Ultra-Low Frequency MRI: Novel Imaging Sequences

Our ultra-low field MRI system operates at a field of 0.132 mT with the signal detected by a Superconducting QUantum Interference Device (SQUID) coupled to an untuned, superconducting, second-derivative gradiometer. Operation at such low fields requires that we prepolarize the protons of our specimen in a field of about 150 mT prior to imaging to increase the sample magnetization. With the ultimate goal of \textit{in vivo} imaging of prostate tissue by taking advantage of the enhanced longitudinal-relaxation-time at low fields, we seek to decrease the imaging time and optimize the signal-to-noise ratio of our imaging pulse sequences to make \textit{in vivo} imaging viable in a clinical setting. To achieve this, we begin with standard imaging sequences used in other frequency domains and adapt them to our specific purposes and requirements, in particular the need to prepolarize. We describe modified inversion recovery and multiple echo imaging sequences, specifically Carr-Purcell-Meiboom-Gill (CPMG) pulse train and fully balanced Steady-State Free Precession (SSFP) sequences. We present the results of applying these sequences to imaging agarose gel phantoms.