Theoretical signal-to-noise ratio and spatial resolution dependence on the magnetic field strength for hyperpolarized noble gas magnetic resonance imaging of human lungs.

In hyperpolarized noble gas (HNG) magnetic resonance (MR) imaging, the available polarization is independent of magnetic field strength and for large radiofrequency (rf) coils, such as those used for chest imaging, the body noise becomes the primary noise source making signal-to-noise ratio (SNR) largely frequency independent at intermediate field strengths (0.1-0.5 T). Furthermore, the reduction in the transverse relaxation time, T2, of HNG in lungs with increasing field strength, results in a decrease in the achievable SNR at higher fields. In this work, the optimum field strength for HNG MR imaging was theoretically calculated in terms of both SNR and spatial resolution. SNR calculations used the principle of reciprocity and included contributions to the noise arising from both coil and sample losses in a chest-sized coil for lung imaging. The effects of susceptibility differences, transverse relaxation time, and diffusion were considered in the resolution calculations. The calculations show that the optimum field strength for HNG MR imaging of human lungs is between 0.1 and 0.6 T depending on gas type (helium or xenon) and sample size. At the field strengths currently used by conventional clinical proton MR imaging systems (1-3 T), the predicted SNR are 10%-50% lower than at the optimum field with only slightly worse spatial resolution (10%-20%). At higher fields (>3 T), however, the SNR degrades considerably reducing the achievable spatial resolution. Although HNG of the lung is still feasible at very low field strengths (<50 mT), the available SNR is much lower than at optimum fields and this reduces the achievable spatial resolution. These findings suggest that HNG imaging may be optimally performed at much lower field strengths (0.1-0.6 T) than conventional clinical proton MR imaging systems. This could considerably decrease cost, improve patient access, and reduce chemical shift and susceptibility artifacts and rf heating.