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Segmentation of 3D Tubular Tree Structures in Medical Images ...

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1.1. Related Work 7<br />

work are discussed <strong>in</strong> appropriate chapters later <strong>in</strong> this work.<br />

1.1.1 Models, Features, and Extraction Schemes<br />

Lesage et al. [79] provide an excellent and up-to-date overview and classification <strong>of</strong> <strong>3D</strong><br />

vessel segmentation techniques that is also valid for airway segmentation techniques. The<br />

different methods known from the literature utilize many different image process<strong>in</strong>g frameworks.<br />

In [79] the authors dist<strong>in</strong>guish between three ma<strong>in</strong> characteristics <strong>of</strong> vessel segmentation<br />

techniques: models, features, and extraction schemes.<br />

Models: Models are used to represent prior knowledge about the target vessel structures.<br />

In this context, one may dist<strong>in</strong>guish between photometric and geometric properties <strong>of</strong> the<br />

vessels. Methods utiliz<strong>in</strong>g pure photometric models make assumptions about the expected<br />

lum<strong>in</strong>ance [1, 40, 117, 175] <strong>of</strong> the vessels and/or the background [40, 52, 93, 125, 126] as<br />

well as the image noise [52, 148] to dist<strong>in</strong>guish between vessel structures and non-vessel<br />

structures. Methods utiliz<strong>in</strong>g geometric models make assumptions about a key characteristic<br />

<strong>of</strong> vessels, namely their specific shape. Therefore, the methods may encode <strong>in</strong>formation<br />

about their elongation, properties <strong>of</strong> their centerl<strong>in</strong>e [15, 42, 72], and/or their cross-section<br />

[26, 27, 42, 43, 150]. Photometric and geometric <strong>in</strong>formation are <strong>of</strong>ten comb<strong>in</strong>ed <strong>in</strong>to hybrid<br />

models, <strong>in</strong>corporat<strong>in</strong>g assumptions about the spatial appearance <strong>of</strong> vessels such as<br />

Gaussian- or bar-like cross section pr<strong>of</strong>iles [64, 70, 135, 166], appearance as a ridge <strong>in</strong> scale<br />

space [2, 12, 45], or template-based approaches [44, 48, 83]. The models typically focus<br />

solely on regular vessel segments, while models about bifurcations or anomalies can only<br />

be found rarely, although exceptions – typically adapted to specific k<strong>in</strong>ds <strong>of</strong> disease – can<br />

be found [1, 42, 83, 159, 162].<br />

As a general note, these models are <strong>of</strong>ten embedded implicitly <strong>in</strong> the features and<br />

extraction schemes discussed below.<br />

Features: Features are the actual detectors/filters used to evaluate a vascular model on<br />

the image data. Therefor the methods utilize <strong>in</strong>formation about the basic image <strong>in</strong>tensity,<br />

first order, and/or second order image derivatives to match aga<strong>in</strong>st the expected model,<br />

<strong>of</strong>ten embedded <strong>in</strong> a Gaussian scale-space framework [84, 143] to account for tubular<br />

structures <strong>of</strong> vary<strong>in</strong>g size. Features used <strong>in</strong> the literature are isotropic features where no<br />

assumption about the directionality <strong>of</strong> the vessel is made [3, 74, 75, 138], features mak<strong>in</strong>g<br />

use <strong>of</strong> the local geometry typically based on derivative features [1, 44, 73, 83, 89, 121] or<br />

model fitt<strong>in</strong>g [48, 72, 148, 166] with few other techniques. Another approach is utiliz<strong>in</strong>g

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