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MR image contrast at high field strength

The T1 of soft tissues increases with magnetic field strength. Some tissue contrast may be diminished on high‐field‐strength magnetic resonance (MR) images when conventional TRs are used, because of altered T1 effects on the MR signals. This necessitates longer TRs in techniques that use long TRs, which prolongs the examination excessively. Behavior of macroscopic magnetization is governed by the Bloch equations. Therefore, T1 contributions to the MR signal can be modulated by means of both timing intervals and radio‐frequency pulses. The analytic solution to the Bloch equations allowed calculation of white matter/gray matter and gray matter/cerebrospinal fluid contrast in both spin‐echo and inversion‐recovery (IR) imaging. Rabbit brains (normal and tumor‐containing) were then imaged in vivo at 1.5 and 4.7 T. In addition, MR images of a human head were obtained at 4.O T. Experimental results supported the theoretical predictions that brain contrast on long TR spinecho or IR images increases with field strength. However, varying the excitation flip angle allowed optimization of the T1 contribution to the MR signals, improving image contrast and/or reducing examination time. Thus, the dependence of T1 on field strength determines the optimum choice of imaging techniques and parameters in a predictable fashion.