# Directive Antenna for Ultrawideband Medical   Imaging Systems

Authors: Amin M. Abbosh
Journal: International Journal of Antennas and Propagation
 (2008-02-17)

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## Abstract

A compact and directive ultrawideband antenna is presented in this paper. The antenna is in the form of an antipodal tapered slot with resistive layers to improve its directivity and to reduce its backward radiation. The antenna operates over the frequency band from 3.1 GHz to more than 10.6 GHz. It features a directive radiation with a peak gain which is between 4 dBi and 11 dBi in the specified band. The time domain performance of the antenna shows negligible distortion. This makes it suitable for the imaging systems which require a very short pulse for transmission/reception. The effect of the multilayer human body on the performance of the antenna is also studied. The breast model is used for this purpose. It is shown that the antenna has more than 90% fidelity factor when it works in free space, whereas the fidelity factor decreases as the signal propagates inside the human body. However, even inside the human body, the fidelity factor is still larger than 70% revealing the possibility of using the proposed antenna in biomedical imaging systems.


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## Body

## 1. Introduction

Ultrawideband (UWB) (3.1–10.6 GHz)
microwave imaging is a promising method for biomedical applications
such as cancer detection because of their good penetration and
resolution characteristics. The underlying principle of UWB cancer detection is a significant contrast in dielectric properties, which is estimated to be greater than2:1 between normal and cancerous tissue. UWB imaging
systems have shown encouraging results in the detection of tumors for early
breast-cancer detection [1].In the UWB imaging systems, a very narrow pulse is transmitted from a UWB
antenna to penetrate the body. As the pulse propagates through the various
tissues, reflections and scattering occur at the interfaces. A particular
interest is in the scattered signal from a small size-tissue representing a
tumor. The reflected and scattered signals can be received using an UWB antenna, or array of antennas, and
used to map different layers of the body.
For an accurate imaging system with high resolution and dynamic range,
the transmitting/receiving UWB antenna should be planar, compact in size, and
directive with high-radiation efficiency and distortionless pulse
transmission/reception.The majority of the compact UWB antennas presented in the literature
exhibit omnidirectional radiation patterns with relatively low gain and an
impulse response with observable distortion [2]. These types of UWB antennas
are suitable for the short-range indoor and outdoor communication. However, for
radar systems, such as an UWB microwave imaging system for detection of tumor
in woman’s breast, a moderate gain directional antenna is advantageous. In
addition to an UWB impedance bandwidth, as defined by the minimum return loss
of the 10 dB, the UWB antenna is required to support a very short pulse
transmission with negligible distortion. This is necessary to achieve precision
imaging without ghost targets. The unipolar and antipodal Vivaldi antennas
presented in the literature [3–5] satisfy the requirements for imaging systems
in terms of bandwidth, gain, and impulse response. However, the achieved
performance is at the expense of a significant size, which has a length of
several wavelengths. Therefore, the challenge is to reduce their physical
dimensions such that it can be incorporated in a compact microwave imaging
detection system, while maintaining its broadband, high-gain, and distortionless
performance.Several UWB antenna designs with compact size and low distortion have
been proposed for the use in the medical imaging systems [6–8]. Each has its
own merits and drawbacks. Some of the proposed antennas have a nonplanar
structure, whereas others have low-gain and/or low-radiation efficiency. The
low-radiation efficiency is a major impairment that limits the dynamic range of
the imaging system, whose major objective is to detect a weak backscatter from
a tumor.In the presented work, a compact(5cm×5cm) elliptical tapered slot UWB
antenna is described. A clear design guideline is given in order to show how to
calculate values of the different design parameters of the antenna. Resistive
layers were incorporated with the radiating elements of the antenna to improve
its directivity and reduce any backward radiation which may affect the accuracy
of the imaging system. The measured and simulated results of the proposed
antenna show an ultrawideband behavior with a moderate gain and distortionless
pulse transmission/reception.


## 2. Design

The antenna presented in
this paper is to be used in a microwave imaging for breast-cancer detection.
The imaging system includes a circular array of the proposed ultrawideband antenna.
In this system, one of the antennas is used to transmit a microwave signal
while the rest of the antennas in the array receive the scattered signal. The
measured data is collected and then the measurement procedure is repeated with
the second transmitting the signal while the remaining are used for receiving
the scattered signal. This process is repeated until all antennas in the array
perform the transmitting role. The antenna array can be moved up and down
automatically via a computer-controlled high-precision linear actuator. This
facilitates the collection of multiple planar data for 3D object imaging.The proposed ultrawideband antenna for inclusions in the UWB microwave
imaging system is shown in Figure1. It resembles an antipodal tapered slot
antenna fed by a parallel strip line.Figure 1
Configuration of the proposed antenna.The radiating element is in the form of an antipodal planar tapered slot
with an elliptical curvature. Rogers RO4003 with 3.38 dielectric constant and
0.508 mm thickness was used as a substrate. A resistive layer of50Ω/□  was sprayed at
the designated areas at the lower end of the radiating structure in the top and
bottom layers to improve the front-to-back ratio, and thus the detection
capabilities of the UWB imaging system.The design objective is to obtain a directive antenna with a compact size,
while maintaining the bandwidth requirement of 3.1 to 10.6 GHz. The following
design procedure is proposed and utilized in developing the proposed antipodal antenna.Step 1.
Given the lowest frequency of operation (fl), thickness of the substrate (h) and its dielectric constant (εr), the width (w) and length (l) of the antenna structure, excluding the
feeder, can be calculated using the following equation [9]:
(1)w=l=cfl2εr+1,
where c is the
speed of light in free space.
It is worthwhile to mention that (1) indicates
that the antenna’s length and width is chosen to be equal to the effective
wavelength calculated at the lowest frequency of operation.Step 2.
The radiating structure of the antenna is
formed from the intersection of quarters of two ellipses. The major radii (r1 and r2) and the secondary radii (rs1 and rs2) of the two ellipses are chosen according to
the following equation:
(2)r1=w2+wm2,r2=w2−wm2,rs1=l,rs2=0.5r2.
According to (2), dimensions
of the radiating element are chosen such that the far-end distance between the
top and bottom radiators is equal to the effective wavelength at the lowest
frequency of operation. Length of each
of the radiators at the left and the right end of the antenna’s structure shown
in Figure 1 is equal to half of the effective wavelength calculated at the
lowest frequency of operation.Step 3.
The width of the microstrip
transmission feeder (wm) to give the characteristic impedance, Zo equal to  50Ω, can be calculated using the following
equations [10]:
(3)wm=120πεrhZo.Step 4.
A metallization layer, with a50Ω/□  surface
resistivity, is added to the top and
bottom radiating parts. Shape of the resistive layers is chosen to be a quarter
of an ellipse with major and secondary diameters equal to r2 and rs2, respectively.Step 5.
A transition
is added to the structure of the antenna. This is required because the antenna’s
radiating element shown in Figure1 is connected to a parallel strip line,
which is a balanced transmission line, whereas the antenna is to be connected
to the other devices of the imaging system using a suitable coaxial cable,
which is an unbalanced transmission line. The transition from the parallel
strip line to the microstrip line is shown in Figure 2, which is adopted from the transitions presented in [11]. The strip line, which is located at the top layer, is
connected using a tapered transmission line to the microstrip line, while width
of the strip line at the bottom layer is gradually increased to form the ground
plane required for the microstrip feeder.Figure 2
Configuration of the parallel strip line to microstrip transition.


## 3. Results

The ultrawideband antenna designed according to the above mentioned
procedure was manufactured using Rogers RO4003C(εr=3.38,h=0.506mm) as a substrate. Values of the design parameters w,l,r1,r2,rs1,rs2,  and wm (shown in Figure 1) are 50 mm, 50 mm, 26 mm, 24 mm, 50 mm, 12 mm, and 2 mm, respectively. A photo for the developed antenna
is shown in Figure 3.Figure 3
Photo of the developed antenna: (a) top view, and (b) bottom view.Concerning the resistive layers, a parametric analysis using the software
Ansoft HFSSv10 indicated that the best performance concerning the bandwidth and
the front-to-back ratio can be achieved when the resistivity of the added
resistive layer is in the range from 50 to100Ω/□  . The lower value
was used because of the availability of the 50Ω/□  chemical mixture
to the author.The validity of the proposed design methodology is verified using the
commercial software package, Ansoft HFSSv10, and experimental tests by using a
vector network analyzer.Figure4 shows the simulated and measured return loss of the manufactured
antenna. As can be seen from Figure 3, the 10 dB return loss bandwidth extends
from 3.1 GHz to more than 11 GHz covering the required UWB band of 3.1 GHz–10.6 GHz. The
simulated result closely resembles the measured result validating the design
procedure of the antenna.Figure 4
The measured and simulated return loss.The far-field radiation patterns of the antenna were calculated using the
software HFSS. They are shown in Figure5 for the frequencies 4 GHz, 6 GHz, 9 GHz, and 11 GHz. The antenna shows directive properties with an average front-to-back
ratio which is greater than 13 dB across the whole band, making it a good
candidate for microwave imaging applications. It is worthwhile to mention that
without the use of the resistive layers, the front-to-back ratio is around 10 dB.The simulated three-dimensional
radiation pattern.
(a)
4 GHz(b)
6 GHz(c)
9 GHz(d)
11 GHzThe measured gain of the antenna is shown in Figure6, which reveals a
moderate gain antenna. The gain is equal to 4.3 dBi at 3 GHz and it increases
with frequency till it becomes 10.8 dBi at 10.6 GHz. It is to be noted that the gain measurements
were done in comparison with a reference-gain antenna which is the corrugated
horn antenna in this case.Figure 6
The measured gain and
calculated radiation efficiency of the antenna.As the use of the resistive layer can be responsible for the reduction in
the radiation efficiency, suitable calculations with the help of the software
HFSS were performed with respect to this parameter. From Figure6, it is
apparent that despite the use of the resistive layers to minimize the backward
radiation (and hence enhance the front-to-back ratio), the proposed antenna has
a good efficiency, which is more than 80% across the whole band. This
performance is superior in comparison with the antennas reported for use in a microwave
imaging system,  where 47% efficiency was noted [8].The time-domain performance of the proposed antenna was also measured. A
narrow pulse was synthesized in the network analyzer using the discrete Fourier
transform module of the device. The pulse was synthesized after assuming that
its frequency spectrum is a rectangular function that extends from 3.1 to 10.6 GHz. Shape of the resulted synthesized pulse is shown in Figure7. Two copolarized
antennas were separated by a distance of 50 cm and the results of the measurement
are shown in Figure 7. Note that the excited pulse and the received pulse are
normalized with respect to their peak values. The figure reveals that the pulse
duration of the antenna is 0.6 nanoseconds. The pulse distortion occurs at the
0.15 level with respect to the peak level of 1, and thus it is almost
negligible. The observed results indicate that the developed antenna supports distortionless
narrow pulse which makes it an excellent radiator for the purpose of a microwave
imaging with high resolution.Figure 7
The measured impulse response
of the antenna.As
the antenna is to be put on or near the human body, specifically the breast for
the case of breast-cancer detection, a study of the effect of the distance from
the skin to the antenna on its return loss is investigated. The electromagnetic
model used to simulate the breast contains two layers: the first layer is the
skin layer with thickness  =  2 mm, dielectric constant = 36, and conductivity = 4 S/m.
The second layer is the breast tissue, which extends to a width of 10 cm, with
a dielectric constant = 9 and conductivity  =  0.4 S/m [12]. Results of simulation using the software HFSS
are shown in Figure 8 for two different distances between the antenna and the
human body. Figure 8 indicates clearly that the antenna maintains its ultrawideband
performance in spite of being very close to the human body.Figure 8
The simulated return loss of
the antenna for different distances from the human body.The
imaging system in which the antenna is to be used contains an array of
antennas. Hence, it is important to investigate the value of the mutual
coupling between these antennas. The mutual coupling between two identical
antennas at different frequencies was calculated using the software HFSS. In
the calculations, two antennas were assumed to be parallel to each other and
the distance between them was changed. The mutual coupling was calculated at
each distance and the results are shown in Figure9. These results show that
the coupling decreases as the distance between the two antennas increases. For
a certain distance between the two antennas, the mutual coupling is less for a
higher frequency. This is because increasing the frequency means a lower
wavelength. Therefore, the distance between the coupled antennas relative to
the wavelength is larger. The results depicted in Figure 9 reveal that the
mutual coupling between the neighboring antennas at any frequency within the
ultrawideband range is less than −20 dB when the distance between the antennas
is more than half a wavelength.Figure 9
Variation of the simulated
mutual coupling with the distance between the antennas at different
frequencies.It is also important to study the distortion when the radiated pulse
propagates through the human body, that is, the skin and the breast tissue in
the case of the breast-cancer-detection system. The antenna fidelity is used as
an indication of that distortion. The fidelity factor is the maximum magnitude
of the cross correlation between the observed pulse at a certain distance and
the excitation pulse [13]. The finite difference time-domain method was used
for this purpose [14]. In
order to reduce the computation domain, Berenger’s perfectly matched layer
(PML) is applied as an absorbing boundary condition [15]. To include the
frequency dependence of the dielectric constant εi and the
conductivity σi of the breast tissue over the UWB, the first-order
Debye dispersion model was applied [12]:
(4)εi−jσi2πfεo=ε∞+εΔ−ε∞1+j2πfτ−jσΔ2πfεo,
where τ is the relaxation time, and ε▵,ε∞, and σ▵ are the Debye model parameters which were
selected according to the published data for the breast tissues [12]: normal tissue:
ε▵=10,ε∞=7,τ=7ps,σ▵=0.15S/m, tumor: ε▵=54,ε∞=4,τ=7ps,σ▵=0.4S/m. For the skin: ε=36, and σ=4S/m.The result is shown in Figure10 where the
effect of all the scattered/reflected signals is included. It indicates that as
the signal propagates through the human body, the fidelity factor decreases.
This indicates an increasing pulse distortion inside the human body. For the
antenna presented in this paper, the fidelity factor is within reasonable
values (more than 70%) even inside the human body.Figure 10
The
simulated fidelity factor with the distance from the antenna in the presence of
a human body.


## 4. Conclusion

The design of a directive ultrawideband antenna for use in a microwave
imaging system has been presented. To
minimize the backward radiation, the antenna uses resistive layers behind its
conductive radiation layers. The simulated and measured characteristics of the
antenna have shown that it covers the band from 3.1 GHz to more than 11 GHz. It
has a radiation efficiency of more than 80%, which is higher than the recently
reported other UWB planar antennas employing resistive layers for microwave
imaging applications. The characteristics of the antenna when operating near a
human body have been investigated. The simulated results have shown that the
antenna maintains its ultrawideband performance concerning the return loss even
with the presence of the human body in proximity with the antenna.The time-domain performance of the antenna has also been studied. It has been
shown that the proposed antenna has the ability to send and receive very short
pulses in a distortionless manner. It has been shown that although the fidelity
factor decreases as the signal propagates through the human body, the value of
that factor is still within acceptable limits.The mutual coupling between two identical antennas has been simulated as
the antenna elements are used within an array in the microwave imaging systems.
It has been shown that the mutual coupling is less than −20 dB when the distance
between the neighboring antennas is more than half a wavelength.



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*Source: 854012-2008-03-23.xml*