9.13  Neuroimaging

Computed Tomography (CT)

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:

  1. 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)
  2. 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.
Figure 9.12.2
Bitemporal Hemianopia (Bilateral)
Bitemporal hemianopia secondary to a pituitary macroadenoma.

Bone Windows

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

Soft Tissue Windows

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

Figure 9.13.2a Non-contrast vs Contrast CT

Non-Contrast

Figure 9.13.2b Non-contrast vs Contrast CT

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.

Contents

Figure 9.13.4 Blowout Fracture

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)

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 9.13.7 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

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)

Figure 9.13.10
Mucocele (Orbital)

Axial CT demonstrating a right orbital mucocele.

Figure 9.13.11 Pleomorphic Adenoma of the Lacrimal Gland

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

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

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

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:

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

“This is a…

1. Axial/Coronal/Sagittal …

2. Orbital/Brain…

3. MRI 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. T1 or T2 Weighted

  • Most images are a mix of T1 or T2, but weighted towards one

6. Fat Saturated/Suppressed

7. Gadolinium Enhanced

8. FLAIR

9. DWI

A Typical MRI Orbit Protocol Includes:

Whole Brain

  • T1 (Axial)
  • T1 (Sagittal)

Orbits

  • T1 (Axial)
  • T1 (Coronal)
  • T1 Fat saturated with gadolinium (Axial)
  • T1 Fat saturated with gadolinium (Coronal)
  • T2 Fat saturated (Coronal)
  • T1 fat saturated, gadolinium enhanced MRI’s are most commonly used for orbital pathology. Additional sequences can be performed according to the pathology being evaluated
Adjectives: Hyper/hypointense
  • Intensity depends on the water content of structures and the magnetic relaxation properties of protons in different tissues

T1 Weighted

T2 Weighted

Water (CSF, vitreous, oedema)

T1 Weighted

Black

T2 Weighted

White
(“Tea for two”- milk is white!)

Fat, blood, contrast

T1 Weighted

White

T2 Weighted

Variable

T1 Weighted

Grey matter (superficial) is darker
White matter (deep) is lighter

T2 Weighted

Grey matter (superficial) is lighter
White matter (deep) is darker

Best For

T1 Weighted

Anatomic detail

T2 Weighted

Pathology

T1 Weighted

9.1.3 MRI Table - T1 Axial
9.1.3 MRI Table - T1 Coronal

T2 Weighted

9.1.3 MRI Table - T2 Axial
9.1.3 MRI Table - T2 Coronal

Fat saturated/suppressed
(More commonly T1)

FLAIR (Fluid Attenuated Inversion Recovery)

Fat saturated/suppressed
(More commonly T1)

Suppresses white fat signal (fat is now black).

FLAIR (Fluid Attenuated Inversion Recovery)

Suppresses fluid (CSF and vitreous are black).
Compared with T1, FLAIR images are sharper but have more “noise”.
Look for white rim at anterior border of lateral ventricles.
White is abnormal.

Best For

Fat saturated/suppressed
(More commonly T1)

Recommended for all orbital MRI.
Improved view of: optic nerve, extra-ocular muscles, lacrimal gland, tumours, inflammatory lesions, vascular malformations.

FLAIR (Fluid Attenuated Inversion Recovery)

Demyelinating disease.
Oedema.

Fat saturated/suppressed
(More commonly T1)

9.1.3 MRI Table - T1 Coronal

FLAIR (Fluid Attenuated Inversion Recovery)

9.1.3 MRI Table - T1 Coronal

Gadolinium
(Always T1, usually fat saturated)

White: extra-ocular muscles, venous sinuses.

Best For

Tumours, inflammatory lesions
Remains intravascular unless there is a break-down in the blood brain barrier.

9.13 MRI Table - T1 + Gadolinium - Axial

DWI (Diffusion weighted image)

White = Ischaemia

Best For

Cerebrovascular accidents

9.13 MRI Table - DWI - Axial

NB: Bone and calcification are difficult to assess on MRI

Contents

Figure 9.13.15 Cavernous Haemangioma (Orbital) Non-contrast

Figure 9.13.15
Cavernous Haemangioma (Orbital)
Left: Axial T1 MRI.

Figure 9.13.15 Cavernous Haemangioma (Orbital) Contrast

Right: Axial T1 fat saturated MRI with gadolinium. The lobular nature of the cavernous haemangioma is demonstrated.

Figure 9.13.16 Cortical Blindness
Axial DWI demonstrating ischaemia in the occipital lobes.

Figure 9.13.17 Lymphoma (Orbital)

Figure 9.13.17
Lymphoma (Orbital)

Coronal T2 fat saturated MRI demonstrating multiple bilateral lymphomatous masses.

Figure 9.13.18 Metastatic Melanoma (Orbital)

Figure 9.13.18
Metastatic Melanoma (Orbital)

Axial T1, fat saturated MRI with gadolinium demonstrating a left intra-conal mass with peripheral enhancement. Subsequent biopsy diagnosed metastatic melanoma.

Figure 9.13.19 Multiple Sclerosis

Figure 9.13.19
Multiple Sclerosis

Axial and Sagittal views are shown
FLAIR MRI demonstrating multiple periventricular demyelinating plaques.

Figure 9.13.20 Nasal Carcinoma

Figure 9.13.20 Nasal Carcinoma
Axial T2 MRI demonstrating a large lobulated mass centred in the left ethmoid sinus with extension through the medial wall of the orbit. The globe is proptosed and displaced temporally.

Figure 9.13.21 Optic Nerve Glioma - T1 gadolinium

Figure 9.13.21
Optic Nerve Glioma

Left: Coronal T1 fat saturated MRI with gadolinium.

Figure 9.13.21 Optic Nerve Glioma - axial T2

Right: Axial T2 MRI.

Figure 9.13.22 Optic Nerve Meningioma

Figure 9.13.22
Optic Nerve Meningioma

Coronal T2 fat saturated MRI demonstrating hyperintensity secondary to an optic nerve meningioma in the right optic nerve. Note that only the optic nerve sheath (meninges) is hyperintense. On axial scans this is seen as a “tram-track” sign.

Figure 9.13.23 Optic Neuritis

Figure 9.13.23
Optic Neuritis

Coronal T2 fat saturated MRI demonstrating hyperintensity secondary to oedema in the right optic nerve.

Figure 9.13.24 Thyroid Eye Disease

Figure 9.13.24
Thyroid Eye Disease

Axial (Left) and Coronal (Right) T1 MRI demonstrating gross enlargement of the extra-ocular muscles (particularly the inferior, medial and superior recti). Note that there is fatty infiltration (white) of the extra-ocular muscles.

Figure 9.13.24 Thyroid Eye Disease

Positron Emission Tomography (PET) Scans

PET scans are used to determine the physiological activity of areas of the body. A radiolabelled metabolised substance is administered to the patient and taken up to various degrees by different tissues. Conventional PET scans use fludeoxyglucose, which is glucose with a positron-emitting fluorine-18 atom attached in place of a hydroxyl group. As this substance decays it releases positrons, which immediately collide with normal electrons and release gamma rays, which can be detected and localised. The amount of radiation released at a location corresponds to the amount of glucose utilised, which is a marker of metabolic activity.

These PET images are very useful, however relatively low resolution. By combining the acquisition of PET with CT cross-sectional images, a composite image can be obtained that colourises CT according the radioisotope update (and hence the metabolic activity). Abnormal lesions are then able to be qualitatively assessed as “hot” or “cold”, indicating high or low metabolic activity respectively.

Alternative radioisotopes (for example gallium dotutate, a somatostatin analogue) can give different physiological information and are used in different conditions.

PET scans are widely used in staging of malignancy, to determine if metastases are present, and if so where. Metastatic deposits can be detected on PET scan before they become clinically apparent or obvious on conventional cross-sectional imaging. Other specialised indications are also recognised.

The amount of radiation from a PET scan (approximately 250 chest X-rays) is significant and therefore PET scans should only be requested for patients with serious conditions where the benefits of early detection outweigh the risks of radiation exposure. Often, these scans are requested by a patient’s oncologist.

Candidates could be expected to identify when a PET scan should be requested in the initial workup of a patient with a malignant condition, and could also be asked to interpret a PET scan of an orbit as shown in Figure 9.9.25.[i]

While PET imaging is not specifically mentioned in the RANZCO Clinical Curriculum Performance Standards 2014 (current at the time of writing), candidates are expected to interpret neuro-imaging of the orbit. Other examination bodies may have different requirements of candidates.

Figure 9.13.25 PET-CT Scan

Figure 9.13.25
PET-CT Scan

3 selected slices from a PET-CT scan of a patient with a right lower eyelid melanoma with distant metastatic deposits in the right pre-auricular lymph nodes and left sacrum. “Hot” colours indicate high FDG uptake and metabolic activity. The update in the brain is physiological, but the hot areas in the anterior right orbit, right neck, and left pelvis are indicative of metastases.

          

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