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Human vitreous: MR imaging of oxygen partial pressure.

ABSTRACT: To develop a magnetic resonance (MR) imaging approach to noninvasively image quantitative Po(2) in the human vitreous.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.Calibrations of phantoms and ex vivo vitreous yielded an R1 association with oxygen of 0.209 sec(-1) + Po(2) ⋅ 2.07 × 10(-4) sec(-1)/mm Hg at 37°C, and an association with temperature (Δ[1/R1]/ΔTemperature) of 0.106 sec/°C ± 0.009 (standard deviation). A difference in R1 was found between the phantoms and vitreous. If uncorrected, vitreal Po(2) would be significantly overestimated (P < .001). In vivo human vitreous Po(2) maps were spatially heterogeneous, with a whole vitreous Po(2) of 16.7 mm Hg ± 6.5 (eye closed). Measurements between open and closed eyes showed spatially dependent R1 differences, which translated to temperature differences of 0.34°-0.83°C across the eye.This study established an MR imaging protocol to image quantitative vitreous Po(2) noninvasively and evaluated effects from vitreal macromolecules, temperature gradients, and ambient air on vitreal Po(2) values. Measurement of vitreous Po(2) with MR imaging has the potential to be used to study eye diseases noninvasively.

SUBMITTER: Muir ER

PROVIDER: S-EPMC3579170 | biostudies-other | 2013 Mar

REPOSITORIES: biostudies-other

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Project description:Purpose To generate magnetic resonance (MR) imaging-derived, oxygen-hemoglobin dissociation curves and to map fetal-placental oxygen-hemoglobin affinity in pregnant mice noninvasively by combining blood oxygen level-dependent (BOLD) T2* and oxygen-weighted T1 contrast mechanisms under different respiration challenges. Materials and Methods All procedures were approved by the Weizmann Institutional Animal Care and Use Committee. Pregnant mice were analyzed with MR imaging at 9.4 T on embryonic days 14.5 (eight dams and 58 fetuses; imprinting control region ICR strain) and 17.5 (21 dams and 158 fetuses) under respiration challenges ranging from hyperoxia to hypoxia (10 levels of oxygenation, 100%-10%; total imaging time, 100 minutes). A shorter protocol with normoxia to hyperoxia was also performed (five levels of oxygenation, 20%-100%; total imaging time, 60 minutes). Fast spin-echo anatomic images were obtained, followed by sequential acquisition of three-dimensional gradient-echo T2*- and T1-weighted images. Automated registration was applied to align regions of interest of the entire placenta, fetal liver, and maternal liver. Results were compared by using a two-tailed unpaired Student t test. R1 and R2* values were derived for each tissue. MR imaging-based oxygen-hemoglobin dissociation curves were constructed by nonlinear least square fitting of 1 minus the change in R2*divided by R2*at baseline as a function of R1 to a sigmoid-shaped curve. The apparent P50 (oxygen tension at which hemoglobin is 50% saturated) value was derived from the curves, calculated as the R1 scaled value (x) at which the change in R2* divided by R2*at baseline scaled (y) equals 0.5. Results The apparent P50 values were significantly lower in fetal liver than in maternal liver for both gestation stages (day 14.5: 21% ± 5 [P = .04] and day 17.5: 41% ± 7 [P < .0001]). The placenta showed a reduction of 18% ± 4 in mean apparent P50 values from day 14.5 to day 17.5 (P = .003). Reproduction of the MR imaging-based oxygen-hemoglobin dissociation curves with a shorter protocol that excluded the hypoxic periods was demonstrated. Conclusion MR imaging-based oxygen-hemoglobin dissociation curves and oxygen-hemoglobin affinity information were derived for pregnant mice by using 9.4-T MR imaging, which suggests a potential to overcome the need for direct sampling of fetal or maternal blood. Online supplemental material is available for this article.

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Human vitreous: MR imaging of oxygen partial pressure.

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