Radiofrequency ablation (RFA) is increasingly used as a nonsurgical option in the management of primary and secondary lung tumors.1 RFA in the lung often has minimal adverse effects with predicable outcomes.1,2 With conventional percutaneous RFA techniques, determining the required needle size, type, position of needle puncture, trajectory, and number of ablations is on the basis of a preprocedural and/or multiple periprocedural computed tomography (CT) scans. This process is nonstandardized, and it requires the interventional radiologist to develop a mental 3-dimensional (3D) map to guide the ablation process. Accurate needle placement can be difficult, even with ideal image guidance. In addition, prolonged use of CT or fluoroscopy during image acquisition exposes the patient (and potentially the operator) to ionizing radiation.
Electromagnetic tracking (EMT) of inner needle tips is a new application for real-time needle-tip positioning that is used in conjunction with standard imaging modalities. It has been enabled by the development of miniaturized sensor coils that can be embedded within the needle itself. EMT requires creating an electromagnetic field around the anatomic region of interest (Fig. 1A). A weak current is induced within the coil when an instrument moves within this electromagnetic field. The current in relation to multiple magnetic generators is detected by a computer and processed into reproducible, position coordinates. This location can be overlaid (in a software process known as “registration”) upon prior CT, magnetic resonance, or positron emission tomography for real-time navigation during RFA.3 Analysis of such an EMT system in a previous 20-patient case series has demonstrated the accuracy of this system in retrospectively correlating virtual needle position with actual needle position during conventional freehand needle insertion. The minimal extra time required to set up the system was also documented.4 The relative accuracy of this technique compared with freehand needle insertion is shown. Further, the ability of an EMT system to prospectively guide needle insertion under clinical circumstances without the use of regular imaging input is assessed in this case.
MATERIALS AND METHODS
An EMT system was used to guide RFA of a 12-mm-right lung melanoma metastasis in a 56-year-old patient (Fig. 1B). The patient opted for RFA over surgery because of the potential morbidities in light of multiple previous surgeries and the presence of slower growing extrapulmonary metastases. The use of EMT was approved by the institutional investigational review board to assess the feasibility and accuracy of this technology. The patient gave written informed consent. The system consisted of a field generator (Fig. 1A), a control unit, and a sensor device (Aurora; Northern Digital Inc., Waterloo, ON), interfaced with registration and display custom software (Philips Research, Briarcliff, NY), and a commercially available tracked 22-G stylet inside a standard 19-G outer biopsy guider needle (Philips Healthcare, Toronto, ON). The patient was first anesthetized, intubated, and ventilated. Five adhesive active fiducial markers with 2 embedded 5 degree-of-freedom tracking sensors per fiducial were placed near the expected skin entry point (Fig. 1A). The tracking coordinates of these sensors, which are necessary for the registration process, are obtained automatically during interruption of ventilation. Moreover, these sensors facilitate dynamic motion compensation during breathing. Seven passive conical fiducial markers (Pinpoint; Beekley, Bristol, CT) were also placed as a backup method of manual registration, which was performed by placing the tracked needle sequentially in each sterile cone, a process that takes, on average, 35 seconds,4 and that can be carried out automatically through skin patches as well. A planning CT scan (3-mm-thick sections, 1.5-mm overlap, and 16-slice CT MX 8000; Philips Medical Systems, Cleveland, OH) was obtained during interruption of ventilation. Using custom software on the workstation, the skin fiducials were identified manually on the CT scan (Fig. 1B). The skin entry site was estimated using a radio-opaque grid (E-Z-EM Inc., Lake Success, NY).
Both conventional techniques and tracking-assisted techniques were used sequentially to estimate needle angulations. The conventional technique involved using the most recent axial CT image of the target and the skin entry point, as determined from the CT skin grid. The 19-G guider needle was placed 5 mm below the skin surface by an interventional radiologist experienced in RFA. Thereafter, the appropriate angle for straight needle advancement was produced using freehand manipulation of the needle. In the usual conventional manner, once it was felt that the appropriate needle trajectory had been achieved, the virtual position of the needle and the expected straight-line trajectory were captured using tracking software (Fig. 2). Ventilation was interrupted during trajectory tracking. The operator was blinded to the tracking display throughout this manual process.
To assess and compare the accuracy of EMT guidance, the physician was then unblinded and the tracking-assisted approach was attempted. The 19-G guider needle was removed and reinserted using tracking software guidance only. Through real-time updated display of the virtual needle position, and by assuming a straight insertion, the operator aligned the guider needle with the tracked target center (Fig. 3). Once a satisfactory needle trajectory was achieved, a CT scan of the tracked guider needle in the chest wall was performed. To estimate the accuracy of tracking, the virtual position was retrospectively compared with this actual CT position (Fig. 4).
A single 3-cm 17.5-G impedance-controlled active-tip Cooltip RFA electrode (Valley Lab, Covidien, Boulder, CO) was then placed immediately adjacent to the guider needle (tandem technique) and inserted in a single motion into the lung and tumor (Fig. 5A). CT confirmation of RFA needle position was performed at 3-mm collimation with 50% overlap. RFA of the tumor was followed for 12 minutes, using a sequentially increasing current ramping algorithm with 5 W power increments delivered. A CT scan taken immediately after the ablation illustrated a hazy density surrounding the original location of the tumor (Fig. 5B). A review CT scan taken 22 months post-RFA shows a stable scar and a complete treatment (Fig. 5C).
After the procedure, the final virtual needle position was compared with the actual needle position on the postinsertion CT scan to assess the accuracy of the EMT system. There was near-perfect alignment of the virtual and actual guider needles on the postinsertion CT (Fig. 4). The overall accuracy of EMT-guided RFA can be quantitatively represented by the tracking error (target to registration error), which is the difference between the actual and the virtual (EMT-derived) needle-tip positions. This error was 3.9 mm.
We then compared the relative accuracy of conventional and tracking-assisted approaches by assessing how closer the trajectories passed relative to the center of the tumor. Figure 2 demonstrates that the conventional technique of freehand positioning of the needle for insertion would have resulted in significant needle misalignment with the tumor in 2 orthogonal planes, had the procedure occurred in conventional manner, perhaps even requiring further needle repositioning. Using tracking-assisted navigation (Fig. 3), the operator viewed the trajectory of the needle in 3 planes and accurately targeted the tumor in a single insertion along the intended trajectory.
The lung is an ideal tissue for RFA, because the inherent airspace outside the tumor provides an insulating effect for the thermal energy produced during the procedure, which may help concentrate heat and energy in the tumor.1,2 Numerous clinical trials on lung tumor RFA have been reported.2 Indications for RFA in the lung are often based upon the size of the tumor and the staging characteristics. RFA may eradicate locally confined disease, and local, short-term control is high in tumors <3 cm in diameter and in favorable locations.1 Tumors >3 cm in diameter may require overlapping ablations.
The EMT system was successfully used in prospectively guiding needle insertion in a patient without the use of regular imaging input. Assuming a straight trajectory, tracking-assisted needle insertion targeted the tumor more accurately compared with that by the conventional freehand methods in this patient. This could be because of 3D multiplanar reconstruction and real-time feedback. Multiplanar reconstructions are produced easily by rotating the plane around the needle, allowing the operator to appreciate needle trajectory in 3D, and view the target and any other critical structures in 3D. In addition, multiplanar reconstruction allows the operator to perform preprocedural path planning. This is an advantage that can be particularly helpful in complex needle insertions, which require steeper angles of approach, where it may be difficult to keep the needle in the same image plane as the target.5 Meanwhile, real-time feedback of needle position relative to the ideal trajectory improves the operator’s ability to accurately redirect the needle as planned. Earlier feasibility studies of this system on both phantom and swine models have shown a statistically significant reduction of both the number of required needle passes and the overall radiation exposure.5
The 3.9-mm tracking error quantifies the accuracy of EMT alone by measuring the distance between the virtual and actual positions of the guider needle tip in 3 different planes (Fig. 4). It is within acceptable limits for clinical utility and correlates well with a tracking error of 3.6 to 5.8 mm from the larger retrospective series.4 The causes for this error include inherent inaccuracy of the registration process (eg, the nonuniform spatial accuracy of tracking within the magnetic field, interference from nearby metal structures) and intraprocedural causes such as respiratory motion or patient or organ shift.3,4,6,7 It is unknown at this time whether this error could increase with more central location.
EMT is a robust and relatively inexpensive and accurate technology that is widely used in nonmedical fields. It is a potentially useful tool that has yet to be fully applied to interventional radiologic and percutaneous procedures. Miniaturization of sensor coils has allowed the tracking of instruments as fine as 22-G needles and 0.018-inch guide wires required for interventional radiology.3 Future research is required to enhance respiratory motion modeling, reduce susceptibility to metal interference, and develop deformable modeling algorithms to better predict organ shift. Further, real-time mapping and feedback of overlapping ablation zones could reduce treatment gaps when treating larger tumors.4,7,8 Although speculative, EMT could potentially improve clinical outcomes by reducing the number of confirmation CTs to monitor needle progression or minimizing the number of needle insertions.
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