CPT is an important parameter for evaluation in macular disease because it represents foveal thickness. An important aspect to quality assessment is to confirm whether the CPT truly represents the foveal thickness. If the radial lines of the scan pattern have not been centered on the fovea, the scan is considered decentered and the CPT would not represent the foveal thickness. The center of the macula corresponds to A-scan 64 as shown in and the central subfield of the numeric plot is assumed to correspond to the macula in well-centered scans as shown in . In decentered scans, the CPT does not represent true foveal thickness. The numeric plot is also shifted and the values of the subfields do not represent their presumed location on the fundus.
Decentration artifact can be assessed using a combination of clues . In scans with a visible foveal depression, if the fovea is not located at A-scan 64 the scan is assumed to be decentered. The OCT image on the map report is truncated and should not be used for assessing decentration. The six underlying B-scans from the retinal thickness reports can be used to evaluate decentration. The radial lines should intersect at the center of the macula in the fundus image. However, it should be kept in mind that the fundus image is taken after the OCT scan is completed. In thin retinas with normal foveal depression, the fovea is represented by a blue circular area on the pseudocolor map and is located within the central subfield if well centered. The standard deviation may or may not be high in decentered scans. If the foveal depression is not clearly visible, alternative clues are used to identify the fovea such as change in the reflectivity of the inner layer due to loss of ganglion cell layer at the fovea, proximity of the largest cyst in cystoid macular edema (assumed to be close to the fovea), and examination of the cross hair scans which may better display the fovea and can be used to reference the location on the fast macular scans.
Figure 6 Decentration artifact. The location of the fovea on the B-scan is shifted from A-scan 64 (blue line). The fundus image shows that the intersecting point of the radial lines does not correspond to the anatomical location of the center of the macula (blue (more ...)
Decentration is mostly an operator-dependent error. Poor patient fixation or inability to identify the fovea in a distorted retina could be contributory factors. However, if identified, rescanning could help avoid this artifact. In decentered scans, remeasurement of foveal thickness using the software calipers (Stratus review software) is a possibility. The retinal thickness report with an identifiable fovea is selected and the calipers are placed at the inner and outer boundary lines of the point identified as fovea. This gives the true foveal measurement. The remaining data in the subfields cannot be reclaimed.
In summary, artifacts on OCTs performed using the FMTM scan protocol of the Stratus OCT are common. Identification of operator-dependent artifacts and retaking images if required reduces the frequency of poor quality OCTs in clinical trials. Simple measures such as centering the scan in a well-dilated pupil, focus and z offset adjustment, and having the patient close their eyes to wet the cornea before taking the image also improves image quality. Poor quality OCTs with erroneous numeric data may still be useful for qualitative analysis of morphological abnormalities.