What Is the Difference Between A-Scan and B-Scan?

Reviewed on 1/29/2021

A-scan and B-scan are both ultrasound exams used to evaluate the eye.
A-scan and B-scan are both ultrasound exams used to evaluate the eye.

  • A-scan and B-scan are both types of ultrasound examinations performed for eye evaluation.
  • A detailed eye examination tells not only about the structures of the eyes but also about any underlying health conditions such as high blood pressure and diabetes
  • Several techniques have evolved for the routine examination of the eyes or to plan eye surgeries (such as cataract surgeries). Ultrasonography techniques are important tools in evaluating eye diseases and performing eye surgery (ophthalmology). 
  • A-scan and B-scan are both eye ultrasonography scans. Although both the scans are based on the principle of ultrasonography, they differ in certain aspects.

A-scan

  • A-scan is the short form for amplitude scan. This eye ultrasound gives details about the length of the eye. 
  • It is a one-dimensional scan of the eye. 
  • The measurement of the axial length of the eye through an A-scan is necessary for placing intraocular lens (IOL, artificial lens) during a cataract surgery
  • It may also be used to assess vision abnormalities of the eye and other diseases involving the eye such as tumors. 

B-scan

  • B-scan is considered the brightness scan. It is used for producing a two-dimensional cross-section of the eye and its orbit.
  • A B-scan is generally used to evaluate diseases involving the posterior segment (the hind two-third of the eye) and orbit, typically when the ocular media (fluids within the eye) are cloudy and a direct visualization is not possible. 
  • They may diagnose eye ailments such as 

How do A-scan and B-scan work?

  • A-scan and B-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). 
  • Ultrasound uses a sound of frequency of more than 20 kHz. When the sound of this frequency comes in contact with an object, it bounces back. These “echoes” then touch the ultrasound probe
  • The probe converts the energy of the sound into an electrical signal that can then be interpreted as an image on a monitor. 
  • The doctor or sonologist will use ultrasounds of different frequencies to control the depth of the image depending upon the structures to be examined. 
  • For an eye examination through A-scan and B-scan, an ultrasound of frequency of around 10 MHz is used.

What happens during A-scan and B-scan?

A-scan and B-scan are safe and painless procedures.

  • During an A-scan, the doctor will put anesthetic drops in your eyes to make them numb. You will be asked to sit in a chair and place your chin on a chin rest. The doctor will then ask you to look straight ahead. They will place an ultrasound wand (a device that produces ultrasound waves that bounce off eye tissues and make echoes) on the front surface of the eye.
  • During a B-scan, the doctor will ask you to close your eyes. A gel will be applied to the eyelids before using the probe. The doctor may ask you to move your eyes in multiple directions.

Both the tests are generally quick and cause minimal discomfort. Because an A-scan uses anesthetic eye drops, you must be careful about touching the eyes until the numbness subsides to avoid any injury.

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References
https://emedicine.medscape.com/article/1228865-overview

https://emedicine.medscape.com/article/1228447-overview#a2

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