Maps of the apparent transverse relaxation time (T 2 † ) were collected on a transaxial plane across the basal ganglia in 54 healthy subjects at 4.7T using a multiecho adiabatic spinecho (MASE) imaging sequence. We attempted to quantify the nonhemin iron concentration (

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Apparent transverse relaxation rate constants (R2† = 1/T2†) were measured in various regions of the healthy human brain using a multi-echo adiabatic spin echo (MASE) sequence at five different magnetic fields, 1.5, 1.9, 3, 4.7, and 7T. The R2† values showed a clear dependence on magnetic field strength (B0). The regional distribution of the R2† was well explained by the sum of three components: 1) regional non-hemin iron concentration ([Fe]), 2) regional macromolecular mass fraction (fM), 3) a region independent factor. Accordingly, R2† = α[Fe] + βfM + γ, where coefficients α, β, and γ were experimentally determined at each magnetic field, by a least square fitting method using multiple regression analysis. Whereas the coefficient α linearly increased with B0, β showed a quadratic dependence on top of a field independent component. The coefficient γ also increased slightly with B0, on top of a field independent component. The linear dependence of α on B0 was consistent with that observed for the transverse relaxation rate of water protons in ferritin solutions, as found previously by others. The quadratic dependence of β on B0 was accounted for by iso- and anisochronous exchange mechanisms using intrinsic relaxation parameters obtained from the literature.

Multiple pairs of adiabatic passage pulses were implemented in a spin-echo sequence to achieve accurate measurements of the apparent transverse relaxation time (T 2 † ) in a short scan time. In experiments on agarose gel phantoms with T 2 values ranging from 30 to 105 ms, the measured T 2 † values were in good agreement with transverse relaxation times measured with a nonselective Carr-Purcell-Meiboom-Gill sequence. In experiments on normal human brain at 4.7 T, T 2 † values in five different gray matter regions were found to range from 38 ؎ 2 ms (globus pallidus) to 64 ؎ 2 ms (frontal cortex). The apparent relaxation rate (1/T 2 † ) in these five regions showed strong correlation (r ‫؍‬ 0.97) with published levels of iron ( The transverse relaxation time T 2 of the water molecule is an important parameter that reveals the microscopic properties of in vivo water, such as its mobility and magnetic environment. Thus, T 2 -weighted contrast is routinely used for diagnosing various diseases. Moreover, a small change in T 2 or T 2 * provides the source of contrast that is exploited in functional MRI (fMRI) to reveal brain activation (1,2). In contrast, quantitative measurements of T 2 have been pursued to only a limited extent, due in part to obstacles in obtaining accurate T 2 values with slice-selective spin-echo sequences. In particular, imperfections in the slice profile produced by the refocusing pulse result in a loss of coherence, and when multiple echoes are collected the loss is cumulative at each refocusing step, leading to erroneous T 2 estimations. When T 2 is measured in a slow manner by stepping TE values in a single (Hahn) spin-echo sequence, other mechanisms of loss of phase coherence occur during the longer TE periods due to diffusion in nonuniform B 0 and exchange of the water molecule. At higher fields these two types of effects are exacerbated by increased B 1 inhomogeneity and larger microscopic susceptibility gradients. As B 0 increases, the B 1 distribution in conductive objects becomes less homogeneous as a result of increased RF interference effects. This B 1 inhomogeneity can lead to further imperfections in slice profile at high field. In addition, as B 0 increases tissue microstructure and the distribution of paramagnetic substances such as Fe become more significant contributors to the T 2 decay. If properly measured, the T 2 at high field could be more sensitive to the tissue microstructure due to increased microscopic susceptibility effects. However, with long and/or variable TEs adopted in the conventional spin-echo measurements this effect can influence the measured value of T 2 .Generally, the T 2 value in cortical gray matter (GM) in brain is thought to be longer than that in white matter (WM). Recently, however, it has been reported that the contrast is inverted between GM and WM in the occipital lobe, and the distribution of stored Fe, especially in the form of ferritin, was suggested to cause the inverse contrast (3,4). It is also well recognized that T 2 values are relatively...

Light-induced ATP synthesis was studied in intact cells and chromatophores of Erythrobacter sp. strain OChll4. ATP synthesis was measured by both the pH method and the luciferin-luciferase luminescence method. The rate of ATP synthesis was moderate (a typical value of 0.65 mol of ATP per mol of bacteriochlorophyll per min), and synthesis was inhibited by antimycin A. ATP was synthesized under illumination only under aerobic conditions and not under anaerobic conditions. This characteristic was similar to that of other light-induced energy transduction processes in this bacterial species, such as oxidation of reaction center, oxidation of cytochrome c55l, and translocation of H+, which were not observed under anaerobic conditions. This phenomenon was reconciled with the fact that the Erythrobacter sp. could not grow anaerobically even in the light. The characteristics of oxidative phosphorylation and ATP hydrolysis were also investigated. The respiratory ratio of chromatophores was 2.3. Typical rates of oxidative phosphorylation by NADH and by succinate were 2.9 mol of ATP per mol of bacteriochlorophylH per min (P/O = 0.22) and 1.1 mol of ATP per mol of bacteriochlorophyll per min (P/O = 0.19), respectively. A typical rate of ATP hydrolysis was 0.25 mol of ATP per mol of bacteriochlorophyHl per min in chromatophores. ATPase and adenylate kinase are also involved in the metabolism of adenine nucleotides in this bacterium.