The intrinsic signal-to-noise ratio in NMR imaging

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Abstract

The fundamental limit for NMR imaging is set by an intrinsic signal-to-noise ratio (SNR) for a particular combination of rf antenna and imaging subjects. The intrinsic SNR is the signal from a small volume of material in the sample competing with electrical noise from thermally generated, random noise currents in the sample. The intrinsic SNR has been measured for a number of antenna-body section combinations at several different values of the static magnetic field and is proportional to B0. We have applied the intrinsic and system SNR to predict image SNR and have found satisfactory agreement with measurements on images. The relationship between SNR and pixel size is quite different in NMR than it is with imaging modalities using ionizing radiation, and indicates that the initial choice of pixel size is crucial in NMR. The analog of "contrast-detail-dose" plots for ionizing radiation imaging modalities is the "contrast-detail-time" plot in NMR, which should prove useful in choosing a suitable pixel array to visualize a particular anatomical detail for a given NMR receiving antenna.

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The sensitivity performance of the human eye on an absolute scale.

George A. Rose (1948)

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A review of normal tissue hydrogen NMR relaxation times and relaxation mechanisms from 1-100 MHz: Dependence on tissue type, NMR frequency, temperature, species, excision, and age

Paul A Bottomley, Thomas H. Foster, Raymond Argersinger … (1984)

The longitudinal (T1) and transverse (T2) hydrogen (1H) nuclear magnetic resonance (NMR) relaxation times of normal human and animal tissue in the frequency range 1-100 MHz are compiled and reviewed as a function of tissue type, NMR frequency, temperature, species, in vivo versus in vitro status, time after excision, and age. The dominant observed factors affecting T1 are tissue type and NMR frequency (V). All tissue frequency dispersions can be fitted to the simple expression T1 = AVB in the range 1-100 MHz, with A and B tissue-dependent constants. This equation provides as good or better fit to the data as previous more complex formulas. T2 is found to be multicomponent, essentially independent of NMR frequency, and dependent mainly on tissue type. Mean and raw values of T1 and T2 for each tissue are tabulated and/or plotted versus frequency and the fitting parameters A, B and the standard deviations determined to establish the normal range of relaxation times applicable to NMR imaging. The mechanisms for tissue NMR relaxation are reviewed with reference to the fast exchange two state (FETS) model of water in biological systems, and an overview of the dynamic state of water and macromolecular hydrogen compatible with the frequency, temperature, and multicomponent data is postulated. This suggests that 1H tissue T1 is determined predominantly by intermolecular (possibly rotational) interactions between macromolecules and a single bound hydration layer, and the T2 is governed mainly by exchange diffusion of water between the bound layer and a free water phase. Deficiencies in measurement techniques are identified as major sources of data irreproducibility.