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

Abstract

Assessment of lung effective transverse relaxation time (T(2)*) may play an important role in the detection of structural and functional changes caused by lung diseases such as emphysema and chronic bronchitis. While T(2)* measurements have been conducted in both animals and humans at 1.5 T, studies on human lung at 3.0 T have not yet been reported. In this work, ultrashort echo time imaging technique was applied for the measurement and comparison of T(2)* values in normal human lungs at 1.5 T and 3.0 T. A 2D ultrashort echo time pulse sequence was implemented and evaluated in phantom experiments, in which an eraser served as a homogeneous short T(2)* sample. For the in vivo study, five normal human subjects were imaged at both field strengths and the results compared. The average T(2)* values measured during free-breathing were 2.11(±0.27) ms at 1.5 T and 0.74(±0.1) ms at 3.0 T, respectively, resulting in a 3.0 T/1.5 T ratio of 2.9. Furthermore, comparison of the relaxation values at end-expiration and end-inspiration, accomplished through self-gating, showed that during normal breathing, differences in T(2)* between the two phases may be negligible.

Figure 1

Diagram of the 2D UTE sequence for lung T₂* measurement. The sequence combines FID radial acquisition, half-sinc RF pulse excitation, ramp sampling and VERSE techniques to achieve an ultrashort echo time below 100us. δ (150us): ramp-down time of slice selective gradient; τ1 (30us): duration of ADC in advance of the readout gradient; τ2 (150us): ramp-up time of the readout gradient.

Figure 2

Eraser phantom experiment evaluating the 2D UTE sequence for short T₂* measurements. (a)-(c) are images acquired at 1.5T with UTE sequence at TE = 0.05, 0.6, 3.0ms, respectively. The region-of-interest (ROI) selected for T₂* measurement is depicted as a dotted square region in (c). (d) shows the slice profile.

Figure 3

Signal decay vs. echo time for the (a) eraser phantom and (b) a voxel in lung region 2 of subject 2 at 1.5T and 3.0T.

Figure 4

Lung images of subject 2 acquired with UTE sequence at 3.0T and 1.5T. (a)-(c) are images of TE = 0.03, 0.2, and 0.6ms at 3.0T. (f)-(g) are images of TE = 0.2, and 0.6ms at 1.5T. (d) and (h) are the voxel-by-voxel fitted T₂^* maps for 3.0T and 1.5T, respectively. (e) shows the six regions of the lungs in Table 1 where the T₂^* values are listed.

Figure 5

Time evolution of signal magnitude and phase at the center of k-space acquired with the 2D UTE sequence during free breathing for subject 2 at 3.0T. (a) and (c) show the signal curves at TE = 0.03 and 0.7ms (phase in bold curves), respectively. (b) and (d) are the corresponding FFT curves in the frequency domain. At TE = 0.03, the respiratory cycle can be clearly observed in (b) as a peak around 0.25Hz in both magnitude (dotted curve) and phase (circled curve) spectra; while in (d) the respiratory cycle can only be observed in the phase spectrum. The peak near 1Hz in magnitude spectrum corresponds to the cardiac cycle.