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 pool 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 relaxation times. However, these restricted protons modulate the observed signal from free water by dipolar and chemical exchange interactions. In MT imaging, specifically tailored RF pulses are applied to saturate selectively the restricted macromolecular pool. This saturation is "transferred" to the free protons, causing their signal amplitude to decrease [1]. Increased signal intensity due to T1 shortening caused by gadolinium administration does not depend upon macromolecular interactions and is not appreciably suppressed by MT pulses (Fig. 1). Consequently, MT pulses act synergistically with gadolinium to increase the visibility of enhancing lesions by preferentially suppressing nonenhancing background tissue [2]. The purpose of this paper is to demonstrate the principles underlying the synergistic effects of MT saturation and paramagnetic contrast agents and to illustrate these effects in clinical MR imaging and MR angiography.