Day 8 (3): Ultrasonography of the Posterior Segment Flashcards
What is ultrasound?
- sound waves with HIGHER frequency than upper audible limit of human hearing
- frequency: > 20 KHz
What are the indications, advantages and disadvantages of ophthalmic B scan ultrasound?
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
What is the Piezoelectric effect?
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
What is the Pulsed-Echo Principle?
Works similarly to the sonar of a boat or submarine
- Piezoelectric crystals contained within the probe vibrate and create high-frequency sound waves which are transmitted through the medium.
- After striking the intraocular structures, sound waves are either absorbed by the structure or reflected back to the probe.
- 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
What are the factors affecting the ultrasound image?
- Frequency: DIRECTLY proportional to image quality
- Wavelength: INVERSELY proportional to image quality
- Velocity
- Reflectivity
- Angle of Incidence of probe
- Absorption by intervening dense structures: shadowing
What is the relationship between wave frequency, wavelength, depth of penetration and image resolution?
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)
What is the relationship between medium density and sound wave velocity?
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
What is the relationship between structure reflectivity and object density?
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
What is A (Amplitude) Scan Biometry?
- 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
Principle behind Amplitude (A) Scan Biometry.
- 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
- 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 - Smoothness or regularity of interface
- 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)
- Semi-solid structure: most transmitted, some reflected back
- A scan: medium to small spikes
- B scan: hypoechoic or mixed echogenicity - Liquid: all waves transmitted through
- A scan: no spikes/flat
- B scan: anechoic (black)
What measurements are derived from the A scan?
- 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% - Internal Structure: Regularity of Height
- choroidal melanoma: medium to low internal echoes with vascular pulsations - Sound Attenuation: Angle of decline in spike height
- choroidal melanoma: strong, smooth sound attenuation
- choroidal hemangioma: weak sound attenuation
What structures are denoted by the tall spikes in the A Scan Biometry?
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]
Three methods employed in doing A Scan Biometry.
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
Compare the accuracy of the different methods of A Scan Biometry.
Applanation: +/- 0.24 mm (huge range of values; most inaccurate)
Immersion: +/- 0.12 mm
Optical (IOLMaster): +/- 0.01 mm (most accurate)
What is gain setting?
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
What is the correct probe positioning?
- Ensure probe is perpendicular to the macula and passes through the visual axis by positioning probe perpendicular to the cornea.
- 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
What are the causes of probe-visual axis misalignment?
Causes some waves to scatter and not make it back to the probe
–> decrease in spike height/amplitude
- Probe is not perpendicular to the cornea
- Macular surface is NOT SMOOTH
- Macular degeneration
- Epiretinal membranes - Macular surface is CONVEX
- Macular edema
- Pigment epithelial detachment
Criteria for a good A Scan?
- 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
- Spikes are tall and rises STEEPLY esp. RETINAL (5th) SPIKE
- (-) sloping, jags, humps or steps - No spikes in front of the retina (vitreous: should be flat) and should be clearly separate from the scleral spike.
- Reproducible measurements
- Repeated 3-5x
- Same eye: difference within +/- 0.10 mm only
- Both eyes: difference within +/- 0.30 mm only
Most common error encountered in contact method of A Scan biometry?
Corneal Compression = Myopic Surprise
- Cornea is naturally pliable; aggravated by lower IOP
- Avoided by the IMMERSION method
- Inaccurately SHORTER AL measurement
What to check?
- INCREASING Axial Length
- Distance between 1st (probe/cornea) and 4th (retinal) spikes
- N: 23.5 mm (22.0 - 24.5 mm) - 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.
What are the biometry findings if the probe is misaligned?
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
What is the B scan?
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
Reminders when doing B scan ultrasound of the eye.
- Marker position corresponds to TOP part of screen
- always orient marker SUPERIORLY or NASALLY - Apply probe directly on the anesthetized eye except:
- trauma
- children
–> performed over the eyelid with coupling jelly - 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 - Patient should look TOWARDS the direction of the quadrant/area to be evaluated.
- Ensure structure under investigation is CENTERED to obtain the best quality image.
- 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
What do the hyperechoic line and anechoic space in the B scan image denote?
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
What are the different TRANSVERSE probe views utilized in B scans?
- 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
