Ultra-Wideband Imaging Systems for Breast Cancer Detection - Ultra-Wideband and 60 GHz Communications for Biomedical Applications - page 84

All of above limitations have led to efforts to find an alternative to x-ray mam-
mography. One of the most promising alternatives is using microwave imaging with
UWB systems. As mentioned before, for the radio frequency (RF) and the microwave
range of frequencies, differences in electrical characteristics of malignant and nor-
mal tissues are greater than 2:1. Also, dielectric constant, ε r , and conductivity, σ ,
change with increasing water content in neoplastic tissues due to increased protein
hydration [ 11 ]. This fact will result in exploiting strong indicators of malignancy
associated with physical or physiological factors of clinical interest such as water
content, vascularization, blood flow rate, and temperature.
This advantage over x-ray can compensate for spatial resolution of UWB systems
which is not as good as x-ray imaging because of the wavelength difference. In fact,
high contrast exists for both high and low frequencies. At lower frequencies, the
contrast is due to breakdown of narcotic cell membranes which are being used for
electrical impedance topography [ 12 , 13 ]. On the other hand, at higher frequencies,
there will be high contrast due to increase in DNA, protein, and hemoglobin ab-
sorption in malignant tissues [ 14 ]. Considering the above advantages of working in
different frequencies and after studying parameters which are important in an imag-
ing procedure, it has been concluded that the frequency range between 1 and 10 GHz
is the optimum band for this application. This band is a range that includes both
advantages of having high spatial resolution which is better at higher frequencies
and also good penetration depth which is better for lower frequencies.
Today, it is well known that microwave imaging is a promising substitute for
conventional mammography. It is also known what frequency range should be used
for imaging. From this perspective, there have been two main techniques for active
microwave imaging: radar and tomography. Both techniques are based on the dielec-
tric contrast in this range of frequency. In both techniques, low-power microwave
signals are transmitted from an array of antennas into the breast. The tomographic
image reconstruction technique is the most commonly attempted technique in this
frequency range that can be performed using narrowband signals. In this technique,
an image reconstruction problem should be solved, which needs regularization to
converge and have a meaningful solution. However, many of these concerns are
minimized in smaller form-factor cases like that in breast cancer detection. On the
other hand, there is UWB radar in which there is no effort to reconstruct the complete
profile of dielectric properties of the breast. In fact, in this technique, the goal is to
find the presence and location of significant backscattered energy due to difference
in dielectric properties of normal and malignant tissues.
In general, the microwave frequency domain for breast imaging is associated with
the following features that are expected from a UWB system as an imaging system
working in this range of frequency:
1. Significant contrast between dielectric properties of normal and malignant tissues
in this frequency range. In normal tissues of the breast, microwave attenuation is
low enough that we can image the entire breast.
2. Avoiding breast compression, as well as using low-power signals which result in
little health risk.