The development of high-resolution MRI imaging should allow better imaging of the whole eye. Until recently, MRI scans of the eyes had high in-plane resolution, but the sensitivity of MRI to eye movement prevented sufficient resolution in the out-of-plane direction for a full 3D evaluation of the eye.
19,28 Using a custom-made eye coil and an in-house–developed blinking control protocol, however, 3D images could be obtained. We were able to create retinal topographic maps, reconstructed from the high-resolution 3D MRI data, that accurately quantify the shape of the peripheral retina. This new technique opens up new clinical possibilities such as quantification of staphyloma and study of the impact of the retinal shape on the peripheral refraction.
As we have studied several myopic eyes, we looked at the individual images. The retinal topographic maps of the three subjects of
Figure 3 show how the distance between the lens and the retina decreases toward the periphery, a trend that was more pronounced for longer axial lengths (
Fig. 3F). A group analysis of the individual horizontal retinal profiles confirms this trend and shows a decrease of more than 1.5 mm in retinal distance at 30° off-axis for the more myopic eyes (
Fig. 6). This could explain the previously described trend in the periphery for the refraction of myopic eyes to be relatively more hyperopic compared to their foveal refraction.
6,15,29
We subsequently determined whether the MRI-derived values corresponded to the PCI-derived central measurements: A comparison of the retinal distance as measured by PCI and by MRI shows good agreement between the two methods over the entire range of values, with a mean difference of SD = 0.23 mm, which is lower than the resolution of the MR image (pixel size 0.5 mm). The small discrepancy is mainly the result of two assumptions in the image-processing algorithm. Firstly, the precise location of the boundary between the retina and vitreous body is currently defined as the location of the maximum pixel-to-pixel increase in signal intensity. The image spatial resolution limits this to 0.5 mm. In the future, additional modeling of ocular MRI data could potentially allow a more specific definition of the boundary between the retina and the vitreous body, which would enable a further increase in the precision of the retinal shape.
The second assumption involves the definition of the central axis. The visual axis is defined as the line between the fixation point and the fovea. It is, however, generally known that not all the optical elements of the eye are centered on the same axis.
30–33 The pupil center is, for example, not centered on the visual axis, resulting in the definition of the pupillary axis as the line though the center of the pupil perpendicularly to the cornea. The difference between the visual axis and the pupillary axis is quantified by the angle kappa and is on the order of 5°.
34,35 The current MRI methods do not provide a high enough spatial resolution either to detect the foveal pit, needed to define the visual axis, or to measure the center of the pupil and the perpendicular intersection with the cornea, needed to reconstruct the pupillary axis. Therefore a different axis is defined, which can be geometrically reconstructed from the MR images. This central axis definition assumes that all elements of the eye are symmetric around the pupillary axis. In this case a line through the centers of gravity of two parts of the eye would define the pupillary axis. Our data already show that this is a robust way to define a central axis, but they also show that the assumption of symmetry around the central axis is not completely valid. The retinal topographic maps are, for example, not symmetric around the center. This observation is quantified by fitting a paraboloid through the retinal map.
Figure 7 shows how such a fit would determine the center of the curved retina. These fits show a systematic difference, of on average 2°, between the central axis and the apex of the retina. Furthermore, the retinal shape of a subset of subjects appears to be asymmetric around the center, as has also been observed by others.
4,36
Other geometrical definitions, using, for example, the apex of the cornea and the center of the pupil, could possibly result in a better-defined axis that corresponds more accurately to the physiology of the eye. The increased spatial resolution images of the anterior segment, needed for such an axis definition, could be measured with MRI by making a separate scan of only the anterior segment of the eye with an increased resolution. Another, more promising possibility would be to combine the MR images with other anterior segment imaging modalities such as optical coherence tomography or Scheimpflug imaging. Such an improved axis definition will, however, have a minor effect on the measured axial length, since the retinal distance is relatively constant near the fovea. It will furthermore not influence the overall shape of the segmented retina but result in only a slight shift, which is clinically considered not relevant.
Another potential source of the discrepancy between MRI and PCI could be the fact that PCI measures an optical path length. This is internally converted to an actual distance using an averaged refractive index that is, however, assumed to be same for the complete eye.
The Bland-Altman plot shows that MRI-derived retinal topographic maps tend to underestimate the central axial length for subjects with longer eyes. A small error in the central axis definition will result in an underestimation of the central retinal distance. The size of this underestimation is directly related to the concavity of the retina, which is increased for subjects with longer eyes,
6,15,29 explaining the observed trend.
The possibility to quantitatively characterize 3D retinal shape by MRI offers new ophthalmologic possibilities. Current studies on myopia, for example, are keenly interested in the question of how refractive errors affect the peripheral shape of the retina.
37–39 Previous studies have already used MRI to quantify the ocular shape as a function of refraction
15,18,19; but until now, the MRI methods were limited to either high-resolution two-dimensional images or low-resolution 3D image stacks, neither of which allows for a full 3D description of the ocular shape. The availability of high-resolution 3D data is of further value because of the automatic image processing, which increases the reproducibility of analysis, making it possible to three-dimensionally quantify small changes in ocular shape over time.
This new MRI-based technique can improve the diagnosis of patients with a staphyloma. The limited field of view of current techniques, such as ultrasound, does not offer the clinician the possibility to assess the complete protrusion with respect to the rest of the retina. A retinal topographic map, however, will allow for quantitative determination of the size and location of the complete staphyloma. For these patients, these data would be a valuable addition to a visual field measurement, as they could confirm the link between the local loss of visual acuity and the retinal shape.