Figure 3. (A) (Main plot) Representative T²relaxation measurements of whole head using the birdcage coil. The measurements were made at 2.5 (open triangle), 7.5 (open square), and 62 (open circle) min, respectively, after beginning sevoflurane uptake. The relative signal intensities, in arbitrary units, are plotted as a function of spin-echo time. The decays of the overall intensity are best fit by biexponential functions of Equation 2in the Appendix, yielding two time constants in the ranges of T^2fast= 3 [nearly equal] 5 ms and T^2slow= 105 approximately 130 ms. (Insert) Comparison of T²relaxation measured with birdcage head probe ([diamond]) and surface coil probe ([diamond]). Measurements were made on the same animal killed 6.5 min after the sevoflurane administration. (B) The percentage of T^2fastcomponent in the total intensity is plotted as a function of uptake time. The 12 rats studied are denoted by the different symbols on the plot. Each point on the plot resulted from nonlinear regressions of 16 or 18 spin-echo measurements with different TE values, similar to those shown in A. Note the logarithmic scale on the ordinate and the difference in the two slopes. (C) A typical sevoflurane uptake curve in the head region measured with the birdcage coil. The total sevoflurane intensity (closed cirle) and the T^2slowcomponent (open circle) did not plateau within the time range used in the study, but the T^2fastcomponent (open square) did.