Nonlinear magnitude and linear phase behaviors of complex T2* imaging

International Conference on Medical Physics

August 03-05, 2015 Birmingham, UK

Zikuan Chen and Vince Calhoun

Posters-Accepted Abstracts: J Nucl Med Radiat Ther

Abstract :

The underlying source of brain imaging by T2*-weighted magnetic resonance imaging (T2*MRI) is the intracranial inhomogeneous tissue magnetic susceptibility (denoted byÏ?) that causes an inhomogeneous field map (via. magnetization) in a main field. By decomposing T2*MRI into two steps, we understand that the 1st step from aÏ?source to a field map is a linear but non-isomorphic spatial mapping, and the 2nd step from the field map to a T2* image is a nonlinear mapping due to the trigonometric behavior of spin precession signals. The magnitude and phase calculations from a complex T2* image introduce additional nonlinearities. In this report, we look into the magnitude and phase behaviors of the T2* image (signal) by theoretical approximation and Monte Carlo simulation. We perform a 1st-order Taylor expansion on the intravoxel dephasing formula of T2* signal and show that T2* magnitude is a quadratic mapping of the field map and T2* phase is a linear isomorphic mapping. By Monte Carlo simulation of T2*MRI for a span of echo times (with B0=3T and TE= [0,120] ms), we first confirm the quadratic magnitude and linear phase behaviors in the small phase angle regime (at TE<30ms), and then provide more general magnitude and phase nonlinear behaviors in large phase angle scenarios (at TE>30ms). By solving the T2*MRI inverse, we perform Ï?tomography by reconstructing the Ï?source from a T2* phase image. For large phase angle scenarios, we show that imperfect phase unwrapping imposes additional distortions on the Ï? tomography.