Oxygen‐enhanced functional lung imaging using a contemporary 0.55 T MRI system

The purpose of this study was to evaluate oxygen‐enhanced pulmonary imaging at 0.55 T with 3D stack‐of‐spirals ultrashort‐ T E (UTE) acquisition. Oxygen‐enhanced pulmonary MRI offers the measurement of regional lung ventilation and perfusion using inhaled oxygen as a contrast agent. Low‐field MRI systems equipped with contemporary hardware can provide high‐quality structural lung imaging by virtue of the prolonged T 2 *. Fortuitously, the T 1 relaxivity of oxygen increases at lower field strengths, which is expected to improve the sensitivity of oxygen‐enhanced lung MRI. We implemented a breath‐held T 1 ‐weighted 3D stack‐of‐spirals UTE acquisition with a 7 ms spiral‐out readout. Measurement repeatability was assessed using five repetitions of oxygen‐enhanced lung imaging in healthy volunteers ( n  = 7). The signal intensity at both normoxia and hyperoxia was strongly dependent on lung tissue density modulated by breath‐hold volume during the five repetitions. A voxel‐wise correction for lung tissue density improved the repeatability of percent signal enhancement maps (coefficient of variation = 34 ± 16%). Percent signal enhancement maps were compared in 15 healthy volunteers and 10 patients with lymphangioleiomyomatosis (LAM), a rare cystic disease known to reduce pulmonary function. We measured a mean percent signal enhancement of 9.0 ± 3.5% at 0.55 T in healthy volunteers, and reduced signal enhancement in patients with LAM (5.4 ± 4.8%, p  = 0.02). The heterogeneity, estimated by the percent of lung volume exhibiting low enhancement, was significantly increased in patients with LAM compared with healthy volunteers (11.1 ± 6.0% versus 30.5 ± 13.1%, p  = 0.01), illustrating the capability to measure regional functional deficits.