Susceptibility effects in hyperpolarized ³He lung MRI at 1.5T and 3T

Abstract

Purpose:To compare susceptibility effects in hyperpolarized 3 He lung MRI at the clinically relevant field strengths of 1.5T and 3T. Materials and Methods:Susceptibility-related B 0 inhomogeneity was evaluated on a macroscopic scale by B 0 field mapping via phase difference. Subpixel susceptibility effects were quantified by mapping T * 2 . Comparison was made between ventilation images obtained from the same volunteers at both field strengths. Results:The B 0 maps at 3T show enhanced off-resonance effects close to the diaphragm and the ribs due to susceptibility differences. The average T * 2 from a voxel (20 ϫ 4 ϫ 4) mm 3 was determined as T * 2 ϭ 27.8 msec Ϯ 1.2 msec at 1.5T compared to T * 2 ϭ 14.4 msec Ϯ 2.6 msec at 3T. In ventilation images the most prominent effect is increased signal attenuation close to the intrapulmonary blood vessels at higher B 0 . Conclusion:Image homogeneity and T * 2 are lower at 3T due to increased B 0 inhomogeneity as a consequence of susceptibility differences. These findings indicate that 3 He imaging at 3T has no obvious benefit over imaging at 1.5T, as signal-to-noise ratio (SNR) was comparable for both fields in this work. IN MRI OF HYPERPOLARIZED NUCLEI the contribution of the Boltzmann polarization, which depends on the field strength (B 0 ) of the static magnetic field, to the longitudinal magnetization is negligible, with the nuclear polarization governed by the external (hyper)polarization physics. While in conventional proton MRI the signal-to-noise ratio (SNR) increases with higher B 0 field strength, the influence of B 0 on imaging of hyperpolarized species is less obvious (1), and imaging at low field strengths becomes feasible and possibly preferable due to longer transverse relaxation times (2-5).Nevertheless, the current trend for multinuclear MR development on whole-body scanners is toward field strengths Ն3T, driven by the need for improved SNR in new applications of low-abundance nuclei such as 13 C and 23 Na. As a consequence, imaging of hyperpolarized nuclei at 3T is emerging (6,7), and there is the need to evaluate the performance at this field strength.In addition to its influence on SNR (1,8), the B 0 field strength has an effect on the field homogeneity via localized magnetic susceptibility differences both on a microscopic (subpixel) and macroscopic (larger than pixel) length scale. The susceptibility difference between lung tissue and air is on the order of ⌬ Ϸ 9 ppm (9). Susceptibility-related field gradients have a bearing on image appearance and the local effective transverse relaxation time T * 2 measured over a given voxel size. Previously, susceptibility effects in 3 He lung MRI at different field strengths have been studied by ramping down a 1.5T scanner to 0.54T (10), and a method to compensate for susceptibility artifacts in gradient-echo imaging at 1.5T has been proposed (11).In this work the influence of susceptibility differences between tissue and gas spaces in 3 He lung imaging at 1.5T and 3T was studied on the macroscopic scale by empl...

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