Introduction: A-Scan testing is one of the biometry tests that is used in ophthalmology. The A Scan test (also called A Scan ultrasonography) measures the axial length of the eye- that is the length of the eye from the front of the eye (the cornea) to the back of the eye (the retina).
The test is conducted using ultrasound. The axial length is needed prior to cataract surgery if an intraocular lens is going to be used to replace the cataractous lens. Keratometry measurements are used in conjunction with the axial length in one of several mathematical formulas to figure out the best intraocular lens power to insert in the eye. Often you will hear the term A’s and K’s to describe this test- meaning that axial length and Keratometry measurements are needed.
Facts and properties behind ultrasound: The following terms are important to understand when learning about ultrasound. We will look at each term and how it applies to ultrasound.
1. Frequency- Frequency refers to how many wavelengths pass by a certain point in a certain time. Think of how we use the word frequency- the frequency of how often you go to the grocery store- 1 or 2 times per week? Frequency describes how often something happens in a given time period. When talking about ultrasound, we use frequency to describe how often a wave peak passes by a certain point in a certain time. The frequency is measured in number of wavelengths per second (or cycles per second). 1 cycle per second is known as a Hertz (Hz). Ultrasound has sound waves with a frequency of greater than 20,000 Hz (20 KHz). The frequency of ultrasound used in biometry is normally around 10MHz or 10 million Hz. This frequency is set by the manufacturer of the ultrasound machine when they make the machine. The more oscillations (greater frequency) the less deep the sound wave will travel into the medium, but the better the resolution is. Alternatively, the lower the frequency the deeper the sound wave will travel into the medium, but the less the resolution will be. See the image below:
2. Wavelength- Ultrasound travels in waves- like in the frequency diagram above. A wavelength is a complete wave cycle. See the diagram here:
3. Velocity- The velocity refers to how fast the waves are traveling. This corresponds to the speed of a car- how fast you are going. Velocity is measured in units of distance per time. We are usually going to be talking about velocity in the units of meters per second.
4. Amplitude- Amplitude refers to the height of the waves- how high do the peaks go. Look at the following image:
Method of testing: As we mentioned above, A Scans measurements are taken using ultrasound. The A Scan machine contains a probe with a crystal that oscillates with a certain frequency (usually 10 MHz). This emits an ultrasonic wave which is aimed back the visual axis to the retina. As it travels through the different structures of the eye, portions of it will be reflected back (echoed) to the probe and this reflectance will be measured.
See the image here to demonstrate how reflectance occurs. As the ultrasound beam travels into a different medium, part of it is reflected back:
How does this relate to measuring the length of the eye? The ultrasound beam travels through the eye and parts of it are reflected back to the ultrasound probe as it goes through the changes of medium in the eye. Each time the ultrasound enters a different medium in the eye and part of it is reflected the A Scan probe detects this echo and records this as a spike in a graph. These spikes will correspond to the following parts of the eye: Cornea, front of the lens, back of the lens, retina, sclera, orbital fat. The A Scan machine will already be programmed with the speed (velocity) that the sound waves will travel through the following media:
It can then calculate the distance/length each of those structures are by measuring how long it takes the ultrasound to reach them. Remember that at each change in media a portion of the ultrasound wave will be reflected/ echoed back and you will see a spike at these points.
There are 2 methods used in measuring Axial length:
Contact method- the ultrasound probe actually touches the patient’s cornea. This test may be conducted in either of the following ways:
Slit lamp mounted probe- the probe is mounted in the slit lamp or other device that holds the probe. The patient rests her chin in a chin rest and forehead against a forehead bar. This allows for the probe to remain steadier throughout the test. This also gives the patient a place to hold her head still to help minimize patient head movement.
Hand held probe- the probe is held by the examiner against the cornea. This allows for more control of patient’s head placement; however it may make it more difficult to keep the patient’s head still. It may also be more difficult to control corneal compression by the probe.
Note that it is very important that the examiner has a place to rest her arm and steady her hand. Often the patient’s cheek will be used for helping to steady the examiner’s hand.
Non contact- the probe does not touch the patient’s cornea- this may be done with the following methods:
Immersion technique- the probe is immersed in fluid which is contained in a shell or cup that is placed on the patient’s eye. This cup or shell allows the probe to be placed in it in a fixed position and fluid (usually BSS) is added to the cup or shell. The probe sits in the fluid and the ultrasound waves pass thru the BSS to the eye. There are a few different types of shells on the market, 2 of them are the Hansen shell and the Prager Shell. Both shells are placed on the patient’s eye and allow for the probe to be placed inside the shell which is filled with BSS. The probe does not touch the patient’s cornea, rather it sits immersed in the BSS.
IOL master technique- the IOL master is a machine that measures axial length, Keratometry, anterior chamber depth, and white to white (visible iris diameter) measurements. This machine uses a laser to take the measurements rather than ultrasound. The patient’s eye is not touched at all during testing. The laser beam is aimed back the visual axis and measurements are taken by the machine at the press of a button by the examiner. These measurements are very consistent and reproducible. The measurements are taken without touching the patient and therefore there is little worry about corneal compression. Here is a picture of the IOL master:
Lenstar-This method of optical biometry takes 9 measurements of the eye in a 30 second period. It is the newest technology that is available to measure axial length. Here are some images showing the Lenstar.
The non contact methods are currently recognized as the most accurate methods and are the medical standard for axial length measurement. These methods have the important benefit that you will not compress the cornea and create an artificially shorter than true axial length. This is a real problem that the examiner must be aware of when measuring axial length using the contact method as it can cause an unwanted post operative refractive outcome.