COT Biometry and A-Scans 3 — Questions and Answers
Question 1: In A-scan biometry, the velocity used for the vitreous cavity is approximately:
- 1532 m/s (Correct answer)
- 1641 m/s
- 1550 m/s
- 1485 m/s
Correct answer: 1532 m/s
The vitreous humor has a sound velocity of approximately 1532 m/s, similar to water.
Question 2: Which measurement does optical biometry (IOLMaster) rely on rather than ultrasound?
- Doppler shift
- Partial coherence interferometry (Correct answer)
- Time-domain reflection
- Infrared spectrometry
Correct answer: Partial coherence interferometry
Optical biometry uses partial coherence interferometry (PCI) to measure axial length with high precision.
Question 3: A-scan biometry measures axial length as the distance from the:
- Corneal vertex to the fovea (Correct answer)
- Corneal epithelium to the retinal pigment epithelium
- Corneal endothelium to the internal limiting membrane
- Limbus to the optic nerve head
Correct answer: Corneal vertex to the fovea
Axial length is measured from the anterior corneal surface (vertex) to the fovea (macula).
Question 4: When performing A-scan on a silicone oil-filled eye, the technician must:
- Use a higher gain setting
- Use a silicone-specific velocity setting (~980 m/s) (Correct answer)
- Use the standard vitreous velocity
- Avoid A-scan entirely
Correct answer: Use a silicone-specific velocity setting (~980 m/s)
Silicone oil has a much lower sound velocity (~980 m/s) than vitreous, requiring adjustment to avoid axial length errors.
Question 5: The A-constant in IOL power calculation is specific to:
- The patient's corneal curvature
- A particular IOL model and its position in the eye (Correct answer)
- The axial length measurement technique
- The surgeon's phaco technique
Correct answer: A particular IOL model and its position in the eye
The A-constant is lens-specific and accounts for the IOL's design and expected effective lens position.
Question 6: Which condition is most likely to cause a falsely HIGH axial length on A-scan?
- Dense nuclear sclerosis
- Posterior staphyloma with probe misalignment (Correct answer)
- High myopia with corneal irregularity
- Small pupil during measurement
Correct answer: Posterior staphyloma with probe misalignment
Posterior staphyloma creates an abnormal retinal contour; if the probe misses the fovea, it records a longer path length.
Question 7: The ACD (anterior chamber depth) in biometry refers to the distance from the:
- Corneal epithelium to the anterior lens surface
- Corneal endothelium to the anterior lens surface (Correct answer)
- Corneal vertex to the posterior lens capsule
- Iris plane to the anterior lens surface
Correct answer: Corneal endothelium to the anterior lens surface
ACD in biometry is measured from the posterior corneal surface (endothelium) to the anterior lens surface.
COT Biometry and A-Scans 3
In A-scan biometry, the velocity used for the vitreous cavity is approximately: