Quantitative T1ρ and magnetization transfer magnetic resonance imaging of acute cerebral ischemia in the rat

Abstract

It has been previously shown that T1 in the rotating frame (T1p) is a very sensitive and early marker of cerebral ischemia and that, interestingly, it can provide prognostic information about the degree of subsequent neuronal damage. In the present study the authors have quantified T1p together with the rate and other variables of magnetization transfer (MT) associated with spin interactions between the bulk and semi-solid macromolecular pools by means of Z spectroscopy, to examine the possible overlap of mechanisms affecting these magnetic resonance imaging contrasts. Substantial prolongation of cerebral T1p was observed minutes after induction of ischemia, this change progressing in a time-dependent manner. Difference Z spectra (contralateral nonischemic minus ischemic brain tissue) showed a significant positive reminder in the time points from 0.5 to 3 hours after induction of ischemia, the polarity of this change reversing by 24 hours. Detailed analysis of the MT variables showed that the initial Z spectral changes were due to concerted increase in the maximal MT (+3%) and amount of MT (+4%). Interestingly, the MT rates derived either from the entire frequency range of Z spectra or the time constant for the first-order forward exchange (ksat) were unchanged at this time, these variables reducing only one day after induction of ischemia. The authors conclude that T1p changes in the acute phase of ischemia coincide with both elevated maximal MT and amount of MT. These changes occur independent of the overall MT rate and in the absence of net water gain to the tissue, whereas in the consolidating infarction the decrease in the rate and amount of MT, as well as the extensive prolongation of T1p, are associated with water accumulation.