Application of computational methods as an adjunct to upper airway assessment

Abstract

This work comprises of an investigation into the application of virtual geometric reconstruction and computational fluid dynamics to the upper respiratory tract, to investigate how their anatomical form affects airflow and to examine the potential of 3D modelling of the airway as a potential surgical adjunct. For the latter purpose, the technology would need to have the ability to produce clinically relevant flow information in a timely, cost-effective fashion. Furthermore, it would need to ensure accurate model reproducibility, with specified limits to inter-user variability; thereby reducing costly post-processing in order for computational simulations to be performed. In this work, computational analysis of airway flow is subjected to critical assessment, examining each stage in the process of model building, testing and validation. The first stage is that of translating clinical scan data (usually CT or MRI) into a virtual 3D model for geometric analysis and flow simulation. In defining the airway geometry, key issues are variations in the quality of scan data, together with variations in the procedures used for image analysis and segmentation. Either may compromise the accuracy and the reproducibility of results and some of the results to be presented will indicate that there is a need for systematic research into threshold choices used for image segmentation of both normal and pathological geometries. A further issue is dynamic airway movement: most current scan data does not capture such data, but examples are shown to indicate the scale of such neglected effects. Having obtained a virtual anatomy, in the following stage computational fluid dynamics simulations can be performed to assess flow dynamics. Even though 3D virtual modelling is already used clinically by cardiothoracic and maxillofacial surgeons, knowledge of specificity and sensitivity of measures applied to geometries as complex as the nasal airway as indicators of physiological performance or markers of pathology is still unknown (Rao and Menon, 2015, Saad et al., 2013). In particular, uncertainties in determining the original geometry affect the predictions of flow, which is an issue as yet scarcely addressed. Here, results of a pilot study detailing a methodology to investigate the scale of such effects in computational prediction of nasal airway flows will be described. Whilst completing the two stages, namely model building and computational simulation provide the required output of air flow prediction, a third stage necessary for developing the technology is validation. In this work, an experimental procedure based on rapid prototype manufacture of replica airways, introduced as part of an investigation of the effects of glottis aperture on pressure loss in the trachea, provides a means for validating the computational methodology. Indeed, such replica models may offer an alternative to computational methodologies for more complex problems. Finally, the processes by which models are created and simulations performed are discussed in the context of requirements for validation and streamlining of the process for clinical acceptability.