The effects of polymer structure and water-macromolecule incan be achieved. Although water relaxation in solutions of teractions on proton relaxation in an aqueous model polymer have diamagnetic proteins is relatively well documented, there is been investigated using quantitative measurements of magnetizaa growing realization that tissue relaxation cannot be considtion transfer. Polyacrylamide gels composed of 95% water, 5% ered as a simple sum of the effects of isolated macromolecomonomers acrylamide and N,N-methylene-bis-acrylamide cules (7). Instead, the dominant relaxation effect comes were studied. The structure and rigidity were varied by changing from supramolecular organization, such as distinguishes sothe cross-linking density of the polymer. The polymer showed a lutions of albumin from its heat-treated and cross-linked biphasic change in transverse relaxation with increasing crosscounterparts (3). However, the degree of macromolecular linking density which was accompanied by a sudden increase in organization necessary or important for some relaxation promagnetization transfer above 40% cross linking. This change may cesses such as magnetization transfer to be effective has not be attributed to the formation of rigid domains in the polymer which exhibit solid-like behavior with a short T 2 (11 ms) and a been established. We have, therefore, sought to study a sim-Gaussian lineshape. Water-macromolecule interactions were conple polymer system which also shows many relaxation and trolled by varying the pH of the gel. At high pH ( ú8), there was MT properties that are similar to biological tissue but in an increase in magnetization transfer and transverse relaxivity which aspects of the composition can be controlled. Furtherconsistent with a chemical-exchange-mediated interaction between more, recent studies have shown that chemical exchange water protons and the polymer. By analyzing the system as two can play an important role in relaxation and magnetization proton reservoirs coupled by magnetization exchange, the proton transfer at the water-polymer interface (8, 9). Both cross populations, intrinsic relaxation rates, and exchange rates were relaxation and chemical exchange may be important in pracestimated, for different degrees of cross linking and pH. This tice.
model affords useful insights into the relevance of both supramo-
In this paper, we report the results of a study of proton lecular structure and chemical exchange on relaxation in tissues.
NMR relaxation and magnetization transfer in a model poly-