129Xe MRI in Pediatric Population With BPD

The most common respiratory complication of preterm birth, bronchopulmonary dysplasia (BPD), defined by a clinically assessed need for supplemental oxygen support at 36 weeks post-menstrual age, has actually increased in incidence as advancements in clinical respiratory care have improved initial survivability for very premature neonates. However, the burden of pulmonary disease continues beyond the NICU; the survivors are at greater risk for respiratory-related rehospitalization and diminished pulmonary capacity. Pulmonary imaging of the neonate has been limited to the clinical assessment of acute changes in respiratory status. The most widely accessible clinical imaging modalities, radiograph and computed tomography (CT), have significant limitations. Chest radiograph's sensitivity in the acute setting is limited because patients with significant respiratory dysfunction may exhibit only minor radiographic abnormalities, and although CT is considered the gold standard for clinical pulmonary imaging, it is not widely implemented because neonates may require sedation, especially for high-resolution CT, and are especially vulnerable to damage from ionizing radiation. Furthermore, CT is not appropriate for longitudinal assessment because of the link between serial radiation exposure and increased cancer risk.

As a nonionizing technique, magnetic resonance imaging (MRI) is an ideal modality for pulmonary imaging; in particular in the infant and pediatric population. Nevertheless, due to the low proton density of the lung parenchyma (only \~20% that of solid tissues), numerous air-tissue interfaces that lead to rapid signal decay, and cardiac and respiratory sources of motion that further degrade image quality, MRI has played a limited role in the evaluation of lung pathologies. Pulmonary MRI of the neonate is additionally confounded by small patient size and the delicate nature of transporting a NICU patient to the scanner. To overcome these limitations, the use of inhaled, hyperpolarized (HP) noble gases such as helium-3 (3He) and xenon-129 (129Xe) has come into play. Filling the air spaces within the lungs with either of these HP gases provides enough signal and contrast to obtain quality images on MRI.

There has been extensive work with HP 3He MRI in both the adult and pediatric population, but this gas is in extremely limited supply, making it increasingly expensive. 129Xe, on the other hand, is part of the atmosphere and as such does not suffer from supply constraints. Also, xenon dissolves in the lung tissue and blood, a process that is associated with characteristic shifts in the resonance frequency of 129Xe. As a result, the uptake and subsequent transport of 129Xe gas by the pulmonary circulation can be monitored, quantified and analyzed with regard to lung function at a temporal and spatial resolution that is infeasible with any other existing non-invasive modality.