Potentials for Studies of Eleven Species Other Than Protons

Whereas MRI and fMRI have been based on imaging proton spin density and intrinsic tissue relaxation rates as well as injected contrast-based relaxation rate changes, a major medical science window is opened by studies of other nuclei such as the spin ½ and spin 3/2 nuclei of carbon-13, oxygen-17, sodium-23, phosphorus-31, potassium-39, and other nuclei present in the mammalian body (see Table 4.1) along with the resonant frequencies at 20 T. Note that many of the anticipated problems for proton studies at 20 T disappear for the other nuclei listed, as they have lower gyromagnetic ratios, hence lower NMR frequencies. Physics of NMR for low gamma nuclei shows the time to acquire equivalent SNR data at 20 T will be reduced by a factor of 8 from that at 7 T and of 33 from that at 3 T, and spectral dispersion and relaxation time changes will allow investigations of metabolites in vivo that cannot be observed by any other method. As shown in Table 4.1, the sensitivities for detection of these nuclei of interest are very much lower than the sensitivity of protons. Moreover, not shown are the abundances in tissues of these nuclei, so it is important to adjust our expectations for applications based on knowledge of the local concentration of nuclei of interest. For example, sodium concentration within cells is 10 M and in extracellular fluid is 130 mM,

TABLE 4.1 Resonant Frequencies of Relevant NMR Nuclei at 20 Tesla

Nucleus Frequency (MHz) Sensitivity Spin
1H 852 1.00 ½
7Li 331 0.29 3/2
13C 214 0.02 ½
15N 184 0.001 ½
17O 115 0.03 5/2
23Na 225 0.09 3/2
31P 345 0.07 ½
37Cl 83 0.03 3/2
39K 40 0.0005 3/2
57Fe 28 0.00003 ½
63Cu 226 0.09 3/2
67Zn 53 0.003 5/2
87Rb 279 0.175 3/2

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