Pathologies of the tracheobronchial tree in children are usually either due to a cartilaginous instability or an extrinsic compression. In children, the primary diagnostic tool for assessment of tracheomalacia is bronchoscopy or CT. However, if an external compression due to aberrant vasculature is suspected, MRI can be used, although the functional degree of stenosis is difficult to assess. MRI comes routinely into play after the trachea has reached a sufficient calibre to be visualized, in our experience at an age of 8 years. After surgery, MRI is capable of visualizing the tracheal lumen and assess for residual instability. Visualization in axial and coronal orientation should be accomplished (Fig. 1). T1-weighted sequences, like VIBE (volume-interpolated breath-hold examination) provide a spatial resolution of approximately 1.4 × 1.4 × 4 mm³ in a breath-hold of 18 s. T2-weighted TSE sequences have a spatial resolution of 0.9 × 0.7 × 3 mm³ and image acquisition is done during free respiration (Ley et al. 2010). After stent implantation, MRI is less useful as the metal leads to strong signal artifacts and loss of signal.

For assessment of tracheal and diaphragmatic motion during free or forced respiration, dynamic, time-resolved techniques can be applied. Usually, a steady-state free precession sequence (spatial resolution 3 × 2 × 6 mm³, temporal resolution 3 images/s) is used. This allows easy visualization of diaphragmatic paresis after cardiothoracic surgery or lack of expiration in obstructive disease, as in patients with Swyer–James syndrome.

As pointed out above, pathologies with an increase in soft tissue or fluid are easier to visualize on MRI than diseases with a deficiency of parenchymal structures (like alveolar simplification in patients with bronchopulmonary dysplasia). Therefore, the majority of publications on lung MRI are on the topic of pneumonia or cystic fibrosis.

MRI had a high agreement with CT (95 %) in the diagnosis of pneumonia in a study of adults (Eibel et al. 2006). In neutropenic patients, MRI showed the same findings as CT in 91 % of cases (Rieger et al. 2008). These positive reports about the capability of MRI are confirmed in our own experience (Fig. 2). However, although pediatric patients are radiosensitive subjects the use of MRI for pneumonia detection is rare. Usually, it is used in cases with a prolonged history or persistence despite antibiotic therapy (Fig. 3). In these cases, evaluation for a pulmonary abscess is needed and can be easily done by MRI. Also, characterization of pleural effusions is possible by MRI without contrast media, as well as planning of drainage placement.

Cystic fibrosis leads to chronic lung infection, airway obstruction, and progressive destruction of the lung parenchyma. These pulmonary pathologies are the main reason for hospitalization and reduced quality of life in cystic fibrosis patients. Frequent imaging studies, usually chest radiographs, are performed beginning in early childhood. This is a group of patients who could greatly benefit from MRI examinations of the lung as it allows morphological and functional assessment of disease activity without radiation.