**5.2 Aiming for parallelism**

To transfer the predefined ablation plan from theory to practice, a navigation system can take remedial action. Visual guidance information provided by the navigation system aids the clinician to position the ablation needle relative to the defined path. The computer-assisted navigation solutions proved to be advantageous for IRE needle guidance in terms of procedural accuracy compared to the manual, CT-fluoroscopically guided approach [38, 39]. Whereas these navigation systems make use of a mechanical guide, navigation systems such as the IMACTIS® CT require the user to actively align and insert the needle. This might cause a challenge for less experienced clinicians in needle guidance. We have therefore compared the two computer-assisted navigation approaches discussed in Section 4.3 by means of a phantom study with a similar setup as in [43]. Firstly, the needle was actively tracked (freehand approach), and secondly the mechanical arm (aiming device approach) was used. Seven participants were requested to place three needles (17 g, 15 cm) using each approach around a tumor encompassing the celiac artery in an artificial, flexible phantom. Three trajectories were predefined on the navigation system and each user received an instructional training. The calibration procedure for the freehand approach was conducted upfront to minimize the calibration error. The starting sequence was randomized, the needle control scan was conducted with a mobile C-arm, and the validation again conducted with the CAScination system by means of image to image fusion.

The mean lateral error for the aiming-device approach (2.4 ± 0.7 mm) was found to be similar compared to the lateral error (2.3 ± 1.3 mm) obtained in [43]. The error for the freehand approach resulted in a larger mean error (6.7 ± 1.7 mm) compared to the results from [43] (4.2 ± 2.0 mm), which can be explained by the larger group of participants with varying experience in manual needle placement. Parallelism of the needles for both approaches was calculated according to the method used in [38] where an oblique slice was placed at the needle closest to the tumor with the normal pointing along the needle direction. Each needle tip was successively projected on the plane translated 3 cm from the tip toward the handpiece to calculate the distance to the needle center. With a total of 14 samples per approach, the resulting mean Euclidean error was significantly lower for the aiming device than for the freehand approach (1.5 ± 0.7 mm versus 2.4 ± 1.1 mm) with a p-value of 0.014 (see **Figure 7**). However,

*Computer Assistance in the Minimally Invasive Ablation Treatment of Pancreatic Cancer DOI: http://dx.doi.org/10.5772/intechopen.93226*

**Figure 7.**

*Comparison of aiming device versus freehand for a specific participant. The top row visualizes the calculation of the Euclidean error between the aiming device (A) and freehand (B) approach. Volume rendering from control CT of aiming device (C) and freehand (D) guidance highlights the bending of the needle in the freehand approach.*

these results should not be compared with the findings by Beyer et al. [38, 39] due to the setup of the studies (phantom vs. in vivo), yet their reported decrease of the error using the computer-assisted navigation goes along with the findings by our group.
