Rapid MRI of Short T2 Species with Multiple Small Angle Excitations and Time-Varying Gradients

Abstract

Images weighted by transverse relaxation time T2 have established efficacy in tissue characterization and enhanced the sensitivity and specificity of the diagnosis of pathology in many organ systems. While relatively long echo times (TE) on the order of 50-100 ms appear to improve image contrast, they do so at the expense of a loss of information, namely short T2 species. This precludes the correlation of complex determinants of tissue T2 decay such as the modulation of intrinsic dipolar interactions by molecular processes like proton exchange, proton transfer, and diffusion, with biological factors such as the degree of tissue hydration (compartmentalization), oxygen content, and temperature. We present a rapid MRI technique that employs multiple small angle excitations synchronized to periodic gradient fields to image both a short T2 species that is not visualized and several long T2 species that are seen with a current spin echo and gradient recalled echo technique (TE = 12.5 ms). The unique phase modulation to which each point in space is subjected is decoded through long term integration of the product of the observed periodic steady-state (dynamic equilibrium) signal and a phase demodulation kernel. Whereas gradient recalled echo techniques with static readout gradients are extremely sensitive to main magnet inhomogeneities, susceptibility changes, and chemical shifts, the decoding operation above can be modified to provide a relatively high degree of immunity to these variations. Furthermore, continuous cosinusoidal gradients enable the use of resonant gradient circuitry to eliminate switching transients and to dissipate less power for comparable amplitudes.