K⁺ dynamics in ischemic rat brain in vivo by ⁸⁷Rb 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.