T1 and magnetization transfer at 7 Tesla in acute ischemic infarct in the rat

Abstract

T1 and magnetization transfer at a field strength of 7 Tesla were used to discriminate between water accumulation and protein mobilization in tissue undergoing infarction. Twelve rats subjected to acute stroke via intralumenal suture occlusion of the middle cerebral artery, and 19 controls, were studied. In MRI studies to 6 hr post‐ictus, serial data acquisition allowed the measurement of cerebral blood flow (CBF), apparent diffusion coefficient of water (ADCw), equilibrium magnetization (M0) and T1, and equilibrium magnetization and T1 under an off‐resonance partial saturation of the macromolecular pool (Msat and T1sat). Using these parameters, the apparent forward transfer rate of magnetization between the free water proton pool and the macromolecular proton pool, kfa, was calculated. Regions of interest (ROIs) were chosen using depressed areas in maps of the ADCw. T1 measurements in bovine serum albumin at 7T were not affected by the mobility of the macromolecular pool (P > 0.2), but magnetization transfer between free water and protein depended strongly on the mobility of the macromolecular pool (P < 0.001). For 6 hr after ictus, kfa uniformly and strongly decreased in the region of the infarct (P < 0.0001). Ratios (ischemic/non‐ischemic) of parameters M0, Msat, T1, and T1sat all uniformly and strongly increased in the infarct. The ratio T1/T1sat in the region of infarction showed that a progressive accumulation of free water in the region of interest was the major (>80%) contribution to the decrease in kfa. There also existed a small contribution due to changes at the water‐macromolecular interface, possibly due to proteolysis (P = 0.005). Magn Reson Med 41:696–705, 1999. © 1999 Wiley‐Liss, Inc.

Acute blood–brain barrier opening in experimentally induced focal cerebral ischemia is preferentially identified by quantitative magnetization transfer imaging

“…After reperfusion, the rise in T 1sat of the BBB-damaged ROIs was approximately three times greater than that in T 1 . While both parameters are sensitive to changes in tissue water content, T 1sat can also be influenced by the mobilization of macromolecules promoted by proteolysis of such large membrane-bound proteins as tubulin (39).…”

Quantitative Evaluation of BBB Permeability after Embolic Stroke in Rat Using MRI

“…For efficient T 1 measurements, we used a variant of the TOMROP (Brix et al, 1990) pulse sequence with an increment of the initial phase step throughout each image acquisition. We have named this (single-slice) method PIPS (phase incremented progressive saturation) (Ewing et al, 1999). Sixty-four images (matrix 128 Â 64, FOV 32 mm, slice thickness 2 mm) were taken using an RF-spoiled gradient echo sequence with TE/TR ¼ 7/40 ms, tip angle, y, approximately 181.…”

Single‐coil arterial spin‐tagging for estimating cerebral blood flow as viewed from the capillary: Relative contributions of intra‐ and extravascular signal

“…MT properties under the conditions of the arterial spintagging off-resonance pulse (6 kHz off-resonance, 0.3 kHz amplitude) have been studied (25). Dipole-dipole interactions and (possibly) chemical exchange allows the transfer of magnetization at a considerable rate from the (MRI visible) protons of the free water of the tissue to the (MRI invisible) protons of the macromolecular and "bound water" proton pools.…”

“…We call this decreased equilibrium magnetization M sat and the shorter relaxation time T 1sat to distinguish them from M 0 and T 1 . M 0 , T 1 , M sat , and T 1sat have been measured in the central imaging slice using a phase-incremented progressive saturation (PIPS) (25,26), a variant of TOMROP (27). We will use values of T 1 and T 1sat typical of experiments in rat cerebral ischemia as parameters in the model calculations presented below.…”

“…The first assumption, saturation, may not be true under the conditions of the experiment (25,29,30). It may be, for instance, that the macromolecular pool is not fully saturated.…”