SKULL BASE
| Preparatory steps: | |||
| - | Indications: neurological diseases (cranial nerves), trauma, malformations, metastasis and bone diseases | ||
| - | Advisable preliminary investigations: x-ray examination of the skull and base 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 C2 to skull vertex | ||
| 1. | DIAGNOSTIC REQUIREMENTS | ||
| Image criteria: | |||
| 1.1 | Visualization of | ||
| 1.1.1 | Entire skull base from C1 to the suprasellar region | ||
| 1.1.2 | Entire cerebellum | ||
| 1.1.3 | Basal part of the frontal lobes | ||
| 1.1.4 | 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 air filled compartments | ||
| 1.2.3 | Visually sharp reproduction of the sella turcica | ||
| 1.2.4 | Visually sharp reproduction of the cerebellar contours | ||
| 1.2.5 | Reproduction of the border between the white and grey matter (cerebellum) | ||
| 1.2.6 | Visually sharp reproduction of the cerebrospinal fluid space around the brain stem | ||
| 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 |
| 3.2 | Volume of investigation | : | from C1 to the suprasellar region |
| 3.3 | Nominal slice thickness | : | 2-5 mm |
| 3.4 | Inter-slice distance/pitch | : | contiguous or a pitch = 1.0 |
| 3.5 | FOV | : | head dimension (about 24 cm) |
| 3.6 | Gantry tilt | : | OM line |
| 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 soft tissue/standard |
| 3.10 | Window width | : | 2000-3000 HU (bones) 70-90 HU (supratentorial brain) 100-160 HU (brain in posterior fossa) |
| 3.11 | Window level | : | 200-400 HU (bones) 40-45 HU (supratentorial brain) 30-40 HU (brain in posterior fossa) |
| 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, enhancing lesions and alterations of blood-brain barrier |
| 4.3 | Problems and pitfalls | - | calcifications versus contrast enhancement |
| - | interpetrous beam 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 | ||