Choroidal line scan measurements in swept-source optical coherence tomography can represent an entire macular volume

Abstract

Purpose: This study aims at a patient friendly (little acquisition time) and an examiner friendly (little evaluation time) optical coherence tomography (OCT) scanning protocol with highest possible validity for representing the choroid. Earlier studies often measured only single subfoveal centerpoint (CP) choroidal thickness (CT) as a representative value for the entire choroid. This study was performed to compare two methods (cube and line scan) and to assess whether the CP or the mean central millimeter (CMM) thickness are representative for the total choroid.

Methods: 60 eyes of 34 patients with uveitis underwent 1050nm OCT scanning (Topcon DRI OCT-1, swept-source). For each eye a 12x9 millimeter (mm) macular cube scan and a 12mm horizontal line scan were performed. The scans were exported and analyzed using custom Vienna Reading Center software. In the cube scan the choroid was segmented by an automated algorithm and manually checked by an expert reader. The line scan was manually segmented by the same expert reader. All scans were analyzed in random order. The retinal CP was set manually in both scans and an Early Treatment Diabetic Retinopathy Study (EDTRS) grid was positioned. 1mm, 3mm temporal (T), 3mm nasal (N), 6mm T and 6mm N averaged subfield CT of the cube scan were compared to corresponding areas on the line scan and to the single subfoveal CP measurement. Statistical analysis was Passing and Bablok regression.

Results: Analysis of the five subfields measured by cube and line scan methods showed no significant deviation (95% confidence interval of slope including 1): 6mm N: 0.92-1.11, 3mm N: 0.88-1.06, 1mm: 0.94-1.11, 3mm T: 0.95-1.12, 6mm T: 0.93-1.17). For the comparison of the single point subfoveal measurement of the CP to the mean CMM CT also no significant deviation was found (0.89-1.08). However, both the CP and the CMM thickness showed significant deviation compared to the entire 6mm area (CP: 1.01-1.53, CMM: 1.03-1.38), that is a consistent overestimation of CT.

Conclusions: Subfoveal CP CT measurements and CMM measurements are not sufficient for representing a 6mm choroidal region without adjustment for overestimation. Subfoveal CT measurements, however, represent the CMM region. Horizontal line scans provide sufficient information on CT of the correspondent ETDRS subfields as obtained by a cube scan.