Topic 6 Ocular Biometry

Question 1

what is ultrasonography?

Answer 1

Ultrasonography uses ultra high frequency sound waves (> 20 000 Hz) for diagnostic or therapeutic purposes, to image an internal body structure, monitor a condition or generate deep heat in tissues.

Question 2

What are the 2 types of ultrasound scans

Answer 2

A- scan: imagine a laser beam of light passing through the eye. (1 dimensional)

B- scan: imagine a line of light passing through the eye. (2 dimensional; x & y axis info)
Useful when there is media opacity and light-based imaging systems such s fundus camera, OCT cannot be used.

Question 3

what is interferometry?

Answer 3

Infrared light is split into two beams that travel in different optical paths, which are then combined again to produce interference (constructive or destructive interference). The resulting interference fringes give information about the difference in optical path lengths.

Question 4

The advantages of partial coherence tomography (IOL) over ocular ultrasonography for measuring axial length are

Answer 4

Non-contact (unlike ultrasonography where a probe will touch the eye)

Built-in software helps to calculate IOL power to use for cataract surgery

Faster and ‘cleaner’ than ultrasound
Ultrasound needs a gel or liquid to conduct the sound waves from the probe to the eye
Using an IOL Master is quite similar to using an auto-refractor (“point and shoot”)

Higher accuracy

Less skill required

Question 5

How are purkinje images formed?

Answer 5

. At least 4 Purkinje images are usually visible and they are reflections from the anterior and posterior surfaces of the cornea and crystalline lens.

The 4th purkinje image (posterior lens surface) is upside down.

The 1st Purkinje image is the corneal reflex. It is the brightest, followed by the 3rd/4th and the 2nd.

Question 6

how can purkinje images be used to calculate radius of curvature of cornea and lens?

Answer 6

Keratometers measure the size of the first Purkinje image and use it to calculate the curvature of the corneal surface.

The anterior corneal curvature is calculated using the convex mirror formula, and the corneal power is estimated empirically based on Snell’s law of refraction with simplified optics

Question 7

What machine can be used for axial length measurement?

Answer 7

Ocular ultrasonographer

IOL master

Question 8

Which is a superior (better) machine for measuring corneal curvature?

Answer 8

Corneal topographer