There is no one right answer to the question of which imaging test is best for patients with hearing loss who are candidates for cochlear implantation (CI). Age, underlying pathology and the ability to tolerate radiation and sedation are just a few of the variables that can determine whether magnetic resonance imaging (MRI), computed tomography (CT) or both are chosen in this clinical setting.
Explore This IssueMay 2012
At least, that’s the current state of affairs in pre-operative CI staging. Twenty years ago, the choice was far simpler, according to Blake C. Papsin, MD, director of the Cochlear Implant Program at the Hospital for Sick Children in Toronto, Canada. “CT scan was the go-to diagnostic tool,” Dr. Papsin said. “It gave us a very detailed picture of the bony structures of the inner ear, and it helped us diagnose the likely cause of the hearing loss, albeit with some radiation exposure. Basically, it gave us a roadmap for how to proceed with CI implantation or other interventions.”
But all of that changed in the mid-2000s, Dr. Papsin noted, when MRI technology had advanced to the point where its advantages, primarily the lack of any radiation exposure and the ability to yield much better images of soft tissue, resulted in serious consideration of the newer imaging test.
To nail down the relative strengths of each scan, Dr. Papsin and colleagues conducted a head-to-head trial of both imaging modalities and published the results in 2007 (Otol Neurotol. 28(3):317-324). In the prospective study, 92 pediatric patients with hearing loss underwent pre-operative imaging of the petrous temporal bone using both CT and MRI. The study showed that each scan did have some shortcomings. In the case of CT, for example, the test failed to consistently detect early obliterative labrynthitis and the presence of the cochlear nerve in the internal auditory canal. The MRI test often missed the presence of enlarged vestibular aqueducts and narrow cochlear nerve canals. The study’s conclusion that “dual-modality imaging,” using both CT and MRI, made sense for many patients because the dual-scan approach “detects abnormalities related to deafness that would not otherwise be found using either modality alone.”
But do the respective abnormalities missed by each scan truly impact the surgical approach in CI patients? In Dr. Papsin’s experience, not always. In fact, during the study, he and his colleagues initially only looked at the MRI results to see if they would yield enough diagnostic information to guide their hands during surgery. “We agreed, based on the design of the study, that if those MRI scans sufficed, we would not ‘break the code’ and ask to see the CT results,” he said. “Well, we never broke the code during the study. Yes, the MRIs missed some things, as the findings pointed out. But those diagnostic ‘misses’ were not enough to change how we treated the patients.”