Auteurs
Christ A, Kainz W, Hahn EG, Honegger K, Zefferer M, Neufeld E, Rascher W, Janka R, Bautz W, Chen J, Kiefer B, Schmitt P, Hollenbach HP, Shen J, Oberle M, Szczerba D, Kam A, Guag JW and Kuster N. 2010. The Virtual Family—development of surface-based anatomical models of two adults and two children for dosimetric simulations. Phys Med Biol 55: N23-N38.

Background
Anatomical computer models of the human body have been used for more than three decades for dosimetric applications in electromagnetic (EM) and in medical radiation physics. Currently, approximately 20 whole and partial body models of adults and 14 models of children are available to the scientific community. However, the segmentation of tissue types in these models is not always accurate or effective for measuring EM exposure.

Objective
The objective of this study was to develop anatomically correct whole-body human models of an adult male (34 years old), an adult female (26 years old) and two children (an 11-year-old girl and a 6-year-old boy) for the optimized evaluation of electromagnetic exposure.

Methods
Anatomical models were developed based on image sets of whole MR body scans of two adults and two children. The scanning sequences were chosen to meet the requirements on image contrast and resolution posed by the segmentation while minimizing the measurement time.  All images were taken in transversal orientation and the entire scanning time was approximately 6 hours for the adults and between 90 minutes and 3 hours for the children depending on their body sizes. The tissues and organs in the MR images were identified by an expert team of biologists and physicians and manual segmentation was facilitated by iSEG software. Segmentation was carried out organ by organ or tissue by tissue. After segmentation, tissue boundaries were reconstructed as unstructured triangulated surfaces using the marching cubes algorithm and were smoothed using the spring-model and by reducing the number of surface triangles.

Results
The four models referred to as the Virtual Family were reconstructed. Approximately 80 different tissues and organs were identified and segmented for each model. There was good agreement between the organ masses of the ICRP reference phantoms and the obtained models for most tissues and organs. Certain deviations can be attributed to segmentation uncertainties, mainly of the digestive tract. Deviations of the masses of those tissues, which show up clearly in the MR images, are due to individual properties of the volunteers.

Interpretation and Limitations
In spite of the high contrast and resolution of the MR images, certain compromises during the segmentation were inevitable. For example, although the outlines of most of the tissues could be constructed with an accuracy of 1 to 2 pixels of the original images, blood vessels with a diameter of less than 2 mm were not included. Nerve tissues only included the spinal cord and the optic nerve. The imaging quality of the digestive tract was not sufficient for a complete segmentation. The intestinal ducts and the lumina could not be reconstructed completely and the pancreas could only be reconstructed for the male adult model. The cortex of the kidneys and the walls of the stomach and the gall bladder were indistinguishable from the medulla and the lumen. They were segmented using a fixed thickness of five pixels of the MR images. The salivary glands have not been segmented separately. Only small fractions of the diaphragm were visible in the images therefore, it was reconstructed with the help of anatomical atlases. The bone marrow of the two female models was not segmented for the entire body and since white and red marrow cannot be told apart in the MR images, the bone marrow was segmented as red marrow in the male models. The periosteum was not segmented separately for all models and the extrathoracic region was not identified as a separate region.

Conclusions
The anatomical accuracy of the Virtual Family Models with respect to their individual features enhances their usability for numerous numerical applications. The models are being widely applied in several studies on electromagnetic exposure, device optimization and medical applications. All four models and the software for their discretization are freely available to the scientific community.


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