Purpose or Case Report:

Tracheomalacia (TM) is a common morbidity associated with prematurity and manifests as dynamic collapse of the trachea lumen due to cyclic changes of intrathoracic pressure during breathing. Premature infants often have elevated work of breathing (WOB) related to their distal, small airway and lung abnormalities. The large airway contribution to WOB can be determined using computational fluid dynamics (CFD), which is a well-known technique to calculate the resistance and WOB in the human airway. However, previous studies are based on static airway geometry without motion. Using the novel technique of ultrashort echo time (UTE) magnetic resonance imaging (MRI), the tidal volume and airway motion can be used to create a dynamic model for use in CFD. Our aim is to calculate the estimated WOB in a dynamic trachea with neonatal TM compared with a stable, static trachea.

Methods & Materials:
Ten neonatal intensive care unit (NICU) patients diagnosed by bronchoscopy (5 with TM, 5 without TM) were imaged using UTE MRI (resolution 0.7x0.7x0.7 mm) in a neonatal-sized 1.5T scanner sited within the NICU. 7 patients were breathing room air at the time of MRI (postmenstrual age at MRI ~41 weeks), and 3 patients were on high flow nasal cannula. Images were retrospectively respiratory-gated based on the motion-modulated k-space center to generate 8 distinct images throughout the breathing cycle, from which tidal lung volumes were measured and airway surfaces generated via segmentation. The airflow rates were determined also using the k-space center Airway motion was defined via registration of each airway surface. STAR-CCM+ (Siemens) 11.06 software was used to simulate the airflow throughout the breathing cycle. The contribution of the airway (nasopharynx to carina) to WOB was calculated for each patient using patient specific CFD simulations. CFD simulations were run without airway motion using a static geometry (largest airway geometry detected during the breathing cycle) to compare the increase in the breathing effort due to motion.
Results:
On average, TM caused 364% increased WOB compared to the static CFD model. Whereas, subjects without TM increased WOB only 36% due to less dynamic changes in the trachea.

Conclusions:
This study demonstrates that WOB related to the large airway movement can be evaluated using patient-specific CFD based on retrospectively gated UTE MRI. Neonates with TM expend nearly 5 times more energy for breathing than if their airway was static.