K + dynamics in ischemic rat brain in vivo by 87 Rb MRI at 7 T

The aims of the present study were as follows: (i) to perform the first 87 Rb MRI in live rats with focal ischemic stroke; and (ii) to test the hypothesis that K + egress from the brain in this model is quantifiable in individual animals by high‐field (7‐T) K/Rb substitution MRI. Rats preloaded with dietary Rb + (resulting in Rb/(K + Rb) replacement ratios of 0.1–0.2 in the brain) were subjected to permanent occlusion of the middle cerebral artery, and 87 Rb MRI was implemented with 13‐min temporal resolution using a dedicated RF coil and a spiral ultrashort‐TE sequence (TR/TE = 3/0.07 ms). The ischemic core was localized by apparent diffusion coefficient mapping, by microtubule‐associated protein‐2 immunohistochemistry and by changes in surface reflectivity. [K], [Na] and [Rb] were determined independently in the micropunched samples by post‐mortem flame photometry. Both techniques were generally in agreement in the nonischemic cortex; however, the MRI‐assessed [K +  + Rb + ] drop in ischemic brain was less pronounced (average efflux rate of 4.8 ± 0.2 nEq/mm 3 /h versus 10 ± 1 nEq/mm 3 /h by flame photometry; p  < 0.0001). The use of higher field gradients for better spatial resolution, and hence more accurate quantification, is suggested. Copyright © 2011 John Wiley & Sons, Ltd.