A signal to noise calibration procedure for NMR imaging systems

A nuclear magnetic resonance (NMR) imaging system signal to noise calibration technique based on an NMR projection of distilled water in a cylindrical bottle is proposed. This measurement can characterize any arrangement of rf coils in any magnetic field as signal to noise per ml times root Hz. Inductive losses in a typical patient must be included in the calibration, and such losses can be simulated in a particular system by an externally attached resistor(s) appropriate to that system. Alternatively, an rf inductive damping phantom consisting of a conducting loop of wire containing an appropriate resistor is suggested that can be inserted into any NMR imaging coil to simulate subject Q damping. The same resistor can be used, independent of the details of the coil construction. Furthermore, if the loop inductance is tuned out at each frequency with a series capacitor, then the same loop resistance will serve for all frequencies as a good approximation to human subject damping. This “projection method” signal to noise ratio is related to the conventional signal to noise ratio measured from a Lorentzian shaped spectral line as ψ_P=ψ_L[2/T₂]^1/2, where ψ stands for signal to noise ratio, subscripts P and L stand, respectively, for the projection and “Lorentzian” methods, and T₂ is the transverse relaxation time of the spectral line used in the Lorentzian method.