Comparison of lung T2* during free-breathing at 1.5 T and 3.0 T with ultrashort echo time imaging.

The dependence of T2* on the magnetic susceptibility difference between the airway and lung tissue suggests that the parameter has the potential to characterize the morphometric structure of micro-architecture of lung at the alveolar level. Structural and functional changes occurring in various pulmonary disorders that change the dimensions, density and composition of lung alveolar tissue may be reflected in the T2* values. However, magnetic resonance imaging of lung parenchyma is challenging due to the inherent limitations from low proton density, relatively short T2* relaxation time and respiratory motion(1,2). The multi-level airway structure, in which water concentration is only ~30%, makes the achievable signal intrinsically lower than other organs. The extremely short T2* of lung also demands a fast signal acquisition following excitation before signal decays into the noise level. Respiratory motion can also cause ghosting artifacts and degrades the lung image quality. While T2* measurements of the lungs have been conducted in both animals and humans at 1.5T, studies on human lung at 3.0T have not yet been reported. The magnetic susceptibility effect is linearly proportional to the B0 field strength, which suggests it may be more sensitive to subtle lung pathologic changes and may have the advantage in detecting early stages of lung diseases. However, lung T2* at 3.0T is expected to be considerably shorter than that at 1.5T. The ultrashort T2* at 3.0T precludes the use of Cartesian-based sequences, even when optimized to achieve very short TEs. Originally proposed by Pauly et.al. to image very short T2* species, ultrashort echo time (UTE) imaging is based on radial k-space sampling of the free induction decay(3,4). The uniqueness of UTE is the self-refocusing RF excitation, in which a half-sinc RF pulse in conjunction with a pair of gradients with opposite polarity is used for slice selective excitation, and the slice profile after combination is that of the tradition full-sinc excitation. With the elimination of slice selective rewinding gradient and with the application of FID acquisition, the effective echo time, which is defined as the delay between the end of excitation and collection of center of k-space signal, can readily be reduced to less than 100 us. In this work, we present a comparison of lung T2* measurements in normal human subjects at 1.5T and 3.0T with a 2D UTE pulse sequence. The UTE sequence utilizing the golden-angle view increment strategy is first briefly described in the following section. A phantom experiment with an eraser serving as homogeneous short-T2* phantom is then performed to evaluate the UTE sequence. In the in-vivo lung imaging experiments, five normal human subjects were imaged in the axial orientation at the two magnetic field strengths. Based on the measurements performed at multiple echo times, the T2* values measured during free-breathing were calculated in six anterior and posterior regions, and the results compared. In addition, T2* values were compared at end-expiration and end-inspiration by utilizing the self-gating strategy to detect the respiratory cycle.