2 research outputs found
Simulation and Design of an UWB Imaging System for Breast Cancer Detection
Breast cancer is the most frequently diagnosed cancer among women. In recent
years, the mortality rate due to this disease is greatly decreased thanks to both
enormous progress in cancer research, and screening campaigns which have allowed
the increase in the number of early diagnoses of the disease. In fact, if the tumor is
identied in its early stage, e.g. when it has a diameter of less than one centimeter,
the possibility of a cure can reach 93%. However, statistics show that more young
aged women are suered breast cancer.
The goal of screening exams for early breast cancer detection is to nd cancers
before they start to cause symptoms. Regular mass screening of all women at risk
is a good option to achieve that. Instead of meeting very high diagnostic standards,
it is expected to yield an early warning, not a denitive diagnosis. In the last
decades, X-ray mammography is the most ecient screening technique. However,
it uses ionizing radiation and, therefore, should not be used for frequent check-ups.
Besides, it requires signicant breast compression, which is often painful. In this
scenario many alternative technologies were developed to overcome the limitations
of mammography. Among these possibilities, Magnetic Resonance Imaging (MRI)
is too expensive and time-consuming, Ultrasound is considered to be too operatordependent
and low specicity, which are not suitable for mass screening. Microwave
imaging techniques, especially Ultra WideBand (UWB) radar imaging, is the most
interesting one. The reason of this interest relies on the fact that microwaves are
non-ionizing thus permitting frequent examinations. Moreover, it is potentially lowcost
and more ecient for young women. Since it has been demonstrated in the
literatures that the dielectric constants between cancerous and healthy tissues are
quite dierent, the technique consists in illuminating these biological tissues with
microwave radiations by one or more antennas and analyzing the re
ected signals.
An UWB imaging system consists of transmitters, receivers and antennas for
the RF part, the transmission channel and of a digital backend imaging unit for
processing the received signals. When an UWB pulse strikes the breast, the pulse is
re
ected due to the dielectric discontinuity in tissues, the bigger the dierence, the
bigger the backscatter. The re
ected signals are acquired and processed to create
the energy maps. This thesis aims to develop an UWB system at high resolution for the detection of carcinoma breast already in its initial phase. To favor the adoption
of this method in screening campaigns, it is necessary to replace the expensive and
bulky RF instrumentation used so far with ad-hoc designed circuits and systems.
In order to realize that, at the very beginning, the overall system environment must
be built and veried, which mainly consists of the transmission channel{the breast
model and the imaging unit. The used transmission channel data come from MRI
of the prone patient. In order to correctly use this numerical model, a simulator was
built, which was implemented in Matlab, according to the Finite-Dierence-Time-
Domain (FDTD) method. FDTD algorithm solves the electric and magnetic eld
both in time and in space, thus, simulates the propagation of electromagnetic waves
in the breast model. To better understand the eect of the system non-idealities,
two 2D breast models are investigated, one is homogeneous, the other is heterogeneous.
Moreover, the modeling takes into account all critical aspects, including
stability and medium dispersion. Given the types of tissues under examination, the
frequency dependence of tissue dielectric properties is incorporated into wideband
FDTD simulations using Debye dispersion parameters. A performed further study
is in the implementation of the boundary conditions. The Convolution Perfectly
Matched Layer (CPML) is used to implement the absorbing boundaries.
The objective of the imaging unit is to obtain an energy map representing the
amount of energy re
ected from each point of the breast, by recombining the sampled
backscattered signals. For this purpose, the study has been carried out on various
beamforming in the literature. The basic idea is called as "delay and sum", which
is to align the received signals in such a way as to focus a given point in space and
then add up all the contributions, so as to obtain a constructive interference at that
point if this is a diseased tissue. In this work, Microwave Imaging via Space Time
(MIST) Beamforming algorithm is applied, which is based on the above principle
and add more elaborations of the signals in order to make the algorithm less sensitive
to propagation phenomena in the medium and to the non-idealities of the system.
It is divided into two distinct steps: the rst step, called SKin Artifact Removal
(SKAR), takes care of removing the contributions from the signal caused by the
direct path between the transmitter and receiver, the re
ection of skin, as they are
orders of magnitude higher compared to the re
ections caused by cancers; the second
step, which is BEAmForming (BEAF), performs the algorithm of reconstruction by
forming a weighted combination of time delayed version of the calibrated re
ected
signals.
As discussed above, more attention must be paid on the implementation of the
ad-hoc integration circuits. In this scenario, due to the strict requirements on the
RF receiver component, two dierent approaches of the implementation of the RF
front-end, Direct Conversion (DC) receiver and Coherent Equivalent Time Sampling
(CETS) receiver are compared. They are modeled behaviorally and the eects of
various impairments, such as thermal, jitter, and phase noise, as well as phase inaccuracies, non-linearity, ADC quantization noise and distortion, on energy maps
and on quantitative metrics such as SCR and SMR are evaluated. Dierential
Gaussian pulse is chosen as the exciting source. Results show that DC receiver
performs higher sensitivity to phase inaccuracies, which makes it less robust than
the CETS receiver. Another advantage of the CETS receiver is that it can work
in time domain with UWB pulses, other than in frequency domain with stepped
frequency continuous waves like the DC one, which reduces the acquisition time
without impacting the performance.
Based on the results of the behavioral simulations, low noise amplier (LNA)
and Track and Hold Amplier (THA) can be regarded as the most critical parts
for the proposed CETS receiver, as well as the UWB antenna. This work therefore
focuses on their hardware implementations. The LNA, which shows critical performance
limitation at bandwidth and noise gure of receiver, has been developed based
on common-gate conguration. And the THA based on Switched Source Follower
(SSF) scheme has been presented and improved to obtain high input bandwidth,
high sampling rate, high linearity and low power consumption. LNA and THA
are implemented in CMOS 130nm technology and the circuit performance evaluation
has been taken place separately and together. The small size UWB wide-slot
antenna is designed and simulated in HFSS.
Finally, in order to evaluate the eect of the implemented transistor level components
on system performance, a multi-resolution top-down system methodology
is applied. Therfore, the entire
ow is analyzed for dierent levels of the RF frontend.
Initially the system components are described behaviorally as ideal elements.
The main activity consists in the analysis and development of the entire frontend
system, observing and complementing each other blocks in a single
ow simulation,
clear and well-dened in its various interfaces. To achieve that the receiver is modeled
and analyzed using VHDL-AMS language block by block, moreover, the impact
of quantization, noise, jitter, and non-linearity is also evaluated. At last, the behavioral
description of antenna, LNA and THA is replaced with a circuit-level one
without changing the rest of the system, which permits a system-level assessment
of low-level issues
Simulation and design of an UWB imaging system for breast cancer detection
In Ultra-Wideband (UWB) imaging for breast cancer detection, UWB pulses illuminate the breast and the echoes are processed to create images of reflected energy. Experiments with RF instrumentation proved this method to be capable of detecting small tumors, but adoption in screening campaigns requires the replacement of RF tools with ad hoc circuits and systems. We report on our experience with the design of an UWB imaging system and assess its feasibility in various steps. We develop a simulation environment that includes breast electromagnetic model, antennas, transmitters, receivers, and digital processing. Simulations produce energy maps, which help identify qualitatively the presence of cancerous tissue, and more quantitative metrics such as Signal-to-Clutter and Signal-to-Mean ratios. Initially the system components are described behaviorally as ideal elements. We then focus on the most critical component, the receiver, and evaluate the impact o fquantization, noise, jitter, and non-linearity. We then design three receiver key elements: a slot antenna, the LNA, and the track-and-hold amplifier, the latter two implemented in CMOS 130nm. In our simulator we can replace their behavioral description with a circuit-level one without changing the rest of the system, which permits a system-level assessment of low-level issue