PETROUS BONE
| Preparatory steps: | |||
| - | Indications: hearing deficits, inflammation, vertigo, facial or acoustic nerve diseases, malformations, bone diseases and trauma | ||
| - | Advisable preliminary investigations: examination of acoustic and labyrinth function, evoked potentials; appropriate x-ray examination of skull, base and petrous bone may only occasionally be necessary; MRI may be an alternative examination without exposure to ionising radiation | ||
| - | Patient preparation: information about the procedure; restraint from food, but not fluid, is recommended, if intravenous contrast media are to be given | ||
| - | Scan projection radiograph: lateral from mastoid to above skull base | ||
| 1. | DIAGNOSTIC REQUIREMENTS | ||
| Image criteria: | |||
| 1.1 | Visualization of | ||
| 1.1.1 | Entire petrous bone | ||
| 1.1.2 | Vessels after intravenous contrast media | ||
| 1.2 | Critical reproduction | ||
| 1.2.1 | Visually sharp reproduction of the cortical and trabecular bone structures | ||
| 1.2.2 | Visually sharp reproduction of the bone structures of the temporal bone such as the cochlea: ossicular chain, fenestra ovale, facial canal and labyrinth | ||
| 1.2.3 | Visually sharp reproduction of the air filled compartments | ||
| 1.2.4 | Visually sharp reproduction of the adjacent cerebellum | ||
| 1.2.5 | Visually sharp reproduction of the adjacent cerebrum | ||
| 1.2.6 | Reproduction of border between the white and grey matter | ||
| 1.2.7 | Visually sharp reproduction of the great vessels and choroid plexuses after intravenous contrast media | ||
| 2. | CRITERIA FOR RADIATION DOSE TO THE PATIENT | ||
| 2.1 | CTDIW | : | no specific value as yet available (for information: routine head: 60 mGy) |
| 2.1 | DLP | : | no specific value as yet available (for information: routine head: 1050 mGy cm) |
| 3. | EXAMPLES OF GOOD IMAGING TECHNIQUE | ||
| 3.1 | Patient position | : | supine, for axial scans; supine or prone for coronal scans |
| 3.2 | Volume of investigation | : | from 0.5 cm below to 0.5 cm above the petrous bone |
| 3.3 | Nominal slice thickness | : | 1-3 mm |
| 3.4 | Inter-slice distance/pitch | : | contiguous or a pitch = 1.0 |
| 3.5 | FOV | : | head dimension (about 24 cm); secondary reduction of FOV is necessary for evaluation of subtle pathology |
| 3.6 | Gantry tilt | : | OM line or tilted above OM line for axial scanning; according to the patient position for coronal scanning |
| 3.7 | X-ray tube voltage (kV) | : | standard |
| 3.8 | Tube current and exposure time product (mAs) | : | should be as low as consistent with required image quality |
| 3.9 | Reconstruction algorithm | : | high resolution or standard |
| 3.10 | Window width | : | 2000-3000 HU (bones) 140-160 HU (soft tissue) 1500-2500 HU (middle setting) |
| 3.11 | Window level | : | 200-400 HU (bones) 30-40 HU (soft tissue) 150-250 HU (middle setting) |
| 4. | CLINICAL CONDITIONS WITH IMPACT ON GOOD IMAGING PERFORMANCE | ||
| 4.1 | Motion | - | movement artefact deteriorates image quality (prevented by head fixation or sedation of non-cooperative patients) |
| 4.2 | Intravenous contrast media | - | useful to identify vascular structures and enhancing lesions |
| 4.3 | Problems and pitfalls | - | calcifications versus contrast enhancement |
| - | interpetrous bone hardening artefacts | ||
| 4.4 | Modification to technique | - | subtle irregularity can be checked with slices in the area of suspected pathology, before considering contrast administration |
| - | higher mAs may be required if artefacts degrade the image quality in the posterior fossa | ||
| - | coronal scans may be used to reduce artefacts | ||
| - | intrathecal contrast may be useful to detect small accustic neuromas | ||