Biometry Methods Explained.
Routine A-scan biometry is an indispensable tool for ophthalmology, but
has limitations in resolution and an inability to consistently direct the
sound beam to a known location. And although we have accepted ultrasound-based
biometry as our main methodology for the measurement of axial length, it's
important to keep in mind exactly what we are doing.
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 50-MHz ultrasound to more precisely image
somewhat smaller anterior segment structures, rather than 10-MHz ultrasound.
Things work best when the measuring wavelength is many times shorter than
the distances being measured, or the resolution desired.
Principle #1: Resolution is directly linked to wavelength.
accuracy of a measurement is tied 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 extremely small
wavelength (0.0000000975 mm) and 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 magnitude. It is simply
a different tool, operating differently. However, we can still do perfectly
acceptable work with 10-MHz ultrasound, but it's helpful to keep in mind
what's actually going on.
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 adjust this technology of ultrasound to fit our purposes
Where people sometimes get confused is with semantics,
replacing the common usage of words for 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.
The five basic
limitations of applanation A-scan biometry are:
1. Variable corneal
2. Broad sound beam without precise localization.
4. Incorrect assumptions regarding sound velocity.
for incorrect measurement distance.
Variable corneal compression.
The first concept that is helpful to understand is that the measurement
accuracy of a 10-MHz sound wave is exactly the same for an applanation technique
and an immersion technique. 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 the position of the corneal vertex
relative to the vitreoretinal interface is not constant. Where the immersion
technique does a much better job is that this troublesome artifact of variable
corneal compression has been removed. That is the one and only difference
between the two techniques, because the measuring device is exactly the same.
Said another way, an immersion technique is more consistent.
If an applanation
technique compressed the cornea by exactly 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 is consistency and 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:
Even though all five of these measurements give an incorrect axial length,
because they are using the same 10-MHz transducer (with the same 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 the artifact
of variable corneal compression results in a different starting point.
look at this another way. Let's say that we have a special stainless steel
ruler. And this ruler has been calibrated so that it is accurate to within
0.1 mm. We are asked to measure the length of a femur bone for some important
forensic project. We measure from the top of the head of the femur 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 is still within 0.1 mm, but we simply measured to the wrong point.
What we have is an incorrect 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, this is where the immersion technique is
a significant improvement over the applanation technique. The measurements
from the corneal vertex to the vitreo-retinal interface are much more consistent.
Not to belabor the point, but the resolution is exactly the same for both,
as they both use the same 10-MHz transducer. And the accuracy for each applanation
measurement is exactly the same, as all measurements have the same resolution.
What is different is that the starting point for each and every applanation
measurement is slightly different. In the parlay of measurement science,
this is known as variability.
Broad sound beam without precise localization
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 no-where else. Instead the sound beam is reflected back
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
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 large error, but it's helpful to keep in mind that these
types of errors are cumulative.
As mentioned above, a 10-MHz
sound bean 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.
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.
between the corneal vertex and the anterior surface of the crystalline lens
has a sound velocity of 1,532 m/sec.
- The crystalline lens
has a sound velocity of 1,641 m/sec.
The lens of a young person, without
a cataract has a sound velocity that is close to 1,641 m/sec. 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/sec. and
these distances are being measured correctly.
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 large errors in axial length occur;
all of which remain unknown to the operator.
Potential for incorrect measurement
For example. If the transducer is off axis, it is measuring
to the wrong location and may give a falsely short axial length. But, the
accuracy of the measurement from the corneal vertex to that wrong position
is unchanged. The measurement is accurate, it's just that the position (stopping
point) that is wrong. 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
So, an immersion A-scan gives much better results. We all know that.
And an applanation A-scan gives less reliable results, which is understood
by everyone. But the resolution and the accuracy of both types of measurements
is still the same. Assuming that we are always "on axis," the one
and only difference between the two is the consistency of the starting point.
- Resolution determines accuracy.
Just because someone measures to the wrong place, the accuracy of that measurement
from one spot to another is still unchanged.
presence or absence of variability (position) determines consistency.
(measuring from and to the same position every time) determines the final
For further reading, we highly recommend the
book A-scan Axial Length Measurements by Sandra Frazier Byrne.
Also, there is an excellent, national certification
program in Ophthalmic Biometry available for your technicians:
American Registry of Diagnostic Medical