| Simple yet effective cylindrical transmit and receive coils for low frequency magnetic resonance imaging (MRI) can be fabricated from a sine-phi distribution of windings. This thesis introduces the novel concept of an actively-shielded transmit coil. This coil produces a uniform oscillating magnetic field (a "B1 field") transverse to the bore of a cylindrical volume (the patient volume) yet at the same time produces very little magnetic field outside of this region. In practice, this allows one to minimize undesirable inductive coupling between the B1 coil and the myriad of other coils (B0, gradient, shim) that are needed to implement MRI. Exact analytical solutions are derived for active shielding of the elementary building blocks of typical B 1 coils. Specifically, solutions for a single straight wire, a complete current loop, and a sine-phi current distribution on the surface of an infinite cylinder are derived. Following this, the results of an experimental study examining the magnetic fields produced by actively shielded B 1 coils is presented. The concepts described in this thesis are expected to be useful for emerging applications in low field nuclear magnetic resonance and magnetic resonance imaging. |
