Purpose or Case Report: Dynamic 4D CT (D4DCT) can replace bronchography in the assessment of tracheobronchomalacia (TBM) but setting up a new D4DCT service for infants with TBM poses unique challenges due to their venerability. Simulation prior to implementation is essential as D4DCT involves continuous volumetric CT scanning through the breathing cycle, potentially delivering high radiation doses. Radiation dose, gantry rotation and scan dynamics can be customized.
Our purpose is to describe the development and implementation of CT dynamic airway protocol using a 3D printed Infant Dynamic Airway Phantom (phantom) for simulation of D4DCT in tachnypnoea and collapsible airways, thereby validating image quality and estimating radiation dose prior to clinical implementation.
Methods & Materials: Materialise Mimics^TM was used with chest CT data of one infant to segment the air contained in the airway, which was wrapped in a 2mm offset to create a lumen model. The proximal airway was cut open for connecting to an ET-tube. The phantom was printed on a Stratasys Connex 500 in Tango+.
Siemens FLASH CT was used for test imaging at rapid respiratory rates and various combinations of CT settings (kVp, mAs, cycle time, rotation time, and total number of rotations). The phantom was surrounded with 5” PMMA acrylic glass for attenuation and was connected via ET tube with an inflated cuff to a wall oxygen outlet for constant air flow. Infant respiratory rates and airway collapsibility were simulated by rhythmically occluding the tubing to a metronome set at 40bpm, causing expansion and collapsibility of the flexible phantom during inspiration and expiration respectively.
Results: A fixed technique setting at 80kVp, 6mAs, 0.28sec rotation time, 0.43sec cycle time, and a total number of 6 scan cycles provided satisfactory dynamic airway CT images capturing just over one breathing cycle. Settings are at the minimum tube output equivalent to a total of 0.74 mGy/6 scan cycles in CTDI, predicted as a total clinical radiation dose of 0.2 mSv for a 4cm length of airway D4DCT.
Conclusions: The Phantom, combined with a set-up simulating a rapid respiratory rate and TBM, allowed us to design and validate a clinical CT protocol for D4DCT of an infant airway. It also allowed estimation and optimization of radiation dose prior to patient scanning. Clinical applications have further supported the predictions of image quality and dose by the phantom study.