Quantitative T_1ρ 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 (T(1rho)) 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 T(1rho) together with the rate and other variables of magnetization transfer (MT) associated with spin interactions between the bulk and semisolid macromolecular pools by means of Z spectroscopy, to examine the possible overlap of mechanisms affecting these magnetic resonance imaging contrasts. Substantial prolongation of cerebral T(1rho) 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 (k(sat)) were unchanged at this time, these variables reducing only one day after induction of ischemia. The authors conclude that T(1rho) 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 T(1rho), are associated with water accumulation.

Dynamics of paramagnetic agents by off-resonance rotating frame technique in the presence of magnetization transfer effect

“…3 shows the magnetization maps for the magnetization transfer effect with exchange rate constant R=2, 100 by using Eq.18. R for biological tissues has wide distributions [35][36]. R=2 is close to the value reported for cross-linked 10% BSA gels [14,29,34]; while R=100 is relevant to the in vivo systems [35][36].…”

“…R for biological tissues has wide distributions [35][36]. R=2 is close to the value reported for cross-linked 10% BSA gels [14,29,34]; while R=100 is relevant to the in vivo systems [35][36]. The Lorentzian lineshape is used to calculate R rfB , which can provide accurate magnetization dependence of θ at small θ angle, as discussed in the section 2.3.…”

Quantitative Assessment of Water Pools by T1p and T2p MRI in Acute Cerebral Ischemia of the Rat

“…In this model, the immobile fraction is named “semi-solid pool” and it includes protons in macromolecules within tissue that have a T 2 relaxation time in the order of 10 μs (Henkelman et al 1993) and a typical τ c in the range of 10 -3 - 10 -6 s. In the present study the more immobile fraction is named “water associated with macromolecules” and has an effective τ c in the order of 10 -9 s indicating much higher mobility. Accordingly, the pool size for semi-solid pool in MT based models is typically between 3 - 15% (Mäkelä et al 2002; Portnoy and Stanisz 2007; Sled and Pike 2001) while we obtained 34 - 38% for this pool. We initially tried to constrain the pool sizes in the 2SX model fitting to be more in line with MT literature, but this resulted in a physically impossible (negative) parameter estimate for the exchange correlation time.…”

Acute cerebral ischemia in rats studied by Carr‐Purcell spin‐echo magnetic resonance imaging: Assessment of blood oxygenation level‐dependent and tissue effects on the transverse relaxation

“…These data clearly show that despite strong extravascular field gradients generated by intravascular AMI-227, T 1 is affected only by the direct signal drop due to the blood pool. Thus, it is very likely that T 1 MRI reveals exclusively tissue responses to ischemia caused by factors such as pH change, altered macromolecular mobility, and (at later time points) magnetization transfer (22,36,38). In this regard, it is very interesting to note that the short CP CP R 2 behaves in a fashion similar to that of R 1 , and thus shows reduced sensitivity to the dynamic dephasing effects relative to the long CP CP R 2 .…”