Reconstruction of the Human Airways - Computational Fluid and Particle Dynamics in the Human Respiratory System - page 51

Hyperpolarized Helium gas MRI (HPHe MRI): Studies have shown promising re-
sults in detecting regional ventilation defects and that it is a reliable method for
measuring the lung airways (Eric et al. 2011). It has greater resolution allowing
imaging ventilation defects in the small airways and alveoli (Wang et al. 2008). The
contrast between the gas in the airway and the lumen walls are quite different to MRI
and CT and therefore a specific algorithm is needed for the HPHe MRI.

Micro-CT, Micro-MRI: Tiny structures associated with the alveolar region presents
challenges for morphological and functional analysis using standard CT and MRI.
Micro versions of these imaging techniques provide, less noise, and higher spatial
resolution to allow greater detail for airway reconstruction. Some drawbacks of the
micro systems include the higher costs of the system, the increased image acquisition
time (spanning into hours), and images typically capture a snapshot of the body
organs in time, thus it is unable to study the dynamic respiration cycle. However,
4D micro-CT has been used to study the cardiac structure and function of a mouse
heart, with a temporal resolution of 10 ms (Badea et al. 2005).

3.3 Image Segmentation

3.3.1 Overview

When an image is read digitally, it is defined as a 2D function, f ( x, y ) over a rectan-
gular grid, where x and y represent the spatial 2D coordinates, and the result f is a
value of the point at x, y . In 3D volumetric images obtained by CT or MRI, each slice
has a thickness (in the z -direction) that represents the slab section at the location of
the scan. For medical images, the pixel value is often referred to as the greyscale
value or intensity Fig. 3.4 . This value represents the attenuation of the x-ray beam
caused by the internal structures of the respiratory system for CT-scanned images,
while for MRI scans it represents the proton density.

Fig. 3.4 A greyscale image represented digitally by row ( x ) and column ( y ), and the greyscale
function f ( x, y ) applied over each pixel