A digital reference model of the human bronchial tree Andreas Schmidt a , Stephan Zidowitz b , Andres Kriete a, * , Thorsten Denhard a , Stefan Krass b , Heinz-Otto Peitgen b a Image Processing Laboratory, Institute of Anatomy and Cell Biology, Justus-Liebig-University, Aulweg 123, 35385 Giessen, Germany b Center for Medical Diagnostic Systems and Visualization (MeVis), Universita ¨tsallee 29, 28359 Bremen, Germany Received 10 July 2003; accepted 23 December 2003 Abstract In-vitro preparations of the human lung combined with high-resolution tomography can be used to derive precise models of the human lung. To develop an abstract graph representation, specially adapted image processing algorithms were applied to segment and delineate the bronchi. The graph thus obtained contains topological information about spatial coordinates, connectivities, diameters and branching angles of 1453 bronchi up to the 17th Horsfield order. The graph was analyzed for statistical and fractal properties and was compared with current models. Results indicate a model that exhibits asymmetry and multifractal properties. This newly established reference model is an important step forward in geometrical accuracy of the bronchial tree representation that will improve both analysis of lung images in clinical imaging and the realism of functional simulations. q 2004 Elsevier Ltd. All rights reserved. Keywords: Bronchial tree; Lung preparation; High-resolution computer tomography; Lung modeling 1. Introduction A widely-used model for simulations and predictions of gas transport, particle deposition and dosimetry is that of a regular, dichotomic branching pattern [1–4], a represen- tation that has its origin in the so-called trumpet model [5] that is used with geometric adjustments introduced later [6]. This is a simplification of the lung geometry that assumes a spatially symmetric branching, a fixed relation of the cross- sections of a parent branch with the daughter branches, and a constant length of all possible spatial trajectories between the trachea and any terminating bronchioli. This model is self-similar regarding spatial scaling properties and as such it reveals fractal properties. Even though the human bronchial tree, at least for the first six generations, exhibits a most symmetrical branching if compared to a wide variety of species, irregular and asymmetric branching pattern cannot be neglected [7–9]. Therefore, regular symmetric models do not predict gas flow and particle deposition reliably, and more realistic models are needed to increase accuracy. Dichotomic branching models also tend to fill space in a spheric fashion and can not be adapted to the form and volume of the human lung [3]. For the local analysis of air flows, more sophisticated models are in use to show that many of the transport processes within the airways depend quite sensitively on the geometry of the bronchial bifurcations and the structure of the boundaries [10–12]. But in the global modelling of the tracheobronchial tree, recent efforts rely on stochastic models [13,14] or synthesize the underlying structure from fixed scaling laws [4,15,16] based on a small number of anatomic examinations. These models meant to represent a mean adult lung, rather than describing the lung of a specific individual. We present the development of a digital reference model of the bronchial tree of a normal lung that is based on high-resolution computer tomography (HRCT) imaging of an in-vitro preparation. The image data are suitable for an in-depth computer-based investigation of the branching pattern of the bronchial tree and statistical analysis of geometry. A new reference model is derived, which will allow further improvements of functional simulations including particle transport and digital dosimetry. 0895-6111/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.compmedimag.2004.01.001 Computerized Medical Imaging and Graphics 28 (2004) 203–211 www.elsevier.com/locate/compmedimag * Corresponding author. Address: School of Biomedical Engineering, Science and Health Systems, Drexel University, 3141 Chestnut Street, Philadelphia, PA 19104-2875. Tel.: þ 1-215-895-2215; fax: þ 1-215-895- 4983.