Variable flip angle 3D ultrashort echo time (UTE) T mapping of mouse lung: A repeatability assessment.

Background: Lung T is a potential translational biomarker of lung disease. The precision and repeatability of variable flip angle (VFA) T mapping using modern 3D ultrashort echo time (UTE) imaging of the whole lung needs to be established before it can be used to assess response to disease and therapy.

Purpose: To evaluate the feasibility of regional lung T quantification with VFA 3D-UTE and to investigate long- and short-term T repeatability in the lungs of naive mice.

Study Type: Prospective preclinical animal study.

Population: Eight naive mice and phantoms.

Field Strength/sequence: 3D free-breathing radial UTE (8 μs) at 4.7T.

Assessment: VFA 3D-UTE T calculations were validated against T values measured with inversion recovery (IR) in phantoms. Lung T and proton density (S ) measurements of whole lung and muscle were repeated five times over 1 month in free-breathing naive mice. Two consecutive T measurements were performed during one of the imaging sessions.

Statistical Tests: Agreement in T between VFA 3D-UTE and IR in phantoms was assessed using Bland-Altman and Pearson 's correlation analysis. The T repeatability in mice was evaluated using coefficient of variation (CV), repeated-measures analysis of variance (ANOVA), and paired t-test.

Results: Good T agreement between the VFA 3D-UTE and IR methods was found in phantoms. T in lung and muscle showed a 5% and 3% CV (1255 ± 63 msec and 1432 ± 42 msec, respectively, mean ± SD) with no changes in T or S over a month. Consecutive measurements resulted in an increase of 2% in both lung T and S .

Data Conclusion: VFA 3D-UTE shows promise as a reliable T mapping method that enables full lung coverage, high signal-to-noise ratio (∼25), and spatial resolution (300 μm) in freely breathing animals. The precision of the VFA 3D-UTE method will enable better design and powering of studies.

Level Of Evidence: 1 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2018.