Development of physical and electronic whole-body shell phantoms for SAR and radio propagation measurement.

The starting point for this phantom was the existing Standard Anthropometric Model (SAM) head that is specified in assessment standards for mobile phones and radios used in the standard ear position. SAM was originally developed as part of Project SARSYS, a European Collaborative Research Programme under the EUREKA scheme. The SARSYS consortium members were MCL, SPEAG and the University of Gent. To guide the specification of a realistic shape phantom, anatomical shapes and sizes were obtained from data generated by a large anthropometric study of men and women of various ethnic origins, aged over 20 years, randomly selected among US Army personnel. The study was carried out in the late 1980s; its findings are considered to be still representative today's society. The dimensions of a 90th percentile male head were selected for the phantom. The lines, arcs and circumferences that determine the shape of the SAM head were applied to a clay sculpture of a head and shoulder model. The sculpture was then used to produce a lightweight, fibreglass shell which was digitised and the three dimensional data stored as a CAD file. An outer shape was generated, incorporating a well defined shell thickness and outer ear in accordance with the shape of the model. The CAD file was translated to a physical model an accuracy of 0.1 mm using a computerised stereolithographic procedure.

Because the phantom shape was intended to be available in digital format, it was decided to construct the whole body phantom from the start as a CAD file, rather than use sculpting and digitisation.

The basic male body shape was developed in a program called Poser, from Curious Labs. This is a powerful professional package designed to allow the construction of anatomically-realistic CGI figures for film and TV. Poser allows any dimension of the body to be adjusted (eg length from elbow to wrist, waist or chest diameter etc) and also any pose to be struck. A muscular male body type was selected because the pronounced areas of convexity and concavity would be expected to give a good range of measured SARs as a result of their effect on antenna-surface distance. A more stylised, smoother, body would be expected to give SAR results similar to a flat tank phantom.

The following dimensions of the Poser body were adjusted one-by-one to match the whole-body dataset that corresponds to the SAM head:

• Height
• Thigh length
• Lower leg length
• Back length
• Upper arm length
• Lower arm length
• Chest circumference
• Waist circumference
• Thigh circumference
• Neck circumference

The result is a whole body SAM dataset. The SAM head is based on the 95 percentile (largest) head dimensions of the dataset, as is the body, which makes it tall: approximately 1.85 m. The Poser program allowed the basic body shape to be adapted from standing to seated so that both posture variants could be developed.

This software manipulation produced surface files for the body; the phantom shell itself has to have a defined thickness and to be capable of being manufactured. This requires separate inner and outer CAD files with a scaling factor applied to give the required wall thickness. It was decided that a 3 mm wall thickness would be appropriate; the SAM head has a 2 mm wall thickness, but our experience of manufacture indicated that this would not be sufficiently strong for the body when filled with tissue-equivalent material, particularly if it were to be used with lower-frequency tissue-equivalent liquids with a high relative density.

The file offset was carried out under contract by a commercial CAD company, and at the same time the original Poser 5 head was replaced on both inner and outer body files with the SAM head, using a manufacturable version of the public-domain head CAD file that MCL developed some years ago. The inner and outer head files are slightly different in shape around the ear as well as having a 2 mm offset. This replacement required the neck region to transition from the 3 mm body wall thickness to the standard 2 mm SAM head wall thickness. Finally the whole-body files were split into separate but contiguous files corresponding to the inners and outers of the head, torso and neck, pelvic region and the arms and legs.

The CAD files were used to make resin mould negatives by CNC machining, and the mould negatives use as formers for metal foam mould positives. MCL's manufacturing facility routinely makes SAM head phantoms, and the same specialist hand-lay-up and finishing techniques were employed to make several types of physical whole-body phantoms. One of these is designed to be instrumented for making SAR measurements in situ using a purpose-made internal measurement system, one is designed for making measurements on motorcycles one is designed to be non-instrumented. In addition a torso phantom has been developed with the same dataset; this was designed to be used with existing laboratory SAR measurement systems such as SPEAG's DASY4.

Each of the whole-body phantoms is sectional, with removable arms and legs and a split at waist level. Each section separately can be filled with liquid through a port, and sealed. The motorcycle phantom is not sectional: it does not have any arms and the torso and pelvic region are contiguous.

The arms have a small bellows arrangement built into the filling cap to allow for temperature-driven change in fluid volume; the larger sections of the whole-body would require a very large bellows indeed and for these a special valve is fitted instead. The valve can be connected to a medical blood bag, allowing the phantom fluid to vent safely if the pressure inside the phantom builds up. In practice this is an issue only for storage, and in use the bags are not connected.

The phantoms have been used by MCL for a range of investigations including SAR measurement inside vehicles, and SAR from body-mounted devices using the torso variant.

The pictures below show the standing phantom as both outer CAD files and in physical form, a DASY4 SAR scan in the torso phantom and two seated-variant whole-body phantoms being used for the measurement of SAR inside a car.

The MCL-T whole body phantom datasheet gives more information on the dimensions of the whole body SAM phantom.

MCL-T is now making solid SAM whole bodies from carbon-loaded silicone, with electrical properties matching tissue over a wide frequency range.