Electromagnetic Needle Tracking Research Articles

Smart goggles augmented reality CT-US fusion compared to conventional fusion navigation for percutaneous needle insertion.

Targeting accuracy determines outcomes for percutaneous needle interventions. Augmented reality (AR) in IR may improve procedural guidance and facilitate access to complex locations. This study aimed to evaluate percutaneous needle placement accuracy using a goggle-based AR system compared to an ultrasound (US)-based fusion navigation system. Six interventional radiologists performed 24 independent needle placements in an anthropomorphic phantom (CIRS 057A) in four needle guidance cohorts (n = 6 each): (1) US-based fusion, (2) goggle-based AR with stereoscopically projected anatomy (AR-overlay), (3) goggle AR without the projection (AR-plain), and (4) CT-guided freehand. US-based fusion included US/CT registration with electromagnetic (EM) needle, transducer, and patient tracking. For AR-overlay, US, EM-tracked needle, stereoscopic anatomical structures and targets were superimposed over the phantom. Needle placement accuracy (distance from needle tip to target center), placement time (from skin puncture to final position), and procedure time (time to completion) were measured. Mean needle placement accuracy using US-based fusion, AR-overlay, AR-plain, and freehand was 4.5 ± 1.7mm, 7.0 ± 4.7mm, 4.7 ± 1.7mm, and 9.2 ± 5.8mm, respectively. AR-plain demonstrated comparable accuracy to US-based fusion (p = 0.7) and AR-overlay (p = 0.06). Excluding two outliers, AR-overlay accuracy became 5.9 ± 2.6mm. US-based fusion had the highest mean placement time (44.3 ± 27.7s) compared to all navigation cohorts (p < 0.001). Longest procedure times were recorded with AR-overlay (34 ± 10.2min) compared to AR-plain (22.7 ± 8.6min, p = 0.09), US-based fusion (19.5 ± 5.6min, p = 0.02), and freehand (14.8 ± 1.6min, p = 0.002). Goggle-based AR showed no difference in needle placement accuracy compared to the commercially available US-based fusion navigation platform. Differences in accuracy and procedure times were apparent with different display modes (with/without stereoscopic projections). The AR-based projection of the US and needle trajectory over the body may be a helpful tool to enhance visuospatial orientation. Thus, this study refines the potential role of AR for needle placements, which may serve as a catalyst for informed implementation of AR techniques in IR.

The learning curve and experience of a novel multi-modal image fusion targeted transperineal prostate biopsy technique using electromagnetic needle tracking under local anesthesia.

Prostate cancer is the most common malignant tumor of male genitourinary system, and the gold standard for its diagnosis is prostate biopsy. Focusing on the methods and skills of prostate biopsy, we explored the learning curve and experience of a novel magnetic resonance imaging and transrectal ultrasound (mpMRI-TRUS) image fusion transperineal biopsy (TPB) technique using electromagnetic needle tracking under local anesthesia. The clinical and pathological data of 92 patients who underwent targeted TPB from January 2023 to July 2023 in our center were prospectively collected. The cumulative sum (CUSUM) analysis method and the best fitting curve were used to analyze the learning curve of this novel technique, and the clinical characteristics, perioperative data and tumor positive rate of prostate biopsy of patients at different stages of the learning curve were compared. With the increase of the number of surgical cases, the overall operative time showed a downward trend. The best fitting curve of CUSUM reached its peak at the twelfth case, which is the minimum cumulative number of surgical cases needed to cross the learning curve of the operation. Taking this as the boundary, the learning curve is divided into two stages: learning improvement stage (group A, 12 cases) and proficiency stage (group B, 80 cases). The surgical time and visual analog scale score during prostate biopsy in group A were significantly higher than those in group B. The visual numerical scale score during prostate biopsy in group A was significantly lower than that in group B. There was no statistically significant difference between group A and group B in the detection rate of csPCa and the incidence of perioperative complications. The novel targeted TPB technique is divided into learning improvement stage and proficiency stage, and 12 cases may be the least cumulative number.

Vector Prostate Biopsy: A Novel Magnetic Resonance Imaging/Ultrasound Image Fusion Transperineal Biopsy Technique Using Electromagnetic Needle Tracking Under Local Anaesthesia

BackgroundProstate magnetic resonance imaging (MRI) is now standard for assessment of suspected prostate cancer (PCa). A variety of approaches to MRI-based targeting has revolutionised prostate biopsies. ObjectiveTo describe the procedure and show the accuracy and tolerability of a novel Vector MRI/ultrasound fusion transperineal (TP) biopsy technique that uses electromagnetic (EM) needle tracking under local anaesthesia (LA). Design, setting, and participantsVector prostate biopsy using BiopSee fusion software, EM tracking technology, and transrectal ultrasound was performed in 69 patients meeting the biopsy criteria in two UK centres between September 2020 and August 2022. Surgical procedureStepper-mounted rectal ultrasound images were fused with MRI scans. LA was applied into two defined perineal tracks and a needle sheath with an EM sensor was inserted. The biopsy needle was directed precisely through the sheath to MRI targets under EM tracking. Biopsies were taken without antibiotic prophylaxis. MeasurementsCancer detection (any PCa; grade group ≥2), side effects, and patient experience measures were recorded. Results and limitationsCancer detection in patients with Likert 4–5 lesions was 98% for any PCa and 83% for grade group ≥2. According to the 50 questionnaires returned, 42 patients (84%) reported no or minimal pain, while 40 (80%) reported no or minimal discomfort. No episodes of postoperative urinary retention occurred, and only one patient required treatment for infection. Limitations include the low patient number and incomplete responses to questionnaires. ConclusionsThis novel Vector technique provides a feasible and tolerable procedure for MRI/ultrasound fusion TP biopsy under LA, with high cancer detection rates. This is achieved while maintaining patient comfort and with minimal rates of complications. Patient summaryWe report a novel technique that uses electromagnetic needle tracking to perform highly accurate and comfortable prostate biopsies through the perineum under local anaesthetic.

Ultrasonography-guided electromagnetic needle tracking in laryngeal electromyography

The clinical evaluation of vocal fold movement disorders should contain the electromyography of the laryngeal muscles (LEMG). The most challenging point in LEMG is the right positioning of the EMG needle in the small target muscles. As the results of the EMG have great influence in the decision of treatment it is important to confirm the results of this examination. Anatomical structures of the larynx should be identified with laryngeal ultrasonography and the ultrasonography (US)-guided electromagnetic needle tracking should guide the LEMG needle to the target muscle. The thyroarytenoid (TA) and cricothyroid (CT) muscles had been evaluated in 19 patients (20 examinations). The US-guided transcutaneous LEMG using electromagnetic needle tracking was performed by one ENT doctor and all examinations had been video monitored. The videos were analyzed for the accuracy rate and the visibility of the important laryngeal structures. The laryngeal structures were identified in all the cases using laryngeal ultrasonography. The examination times of the US-guided LEMG were acceptable (8min, 32s). The US-guided LEMG was feasible in 56 (36 TA, 20 CT) examinations. The TA and CT could be visualized successfully but in 17 examinations (30%) the signal was not stable. We could still reach the target muscles in more than 50% of these cases. US-guided electromagnetic needle tracking in LEMG helps to determine the exact position of the laryngeal structures. With further technical improvement of the stability of the electromagnetic needle tracking signal the US-guided electromagnetic needle tracking of the target muscles in the larynx could help to improve the accuracy of the transcutaneous LEMG.

04:12 PM Abstract No. 294 Virtual reality (VR), dynamic holographic, display of the vascular anatomy and a co-registered angiographic catheter with electromagnetic (EM) tracking for the guidance of endovascular procedures: a pilot phantom study to evaluate the feasibility of angiography-free endovascular procedures

The purpose of this study was to demonstrate feasibility of using holographic 3D Virtual Reality (VR) to visualize an angiographic catheter with electromagnetic (EM) sensors while selecting vessels in a co-registered vascular phantom. Electrophysiologists have nearly replaced angiography with EM catheter tracking. EM needle tracking is used during tumor ablation. Interventional Radiologists have not adopted this to endovascular catheters. The novel ability to visualize the catheter in coregistered dynamic VR allows one to view the 3D vascular anatomy while steering the catheter as never before possible. A 3D hologram of the abdominal/pelvic vasculature was created from a CTA using 3D Slicer (slicer.org) and proprietary software (pyrusmedical.com). A 3D printed model of the abdominal/pelvic vasculature was held in anatomic orientation in a scaffold, for registration of the VR holographic representation to the position of the physical model in space. A Simmons 1 catheter and microcathter with EM sensors was advance in the 3D printed vascular model. A trakSTAR tracking system (Northern Digital Inc., Ontario, Canada) was used to visualize the image of the catheter in the co-registered VR displayed holographic image as mesenteric branches were selected. The right hepatic, splenic and gastroduodenal arteries were selected 6 times each using only VR guidance, then again using only fluoroscopic guidance. The time taken to steer the catheter from the common femoral artery to the target vessels was compared and also compared to times retrospectively collected from actual clinical angiographic procedures. A survey regarding VR was then completed. The mean time to select the GDA, splenic, and right hepatic artery using VR display versus fluoroscopic guidance was 17.6, 18.6, 22.6 versus 70.3, 66.1, 73.5 seconds, respectively (P<.05). During clinical cases the mean time to select these vessels was 171.2, 92.3, 188.4 (P<.05). Survey responses suggested improved ease, precision, confidence and efficiency. VR with EM catheter tracking enables augmented intra-procedural guidance for catheter steering with reduced radiation exposure and procedure time.

The Value of Needle‐Guidance Technology in Ultrasound‐Guided Percutaneous Procedures Performed by Radiology Residents: A Comparison of Freehand, In‐Plane, Fixed‐Angle, and Electromagnetic Needle Tracking Techniques

Radiology residents typically learn ultrasound-guided procedures by performing supervised procedures on patients who may experience longer procedure times and higher complication rates. The purpose of this study was to determine if existing technologies, such as in-plane, fixed-angle guidance (IPFA) and electromagnetic needle tracking (ENT), can improve resident procedure time and accuracy. Radiology residents (18 total) were randomized to 1 of 3 ultrasound-guidance technique groups-freehand, IPFA, or ENT-and instructed to place a needle into 4 liver lesions in a humanoid phantom, each increasing in difficulty. For each lesion, residents were timed from skin puncture to needle placement, and the number of times the needle was pulled back and redirected (pullbacks) was recorded. Primary outcomes were total time and total number of pullbacks for all 4 lesions. Secondary outcomes were individual time and number of pullbacks for each lesion. Compared to the freehand group, the IPFA and ENT groups demonstrated lower procedural time and number of pullbacks both in total and for each individual lesion. Differences in total time and total number of pullbacks were significant (P < .001), as were differences for lesion 3 (P = .002-.02) and lesion 4 (P < .001). Differences for lesions 1 and 2 were not statistically significant. Radiology resident procedure time and procedure accuracy (as judged by number of pullbacks) are significantly improved by the use IPFA and ENT guidance technologies.

Electromagnetic needle tracking during simulated right internal jugular cannulation.

This study used three-dimensional information from Stealth navigation technology during simulated right internal jugular vein cannulation to define the initial needle trajectory taken when using three approaches: landmark (LM), short-axis (SAX) ultrasound and long-axis (LAX) ultrasound. Nineteen volunteers indicated the entry site and needle direction (track) they would use in performing right internal jugular vein cannulation by the three approaches. The likelihood of cannulation success, arterial puncture and needle direction were recorded. Volunteers were asked to assess the suitability of the simulation system for validity and educational benefit. The SAX track crossed the jugular vein more frequently than the LAX and LM tracks (SAX: 94%; LAX: 80%; LM: 47% [SAX versus LM, P <0.01]). The mean indicated needle direction in the coronal plane for LM, SAX and LAX were -4, 13 and 11 degrees, respectively. The track associated with the LAX technique would have entered the carotid artery by 16% of volunteers. At needle depths of over 40 mm, the track crossed the vertebral artery at the following rates (LM: 11%; SAX: 16%; LAX: 16%). The use of Stealth technology to provide three-dimensional feedback of the needle path taken during simulated right internal jugular cannulation was considered realistic (16/19) and of benefit for 18 of 19 (95%) respondents. The SAX track was associated with the highest likelihood of successful jugular cannulation and the lowest cross rate of the carotid artery. The simulation model using Stealth was considered to be valuable and realistic by participants despite some limitations.

Performance and suitability assessment of a real-time 3D electromagnetic needle tracking system for interstitial brachytherapy.

PurposeAccurate insertion and overall needle positioning are key requirements for effective brachytherapy treatments. This work aims at demonstrating the accuracy performance and the suitability of the Aurora® V1 Planar Field Generator (PFG) electromagnetic tracking system (EMTS) for real-time treatment assistance in interstitial brachytherapy procedures.Material and methodsThe system's performance was characterized in two distinct studies. First, in an environment free of EM disturbance, the boundaries of the detection volume of the EMTS were characterized and a tracking error analysis was performed. Secondly, a distortion analysis was conducted as a means of assessing the tracking accuracy performance of the system in the presence of potential EM disturbance generated by the proximity of standard brachytherapy components.ResultsThe tracking accuracy experiments showed that positional errors were typically 2 ± 1 mm in a zone restricted to the first 30 cm of the detection volume. However, at the edges of the detection volume, sensor position errors of up to 16 mm were recorded. On the other hand, orientation errors remained low at ± 2° for most of the measurements. The EM distortion analysis showed that the presence of typical brachytherapy components in vicinity of the EMTS had little influence on tracking accuracy. Position errors of less than 1 mm were recorded with all components except with a metallic arm support, which induced a mean absolute error of approximately 1.4 mm when located 10 cm away from the needle sensor.ConclusionsThe Aurora® V1 PFG EMTS possesses a great potential for real-time treatment assistance in general interstitial brachytherapy. In view of our experimental results, we however recommend that the needle axis remains as parallel as possible to the generator surface during treatment and that the tracking zone be restricted to the first 30 cm from the generator surface.

Sci-Fri PM: Topics - 08: The Role and Benefits of Electromagnetic Needle-Tracking Technologies in Brachytherapy

In modern brachytherapy, application of large doses of ionizing radiation in a limited number of fractions is frequent. Furthermore, as with any surgical procedures, brachytherapy is subject to learning curve effects. In this context, there could be advantages of integrating real-time tracking of needles/catheters to existing protocols given the recent prominent advances in tracking technologies. In this work, we review the use of an electromagnetic tracking system (EMTS) based on the second generation Aurora® Planar Field Generator (Northern Digital Inc) and custom design needles (Philips Healthcare) for brachytherapy applications. The position and orientation information is obtained from 5 degrees of freedom sensors. Basic system performance characterization is performed in well-controlled conditions to establish accuracy and reproducibility as well as potential interference from standard brachytherapy equipment. The results show that sensor locations can be tracked to within 0.04mm (la) when located within 26cm of the generator. Orientation accuracy of the needle remained within ±1° in the same region, but rose quickly at larger distances. The errors on position and orientation strongly dependent the sensor position in the characterization volume (500×500×500mm{sup 3}). The presence of an ultrasound probe was shown to have negligible effects on tracking accuracy. The use of EMTS formore » automatic catheter/applicator reconstruction was also explored. Reconstruction time was less than 10 sec/channel and tips identification was within 0.69±0.29mm of the reference values. Finally, we demonstrate that hollow needle designs with special EM adaptation also allow for real-time seed drop position estimation. In phantom experiments showed that drop positions were on average within 1.6±0.9mm of the reference position measured from μCT. Altogether, EMTS offer promising benefits in a wide range of brachytherapy applications.« less

Prospective Comparison of Freehand and Electromagnetic Needle Tracking for US-guided Percutaneous Liver Biopsy

PurposeTo prospectively compare electromagnetic needle tracking (EMT) and freehand ultrasound (US)-guided liver biopsies. Materials and MethodsAmong 60 consecutive US-guided liver biopsies performed by staff radiologists (senior operators) and residents (junior operators), 30 were performed freehand and 30 with EMT. Needle placement time, numbers of needle punctures and pullbacks, and subjective scores of procedure difficulty were compared by χ2 or Student t test. ResultsDiagnostic success rates, defined by the procurement of an adequate histopathologic specimen, were 96.6% for freehand biopsy and 100% with EMT. Needle placement time was significantly lower for EMT (mean ± standard deviation, 45.8 s ± 48.1) than for freehand procedures (143.2 s ± 122.1; P < .01). In the freehand group, needle placement times were 179.6 seconds ± 133.3 for junior operators and 106.8 seconds ± 101.3 for senior operators (P = .15). In the EMT group, needle placement times were 49.2 seconds ± 55 for junior operators and 42.5 seconds ± 41.2 for senior operators (P = .53). The number of needle pullbacks was significantly lower for senior operators (1.2 ± 0.80) compared with junior operators (2.4 ± 1.4) in the freehand group (P = .01), with no significant difference (junior, 0.47 ± 0.92; senior, 0.67 ± 0.72; P = .24) in the EMT group. The postprocedural difficulty score was lower in the EMT group (1.5 ± 0.7) than in the freehand group (2.1 ± 1.1; P = .02). Needle placement time and number of needle pullbacks were lower in the EMT group, even after taking into account tumor size and depth and operator experience. ConclusionsThe EMT procedure shortens needle placement time and reduces the number of needle pullbacks needed for redirection, regardless of operator experience.

Real-time ultrasound-guided spinal anesthesia using the SonixGPS® needle tracking system: a case report

The SonixGPS® is an electromagnetic needle tracking system for ultrasound-guided needle intervention. Both current and predicted needle tip position are displayed on the ultrasound screen in real-time, facilitating needle-beam alignment and guidance to the target. This case report illustrates the use of the SonixGPS system for successful performance of real-time ultrasound-guided spinal anesthesia in a patient with difficult spinal anatomy. A 67-yr-old male was admitted to our hospital to undergo revision of total right hip arthroplasty. His four previous arthroplasties for hip revision were performed under general anesthesia because he had undergone L3-L5 instrumentation for spinal stenosis. The L4-L5 interspace was viewed with the patient in the left lateral decubitus position. A 19G 80-mm proprietary needle (Ultrasonix Medical Corp, Richmond, BC, Canada) was inserted and directed through the paraspinal muscles to the ligamentum flavum in plane to the ultrasound beam. A 120-mm 25G Whitacre spinal needle was then inserted through the introducer needle in a conventional fashion. Successful dural puncture was achieved on the second attempt, as indicated by a flow of clear cerebrospinal fluid. The patient tolerated the procedure well, and the spinal anesthetic was adequate for the duration of the surgery. The SonixGPS is a novel technology that can reduce the technical difficulty of real-time ultrasound-guided neuraxial blockade. It may also have applications in other advanced ultrasound-guided regional anesthesia techniques where needle-beam alignment is critical.

Drainage of Air-Containing Cavities Guided by Image Fusion involving Ultrasound and Electromagnetic Needle Tracking

Drainage of Air-Containing Cavities Guided by Image Fusion involving Ultrasound and Electromagnetic Needle Tracking

Abstract No. 67: Prospective comparison of US guided percutaneous liver biopsy comparing free-hand and electromagnetic needle tracking in a single center

Abstract No. 67: Prospective comparison of US guided percutaneous liver biopsy comparing free-hand and electromagnetic needle tracking in a single center

Freehand Biopsy Guided by Electromagnetic Needle Tracking: A Phantom Study

To evaluate the overall accuracy and time spent on biopsy guided by electromagnetic needle tracking in a phantom compared with the standard technique of US-guided biopsy with an attached steering device. Furthermore, to evaluate off-plane biopsy guided by needle tracking. Three different series of biopsy were performed in a phantom: one with a steering device attached to the transducer without needle tracking, simulating the standard ultrasound-guided biopsy procedure (series 1), one freehand in the scan plane using electromagnetic needle tracking (series 2), and one freehand off-the-scan plane using electromagnetic needle tracking (series 3). The phantom contained spheres of 1 cm in diameter filled with red dye. Each time of the phantom surface was perforated counted as an attempt. 180 biopsies were performed. The mean time spent on each biopsy in series one was 19.9 seconds (SD: 9.1), in series two 34.1 seconds (SD: 17.9) and in series three 34.4 seconds (SD: 14.0). The overall rate of success was: 88 % for in-plane needle-guided biopsy, 87 % for in-plane needle tracking, and 92 % for off-plane needle tracking. No statistically significant difference between the methods was shown. Needle navigation is a potentially valuable tool for image-guided biopsy with an equal rate of success compared with conventional image-guided biopsy. Furthermore, it enables off-plane image-guided biopsy.

Electromagnetic-Tracked Biopsy under Ultrasound Guidance: Preliminary Results

This study was designed to evaluate the accuracy and safety of electromagnetic needle tracking for sonographically guided percutaneous liver biopsies. We performed 23 consecutive ultrasound-guided liver biopsies for liver nodules with an electromagnetic tracking of the needle. A sensor placed at the tip of a sterile stylet (18G) inserted in a coaxial guiding trocar (16G) used for biopsy was localized in real time relative to the ultrasound imaging plane, thanks to an electromagnetic transmitter and two sensors on the ultrasound probe. This allows for electronic display of the needle tip location and the future needle path overlaid on the real-time ultrasound image. Distance between needle tip position and its electronic display, number of needle punctures, number of needle pull backs for redirection, technical success (needle positioned in the target), diagnostic success (correct histopathology result), procedure time, and complication were evaluated according to lesion sizes, depth and location, operator experience, and "in-plane" or "out-of-plane" needle approach. Electronic display was always within 2 mm from the real position of the needle tip. The technical success rate was 100%. A single needle puncture without repuncture was used in all patients. Pull backs were necessary in six patients (26%) to obtain correct needle placement. The overall diagnostic success rate was 91%. The overall true-positive, true-negative, false-negative, and failure rates of the biopsy were 100% (19/19) 100% (2/2), 0% (0/23), and 9% (2/23). The median total procedure time from the skin puncture to the needle in the target was 30 sec (from 5-60 s). Lesion depth and localizations, operator experience, in-plane or out-of-plane approach did not affect significantly the technical, diagnostic success, or procedure time. Even when the tumor size decreased, the procedure time did not increase. Electromagnetic-tracked biopsy is accurate to determine needle tip position and allows fast and accurate needle placement in targeted liver nodules.

Documenting the location of prostate biopsies with image fusion.

OBJECTIVE To develop a system that documents the location of transrectal ultrasonography (TRUS)-guided prostate biopsies by fusing them to MRI scans obtained prior to biopsy, as the actual location of prostate biopsies is rarely known. PATIENTS AND METHODS Fifty patients (median age 61) with a median prostate-specific antigen (PSA) of 5.8 ng/ml underwent 3T endorectal coil MRI prior to biopsy. 3D TRUS images were obtained just prior to standard TRUS-guided 12-core sextant biopsies wherein an electromagnetic positioning device was attached to the needle guide and TRUS probe in order to track the position of each needle pass. The 3D-TRUS image documenting the location of each biopsy was fused electronically to the T2-weighted MRI. Each biopsy needle track was marked on the TRUS images and these were then transposed onto the MRI. Each biopsy site was classified pathologically as positive or negative for cancer and the Gleason score was determined. RESULTS The location of all (n= 605) needle biopsy tracks was successfully documented on the T2-weighted (T2W) MRI. Among 50 patients, 20 had 56 positive cores. At the sites of biopsy, T2W signal was considered 'positive' for cancer (i.e. low in signal intensity) in 34 of 56 sites. CONCLUSION It is feasible to document the location of TRUS-guided prostate biopsies on pre-procedure MRI by fusing the pre-procedure TRUS to an endorectal coil MRI using electromagnetic needle tracking. This procedure may be useful in documenting the location of prior biopsies, improving quality control and thereby avoiding under-sampling of the prostate as well as directing subsequent biopsies to regions of the prostate not previously sampled.

Electromagnetic Tracking for Thermal Ablation and Biopsy Guidance: Clinical Evaluation of Spatial Accuracy

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. 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. 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. 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.