Parra-Robles Juan, Cross Albert R, Santyr Giles E
Department of Physics, Carleton University, Ottawa, Ontario K1S 5B6, Canada.
Med Phys. 2005 Jan;32(1):221-9. doi: 10.1118/1.1833593.
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.
在超极化惰性气体（HNG）磁共振（MR）成像中，可用极化与磁场强度无关，对于大型射频（rf）线圈，如用于胸部成像的那些线圈，人体噪声成为主要噪声源，使得在中场强（0.1 - 0.5 T）下信噪比（SNR）在很大程度上与频率无关。此外，随着场强增加，肺部中HNG的横向弛豫时间T2缩短，导致在更高场强下可实现的SNR降低。在这项工作中，从SNR和空间分辨率两方面对HNG MR成像的最佳场强进行了理论计算。SNR计算采用互易原理，包括肺部成像胸部大小线圈中线圈和样品损耗产生的噪声贡献。在分辨率计算中考虑了磁化率差异、横向弛豫时间和扩散的影响。计算表明，根据气体类型（氦或氙）和样品大小，人体肺部HNG MR成像的最佳场强在0.1至0.6 T之间。在传统临床质子MR成像系统目前使用的场强（1 - 3 T）下，预测的SNR比最佳场强时低10% - 50%，而空间分辨率仅略差（10% - 20%）。然而，在更高场强（>3 T）下，SNR显著下降，降低了可实现的空间分辨率。尽管在非常低的场强（<50 mT）下肺部的HNG成像仍然可行，但可用的SNR远低于最佳场强时的SNR，这降低了可实现的空间分辨率。这些发现表明，HNG成像可能在比传统临床质子MR成像系统低得多的场强（0.1 - 0.6 T）下实现最佳效果。这可以显著降低成本，改善患者就医机会，并减少化学位移和磁化率伪影以及射频加热。
