Active shielding of cylindrical transmit-coils for low-field magnetic resonance imaging

Simple yet effective cylindrical transmit and receive coils for low frequency magnetic reso nance imaging (MRI) can be fabricated from a sine-phi distribution of windings. This thesi introduces the novel concept of an actively-shielded transmit coil. This coil produces a uni form oscillating magnetic field (a "B1 field") transverse to the bore of a cylindrical volum (the patient volume) yet at the same time produces very little magnetic field outside of thi region. In practice, this allows one to minimize undesirable inductive coupling betweei the B1 coil and the myriad of other coils (Bo, gradient, shim) that are needed to implemen MRI. Exact analytical solutions are derived for active shielding of the elementary build ing blocks of typical B1 coils. Specifically, solutions for a single straight wire, a completl current loop, and a sine-phi current distribution on the surface of an infinite cylinder ar~ derived. Following this, the results of an experimental study examining the magnetic field produced by actively shielded B1 coils is presented. The concepts described in this thesi are expected to be useful for emerging applications in low field nuclear magnetic resonance and magnetic resonance imaging.