He emphasized the importance of oblique imaging. For instance, when looking for dehiscence of the superior canal, radiologists look for the highest bony resolution, he said, and “we try to do images that are perpendicular and parallel to the actual little bit of bone that we’re trying to assess.”
Explore this issue:July 2015
One way MRI is used is in non-echo-planar diffusion-weighted imaging (DWI) in cases of recurrent types of cholesteatoma. The initial evaluation will be done with CT, because of the attention to bone erosions. But non-echo-planar DWI can be useful in assessing potential changes when those images are cross-referenced with others, Dr. Curtin said.
Congenital Hearing Loss
Caroline Robson, MD, chief of neuroradiology at Boston Children’s Hospital, reviewed considerations in imaging of children with congenital hearing loss.
Multi-detector CT, she said, is best for assessing children with external and middle ear anomalies who present with conductor or mixed hearing loss, as well as those suspected of having congenital cholesteatoma. “Both CT and MR provide excellent information regarding the evaluation of children with sensorineural hearing loss, and the choice of the modality is going to depend on several factors,” she said.
CT would probably be best for first-line imaging for a child who could get through the exam easily without sedation but would need sedation or general anesthesia for an MRI exam.
But MRI would be best for a child who is a potential cochlear implant candidate, due to the need for information on the integrity of the membranous labyrinth and the presence and size of or absence of the cochlear nerves. And MRI is definitely best for children with sensorineural hearing loss suspected of having concurrent brain anomalies, she said.
“Whenever possible, we attempt to avoid sedation or general anesthesia,” Dr. Robson said. “The newest generation multi-slice CT units obtain images in a matter of seconds. And so, provided a patient is either asleep or cooperative, many times CT can be obtained without sedation.”
Both CT and MR are conducted, when possible, with children napping, something most successful with infants younger than nine months of age. Motion-reduction sequences and motion-tracking devices are great when imaging the brain but not so useful for the fine detail required when imaging the temporal bone region. “In more cooperative children, we use video goggles and the ministrations of a child-life specialist to encourage us to get through MR without sedation,” Dr. Robson said.
She also touched on the sometimes-difficult subject of what constitutes an enlarged vestibular aqueduct. Based on findings from a Cincinnati group involving a large number of measurements, it’s agreed that the midpoint measurement of a normal vestibular aqueduct should be less than 1 mm, and the opercular measurement should be less than 2 mm.