” patient-specific tumor ablation. Such automation, navigation, and visualization tools could eventually optimize radiofrequency ablation and other needle-based ablation procedures and decrease variability among operators, thus facilitating the translation of novel image-guided therapies. Much of this new technology is in use or will be available to the interventional radiologist in the near future, and this brief introduction will hopefully encourage research in this emerging area.

This paper investigates the possibility of using the motion of a patient's anterior surface in combination with a motion model to compensate for internal respiratory motion during tracked biopsies. Position data from two electromagnetically tracked sensors, one placed on the patient's sternum, the other incorporated into a biopsy needle, were acquired during a liver biopsy. The data were used to evaluate the correlation between the position measurements of the two sensors and to derive an affine motion model to assess respiratory motion compensation for image-guided interventional procedures. The correlation reached up to 94% for ranges of steady respiration. The residual motion of the internal sensor after compensation is reduced by a factor of approximately four.

An experimental workstation and surgical tools are presented utilizing electro-magnetic tracking to register real-time laparoscopic ultrasound (LUS) images with static, pre-acquired CT volumes. The registration and image fusion were used to guide tracked needles to target locations in an abdominal phantom. A custom-made sleeve equipped with magnetic sensor coils and attached to the steerable tip of a commercial LUS probe allowed precise localization of the imaging array without line-of-sight restrictions. For registration with CT image space, landmarks in physical space were identified using three different methods: (1) by pointing with a separate, electro-magnetically tracked pointer, (2) by pointing with the tip of the laparoscope, or (3) by imaging and subsequent manual identification of the landmark with LUS. The root-mean square (RMS) registration error using the three methods was 1.5 mm, 1.4 mm, and 1.1 mm, respectively. Overlays of real-time LUS with pre-acquired CT demonstrated the registration quality. Based on these registrations, tracked needles were guided into user-defined target locations. The RMS placement accuracy was 1.8 mm, 1.3 mm, and 1.9 mm, respectively. Accuracy is sufficient for planned clinical trials.

Position data from two electromagnetically tracked sensors, one placed on the patient's sternum, the other incorporated into a biopsy needle, were acquired during a liver biopsy. The data were used to evaluate the correlation between the position measurements of the two sensors and to derive an affine motion model to assess the possibility of using an external sensor for respiratory motion compensation for image-guided interventional procedures.

To assess the feasibility of the use of preprocedural imaging for guide wire, catheter, and needle navigation with electromagnetic tracking in phantom and animal models.An image-guided intervention software system was developed based on open-source software components. Catheters, needles, and guide wires were constructed with small position and orientation sensors in the tips. A tetrahedral-shaped weak electromagnetic field generator was placed in proximity to an abdominal vascular phantom or three pigs on the angiography table. Preprocedural computed tomographic (CT) images of the phantom or pig were loaded into custom-developed tracking, registration, navigation, and rendering software. Devices were manipulated within the phantom or pig with guidance from the previously acquired CT scan and simultaneous real-time angiography. Navigation within positron emission tomography (PET) and magnetic resonance (MR) volumetric datasets was also performed. External and endovascular fiducials were used for registration in the phantom, and registration error and tracking error were estimated.The CT scan position of the devices within phantoms and pigs was accurately determined during angiography and biopsy procedures, with manageable error for some applications. Preprocedural CT depicted the anatomy in the region of the devices with real-time position updating and minimal registration error and tracking error (5 mm). PET can also be used with this system to guide percutaneous biopsies to the most metabolically active region of a tumor.Previously acquired CT, MR, or PET data can be accurately codisplayed during procedures with reconstructed imaging based on the position and orientation of catheters, guide wires, or needles. Multimodality interventions are feasible by allowing the real-time updated display of previously acquired functional or morphologic imaging during angiography, biopsy, and ablation.

We developed and tested a small electromagnetically tracked 5DOF bone screw. We examined accuracy compared to conventional optical tracking systems and developed devices and techniques for implanting the sensor during an interventional percutaneous procedure. Finally we conducted “proof-of-concept” experiments for percutaneous spine procedures by implanting the tracking sensors in the vertebral body of an interventional phantom and cadaver. Experiments were done in an interventional fluoroscopy suite to evaluate the susceptibility of the magnetic tracking system to potential interference sources in a hospital setting. Despite the presence of environmental metal, we were able track it robustly throughout the volume of interest. By using two or more of these screws, a rigid body reference definition was created. In image space, fiducials embedded in the screws were easily visualized on fluoroscopic images. Orientation errors of the new screw compared to the RCM-PAKY robot averaged 0.8° compared to 0.2° for the bare coil. Positioning error compared to the robot averaged 1.7 mm for the screw and 1.9 mm for the bare coil. Largest discrepancies occurred at the extremes of the volume. When using six DOF targets (employing two coils), positioning error averaged 1.5 mm. The magnetically tracked bone screw has good potential for reducing the invasiveness of many procedures that currently require separate registration and tracking. Accuracy is acceptable in the operating room, and the technique is not a major departure from current clinical practice.

A general-purpose augmented reality apparatus was developed. The system uses rapidly scanned lasers to display information directly onto the patient using the patient as a “projection screen”. The device is also capable of measuring the location of projected laser spots with a 3D camera to ensure that the display is accurate, to apply corrections to the projection path and to assist in registration. The projection system has its own Application Programmer's Interface (API) and is a stand-alone add-on unit to any host computer system. An initial host application was developed to demonstrate the new unit. Fiducials representing vertices along a proposed craniotomy were embedded into a plastic skull and a projection path was calculated. A feedback-based optimization of the plan was performed by comparing the measurement taken by the camera of these coordinates. The optimized plan was projected onto the skull. On average, the projection deviated by approximately 1 mm from the plan. Possible applications include identification of critical anatomical structures, visualization of preplanned paths and targets, and telesurgery or teleconsultation applications.

To assess the feasibility of combined electromagnetic device tracking and computed tomography (CT)/ultrasonography (US)/fluorine 18 fluorodeoxyglucose (FDG) positron emission tomography (PET) fusion for real-time feedback during percutaneous and intraoperative biopsies and hepatic radiofrequency (RF) ablation.In this HIPAA-compliant, institutional review board-approved prospective study with written informed consent, 25 patients (17 men, eight women) underwent 33 percutaneous and three intraoperative biopsies of 36 FDG-avid targets between November 2007 and August 2010. One patient underwent biopsy and RF ablation of an FDG-avid hepatic focus. Targets demonstrated heterogeneous FDG uptake or were not well seen or were totally inapparent at conventional imaging. Preprocedural FDG PET scans were rigidly registered through a semiautomatic method to intraprocedural CT scans. Coaxial biopsy needle introducer tips and RF ablation electrode guider needle tips containing electromagnetic sensor coils were spatially tracked through an electromagnetic field generator. Real-time US scans were registered through a fiducial-based method, allowing US scans to be fused with intraprocedural CT and preacquired FDG PET scans. A visual display of US/CT image fusion with overlaid coregistered FDG PET targets was used for guidance; navigation software enabled real-time biopsy needle and needle electrode navigation and feedback.Successful fusion of real-time US to coregistered CT and FDG PET scans was achieved in all patients. Thirty-one of 36 biopsies were diagnostic (malignancy in 18 cases, benign processes in 13 cases). RF ablation resulted in resolution of targeted FDG avidity, with no local treatment failure during short follow-up (56 days).Combined electromagnetic device tracking and image fusion with real-time feedback may facilitate biopsies and ablations of focal FDG PET abnormalities that would be challenging with conventional image guidance.

Purpose To determine the feasibility of electromagnetic tracking as a method to augment conventional imaging guidance for the safe delivery, precise positioning, and accurate deployment of thoracic aortic endografts. Materials and Methods Custom guide wires were fabricated, and the delivery catheters for thoracic aortic endoprostheses were retrofitted with integrated electromagnetic coil sensors to enable real-time endovascular tracking. Preprocedure thoracic computed tomographic (CT) angiograms were obtained after the placement of fiducial skin patches on the chest wall of three anesthetized swine, enabling automatic registration. The stent-graft deployment location target near the subclavian artery was selected on the preprocedure CT angiogram. Two steps were analyzed: advancing a tracked glidewire to the aortic arch and positioning the tracked stent-graft assembly by using electromagnetic guidance alone. Multiple CT scans were obtained to evaluate the accuracy of the electromagnetic tracking system by measuring the target registration error, which compared the actual position of the tracked devices to the displayed “virtual” electromagnetic-tracked position. Postdeployment CT angiography and necropsy helped confirm stent-graft position and subclavian artery patency. Results A stent-graft was successfully delivered and deployed in each of the three animals by using real-time electromagnetic tracking alone. The mean fiducial registration error with autoregistration was 1.5 mm. Sixteen comparative scans were obtained to determine the target registration error, which was 4.3 mm ± 0.97 (range, 3.0–6.0 mm) for the glidewire sensor coil. The mean target registration error for the stent-graft delivery catheter sensor coil was 2.6 mm ± 0.7 (range, 1.9–3.8 mm). The mean deployment error for the stent-graft, defined as deployment deviation from the target, was 2.6 mm ± 3.0. Conclusions Delivery and deployment of customized thoracic stent-grafts with use of electromagnetic tracking alone is feasible and accurate in swine. Combining endovascular electromagnetic tracking with conventional fluoroscopy may further improve accuracy and be a more realistic multimodality approach.

In this paper, a system for fusion of realtime transrectal ultrasound (TRUS) with pre-acquired 3D images of the prostate is presented with a clinical demonstration on a cohort of 101 patients with suspicion of prostate cancer. Electromagnetically tracked biopsy guides for endocavity ultrasound transducers were calibrated and used to fuse MRI-based suspicious lesion locations with ultrasound image coordinates. The prostate shape is segmented from MRI in a semi-automated fashion via a model-based approach, and intraoperative image registration is performed between MR and ultrasound image space to superimpose target fiducials markers on the ultrasound image. In order to align both modalities, a surface model is automatically extracted from 2D swept TRUS images using a partial active shape model, utilizing image features and prior statistics. An automatic prostate motion compensation algorithm can be triggered as needed. The results were used to display live TRUS images fused with spatially corresponding realtime multiplanar reconstructions (MPRs) of the MR image volume. In this study, all patients were scanned with 3T MRI and TRUS for biopsy. Clinical results show significant improvement of target visualization and of positive detection rates during TRUS-guided biopsies. It also demonstrates the feasibility of realtime MR/TRUS image fusion for out-of-gantry procedures.

This paper presents an ultrasound guidance system for needle placement procedures. The system integrates a real-time 3D ultrasound transducer with a 3D localizer and a tracked needle to enable real-time visualization of the needle in ultrasound. The system uses data streaming to transfer real-time ultrasound volumetric images to a separate workstation for visualization. Multi-planar reconstructions of the ultrasound volume are computed at the workstation using the tracking information, allowing for real-time visualization of the needle in ultrasound without aligning the needle with the transducer. The system may simplify the needle placement procedure and potentially reduce the levels of skill and training needed to perform accurate needle placements. The physician can therefore focus on the needle placement procedure without paying extra attention to perfect mid-plane alignment of the needle with the ultrasound image plane. In addition, the physician has real-time visual feedback of the needle and the target, even before the needle enters the patient's skin, allowing the procedure to be easily, safely and accurately planned. The superimposed needle can also greatly improve the sometimes poor visualization of the needle in an ultrasound image (e.g. in between ribs). Since the free-hand needle is not inserted through any fixed needle channel, the physician can enjoy full freedom to select the needle's orientation or position. No cumbersome accessories are attached to the ultrasound transducer, allowing the physician to use his or her previous experience with regular ultrasound transducers. 3D Display of the target in relation to the treatment volume can help verify adequacy of tumor ablation as well.

Organ motion was quantified and motion compensation strategies for soft-tissue navigation were evaluated in a porcine model. Organ motion due to patient repositioning, and respiratory motion during ventilated breathing were quantified. Imaging was performed on a 16-slice CT scanner. Organ motion due to repositioning was studied by attaching 7 external skin fiducials and inserting 7 point fiducials in the livers of ventilated pigs. The pigs were imaged repeatedly in supine and decubitus positions. Registrations between the images were obtained using either all external fiducials or 6 of the 7 internal fiducials. Target registration errors (TRE) were computed by using the leave-one-out technique. Respiratory organ motion was studied by inserting 7 electromagnetically (EM) tracked needles in the livers of 2 pigs. One needle served as primary target, the remaining six served as reference needles. In addition, 6 EM tracked skin fiducials, 5 passive skin fiducials, and one dynamic reference tracker were attached. Registrations were obtained using three different methods: Continuous registration with the tracking data from internal and external tracked fiducials, and one-time registration using the passive skin fiducials and a tracked pointer with dynamic reference tracking. The TRE for registering images obtained in supine position after an intermittent decubitus position ranged from 3.3 mm to 24.6 mm. Higher accuracy was achieved with internal fiducials (mean TRE = 6.4 mm) than with external fiducials (mean TRE = 16.7 mm). During respiratory motion, the FRE and TRE were shown to be correlated and were used to demonstrate automatic FRE-based gating. Tracking of target motion relative to a reference time point was achieved by registering nearby reference trackers with rigid and affine transformations. Linear motion models based on external and internal reference trackers were shown to reduce the target motion by up to 63% and 90%, respectively.

OBJECTIVE To evaluate the feasibility and utility of registration and fusion of real-time transrectal ultrasonography (TRUS) and previously acquired magnetic resonance imaging (MRI) to guide prostate biopsies. PATIENTS AND METHODS Two National Cancer Institute trials allowed MRI-guided (with or with no US fusion) prostate biopsies during placement of fiducial markers. Fiducial markers were used to guide patient set-up for daily external beam radiation therapy. The eligible patients had biopsy-confirmed prostate cancer that was visible on MRI. A high-field (3T) MRI was performed with an endorectal coil in place. After moving to an US suite, the patient then underwent TRUS to visualize the prostate. The US transducer was equipped with a commercial needle guide and custom modified with two embedded miniature orthogonal five-degrees of freedom sensors to enable spatial tracking and registration with MR images in six degrees of freedom. The MRI sequence of choice was registered manually to the US using custom software for real-time navigation and feedback. The interface displayed the actual and projected needle pathways superimposed upon the real-time US blended with the prior MR images, with position data updating in real time at 10 frames per second. The registered MRI information blended to the real-time US was available to the physician who performed targeted biopsies of highly suspicious areas. RESULTS Five patients underwent limited focal biopsy and fiducial marker placement with real-time TRUS-MRI fusion. The Gleason scores at the time of enrolment on study were 8, 7, 9, 9, and 6. Of the 11 targeted biopsies, eight showed prostate cancer. Positive biopsies were found in all patients. The entire TRUS procedure, with fusion, took ≈10 min. CONCLUSION The fusion of real-time TRUS and prior MR images of the prostate is feasible and enables MRI-guided interventions (like prostate biopsy) outside of the MRI suite. The technique allows for navigation within dynamic contrast-enhanced maps, or T2-weighted or MR spectroscopy images. This technique is a rapid way to facilitate MRI-guided prostate therapies such as external beam radiation therapy, brachytherapy, cryoablation, high-intensity focused ultrasound ablation, or direct injection of agents, without the cost, throughput, or equipment compatibility issues that might arise with MRI-guided interventions inside the MRI suite.

Multi-modality fusion imaging for targeted prostate biopsy is difficult because of prostate motion during the biopsy procedure. A closed-loop control mechanism is proposed to improve the efficacy and safety of the biopsy procedure, which uses real-time ultrasound and spatial tracking as feedback to adjust the registration between a preoperative 3D image (e.g. MRI) and real-time ultrasound images. The spatial tracking data is used to initialize the image-based registration between intraoperative ultrasound images and a preoperative ultrasound volume. The preoperative ultrasound volume is obtained using a 2D sweep and manually registered to the MRI dataset before the biopsy procedure. The accuracy of the system is 2.3 +/- 0.9 mm in phantom studies. The results of twelve patient studies show that prostate motion can be effectively compensated using closed-loop control.

Purpose To evaluate the spatial accuracy of electromagnetic needle tracking and demonstrate the feasibility of ultrasonography (US)–computed tomography (CT) fusion during CT- and US-guided biopsy and radiofrequency ablation procedures. Materials and Methods The authors performed a 20-patient clinical trial to investigate electromagnetic needle tracking during interventional procedures. The study was approved by the institutional investigational review board, and written informed consent was obtained from all patients. Needles were positioned by using CT and US guidance. A commercial electromagnetic tracking device was used in combination with prototype internally tracked needles and custom software to record needle positions relative to previously obtained CT scans. Position tracking data were acquired to evaluate the tracking error, defined as the difference between tracked needle position and reference standard needle position on verification CT scans. Registration between tracking space and image space was obtained by using reference markers attached to the skin (“fiducials”), and different registration methods were compared. The US transducer was tracked to demonstrate the potential use of real-time US-CT fusion for imaging guidance. Results One patient was excluded from analysis because he was unable to follow breathing instructions during the acquisition of CT scans. Nineteen of the 20 patients were evaluable, demonstrating a basic tracking error of 5.8 mm ± 2.6, which improved to 3.5 mm ± 1.9 with use of nonrigid registrations that used previous internal needle positions as additional fiducials. Fusion of tracked US with CT was successful. Patient motion and distortion of the tracking system by the CT table and gantry were identified as sources of error. Conclusions The demonstrated spatial tracking accuracy is sufficient to display clinically relevant preprocedural imaging information during needle-based procedures. Virtual needles displayed within preprocedural images may be helpful for clandestine targets such as arterial phase enhancing liver lesions or during thermal ablations when obscuring gas is released. Electromagnetic tracking may help improve imaging guidance for interventional procedures and warrants further investigation, especially for procedures in which the outcomes are dependent on accuracy.

A system for fusion of realtime transrectal ultrasound (TRUS) with pre-acquired 3D images of the prostate was designed and demonstrated in phantoms and volunteer patients. Biopsy guides for endocavity ultrasound transducers were equipped with customized 6 degree-of-freedom (DoF) electromagnetic (EM) tracking sensors, compatible with the Aurora EM tracking system (Northern Digital Inc, NDI, Waterloo, ON, Canada). The biopsy guides were attached to an ultrasound probe and calibrated to map tracking coordinates with ultrasound image coordinates. Six cylindrical gold seeds were placed in a prostate phantom to serve as fiducial markers. The fiducials were first identified manually in 3T magnetic resonance (MR) images collected with an endorectal coil. The phantom was then imaged with tracked realtime TRUS and the fiducial markers were identified in the live image using custom software. Rigid registrations between MR and ultrasound image space were computed and evaluated using subsets of the fiducial markers. Twelve patients were scanned with 3T MRI and TRUS for biopsy and seed placement. In ten patients, volumetric ultrasound images were reconstructed from 2D sweeps of the prostate and were manually registered with the MR. The rigid registrations were used to display live TRUS images fused with spatially corresponding realtime multiplanar reconstructions (MPRs) of the MR image volume. Registration accuracy was evaluated by segmenting the prostate in the MR and volumetric ultrasound and computing distance measures between the two segmentations. In the phantom experiments, registration accuracies of 2.2 to 2.3 mm were achieved. In the patient studies, the average root mean square distance between the MR and TRUS segmentations was 3.1 mm, the average Hausdorff distance was 9.8 mm. Deformation of the prostate during MR and TRUS scan was identified as the primary source of error. Realtime MR/TRUS image fusion is feasible and is a promising approach to improved target visualization during TRUS-guided biopsy or therapy procedures.

Electromagnetic tracking has great potential for assisting physicians in precision placement of instruments during minimally invasive interventions in the abdomen, since electromagnetic tracking is not limited by the line-of-sight restrictions of optical tracking. A new generation of electromagnetic tracking has recently become available, with sensors small enough to be included in the tips of instruments. To fully exploit the potential of this technology, our research group has been developing a computer aided, image-guided system that uses electromagnetic tracking for visualization of the internal anatomy during abdominal interventions. As registration is a critical component in developing an accurate image-guided system, we present three registration techniques: 1) enhanced paired-point registration (time-stamp match registration and dynamic registration); 2) orientation-based registration; and 3) needle shape-based registration. Respiration compensation is another important issue, particularly in the abdomen, where respiratory motion can make precise targeting difficult. To address this problem, we propose reference tracking and affine transformation methods. Finally, we present our prototype navigation system, which integrates the registration, segmentation, path-planning and navigation functions to provide real-time image guidance in the clinical environment. The methods presented here have been tested with a respiratory phantom specially designed by our group and in swine animal studies under approved protocols. Based on these tests, we conclude that our system can provide quick and accurate localization of tracked instruments in abdominal interventions, and that it offers a user-friendly display for the physician.

Rationale and Objectives Electromagnetic tracking potentially may be used to guide percutaneous needle-based interventional procedures. The accuracy of electromagnetic guided-needle puncture procedures has not been specifically characterized. This article reports the functional accuracy of a needle guidance system featuring real-time tracking of respiratory-related target motion. Materials and Methods A needle puncture algorithm based on a “free-hand” needle puncture technique for percutaneous intrahepatic portocaval systemic shunt was employed. Preoperatively obtained computed tomographic images were displayed on a graphical user interface and registered with the electromagnetically tracked needle position. The system and procedure was tested on an abdominal torso phantom containing a liver model mounted on a motor-driven platform to simulate respiratory excursion. The liver model featured two hollow tubes to simulate intrahepatic vessels. Registration and respiratory motion tracking was performed using four skin fiducials and a needle fiducial within the liver. Success rates for 15 attempts at simultaneous puncture of the two “vessels” of different luminal diameters guided by the electromagnetic tracking system were recorded. Results Successful “vessel” puncture occurred in 0%, 33%, and 53% of attempts for 3-, 5-, and 7-mm diameter “vessels,” respectively. Using a two-dimensional accuracy prediction analysis, predicted accuracy exceeded actual puncture accuracy by 25%–35% for all vessel diameters. Accuracy outcome improved when depth-only errors were omitted from the analysis. Conclusions Actual puncture success rate approximates predicted rates for target vessels 5 mm in diameter or greater when depth errors are excluded. Greater accuracy for smaller diameter vessels would be desirable for implementation in a broader range of clinical applications.

Electromagnetic tracking systems have the potential to track instruments inside the body because they are not limited by the line of sight constraints that characterize optical tracking systems. To integrate an electromagnetic tracking device into a surgical navigation system, accurate registration is required. We present a two-stage registration mechanism designed to be more accurate than the widely used global fiducial-based registration method. The first stage uses a hybrid Iterative Closest Point (ICP) registration method and the Simulated Annealing (SA) optimization algorithm, to increase the initial registration accuracy. The second stage exploits multiple implanted tracking needles that are used to calculate the affine transform based on the initial transform information, and thereby to compensate for the deformation in real time. Phantom and swine studies have demonstrated the utility of this technique.

Computer-aided surgery, in which computers are used to provide image guidance to physicians for invasive procedures, is still a relatively new field. Most developments have come over the last 10 years. Systems based on bony landmarks for applications such as the brain, spine, and ear, nose, and throat are now commercially available, and some institutions use them as the standard for care. However, systems for abdominal interventions and for other procedures with substantial organ motion and deformation have not yet been developed, and they constitute an area of current research. A typical computer-aided surgery system has three major components: a control computer, a localizer for tracking instruments, and software for image processing and display. The localizer can be optical or electromagnetic; however, most commercial systems use optical localizers. Electromagnetic localizers show promise for future applications, including tracking of internal organs, but recent technological advances have only now rendered them serious contenders for this purpose. Computer-aided surgery is an active area of research and emphasizes collaborations between biomedical engineers and clinical experts. Research areas include integration of real-time modalities, such as ultrasound, to provide intraoperative updates; modeling of soft-tissue deformation and respiratory motion; new registration and tracking techniques; multimodal image fusion; and improvement of visualization methods. Keywords: localization; tracking; image overlay

Electromagnetic tracking systems have the potential to track instruments inside the body because they are not limited by the line of sight constraints that characterize optical tracking systems. To integrate an electromagnetic tracking device into a surgical navigation system, accurate registration is required. We present a two-stage registration mechanism designed to be more accurate than the widely used global fiducial-based registration method. The first stage uses a hybrid Iterative Closest Point (ICP) registration method and the Simulated Annealing (SA) optimization algorithm, to increase the initial registration accuracy. The second stage exploits multiple implanted tracking needles that are used to calculate the affine transform based on the initial transform information, and thereby to compensate for the deformation in real time. Phantom and swine studies have demonstrated the utility of this technique.

We have developed an augmented reality system capable of projecting preoperative plans directly onto a patient using rapidly scanned laser beams. Projected contours can typically represent cut paths, tumors or delineate boundaries of interest. The system can be used as part of, or a replacement for, conventional robotic Telesurgery systems. Because the graphics are projected, there is no degradation in surgeon's view due to optical components interposed between the surgeon's eye and the patient. This system has been designed to work with a common infrared 3D camera system used in image-guided surgery, and projects both visible and infrared beams. The IR beam enables surface digitization functions to be carried out using the camera. The clinical accuracy is in the range required by CAS procedures, around 1-2mm. The device will be particularly useful for executing precise preoperative plans and for teleconsultation applications, where planned or live consultations can be efficiently communicated to a less skilled local caregiver.

This paper presents our preliminary experience validating a needle guidance algorithm featuring magnetic tracking and respiratory gating for accurate needle placement in the liver using a specially designed phantom. Experimental results suggest that our implementation can facilitate accurate needle placement in this phantom during simulated respiratory-related liver motion.

The purpose of this study was to evaluate our ability to insert magnetically tracked needles into liver phantom tumors which move simulating physiologic respiration. First, a novel image-guided platform based on a new magnetic tracking device (AURORA?) was constructed. Second, an accuracy evaluation of a compatible magnetically tracked needle (MagTrax) was performed. Finally, 16 liver tumor punctures were attempted using only the image-guided platform for guidance. The inherent MagTrax needle positional error was 0.71?0.43 mm in the non-surgical laboratory setting. Successful puncture of liver tumors was achieved in 14 of 16 attempts (87.5%) by two users. The average time of each procedure was short (163?57 seconds.) The system adequately displayed the moving liver allowing for tumor target visualization and targeting. The AURORA based navigation platform and the compatible MagTrax needle appear promising for more rigorous phantom accuracy studies and in vivo tumor puncture testing in a respiring animal.

This was an initial study to evaluate the accuracy of a new magnetic localizing system (AURORA™) in the interventional radiology environment. This work is part of our research effort to incorporate magnetic tracking in an image-guided system for minimally invasive abdominal interventions.

As a demonstration platform, we are developing a robotic biopsy testbed incorporating a mobile CT scanner, a small needle driver robot, and an optical localizer. This testbed will be used to compare robotically assisted biopsy to the current manual technique, and allow us to investigate software architectures for integrating multiple medical devices. This is a collaboration between engineers and physicians from three universities and a commercial vendor. In this paper we describe the CT-directed biopsy technique, review some other biopsy systems including passive and semi- autonomous devices, describe our testbed components, and present our software architecture. This testbed is a first step in developing the image-guided, robotically assisted, physician directed, biopsy systems of the future.

Study design In vitro and in vivo assessment of the accuracy of devices proposed for tracking spine motion during surgery; in vivo assessment of vertebral motion during spine surgery. Objectives 1) To quantify the accuracy of newly designed vertebral body trackers; 2) to demonstrate the feasibility of tracking vertebral motion in a cadaveric model; and 3) to quantify the vertebral motion that occurs during spinal surgery. Summary of background data Computer techniques are beginning to be applied to spine surgery. Validation of accuracy of methods of spinal tracking has not been reported. No information exists on the amount of vertebral motion that occurs during surgery. Because the new techniques require accurate positional information for the vertebral body, it is important to understand and evaluate methods of tracking vertebrae. Methods An optical tracking system (Northern Digital, Waterloo, Ontario, Canada) was used to track custom-designed trackers. The reliability and accuracy of the trackers were evaluated in vitro. The proposed tracking methodology for human testing was performed using a cadaveric model, and after successful completion, human testing was done in the operating room to evaluate the motion of two vertebral bodies during exposure for instrumentation of the lumbar spine. This technique was used to evaluate the custom designed trackers effectiveness for tracking vertebral bodies for pedicle screw insertion. Results The trackers developed were accurate and capable of tracking the motion of the spine. Measured motion of L3 and L4 during breathing was 1.3 mm, peak to peak. Maximal intraoperative motion of the vertebral bodies was 12.3 mm during maneuvers simulating dissection of soft tissue and targeting of spinal pedicles. Conclusions Significant motion occurs in lumbar vertebral bodies during surgery. Breathing motion alone is up to 1.3 mm, and surgeon-induced motion up to 10 times greater. Vertebral body trackers for use with an optical position sensor were capable of measuring this motion.

The use of an image guided surgical (IGS) technique to assist in the placement of iliosacral lag screws has been demonstrated in a pig cadaver. A marker carrier plate was first attached to the pig's sacrum, and used to register a commercial IGS system. K-wires were inserted percutaneously through the ilium and into the sacrum using only the workstation. Postoperative CT confirmed wires were an average of 2.5 mm and 9.1 degrees from the intended position.

We performed a biomechanical comparison of two ro tator cuff repair techniques using fresh-frozen human cadavers. Nine pairs of cadaveric shoulders had stan dardized full-thickness tears made at the supraspina tus tendon insertion. One of each pair of the cadaveric shoulders was repaired by pulling the tendon into a bone trough in the humeral head using standard su tures. The remaining half of the pairs was repaired using anchor sutures. The repairs were tested using a servohydraulically operated material testing system. The anchor suture repair was significantly stronger than the standard suture technique irrespective of bone quality. Failure occurred predominantly through bone in the suture repairs and as a result of suture breakage in the anchor repairs. The anchors should be placed into the edge of the subchondral bone adjacent to the articular surface. The surgeon should direct the anchor so that the direction of the pull is approximately 90 degrees to the anchor, with the humerus at 30 degrees of abduction.

The registration of cross-sectional medical images with patient anatomy is an important problem in image-guided surgery. Current registration methods such as paired-point matching can be tedious and require large incisions. Improved methods of registration for percutaneous spine procedures could potentially improve the accuracy of minimally invasive spine procedures. In this poster, we will describe a combination of hardware and software for the automatic registration of a vertebral body with cross-sectional medical images. The technique is based on a fiducial carrier attached directly to the spinous process and does not require the user to identify anatomical landmarks.