21-06-2013, 12:37 PM
PHASED-ARRAY NEAR FIELD RADIOMETRY FOR BRAIN INTRACRANIAL APPLICATIONS
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Abstract
During the past decades there has been a tremendous
increase throughout the scienti¯c community for developing methods of
understanding human brain functionality, as diagnosis and treatment
of diseases and malfunctions, could be e®ectively developed through
understanding of how the brain works. In parallel, research e®ort
is driven on minimizing drawbacks of existing imaging techniques
including potential risks from radiation and invasive attributes of
the imaging methodologies. Towards that direction a new near ¯eld
radiometry imaging system has been theoretically studied, developed
and experimentally tested and all of the aforementioned research
phases are herein presented. The system operation principle is based
on the fact that human tissues emit chaotic thermal type radiation at
temperatures above the absolute zero. Using a phase shifted antenna
array system, spatial resolution, detection depth and sensitivity are
increased. Combining previous research results.
INTRODUCTION
The present paper discusses the potential of a newly constructed
prototype microwave radiometric system for use in intracranial
diagnostic applications. Radiometry is based on the measurement of
thermal electromagnetic, noise-type, radiation emanating from lossy
media, at temperatures above the absolute zero.
MATERIALS AND METHODS
Phased Array Radiometer
As already mentioned, the thermal type radiation is measured by
placing the antennas around the subject. The received signals, after
being digitally phase-shifted are combined and driven to a sensitive
radiometric receiver.
The radiometer used for the proposed system was designed for
the purposes of the present research and operates at central frequency
fr = 3:2 § 0:45 GHz.
The receiver has four inputs one for each antenna used. Then
there's a two stage low noise ampli¯er and a microwave strip-line
band-pass ¯lter which rejects the out of band spurious signals. The
output is driven to a 6-bit low loss digital phase shifter in order to
beam form the antenna patterns. There are four similar chains which
are combined through a power combiner and the combined signal is
ampli¯ed using another low noise ampli¯er.
Patch Antennas
The appropriate antennas for the designed systems should ful¯ll a
number of requirements small size, low cost as well as being su±ciently
conformal in order to be placed around the head. Several versions of
the designed antennas [16, 17] have been simulated and tested in order
to achieve all requirements. The antennas used for the experiments are
conformal patch antennas with resonance frequency of approximately
f = 3:1 GHz and are connected to the radiometer via coaxial feed.
EXPERIMENTAL PROCEDURES AND RESULTS
The radiometer is sensitive to external interference. To avoid
measuring random electromagnetic waves and to eliminate the
in°uence of external electromagnetic noise from other sources, all
measurements took place into a Faraday chamber. Numerous tests
have been carried out using water based phantoms in the form of
cylindrical containers. The latter are placed in various positions
relevant to each separate antenna element position, based on its phase.
Temperature Resolution
The ¯rst experiment is carried out using a small vessel with capacity
200 mL, containing warm water. Its initial temperature is 44±C. The
phantom is placed in front of the antennas and remains still throughout
the measurement. The temperature is measured using the electronic
thermometer. Real time measurements are performed over a period
of 40 sec, for each temperature. This period is calculated and limited
by the fact that the container is not heat-insulated thus, there's rapid
degradation of its temperature, especially at higher values. Separate
sets of measurements are performed in the region 36{44±C with a
step of 2±C. The average measured background noise throughout the
experiment is Vbgrd = 2:6587 § 0:0001 V.
DISCUSSION AND CONCLUSIONS
In the present paper the theoretical electromagnetic analysis, the
design and implementation as well as the experimental results that
verify the use of a novel portable phased array system are presented.
Simulation results have shown that the proposed phased array system
may provide detection of temperature changes in biological tissue for
in-depth regions of up to 5 cm. A novel compact portable radiometric
system with four inputs has been developed and its overall performance
has been tested using phantoms. Extensive measurements have shown
that by selecting appropriate phase shifts it is possible to identify
a single focusing area in numerical and experimental phantoms.
Experiments have shown that the system can identify di®erent hot
spots, as long as they are at least 1 cm apart, thus allowing reliable
surface monitoring of in-depth tissues from surface antenna sensors
via step measurements. The system's temperature resolution is less
than 1±C. One of its advantages is that the focus point can be easily
controlled via the phase shifting mechanism, which could render it
a convenient tool for examining contiguous spots separated by 1 cm
or more.