Optimal imaging protocols for cochlear implantation have yet to be determined. Pre-operative computed tomography (CT) and magnetic resonance image (MRI) scans are used to assess cochlear anatomy and patency, to delineate surgical access, and to aid in choice of side for implantation. However, opinion still differs as to which modality provides more information in pre-operative assessment, or if, indeed, a combination of the two is superior.
The first 88 patients on the Irish National Cochlear Implant Programme (NCIP) were retrospectively studied to determine the accuracy of pre-operative CT and MRI in predicting abnormalities at the time of surgery. Correlation with surgical findings was determined in three separate groups of patients (those who had CT only, those who had MRI only, and those who had both CT and MRI performed).
Of the 24 patients that had both CT and MRI performed, both modalities had a 79 per cent correlation with surgical findings. CT and MRI reports concurred in 75 per cent of cases. Specificity and negative predictive value were high (86 per cent and 90 per cent, respectively). CT alone (47 cases) correlated with surgery in 39 cases (83 per cent); MRI alone (17 cases) correlated in 15 cases (88 per cent).
The findings of this study suggest that CT and MRI are effective at predicting normal inner ear anatomy, and thus at predicting the patient and the cochlea most suitable for implantation. Both modalities are useful in determining the side of implantation, thus avoiding potential surgical difficulties in cases of unilateral abnormalities. There was no significant difference between the ability of MRI and CT to detect abnormalities at the time of surgery. In this series the combination of CT and MRI has not been shown to be superior to either modality used alone, although anecdotal evidence to the contrary was noted.