Living tissue is electromagnetically interesting because the state of the tissue can be discerned from the dissipation and dispersion of externally applied electromagnetic fields.  So here are two words for you to understand:

                        Dissipation:       the absorption of energy as it proceeds through a medium.

Dispersion:       The frequency dependent slowing of energy while it is traveling through a medium.

In this case, “the medium” refers to biological tissue. 

Tissue is composed primarily of water.  The human body is between 50% and 70% water by weight.  Certain areas where the water content is particularly interesting are skeletal muscle and the fatty subcutaneous layer of the skin. 

Information about the water content in the skeletal muscle is a key indicator for dehydration.  Today, body water is determined by bioelectrical impedance analysis (BIA) where the electrical impedance of the body is measured using AC signals in the frequency range of 5 KHz to 500 KHz.  However, redistribution of body fluid and changes in electrolyte content during physical exertion result in inaccurate BIA results[1].  More accurate methods to determine body water content are needed for medical use as well as for use in the physiology lab.  At higher frequencies, where current techniques do not operate, dissipation and dispersion are not affected by such things as electrolyte content, and depends entirely on the amount of free water in the path of the signal. 

People susceptible to heat illness include athletes, the elderly, people with certain medical conditions, and workers such as miners, firefighters and agricultural workers.  Each year 240 people die from heat related illnesses in the United States[2].  

Finding Cancer.  By far, the most common type of cancer is non-melanoma skin cancer with 1.3 million new cases reported in the US every year.  After that, there are 183,000 new cases of breast cancer and 180,000 new cases of prostate cancer every year.  Tumors are detected by performing a biopsy.  First, your doctor needs to have a reason to cut tissue out of you for examination, so there must be symptoms, either visible or physical.  The time it takes to determine the presence of a tumor is critical to successful treatment, and especially to the prevention of metastasis.  Physicians who are on the first line of defense in the war on cancer are welcoming of any way to more rapidly identify tumors.  There is a lot of additional fluid in tumors, especially after angiogenesis begins and blood begins to flow through the cancer.  The elevated fluid levels in tumors provide an opportunity to detect the presence of the tumors by measurement of the dissipation and dispersion of electromagnetic signals.  This goes beyond tumors on the skin.  Some day hopefully active electromagnetic scanning can be used to find tumors throughout the body.  There has already been some success finding breast cancer tumors this way[3].  Studies at the Universities of Wisconsin and Calgary are showing a large contrast between the microwave dispersions of malignant and normal breast tissues.  With considerably more contrast than is seen with X-Rays as well as less physical discomfort, microwave imaging may some day replace today’s mammogram.

The dissipation and dispersion of living tissue are unfortunately affected by things other than water.  However, if there is a region of the spectrum where water content alone determines these properties, then we would be able to perform measurements.  Figure 1 shows the dielectric constant (dispersion) of biological tissue and compares it to pure water[4].  At audio frequencies, the alpha dispersion is dominated by counterion polarization effects.  At HF frequencies, the beta dispersion is dominated by interfacial polarization of the cell walls.  At radio and microwave frequencies above about 100 MHz, the dispersion characteristics of water and tissue are well matched.  This is the so called gamma dispersion where the Debye relaxation of water molecules dominates the dispersion of tissue.  In this range, it is possible to get information about tissue from measuring the tissue’s effect on an electromagnetic signal.  This in fact was demonstrated more than 20 years ago by Stuchly, et al[5].

In my own experiments, I asked whether I could determine the amount of water in a sample using microwave resonance.  I expect resonance to be more sensitive, because I can see very small changes in resonant frequency, and relate those changes to the water content.  Figure 2 shows the test set-up and Figure 3 shows the result.  Clearly, in the biologically important region of 50% by weight water, the microwave resonator is very sensitive to water content.  Future work will explore how to make a device for medical diagnostics based on this simple principle.