Introduction

Abnormalities in the water flow in the anterior chamber cause eye diseases such as glaucoma [1]. Gadolinium, as an MRI contrast agent, does not readily pass through the blood-retinal barrier or the blood-aqueous barrier of capillaries that supply water to the eye; therefore, it is difficult to visualize the flow of ocular aqueous humor. The protons in H₂¹⁷O have various Larmor precession frequencies due to the scalar coupling with ¹⁷O [2]. The chemical exchange of protons between H₂¹⁷O and H₂¹⁶O shortens the T₂ of the water protons. As a result, the T2WI signal intensity in the region with H₂¹⁷O is lower than that with only H₂¹⁶O. Recently, an in vivo experiment on the human brain using dynamic T2W ¹H-MRI of H₂¹⁷O was reported [3]. Our study using eye drops of H₂¹⁷O saline was designed to observe the distribution of H₂¹⁷O in the eye and its flow into and out of the anterior chamber using dynamic T2W ¹H-MRI.

Methods

Three healthy female volunteers (20–31 years old) participated in our study. The parameters of the dynamic T2W ¹H-MRI (3T, Siemens, Erlangen, Germany) were as follows: a half-Fourier, single-shot, turbo-spin echo sequence [4], TR/TE, 3000/444 msec; field-of-view (FOV), 180×180 mm; slice thickness, 3 mm; and matrix, 320×320 pixels, reconstructed to 640×640. The total scan time was 42 min, and each subject performed the following sequence of actions: 1-min, rest (Eyes_closed); 1-min, stare at a single point to avoid eye movement (Eyes_open); 1-min, applied eye drops to her right eye with 10 mol% H₂¹⁷O saline (0.92–1.43 mL; TAIYO NIPPON SANSO, Tokyo, Japan); 9-min, Eyes_open; 5-min, Eyes_closed; 10-min, Eyes_open; 5-min, Eyes_closed; 10-min, Eyes_open. In-house software developed in MATLAB (MathWorks, MA, USA) was used for the data analysis. Images obtained during Eyes_closed periods and those with motion artifacts were not used in the analysis. Three averaged images were created from 10 consecutive images obtained at the following time points: i) just after the application of the eye drop, ii) when the signal intensity was the lowest, and iii) during the signal recovery; then the image obtained prior to the eye drops was subtracted from each of them. To obtain the normalized signal intensity of the right anterior chamber (nAC), in a single image, the right and left anterior chambers were manually selected as regions-of-interest (ROIs), each of size 1.66 mm² (7×3 pixels), and the right-ROI signal intensity was divided by that of the left. For each of all dynamic images, the ROIs were set at the same position as the two selected ROIs. A linear relationship between H₂¹⁷O concentration and the relaxation rate change (ΔR₂) of MR signal intensity has been reported [5, 6]. The ΔR₂ of nAC at time t was calculated using the following equation:

• ΔR₂(t) = [−ln{nAC(t)/nAC(0)}]/TE, (1)

where nAC(0) is the signal intensity before the eye drop. Then, a biexponential curve was fitted to the time variation of ΔR₂:

• ΔR₂(t) = A{−exp(−Bt) + exp(−Ct)}, (2)

where A is a constant coefficient, and B and C are the inflow and outflow constants, respectively.

Results

The variations in nAC across time were similar for all subjects: the signal intensity decreased after the eye drop, to a minimum at 7–9 min, and then began to recover, reaching a value close to nAC(0) at 40 min. The subtracted images showed decreases in signal intensity in the anterior and posterior chambers but not in the vitreous body (Fig. 1). The inflow and outflow constants for the three subjects ranged 0.195–0.326 min^-1 and 0.026–0.240 min^-1, respectively (Fig. 2).

Discussion

The time changes of nAC and the inflow and outflow constants were similar for the three subjects. These results show that the H₂¹⁷O saline eye drops flow smoothly into the anterior chamber and flow slowly out again, as can be observed using dynamic T2W ¹H-MRI. There have been no reports of H₂¹⁷O being administered to human eyes in vivo, but there are a few reports in animals [6, 7]. Kwong et al. administered H₂¹⁷O to the rabbit eye [6]. Similar observations in rabbit eyes were reported by Obata et al. in ²H-MRI with D₂O [8]. With the application of eye drops to the rabbit eye in those studies [6, 8], outflow of H₂¹⁷O and of D₂O from the anterior chamber gave similar results, with a constant of about 0.1 min^-1. Our outflow constants ranged from 0.026 to 0.240 min^-1, which include the above value, but with such a large range in our values, 0.1 cannot clearly be considered ‘similar’. Our large range may indicate that there are large individual differences in humans, including the instability due to the very long scan time. For T2WI, the D₂O study reported signal changes in the vitreous body [8], therefore in our study, the possibility of signal changes at the noise level due to the flow of H₂¹⁷O into the vitreous body cannot be ruled out.

Conclusions

Our results using dynamic T2W ¹H-MRI showed that H₂¹⁷O saline eye drops were distributed in the human anterior and posterior chambers and smoothly flowed into, and slowly out of, the anterior chamber. Further measurements on healthy subjects are needed to confirm differences in age and other factors.