Biometry uses ultrasound A-scans to measure the eye's dimensions, which are used to calculate the ideal intraocular lens power for cataract surgery. The A-scan procedure involves taking a patient history, preparing the patient, and using ultrasound to measure the axial length of the eye. Accurate measurements require using the proper gain and sound velocity settings for each patient's eye condition, and ensuring the probe is correctly positioned on the cornea pointing towards the macula. The immersion technique, where the probe is submerged in fluid coupled to the eye, provides more precise measurements than the contact technique.

1. Biometry or A-Scan
Procedure
Md.Asaduzzaman
Associate Optometrist

2. Definition of Biometry
Biometry is the process of measuring the power of the
cornea (keratometry) and the length of the eye, and
using this data to determine the ideal intraocular lens
power by A scan. A-scan ultrasound biometry, commonly
referred to as an A-scan (short for Amplitude scan), is
routine type of diagnostic test used in optometry
or ophthalmology.The A-scan provides data on the
length of the eye, which is a major determinant
in common sight disorders.

3. Examination Procedure
 History taking
 Patient preparation
 Biometry technique or types

5. Measurement of AL by Ultrasound
 A-Scan ultrasonography to measure
AL of the eyeball and a software to
calculate IOL power various
formula.Various methods of A- Scan
examination include water bath
technique, uese of water –filled
probes and that of solid probes.
 However , the immersion technique
is more accurate than the contact
technique of measuring AL.

6. Gain or sensitivity Setting
 Gain refers to the electronic
amplification of the sound waves
received by the transducer.
 Increase in gain may be required when
the height of echoes achieved is
inadequate as in very dense
cataracts,other ocular opacities and high
myopia.Increase in the gain produces
taller echoes.
 Decrease in the gain is done when
artefacts are seen near the retinal
echoes, e.g.in slicone filled eyes and
pseudophakic eyes.

7. Sound velocity setting
 Sound velocity setting should be done
appropriately according to the type of
eye, i.e.
 Normal
 Cataractous (mild,moderate or dense)
 Aphakic
 Pseudophakic (Sliicone or acrylic)
Note :Change in the sound velocity due
to materials like silicone oil in the eye
can produce an error of 3-4D

8. Operative instructions
 Operative instructions to be kept in mind while
performing the A-Scan for measuring AL are as
follows
 Placing of probe on the anaesthetized cornea
should be such that:
 It points towards macula.
 This is specially important in myopes,who may have
staphyloma.
 There is no fluid bridge between the probe and
cornea.
 Cornea Should not be depressed, otherwise it will
result in advertent shortening of AL.

9. Characteristics of a good scan
 A-Scan produce one dimensional image in which echo strength are
displayed as vertical deflections or spikes of verying heights,on a
display screen. Characteristics of a good scan are:
 Corneal echo is seen as a tall single peak.
 Aqueous chamber does not produce any echo.
 Anterior and posterior lens capsules produce tall echoes.
 Vitreous cavity produces few to no echoes.
 Retina produces tall echoes.
 Orbital fat produces medium to low echoes.

10. Immersion Technique
 The immersion technique requires the use of
a Prager Scleral Shell or a set of Ossoinig or
Hansen Scleral Shells.
 The patient lies supine, looking up at the
ceiling and the scleral shell is placed between
the eyelids and centered over the cornea. The
scleral shell is then filled with a 40-60 mixture
of Goniosol and Dacriose and the probe tip is
placed into the solution. Align the ultrasound
beam with the macula by having the patient
look at the probe tip fixation light, then
simply take your readings as usual.

11. Immersion technique

12. IOL Master
 A non-contact optical device that
measures the distance from the
corneal vertex to the retinal
pigment epithelium by partial
coherence interferometry, the IOL
Master is consistently accurate to
within ±0.02 mm or better.
The IOL Master is the first such
device to be widely used in clinical
ophthalmology.

13. Home Messages
 Increase the patient comfort.
 See refractive status before doing
biometry.
 Biometry is an important step in
successful IOL implantation.So
you will do it carefully.

14. Any Questions