As a rule, an interference signal is produced if the measuring light
is reflected by the tear film and the retinal pigmented epithelium of
the eye. This signal is utilized for axial length measurement.
Note:
Ultrasonic biometrical instruments measure the axial length as that
distance between the cornea and the inner limiting membrane, because
the sound waves are reflected at this membrane. To ensure that the
measured values obtained with the IOL Master are compatible with those
obtained through acoustic axial length measurement, the system
automatically adjusts for the distance difference between the inner
limiting membrane and the pigmented epithelium. The displayed axial
length values are thus directly comparable to those obtained by
immersion ultrasound.
It is absolutely necessary to re-personalize the "lens constants" for
use with the IOL Master prior to applying its calculation values to
determine lens power and surgical technique. Refer to the specialist
literature and the publications of the authors of the IOL formulas
regarding the personalization of constants. Updated information is available at the following web sites:
With an optimally aligned instrument, a good SNR, and weak ametropia
(approx. < 6 D), the secondary maxima located symmetrically on each
side of the measuring peak will be seen. These maxima are an artifact of the
light source. The distance form each of the secondary maxima to the peak signal
is approximately 0.8 mm. The secondary maxima are likewise always visible in
measurements of the provided test eye. The measuring system of the IOL Master
is capable of resolving fine structures on the fundus of the eye.
Depending on the anatomical conditions of the measured eye it may
happen that the measuring beam produces interferences when being
reflected at the inner limiting membrane and/or the choroid. This is indicated by:
Broader signal peaks of the measuring curve,
Variations of axial length data of approx. 0.15 -0.35 mm in one measurement series and
Display of "Evaluation" in place of mean value
Example:
Such measuring curves or measurement series absolutely require
verification. This can be done both between the individual
measurements (in LAM mode) and in post measurement editing (without
the patient sitting in front). Interpretation, or post measurement
editing, should always be done with the help of the zoom function.
Note:
The resolution of fine retinal structures is clearly distinguishable
from the previously mentioned secondary maxima. The secondary maxima
(2) are much further distant from the retinal pigment epithelium
signal peak and they are symmetrical to it.
The distance between the maximal peak and internal limiting membrane
or choroid is 0.1 -0.35 mm (whereas the secondary maxima are about 0.8
mm displaced for the maximal peak).
Signals from the inner limiting membrane (ILM) -
Measuring light is relatively often reflected at the inner limiting
membrane thus also producing an interference signal. The
corresponding ILM signal peak appears to the left of the measuring
peak for the RPE by a distance of 0.15 -0.35 mm. If the measuring
cursor is placed at the ILM it will produce a shorter inaccurate axial
length value by 0.15 to 0.35 mm. This is the most likely reason that
no average axial length value is generated for a series. In the
zoomed view of the measuring curve, both peaks may be clearly
distinguished from each other.
Usually, the signal from the amplitude of the peak produced by the
inner limiting membrane is smaller that that of the reflectance from
the pigmented epithelium. In such a case the automatic algorithm
finds the correct axial length.
Important:
Never move the measuring cursor manually to the (left) peak produced
by the inner limiting membrane.
In rare cases it may happen that the amplitude of the signal from the
inner limiting membrane is higher than that of the reflected light
from the pigmented epithelium. In this case automatic peak detection
will place the cursor incorrectly at the peak from the ILM.
In measurement series such individual measurements stand out by
deviations in the range of approx. 0.15 - 0.35 mm toward shorter axial
lengths. You may correct the measured value by moving the measuring
cursor to the right to the smaller peak (that produced by the
pigmented epithelium). This manipulation is permissible only with the
other signal curves of this measurement series.
Signals from the choroid -
Triple peaks:
In rare cases it may happen that the measuring light is also reflected
from the vessels of the choroid.
The measuring peak produced by the choroid appears shifted towards
longer axial lengths by approximately 0.15 - 0.25 mm from the peak of
pigmented epithelium.
In the example illustrated above, the signal from the RPE (middle
peak) has the highest amplitude. The automatic peak detection system
has correctly placed the cursor here to detect the correct axial
length. No further movement of the cursor is needed. This type of
rare triple peak clearly differs form the secondary maxima produced
through the light source by the distance form the RPE reflected peak.
In rare cases, depending on the anatomical condition of the measured
eye, it may occur that the signal produced by the pigmented
epithelium does not have the highest amplitude.
In the graphic above, automatic peak detection system will find a
axial length value that is too short by approximately 0.15 -0.35 mm.
After comparing all measured values and curves for this eye, the
measuring cursor must be moved manually to the middle (small RPE) peak
produced by the RPE. This measured value may be corrected (with
corresponding asterisk) or deleted.
Double peaks:
In very rare cases, it may occur that signals are produced by both the
pigmented epithelium and the choroid.
In the above graphic, the automatic peak detection system has placed
the measuring cursor in the correct position at the RPE and the
measuring cursor must not be shifted manually.
Note:
Such a curve may only be evaluated correctly by viewing all measuring
curves of this eye. Such a curve must be clearly distinguished from
double peaks produced by the inner limiting membrane and the RPE. It
may be necessary to delete suspicious curves and take additional
measurements are maximally possible per eye on one day).
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Note:
Ultrasonic biometrical instruments measure the axial length as that
distance between the cornea and the inner limiting membrane, because
the sound waves are reflected at this membrane. To ensure that the
measured values obtained with the IOL Master are compatible with those
obtained through acoustic axial length measurement, the system
automatically adjusts for the distance difference between the inner
limiting membrane and the pigmented epithelium. The displayed axial
length values are thus directly comparable to those obtained by
immersion ultrasound.
Important:
Never move the measuring cursor manually to the (left) peak produced
by the inner limiting membrane.
