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

Abstract

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.

A system for open‐access 3He human lung imaging at very low field

“…The SNR for these first images was lower than theoretical predictions, which should have approached the imaging quality obtained in clinical MRI scanners (14). This prevented us from using a slice selection gradient to obtain finer details of lung structure and resulted in fullprojection images with SNR ~10.…”

“…In contrast, susceptibility-induced gradients are negligible for B 0 ~10 mT; as a result, T 2 and may be over an order of magnitude larger at very low fields, helping to offset the reduction in NMR signal obtained from operating at lower RF frequency. Laser-polarized 3 He lung imaging at B 0 ~5-10 mT can thus yield MRI images with resolution and SNR only about three times lower than values obtained at 1.5 T (14), assuming a similar 3 He polarization level and absolute B 0 homogeneity.…”

“…For prepolarized samples, the ideal B 0 value is that where sample noise has not yet begun to dominate, while NMR signal has been maximized with respect to coil (or Johnson) noise. For laser-polarized 3 He in samples the size of human lungs, this value has recently been determined to be ~0.1-0.2 T (14). At B 0 above this value, signal is lost due susceptibilityinduced background gradients, which result in significantly reduced , line broadening and inherent loss of resolution at B 0 as high as that commonly encountered in clinical MR scanners.…”

Functional lung imaging using hyperpolarized gas MRI

“…Low-field systems (Fig. 7) have potential for both reducing cost and optimizing SNR and spatial resolution for HP gas and HP contrast agents in general because the signal strength will depend only on the gyromagnetic ratio and the polarization level of the gas (50). Fast gradient systems will likely still be required for these systems to take full advantage of data acquisition within a single breathhold, but parallel imaging techniques combined with variable flip angle (60) or balanced SSFP (55) will likely provide the SNR and time-efficiency necessary for clinical applications.…”

“…The ␥ of He-3 is a factor of 3 larger than the ␥ of Xe-129 (Table 1), which generally leads to higher signal. However, noise characteristics and spatial resolution depend on resonant frequency and therefore field strength does influence SNR differently for the two gases (50). At a field strength of 2 T, T2 values of 2 to 3 seconds and T2* values of 20 msec (Table 2) have been measured for HP He-3 in the lung parenchyma of rats (51), while T2* for HP He-3 was measured at ϳ10 msec in human lung parenchyma at 1.5 T (52).…”