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Fundamental Issues in Antenna design for
Microwave Medical Imaging Applications
M. Fernando, K. Busawon, M Elsdon and D. Smith
School of Computing Engineering and Information Sciences,
Northumbria University, NE1 8ST, UK
e-mail: michael.fernando@northumbria.ac.uk
Abstract—
This paper surveys the development of
microwave medical imaging and the fundamental challenges
associated with microwave antennas design for medical
imaging applications. Different microwave antennas used in
medical imaging applications such as monopoles, bow-tie,
vivaldi and pyramidal horn antennas are discussed. The
challenges faced when the latter used in medical imaging
environment are detailed. The paper provides the possible
solutions for the challenges at hand and also provides
insight into the modelling work which will help the
microwave engineering community to understand the
behaviour of the microwave antennas in coupling media.
1. Introduction
Microwave imaging for medical application has been a
subject of research for many years. In the last decade,
however, there has been a renewed interest in the topic
due to its viable and advantageous approach for many
medical applications. Basically, microwave images are
maps of electrical property distributions in the body. The
changes in electrical property indicate the deposition of
heat in the tissues [1]. Cancer detection using microwave
imaging is based on such contrast in electrical properties.
Recently, microwave imaging for breast cancer detection
has gained attention due to advances in imaging
algorithms, microwave hardware and computational
power [2]. Microwave breast cancer detection is based on
differences in electrical properties between healthy and
malignant tissues at microwave frequencies [3, 4]. Breast
cancer is a significant health issue for women and affects
one in every seven women. Early detection and timely
medical intervention is the key to successful treatment,
long-term survival and quality of life for patients [5].
Currently, X-ray mammography is the most
effective detection technique, and women are encouraged
to participate in breast cancer screening programs that
involve regular mammograms [6, 7]. A mammogram is a
map of the densities of the breast, and has proven to be
quite sensitive to the presence of lesions in the breast.
According to the reports published on the X-ray
mammography reviews by U.S. Institute of Medicine
(IOM) [7], the limitations of the mammography includes
missing up to 15% of breast cancers together with false
negative rates ranging from 4% to 34% [8].
Mammography has a recall rate of 11% [9] and the
diagnosis of suspicious lesions identified on
mammograms often involves waiting for further imaging
or biopsies. From a patient viewpoint, this modality also
involves uncomfortable compression of the breast. X-rays
are also ionising and this poses limitations on the
frequency of screening [10].
Microwave imaging of breast tumours offers an
alternative approach to mammography. X-rays detect
structural changes in tissue cells whilst microwaves
detect changes in dielectric properties. Also microwaves
do not have any ionisation properties and hence this
technology is ideal for breast imaging that results in safer
and more comfortable examinations [1]. It is also less
expensive than MRI and nuclear medicine methods. The
advantages of the microwave imaging system are that the
process is very rapid, sensitive and specific. It has the
ability to detect small tumours by measuring the
difference in the electrical permittivity of malignant and
normal tissues. The typical difference in the permittivity
between the normal and malignant tissues is 10 – 20 %
[11]. Different techniques are employed by different
microwave research groups around the world in the hope
of developing an efficient tool for early breast cancer
detection. Three different methods of microwave breast
imaging methods are discussed briefly below.
1. Passive microwave Imaging: Passive methods
incorporate radiometers to measure temperature
differences in the breast, detecting tumours
based on their increased temperature compared
to normal tissue. Microwave radiometry has
been explored for breast cancer detection as an
adjuvant to mammography [12-14]. Two
examples of microwave radiometers are
Oncoscan [14] and the system reported by S.
Mouty et al. [15].
2. Hybrid Microwave-Acoustic Imaging: Hybrid
methods use microwave energy to select and
rapidly heat tumours and ultrasound transducers
to detect pressure waves generated by the
expansion of the heated tissues. Due to higher
conductivity of malignant breast tissue, more
energy is deposited in tumours, resulting in
selective heating of these lesions. The tumours
expand and generate pressure waves which are
detected by ultrasound transducers. Two
methods of image reconstruction proposed are
Computed Thermo-acoustic Tomography (CTT)
[16, 17] and Scanning Thermo-acoustic
Tomography (STT) [18, 19].
3. Active Microwave Imaging: Active methods
involve illuminating the breast with microwaves
and then measuring transmitted or reflected
microwave signals, and forming images with
these data. Active microwave methods for breast
imaging can be classified as tomography and
radar- based. Meaney et al. [20, 21] at
Dartmouth College have successfully
implemented a clinical prototype of tomography
imaging for active microwave imaging of the
breast. Hagness et al. [22] proposed the first
radar-based breast cancer detection in 1998.
Since then, two systems have been developed:
Microwave Imaging via Space Time
beamforming (MIST) developed by Hagness et
al.[23, 24] in 2003 and Tissue Sensitive
Adaptive Radar (TSAR) developed by Fear et
al.[3, 25] in 2003.
So far in this section the development of microwave
medical imaging for breast cancer detection was
presented along with the different imaging approaches.
All these microwave medical imaging approaches use
microwave antennas to transmit and receive
signals/energy. The characteristics of the microwave
antenna differ considerably in freespace and coupling
media. Most of the imaging techniques employ dielectric
medium to nullify the reflections at the air-skin interface.
So it is paramount to study the behaviour of the antenna
used in relation to that of the lossy medium employed.
The following sections detail the different types of
microwave antenna employed and the design challenges
they present in medical imaging application.
2. Microwave Antennas employed in Medical Imaging
Ever since engineers started using microwaves for
medical applications, the search for a suitable microwave
antenna has been underway. Various microwave antennas
are used across the globe by different microwave medical
imaging groups. This section details four such antennas
which are either used in medical imaging applications or
are identified as potential solutions to be used; namely:
the monopole antenna, the vivaldi antenna, the bow-tie
antenna and the pyramidal horn antenna. In what follows,
a discussion on each of these antennas will be made.
2.1. Monopole Antenna
By using monopole antennas the entire imaging region
can be illuminated by placing them close to the target,
whereas in other antennas the distance has to be greater in
order to provide sufficient illumination coverage. Space
advantage offered by the monopole transmitters can
prove to be very useful for systems using multiple
transmit/receive channels. Meaney et al. [26] have
designed a configuration which utilizes the monopole
antennas to both transmit and receive elements. The
monopole were constructed by having the centre
conductor of a semi rigid cable of quarter wavelength
(physical length was 2.5cm) exposed in a medium at
500MHz. The figure of a typical Monopole antenna
constructed using semi rigid coax is shown in Figure 1. In
a medium such as air or deionised water this type of
antenna is notorious for producing exciting currents. Due
to the lack of any balun arrangement, the characteristic
impedance of the monopole antenna in deionised water is
uneven. Meaney et al. [26] capitalized on the high
attenuation of the surrounding saline solution to limit this
effect. The characteristic impedance of the monopole
antenna in the saline solution (0.9%) is considerably
different; it exhibits a nominal return loss of 9dB for the
frequency range of 300–1100MHz [26].
Figure.1 Monopole Antenna constructed using Semi Rigid
Coax
Through this finding Meaney et al. [26] demonstrate that
the isotropic radiation pattern of the monopole does not
serve to degrade imaging performance in the near field
context, rather it actually increases the image quality
obtained. In order to realise a clinically viable system a
fixed array data acquisition design may be desired.
Because of the physical advantages offered by the
monopole transceiver arrangement, by eliminating the
more bulky waveguides, they can be conducive to a fixed
array design thereby making this arrangement more
suitable for medical applications.
2.2. Bow-tie Antenna
G. Bindu et al. [27] designed an efficient wideband
coplanar stripline fed bow-tie antenna with improved
bandwidth, low crosspolarisation and reduced
backradiation. The new antenna is constructed by
structurally modifying the conventional microstrip bow-
tie antenna design; this is achieved by attaching an image
plane. The antenna is designed as a patch on a single
layered substrate with ε
r
= 4.28 and thickness of 1.6mm.
The coplanar stripline is designed to have an input
impedance of 50Ω in order to couple the antenna
effectively with the measurement system. The
parameters, such as the distance to the image plane, flare
angle of the bow, and dimensions of the antenna, are
found to affect the bandwidth. These parameters are
optimised to enhance the performance.
The antenna exhibits unidirectional radiation pattern with
enhanced bandwidth reduced backradiation and low
crosspolarisation in the operational band and thus making
it suitable for Confocal Microwave Imaging (CMI). A
typical wideband bow-tie antenna with coplanar stripline
feed for CMI is shown in Figure 2. CMI employs back
scattering to locate breast cancer tumours, so the antenna
employed is required to focus the microwave signal
towards the target and collect the back scattered energy
[4]. A 2:1 Standing Wave Ratio (SWR) bandwidth of
45.9% is obtained for the designed 4x4cm bow-tie
antenna in air, which has a flare angle of 90˚. The antenna
operates in the band of 1850MHz - 3425 MHz with a
return loss of -53dB. It is reported that in corn syrup the
bandwidth is enhanced to 91% in the range of 1215 MHz
– 3810 MHz with resonant frequency of 2855MHz and
return loss of -41dB[28].
Figure.2 Wideband Bow-tie antenna
2.3. Vivaldi Antenna
The Vivaldi antenna, a form of the tapered slot radiator,
has been shown to produce performance over a wide
bandwidth limited only by the traditionally used slotline
to microstrip transition [29]. Langley et al. [30] designed
a Vivaldi antenna that satisfies the requirements for
imaging systems in terms of bandwidth, gain and impulse
response, albeit at the expense of significant volumetric
size. In addition to the bandwidth requirement, the
antenna supports the sub nanosecond pulse transmission
with negligible distortion to achieve precision imaging
without ghost targets. Later in 2006, Abbosh et al.[31]
designed a Vivaldi antenna that reduced its physical
dimensions such that it can be incorporated in a compact
microwave imaging detection system whilst maintaining
its distortionless performance.
Figure.3 Antipodal Vivaldi antenna
A typical Ultrawideband Antipodal Vivaldi antenna is
shown in Figure 3. The antenna operates over an
Ultrawideband (UWB) from 3.1GHz to 10.6GHz with a
peak gain of 10.2dBi at 8GHz. These characteristics show
that the Antipodal Vivaldi antenna has the potential to be
used in medical imaging applications.
2.4. Pyramidal Horn Antenna
Horn Antennas are known for their higher aperture
efficiencies but are constrained to certain applications due
to their limited bandwidths. However, the bandwidth of
the horn antennas can be increased significantly by
adding metallic ridges to the waveguide and flared
sections[32]. Numerical and experimental investigations
of pyramidal horn antennas with double ridges have been
reported[33]. E.T. Rosenbury et al.[34] designed a
modified version of the ridged horn antenna in which the
waveguide section is eliminated and one of the two ridges
is replaced by a curve metallic plane terminated by
resistors. Later in 2003 Susan C. Hagness and her team
presented a complete numerical and experimental study
of a specific realisation of this design, wherein the
antenna is customized to centimetre scale dimensions for
operation in the microwave frequency range 1 to 11
GHz[35].
The antenna consists of a pyramidal horn radiation cavity,
a metallic ridge, and a curve metallic launching plane
terminated with resistors. The pyramidal horn is
connected to the outer conductor of the coaxial feed and
serves as the ground plane, providing a current return
path. Because of the coaxial feed, the ground plane
configuration eliminated the need for a UWB Balun. The
launching plane is a curved plane structure connected to
the central conductor of the coaxial feed.
Termination resistors are attached between the end of the
launching plane and the side wall of the pyramidal horn.
Microwave energy is directed and launched by this
curved plane into the surrounding medium. The
termination resistors suppress reflections from the end of
the launching plane. The top surface of the ridge curves
toward the antenna aperture. The dimensions of the horn
antenna are chosen according to the physical size
required and operating frequency range.
Figure.4 Ridged Pyramidal Horn Antenna
A typical Ridged Pyramidal Horn antenna is shown in
Figure 4. The curvature and shape of the launching plane,
the thickness and the contour of the curved side of the
ridge and the termination resistors are the main factors
influencing the input impedance of the antenna. The
pyramidal horn has a depth of 13mm with a 25mm x
20mm aperture. The maximum width of the launching
plane is 12mm and the thickness of the ridge is 2mm.
This antenna yields VSWR of less than 1.5 over the
frequency range and fidelity of approximately 0.96 in
both the simulation and experiment[35]. The antenna has
been tested under low loss immersion medium and
achieved similar VSWR and fidelity. Overall it is evident
that this type of antenna can be useful for biological
sensing and imaging application.
3. Antenna Design Challenges in Medical Imaging
Applications
In order to develop a clinically viable medical imaging
system, it is important to understand the characteristics of
the microwave antenna under coupling media. One of the
major requirements of the microwave medical imaging is
that the whole arrangement is to be immersed in a
coupling medium in order to account for reflections at the
air-skin interface. It is essential that the system designers
take into consideration all the changes to the antenna
characteristics used in comparison to its freespace
behaviour. Most imaging systems work on the principle of
transmitting and receiving signal/energy to and from the
object. The signal propagation from the microwave
antenna to the object and the reflected/scattered signal to
the receiving antenna will be altered depending on the
medium it propagates in relation with freespace
propagation. The microwave signal propagation is
characterised by a constant k, known as the propagation
constant. In freespace the propagation constant k is related
to the angular frequency ω, the permeability µ
o
and
permittivity ε
o
of freespace and it is given in (1)
2
(1)
The permittivity of the coupling medium ε
r
is given as
′
′′
where
′
and
′′
are the real part and
imaginary part of the dielectric constant respectively. The
conductivity of the coupling medium is given as
′′
. Ideally for medical applications coupling media
with no losses are preferred, i.e., the imaginary part in the
permittivity equation will be zero and the propagation
constant
will given as
However, practically it is impossible to have a coupling
medium without any losses. Because of the conductivity
values of the coupling medium the propagation constant
′
will be a complex value and this will change the
wavelength λ to λ
r
in coupling medium. The propagation
constant k for a lossy medium is given as (2)
′
′
′′
(2)
In microwave antenna design, the size of the antenna (l)
will always be specified in terms of wavelength, for
example l can be /4 long (quarter wavelength). This
relationship between the wavelength and size of the
length will affect the length of the antenna in coupling
medium when compared with freespace length. The input
impedance of the antenna will also be affected by the
coupling medium.
Figure 5 Illustration of the power decay component
difference in freespace and coupling medium
The input impedance Z is normally derived as the ratio
between the voltage applied and the current distribution
along the antenna. The current distribution of the antenna
in the coupling medium will depend on the new
wavelength λ
r
and thereby altering the input impedance of
the antenna. In order to match the antenna properly in the
coupling medium the designer needs to take into account
the input impedance in the coupling medium. This
variation caused by the conductivity values in the
radiation pattern of the microwave antenna will affect the
performance of the imaging system. In freespace the
power decay in far field is proportional to 1/R
2
where R is
the distance between the origin and the observation point.
However, in lossy media this decay factor will be
increased by a factor
′
this exponential term accounts
for the additional loss in the system because of the
coupling medium. Thereby, the radiated signal from the
antenna cannot illuminate the whole object or reach the
required depth of penetration. Figure 5 shows the
difference in the power loss in freespace and coupling
medium. This presents the designer with the challenge to
fully understand the antenna behaviour under the lossy
medium and comprehend the situation by altering the
algorithm to accommodate these changes or to modify the
design parameters of the antenna to enhance its
performance.
4. Proposed Solutions
As stated above one of the most important aspects of the
proposed solution has to be the study of the antenna
behaviour in coupling media. The study should involve
analysing the difference in impedance and radiation
pattern of an antenna in coupling media and freespace.
However the traditional analysis for determining the
impedance and radiation pattern become computationally
cumbersome once we extend the surrounding beyond
