Fundamental SNR and SAR limitations in very low field MRI

New techniques for magnetic resonance imaging (MRI) at low field strength (and hence low frequency) are currently being developed. Two important factors related to the weak electrical conductivity of the human body remain uncharacterized in this regime: (i) the intrinsic signal-to-noise ratio (ISNR) and (ii) the specific absorption rate (SAR) for electromagnetic energy associated with radiofrequency (RF) tipping pulses. This thesis presents experiments--performed over the frequency range 0.01 to 1.25 MHz--in which saline phantoms and human subjects were exposed to low-level oscillating magnetic fields produced by RF coils appropriate for low field MRI. The sample perturbs the quality factor and field map of the coil, and measurements of these properties were used to quantify ISNR and SAR. Results obtained from phantoms show excellent agreement with analytical predictions; results obtained from human subjects are directly applicable to design of low field MRI devices and pulse sequences.;Keywords. Signal-to-noise ratio (SNR); specific absorption rate (SAR); low field nuclear magnetic resonance (NMR); magnetic resonance imaging (MRI); very low field imaging; hyperpolarized noble gases.