Hand held scans are inherently inaccurate due to problems with positioning the probe
perpendicular to the eye and indentation of the cornea. Remember that accuracy to
0.5D requires about 150 micron A-scan accuracy. Hence the past interest in immersion
A-scans which are messy and do not solve the alignment problem. Similarly, an A-scan
probe attached to the tonometer arm of the slitlamp solves the alignment issue but
can indent the cornea. The best arrangement is a dedicated A-scan chinrest which
allows the probe to slip and solves the alignment issues (see figure).
The technique involves moving the tip toward and away from the eye while watching
the monitor (since alignment issues are solved by the chinrest). Initially bring
the probe into contact with the eye and pull back observing the largest number (longest
axial length) until contact is lost. That number is the goal and represents the moment
when a meniscus of fluid separates the tip from the probe. Remember, the ideal scan
shows sharp spikes (see figure) rather than flattened tops and should be “clean”
between spikes (indicating the scan is done through the center of the pupil.