Determining optimum imaging parameters for SWIFT: application to superparamagnetic iron oxides and magnetized objects.

Abstract

A relatively new pulse sequence in MRI known as SWIFT, sweep imaging with Fourier transformation, has been shown to effectively image spins with both short and long transverse and longitudinal relaxation rates. It is desirable to have equations that accurately describe the signal of spins when excited by SWIFT; however the Bloch equations are not easily solvable for the SWIFT sequence for all relaxation rates and flip angles. The purpose of this work is to determine a set of optimization equations for the SWIFT sequence through comparison to the Ernst energy equations via a Bloch simulator. An innovative contrast technique is also developed. The optimization equations are then tested experimentally and applied to imaging of superparamagnetic iron oxides. Susceptibility artifacts distort images around metal objects. In SWIFT images the susceptibility artifacts manifest as signal voids surrounded by pileup artifacts. This work develops predictive equations for the pileup artifacts around metallic spheres. A technique called ROC, radial off-resonance correction, is developed to reconstruct distorted images by utilizing the pileup predictive equations in post-processing.