A navigation system for radiological interventions (CAS-One IR, CAScination, Bern) has been used to evaluate three different needle insertion methods on a non-rigid liver phantom. The insertion methods under investigation include: navigated free-hand needle insertion, aiming device-based insertion with active depth control and aiming device-based needle insertion with passive depth control. For each method a series of 25 punctures was performed and assessed by computing residual error (RE) given by the system and target positioning error (TPE) given from control CT scans.
measurement system (NDI Vicra, Northern Digital, Canada) and a set of custom-made marker shields with retroreflective passive markers that can be adapted to a variety of tools, enabling their accurate tracking within the operating room.
Real-time patient tracking is done using a set of single retro-reflective marker spheres (SM) that are attached to the patient skin around the area of the expected needle incisions (Fig.1a). A sterilizable plastic shell around the marker and biocompatible tape allow integration of SM into clinical scenarios.
Navigation proceeds as follows: when the needle tip is placed at the entry of the planned trajectory, a targeting viewer is enabled (Fig.1b). By projecting the needle tip (red dot) and shaft (green dot) on a 2-dimensional plane placed at the target, the operator is visually aligning the needle with the planned trajectory. A depth bar on the right indicates, the distance from the needle tip to the planned target as well as the following supplementary errors: residual error separated into longitudinal and later components, angular error. A cross-sectional view at the needle axis visualizes anatomical obstacles along the trajectory.
Three series of 25 punctures were performed on a non-rigid liver phantom. The phantom was produced by rapid prototyping using a segmented 3D model of the liver (MeVis Distant Services, Bremen, Germany) that included several anatomical obstacles (portal vein, hepatic vein) and tumors, which require careful planning and needle placements. 1 mm metal screws were fixed on the liver phantom and used as targets. The liver phantom was placed under deformable plastic foam (Fig. 2) in order to simulate the patient skin. Six single markers were attached to the surface of the foam in a non-symmetrical configuration for the automatic registration. The following needle insertion methods were then evaluated on the presented phantom: 1) Navigated free-hand needle insertion: The procedure does not utilize any stabilization device during the puncture.
After definition of the trajectory, the marker shield is placed on the needle shaft. The position and axis orientation of the needle are obtained from calibration. The operator moves the needle above the entry and guides the needle towards the target based on the information shown on the targeting viewer and depth bar (Fig. 2a). 2) Navigated needle insertion with the aiming device and active depth control: The procedure uses an aiming device (ATLAS, Elekta AB, Sweden) to provide stabilization during the adjustment of the needle to the planned trajectory and allow fixation of the final orientation of the needle during the insertion. While screwed to the CT table, an 8mm-thick, rigid, medical-grade titanium cylinder, with an attached marker shield, is used to guide the insertion brackets of the aiming device to the planned entry point. Once the position of the aiming device is correct, the cylindrical tool is removed from the aiming device and replaced with a calibrated needle. During insertion, depth information is shown on the targeting viewer (Fig. 2b). 3) Navigated needle insertion with the aiming device and passive depth control: The procedure differs from the method presented above in that the needle insertions are not monitored by the navigation system. Once the position and orientation of the aiming device correspond to the planned trajectory, the distance between the tip of the cylinder and the target is displayed on the navigation system. This distance is then marked on the needle with a biocompatible pen. The cylinder is removed from the aiming device and the needle is inserted up to mark on the needle (Fig. 2c).
Before each needle insertion, the fiducial registration error (FRE) was measured from the SM tracking system. After needle insertion, the residual error (RE) was stored and the target positioning error (TPE) was evaluated on a control CT dataset. RE measures the distance between the needle tip and the target in the navigation coordinate system. This error expresses how accurately the operator may transfer a trajectory to the patient based on a given visualization scheme. FRE was computed by the system as the root mean square error between registered corresponding points. TPE was measured on the control scans as the distance between the needle tip and the planned target [10]. TPE was separated into longitudinal (along the planned trajectory) and lateral components (along the orthogonal direction). The angular error of each needle insertion was also computed. 3
The average FRE measured among all methods just before needle insertion was 0.7 ± 0.1 mm, with a maximum value of 0.9 mm.
Targeting accuracy is described in Table 1. The Euclidean TPE is similar for each method (ranging from 4.6 ± 1.2 to 4.9 ± 1.7 mm), however lateral error components are significantly lower for the method using the aiming device (unpaired t-test, p = 0.01). The longitudinal error component is markedly lower for the free-hand method without utilization of the aiming device. The highest average angular error was measured for the free-hand insertion. Methods using the aiming device have a lower angular error but the difference was not significant. RE represents the error given by the navigation system at the final needle position and is similar for both methods with needle guidance (free-hand needle insertion and aiming device-based with active depth control). RE values are not available for the method with passive depth control as needle insertion was not monitored by the navigation system.
An in vitro accuracy evaluation of three needle insertion methods was performed with a navigation system dedicated for percutaneous needle interventions utilizing a non-rigid single sphere-based method for patient tracking. TPE represents the final positioning error of the needle; it includes registration, tracking, user, and process errors such as needle bending, however in this case it does not include errors introduced by patient motion. Average measured Euclidean TPE (4.6 ± 1.2 mm, maximum 8.1 mm) was similar for all methods and comparable to previously reported accuracy. For example, Maier-Heinet al. [9] performed 32 free-hand punctures in an in vivo experiment with ventilated swine and reported an overall error of 3.7 ± 2.3 mm and maximum error of 11.1 mm. Neither lateral, longitudinal nor angular error components were computed.
Separating TPE into longitudinal and lateral components is of clinical relevance because the correction of lateral placement errors, unlike longitudinal errors, requires replacement of the needle, which is time-consuming and increases the risk for complications. The lowest lateral errors were achieved using the aiming device (2.3 ± 1.3 mm and 2.8 ± 1.6 mm respectively). These results are statistically significantly better than in the free-hand case. The reason behind this may be that during free-hand insertions it is difficult to maintain the correct needle trajectory angles while advancing the needle into the phantom. The accuracy of the navigated free-hand needle placement depends largely on the surgeon’s hand-eye coordination and ability to guide the needle based on the feedback provided by the navigation system. It is essentially impossible to effectively correct the needle path once insertion has commenced; any attempts to correct needle position will cause bending and displacement at the target.
Additionally, these results show that active depth control does not provide accuracy improvements when compared to passive depth control. However, tracking of the needle during insertion may allow detection of potential damage to critical anatomical structures while inserting a needle.
This study has presented an in vitro comparison of three needle insertion methods. The results obtained demonstrate that usage of an aiming device leads to increased lateral accuracy during needle insertion. 5