Biometry and A-Scans Flashcards

In A-scan biometry, the velocity used for the vitreous cavity is approximately:

Answer: 1532 m/s

The vitreous humor has a sound velocity of approximately 1532 m/s, similar to water.

Which measurement does optical biometry (IOLMaster) rely on rather than ultrasound?

Answer: Partial coherence interferometry

Optical biometry uses partial coherence interferometry (PCI) to measure axial length with high precision.

A-scan biometry measures axial length as the distance from the:

Answer: Corneal vertex to the fovea

Axial length is measured from the anterior corneal surface (vertex) to the fovea (macula).

When performing A-scan on a silicone oil-filled eye, the technician must:

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.

The A-constant in IOL power calculation is specific to:

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.

Which condition is most likely to cause a falsely HIGH axial length on A-scan?

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.

The ACD (anterior chamber depth) in biometry refers to the distance from the:

Answer: Corneal endothelium to the anterior lens surface

ACD in biometry is measured from the posterior corneal surface (endothelium) to the anterior lens surface.