CT Sinuses and Temporal Bones
CT of the paranasal sinuses and CT of the temporal bones are high resolution bone algorithm studies of the facial and skull base air spaces and of the ear.
Both are done without contrast for most indications, on a multidetector scanner with isotropic voxels of 0.4 to 0.625 mm, reconstructed in axial, coronal and sagittal planes (for the temporal bone, additional oblique planes along and perpendicular to the superior semicircular canal and the long axis of the stapes).
For the sinuses the field runs from the frontal sinus to the hard palate (and the mandibular alveolar ridge when odontogenic disease is suspected); for the temporal bones it runs from the arcuate eminence to the mastoid tip, with a small field of view of 9 to 12 cm for each ear.
The scan takes under 20 seconds. Effective dose is about 0.1 to 0.5 mSv for sinus CT with low dose settings (a routine dose is about 1 to 2 mSv) and about 0.5 to 1.5 mSv for temporal bones. The lens dose is reduced by gantry angulation and low tube current.
Chronic rhinosinusitis affects about 5 to 12% of adults, and CT is the reference for anatomy before endoscopic sinus surgery; sinus CT has a sensitivity of over 95% for sinus opacification and bone erosion.
Hearing loss affects over 1.5 billion people in some degree, and CT is first line for conductive loss, chronic otitis media and cholesteatoma (sensitivity about 85 to 90% for cholesteatoma with ossicular erosion), congenital malformations, and trauma.
Intravenous contrast is added, or MR is used, for orbital and intracranial complications, tumours, and vascular lesions.
Indications. Sinuses: chronic or recurrent rhinosinusitis not responding to medical treatment, planning of functional endoscopic sinus surgery and image directed surgery, nasal polyposis, complications of sinusitis (orbital cellulitis, subperiosteal abscess, osteomyelitis of the frontal bone, intracranial extension), facial trauma, mucocele, fungal sinusitis,
tumours and skull base lesions (with contrast), cerebrospinal fluid leak, epistaxis, and anatomical variants before surgery.
Temporal bones: conductive and mixed hearing loss, chronic otitis media and cholesteatoma, complications of otitis (mastoiditis, petrous apicitis, labyrinthitis, sigmoid sinus thrombosis, intracranial abscess), temporal bone trauma, congenital ear malformations and cochlear implant planning, otosclerosis, pulsatile tinnitus, facial nerve palsy of unknown cause, vestibular symptoms (superior canal dehiscence), glomus tumours, and post-operative assessment.
Not for uncomplicated acute sinusitis (clinical diagnosis) or for sensorineural hearing loss without conductive features, where MR of the internal auditory canals is the study of choice.
Contraindications and precautions. No absolute contraindications. Pregnancy: avoid unless essential; low foetal dose from a head scan, so urgent indications are acceptable. Children: low dose settings, with sedation if necessary, and attention to lens dose.
Contrast for suspected abscess, tumour or vascular disease follows the usual precautions for allergy and renal function. Patients must stay still, and cooperation is checked in children.
Preparation and technique. No preparation. For the sinuses the patient lies supine with the head in a holder (or prone with the chin extended for direct coronal acquisition on older scanners).
The volume is acquired in an axial plane parallel to the hard palate, with coronal and sagittal reconstructions at 1 mm and a bone algorithm with a wide window (width 2000, level 200 to 300 HU) and a soft tissue window (width 400, level 40 HU).
For the temporal bones thin slices of 0.4 to 0.625 mm with a sharp bone kernel are used, with reformats in the plane of the lateral semicircular canal and the sagittal oblique along the facial nerve canal.
Contrast is given when the soft tissue question needs it (50 to 80 mL, with scan at 60 to 70 seconds). Dose is minimised with iterative reconstruction.
