Prepolarized fast spin-echo pulse sequence for low-field MRI.

Clinical MRI systems use magnetic fields of at least 0.5T to take advantage of the increase in signal-to-noise ratio (SNR) with B(0). Low-field MRI apparatus is less expensive and offers the potential benefit of improved T(1) contrast between tissues. The poor inherent SNR at low field can be offset by incorporating prepolarizing field pulses with the MRI pulse sequence. The prepolarizing field does not need to be as homogeneous as the detection field, so it can be generated by a relatively inexpensive electromagnet. Prepolarizing hardware for a 0.01T MRI system was developed together with a prepolarized MRI pulse sequence that incorporates fast imaging techniques to reduce acquisition times by a factor of 5 relative to standard methods. Comparison images of test objects show that most of the enhanced SNR is retained with the fast method. Low-field images of a human wrist acquired using the fast prepolarized method are also shown.