Many factors affect the best ways to encode and detect information about a particular sample, whether it's a living organism, a sample of tissue on a slide, a gas, liquid, or mineral sample, or even a solid surface.
During encoding, the RF coil has to be of the same dimension as the sample and often surrounds it; in addition, the main magnet has to be big enough to bathe the sample in a magnetic field that is typically very strong. In hospitals, for example, MRI equipment big enough to examine the head or lungs is bulky and expensive.
For both encoding and detection, the proportion of target nuclei in the sample is another important consideration. If the sample is large but the proportion of target nuclei is small, this small "filling factor" makes for a weak signal.
Polarization -- the difference in the number of spin-up versus spin-down nuclei -- is also vital. Even in a strong magnetic field, the excess of spin-up hydrogen is at best 1 in 100,000.
Xenon-129, unlike hydrogen, can be optically "hyperpolarized" before it is introduced into the sample, where its nuclei interact with the surroundings to encode NMR and MRI information. Because xenon is a noble gas, chemically inert and nontoxic, it is ideal for many biological applications. In hyperpolarized xenon some 20 percent of nuclei are spin-up, "so we don't waste all those spins," Han remarks.
For optimum signal encoding, then, an NMR/MRI set-up may include a big RF coil and a strong magnetic field, while the best detection set-up for the same sample might require a more sensitive magnetic field and a smaller RF coil -- or even a supersensitive, non-MRI detector like a superconducting quantum interference device (SQUID) or a laser
Contact: Paul Preuss
DOE/Lawrence Berkeley National Laboratory