A Novel MR Signal Detection Method Based on Optical Pumping

Abstract

Instead of using a RF coil, a novel MR signal detection method based on optical pumping is introduced. The simulated MR signals are generated via a coil which is mounted around the Rubidium vapor cell. By measuring the transmitted light intensity, magnetic fields generated by the coil could be obtained. First of all, a FID signal is detected by this method. Then 1D imaging of two point spins is also presented. Future improvements are discussed at the end of the paper. In conventional NMR or MRI techniques, the physical principles of MR signal detection are derived from Faraday's law of electromagnetic induction. However, as the MR signal is proportional to the magnetic field strength, routine detection method, which makes use of the RF coil, suffers from poor sensitivity in low magnetic field. Optical pumping technique was first developed by Alfred Kastler in 1950's and he was awarded the Nobel Prize for this work in 1966. It is a process involving absorbing and emission of radiation which produces a highly non-thermal population distribution among atomic energy levels. Optical pumping is important for the develop- ment of modern atomic physics. Its applications include lasers, optically pumped magnetometers, atomic clocks and so on. Optically pumped magnetometers could be used to measure very weak magnetic fields. Recently, they have been applied in NMR or MRI to replace the RF coil(1)(2). Although the sensi- tivity of the optically pumped magnetometer is comparable to SQUID, it is difficult to build. By analyzing the experiments, a novel detection method is developed. The new method is also based on optical pumping, however, signals could be obtained just by measuring the transmitted light intensity. A brief review for optical pumping is given in the next section, followed by details of the experimental setup and the results.