Lecture 6: Ocular Biometry

Question 1

Explain how ultrasound captures internal body images

Answer 1

a sonogram is created from echos.

through ultra high frequencies (>20,000 Hz). these sound waves hit a structure of different refractive index, & some sound waves bounce back to ultrasound probe tip and detected by a sensor.

therapeutic or diagnostic purposes

+generate deep heat in tissues, monitor conditions

Question 2

State and explain the two types of ultrasound scans

Answer 2

(1) A scan: 1 dimensional, can capture axial lengths

(2) B scan: 2 dimensional (used when there is media opacity, when light-based systems cannot be used ie. OCT, fundus cam

Question 3

Explain how interferometry captures its measurements

Answer 3

through light/laser. infrared light splits into two beams to travel different optical paths, then combines to produce interference (destructive/constructive). Resulting interference fringes show info. of difference in optical path lengths

Question 4

Explain examples of interferometry in optometry machines

Answer 4

ZEISS IOL uses partial coherence interferometry (gold standard for ocular biometry)

Question 5

Explain advantages of interferometry in ocular biometry compared to ultra sound

Answer 5

(1) non-contact (versus ultrasound that uses probe to touch eye, which can cause compression of cornea, disrupting accuracy of measurement)
(2) built in software for calculating IOL pressure for cataract surgery
(3) Faster and ‘cleaner” (does not need gel like ultrasound, just ‘point and shoot’)

Question 6

Describe purkinje images

Answer 6

formed by reflections of objects in the structures of the eye

aka purkinje-sanson images, purkinje reflexes

At least 4 visible by anterior and posterior poles of cornea and crystalline lens - 4th: posterior lens surface (upside down), 1st: corneal reflex (brightest. followed by 3rd, 4th, 2nd)

First image reflected off anterior cornea surface, Second off posterior cornea surface, Third off anterior lens surface, Fourth off posterior lens surface

In normal eye, P1-2-4 lie on single line

Question 7

Describe clinical application: keratometry

Answer 7

for obtaining central corneal curvature for contact lens fitting (using first purkinje image)

Question 8

Explain limitations to keratometer

Answer 8

(1) measures small region of cornea (only two points at 3-4mm zone)
(2) will measure different regions for corneas of different powers
(3) assumes cornea is spherocylindrical/ regular astigmatism
(4) ignores spherical abberations
(5) susceptible to misalignment and focusing errors

Hence corneal topography may be better