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.

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.

How does an A scan work?

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.

Applanation A-scan Biometry.

 

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-Held Method

  • 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
  • A probe is placed on the patients cornea.
  • The probe is attached to a device that delivers adjustable sound waves.
  • The measurements are displayed as spikes on the screen of an oscilloscope (Visual monitor).
  • The appearance of the spikes and the distance between them can be correlated to structures within the eye and the distance between them.

There are Five basic limitations of Applanation A-scan biometry 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.

Immersion A-scan Biometry

 
  • 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

  • Note: Be sure to set your ultrasound machine to immersion mode, if it doesn’t automatically do so, or you will get meaningless readings that are several mm too long.
  • 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.
  • The immersion technique requires the use of a Prager Scleral Shell (pictured at the top of this page), or a set of Ossoinig or Hansen Scleral Shells.


  • We have found that the Prager Scleral Shell is 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 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.
 

Use of Gain in a difficult situation

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).

IOL Master

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 IOLMaster is 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.

Why we’ve invested in the IOLMaster:
  • 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 IOLMaster is 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 IOLMaster takes less time than using ultrasound, the traditional form of measurement that requires anesthesia.

IOL Calculation Formula

 

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 – 0.9 K – 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.

Why do things go wrong?

 
No matter how good the system is, people will still make mistakes. Some reasons include:
  • people in a hurry

  • lack of training or accessible guidelines

  • reliance on others

  • technical failure (rarely)

  • human error (often).

Some common mistakes (collected from the UK and overseas departments):
  • 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).