3D Modelling of the Mastoid Bone. Structural and developmental analysis of the human air cell system.
The mastoid bone contains a set of numerous small air cells, which are connected together and located in the backside of the middle ear underneath the cranium. Development of middle ear infections (otitis media = OM) together with sequelae concerning hearing lost are a common factor for a net absorption of gas from the mastoid bone that leads to a negative pressure in the middle ear (MEP). The origin of this “gas exchange” results in a difference in gas concentration between the air phase and the blood phase (via the mucosa). The structure of the air cell system in the mastoid bone with its numerous ramifications implies a relative high surface area compared to its volume which results in a high surface area to volume ratio, and is therefore considered to be very effective for gas exchange. The related clinical problems are important and lead to a main case for ear surgery treatment in order to improve hearing. <p> Similarly, the lungs have numerous ramifications (bronchi) and air cells (alveoli), which also lead a high surface area to volume ratio leading to an effective gas exchange, by inhalation of oxygen through the trachea. Other similarities can be found between the structure and the functionalities of the lungs and the mastoid: 1) histologi-cal structure of the flat epithelium with rich vascularization of the underlying network of capillaries, and 2) the central neural feedback regulation for the both the lungs and the mastoid bone is performed in the same cranial nerve (IX and X) and located in the same area of the brainstem. These similarities have led to the naming of the mastoid bone as a miniature lung. <p> In spite of the important role of the mastoid bone in the development of OM and sequelae, research concerning the structure and the functionality of the mastoid bone is rather poor because of its inaccessibility. Improved CT scanning methods generate more detailed pictures, which can be used as the basis for advanced image processing techniques with possible 3D reconstruction and derivation of quantitative data describing the inherent structure of study. Such investigations have been performed for the lungs and lead to a better analysis of the lung functionality. Due to the similarities of the mastoid bone structure and function with the ones from the lungs, the use of a similar method, new knowledge concerning the development of OM as well as corresponding sequelae could be derived in order to improve treatments about preserving hearing.
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IntroductionSecretory otitis media (SOM) is the second most frequent children disease next to and related to common colds. Traditionally the background of the disease has been perceived as changes in the middle ear (ME) gas exchange (GE) in combination with a reduced function of the Eustachian tube (ET) leading to the development of a negative middle ear pressure (MEP). This results in a ME transudation, decreased hearing and pressure sensation in the ear. Treatment is most often accomplished by insertion of a ventilation tube into the tympanic membrane (TM), which counterbalance the negative MEP to ambient pressure and normalize the hearing.
The mastoid bone with its numerous air cells plays a very important role in the development of a negative MEP, but its overall role has been attributed only to a passive GE over the ME mucosa, and thus, most of the current research focus has been aimed at an active regulation of the MEP through ET openings. However, some researchers have stressed the importance of the mastoid bone, and call the mastoid the “miniature lung”.
However, the exact role of the mastoid is relatively unknown due to its inaccessibility, but recent experiments, where MEP is being measured and monitored directly with a catheter inserted into the cells indicate that active regulation of the pressure actually takes place at least for smaller pressure changes. Due to mentioned similarities with respiratory physiology, these pressure changes are suggested to be effected by changes in perfusion of mastoid mucosa, which leads to changes in diffusion of gases, and thus, MEP.
Further, the role of the mastoid also seem important considering clinical observations, where sclerotic mastoids with poor pneumatization are very often found in patients with chronic otitis media and impaired regulation of the MEP. In normal ears the mastoid develops throughout childhood increasing its number of air cells, and hence its degree of pneumatization, and full maturation is reached at the age of 14 to 16 years of age (21). This has lead to the hypotheses that either children with genetically small mastoids are especially prone to developing otitis media, or that otitis media affects a normal pneumatization of the mastoid. Though this discussion is older than CT imaging techniques, it is still vivid.
Altogether, the role of the mastoid in MEP regulation and ME diseases is basically unknown, but new physiological experiments monitoring pressures directly add important new knowledge on its functional properties. Improved CT scanning techniques and imaging analysis may add valuable information on its structural properties including developmental.
Due to the high similarity between lung and mastoid structure, it is seems obvious to suggest that the mastoid is also developed by dichotomous budding. Quantitative analysis of the tree geometry from intrathoracic airway has been investigated in many different studies and is considered important for the evaluation of the bronchial tree structure and its function. This development can be described by image processing applied on CT scans, where the branches (ramifications) of the lung part can be determined, and the tree structure can be described quantitatively in details; moreover the surface area and the volume of the lungs can be computed..
A similar application of this technique to the mastoid bone has never been attempted before, and the combined determination of the proposed tree structure and analysis of connected air cells will improve our analysis be determining its functional area and volume. Previous methods merely determining the total area and volume of the air cells will also include isolated air cells, which do not contribute to GE and MEP regulation.
In spite of the huge clinical relevance for the study of the mastoid structure and its function, there is no quantitative studies describing the tree structure of a mastoid bone and its development for healthy and for patients having Otitis Media. The reason of this non existence of results is, as explained above, the inaccessibility of the mastoid bone. But new high resolution CT scans allow for detailed and more precise views of the mastoid bone, which can be used as input for modern and advance computer image processing tools. Such methods are used in the description of lung ramifications and its tree structure, and which are used for the analysis of the lung functions. Correspondingly, these methods could be modified in order to be used for the description and the analysis of the mastoid bone.
Hypothesis and AimsHypothesis: The structure of the human air cell system of the temporal bone can be described by quantitative imaging analysis of high resolution CT scans, which can contribute to understand its function in normal and pathological ears.
This hypothesis leads to four aims of our studies:
Segmentation. As discussed in this protocol, simple thresholding technique (like the binary) is not enough to obtain a detailed representation of the air cells in the mastoid bone. Followed by a number of filtering techniques, more advanced segmentation methods based on edge detection and level set propagation will be used. Extraction of the air cells in the mastoid bone is rather a challenge, especially in “temporal bone” CT scans where it seems that some air cells are filled with a liquid or by thickened mucosa (gray value being different from air).
Tree Structure. Once the optimal segmentation tool has been applied on the dataset(s), a tree structure can be extracted from the segmented air cells in the temporal bone. Important information can be extracted from the tree structure (such as the number of divisions, number of branches, length of a particular branch, distance of a branch from the root). Being able to extract five generations of branches from the root and compare it to the structure of the lungs, we could confirm the close similarity between the air cells of the mastoid bone with the lung structure.
Surface Area and Volume. From the results of the segmentation, measurements of the surface area, the volume and the A/V-ratio can be obtained, which reflects the efficacy of GE. Thus, these variables may be used for correlations with clinical course of ME diseases.
Healthy vs. Pathological mastoids. By having at disposition two types of CT scans (“routine” healthy) and “temporal bone” (often pathology on one side or on both mastoid bones) CT scans, a quantitative analysis will be performed on both datasets to investigate the differences between healthy versus pathologic mastoid bones, variation in surface area, variation on volume, and variation in the A/V ratio, as well as variation of the corresponding tree structure (smaller tree structure, less children branches, shorter branches, etc).
In the present study, the ultimate goal is to get: an accurate surface area, a volume, and a tree structure of the air cell system as possible to better understand its structure and its function.