Determination of magnetization transfer contrast in tissue: an MR imaging study of brain tumors

In this study, the magnetization transfer contrast (MTC) on MR images of several brain tumors and the correlation between MTC and tumors' histologic features were investigated. MTC depends on the extent of magnetization transfer, or cross-relaxation, from tissue water protons to macromolecular protons. On the basis of the known increase of the cross-relaxation rate with increasing molecular weight of protein in protein solutions, the hypothesis that changes in MTC correlate directly with the macromolecular composition of various tumors was tested.

Preoperative MR images were obtained with a 0.1-T MR system in 40 patients with brain tumors. MTC was correlated with the histologic features and the dry weight of the tumors. The tumors studied included astrocytomas (10), acoustic schwannomas (three), meningiomas (12), pituitary adenomas (10), craniopharyngiomas (two), and hemangioblastomas (three).

MTC was 0.43 in normal white matter and 0.42 in normal gray matter, and varied from 0.11 to 0.37 in the tumors. The mean MTC in astrocytomas (0.21 +/- 0.09) was smaller than the mean MTC in the gray matter (p = .0001) or in the other solid tumors (0.34 +/- 0.07 to 0.37 +/- 0.09, p < .002). MTC was larger in high-grade than in low-grade astrocytomas (0.28 +/- 0.05 vs 0.14 +/- 0.04, p = .0005). In meningiomas, MTC correlated with the collagen content of the tumor tissue (r = .95, p = .01). The differences in contents of solids between the solid tumor groups were not significant (p > or = .1, NS).

When the previously demonstrated correlations between solid content and 1/T1 of the types of tumors studied are taken into consideration, the present results suggest a larger relative contribution from hydrodynamic vs cross-relaxation effects in astrocytomas than in benign tumors or in gray matter. The twofold difference in MTC between low- and high-grade astrocytomas probably reflects the amount of nuclear material in the tumor cells. Collagen content determined the differences in MTC among meningiomas. These results indicate that the major determinant of differences in MTC within these tumor groups is the high-molecular-weight tissue macromolecules, suggesting higher specificity for MTC than for T1 in discriminating between tissues on MR images.