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4 Medical and Life Science Studies (MRI, fMRI, MRS) Enabled by 20 Tesla
Pages 80-99

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From page 80...
... The opportunities opened by much higher magnetic fields than exist today are tremendous, because many human health conditions cannot be approached by any other methods, as discussed in the body of this chapter. The technologies presented here are meant for research and not for routine clinical use.
From page 81...
... Safety and health effects studies commenced in the late 1970s and continue to the present time while keeping pace with new methods of acquisition of the magnetic resonance signals and the increases in magnetic field strength (Appendix F)
From page 82...
... Springer, 2007, The magnetic field and tissue dependences of human brain 1H2O longitudinal relaxation in vivo, Magnetic Resonance in Medicine 57:308-318. et al., 1988; Schenck et al., 1992; Ugurbil et al., 1993)
From page 83...
... transmission and reception coils; gradient coils and power supplies; spectrometer design and performance; and the associated pulse sequences needed FIGURE 4.2  Increase in field and resolution for human brain proton MRI from 1984 to 2008.
From page 84...
... The main driver for development has been proton MRI, which largely depicts varia tions between tissues in proton (mainly water hydrogen nuclei) density and NMR relaxation times and provides exquisite anatomical images.
From page 85...
... 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.
From page 86...
... . In addition, relaxation times are field independent, so that 17O sensitivity gain with higher magnetic fields will enable imaging the dynamics of H217O in vivo at 9.4 T and above (Zhu et al., 2001; Atkinson and Thulborn, 2010)
From page 87...
... . It is possible to discriminate cation resonances using the difference in longitudinal relaxation values, which if sufficiently different in intracellular and extracellular environments allow simple inversion recovery pulse sequences (e.g., potassium in perfused heart studies)
From page 88...
... In addition, 13C-CEST has the potential to redefine metabolic pathway pheno types in human beings. Furthermore, 20 T will allow measurement of pH, redox, metabolite levels, temperature, reactive oxygen species (ROS)
From page 89...
... Terpstra, et al., 1998, Resolution improvements in in vivo H NMR spectra with increased magnetic field strength, Journal of Magnetic Resonance 135:26-264. There would be considerable value to being able to routinely image cortex with resolutions 2-4 times smaller -- for example, to visualize cortical columns and cortical layers.
From page 90...
... . However, magnetic fields much higher than 7 T are needed to achieve the SNR and data acquisition times required to decipher the neural code at the scale of fundamental computations.
From page 91...
... For PARACEST agents, the larger chemical shifts at 20 T would make multiplexing less challenging. Proton Transverse Relaxation Rate Changes with Field The anomalous behavior of the proton transverse relaxation rate (R2 = 1/T2)
From page 92...
... to new classes of molecular imaging techniques, including CEST and T1rho imag ing. The manifestation of this mechanism will be at high magnetic fields.
From page 93...
... Susceptibility anisotropy within macromolecules and assemblages of molecules is discussed in Appendix F Some of the potential benefits are related to the image contrast that results from bulk magnetic susceptibility differences in adjacent tissues due to compounds such as ferritin and myelin, both of which are found throughout brain tissue.
From page 94...
... Traveling wave methods may be adapted for such acqui sitions, and the fact that image data can be detected at a distance from the object provides a compelling reason to continue to develop these methods at higher fields. Anticipated Problems from Interactions Between High Magnetic Fields and MRI Hardware and Human Subjects Interactions of High Magnetic Fields with Imaging Gradients Because of the well-known frequency vs.
From page 95...
... Gradient Amplitude Requirements for Low γ Nuclei Whereas very high fields open opportunities for imaging nuclei other than protons because the frequencies allow body penetration and to some extent reduce the RF power deposition, there is a penalty: The spatial resolution requires a spatially dependent frequency shift, and the magnitude of this shift is dependent on the product of the gyromagnetic ratio and the gradient. But this is not a serious consequence, because volumetric resolutions 10-50 times less than enjoyed from proton imaging can provide metabolic information not available by any other noninvasive method.
From page 96...
... At 20 T, imaging the human cortex using proton MRI at 50-µm resolution will be possible even though RF penetration at 852 MHz is limited to a few centimeters. The susceptibility differences between Alzheimer's plaques and adjacent tissues size should allow visualization of plaque-invested tissues even for particles of 20-µm
From page 97...
... . In parallel, an engineering feasibility study should be undertaken to identify appropriate radio frequency, gradient coils, and power supplies that will enable MRI and MRS and an extension of current health and safety research currently being conducted at lower fields.
From page 98...
... 2008. Chemical exchange saturation transfer contrast agents for mag netic resonance imaging.
From page 99...
... 1999. Functional mapping in the human brain using high magnetic fields.


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