The research adhered to the tenets of the Declaration of Helsinki and was approved by both the Queensland University of Technology (QUT) University Human Research Ethics Committee and Prince Charles Hospital Human Research Ethics Committee. Informed consent was obtained from all participants. They were all in good general and ocular health, with corrected visual acuity of 6/6 or better in the tested eye. They consisted of 22 emmetropes (best sphere corrections −0.50 to +0.75D) and 66 myopes to −12.00 D, aged between 18 and 36 years, with small amounts of astigmatism (≤0.75 D) except for five participants (astigmatism 1.00–2.00 D). There were 53 females and 35 males. Eighty-four percent (74/88) and 16% (14/88) of participants were white and Asian, respectively. Right eyes were used, unless they were outside refraction or acuity criteria. Contralateral eyes were occluded during MRI.
MRI was performed on a 1.5-T clinical system (Signa Twin Speed; GE Medical Systems, Milwaukee, WI) with a 7.5-cm receive-only surface coil positioned over the eye. To ensure minimum eye movement during imaging, participants were scanned while lying supine and looking vertically upward at the image of a 5-mm diameter light emitting diode (LED) reflected in a mirror mounted at an angle 45° to the horizontal inside the bore of the magnet. The LED was placed approximately 6 m in front of the magnet along the axis of the magnet tunnel and patient table. Each subject was advised to restrict blinking while keeping the eye as still as possible and focused on the LED (46% of eyes wore a best sphere contact lens). A series of scout scans were obtained and examined to check that the subject was looking vertically upward. Failing this, the mirror was adjusted and scout images rescanned. Sagittal and axial images containing the line of sight, were acquired using a fast spin-echo (FSE) sequence with the following imaging parameters: receiver bandwidth of ±15 kHz, slice thickness 3 mm, field of view 80 mm2, pulse flip angle 90°, repetition time (TR) 400 ms, echo time (TE) 16.9 ms, echo train length 4, four averages, acquisition matrix 320 × 320 (interpolated with zero-filling to 512 × 512). Acquisition time was 130 seconds. Sagittal FSE images were acquired with fat suppression to minimize chemical shift artifact in the inferior region of the sclera.
Before the study, tests were performed to determine the extent to which image artifacts arising from the proximity of air in the sinuses, might generate spatial distortion of the images. The significant magnetic susceptibility difference between air and soft tissue is known to result in significant image distortion in MRI.
12 Images were obtained of a phantom constructed from a 12-cm diameter Perspex cylinder filled with 0.7 mM CuSO
4 solution to simulate the head, enclosing a 1.9-cm precision diameter glass tube, also filled with 0.7 mM CuSO
4 (simulating the orbit), with a piece of bubble wrap tethered approximately 1 cm from the glass test tube to simulate air in the sinuses. In spin-echo images acquired with bandwidths between ±13.9 and ±62.5 kHz, no distortion in the apparent shape or position of the glass tube was detected. We concluded that differences in magnetic susceptibility between the sinus and surrounding tissue did not significantly affect the orbit region in our study.
Measures of ocular dimensions were made from the axial and sagittal images at approximately 50× magnification on a computer monitor with a standard resolution (1024 × 768 pixels). Using a graphic device interface program written using a commercial application programming interface (WIN32; Microsoft, Redmond, WA), the DICOM (Digital Imaging and Communications in Medicine) image of the cross section through the eye was displayed and contrast adjusted until the edges of interest were most clearly defined. Distances were measured with a line caliper, and the distance between the two points in pixels was converted to the distance in millimeters (using the resolution of the original DICOM image and its magnification on screen).
For both the axial and sagittal sections of the eye, eye length was recorded as the distance between the anterior cornea and the approximate location of the fovea along the line that bisected the eye in the axial plane
(Fig. 2) . Eye length dimensions reported are those measured from the axial image (unless otherwise stated). Eye width was measured retina to retina across the axial image at the point that visually appeared the widest
(Figs. 2b 2d) . Eye height (retina to retina) was measured from the image through the sagittal section of the eye
(Figs. 2a 2c) .
Measurements were made to two decimal places and averages were taken of three measures of each dimensional parameter (eye length, height, and width). The mean standard deviation of the three repeated measures of each parameter from one scan was approximately 0.1 mm. To investigate differences between two different scans (images) of the same section, we examined three repeated measures of alternate images from five participants. The mean difference between images was approximately 0.3 mm for eye length, height, and width. To investigate observer repeatability, differences (of another five participants) in eye length, height, and width were compared in the two observers. The mean difference between observers was approximately 0.2 mm. From these data, we concluded our measures were accurate to approximately 0.3 mm, as this was the maximum amount of error we observed (i.e., between image repeatability). For 83 of the 88 participants, axial length measurements were also made by A-scan ultrasonography (Echograph Axis-II; Quantel Medical, Clermont-Ferrand, France) for comparison to the MRI data. The means of 10 measures were recorded.
Measurements of the eye dimensions of length, height, and width were recorded in mm and are expressed as the mean ± standard deviation, unless stated otherwise. A subjective refraction was performed on each participant. Regression was performed to determine the relationships between eye dimensions and best sphere correction. Refractive correction groups in 1-D steps up to −6.50 D were also used to describe the population, with the five participants with higher refractive corrections (−7.25 to −12.00 D) combined into one group. Paired t-tests were used to compare eye dimensions within the same subjects and to compare axial length dimensions measured using different techniques. ANOVA was used to make comparisons between refractive correction groups. Probabilities <0.05 were considered to be statistically significant.