*2.4. Electrode Depth-to-Angular Coverage Prediction*

The centerline can be discretized based on angles (in cylindrical coordinates), which can be used to predict a priori the angular coverage an electrode array is expected to reach as a function of the number of electrodes inserted beyond the RW. Shurzig et al. [50,51] proposed an ideal trajectory for the electrode, to be computed by subtracting the radius of the electrode from the radius of the cochlear spiral. A retrospective analysis of our predictions carried out on 98 images from our clinical dataset hinted that, on average, the CI electrode only follows an ideal trajectory after hitting the lateral wall around 150 deg. This observation leads us to propose the following statistical predictive model:

$$\delta\_{i} = \rho - \begin{cases} 1.3 - 0.007\theta\_{i\nu} & \text{if } i \le 150^{\circ} \\ r\_{i\nu} & \text{otherwise} \end{cases} \tag{1}$$

where *ρ* is the radius of the centerline in cylindrical coordinates, and *r* and *θ* represent the radius of the *i*th electrode. Figure 4 depicts the angular errors based on Equation (1). Our predictions fall, on average, within 20◦ of the observed insertion angular coverage (n = 58).
