Tomey OA-2000 - Ultrasound Conversion Formula

■ 4-12
4.3 Ultrasound conversion formula
The axial length measurement displayed in Contact mode or Immersion mode of OA-2000 is
the value calculated by converting the optically obtained axial length to an ultrasound axial
length value, using a correction formula developed based on clinical results. Therefore, the
converted ultrasound axial length value includes measurement errors due to the difference
between the optical measurement principle/method and ultrasound measurement
principle/method.
- Formula for conversion to ultrasound axial length in Contact mode
AXIAL(Contact) = {OptLength － 1.4687} / 0.9581
When {OptLength - AXIAL(Contact)} ≧ 0.75mm
AXIAL(Contact) = OptLength － 0.75
- Formula for conversion to ultrasound axial length in Immersion mode
AXIAL(Immersion) = {OptLength － 1.3304} / 0.9573
OptLength = OPL/Naxl
OPL : Optical path length
Naxl : Average refractive index of eye
Difference between optical and ultrasound measurement principles/methods
- Retina thickness
The thickness from the corneal epithelium to the retinal pigment epithelium is
measured by the optical method, while the thickness from the corneal epithelium to
the internal limiting membrane is measured by the ultrasound method. Therefore, the
retina thickness and axial length measured by the optical method become longer than
the values measured by the ultrasound method.
- Pressure on retina
The optical method performs non-contact measurement, but the probe contacts the
retina directly or indirectly. Therefore, the axial length measured by the ultrasound
method tends to be shorter due to pressure on the retina.
- Measurement axis
Since the optical method requires fixing the patient’s sight to the light source, a stable
visual axis can be captured during measurement. On the other hand, although the
fixation light is used for the ultrasound method, the optic axis where the reflection of
the retina waveform becomes the strongest is captured during actual measurement.