Sensitivity and noise analysis of solenoidal coils for nuclear magnetic resonance microscopy
Peck, Timothy L.
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https://hdl.handle.net/2142/21647
Description
Title
Sensitivity and noise analysis of solenoidal coils for nuclear magnetic resonance microscopy
Author(s)
Peck, Timothy L.
Issue Date
1992
Doctoral Committee Chair(s)
Magin, Richard
Department of Study
Electrical and Computer Engineering
Discipline
Electrical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Electronics and Electrical
Language
eng
Abstract
Conventional NMR studies involve sample volumes $(\upsilon\sb{s})$ of several cubic millimeters and greater. Much recent interest is directed at the development of microscopic NMR systems for the analysis of smaller samples and the visualization of microscopic biological structures. The objective of the research summarized in this thesis is to extend the analysis of RF coils used for NMR to microscopic domains. Microscopic nuclear magnetic resonance suffers from an inherently low SNR, due to the small number of nuclei available to contribute to the signal. Decreasing the size of the RF receive coil provides better coupling between the sample being examined and the coil, i.e., it increases the sensitivity of the coil. We define the sensitivity of a coil as the magnitude of the RF magnetic field that is produced in the coil when a unit current is passed through the windings of the coil.
However, the sensitivity of the coil to detect signals is not the only consideration when determining the SNR, as the noise must also be considered. Noise originates primarily from the resistance of the coil. Electrical circuit models used to characterize large coils (diameters $\geq$ 1 mm) cannot be used for smaller coils, in which the wire diameter approaches one skin depth at the frequency of operation. Therefore, we extend the electrical circuit models which represent the coil to microscopic domains to characterize accurately the SNR performance of RF coils for submillimeter NMR. It is clear from our results that the loss in microcoils can be characterized accurately and that an accurate calculation of the SNR using microcoils can be achieved. Furthermore, the results demonstrate that an enhanced SNR can be achieved in NMR with microcoils as small as 38 $\mu$m, and support the continued reduction of coil size for further enhancement in the SNR.
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