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.

Accurate placement of needles within the liver for percutaneous interventions may be accomplished using computed tomography (CT), magnetic resonance imaging (MRI), or ultrasound guidance. Modality-specific limitations include requiring ionizing radiation, nonmagnetically susceptible instruments, or adequate acoustical window without interposed osseous or gas-filled structures. In more complex intrahepatic vascular procedures such as transjugular intrahepatic portocaval systemic shunt (TIPS), shunt creation between portal and hepatic veins is most often accomplished without direct real-time guidance, although planar and three-dimensional ultrasound ( ) and MRI guidance has been reported ( ). Alternatively, the target portal vein can be identified fluoroscopically by several techniques, including wedged hepatic venography using iodinated contrast or carbon dioxide ( ), transhepatic portography, or percutaneous placement of target guidewires or markers in the portal vein ( ).

Respiratory motion interferes with accurate needle placement in static CT-guided interventions, although real-time imaging with ultrasound, CT fluoroscopy, or MRI with breath-hold can help compensate for target excursion with respirations. In an alternative approach, static images would be registered with positional data obtained from an electromagnetic tracking system, allowing the position of electromagnetically tracked instruments to be displayed on the static image. Electromagnetic tracking could be enhanced to track the respiratory related motion of the target organ with retrievable embedded fiducials. The electromagnetic tracking system would then provide 1) real-time location of the tracked needle or instrument and 2) real-time location of the target during the respiratory cycle-related target excursion.

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Materials and methods

Determination of Positional Accuracy of the Tracking System

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Determination of Orientation Accuracy of the Tracking System

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Accurate Measurement of the Coil Offset

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Phantom Design and Needle Puncture Procedure

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Results

Manufacturer’s Stated System Accuracy

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Table 1

Manufacturer’s Specifications for the Aurora System

Five-Dimensional Sensor Accuracy positional 1–2 mm 3D root mean square ⁎ Accuracy angular 0.5°–1° root mean square ⁎ Sensor Dimensions 0.9 mm diameter × 8 mm Number of sensors 1–10 Measurement rates 20–60 Hz †

Table courtesy of Northern Digital, Inc.

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Positional Accuracy

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Table 2

Magnetically Measured Displacement by Robot Axis

x-axis 0–100 mm 100.15 ± 0.10 100 mm to 0 mm 100.46 ± 0.58 y-axis 0–20 mm 19.88 ± 0.10 20 mm to −20 mm 39.28 ± 0.02 −20 mm to 0 mm 19.43 ± 0.04 z-axis 0–20 mm 19.99 ± 0.02 20 mm to −20 mm 40.06 ± 0.05

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Orientation Accuracy

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Needle Puncture Experiments

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Table 3

Needle Puncture Experiment

Incidence Total Frequency 3 mm 5 mm 7 mm Unsuccessful punctures Missed superficial vessel 1 1 2 1 4 Missed deep vessel 2 7 2 9 Missed both vessels 2 2 Depth error only 2 5 7 Missed vessel 1 + depth error 1 1 Missed vessel 2 + depth error 4 1 1 6 Missed vessels 1, 2 + depth error 3 3 Successful punctures 0 5 8 Total passes 15 15 15

Table 4

Predicted vs. Actual Puncture Error Analysis

All Puncture Attempts Total Error (mm) Vessel Diameter (mm) Predicted Success Rates Mean Observed Success Rate Difference Total Attempts Perpendicular Case Parallel Case 4.23 3 0.16 0.42 0.290.00 0.29 15 4.23 5 0.44 0.63 0.540.30 0.24 15 4.23 7 0.86 0.89 0.880.53 0.35 15

All Puncture Attempts excluding Depth Errors Total Error (mm) Vessel Diameter (mm) Predicted Success Rates Mean Observed Success Rate Difference Total Attempts Perpendicular Case Parallel Case 4.23 3 0.16 0.42 0.290.00 0.29 15 4.23 5 0.44 0.63 0.540.39 0.15 13 4.23 7 0.86 0.890.880.80 0.08 10

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Discussion

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Acknowledgments

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Appendix

Accuracy Prediction Calculations

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References

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19. Levy E., Cleary K., Banovac F., et. al.: Implementation of a magnetic tracking system for accurate puncture needle guidance.2002. Baltimore, MD, April
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Implementation of an Electromagnetic Tracking System for Accurate Intrahepatic Puncture Needle Guidance Accuracy Results in an In Vitro Model

Rationale and Objectives

Materials and Methods

Results

Conclusions

Materials and methods

Determination of Positional Accuracy of the Tracking System

Determination of Orientation Accuracy of the Tracking System

Accurate Measurement of the Coil Offset

Phantom Design and Needle Puncture Procedure

Results

Manufacturer’s Stated System Accuracy

Positional Accuracy

Orientation Accuracy

Needle Puncture Experiments

Discussion

Acknowledgments

Appendix

Accuracy Prediction Calculations

=cos−1[(d2−2r2)/2r]360

=cos−1[(d2−2r2)/2r]πr2180

References

Further Reading

Abstracts of Funded National Institutes of Health Grants

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Cardiac CT Angiography (CTA) and Nuclear Myocardial Perfusion Imaging (MPI)—A Comparison in Detecting Significant Coronary Artery Disease