Computed Tomography (CT) uses ionising radiation (X-rays) to produce cross-sectional images. It is preferred over Magnetic Resonance Imaging (MRI) for acute haemorrhage, bone delineation, calcification and metallic foreign bodies. CT should be avoided in pregnancy. Communicating to the radiologist the purpose of the scan is essential for correct imaging and interpretation. CT scans of orbits/brain are often shown to examination candidates in conjunction with oculoplastic (e.g. orbital tumours) and neuro-ophthalmic (e.g. multiple sclerosis) cases. Candidates should have a basic understanding of how to describe a CT scan.

“This is a…

1. Axial/Coronal/Sagittal …

2. Orbital/Facial bones/Brain…

3. CT Scan…

4. of Name/Age/Date Taken”

It is important that images be studied in more than one plane, otherwise pathology may be missed/misinterpreted

5. Bone (Brain Black) vs. Soft Tissue Windows

With bone windows, the brain is dark grey and bone is clearly delineated. With soft tissue windows the grey-white matter differentiation in the brain is clearly observed. Bone and soft tissue windows differ in two main ways:

The window width and level used. Bone has a density of 300-3000 Hounsfeld units (HU). Soft tissue has a density of 30-50 Hounsfeld units (HU)
The reconstruction algorithm. Bone windows use a sharp filter that enhances edges. The high resolution is optimal for bony detail, at the expense of increased noise (producing a grainy image). Soft tissue windows use a smoothing filter that reduces image noise at the expense of decreased spatial resolution

6. Contrast vs. Non contrast

Contrast does not cross an intact blood brain barrier, and is therefore useful for suspected inflammation, infection and malignant processes when this barrier may be compromised. (Relative) contraindications of contrast include: allergy to iodine, renal failure, multiple myeloma, diabetes, severe cardiac disease, asthma and active thyroid orbitopathy. A standard post-contrast scan is a venous phase CT. CT angiography (CTA) can be performed to evaluate arteries
Non-contrast CT can be used to evaluate haemorrhage, foreign bodies and orbital fractures

7. Normal (3mm) vs. Fine Slice

Slices as fine as 1mm are useful for imaging optic canal trauma and foreign bodies and should be specifically requested. Most scanners now acquire volumetric information and fine slices can be requested post-scan if required

Adjectives: Hyper/hypodense

Tissue density is represented on a grey-scale ranging from white (maximum, e.g. bone) to black (minimum, e.g. air)

Figure 9.12.2
Bitemporal Hemianopia (Bilateral)
Bitemporal hemianopia secondary to a pituitary macroadenoma.

Bone Windows

Soft Tissue Windows

Figure 9.13.1 CT Bone vs Soft Tissue Windows
Coronal and axial views shown

Non-Contrast

Contrast

Figure 9.13.2
Non-contrast vs Contrast CT
Note that the superior ophthalmic veins highlight in the contrast CT.

Figure 9.13.3
CT Angiography
Axial views shown. Contrast is seen in the middle cerebral arteries.

Figure 9.13.4: Blowout Fracture
Figure 9.13.5: Cavernous Haemangioma (Orbital)
Figure 9.13.6: Cellulitis – Post Septal (Orbital)
Figure 9.13.7: Cellulitis – Pre Septal
Figure 9.13.8: Hydrocephalus
Figure 9.13.9: Idiopathic Orbital Inflammatory Disease
Figure 9.13.10: Mucocele (Orbital)
Figure 9.13.11: Pleomorphic Adenoma of the Lacrimal Gland
Figure 9.13.12: Retrobulbar Haemorrhage
Figure 9.13.13: Ruptured Globe
Figure 9.13.14: Thyroid Orbitopathy

Figure 9.13.4
Blowout Fracture
Coronal CT demonstrating a left orbital floor blow-out fracture with herniation of soft-tissue around the inferior rectus through the fracture.

Figure 9.13.5a Cavernous Haemangioma (Orbital)

Figure 9.13.5b Cavernous Haemangioma (Orbital)

Figure 9.13.5
Cavernous Haemangioma (Orbital)
Top: Pre-contrast
Bottom: Post-contrast

Figure 9.13.6
Cellulitis – Post Septal (Orbital)
The infection has spread posterior to the orbital septum. There is an abscess adjacent to the right lateral orbital wall.

Figure 9.13.7 Cellulitis – Pre Septal

Figure 9.13.8
Hydrocephalus
Axial soft tissue brain CT demonstrating gross dilatation of the ventricles. This patient had gross papilloedema.

Figure 9.13.9
Idiopathic Orbital Inflammatory Disease
Axial CT demonstrating left thickening of the globe wall consistent with posterior scleritis in idiopathic orbital inflammatory disease.

Figure 9.13.10
Mucocele (Orbital)
Axial CT demonstrating a right orbital mucocele.

Figure 9.13.11
Pleomorphic Adenoma of the Lacrimal Gland
Axial CT demonstrating a right lacrimal gland mass causing proptosis of the right globe – Pleomorphic Adenoma

Figure 9.13.12
Retrobulbar Haemorrhage
Axial CT demonstrating a left retro-bulbar haemorrhage with proptosis. This is an ophthalmic emergency. Clinical evidence of retrobulbar haemorrhage indicates immediate decompression via lateral canthotomy and cantholysis prior to radiological scanning.

Figure 9.13.13
Ruptured Globe
Axial CT demonstrating a right globe rupture with loss of the lens (aphakia).

Figure 9.13.14
Thyroid Orbitopathy
Coronal CT demonstrating gross enlargement of the extra-ocular muscles, particularly the inferior rectus

CT orbits is the neuro-imaging modality of choice for most patients with thyroid orbitopathy. Signs that may be present include:

Enlargement of the extra-ocular muscles with tendon sparing (if non-tendon sparing consider other orbital inflammatory processes.
Proptosis (approximately >1/3 of orbit anterior to line through lateral orbital rims)
Increased orbital fat

Look at the orbital apex (coronal sections) for evidence of optic nerve compression Look at the bones/sinuses for orbital decompression

Magnetic Resonance Imaging

Magnetic Resonance Imaging (MRI) uses a large magnetic field to re-arrange protons in water molecules. The energy released by these protons re-equilibrating is then detected by a scanner to generate an image. MRI is preferred over Computed Tomography (CT) for soft tissue visualisation, demyelination and infarction. MRI is contraindicated with metallic foreign bodies and implants. Communicating to the radiologist the purpose of the scan is essential for correct imaging and interpretation. MRI scans of orbits/brain are often shown to examination candidates in conjunction with neuro-ophthalmic cases. Candidates should have a basic understanding of how to describe a MRI scan.