prepolarization

Prepolarization refers to a technique in nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) in which spins are first aligned by exposing the sample to a strong magnetic field before measurement in a weaker field. The aim is to increase the net magnetization available for detection when the measurement occurs at a low magnetic field, where thermal polarization is small.

Principle: The population difference between spin states is proportional to the magnetic field; by briefly applying a high polarizing field Bp for a polarization time tp, the sample acquires a higher initial magnetization M0 which is then carried to the measurement region with field Bm (Bm << Bp). Once in the measurement region, the signal is detected as free induction decay or spin echoes as the spins relax with time constants T1 and T2.

Timing constraints: The transport and hold times must be short relative to T1 to preserve polarization; relaxation during transfer reduces the effective signal; the design balances Bp, tp, and the distance and speed of transfer.

Applications: Prepolarization is widely used in ultra-low-field MRI and portable MRI systems to compensate for reduced Boltzmann polarization. It enables imaging at fields from microtesla to tens of millitesla and is complementary to other polarization techniques such as dynamic nuclear polarization, though it typically yields more modest gains.

Limitations and challenges: The method requires rapid and controlled transfer, specialized hardware to generate and switch fields, and careful management of field inhomogeneities and timing to maintain reproducible signal.

See also: Nuclear magnetic resonance, MRI, ultra-low field MRI, prepolarized MRI, dynamic nuclear polarization.