[Relationship between water content and magnetization transfer ratio in the rat brain using a new cardiac perfusion...

We devised a method to make a gradient water content model of the rat brain by perfusing osmotic NaCl solution from the left ventricule. Using this model we measured the water content and magnetization transfer ratio (MTR) of the frontal lobe and the pons as examples of gray and white matter, respectively. When we perfused five levels of osmotic NaCl solution (0, 0.9, 1.8, 3.6 and 7.2%) to respective groups of four rats, mean water content decreased from 81.5% at 0% to 75.9% at 7.2% NaCl in the frontal lobe and from 74.1% to 61.0% in the pons, while MTR increased from 21.5% to 24.2% in the frontal lobe and from 22.9% to 27.3% in the pons. Thus water content was significantly higher (p < 0.01) in the frontal lobe than the pons at each level of NaCl concentration, and MTR was significantly higher in the pons than the frontal lobe (p < 0.05 approximately 0.01, except at 0%). Further, the grade of alteration was higher in the pons than the frontal lobe for both water content and MTR. Excellent correlations between MTR and water content were observed with coefficients of -0.75 and -0.82 for the frontal lobe and pons, respectively, and the regression lines showed higher position and slope in the frontal lobe than the pons. From these results we concluded that the pons is rich in free water of extracellular fluid that moves easily between the myelin sheath, whereas in the frontal lobe, water is so diffusely mingled with membranes and favorable to bound water that the magnetization transfer effect is more effective, yielding a higher MTR for the same water content and showing greater change in MTR for the same change in water content as the pons.