COMBINED EFFECTS OF MAGNETIZATION-TRANSFER AND GADOLINIUM IN CRANIAL MR-IMAGING AND MR-ANGIOGRAPHY

Magnetization transfer (MT) imaging is an MR technique in which image contrast is altered by applying RF pulses that saturate a restricted p ool of hydrogen protons associated with cell membranes, proteins, and other macromolecules. Protons in this restricted pool, unlike those in tissue-free water, are not visible on MR due to their short T2 relaxa tion times. However, these restricted protons modulate the observed si gnal from free water by dipolar and chemical exchange interactions. In MT imaging, specifically tailored RF pulses are applied to saturate s electively the restricted macromolecular pool. This saturation is ''tr ansferred'' to the free protons, causing their signal amplitude to dec rease [1]. Increased signal intensity due to T1 shortening caused by g adolinium administration does not depend upon macromolecular interacti ons and is not appreciably suppressed by MT pulses (Fig. 1). Consequen tly, MT pulses act synergistically with gadolinium to increase the vis ibility of enhancing lesions by preferentially suppressing nonenhancin g background tissue [2]. The purpose of this paper is to demonstrate t he principles underlying the synergistic effects of MT saturation and paramagnetic contrast agents and to illustrate these effects in clinic al MR imaging and MR angiography.