|Optimization of MEOP experiment performed at elevated 3He pressures (Vol. 42, No. 3)|
(a) Reshape of the gaussian pump laser beam into the annular profile using a pair of axicons, (b) Photograph of the rf discharge plasma afterglow in a transversal plane of helium cell, (c) Steady-state nuclear polarization of 3He gas vs pressure for two laser beam profiles.
It was demonstrated about fifteen years ago that hyperpolarized 3He gas can be used as an inhaled contrast agent in Magnetic Resonance Imaging (MRI) of human lungs. Since then, the technique has proved successful in anatomical and dynamic ventilation studies, which are not feasible by the standard proton MRI. One of the methods to obtain 3He gas of high polarization is Metastability Exchange Optical Pumping (MEOP),which is usually performed at low operating pressure of 1 mbar and at low magnetic field of 1 mT. Recently it has been shown that the MEOP method can be also performed at elevated 3He gas pressure, up to 260 mbar, provided it is performed at high magnetic field of 1.5 T or higher. Under these operating conditions, one of the factors that limits the efficiency of the method is an inhomogeneous density distribution of metastable state atoms produced by the RF discharge in the optical pumping cell (Fig.b). The paper shows how the situation can be improved by matching the spatial profile of the pumping laser beam to the distribution of metastable atoms. It is achieved by using a pair of axicons to produce an annular, instead of conventional Gaussian beam profile (Fig.a). The obtained nuclear polarization of 3He gas was up to 60% higher for pressures above 67 mbar (Fig.c). This result opens the possibility of producing large quantities of highly polarized 3He gas in a compact polarizer working in highmagnetic field of medical MRI scanner.
Optimization of the pumping laser beamspatial profile in
the MEOP experiment performed at elevated 3He pressures