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B-Scan Ultrasonography - Ophthalmology Residency Viva Q&A

BASICS & TECHNIQUE

Q1. What is the frequency of the probe used in B-scan ultrasonography? A: 10 MHz (10 megahertz). This provides a good balance between resolution and tissue penetration for ocular imaging. A-scan standardized probes also use 10 MHz. UBM (ultrasound biomicroscopy) uses 50-100 MHz for anterior segment detail.

Q2. What does B-scan stand for, and how does it differ from A-scan? A: "B" = Brightness mode. B-scan produces a two-dimensional cross-sectional image where echoes are represented as bright dots, giving an anatomical picture of the eye. A-scan ("Amplitude" scan) is one-dimensional, producing spike peaks whose height represents echo amplitude - used for axial length measurement and tissue characterization (standardized A-scan). B-scan is better for morphology; A-scan is better for quantitative reflectivity measurement.

Q3. What is the speed of sound assumed for ocular biometry? A: 1532 m/s in the vitreous (aqueous/vitreous), 1641 m/s in the lens, and an average of 1550 m/s is used in most B-scan systems.

Q4. What are the three scan orientations used in B-scan? A:

Transverse scans - probe marker perpendicular to limbus; images equatorial structures
Longitudinal scans - probe marker parallel to limbus (pointing toward optic nerve); images meridional structures
Axial scans - probe centered over cornea with patient looking straight ahead; images optic nerve head and posterior pole

Q5. What coupling medium is used and why? A: Methylcellulose gel (viscous) when scanning through the eyelid (closed-eye technique), or saline/gel when scanning the cornea directly (open-eye/immersion technique). The coupling medium eliminates the air-tissue interface that would otherwise reflect all sound.

INDICATIONS

Q6. What are the indications for B-scan? A (high-yield list):

Media opacity obscuring fundal view (dense cataract, corneal opacity, vitreous haemorrhage, hyphaema)
Vitreous haemorrhage - to exclude retinal detachment or choroidal melanoma (Kanski's)
Intraocular tumour evaluation (choroidal melanoma, retinoblastoma, haemangioma)
Retinal detachment - extent, type (rhegmatogenous vs tractional vs exudative), funnel configuration
Choroidal detachment - distinguish serous from haemorrhagic
Posterior scleritis diagnosis
Intraocular foreign body (IOFB) - localisation (especially non-metallic/non-radio-opaque)
Endophthalmitis - vitreous membranes, retinal detachment
Optic nerve lesions - disc drusen, papilloedema measurement, optic nerve sheath diameter
Staphyloma assessment
Phthisis bulbi
Pre-surgical planning in opaque media

SPECIFIC FINDINGS - THE CORE VIVA TERRITORY

Q7. Describe the B-scan appearance of a vitreous haemorrhage. How do you assess its significance? A: Shows multiple dot/fleck echoes in the vitreous cavity ("shower of dots"). The key test is the aftermovement test (kinetic echography):

Ask the patient to look left-right rapidly, then fix; observe the echoes
Vitreous haemorrhage alone: echoes move freely and settle slowly (good mobility, high aftermovement)
If a membrane is underlying (e.g., retinal detachment), you see a membrane that inserts at the disc and ora serrata with restricted, undulating movement
A-scan of VH shows low, irregular spikes

The most important rule: always do B-scan in any dense vitreous haemorrhage to exclude retinal detachment or choroidal melanoma (Kanski's).

Q8. How do you differentiate retinal detachment from vitreous detachment and choroidal detachment on B-scan?

Feature	Retinal Detachment	Vitreous Detachment (PVD)	Choroidal Detachment
Echo brightness	Bright, strong	Thin, faint	Very bright, thick
Insertion	Optic disc + ora serrata (tethered)	Not attached at disc	Anterior (ciliary body), not disc
Mobility	Undulating (ripple), limited aftermovement	High mobility, floats freely	Very limited mobility, rigid
Shape	Concave toward vitreous	Irregular, wrinkled	Dome-shaped, lobular
A-scan spike	High (100% reflectivity)	Low to medium	Very high
Crosses midline	No	Yes	No (kissing detachment in bullous)

Retinal detachment inserts at optic disc - this is the distinguishing landmark. Choroidal detachment inserts anterior to the equator and does NOT reach the disc.

Q9. What is the "T-sign" on B-scan and what does it indicate? A: Seen in posterior scleritis. The stem of the T is formed by fluid in the sub-Tenon space (between the optic nerve sheath and the posterior sclera), and the crossbar of the T is formed by the optic nerve shadow. This produces the characteristic T-shaped shadow on axial B-scan. Also see scleral thickening and choroidal folds on B-scan. (Kanski's, Wills Eye Manual)

Q10. What are the classic B-scan features of choroidal melanoma? A:

Mushroom (collar-stud) shape - due to Bruch's membrane rupture - most specific feature
Acoustic hollow/shadowing - the tumour attenuates sound, creating a "choroidal excavation" sign (indentation into choroid)
Low-medium internal reflectivity on standardized A-scan (50-70%)
Orbital shadowing (sound shadow behind the lesion)
Secondary exudative retinal detachment (shallow, mobile)
Solid, dome-shaped mass with smooth surface
High vascularity on Doppler
For choroidal naevus vs melanoma: hollow acoustic appearance = risk factor for progression to melanoma (Kanski's)

Q11. What are B-scan features of retinoblastoma? A:

Irregular, heterogeneous intraocular mass
Calcification - hyperechoic foci with posterior acoustic shadowing (pathognomonic in the right clinical context - child with leukocoria)
Complete retinal detachment may be present
No choroidal excavation (unlike melanoma)
Orbital involvement: optic nerve thickening, extraocular spread

Q12. How do you differentiate serous from haemorrhagic choroidal detachment on B-scan? A:

Serous: Anechoic (optically empty, dark) subspace between detached choroid and sclera; smooth dome-shaped appearance; low A-scan
Haemorrhagic: Echoes within the subspace (clot); high internal reflectivity; often more irregular shape; A-scan shows medium-high irregular spikes. The dome is less smooth.
Clinically important because haemorrhagic detachment changes surgical management (wait for clot liquefaction before drainage, ~2 weeks) (Wills Eye Manual)

Q13. Describe the B-scan of optic disc drusen. A: Calcified drusen appear as a bright, hyperechoic nodule at the optic disc with posterior acoustic shadowing, even at low gain settings. This distinguishes drusen from true papilloedema (which shows no such hyperechoic signal). The buried drusen appear as a bright focus just anterior to the lamina cribrosa. B-scan is more sensitive than CT for detecting buried drusen.

Q14. How is B-scan used to assess papilloedema? A: Papilloedema on B-scan shows disc elevation (mushroom-shaped protrusion of disc surface). The subarachnoid space around the optic nerve sheath can be measured - an optic nerve sheath diameter (ONSD) >5 mm at 3 mm behind the globe is associated with raised intracranial pressure. This is also measurable with B-scan and is used as a bedside tool for ICP assessment.

Q15. What are the B-scan findings in endophthalmitis? A:

Vitreous opacities (multiple echoes, "snowball" or "snowstorm" pattern)
Vitreous membranes
Subretinal/subchoroidal fluid
Retinal detachment (especially tractional)
Choroidal thickening
In severe cases: optic nerve thickening, signs of panophthalmitis
Useful for monitoring resolution and timing of vitrectomy when media is opaque

ADVANCED/TRICKY VIVA QUESTIONS

Q16. What is "aftermovement" in B-scan and what is its clinical significance? A: Aftermovement (kinetic echography) refers to continued oscillation of an echo after the eye stops moving. It indicates membrane mobility:

High aftermovement = freely mobile (vitreous membranes, VH alone) = no or minimal traction
Low aftermovement = rigid (retinal detachment) = tethered at disc and ora
No aftermovement = completely fixed (choroidal detachment, IOFB, calcified lesion) This guides prognosis and surgical planning.

Q17. What is the gain setting and why is it important? A: Gain amplifies the returning echoes. A high-gain setting shows even low-level echoes (sensitive but less specific). A low-gain setting shows only high-reflectivity structures.

High gain: used to look for subtle vitreous opacities, PVD, thin membranes
Low gain: used to confirm solid structures (e.g., disc drusen remain bright at low gain, papilloedema echoes disappear at low gain; melanoma shows choroidal excavation more clearly) The standard approach: start at high gain, then reduce gain to characterize lesions.

Q18. How does B-scan help in an eye with suspected IOFB? A:

Locates the IOFB (shows as bright echo with orbital shadowing)
Determines whether it is anterior to, within, or posterior to the posterior capsule
Assesses associated pathology: retinal tear, vitreous haemorrhage, endophthalmitis
Non-metallic, non-radio-opaque FBs (glass, wood, plastic) that are missed on X-ray and CT can be detected on B-scan
Important caveat: if open-globe injury is suspected, do NOT press the probe on the globe - use the closed-eye technique gently or defer if risk of expulsive pressure

Q19. What is the "ring-down" artifact on B-scan? A: Also called reverberation artifact - occurs with highly reflective surfaces (gas bubbles, metallic IOFB, silicon oil). The echoes "ring" back and forth between the probe and the reflective surface, creating multiple parallel lines behind the structure. Gas bubbles in the vitreous show this distinctly. Silicone oil also produces a hyperechoic interface at the oil-vitreous boundary.

Q20. How do you use B-scan to assess a phthisical eye before evisceration vs enucleation? A:

Phthisis shows a disorganized, small globe with multiple calcified echoes (calcium deposits/bone)
Assess for hidden tumour (e.g., choroidal melanoma causing phthisis) before evisceration - evisceration is contraindicated if intraocular tumour is suspected (risk of tumour seeding)
B-scan helps rule out underlying malignancy

Q21. What is the difference between standardized A-scan and contact B-scan? When do you use each? A:

Standardized A-scan (Ossoinig technique): Fixed amplification (Kφ calibration), specific probe - provides reproducible, tissue-specific reflectivity profiles. Gold standard for characterizing intraocular lesions. Uses 8 MHz probe.
Contact B-scan: Anatomical, morphological imaging. Better for overall picture, extent of pathology, movement studies.
In practice: B-scan gives the map; A-scan gives the tissue signature. Both together are ideal for tumour assessment.

Q22. What are the B-scan findings in posterior staphyloma? A: Outpouching of the posterior sclera, most often at or around the optic disc. B-scan in axial view shows an ectasia (concavity) in the posterior scleral contour. The retina and choroid follow the contour. Associated with high myopia. Important to recognize because it can distort axial length measurements in biometry.

Q23. A patient presents with a white pupil (leukocoria) in infancy. How does B-scan help differentiate retinoblastoma from persistent fetal vasculature (PFV) and Coats disease?

	Retinoblastoma	PFV (PHPV)	Coats Disease
Eye size	Normal or enlarged	Microphthalmia	Normal
Calcification	Yes (strong acoustic shadow)	Absent	Absent
Mass	Solid, irregular	Fibrovascular stalk (vitreous)	No solid mass
Retinal detachment	Variable	Present (tractional)	Exudative (high mobility)
A-scan	High, irregular	Variable	Low to medium

Q24. What is the role of B-scan in assessing silicone oil-filled eyes? A:

The silicone oil creates a bright posterior interface and ring-down artifact
Useful to detect retinal re-detachment under silicone oil (retina detaches while oil is in place)
Can detect proliferative vitreoretinopathy (PVR) membranes
Assess emulsification of silicone oil (multiple small droplets appear as diffuse echoes)
Helps plan silicone oil removal surgery

PEARL QUESTIONS (EXAMINER FAVOURITES)

Q25. Why should you NOT use B-scan in an open globe injury? A: Any pressure on the open globe risks expulsion of intraocular contents (uvea, lens, vitreous). Even gentle probe contact can cause irreversible damage. If assessment is clinically necessary, use CT orbit instead. The eyelid-closed gentle technique may be used with extreme caution only if absolutely essential and no alternative exists.

Q26. What is the "double-arched" appearance on B-scan? A: Seen in kissing choroidal detachments - two large dome-shaped choroidal detachments meet in the midvitreous cavity, forming a "kissing" pattern. This is the most severe form of haemorrhagic choroidal detachment and is associated with very poor visual prognosis. It is a surgical emergency if associated with hypotony or open globe.

Q27. What does B-scan show in diffuse choroidal haemangioma (Sturge-Weber)? A: Diffuse choroidal thickening throughout the posterior segment (rather than a focal dome-shaped mass as in circumscribed haemangioma). Associated with secondary exudative retinal detachment and cystoid retinal degeneration (Kanski's).

Q28. In which posterior segment conditions is the retina tethered at the optic disc on B-scan? A: Retinal detachment (all types) - the retina physically attaches at the disc margin (no bare optic nerve on B-scan when retina is off except for retinoschisis, which shows splitting with tethered inner layer). Choroidal and vitreous detachments do NOT insert at the disc. This is the key discriminating sign.

Sources:

Kanski's Clinical Ophthalmology: A Systematic Approach, 10th ed. - vitreous haemorrhage, choroidal lesions, posterior scleritis (T-sign), PVD/RD differentiation
Wills Eye Manual - choroidal detachment differentiation, posterior scleritis workup, B-scan indications
touchOphthalmology - B-scan in retina practice - structured assessment, endophthalmitis, serial monitoring

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