CCC.4 Output of an Optical A-scan Device

Figure CCC.4-1 demonstrates the waveform-output of a partial coherence interferometry (PCI) device used for optical ophthalmic axial length measurement. This is referenced in the Ophthalmic Axial Measurements IOD in Attribute Referenced Ophthalmic Axial Length Measurement QC Image Sequence (0022,1033).

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Figure CCC.4-1 Waveform Output of a Partial Coherence Interferometry (PCI) Device Example

Physical distance is on the x axis, and signal strength is on the y axis. What is actually measured is phase shift, determined by looking at interference patterns of coherent light. Physical distance is calculated by dividing “optical path length” by the “refractive group index” – using an assumed average refractive group index for the entire eye. The “optical path length” is derived from the phase shift which is actually observed. Similar to ultrasound, this waveform allows clinicians to judge the quality of an axial length measurement.

Figure CCC.4-1 above demonstrates a high quality scan, with tall, straight spikes representing the ocular axial length. The corneal spike is suppressed (outside the frame on the left hand side) and represents the reference 0 mm. The single spike on this display represents the signal from the retinal pigment epithelium (RPE) and provides the axial length measurement value (position of the circle marker). Sometimes smaller spikes can be observed on the left or right side of the RPE peak. Those spikes represent reflections from the internal limiting membrane (ILM, 150-350 µm before RPE) or from the choroid (150-250 µm behind RPE) respectively.

Because all classical IOL power calculation formulas expect axial lengths measured to the internal limiting membrane (as provided by ultrasound devices), axial length measurements obtained with an optical device to the retinal pigment epithelium are converted to this convention by subtracting the retinal thickness.

Figure CCC.4-1 above displays five axial length measurements obtained for each eye (one column for each eye) and the selected axial length value is shown below the line.