There is much subjective variation in the software caliper placement by users of the ASOCT, and inter-observer variations of measurements have been shown to have a SD of 18.0 - 20.2um at +1.0
mm and −1.0
mm from the centre of the cornea.* Recently we compared the interobserver and intraobserver variation of LASIK flaps measured using a time domain and spectral domain machine [6
]. The interobserver and intraobserver results for the time domain machine were similar to that previously published [2
]. The mean limit of agreement (LOA) was worse for the central cornea reading compared to those measurements taken at +1.5
mm and −1.5 from centre. Using the time domain OCT, the interobserver correlation coefficients (r) were 0.73(−1.5
mm from centre), 0.62 (centre) and 0.78 (+1.5
mm from the centre). For spectral domain machines the LOA was much closer for the two observers, and interobserver correlation coefficients were much stronger 0.82 (−1.5
mm from the centre), 0.88 (centre) and 0.88 (+1.5
mm from the centre) [6
]. Hence the improved resolution allowed for improved accuracy in measurements, both inter and intra-observer. However, these are only single point measurements and to get multiple point information an automated/semi-automated system would be more efficient, faster and reduce the inherent error rate.
In this study, we found that the COLGATE program was a highly reproducible tool for graft area measurements and consistent graft area calculations could be obtained both inter and intra-observer from every scan. This was evident by the strong linear correlation coefficient, intraclass and interclass coefficient and the small range of limits of agreement. It was observed that the automated method gave consistently higher values than the semi automated method and this was especially pronounced in thin grafts. In particular graft areas of less than 80000um2 measured by the semi automated method had more than double the measurement by the manual method. This may be due to the fact that the boundary between the anterior graft surface and the underside of the recipient stroma might not be as easily discernible in thin grafts by the COLGATE program. The authors believe that using the semi automated method to calculate the graft area would be better in all grafts.
We found a higher linear correlation of the graft area measurement made between observers (r
0.969) using the COLGATE program than the previously reported inter-observer correlation on the same time domain ASOCT (r
]. The automated/semi-automated system has several advantages over multiple single point measurements as the entire graft thickness is taken into consideration. An alternative would be to take an intergral of multiple points on the graft but the latter option would be laborious and it is not physically possible to include every single point from the graft border. Using the automated system initially allows rapid delineation of the graft boundaries that are identified by the program and this gives an approximate estimate of the graft border. The user is then able to fine-tune the measurements by adjusting the software to delineate the graft border more accurately. This semi-automated programme is akin to the Heidelberg Retina Tomograph software#
used in glaucoma for evaluating the optic nerve head or the Artemis high frequency ultrasound system [10
The COLGATE system may be used as an objective method for ophthalmologists and researchers to obtain graft area measurements. This raises the possibility that DSAEK surgeons may then have a program to preoperatively predict the maximum donor diameter of the graft to be inserted so as to enable transfer of the maximal amount of donor endothelial cells, and with a chart including a range of individual donor thicknesses, be able to select and decide on the exact diameter required at the time of surgery taking into account the actual thickness of the donor tissue supplied for the case, without the fear of encroaching into the chamber angle and risking donor iris contact at the periphery.
Currently, graft thickness is measured using the software calipers on the Visante ASOCT (as well as other OCT systems) at a single point. However, as most grafts are irregularly cut, the central thickness does not provide a good estimate of the entire graft thickness [11
]. There is often a mismatch in thickness between the central and peripheral graft of between 75 to 100 microns [12
]. The Optovue system also utilizes similar caliber software in a similar manner with similar potential errors of measurement. The post op DSAEK total corneal thickness is a surrogate marker for the physiological ‘well-being’ of the donor corneal allograft as a thin graft implies a healthy endothelium and clear cornea free of corneal edema. In corneal grafts with good endothelial function, the grafts are often thinner compared to thickness measurements in the early postoperative period. In cases of early or late graft failure or in cases of graft rejection the graft will become swollen, and thicker. It is believed that as early as one week post DSAEK the surgeon is able to predict the likelihood of graft survival based on ASOCT measurements of the central and peripheral cornea thickness. [13
]. Authors have found that failed DSAEK grafts were significantly thicker at post operative week one onwards compared to successful grafts. There has also been increasing interest in the relationship between DSAEK graft thickness and post-operative refractive error. There have also been other reports on how the difference in thickness between center and periphery of the DSAEK graft induces a change in posterior corneal curvature resulting in a hyperopic shift [14
]. Hence knowing the post operative graft thickness will allow the surgeon to better visually rehabilitate the patient with future graft refractive procedures.
There are other uses of this software. Its use can be extended to evaluating penetrating keratoplasty grafts, as well as anterior and deep anterior lamellar grafts. Future software enhancements will also allow it to be possible to measure the anterior and posterior curvature of the graft, which are useful parameters in assessing post graft refractive evaluation [15
]. Currently this is not possible with any other software and can only be determined from supportive data [14
]. The inbuilt automatic boundary detector can also be extrapolated to evaluate iris profile as well as measurements of the trabecular iris surface area (TISA) [17
] in glaucoma patients.
There are some limitations to this program. Though the graft area is a better surrogate for graft thickness than single point measurements as it incorporates the entire graft, the graft volume would be a more precise parameter than area. However, this would require a three-dimensional imaging acquisition of the corneal graft, which currently is not available. An estimation of graft volume may be calculated from our values by multiplying the graft area by the graft diameter which will be known to the surgeon implanting the graft e.g. if area is 20000 um2
area x size of trephine (ie 8.5
It is also not sufficient to simply rely on the automated readings as we found that the semi automated readings gave a better and consistent graft area calculation.
Lastly, it would also be interesting to demonstrate how image quality changes the inter-observer and intra-observer reproducibility of the measurements.