A-scan Biometry

Routine A-scan biometry is an indispensable tool in ophthalmology; however, it has limitations in resolution and an inability to direct the sound beam to a known location consistently. Although we have accepted ultrasound-based biometry as our primary methodology for measuring axial length, it’s essential to understand precisely what we are doing.

The resolution of a wavelength-based measurement is inversely proportional to the wavelength of the measuring device being used. The longer the wavelength, the lower (worse) the resolution. The shorter the wavelength, the higher (better) the resolution. This is why an electron microscope has much higher resolution than a light microscope. This is also why we use a 50-MHz ultrasound to more precisely image somewhat smaller anterior segment structures, rather than a 10-MHz ultrasound. Things work best when the measuring wavelength is many times shorter than the distances being measured or the resolution desired.

Fundamental Principle #1:

Resolution is directly linked to wavelength.

Therefore, the accuracy of a measurement is directly related to its resolution. In terms of resolution, the 10-MHz sound wave used in ophthalmic ultrasound has a wavelength that is less than one order of magnitude smaller than the resolution desired for the anatomy being measured (0.03 mm). By comparison, a partially coherent light wave (as used in optical coherence biometry) has a tiny wavelength (0.0000000975 mm). It is used to measure distances that are very large by comparison. In terms of resolution, optical coherence biometry surpasses 10-MHz ophthalmic biometry by more than eight orders of magnitude. It is simply a different tool that operates differently. However, we can still produce perfectly acceptable work with a 10-MHz ultrasound, but it’s helpful to keep in mind what’s actually happening.

The correct (also known as the refractive) axial length for IOL power calculations would be defined as the distance from the corneal vertex (surface of the corneal epithelium at the center of the visual axis) to the outer segments of the photoreceptor at the foveal center. Keeping in mind that this is not what A-scan biometry measures, we can begin to understand how we have learned to adapt this ultrasound technology to fit our purposes in ophthalmology.

Where people sometimes get confused is with semantics, where the common usage of words is replaced by the actual science involved. One common mistake is that people interchange the term accuracy, when what they really mean is the correct measurement.

Let’s begin with applanation A-scan biometry, which suffers from five fundamental limitations.

Five fundamental limitations of applanation A-scan biometry:

1. Variable corneal compression

The first concept to understand is that the measurement accuracy of a 10-MHz sound wave is the same for both applanation and immersion techniques. Again, where people get confused is with semantics. This does not mean that applanation and immersion biometry have the same outcomes. Of course not. But what this does mean is that the measurement from the position of the corneal vertex to any area of the vitreoretinal interface is done with the same precision.

For an applanation technique, every first-year resident knows that the position of the corneal vertex relative to the vitreoretinal interface is not constant. Where the immersion technique excels is that it removes this troublesome artifact of variable corneal compression. That is the only difference between the two techniques, as the measuring device is precisely the same. In other words, an immersion technique is more consistent.

If an applanation technique compressed the cornea by the same amount each and every time, there would be no difference in consistency, and the only difference would be that the applanation technique yields a shorter axial length and, as a result, would require a lower lens constant. Again, the difference between applanation and immersion lies in consistency, not accuracy. Semantics sometimes gets in the way of science.

For example, let’s say that a patient’s true axial length is 24.24 mm. We are doing an applanation A-scan, and there is the unavoidable artifact of variable corneal compression. Our first five measurements look like this:

 

	Applanation AL	Corneal Compression
#1	24.02 mm	0.22 mm
#2	24.00 mm	0.24 mm
#3	24.06 mm	0.18 mm
#4	23.96 mm	0.28 mm
#5	24.10 mm	0.14 mm

 

Even though all five of these measurements give an incorrect axial length, because they are using the same 10-MHz transducer (with the exact resolution, and the same gate settings as an immersion A-scan), they are all equally accurate in their measurement from the position of the corneal vertex at the time of the measurement to the vitreo-retinal interface at the time of the measurement. The only thing that makes them different is that the artifact of variable corneal compression results in a different starting point.

Let’s look at this another way. Let’s say that we have a special stainless steel ruler. This ruler has been calibrated to an accuracy of 0.1 mm. We are asked to measure the length of a femur bone for an important forensic project. We measure from the top of the femur’s head to the longest point on the lateral condyle. In reality, the measurement should be to the longest point of the medial condyle. The accuracy of our initial measurement remains within 0.1 mm, but we measured to the incorrect point. What we have is an inaccurate measurement. The accuracy of our special ruler cannot be changed. More correctly stated, the distance between the two points being measured was simply incorrect.

Fundamental Principle #2

The accuracy of a measuring instrument is an inherent quality of that instrument. How it is used determines if the measurement is correct.

Lacking the artifact of variable corneal compression, the immersion technique offers a significant improvement over the applanation technique. The measurements from the corneal vertex to the vitreoretinal interface are much more consistent. To reiterate, the resolution is identical for both, as they both utilize the same 10-MHz transducer. And the accuracy for each applanation measurement is precisely the same, as all measurements have the exact resolution. What is different is that the starting point for each applanation measurement is slightly different. In the parlay of measurement science, this is known as variability.

2. Broad sound beam without precise localization

For both applanation and immersion A-scans, the sound beam is not an infinitely small, thin pencil of sound, like a line on a piece of paper, but a relatively broad beam. For this reason, it is not possible to measure directly to the foveal center and nowhere else. Instead, the sound beam is reflected from some area around the center of the macula. Recall that the definition of the refractive axial length is from the corneal vertex to the photoreceptor outer segments at the center of the fovea. A-scan biometry is offset from the outer segments of the photoreceptors at the center of the macula by the retinal thickness.

However, the retinal thickness at the foveal center is, on average, approximately 165 µm, but the retinal thickness just to the side of the foveal center is closer to 250 µm. We also know that the distance between the center of the fovea and the shoulder of the fovea is smaller than our ability to control the position of the sound beam. This inability to discriminate between the foveal center and the shoulder just outside is a second source of error.

Fundamental Principle #3:

Variability for an on-axis A-scan measurement is an artifact of position.

Here is an OCT-3 macular thickness plot from a normal eye that illustrates this. So, for the exercise of A-scan biometry, we have to take into account the inherent resolution limitations of a 10-MHz sound wave and its inability to discriminate between the foveal center and the foveal shoulder. This may not sound like a significant error, but it’s helpful to keep in mind that these types of errors are cumulative.

3. Limited resolution

As mentioned above, a 10-MHz sound beam has a resolution of approximately 0.03 mm. By comparison, the 780-nm partially coherent light source used in optical coherence biometry has a wavelength of 0.0000000975 mm. And since the smaller the wavelength, the higher the resolution, there is simply no comparison between the two.

4. Incorrect assumptions regarding sound velocity

The typical contact or immersion A-scan makes the following assumptions:

Of course, this is not true. The 550 microns of the cornea actually has a sound velocity of 1,641 m/sec. Using the velocity conversion equation, this would under-estimate the axial length by approximately 0.04 mm.

The lens of a young person, without a cataract, has a sound velocity of approximately 1,641 m/s. But for the aging lens with only a moderate cataract, the sound velocity is actually closer to 1,628 m/sec, which would produce an error of 0.28 mm for a lens of 4.25 mm. And for an eye with a mature cataract, the sound velocity is closer to 1,589 m/sec, which would produce an even larger error for a lens of 4.25 mm.

Fortunately, the sound velocity of the aqueous and the vitreous (the great majority of the eye) is a water velocity of 1,532 m/s. These distances are being measured accurately.

Of course, all of these errors are still relatively small, but when you begin to add them together, it is not difficult to see how small or significant errors in axial length occur; all of which remain unknown to the operator.

5. Potential for incorrect measurement distance

For example. If the transducer is off-axis, it measures from the wrong location and may yield a falsely short axial length. But the accuracy of the measurement from the corneal vertex to the wrong position is unchanged. The measurement is accurate, it’s just that the position (stopping point) is incorrect. Again, here is where people get confused. The ultrasound probe will always measure with the same accuracy (it can do nothing else).

Fundamental principle #4:

A correct measurement is the result of the correct position.

To summarize…

Therefore, an immersion A-scan yields significantly better results. We all know that. An applanation A-scan yields less reliable results, which is generally understood. However, the resolution and accuracy of both types of measurements remain the same. Assuming we are always “on axis,” the only difference between the two is the consistency of the starting point.

How to learn more about A-scan techniques:

We highly recommend the book A-scan Axial Length Measurements by Sandra Frazier Byrne.

There is an excellent, national certification program in Ophthalmic Biometry available for your technicians: