Environmental optimization and shielding for nmr experiments and imaging in the earth's magnetic field

Abstract

For many years, a number of laboratories have been working on the applications of very low field NMR. In 1985, our laboratory presented the first NMR images using the earth's magnetic field. However, the use of this technique was limited by the weakness of the signal and the disturbing effects of the environment on the signal‐to‐noise ratio and on the homogeneity of the static magnetic field. Therefore experiments had to be performed in places with low environmental disturbances, such as open country or large parks. In 1986, we installed a new station in Lyon, in the town's hostile environment. Good NMR signals can now be obtained (with a signal‐to‐noise ratio better than 200 and a time constant T*₂ better than 3 s for 200‐ml water samples and at a temperature of about 40°C). We report here our new technique: A shielded box was built and installed on scaffoldings above the terrace roof of our faculty building. Gradient coils were used to correct the local inhomogeneities of the earth's magnetic field. We show FIDs and MR images of water‐filled tubes made with or without these improvements.