Measuring axial length of the eye from magnetic resonance brain imaging

Abstract

Background: Metrics derived from the human eye are increasingly used as biomarkers and endpoints in studies of cardiovascular, cerebrovascular and neurological disease. In this context, it is important to account for potential confounding that can arise from differences in ocular dimensions between individuals, for example, differences in globe size.

Methods: We measured axial length, a geometric parameter describing eye size from T₂-weighted brain MRI scans using three different image analysis software packages (Mango, ITK and Carestream) and compared results to biometry measurements from a specialized ophthalmic instrument (IOLMaster 500) as the reference standard.

Results: Ninety-three healthy research participants of mean age 51.0 ± SD 5.4 years were analyzed. The level of agreement between the MRI-derived measurements and the reference standard was described by mean differences as follows, Mango - 0.8 mm; ITK - 0.5 mm; and Carestream - 0.1 mm (upper/lower 95% limits of agreement across the three tools ranged from 0.9 mm to - 2.6 mm). Inter-rater reproducibility was between - 0.03 mm and 0.45 mm (ICC 0.65 to 0.93). Intra-rater repeatability was between 0.0 mm and - 0.2 mm (ICC 0.90 to 0.95).

Conclusions: We demonstrate that axial measurements of the eye derived from brain MRI are within 3.5% of the reference standard globe length of 24.1 mm. However, the limits of agreement could be considered clinically significant. Axial length of the eye obtained from MRI is not a replacement for the precision of biometry, but in the absence of biometry it could provide sufficient accuracy to act as a proxy. We recommend measuring eye axial length from MRI in studies that do not have biometry but use retinal imaging to study neurodegenerative changes so as to control for differing eye size across individuals.

Keywords: Axial length; Biometry; MRI; Neurological.

Fig. 1

Procedure for finding the centre of the eye in three orthogonal planes on T2-weighted brain MRI. Example shown is the Mango image analysis tool – the main image component displays the brain in an axial orientation with two supplementary projections (sagittal top and coronal bottom) used to guide the user to the centre of the globe. Zoom tools were then used on the axial image to better visualize the anterior and posterior boundaries of the globe. Maximum eye axial length is then measured on the axial image (pink line). Supplementary Figs. 2, 3, 4, 5, 6, 7, 8 show examples using the Caresteam image analysis tool, magnified on the axial image to better show the anterior and posterior measurement boundaries

Fig. 2

Bland-Altman plots showing agreement between right eye axial length measured in mm in Mango, ITK and Carestream versus the IOLMaster reference data (N = 93). Central solid lines indicate mean difference (MRI measures minus reference standard) and dashed lines indicated lower/upper limits of agreement (±1.96*SD). The points represent the individual deviation of each measurement compared with the reference