Ocular Imaging

Question 1

What are the 3 layers of the globe?

Answer 1

Outer layer = cornea and sclera
Middle layer = iris, ciliary body and choroid`
Inner layer = retina

Question 2

Which imaging modality is best suited for imaging of the globe?

Answer 2

Ultrasound
Imaged from rostral approach through cornea (transcorneal or transpalpberal)

Question 3

What type of probe should be used when ultrasounding the globe and why?

Answer 3

High frequency (10-15MHz) linear probe - gives best anatomic detail

Frequency of the probe affects the spatial resolution and depth of penetration of the ultrasound beam.
Small/superficial structures best imaged with high frequency transducer that does not need large depth of penetration.
Linear probe allows broad field of view for examining the whole of the globe at once.

Question 4

What are the 3 chambers seen on ultrasound of a normal globe? What is their appearance?

Answer 4

Anterior Chamber
Posterior Chamber
Vitreal body
Anechoic in appearance

Question 5

How does the cornea present on ultrasound?

Answer 5

Single or double hyperechoic line with anechoic middle portion
1st line = corneal surface (highly refractive)
Anechoic area = stroma + posterior endothelium

Limbus = junction between highly reflective sclera and low reflective cornea.

Question 6

How does the posterior chamber appear on ultrasound?

Answer 6

Small anechoic area between periphery of the lens and this iris
Iris = thin hyperechoic structure with void in central portion for pupil
Ciliary body = at periphery of lens

Question 7

How does the lens appear on ultrasound?

Answer 7

Capsulated
Internal appearance of normal lens = anechoic
Curvilinear hyperechoic reflections at periphery of lens when scanned perpendicularly.
Normal nucleus same echogenicity as surrounding lens.

Question 8

Where does the vitreal body attach? How does it appear?

Answer 8

Thick and acellular = anechoic
Attached primarily in region of optic disc and where the ciliary bodies meet the choroid (ora serrata)
At posterior pole of lens vitreal body slightly less dense forming hyaloid canal - potential space houses hyaloid artery in the embryonic eye.

Question 9

Can the retina, choroid and sclera by distinguished on ultrasound of the posterior surface of the eye?

Answer 9

No cannot individually identify these layers on ultrasound.

Question 10

Can you use MRI/CT to image the globe? What will you see if you do use this modality?

Answer 10

Can be used but less detail than ultrasound
Will identify wall of globe, lens, ciliary body, anterior chamber, vitreous and region of optic disc.
Good for imaging of globe wall.

Question 11

Which bones make up the bony orbit? Which structure borders the globe laterally ?

Answer 11

Frontal bone (dorsal and medial)
Lacrimal bone (rostromedial)
Palatine bone (ventromedial)
Maxillary bone (ventral)
Zygomatic bone (ventrolateral)

Fibromuscular orbital ligament laterally - suprorbital process of the frontal bone to the zygomatic bone.

Question 12

What are the soft tissue structures of the retrobulbar space?

Answer 12

Optic nerve
External and internal ophthalmic arteries
Orbital plexus
Extrinsic muscles of the eyeball
Peri-orbital fat

Question 13

Which structure is located medial to the zygomatic arch and ventral to the peri-orbita?

Answer 13

Zygomatic salivary gland

Question 14

Where does the nasolacrimal duct begin and flow to?

Answer 14

Begins in lacrimal bone at fossa for lacrimal sac
Passes through lacrimal bone continues in canal on medial surface of maxila bone and finally opens ventral to the basal lamina of the maxilloturbinate scrolls.

Question 15

Discuss the pros/cons of conventional radiography for imaging the orbit. What other use might it be helpful for?

Answer 15

Pros - cheap and readily avaliable
Cons - low sensitivity compared to other modalities for detection of pathology

Oblique views to skyline areas of interest as well as lateral and dorsoventral views

Uses - trauma, detection of neoplasia, metastatic screening (lower sensitivity than CT), contrast imaging e.g dacryocystography

Question 16

How is ultrasound at imaging the retrobulbar space? When imaging this area which probe may be used and what else should you do?

Answer 16

Yes can image retrobulbar space - either via transcorneal approach or dorsolateral approach above rostral aspect of zygomatic arch.
Lower frequency (8-12MHz) preferred - microconvex gives better depth penetration so improves visualisation of deeper structures.

Ability to recognise normal anatomical structures within retrobulbar space limited and sensitivity and specificity quite low for retrobulbar disease detection.

Always image contralateral retrobulbar space for comparison.

Question 17

Which modalities are better for imaging the retrobulbar space? How do these modalities compare?

Answer 17

CT/MRI - allow assessment slice by slice without superimposition of nearby structures.

MRI - under GA (at least 45 mins study time)
CT - either sedation or GA (<10 mins average)

CT thinner slices than MRI - can be reformatted into different planes without losing image quality

Both can identify individual retrobulbar muscles as well as normal optic nerve.

Question 18

What is the modality of choice for assessment of bony structures around the eye?

Answer 18

CT

Question 19

Which modality is the best for detection of inflammatory changes within the retrobulbar space?

Answer 19

MRI - excellent soft tissue detail
Particularly good for optic nerve and can distinguish it from surrounding CSF.

Question 20

List the uses for radiography in ocular cases

Answer 20

Trauma - orbital fractures
Neoplasia detection
Radiopaque foreign bodies
Assessment of adjacent nasal cavity, frontal sinus and dental arcades
Thoracic imaging as part of metastatic screening
Contrast radiography e.g dacryocystography for nasolacrimal duct obstruction

Overall low sensitivity for detection of pathology - absence of radiographic abnormalities does not rule out disease.

Question 21

How may you image the vascular supply to the globe?

Answer 21

Doppler ultrasonography can identify vessels in retrobulbar space and can differentiate arterial supply from venous drainage.

CT superior for imaging vascular anatomy of globe/orbit.

Question 22

If performing radiographs how can we optimise technique?

Answer 22

GA
Patient positioning - no rotation
Standard views - lateral, ventrodorsal or dorsoventral
Skyline oblique views to highlight areas of interest
Increase exposure by 5-10kVP if nasopahrynx area of interest.
Oblique views both L and R markers used
Low KV technique for orbital structures (50-60 kV)
Tight collimation to area of interest to reduce scatter and improve resolution.

When assessing look for asymmetry between affected and non affected side
Look for evidence of osteolysis, osteoproliferation, fractures
Will not detect soft tissue pathology e.g abscesses, cellulitus or soft tissue masses not invading bone.
Ok for assessing for nasal neoplasia invading orbit through nasal cavity.

Question 23

When is dacryocystography performed?

Answer 23

Investigation of chronic conjunctivitis/epiphora and to confirm patency of the lacrimal duct.

Question 24

How is dacryocystography performed?

Answer 24

Done under GA
Survey radiography first with the diseased side closest to cassette to reduce magnification errors.
Nose tilted downwards to encourage flow of contrast medium and avoid retrograde flow into nasal chamber.
Superior punctum cannulated using nasolacrimal duct catheter and remaining punctum occluded using digital pressure.
Catheter advanced as far as possible and flushed with saline
Then 0.5-2ml undiluted non ionic water soluble iodinated contrast medium (iohexol) injected into catheter with gentle pressure.
Contrast injected until visualised at nares.
Lateral view of skull taken and sometimes also dorsoventral intraoral view and a R20degreeV-CdDO

Can also do this as part of CT investigation.

Duct should be of even diameter throughout course of passage to nares.

Occlusions can be partial or complete - secondary to discharge, foreign material, stenosis (mass or cicatrisation), dilation sometimes identified proximal to a region of stenosis.

Stenosis = consistently narrowed region of the duct

Lacrimal cyst - large cystic cavity that communicate within lacrimal system and contact contrast agent.

Question 25

Apart from imaging how else can ultrasound assist investigation of ocular disease?

Answer 25

FNA guided aspirations - e.g retrobulbar lesions

Question 26

How is the image generated for a linear probe vs a microconvex probe respectively?

Answer 26

Linear probe = multiple parallel beams (10-15MHz)
Microconvex = multiple divergent beams (7-10 MHz)

Linear probes = near field and its parallel beams maintain image quality throughout the depth of region imaging. Limitation - not image to the same depth as a similar frequency non linear probe so limited assessment for retrobulbar space.

Question 27

How would you perform ocular ultrasound typically?

Answer 27

Conscious patient (preferred to sedation/anaesthesia as rotation of globe/retraction of eye occurs)
Local anaesthesia drops applied - e.g proxymetacaine
Eyelids open or closed (open generally preferred)
Acoustic gel applied (care if risk perforated eye)
Imaged in horizontal and vertical planes
Eyes washed with saline once study complete

Question 28

What diseases can cause increase in the anterior chamber depth on ultrasound?

Answer 28

Glaucoma
Posterior lens luxation
Absent or abnormally small lens

Question 29

Which diseases can reduce anterior chamber depth on ultrasound?

Answer 29

Anterior lens luxation (secondary to trauma or glaucoma)
Neoplasia

Question 30

What may cause an increase in the echogenicity of the anterior chamber?

Answer 30

Should usually be anechoic

Haemorrhage, fibrin or perforated lens material may increase echogenicity.

Question 31

How do iridociliary cysts appear on ultrasound? What other condition may they be confused with?

Answer 31

Thin walled circular structures with anechoic contents - extend into either anterior chamber or posterior chamber.

Can be confused with iris bombe - circumferential adhesion of the iris to the lens or anterior vitreous resulting in anterior bowing of the iris.

Question 32

Which conditions will increase the reflectivity of the lens on ultrasound? How do you differentiate them on ultrasound?

Answer 32

Nuclear sclerosis and cataract formation

Nuclear sclerosis = thin hyperechoic line running parallel to anterior surface of the lens

Cataract = the lens opacifies. Opacification. that begins at margins of lens and develops towards centre = cortical, opacification that begins centrally = nuclear.

Question 33

What factors may affect the axial depth of the lens on ultrasound?

Answer 33

Increased depth = immature cataracts and diabetic cataracts
Decreased = hypermature cataracts

Question 34

Why is ultrasound used for pre-operative assessment before phacoemulsification surgeries?

Answer 34

Assess for concurrent lens abnormalities e.g lens rupture
Retinal detachments
Vitreous degeneration
Embryonic remnants within vitreous

Question 35

What will increase the echogenicity of the vitreous on ultrasound?

Answer 35

Haemorrhage
Inflammation
Membrane formation
Degeneration (multifocal speckles or linear echogenic regions - syneresis)
Embryonic remnants

Question 36

How does the presence of the hyaloid artery appear on ocular ultrasound? When should it have usually regressed by?

Answer 36

Usually regresses by day 45 of gestation - branch of ophthalmic artery which supplies nutrients to developing foetal lens

Triangular echodense posterior appearance of lens

Often associated with microphthalmia and cataract formation.

Question 37

How does complete retinal detachment appear on ultrasound?
How does it appear if it is a partial detachment?

Answer 37

Complete detachment = long continuous hyperechoic linear regions attached at the optic disc and ora cilliaris retina
Acute detachment may have undulating movement

Partial = more funnel shaped appearance

Question 38

How may intraocular foreign bodies appear on ultrasound?

Answer 38

Most = plant material
Occasionally seen as hyperechoic structures within globe. However not all will show acoustic shadowing if small size or may not be appreciated if surrounding vitreous remains anechoic.

Metal = comet ring artefact

Question 39

How is optic disc swelling seen on ultrasound? What is it associated with?

Answer 39

Rounded projection into the vitreous at the location of the optic disc
Associated with inflammation (optic neuritis), retrobulbar tumours (meningiomas) or can be marker for raised intracranial pressure (papilloedema)

Question 40

What are the most common types of ocular neoplasia seen on ultrasound?

Answer 40

Limbal melanoma - raised pigmented masses located in scleral or sunconjunctival tissue, slow growing nad not expected to spread but need to be differentiated from invasive tumours of the globe.

Anterior uveal melanomas - arise from iris or ciliary body (majority benign in dogs, malignant can spread elsewhere and uveal melanomas may be secondary from primary tumour elsewhere (e.g oral cavity, nail bed)
Cats - very malignant and high rate metastasis to liver/lungs (may not be evident for several years post enucleation)

Adenoma/adenocarcinomas - benign/slow glowing majority

Lymphoma - secondary neoplasia

Question 41

What scenarios can ultrasound be useful for the retrobulbar space?

Answer 41

Abscessation - area of echoic fluid accumulation
Occasionally identification of foreign bodies

Question 42

What are the most common orbital tumours in dogs/cats?

Answer 42

Dogs - osteosarcoma, mast cell tumour, reticulum cell sarcomas, fibrosarcomas, neurofibrosarcomas
Cats - squamous cell carcinoma

Meningiomas (rare) of optic nerve

Orbital neoplasia may be primary or secondary due to extension into orbit
Most malignant and local extension/metastasis common.

Question 43

When is CT preferred over MRI for ocular imaging?

Answer 43

CT primary imaging modality in investigation of retrobulbar disease that may be associated with alterations in positioning of the globe.

Exceptional at detecting bone destruction relating to primary orbital neoplasia or infection or from extension from surrounding areas (tooth root/nasal neoplasia)

Not as good for soft tissue assessment as MRI
Post contrast CT can be used to rapidly identify areas of retrobulbar masticatory muscle abscessation or inflammation.
Migrating grass seeds not easily identified either modality but both may identify an area of abscessation that can then be interrogated by ultrasound.
MRI more sensitive for organic matter FB’s than CT.

Question 44

What are the two types of MRI in the veterinary field and what are the differences between them?

Answer 44

Low field (permanent) magnets - 0.4 Tesla strength or less
High field (superconducting) 1.5 Tesla
A higher field stregth = improved resolution and reduced scanning time

Question 45

Which has thicker slices - MRI or CT?

Answer 45

MRI thicker slices (2.5mm - 4mm) - does not allow for multiplanar reconstruction without loss of detail

Question 46

When is MRI the modality of choice in ocular disease?

Answer 46

Superior for assessment of soft tissue structures and intracranial disease.
Intracranial neoplasia, multifocal inflammatory disease, optic nerve disease (optic neuritis), generalised inflammatory disease of retrobulbar structures and muscles of mastication.