Day 8 (3): Ultrasonography of the Posterior Segment

Question 1

What is ultrasound?

Answer 1

• sound waves with HIGHER frequency than upper audible limit of human hearing
• frequency: > 20 KHz

Question 2

What are the indications, advantages and disadvantages of ophthalmic B scan ultrasound?

Answer 2

Indications
- examine posterior segment when direct visualization of intraocular structures is difficult or impossible
- confirm or differentiate pathologies based on tissue characteristics

Advantages
- accurately image intraocular structures
- shows DYNAMIC characteristics of structures

Disadvantages
- poorer resolution
- operator-dependent

Question 3

What is the Piezoelectric effect?

Answer 3

Piezoelectric Effect

• electromechanical property of certain materials like quartz where an electrical current applied through the object generates vibrations and pulsed sound waves
• electrical energy is converted to mechanical energy

Question 4

What is the Pulsed-Echo Principle?

Answer 4

Works similarly to the sonar of a boat or submarine

1. Piezoelectric crystals contained within the probe vibrate and create high-frequency sound waves which are transmitted through the medium.
2. After striking the intraocular structures, sound waves are either absorbed by the structure or reflected back to the probe.
3. Presence (or absence) and the intervals between the reflected sound waves are converted by the machine into electric signals and displayed as a 2D image: ECHOGRAM

Question 5

What are the factors affecting the ultrasound image?

Answer 5

1. Frequency: DIRECTLY proportional to image quality
2. Wavelength: INVERSELY proportional to image quality
3. Velocity
4. Reflectivity
5. Angle of Incidence of probe
6. Absorption by intervening dense structures: shadowing

Question 6

What is the relationship between wave frequency, wavelength, depth of penetration and image resolution?

Answer 6

HSSS:
HIGHER frequency
SHORTER wavelength
SHALLOWER penetration
SPLENDID image resolution

Abdominal UTZ: 1 - 5 MHz
- lower frequency
- longer wavelength
- deeper penetration
- grainy resolution

Ophthalmic UTZ: 10 MHz
- depth: 4 cm
- resolution: 940 microns

Ultrasound Biomicroscopy: 20 - 50 MHz
- depth: 0.5 - 1.0 cm only (SHALLOWER penetration)
- resolution: 40 microns (BETTER resolution)

Question 7

What is the relationship between medium density and sound wave velocity?

Answer 7

INVERSELY proportional
- the DENSER the medium, the FASTER the propagation of sound
- v.s. light waves which travel faster in less dense media
- Air: 330 m/s
- Water: 1500 m/s
- AH/VG: 1532 m/s
- Cornea/Lens: 1641 m/s

Question 8

What is the relationship between structure reflectivity and object density?

Answer 8

Reflectivity: measured in terms of acoustic impedance

Acoustic impedance = sound velocity x density of medium
- as object density INCREASES
- acoustic impedance INCREASES
- reflectivity of structure INCREASES
- amplitude of spike INCREASES (A scan)
- image WHITER (B scan)

Highly Reflective: 100% of waves reflected back to probe
- retina
- foreign bodies
- calcifications

Moderately Reflective: 50% reflected, 50% transmitted
- membranes
- organized hemorrhage

Poorly Reflective: < 25% reflected; 75% transmitted
- floaters
- minimal hemorrhage

Question 9

What is A (Amplitude) Scan Biometry?

Answer 9

• Measures the TIME it takes for a SINGLE sound wave to travel from the probe to the retina and back
• Uni-dimensional: measures TIME only (hence INDIRECT measure of distance which is calculated from an equation)
• Purpose:
  1. Determine the axial length (DISTANCE) of the eye indirectly
• combined with Keratometry to calculate IOL power
• most common indication
  2. For cases where fundus is obscured from visualization by slit lamp or laser interferometry

EQUATION: Distance (AL) = Time x Velocity
1. Distance: distance from the anterior pole to the posterior pole of the globe)
2. Time: for the sound waves to travel from the probe to the retina and back
3. Velocity: of the sound wave in the given medium

Question 10

Principle behind Amplitude (A) Scan Biometry.

Answer 10

• Measures the TIME it takes for a SINGLE sound wave to travel from the probe to the retina and back
• If an interface is encountered, the wave is either reflected back as echoes or transmitted through
• Echoes that return to the probe are converted to SPIKES
• Spike HEIGHT is proportional to the STRENGTH of the echo
• AXIAL LENGTH = distance between corneal and retinal spike

Factors affecting spike amplitude:
1. Properties of the 2 tissues at the interface
- if very different: majority of wave reflected back = stronger echo = higher spike
- if almost similar: majority pass through = short spike

2. Wave Angle of Incidence: angle of the wave from a line perpendicular to the surface
   - higher angle of incidence = less echo returning to transducer = shorter spike
3. Smoothness or regularity of interface
4. Density of structures the wave passes through
   - denser/solid = more waves reflected = more echoes = tall spikes

Results:
1. Solid/Dense structure: most or all waves reflected back as echoes
- A scan: tall spikes
- B scan: hyperechoic (white)

2. Semi-solid structure: most transmitted, some reflected back
   - A scan: medium to small spikes
   - B scan: hypoechoic or mixed echogenicity
3. Liquid: all waves transmitted through
   - A scan: no spikes/flat
   - B scan: anechoic (black)

Question 11

What measurements are derived from the A scan?

Answer 11

1. Reflectivity: Spike amplitude
   - measured either in absolute value in dB or % between 1st spike and spike of interest
   - retinal detachment: 90 - 100%
   - posterior vitreous detachment: <80%
2. Internal Structure: Regularity of Height
   - choroidal melanoma: medium to low internal echoes with vascular pulsations
3. Sound Attenuation: Angle of decline in spike height
   - choroidal melanoma: strong, smooth sound attenuation
   - choroidal hemangioma: weak sound attenuation

Question 12

What structures are denoted by the tall spikes in the A Scan Biometry?

Answer 12

Taller spikes = Intensity of echoes reflected back to probe = Solid

1st spike: Probe-Cornea interface
FLAT: Aqueous Humor
2nd spike: Anterior Lens Capsule
3rd spike: Posterior Lens Capsule
FLAT: Vitreous Body
4th spike: Retina
5th spike: Sclera
6th spike (decreasing amplitude): Orbital fat and tissues

Axial Length
- Distance from 1st spike to 4th spike (probe tip/cornea –> retina)
- N: 23.5 mm [22 - 25 mm]

Anterior Chamber Depth
- Distance from 1st spike to 2nd spike (probe tip/cornea –> anterior lens capsule)
- N: 3.24 mm [2.5 - 4.0 mm]

Lens Thickness
- Distance from 2nd spike to 3rd spike (anterior lens capsule to posterior lens capsule)
- N: 4.63 mm [up to 7 mm]

Question 13

Three methods employed in doing A Scan Biometry.

Answer 13

A. Contact/Acoustic - Applanation Method
- Handheld: easy, quick BUT compresses cornea
- Tonometer-mounted: minimal compression BUT cumbersome
- Pt can be seated
- Limitations:
1. Variable corneal compression (even with same examiner)
2. No precise localization
3. Limited resolution
4. Incorrect assumptions re: sound velocity
5. Potential for incorrect measured AL

B. Contact/Acoustic - Immersion Method
- uses Praeger (scleral) shell with saline or methylcellulose
- more accurate than applanation method
- PROS:
1. NO corneal compression: probe does not touch cornea
2. Better consistency and reproducibility of measurements
- CONS: more inconvenient
1. Asians with smaller palpebral fissures: difficulty in placing Praeger shell
2. Pt has to lie down else the saline will spill

Difference bet. AL measured by Immersion VS Applanation
- 0.14 - 0.28 mm
- 0.10 mm error ~ 0.25 D difference

C. Non-Contact/Optical Method: IOL Master

Question 14

Compare the accuracy of the different methods of A Scan Biometry.

Answer 14

Applanation: +/- 0.24 mm (huge range of values; most inaccurate)

Immersion: +/- 0.12 mm

Optical (IOLMaster): +/- 0.01 mm (most accurate)

Question 15

What is gain setting?

Answer 15

Electronic amplification of the signal coming back to the probe
Higher gain –> higher sensitivity to signals –> higher spikes
For imaging of smaller lesions or through an obstruction (opacities)
Disadvantage: more noise + merging of interfaces

What is seen in the scan if gain set too high:
1. (+) Spikes with flat tops (esp. RETINAL spike)
2. (+) Blurring or merging of posterior spikes

Recommendation:
Begin with high gain to detect small lesions, and then to reduce the gain to improve sharpness of the image

Question 16

What is the correct probe positioning?

Answer 16

1. Ensure probe is perpendicular to the macula and passes through the visual axis by positioning probe perpendicular to the cornea.
2. Lightly touch the cornea and avoid compression as this will shorten the axial length measurement.

Ensures:
1. Most, if not all, waves will be reflected back to the probe –> maximum spike amplitude/height
2. Least noise

Remember: misalignment causes waves to scatter to other directions –> less waves make it back to the probe –> smaller spikes

Question 17

What are the causes of probe-visual axis misalignment?

Answer 17

Causes some waves to scatter and not make it back to the probe
–> decrease in spike height/amplitude

1. Probe is not perpendicular to the cornea
2. Macular surface is NOT SMOOTH
   - Macular degeneration
   - Epiretinal membranes
3. Macular surface is CONVEX
   - Macular edema
   - Pigment epithelial detachment

Question 18

Criteria for a good A Scan?

Answer 18

1. Presence of various spikes (usually 5 high amplitude spikes):

• 1st spike: Probe-Cornea Interface
• 2nd spike: Anterior Lens Capsule
• 3rd spike: Posterior Lens Capsule
• 4th spike: Retina
• 5th spike: Sclera
• 6th spike (smaller): Orbital fat and tissues

2. Spikes are tall and rises STEEPLY esp. RETINAL (5th) SPIKE
   - (-) sloping, jags, humps or steps
3. No spikes in front of the retina (vitreous: should be flat) and should be clearly separate from the scleral spike.
4. Reproducible measurements
   - Repeated 3-5x
   - Same eye: difference within +/- 0.10 mm only
   - Both eyes: difference within +/- 0.30 mm only

Question 19

Most common error encountered in contact method of A Scan biometry?

Answer 19

Corneal Compression = Myopic Surprise
- Cornea is naturally pliable; aggravated by lower IOP
- Avoided by the IMMERSION method
- Inaccurately SHORTER AL measurement

What to check?

1. INCREASING Axial Length
   - Distance between 1st (probe/cornea) and 4th (retinal) spikes
   - N: 23.5 mm (22.0 - 24.5 mm)
2. DECREASING Anterior Chamber Depth
   - Distance between 1st (probe/cornea) and 2nd (anterior lens capsule) spikes
   - N: 3.24 (2.5 - 4.0 mm)
   - Delete measurement if ACD becomes shorter/shallower
   - LONGER ACD is MORE ACCURATE because you can’t extend AC but you can compress it

How does this cause myopic surprise?
1. Inaccurately SHORT AL measurement
2. To focus light on a shorter eye, higher powered IOL needed
3. Placing the inappropriately higher powered IOL in the pt’s eye with a longer ACTUAL AL will cause light to focus in FRONT of the macula and not at the retina.

Question 20

What are the biometry findings if the probe is misaligned?

Answer 20

Situation 1: PROBE NOT PERPENDICULAR TO CORNEA
- Hence, NOT perpendicular to the center of the lens and macula and DOES NOT pass through the visual axis
- Due to pt not focusing or looking directly at the fixation spot/light
- Light scatters instead of mostly returning back to probe
- Normally:

If waves pass through the visual axis and the center of the lens:
1. BOTH anterior and posterior lens spikes have HIGH amplitude

If waves pass perpendicular to the macula:
2. BOTH retinal spike and scleral spikes have HIGH amplitude
3. Retinal spike rises steeply from baseline (NO SLOPING)
4. NO jags, humps, or steps on the ascent of the retinal spike

• Findings in biometry:
  1. Lens spikes are UNEQUAL (posterior lens spike MUCH SHORTER than anterior)
• not due to cataract because NO smaller spikes in between
  2. SHORT retinal spike which does not rise steeply
  3. (+) Artifactual spikes in front of the retinal spike

Situation 2: PROBE ALIGNED WITH OPTIC NERVE
- Passes through posterior scleral foramen
- Pt cannot see light because ON is a blind spot
- Finding: NO scleral spike because wave passes through ON

Question 21

What is the B scan?

Answer 21

Brightness scan
- multiple A scans sweeping across an arc
- echoes are converted to dots with brightness (B scan) proportional to the echo amplitude (A scan)
- unidimensional A scan (time) is converted into a 2D image
- DENSE = HIGH amplitude = HYPERechoic (white)
- LIQUID = ABSENT echoes/FLAT line = HYPO/ANechoic (black)
- if object is dense enough: (+) posterior shadowing
- investigated parameters:
1. location
2. shape
3. borders
4. size
5. mobility: fixed or mobile; (+/-) aftermovements
6. proximity to the optic disc

Question 22

Reminders when doing B scan ultrasound of the eye.

Answer 22

1. Marker position corresponds to TOP part of screen
   - always orient marker SUPERIORLY or NASALLY
2. Apply probe directly on the anesthetized eye except:
   - trauma
   - children
   –> performed over the eyelid with coupling jelly
3. Start with HIGH gain to visualize weak signals; once lesion is identified, DECREASE gain to improve resolution and lessen noise.
   - small lesions and foreign bodies
   - vitreous opacities and hemorrhages
   - posterior vitreous detachments
4. Patient should look TOWARDS the direction of the quadrant/area to be evaluated.
5. Ensure structure under investigation is CENTERED to obtain the best quality image.
6. Use limbus-to-fornix rocking and rotational motion
   - gently glide the probe from the limbus of the eye to the fornix in a sweeping motion
   - ensures that sound waves always pass through the center and maximizes the amount of retina visualized

Question 23

What do the hyperechoic line and anechoic space in the B scan image denote?

Answer 23

Probe face
- LEFTmost hyperechoic arc on the screen
- marker (dot or line): SUPERIOR portion of the screen

Vitreous cavity
- anaechoic space in the CENTER

Posterior pole +/- lesions
- structures on the RIGHT of variable echogenicity

Question 24

What are the different TRANSVERSE probe views utilized in B scans?

Answer 24

• used for basic SCREENING examinations if no direct view of fundus
• encompasses 6 clock hours to examine the 4 QUADRANTS of the fundus
• provides a “lateral” view or transverse dimensions of a lesion
• probe placed outside the limbus
• labelled according to AREA at the CENTER of the scan

T12/SUPERIOR Quadrant Scan
- probe: INFERIOR aspect pointing SUPERIORLY
- gaze: UP
- marker: NASAL

T3/NASAL (R) or TEMPORAL (L) Quadrant Scan
- R eye
+ probe: TEMPORAL aspect pointing NASALLY
+ gaze: LEFT
- L eye
+ probe: NASAL aspect pointing TEMPORALLY
+ gaze: LEFT
- marker: SUPERIOR

T6/INFERIOR Quadrant Scan
- probe: SUPERIOR aspect pointing INFERIORLY
- gaze: DOWN
- marker: NASAL

T9/TEMPORAL (R) or NASAL (L) Quadrant Scan
- R eye
+ probe: NASAL aspect pointing TEMPORALLY
+ gaze: RIGHT
- L eye
+ probe: TEMPORAL aspect pointing NASALLY
+ gaze: RIGHT
- marker: SUPERIOR

Question 25

What are the different LONGITUDINAL probe views utilized in B scans?

Answer 25

• best for more detailed examinations of pathologies
• shows the RADIAL extent of a lesion by providing a “vertical” view or longitudinal dimensions of a lesion
• visualizes 1 clock hour only BUT extent of the lesion from the posterior pole to anterior retina is imaged
• recommended for: 1) intraocular tumors 2) retinal tears
• probe:
  + placed outside the limbus
  + marker directed TOWARDS the PUPIL
• image:
  + labelled according to the CLOCK HOURS
  + SUPERIOR part: ANTERIOR extent
  + INFERIOR part: POSTERIOR extent

L1: probe pointing to the 1 o’ clock hour
L2: probe pointing to the 2 o’ clock hour
L3:
- probe pointing to the 3 o’ clock hour
- gaze: LEFT

L4: probe pointing to the 4 o’ clock hour
L5: probe pointing to the 5 o’ clock hour
L6:
- probe pointing to the 6 o’ clock hour
- gaze: DOWN

L7: probe pointing to the 7 o’ clock hour
L8: probe pointing to the 8 o’ clock hour
L9:
- probe pointing to the 9 o’ clock hour
- gaze: RIGHT

L10: probe pointing to the 10 o’ clock hour
L11: probe pointing to the 11 o’ clock hour
L12:
- probe pointing to the 12 o’ clock hour
- gaze: UP

Question 26

What is the Longitudinal Macula Scan?

Answer 26

• probe: NASAL aspect pointing CENTRALLY to macula
• gaze: TEMPORAL
• marker: TOWARDS PUPIL
• image:
  + MACULA: at center
  + OPTIC NERVE: below the macula

Question 27

What is the Axial probe view?

Answer 27

• most commonly seen ultrasound image
• images the lens and the posterior pole in one view
• probe: CENTERED on the pupil through the visual axis pointing POSTERIORLY
• disadvantages:
  1. phakic: sound attenuation by the lens compromises image quality
  2. pseudophakic: artifactual reverberations

2 Types:
1. Vertical Axial Scan: marker pointed SUPERIORLY
- longitudinal cut: images lens + macula
2. Horizontal Axial Scan: marker pointed NASALLY
- similar to LMAC but probe centered on the pupil
- transverse cut: images lens + macula + optic nerve

Question 28

What are the 5 Basic Scans obtained on the posterior pole?

Answer 28

Quadrant Views using Transverse technique:
- scan is swept from posterior pole to anterior periphery

1. T12 Quadrant
2. T3 Quadrant
3. T6 Quadrant
4. T9 Quadrant
5. Posterior Pole View
   - to image the macula and optic nerve in one view
   - options:
   + Longitudinal Macula Scan - off-center
   + Horizontal Axial Scan - on-center; with lens view

Note:
If pathology is detected, additional tests may be requested such as:
1. Longitudinal scan: obtain radial or anterior-posterior dimensions of the lesion
2. Axial Scan: document location of pathology with respect to the optic disc
3. A-Scan: further tissue differentiation

Question 29

B scan appearance of vitreous floaters vs asteroid hyalosis?

Answer 29

Floaters:
- mobile, discrete echoes with LOW reflectivity (gray)
- composition: pathologically aggregated collagen

Asteroid bodies:
- mobile, discrete, dense echoes with HIGH irregular reflectivity (white)
- on eye movement: moves with the vitreous and returns to it’s original position
- CLEAR vitreous adjacent to the retina
- composition: calcium soaps

Question 30

A scan and B scan appearance of posterior vitreous detachment vs vitreous hemorrhage vs retinal detachment?

Answer 30

PVD secondary to Vitreous Syneresis
- (+) posterior hyaloid face
- PARTIAL/incomplete: vitreous still attached to the optic disc
- TOTAL/complete: vitreous completely detached and floating
- A scan: LOW amplitude spikes between the 3rd and 4th spikes
- B scan:
+ THIN undulating membrane containing HYPOechoic fluid within
+ (+) aftermovements: slow gel-like movement of the vitreous even after eye movement has ceased
+ DECREASING echogenicity towards PERIPHERY

Vitreous Hemorrhage
- A scan: low to moderate amplitude spikes between the 3rd and 4th spikes
- B scan:
+ dense MIXED ECHOgenic fluid in the vitreous
+ membrane may not be visualized if still attached
+ (+) aftermovements if vitreous detached

Retinal Detachment
- A scan: HIGH amplitude spikes between the 3rd and 4th spikes
- B scan:
+ THICK, dense and HYPERechoic membrane in the vitreous
+ mobile with eye movement BUT NO or minimal aftermovements
+ remains attached to the optic disc
+ UNIFORM 80-100% echogenicity over ENTIRE extent

Choroidal Detachment
- A-scan: HIGH amplitude spike with DOUBLE peaks

Question 31

Difference in B scan appearance of Tractional vs Exudative Retinal Detachment?

Answer 31

Tractional RD
- seen in advanced (proliferative) diabetic retinopathy
- proliferative membranes that contract and elevate the retina
- thick, dense, HYPERechoic and IMMOBILE membrane
- (+/-) funneling: due to multiple intervening THICK bands with MIXED ECHOgenicity within the vitreous

Exudative RD
- subretinal exudation
- dome-shaped elevation outlined by a thick, dense and HYPERechoic membrane containing SHIFTING HYPOechoic FLUID on eye movement

Question 32

A scan and B scan appearance of choroidal detachment.

Answer 32

A-Scan
80 - 100% reflectivity of the choroid showing a HIGH amplitude spike between the 4th (retinal) and the 5th (scleral) spike.

B-Scan
Homogenous thick DOME-shaped membrane NOT attached to the optic disc and MOVES with the eye
- suprachoroidal space may contain:
+ serous fluid: HYPOechoic
+ hemorrhage: MIXED echogenic

Question 33

B scan appearance of intraocular foreign bodies.

Answer 33

A Scan
- High amplitude spike in between the 3rd (posterior lens capsule) spike and the 4th (retinal) spike
- Due to the high object density, sound waves are unable to penetrate the object and cause shadowing where 4th, 5th and 6th spikes become indistinct from each other.

B Scan
- Dense, HYPERechoic object within the vitreous cavity with posterior SHADOWING

Question 34

B scan appearance of dropped IOL.

Answer 34

• Posterior shadowing behind the IOL with eye movement
• (+/-) Artifactual reverberations

Question 35

B scan appearance of dislocated cataract.

Answer 35

HYPERechoic structure, lenticular in shape, floating in the vitreous cavity