Magnetic resonance imaging techniques for physiological contrast visualization at ultra-high magnetic fields.

摘要

The race to build magnets for magnetic resonance imaging in humans at ever-increasing magnetic field strengths is motivated by basic physics: for a given sample, the observable NMR signal is directly proportional to the main magnetic field strength, B₀. Currently, however, most MRI research at ultra-high field has been limited to functional imaging (FMRI) and spectroscopy (MRS) of structures that can be visualized using small surface RF coils a few centimeters in diameter. Whole-brain and body structural and pathological imaging is largely restricted to lower fields primarily due to RF issues, among them limitations on RF power that can be applied (SAR restrictions) and B₁ inhomogeneity, which becomes particularly significant at magnetic fields of 3 Tesla and above for 1H (proton) imaging.; T₂ contrast is perhaps the most interesting form of MRI contrast for visualizing structure and pathology, since most disease states are characterized by elevated T₂. The standard imaging sequence for acquiring T₂-weighted images at low magnetic fields is the spin echo (SE) sequence, which uses one or more 180° refocusing pulses to cancel the T₂ ′ component of the T₂* relaxation process. At ultra-high magnetic fields, however, uniform 180° pulses are extremely difficult to implement due to B₁ inhomogeneity and require high RF power, so alternative techniques would be very useful for T₂ imaging at ultra-high field.; The PSIF sequence is analyzed as such a sequence. The PSIF sequence produces images with T₂ weighting by sampling the steady state free precession (SSFP) echo. The inherent SNR of the PSIF technique is Tower than SE, but the RF power requirements are substantially lower, the time required for each acquisition is shorter, and it is relatively insensitive to B₁ inhomogeneity. These properties are particularly suited to the problems experienced at ultra-high magnetic fields.