A scan biometry | How to Use A-scan? Types of A-Scan Biometry?

A-SCAN BIOMETRY | What is A-Scan Biometry? How To Use It?
A-scan is the short form of amplitude scan.
This eye ultrasound gives details about the length of the eye.
A-Scan is an essential diagnostic tool used in ophthalmology.
The measurement of the eye’s axial length through an A-scan is necessary for placing an intraocular lens (IOL, artificial lens) during cataract surgery.
The total refractive power of the emmetropic eye is approximately 60D. Of this power, the cornea provides roughly 40D, and the crystalline lens 20 diopters.
When a cataract is removed, the lens is replaced by an artificial lens implant. By measuring both the length of the eye (A-scan Biometry) and the power of the cornea (keratometry).
It may also be used to assess vision abnormalities and other diseases involving the eye such as tumors.
A-scan techniques are based on the principles of ultrasonography. Sound travels in a wave pattern. For a sound to be heard by the human ear, the frequency must be between 20 and 20,000 Hz (20 kHz).
For an eye examination through A-scan, an ultrasound of frequency of around 10 MHz is used.

1. What, How & Types
Of A-Scan Biometry
OPTOM.NaeemAhmad

2. A-scan is the short form of amplitude scan
•The Ultrasound gives details about the length of the eye.
•A-Scan is an essential diagnostic tool used in ophthalmology.
•The measurement of the eye’s axial length through an A-scan is necessary for placing an
intraocular lens (IOL, artificial lens) during cataract surgery.
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3. The total refractive power of the emmetropic eye is
approximately 60D. Of this power, the cornea provides
roughly 40D, and the crystalline lens 20D.
•When a cataract is removed, the lens is replaced by an
artificial lens implant. By measuring both the length of
the eye(A-scan Biometry)and the power of the
cornea(keratometry).
•It may also be used to assess vision abnormalities
and other diseases involving the eye such as tumors.
•A-scan techniques are based on the principles ofultrasonography. Sound travels in a wave
pattern. For a sound to be heard by the human ear, the frequency must be between 20 and
20,000 Hz (20 kHz).
•For an eye examination through A-scan, an ultrasound of frequency of around 10 MHz is used.
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4. a.Probe tip. Echo from the tip of the probe,
now moved away from the cornea and has
become visible.
b.Cornea.A double-peaked echo will show
both the anterior and posterior surfaces.
c.Anterior lens capsule.
d.Posterior lens capsule.
e.Retina.This echo needs to have sharp 90-
degree take-off from the baseline.
f.Sclera.
g.Orbital fat.
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5. 5

6. •In A-scan biometry, the sound travels through the solid cornea, the liquid aqueous, the solid lens, the
liquid vitreous, the solid retina, choroid, sclera, and then orbital tissue; therefore, it continually
changes velocity.
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7. There are 3 Types of method
Immersion A-scan
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8. A-scan biometry by applanation requires that the
ultrasound probe be placed directly on the corneal
surface. This can either be done at the slit lamp, or by
holding the ultrasound probe by hand.
PROCEDURE: Hand-HeldMethod
•Explain the procedure
•Anesthetize the patient’s eye with a topical anesthetic
such as Proparacaine. Please refer to the
manufacturer’s instructions for proper use.
•Clean the probe
•Aprobe is placed on the patient‘s cornea.
•Theprobe is attached to a device that deliversadjustablesound waves.
•The measurements aredisplayedasspikeson thescreen of anoscilloscope(Visualmonitor).
•The appearanceof thespikesand the distancebetween them can be correlated to structures
withinthe eye and the distance between them.
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9. There are Five basic types of limitations are :-
1.Variable corneal compression.
2.Broad sound beam without precise localization.
3.Limited resolution.
4.Incorrect assumptions regarding sound velocity.
5.Potential for incorrect measurement distance.
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10. •Preferred over applanation: With the immersion
A-scan
•Technique, the probe tip does not come into
contact with the cornea.
•Instead, the ultrasound beam is coupled to the
eye through the fluid(with saline). Because
there is no corneal compression, the displayed
result more closely represents the true axial
length.
•When the ultrasound beam is properly aligned
with the center of the macula, all five spikes
(cornea, anterior and posterior lens capsule,
retina and sclera) will be steeply rising and of
maximum height.
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Note: Be sure to set your ultrasound machine
toimmersion mode, if it doesn’t automatically
do so, or you will get meaningless readings
that are several mm too long.

11. •The immersion technique requires the use of aPrager
Scleral Shell(pictured at the top of this page), or a set
of Ossoinigor Hansen Scleral Shells.
•We have found that thePrager Scleral Shellis the
easiest to use, and gives very consistent readings.
•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 Goniosoland Dacrioseand 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

12. •Defined as the electronic amplification factor of
the sound waves received by the transducer, it is
measured in decibel (dB). Normal setting is
around 70% -75%. But it may be increased where
high echoes are inadequate (hard cataracts,
dense ocular opacities, high myopia). It is
decreased when artifacts are seen near the
retinal spikes (silicone filled eyes & in
pseudophakics).
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13. •In cataract surgery or other lens-replacement
procedures, vision is improved by replacing the eye’s
natural lens with an intraocular lens (IOL).
•IOLs come in different powers and sizes to compensate
for each patient’s individual needs. For best vision after
cataract surgery, precise measurements must be taken
to determine which IOL to implant.
•The IOLMasteris a high-precision instrument
revolutionizing all previous techniques and setting a
new standard for IOL calculations, called optical
biometry.
•Optical biometry using IOLMaster does not subject our patients to any discomfort. No local anesthesia is required
and there is no risk associated with taking the measurements.
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14. •The difference between good vision and great vision after
cataract surgery can be attributed to the diagnostic
technology used to calculate the appropriate IOL. This
technology provides our patients with the opportunity for
the best possible visual outcome.
•The IOLMasteris a great advancement in IOL calculation.
•The risk of confusing right and left eye measurements is
eliminated with this technology. The IOLMaster
automatically detects the right or left eye while taking the
measurement.
•Axis length, corneal radii and anterior-chamber depth of
the eye are measured with the IOLMaster. Three
instruments integrated into one location provides our
patients with a higher level of comfort.
•Measuring the eye with the IOLMastertakes less time
than using ultrasound, the traditional form of measurement
that requires anesthesia.
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15. Sanders, Retzlaff, and Kraff developed the SRK formula which was widely used. Though now replaced with the newer
generation formulas, it is still useful for understanding the relation of the variables and A-constant to the IOL Power (P).
P = A-0.9K –2.5 AL
•Where
•P= Power of the IOL in diopters
•A= A constant
•K= Average keratometry value in
diopters
•AL= axial length in mm
•This is rarely used to calculate IOL
power manually where other tools are
not available but should be avoided
whenever possible.
Accuracy of axial length by
different machine
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No matter how good the system is, people will
still make mistakes. Some reasons include:
Some common mistakes (collected from the UK
and overseas departments):
➢People in a hurry
➢Lack of training or accessible guidelines
➢Reliance on others
➢Technical failure (rarely)
➢Human error (often).
➢Wrong a-constant selected
➢The wrong formula used
➢Wrong k-readings entered by hand (90 degrees
out)
➢Biometry print-out stuck in wrong patient’s notes
➢Incorrectly labeled IOL
➢Wrong patient in theatre
➢Reversed IOL optic
➢The wrong IOL implanted (25.5 D implanted
instead of 22.5 D or +30 D instead of +3.0 d).

17. Optom.Naeem Ahmad
YouTube Channel
Ahmad infotech