Figure CCC.3-1 demonstrates an A-scan waveform – produced by an ultrasound device used for 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).
Figure CCC.3-1 Scan Waveform Example
Time (translated into distance using an assumed velocity) is on the x-axis, and signal strength is on the y-axis. This waveform allows clinicians to judge the quality of an axial length measurement for use in calculating the power of intraocular lens to place in a patient’s eye in cataract surgery. Figure CCC.3-1 above demonstrates a high quality scan, with tall, even spikes representing the ocular structures of interest. This tells the clinician that the probe was properly aligned with the eye. The first, double spike on the left represents anterior cornea followed by posterior cornea. The second two, more widely spaced spikes represent anterior and posterior lens. The first tall spike on the right side of the display is the retinal spike, and the next tall spike to the right is the sclera. Smaller spikes to the far right are produced by orbital tissues. Arrows at the bottom of the waveform indicate the location of gates, which may be manually adjusted to limit the range of accepted values. Note that in the lower right corner of the display two measurements are recorded. In the column labeled AXL is an axial length measurement, which on this device is the sum of the measurements for ACD (anterior chamber depth), lens, and VCD (vitreous chamber depth). The measured time value for each of the segments and a presumed velocity of sound for that segment are used to calculate the axial length for that segment. An average value for each column is displayed below along with the standard deviation of measurements in that column. The average axial length is the axial length value selected by this machine, although often a clinician will make an alternative selection.