Tissue-equivalent solid phantom

Phantoms on a different wavelength

5 August 2001 9:54 EST

by Sandra Katzman, BioMedNet News

published online at http://news.bmn.com

TOKYO--Phantoms that mimic the scattering of light or sound waves within hands, skin, brain tissue, muscle and other human tissues have been used in medicine for a decade, to measure the performance of medical devices such as those based on lasers or ultrasound. Yesterday Hiroki Kawai of Chiba University (who his lab director Koichi Ito called "the brains behind the phantom") announced a unique attempt at using tissue-equivalent solid phantoms to estimate the diffusion of a different kind of wave--microwave radiation--within human bodies. He was speaking at the 2001 Asia-Pacific Radio Science Conference here.

There are important reasons to measure the impact of radiofrequency radiation on the human body. For instance, police officers and firefighters sometimes wear personal communication devices with antennae attached to the abdomen. There has also been considerable interest in the use of liquid phantoms to measure the effects of mobile phones on the human body.

Wanting to assess the surface specific absorption rate (SAR) of microwaves by human bodies, the Chiba researchers with colleagues at Matsushita Communications Industrial Company of Yokohama and Matsushita Electric Industrial Company of Osaka created a cylindroid body-like solid phantom based on magnetic radio imaging (MRI). They have also created a solid head phantom.

The new tissue-equivalent solid phantoms could improve the evaluation of the diffusion of electromagnetic waves in the human body over the liquid phantoms that are already in use, Kawai asserts. The surfaces of liquid phantoms can't be observed, because they must be held inside containers. But the solid phantom doesn't need a container, so it is possible to observe the surface distribution, Chiba University team member Kazuyuki Saito told BioMedNet News.

The solid phantom, composed mostly of glycerin, approximates the relative permitivity and conductivity of human tissue. In the full-body phantom, distribution patterns were within 10% and 13% agreement between measured results and results calculated by the dominant method for radio-frequency dosimetry, the finite-difference time-domain (FDTD) technique. The Chiba group reports evaluating the SAR in the human head model at 900 MHz, and at 150 MHz in a tissue-equivalent solid phantom for the VHF band.

The phantom of the brain is mostly made of water, and doesn't last as long as the glycerin-based body phantom, which can last for up to six months at room temperature. But it has advantages: For example, it can be split vertically and opened to measure distribution inside.

After working for ten years on solid phantoms, Ito says dynamic phantoms are the next step. Static phantoms can measure only heat and conductivity, he says, but not blood flow. He hopes to add thin water-filled tubes to the phantom to simulate blood circulation.

The new phantoms met some criticism at the meeting today. The team's claims that measurement by infrared cameras on a head phantom agreed with known calculations of SAR brought a quick but friendly challenge from James Lin of the University of Illinois at Chicago department of bioengineering. "If you call it temperature rise, that's fine, but if you call it SAR distribution, it is no longer the same," he said.

Lin, who presented work with Taiwan colleagues who use phantom modelling to experiment with antennas for hyperthermia treatment of brain tumors, says the Chiba team waits too long to before taking a measurement by infrared camera. "The heating time is too long--180 seconds," he told BioMedNet News. "Fifteen seconds [is the time lapse needed]. Otherwise, the measurement will be of heat diffusion." Calculations indeed show that the measurements present a small heat diffusion, the Chiba researchers concede.

C. K. Chou of Motorola Florida Research Laboratories calls the solid phantom a "good tool", but he says he sees a limited ability to measure SAR in the body. "How do you know you are measuring the peak?" he asked.

"It's an opinion," Ito responded.

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