Ridged waveguide antenna submerged in dielectric liquid

A system for remote microwave interrogation and imaging of biological tars comprises at least one microwave, double ridged waveguide antenna probe which operates at S-band frequencies, and a high dielectric liquid medium, preferably water, in which both the probe and the target are completely immersed. For imaging applications, the probe is positioned with respect to the target such that the target is in the near field of the antenna.

FIELD OF THE INVENTION 
The present invention relates generally to remote microwave interrogation 
and imaging of biological targets, and more particularly to interrogating 
and imaging systems incorporating non-contacting microwave antenna probes. 
DESCRIPTION OF THE PRIOR ART AND PRIOR ART STATEMENT 
Many forms of radiation have been utilized for remote interrogation and 
imaging of biological targets. Active imaging systems have utilized 
various forms of radiation, such as x-rays, radioneucliotide, heavy 
particle, and neutron radiation, as well as ultrasonic acoustic radiation 
produced by mechanical disturbances of an elastic medium. Passive imaging 
systems, which depend upon the Planck distribution of emitted radiation, 
have utilized infrared and microwave radiation for thermographic 
measurements. However, active imaging systems for biosystems have 
heretofore not been developed which utilize electromagnetic radiation in 
the microwave region having wavelengths greater than 3 mm. A number of 
factors have frustrated the development of such systems. First, at shorter 
wavelengths in the microwave region, where spatial resolution of an 
imaging system would be best, the attenuation of energy as it passes 
through targets having water dominated dielectrics is so great that the 
detection of transmitted energy is not practical and detection of 
reflected energy becomes increasingly more difficult the deeper is the 
location of the reflecting boundary within the target. 
Second, although the attenuation problem can be overcome quite simply by 
operating at a lower frequency, the resulting increase in wavelength of 
the radiation requires that the physical aperture of the probe must be 
increased in order to efficiently radiate or receive the interrogating 
energy. In the case of microwave imaging systems, the use of a large 
aperture results in degradation of spatial resolution to the point where 
the system is useless. 
Third, multipath propagation is a serious problem in microwave 
interrogation systems if measurements of transmission loss and phase shift 
through a lossy dielectric in a non-anechoic environment are attempted. 
Heretofore, useable data could not be obtained by non-invasive microwave 
interrogation techniques because of the multipath problem unless the 
probes were in contact with the target being interrogated. However, such 
probes suffer from a number of disadvantages. A major disadvantage is that 
no air gap between the probe and the target can be permitted anywhere over 
the surface of the probe, which limits the probes utility with respect to 
irregularly contoured targets. Another disadvantage is that contacting 
probes tend to deform the target and it is difficult to obtain uniform 
surface contact. A further disadvantage is that contacting probes are 
limited to manual scanning over complex surfaces. A still further 
disadvantage is that the multipath internal to the target under study is 
not eliminated, and the bandwidth of such probes is not wide enough to 
allow use of pulsed RF or Microwave Time Delay Spectroscopy techniques. 
Fourth, there is a problem with the dielectric discontinuity encountered at 
the interface of the target surface and the environment in which it is 
situated. In the case of a human subject situated in free space, the 
difference between the dielectric constants of air and skin is such that 
there will be a large reflection of the incident energy at this interface. 
In the probe of the present invention, the wavelength of the interrogating 
radiation is contracted, and the physical aperture is reduced, by 
completely immersing a transmitting microwave antenna, receiving antenna, 
and the target into a liquid medium having a high dielectric constant, 
such as water. It is known in the prior art to load the interior of 
electromagnetic antennas with a high dielectric material, either solid or 
liquid, in order to contract the wavelength thereof. A representative 
example of such antennas is disclosed in U.S. Pat. No. 974,762 
(Fessenden), wherein parabolic transmitting and receiving antennas for a 
spark gap transmitter are filled with a liquid, such as water, having a 
high dielectric constant. A further example of such antennas is the 
contact probe for microwave interrogation of biological targets described 
in an article by Barrett and Myers, entitled "Subcutaneous temperatures: a 
method of noninvasive sensing," published in volume 190 of Science, pp. 
669-671 (1975), wherein a rectangular waveguide antenna is interiorly 
filled with a plastic material containing titanium dioxide. It is to be 
noted that interior loading of an antenna creates an impedance mismatch at 
the interface with the surrounding space and results in a reduced 
bandwidth which becomes so severe at the S-band microwave frequencies 
which are used in microwave imaging systems as to prevent the use of such 
antennas as non-contacting probes. 
Although acoustic imaging systems and sonars do not utilize electromagnetic 
radiation, operate at relatively low frequencies, and involve different 
principles of operation, it is noted that the ultrasonic transducer 
elements used in these acoustic imaging system are completely immersed in 
a water medium. An example of an acoustic imaging system is disclosed in 
U.S. Pat. No. 3,269,173 (von Ardenne). 
There have also been several efforts in the field of communication, 
primarily underwater, which have utilized electromagnetic antennas 
completely submerged in a high dielectric medium. Generally speaking, such 
activity has involved low frequency radiation, much below the microwave 
frequencies used in microwave imaging systems, because of the attenuation 
problem noted above. Further, such antennas have typically been of the 
long wire or loop type and are not suitable for imaging applications. 
There also have been preliminary investigations of the suitability of 
dipole radiation at a frequency of 14 MHz in a water medium for 
communication purposes, using electrically insulated dipole antennas 
submerged in a lake. These activities are described in a paper by Shen, et 
al., "Measured field of a directional antenna submerged in a lake," IEEE 
Trans. Antennas and Propagation, Vol. AP-24, pp. 891-894 (November 1976). 
Finally, very short monopole antennas have been used as invasive probes in 
biosystem interrogation systems, wherein the probe is inserted into the 
target and radio frequency measurements are obtained in the immediate 
vicinity of the probe. Since the probe is physically inserted into the 
target, useful image scans are not possible. Further, a particular probe 
is limited to interrogation of specific tissues since the probe impedance 
is affected by the tissue into which the probe is inserted. In addition, 
such probes are impractical for tissues having low dielectric constants, 
such as bone. 
The prior art cited hereinabove includes, in the opinion of the applicants, 
the closest prior art of which they are aware. However, there is no 
representation that no better art exists. 
SUMMARY OF THE INVENTION 
These and other disadvantages of the prior art are overcome by a system 
constructed in accordance with the present invention for remote microwave 
interrogation and imaging of a biological target, which comprises at least 
one microwave antenna probe capable of operating at S-band frequencies, 
and a liquid medium having a dielectric constant greater than that of air 
in which both the probe and the target are completely immersed. 
Preferably, the liquid medium has a dielectric constant in the range of 
approximately 40 to 80, and advantageously may be water. In accordance 
with a further aspect of the invention, the probe is positioned with 
respect to the target such that the target is in the near field of the 
probe. 
In accordance with another aspect of the invention, a rectangular waveguide 
is preferred over other configurations of the probe, and a double or quad 
ridged waveguide is the preferred form of rectangular waveguide, because 
of bandwidth considerations. 
With a system constructed according to the present invention, significant 
improvements in the spatial resolution of near field imaging of the target 
are achieved. Further, there is no need to provide an anechoic chamber in 
which to perform interrogation or imaging of the target, since a 
reasonably small volume of the high dielectric medium provides sufficient 
attenuation of multipath radiation so as to constitute an inherently 
anechoic environment. Finally, when a water dielectric medium is utilized, 
there is improvement of energy coupling into the biological target, since 
the dielectric constants of external tissues such as skin are much more 
closely matched to water than they are to air. Also, in the case of human 
targets, water hydrates the corified epithelium and improves the impedance 
match at the 100-200 microns of the skin which would otherwise be very low 
in water content. 
Other features and advantages of the invention will be set forth in, or 
apparent from, the detailed description of a preferred embodiment found 
hereinbelow.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A preferred non-contacting probe constructed in accordance with the present 
invention is depicted in FIGS. 1-2. The probe, generally denoted 80, 
comprises a microwave antenna in the form of double ridged waveguide 82, 
which is approximately 6.7 mm in length and 7.7 mm in width. The length of 
waveguide 82 represents a compromise between internal loss and ease of 
impedance matching, since a shorter length, on the order of 3 mm, would be 
preferable from the standpoint of power loss, but would not permit the use 
of tuning screws for impedance matching. It is to be noted that any 
configuration of microwave antenna may be utilized, with a rectangular 
waveguide to be preferred over other forms, and a double or quad ridged 
waveguide to be preferred over a rectangular waveguide. 
The top wall of waveguide 82 is provided with an aperture 83, through which 
the feed probe 84 of a standard 50 ohm impedance coaxial input cable 86 
having a fluorocarbon dielectric such as "Teflon" is inserted by means of 
a standard female connector 85 mounted on the top surface of waveguide 82 
and a standard male connector 87 mounted on the end of cable 86. 
As shown, the top ridge 90 of waveguide 82 extends longitudinally along the 
upper interior surface only from the front end of waveguide 82 to the 
perimeter of aperture 83, while the bottom ridge 91 extends longitudinally 
along the entire lower interior surface from the front end to the rear end 
of waveguide 82. In addition, as shown, the front ends of both top ridge 
90 and bottom ridge 91 are bevelled. Two holes 94 and 95, size 2-56, are 
provided in bottom ridge 91 of waveguide 82 for receiving tuning screws 96 
and 97, respectively, which are used to obtain a broader impedance match. 
Hole 94 is substantially coaxial with aperture 83. It is noted that the 
rear screw 96 does not protrude into waveguide cavity 88, while front 
screw 97 does protrude into cavity 88. 
Feed probe 84 is inserted into cavity 88 and is shorted to tuning screw 96, 
and thus to the bottom ridge 91 of waveguide 82 in order to control the 
VSWR. Preferably, probe 84 is oriented substantially perpendicularly with 
respect to bottom ridge 91. The diameter of probe 84 is also reduced to 
approximately 0.5 mm to provide a better match to the high impedance 
ridges of waveguide 82. Probe 80 further comprises a shorting plate 92 
mounted at the rear of waveguide 82 and positioned with respect to feed 
probe 84 so as to obtain the smoothest impedance match over the operating 
bandwidth of the probe. Shorting plate 92 is provided with a 2.2 mm 
diameter hole 93 to facilitate removal of air bubbles trapped in waveguide 
82 when antenna 80 is immersed in the dielectric medium, and to permit 
alignment of probe 80 with respect to the target. The dimension of hole 93 
is determined by the bandwidth of the radiation to be transmitted, being 
sized so as to be below the cutoff frequency for the bandwidth of the 
radiation. 
The dielectric of feed probe 84 is preferrably inserted into aperture 82 
such that the dielectric is approximately even with the upper interior 
surface of waveguide 82. Final impedance matching is obtained by 
simultaneous adjustment of tuning screws 96 and 97 and penetration of the 
dielectric into aperture 83. 
Probe 80 is advantageously enclosed in a conventional double ridged 
waveguide flange 99, which provides mechanical stability and a means for 
mounting extensions onto probe 80. Preferably, flange 99 is machined with 
notches 100 and 101 to permit connection of feed cable 86 onto connector 
85, and access to tuning screws 96 and 97. 
In use, probe 80 and the associated flange 99 are advantageously mounted at 
the end of a hollow tube 102 which supports probe 80 and provides a 
conduit which protects cable 86. Cable 86 is terminated at the distal end 
of tube 102 in a conventional type N connector 104. To reduce the effect 
of reflection off tube 102, probe 80 is supported 5 cm in front of tube 
102 by means of a metal standoff 106 and connector 107. 
Probe 80 is designed to be operated totally immersed in a dielectric medium 
and have an operating bandwidth of 2000 MHz to 4000 MHz. The dimensions 
which have been cited hereinabove assume that the dielectric medium is 
water, which is preferably distilled, and at a temperature of 32.degree. 
C. If a medium with a different dielectric constant is to be used, then 
the dimensions would need to be altered accordingly. In general, if the 
medium has a dielectric constant lower than that of water, larger 
dimensions would be required, and conversely, if the medium has a 
dielectric constant higher than that of water, smaller dimensions would be 
required. 
The dielectric medium which is employed may be any high dielectric medium 
which is physiologically and electrically acceptable. For biological 
targets, media such as deuterium oxide, ethylene glycol, and methanol may 
advantageously be used in addition to water. In the case of biological 
targets having tissues with dielectric constants in the range of 5 to 80 
at S-band frequencies, such as human subjects, a medium having a 
dielectric constant in the range of approximately 40 to 80 is preferable 
as providing the best impedance match to both the antenna and the target. 
Water is preferred because of its acceptable loss characteristics, 
inertness, and match to tissue, in addition to its high dielectric 
constant. 
The volume of dielectric medium in which probe 80 and the target to be 
interrogated are immersed should be sufficient to constitute an anechoic 
environment for interrogation of the target. Satisfactory results have 
been achieved with a water medium contained in a tank which is 45.7 cm on 
a side and filled to within 6.4 cm of the top, and with probe 80 immersed 
17.8 cm below the surface of the water. 
The use of probe 80 as the antennas of a microwave time delay spectroscopic 
remote interrogation system is described in the copending application 
referred to hereinabove, which is hereby incorporated by reference. For 
microwave imaging, it is important that the target be positioned within 
the near field of the probe(s) which are utilized. 
Tests conducted on the preferred embodiment of probe 80 described 
hereinabove have indicated that the maximum effective range of a probe 80 
with a water dielectric is between approximately 15 to 31 cm. These tests 
further demonstrate that probe 80 has good impedance characteristics (VSWR 
less that 2.3) and reasonable losses (combined loss for two antennas less 
than 6 dB over almost 80% of an octave bandwidth from 2000 to 4000 MHz, 
and less than 14 dB total loss for both antennas at the highest 
frequency). The tests also demonstrate that probe 80 may be used to create 
line scan images of dielectric targets. Objects with a diameter of 1.8 mm 
and spacing of 10 mm are easily detected by interrogating radiation having 
a wavelength of 75 mm. The resolution of the probes 80 (in terms of 
separating two closely spaced objects) is between 5 and 10 mm and appears 
to be limited by the width of the broad dimension of the opening of 
waveguide 82. In addition, the tests demonstrate that all four parts of 
the scattering parameter data, i.e., the magnitude and phase of the 
reflection and transmission coefficients, must be considered in 
formulating an image. 
Although the invention has been described with respect to an exemplary 
embodiment thereof, it will be understood that variations and 
modifications can be effected in the embodiment without departing from the 
scope or spirit of the invention.