Anatomically realistic computer representations of the human body are regularly used when assessing the absorption of electromagnetic fields in people. This article discusses how the MAXWEL model was made.
Computer models of humans, sometimes called computational human or voxel phantoms, are models of the human body utilised in computational physics. These models are anatomically realistic representations of the human body derived from medical imaging data. In the field of electromagnetics, they are used to calculate internal dose quantities such as induced electric fields, induced current densities and specific absorption rates (SAR) in tissues within the body.
At EMFcomp Limited, we have developed a voxel model of an adult male body. The model is known as the MAle voXel Whole-body modEL, or MAXWEL. MAXWEL was created using raw, grey-scale MRI slices from a number of continuous partial body scans of a single subject. These axial slices were of 2 mm thickness in the vertical plane. The grey scale data of the medical images were segmented into different tissue types using volume segmentation.
Code was written to automatically interpret the 8-bit grey-scale images into the different tissues. After this automatic segmentation had taken place, manual segmentation was required to ensure the separation of tissues in regions where voxel intensities were in close range. The resulting human voxel model consists of approximately 10 million voxels segmented into 42 different tissue types
The tissue types include adrenals, bile, bone, bone marrow, brain (white matter), brain (grey matter), breast, background air, gall bladder, cerebrospinal fluid, blood, spleen, small intestine, stomach, stomach (contents), prostate, testis, bladder, urine, thymus, liver, muscle, skin, fat, spinal cord, lower large intestine, upper large intestine, thyroid, eye lens, humour, sclera, heart muscle, lung, tendon, cartilage, oesophagus, kidney and pancreas.
The dimensions of the voxels within MAXWEL were scaled so that the height of the voxel phantom was
1.76 m and the mass was 73 kg. These values agree with the values of Reference Man as defined by the International Commission on Radiological Protection (ICRP) in ICRP 89 (2002). The resulting voxel model is of approximately 2 mm resolution; each cubical 2 mm voxel is given a number that identifies it with the discrete tissue type of that particular voxel.
Other postures of MAXWEL can be derived by manipulating the limbs of the voxel model. At EMFcomp, code has been written in FORTRAN and C++ to move and realign files representing the limbs into the correct orientation. Important factors in this process are to maintain the correct length of the articulated limbs and provide electrical continuity of the different tissues when the limbs were re-attached. Similar to the developed
standing human voxel model, the postured voxel phantoms can be normalised to a mass of 73 kg. This is achieved by adjusting the horizontal dimensions slightly.
The MAXWEL adult voxel model can also be non-linearly scaled to reproduce the heights and masses of reference 1, 5 and 10 year old children. The dimensions as defined by ICRP 89 (2002) are 1 year: height – 0.75 m, mass – 10 kg, 5 year: height – 1.10 m, mass – 20 kg, 10 year: height – 1.38 m, mass – 33 kg
As the models are rescaled, the masses of various tissue types are examined to ensure that the tissue mass, in addition to the dimensions of the model, are in line with published organ mass/total mass ratios for the different ages.