Hybrid magnetoacoustic method for breast tumor detection: An in-vivo and in-vitro modelling and analysis

Abstract

A new breast cancer detection method that is based on tissue bioelectric and acoustic characteristics has been developed by using a hybrid magnetoacoustics method. This method manipulates the interaction between acoustic and magnetic energy upon moving ions inside the breast tissue. Analytical in-vivo and in-vitro modelling and analysis on the system performance have been done on normal and pathological mice breast tissue models. Analysis result shows that, hybrid magnetoacoustic method is capable to give unique characteristics between normal and pathological mice breast tissue models especially in in-vitro application. However, additional parameter such as analysis of current distribution in tissue should be taken into account for in-vivo application due to the redundancy of the existing result.

Full text

Hybrid Magnetoacoustic Method for Breast Tumor Detection: An
In-vivo and In-vitro Modelling and Analysis.
MAHEZA IRNA MOHAMAD SALIM, MOHAMMAD AZIZI TUMIRAN, SITI NOORMIZA
MAKHTAR, BUSTANUR ROSIDI, ISMAIL ARIFFIN, ABD HAMID AHMAD, EKO
SUPRIYANTO
Universiti Teknologi Malaysia,
Faculty of Biomedical and Health Science Engineering
Department of Clinical Engineering and Sciences,
81310 Johor Bahru, Johor,
MALAYSIA
maheza@utm.my
, iziza_366@yahoo.com, bustanur@fke.utm.my, ismail@fke.utm.my,
abhamid@fke.utm.my, eko@utm.my
http://www.biomedical.utm.my
Abstract: A new breast cancer detection method that is based on tissue bioelectric and acoustic
characteristics has been developed by using a hybrid magnetoacoustics method. This method
manipulates the interaction between acoustic and magnetic energy upon moving ions inside the
breast tissue. Analytical in-vivo and in-vitro modelling and analysis on the system performance
have been done on normal and pathological mice breast tissue models. Analysis result shows
that, hybrid magnetoacoustic method is capable to give unique characteristics between normal
and pathological mice breast tissue models especially in in-vitro application. However,
additional parameter such as analysis of current distribution in tissue should be taken into
account for in-vivo application due to the redundancy of the existing result.
K
ey-words: magnetic field, acoustic wave, breast tissue, normal and pathological.
1. Introduction
The emergence of magnetoacoustic method, a
combination between acoustic and magnetic energy has
been explored since 2 decades ago for impedance mapping
of tissue [1-8]. Basically, magnetoacoustic method
manipulates the interaction that rises when acoustic and
magnetic energy acting simultaneously on random ionic
particles inside a tissue. Biological tissue is a conductive
element due to the presence of random charges that is
mainly contributed by intra and extracellular diffusion that
supports cell metabolism [1-5]. Propagation of ultrasound
wave will cause charges inside the breast tissue to move at
high velocity due to the back and forth motion of the wave
[25-27]. Moving charges in the present of magnetic field
will experience Lorentz Force that separate the positive
and negative charges, producing an externally detectable
voltage that can be collected using a couple of skin
electrode [1-3].
This interaction has been manipulated to map conductivity
data of biological tissue especially in impedance imaging.
However, previous researches [1-8] apply
magnetoacoustic method for conductivity mapping
purposes only. The ultrasound wave that is used to
stimulate ionic particle motion is not taken into account
though its output delivers valuable information with
regards to tissue mechanical properties [9]. Table 1 below
summarizes previous research reports that combines
ultrasound and magnetic energy.
In ultrasound imaging, detection of malignant and benign
breast nodules are often complicates by their mechanical
characteristic similarities. Benign and malignant nodules
have very small differences in tissue density [10]. Hence
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additional information such as tissue conductivity will be
very helpful for tissue characterization.
Table 1: Summary of previous research reports that
combines ultrasound and magnetic field for impedance
mapping of tissue.
In this study, a hybrid magnetoacoustic method that
combines magnetic and acoustic energy has been
developed. This system is not only collecting the
magnetoacoustic voltage that rises from the acoustic and
magnetic energy interaction for conductivity evaluation, it
also captured back the ultrasound echo that is initially
used to induce charge motion inside the breast tissue for
acoustic properties evaluation. This paper describes the in-
vivo and in-vitro quantitative analysis through a one
dimensional modelling for normal and pathological breast
tissue evaluation.
1.1 Theory
Consider a one dimensional example of an ion inside a
breast tissue having charge q. An ultrasound transducer
delivers a longitudinal ultrasound wave in the x direction
perpendicular to magnetic field B
0
which is in the y
direction. The longitudinal particle motion of the
ultrasound wave at position x and time t will cause the ion
to oscillate back and forth in the tissue with velocity v(x,t).
In the presence of the constant magnetic field B
0
, the ion
is subjected to Lorentz Force of [1-3]:
This force is equivalent to an electric field of:
That establishes a current density
of:
Total current is derived by integrating (3) over the
transducer beam width W and the ultrasound path [1-3]:
(4)
Hence, the resulting voltage collected by the system
circuitry with impedance R is [1-3]:
From the equation, it is known that the amplitude of
magnetoacoustic voltage is proportional to the tissue
conductivity since another parameter such as v, B
0
, α, R
and W is controlled by the system. In the present study,
the value of R is 600Ω for Ag/AgCl electrodes and α is set
to 100% representing an ideal detection circuit.
Using the equation of wave motion, the resulting voltage
in (5) can also be expressed in terms of ultrasound
pressure and spatial gradient of tissue conductivity and
density as [1-3]:
Since the resulting voltage is proportional to the tissue
conductivity, this information is very valuable to be used
in breast tumor characterization since in general;
pathological tissue will have higher conductivity
compared to normal tissue due to an increased rate of
metabolism.
Besides the magnetoacoustic voltage that rises from the
acoustic and magnetic energy interaction, the transmitted
ultrasound wave packet in the x direction that is initially
used to induce ionic particle motion will further
propagates inside the breast tissue. The ultrasound
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propagation in one dimensional is governed by the wave
equation:
In which c
l
is the longitudinal speed of sound in breast
tissues and is the velocity potential. As the ultrasound
propagates further, part of the wave will be reflected back
when it hits tissue boundary and another part will be
further transmitted and attenuated inside the tissue. The
echoes captured back from the system carry information
on wave attenuation and time of flight that is valuable for
breast tumor acoustic characterization.
2 M
ethodology
Fig. 1: Calculation set up of Hybrid Magnetoacoustic
method.
Figure 1 above shows the calculation set up of the
developed system. This system consists of a set of
permanent magnet, ultrasound pulser and receiver unit as
well as an oscilloscope to collect the voltage data. In this
study, a complete mathematical analysis has been done to
test the efficiency of the hybrid system in differentiating
normal and pathological mice breast tissue models both,
in-vitro and in-vivo. Normal and pathological mice breast
models having breast carcinoma and benign fibrosis were
evaluated in terms of its acoustic and electric
characteristics.
2.1 Magnetic Field.
Static magnetic field having intensity of 0.1T is used in
the calculation. The magnitude is assumed to be
homogenous throughout the breast tissue model. The
magnetic field direction is set in positive y direction,
perpendicular to the ultrasound wave.
2.2 Ultrasound System
The ultrasound system delivers 10 MHz frequency pulses
with amplitude of 200V and repetition frequency of
5000Hz via a PVDF transducer. Since the measured
impedance of the PVDF transducer is 1.39e6 Ω, total
electrical power delivered by the system is 7.19e-9W.
However, total acoustic power received by the tissue is
only 1nW due to the low electroacoustic coupling factor
of the PVDF. Total acoustic intensity delivered by the
system is 3.965e-8 W/cm
2
with 0.0254cm
2
beamwidth.
The ultrasound beam is set to be in x direction. Since
initial ultrasound intensity and pressure delivered to the
tissue is known, total acoustic reflection and attenuation
can be calculated and compared between each tissue
model.
2.3 Tissue Modelling
2.3.1 In-vivo tissue modelling
The mice breast tissue has been modeled to have 5 basic
layers based on Sudershan et al [22]: skin, subcutaneous
fat, normal mammary gland, thoracic muscle and thoracic
wall to represent normal breast. For pathological model,
either benign fibrosis or breast carcinoma lesion layer is
added as an additional layer during calculation. Each
tissue layer is having 0.5mm thickness.
Fig. 2: Model of mice breast tissue used in the calculation
Fig 3: Normal mice breast tissue model
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Fig 4: Mice breast tissue model with breast malignant
lesion
Fig 5: Mice breast tissue model with benign lesion
2.3.2 In-vitro tissue modeling
For in-vitro calculation, the mice breast model has been
designed to have only the fat, mammary gland, benign
fibrosis and breast carcinoma layers. The other layers are
eliminated during dissection. Each tissue layer is having
0.5mm thickness.
Fig 6: Normal mice breast tissue model
Fig 7: Mice breast tissue model with malignant lesion
Fig 8: Mice breast tissue model with benign lesion
Table 2 in appendix shows the acoustic and electrical
parameters that were used during analysis. The properties
such as ultrasound attenuation coefficient (α) and
acoustic
impedance
(Z)
is used to analyze the propagation of
ultrasound wave while tissue conductivity (σ) and tissue
density (ρ) is used to analyze the amplitude of
magnetoacoustic voltage that rise due to the interaction
between ultrasound wave and magnetic field.
2.3 Ultrasound wave propagation analysis
As ultrasound wave enters the tissue and hit tissue
boundary, part of its wave will be reflected and another
part of the wave will be further transmitted. The amount
of reflected and transmitted ultrasound wave intensity is
calculated using the following formula:
% Reflection: (Z
2
-Z
1
/Z
2
+Z
1
)
2
x 100, in which Z is the
acoustic impedance of tissue.
% Transmission: 1- % Reflection
Inside a particular tissue layer, the transmitted ultrasound
intensity is further reduced due attenuation process in that
layer. Attenuation was calculated using the following
formula:
-dB=10 log(I
0
/I),
where I
0
is the initial wave intensity when ultrasound
enters a particular layer and I is the intensity at the end of
that layer.
These calculations were repeated every time the
ultrasound wave passing through different layer of tissue
to calculate the spontaneous ultrasound intensity at each
layer.
2.4 Conductivity and voltage analysis
The amplitude of voltage that rises due to ultrasound wave
and magnetic field interaction is calculated using equation
(6) from the z axis. As instantaneous ultrasound intensity
is known from the ultrasound wave propagation analysis,
that instantaneous intensity is converted to instantaneous
pressure at each layer following the equation: I=p
2
/Z.
Finally, the magnetoacoustic voltage that rises in the
system can be further estimated using Equation (6) with
B
0
equals to 0.1T, beamwidth of 0.0254cm
2
.
3. Result
The Hybrid Magnetoacoustic method produces 2 outputs,
t
he ultrasound echo collected by the ultrasound transducer
and the magnetoacoustic voltage from the skin electrodes
that rises due to the interaction between acoustic and
magnetic energy. The ultrasound echo carries information
with regards to tissue mechanical property such as tissue
density, reflected intensity at tissue boundary and tissue
acoustic attenuation whilst the magnetoacoustic voltage
carries information on tissue conductivity.
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3.1 Calculation Result In-vivo
Fig 9: Ultrasound attenuation in different mice breast
model in-vivo
Table 3: Total attenuated ultrasound intensity in-vivo
Fig 10: Ultrasound reflected at different tissue boundary
in- vivo.
Fig 11: Magnetoacoustic voltage at different tissue
boundary in vivo.
3.2 Calculation Result in vitro
Fig 12: Ultrasound attenuation in different mice breast
model in-vitro
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Table 4: Total Ultrasound intensity attenuation in-vitro
Fig 13: Ultrasound reflected at different tissue boundary in
vitro
Fig 14: Magnetoacoustic voltage at different tissue
boundary in-vitro.
4. Discussion
Figure 9 and 12 above respectively show the attenuation
l
evel of the mice breast models in-vivo and in-vitro. Both
figures indicate that normal breast models highly attenuate
ultrasound intensity within the same propagation distance
followed by benign fibrosis. On the other hand, breast
carcinoma models attenuate the least with about 6%
reduction from normal models in-vivo and 12% reduction
from normal model in-vitro. This result agrees well with
the fundamental theories of ultrasound propagation in
which malignant tissue with denser tissue composition
will ease the propagation of ultrasound and hence, caused
less attenuation. Benign fibrosis, on the other hand, is less
dense than breast carcinoma and hence, attenuates some
amount of sounds wave. Normal breast which is usually
composed of high percentage of adipose tissue resists
sound propagation and finally caused high attenuation.
The total ultrasound intensity attenuated by the models is
summarized in Table 3 and 4.
The ultrasound reflected intensity at tissue boundary is
shown by figure 10 for in-vivo and 13 for in-vitro
analysis. It includes all reflections that occur at each tissue
boundaries for every mice breast models. Theoretically,
percentage of reflection at tissue boundaries is determined
by the level of acoustic impedance mismatch between the
2 adjacent tissues. Higher impedance mismatch will
produce more reflection at the boundary and vice versa. In
figure 10, in-vivo analysis shows that ultrasound is
reflected at similar level for benign fibrosis and breast
carcinoma since both of the tissues have the same value of
acoustic impedance mismatch with their previous adjacent
tissue, mammary gland. However, normal tissue model
produces zero reflection since there is no tissue boundary
as the adjacent layers consist of the same tissue which is
also mammary gland. In-vitro reflection in figure 13 also
shows the same result at the normal gland, benign and
breast carcinoma layer.
The last signal collected by this system is the
magnetoacoustic voltage. As shown by Figure 11 and 14,
the calculation agrees well with previous research in
impedance imaging, in which the amplitude of collected
voltage is in the order of milivolt [1-5]. Magnetoacoustic
voltage is represented by a positive and negative peak
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signal at each tissue boundaries. Positive voltage
amplitude indicates that the current tissue layer at that
particular boundary is more conductive then the next
tissue layer and vice versa as shown by signal ‘GC’ and
‘CW’ for malignant model in figure 14. Signal ‘GC’ is
negative because the first tissue layer in that particular
boundary (mammary gland) is less conductive then the
next tissue layer (breast carcinoma). On the other hand,
signal ‘CW’ is positive since the first tissue layer (breast
carcinoma) is more conductive then the next layer (water).
As stated earlier, magnetoacoustic voltage produced by
this system is proportional to the conductivity difference
between adjacent tissues. Higher conductivity difference
will produce higher voltage amplitude. From figure 14, it
can be seen that, mice breast model with carcinoma
producing the highest (-ve) and (+ve) voltage amplitude at
both: the mammary gland-carcinoma boundary and the
carcinoma-water boundary whilst normal mice breast
model producing the lowest voltage amplitude at the same
boundaries. Benign fibrosis, on the other hand produces
moderate voltage amplitude. Despite a very clear signal
amplitude differences in in-vitro analysis, in-vivo signal
shows that normal mice breast model produce the highest
voltage amplitude at the mammary gland-muscle layer. It
also shows that the signal at malignant-muscle boundaries
for malignant breast model is lower than that of normal
signal because of the conductivity difference of those
tissues. Hence, this amplitude indicates that conductivity
difference between normal gland and muscle is much
higher than the malignant and muscle. However, this
complicated result can be improved by analyzing the
current density within the tissue layer as an addition to
voltage analysis at the tissue boundary for better tissue
recognition.
The magnitude of voltage calculated in this study is an
ideal value in which the direction of magnetic field,
ultrasound wave and tissue interface is perpendicular to
each other due to the orientation of Lorentz Force. In the
real case in which tissue boundary is not fully
perpendicular to the system, lower voltage amplitude may
be obtained depending on the angle of the tissue interface
to the system. In addition, this calculation is done on a
homogenous tissue layer model. In the case of
heterogenous tissue layer such as in invasive breast
carcinoma that boundaries between tissue layers is no
longer distinctive, the ability of this method is not yet
predicted. Modification on calculation set up and
procedure such as the used of focus ultrasound transducer
to focus the beam intensity within a small tissue area and
improving analysis to include current density calculation
will be very helpful.
4. Conclusion
Quantitative analysis on the output of Hybrid
M
agnetoacoustic Method for detection of normal and
pathological breast tissues have been completed. 6 mice
breast tissue model representing normal, breast carcinoma
and benign fibrosis were used for in-vivo and in-vitro
analysis. The calculation shows that, hybrid
magnetoacoustic system is capable to give a distinctive
output especially for in-vitro application. However,
additional parameter should be considered when applying
the system for in-vivo application due to the redundancy
of the existing result such as to include the current density
analysis. The overall output of the system is summarized
in table 5.
Tissue
Acoustic
Properties
Electric
Properties
I
N
V
I
T
R
O
Normal
High
attenuation
level
No
ultrasound
reflection
Very low voltage
amplitude
Malignant
Low
Attenuation
Level
Moderate
Ultrasound
reflection
Very high voltage
amplitude
Benign
Moderate
Attenuation
Level
Moderate
Ultrasound
Moderate voltage
amplitude
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reflection
I
N
V
I
V
O
Normal
High
attenuation
level
No
Ultrasound
reflection
High at mammary
Gland-muscle
Interface (-ve)
Malignant
Low
Attenuation
Level
Moderate
Ultrasound
Reflection
Low at carcin
oma
-muscle layer
(+ve)
High at mammary
Gland-carcinoma
Interface (-ve)
Benign
Moderate
Attenuation
Level
Moderate
Ultrasound
Reflection
Moderate voltage
amplitude
Table 5: Summary of Hybrid magnetoacoustic system
output for every breast model in-vitro and in-vivo.
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Appendix
Tissue
α
reference
value
(dB/cm/MHz)
α
calculated
value
(dB/0.5mm
/10MHz)
Acoustic
Impedance
(Z) Mrayls
Conductivity
(σ), S/m @ (1-
27MHz)
Density
(ρ) kg/m
3
Transducer Matching
layer
-
-
2.41
Water
2.17e
-
3 @2 MHz
[12]
5.425 e
-
4
1.482 [12]
0.0001 [18]
1.000
[12]
Skin
2.25 @ 1MHz
(average dermis
and hypodermis)
[19]
1.125
1.61[17]
0.5 [18]
@10MHz
1000
[9]
Fat
0.738 @ 10MHz
[13]
0.0369
1.327 [12]
0.1 [15] @
10MHz
928 [12]
Gland/parenchyma of
cancerous breast/far
from tumor center
6.845@ 10MHz
[12]
0.3422
1.540 [12]
0.07 [15]
1020 [10]
Normal breast
tissue/Fibrofatty
parenchyma
6.845@ 10MHz
[12]
0.3422
1.540 [12]
0.05 [15]
Benign Fibrosis
3.98@ 10MHz
[13]
0.199
1.8 [10 x
16], [20]
0.393[21]
1030 [10]
Breast Carcinoma
2.33@ 10
MHz[13]
0.1165
1.8 [10 x
16][20]
0.8[24][23][15]
@ 10MHz
1041[10]
Muscle
0.57 @ 1MHz
[12]
0.285
1.645 [12]
0.8 [11] @
10MHz
1041[10]
Bone
3.54@ 1 MHz
[12]
1.77
6.364 [12]
0.05 [11] @
10MHz
1990 [12]
Table 2: Overall acoustical and electrical tissue properties used in the calculation
WSEAS TRANSACTIONS on
INFORMATION SCIENCE and APPLICATIONS
Maheza Irna Mohamad Salim, Mohammad Azizi Tumiran,
Siti Noormiza Makhtar, Bustanur Rosidi, Ismail Ariffin,
Abd Hamid Ahmad, Eko Supriyanto
ISSN: 1790-0832
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Issue 8, Volume 7, August 2010