Human Vitreous: MR Imaging of Oxygen Partial Pressure

Abstract

Purpose:To develop a magnetic resonance (MR) imaging approach to noninvasively image quantitative Po 2 in the human vitreous. Materials and Methods:Human studies were approved by the institutional review board with informed consent obtained from all subjects and were HIPAA compliant. Animal studies were performed with animal care committee approval. An MR imaging method to measure the longitudinal relaxation rate, or R1, of water was implemented with a 3.0-T MR imager. R1 was calibrated in water phantoms at multiple Po 2 and temperature conditions (n = 10) and in ex vivo animal vitreous (n = 2). Vitreous Po 2 was imaged in three human volunteers (age range, 26-28 years) in multiple sessions on separate days to evaluate reproducibility. The effects of temperature and ambient air were evaluated by acquiring data with the eye open and closed. Statistical analysis consisted of t tests, with P less than .05 indicating significant difference.

MRI of brain tissue oxygen tension under hyperbaric conditions

“…The relaxation rate constant R 1 (R 1 = 1/T 1 ) is linearly proportional to the partial pressure of dissolved oxygen. The relation in distilled water at 37° C and 3 Tesla is, 15 …”

“…12, 13 T 1 MRI changes have been used as an index to monitor tissue oxygenation changes via the T 1 changes associated with oxygen or carbogen inhalation in tumors. 9, 14 In the vitreous where the protein content is low and thus the relaxation contribution from dissolved oxygen is more substantial, the mean vitreous pO2 in humans was found to be 16.7 ± 6.5 mmHg 15 and 13.7 ± 7.8 mmHg 16 using water T 1 MRI.…”

Measuring the Effect of Pars Plana Vitrectomy on Vitreous Oxygenation Using Magnetic Resonance Imaging

“…The previously described study by Muir et al reported a mid vitreous pO 2 of 18.3 mm Hg, which is comparable to our result (13.4 6 6.7 mm Hg). 51 Apart from the variability inherent to the technique's limitations, the reasons for the small difference between the results are likely to be the same that accounted for the differences in both studies phantom experiments, namely temperature and magnetic field strength. The fact that our result is closer to our own intraocular measurements, and to the large number of eyes measured by Holekamp et al, 27,28 would suggest that the mid vitreous temperature is lower than Muir's assumed temperature of 378C temperature assumption used by Muir et al 51 However, interestingly Muir et al investigated T 1 changes with eyes open and closed (in our study the participants had their eyes open) and found a difference that suggested vitreous temperature increased a small amount with closed eyes.…”

“…51 Apart from the variability inherent to the technique's limitations, the reasons for the small difference between the results are likely to be the same that accounted for the differences in both studies phantom experiments, namely temperature and magnetic field strength. The fact that our result is closer to our own intraocular measurements, and to the large number of eyes measured by Holekamp et al, 27,28 would suggest that the mid vitreous temperature is lower than Muir's assumed temperature of 378C temperature assumption used by Muir et al 51 However, interestingly Muir et al investigated T 1 changes with eyes open and closed (in our study the participants had their eyes open) and found a difference that suggested vitreous temperature increased a small amount with closed eyes. Although it is not explicitly stated in their article, the 18.3 mm Hg mid vitreous pO 2 reported could be for closed eyes, or an average of open and closed, thereby, increasing the temperature closer to their assumed value, which could explain why the difference between pO 2 in both our studies was only 4.9 mm Hg, far less than would be expected given the 28C difference in our temperature assumptions.…”

“…51 They measured R 1 (1/T 1 ) in distilled water at various oxygen levels, and in the vitreous of two intact globes (one baboon and one rabbit), at three different temperatures, 348C, 378C, and 408C. They reported comparable R 1 intercepts (i.e., representing fully deoxygenated states) to our study, for both water (we used BSS), 0.205 s À1 vs. 0.210 s À1 , and vitreous, 0.209 s À1 vs. 0.218 s À1 .…”