ORBITS
| Preparatory steps: | |||
| - | Indications: structural diseases of the orbits and orbital content, trauma, foreign body | ||
| - | Advisable preliminary investigations: evaluation of visual function; evoked potentials; appropriate x-ray examination of the orbits may occasionally be necessary; MRI and ultrasonography may be alternative examinations 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 jaw to vertex | ||
| 1. | DIAGNOSTIC REQUIREMENTS | ||
| Image criteria: | |||
| 1.1 | Visualization of | ||
| 1.1.1 | Entire orbits | ||
| 1.1.2 | Osseous walls | ||
| 1.1.3 | Vessels after intravenous contrast media | ||
| 1.2 | Critical reproduction | ||
| 1.2.1 | Visually sharp reproduction of the osseous walls | ||
| 1.2.2 | Visually sharp reproduction of the optic nerve canal | ||
| 1.2.3 | Visually sharp reproduction of the globe | ||
| 1.2.4 | Visually sharp reproduction of the optic nerve | ||
| 1.2.5 | Visually sharp reproduction of the orbital muscles | ||
| 1.2.6 | Visually sharp reproduction of the retrobulbar fat | ||
| 1.2.7 | Visually sharp reproduction of the main vessels 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 orbital cavity |
| 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); secondary reduction of FOV is necessary for evaluation of subtle pathology |
| 3.6 | Gantry tilt | : | -6 to -10° from OM or parallel to the optic nerve 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 | : | 140-300 HU (soft tissue) 2000-3000 HU (bones) about 4000 HU (special orbit window) |
| 3.11 | Window level | : | 30-40 HU (soft tissue) 200-400 HU (bones) about 0 HU (special orbit window) |
| 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 |
| - | foreign bodies (beam hardening artefacts) | ||
| - | artefacts from orbital or dental prothesis/fillings | ||
| 4.4 | Modification to technique | - | change of gantry angulation or patient position to avoid artefact |