a scan amplitude scan fir axial -1.pptx

ULTRASOUND BIOMETRY
Applanation biometry
Immersion biometry

ULTRASOUND BIOMETRY
Measure Axial length , AC depth ,Lens thickness
Measure axial length from anterior corneal surface to ILM

They measure the speed of sound when sound waves
travels from each of media of the eye thus each section of
the eye is measured at its correct velocity so as to get
correct axial length of the eye

 Gain increase for dense cataract
 Decrease for artefacts near retinal echoes –silicon oil and
pseudophakis

DIFFRENCE B/W CONTACT AND
IMMERSION TECHNIQUE
 Probe directly to cornea
 Easier possible error due to indentation
 Possible infection
Small scleral shell (ossining shell /prager shell )
Probe separated from cornea coupling fluid
Reduces indentation error
Time consuming
Possible infection

LIMITATION
 Erroneously short measure ment -
 Indentation
 Too high gain
 Misalingened probe
 R/D and macular thickning

 Erroneously long measurement –
 Air bubble in fluid bath
 Rapid sound
 Posterior staphyloma
 Thick lens
 Consequense of error-
 1.0mm -2.5 D
 Longer than 30 mm -1mm error -1.75D
 Shorter than 22 mm -1mm error -3.75D

DRAWBACKS
 Measures anatomical axial length not optical axial length
 Measure up to ILM
 Very short and very long eye
 Posterior staphyloma
 Need experience operator
 Pseudophakia and silicon filled eye

OPTICAL BIOMETRY
 Advantages –
 Accuracy in measurement
 More no of parameters
 Less technician based
 Essential to predict effective lens positioning
 Non contact
 Can measure both AL and k reading
 Integrated iol formulae

 Disadvantages –
 Expensive
 Not suitable for dense cataract –solved with new ss-oct based optical biometry
 Patient needs atleast 6/60 vision for fixation
 Not suitable for the patients with nystagmus ,tremors and lid abnormalities

TECHNOLOGIES
 Optical low cohorence reflectomerty
. Lens star -900
Partial cohorence interferometry
IOL master 500
Swept source OCT
IOL master 700

Lens star 900
Based on optical low cohorence reflectomerty
Uses superillumincent diaode(820nm)
9 parameter

IOL master 500
 Based on partial cohorence interferometry
 Uses diode laser of 780 nm
uses dual beam produces interference signal produced by reflected light from
anterior surface of cornea and another beam from RPE
1. These signals analyse by photodetector to calculate the distance between these
two surfaces
2. Parameter that can be measured are (AL , keratometry, ACD ,White to White
distance pupil size, Lens thickness)

IOL MASTER 700
Uses 1060 nm wavelength
More accurate measurements
Better penetration
Faster scan speed
Measure from ant. cornea to RPE
Examiner bias eliminated
Advantages –provide continuous b scan and full length oct

a scan amplitude scan fir axial -1.pptx

a scan amplitude scan fir axial -1.pptx

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a scan amplitude scan fir axial -1.pptx