This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TBME.2015.2434956, IEEE Transactions on Biomedical Engineering
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REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < B. S-parameter Results Fig. 6-a shows the measured S11 of the single-polarization and Fig. 6-b shows the measured S11 of the dual-polarization antennas for different positions of the antennas in the array as shown in Fig. 1. S11 results show good impedance matching (|S11| below -10 dB) across the 2-4 GHz frequency range for each position. Results demonstrate the flexible antenna is insensitive to bending, and thus is a good candidate for use in the array configuration. The amplitude and phase of S21 (transfer function) was measured for several different antenna pairs within the monopole antenna array. More specifically, the S21 between antenna 3 of array 1 and antennas 1 to 4 of arrays 2 and 3 (array layouts and antenna numbering as shown in Fig. 5) is shown in Fig. 7. The measured S21 results between antenna 3 of array 1 and antennas 1 to 4 of arrays 2 and 3 for single arm spiral antenna array are plotted in Fig. 8. From the S21 results, we conclude that antennas that are further apart communicate with higher insertion loss. Additionally, because the biological tissues have higher loss in higher frequencies, the S21 results follow this behavior (less insertion loss in lower frequency). From the measured S-parameters, it is clear that there are different channel transfer functions between each transmitreceive antenna pair.
6
TABLE II ANTENNA PERFORMANCE FOR MONOPOLE ANTENNA ARRAY Pairs Ant. 3 to
Fidelity
Power Efficiency (%)
Group Delay Variation (ps)
FoM
Ant. 1
0.94
.001
6.2
0.94
Ant. 2
0.87
.0009
11.4
0.78
Ant. 3
0.92
.003
9.1
2.7
Ant. 4
0.97
.013
7.1
12.6
Ant. 1
0.67
.0004
7
0.27
Ant. 2
0.88
.00093
9.7
0.81
Ant. 3
0.86
.0014
10
1.2
Ant. 4
0.95
.01
10.6
9.5
Array 2
Array 3
V. ANTENNA PERFORMANCE In breast cancer applications, the close proximity of the transmitter, receiver antennas, and biological tissues leads to complex propagation and antenna response that is dependent on the electrical properties of the biological tissues. In this section, we discuss the efficiency of antennas from a wireless link viewpoint. We find the received power when a specific signal is transmitted for each antenna. Furthermore, we investigate fidelity, variation of group delay vs. frequency range of interest, and a Figure-of-Merit (FoM) for this application. A. Power Efficiency and Group Delay We investigate power efficiency and group delay of the different antenna pairs via simulation by using the measured result in Figs. 7 and 8 as the received power when our antenna is used with a common UWB pulse shape. The excitation pulse consists of a Gaussian-modulated sinusoidal waveform mathematically described by [25] V(t) = sin[2πf0 (t − t 0 )] × e
(t−t0 )2 − 2τ2
TABLE III ANTENNA PERFORMANCE FOR SPIRAL ANTENNA ARRAY Pairs Ant. 3 to
Power Efficiency (%)
Group Delay Variation (ps)
FoM
Ant. 1
0.85
.00035
12.5
0.3
Ant. 2
0.8
.00034
20
0.27
Ant. 3
0.92
.00054
Ant. 4
0 .84
.002
4.6
1.7
Ant. 1
0.88
.0003
12.5
0.26
Ant. 2
0.7
.0012
13
0.84
Ant. 3
0.61
.00032
19.5
0.2
Ant. 4
0.68
.0001
Array 2
(1)
where f0 = 3 GHz, τ = 450 ps and t0 = 0 s. This pulse has a frequency spectrum centered at 3 GHz and a 40 dB bandwidth from 2 to 4 GHz. MATLABTM was used to characterize the performance of the wireless link between the antenna pairs using the measured S-parameters of each TX and RX antenna-pair. To calculate the received signal, the spectrum of the pulse is multiplied by S21. Table II and III show the portion of the total power which is received (power efficiency) by the RX antennas. The power level varies with antenna pair, due to the different frequency
Fidelity
6.9
0.5
Array 3
11.6
0.07
response associated with the path of each individual antenna pair. In Table II and III, the variation of group delay is calculated for different antenna pairs based on the measured phase of S21
0018-9294 (c) 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TBME.2015.2434956, IEEE Transactions on Biomedical Engineering
> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < in the Figs. 7 and 8. As shown, different antenna pairs have different variations and create different distortions on the transmitted pulse.
Reflector
Reflector
Antenna Array
Antenna Array
c
c
Configuration 1
Configuration 2
(a) Z
Activated Antenna
Activated Antenna
Reflector
Skin
1 cm
Fat
X
0
25
50 (mm)
Y
Z
With Reflector
Ante nna
Without Reflector
Muscle
X
Gland
B. Fidelity Factor of the Wireless Links between the Antenna Pairs System performance is optimized when the transmitted waveform is received with no distortion. Simple design goals for amplitude and group delay attempt to minimize the distortion. Unlike narrowband antennas, UWB antennas can significantly alter transmitted pulses, due to considerably different behavior across the wide frequency band [25]. The fidelity factor of the wireless link captures the similarity between the ideal expected output waveform of an antenna and the actual radiated waveform. The fidelity factor is defined as the maximum cross-correlation between the ideal and the actual radiated waveforms when both waveforms are normalized by their energies [25]. Fidelity varies between “0 and 1”, with “1” representing the minimum and “0” representing the maximum distortion introduced by the antenna. Let r(t) be the received pulse for an ideal link for a given excitation pulse shape. Let Sr(t) be the actual received waveform for that excitation pulse shape. The fidelity is defined as the following, assuming r(t) and Sr(t) have normalized energies [25]
7
X
0
25
50 (mm)
Y
(b) Poynting[W/m^2]
Z Activated Antenna
Without Reflector
5.0000 4.6436 4.2871 3.9307 3.5743 3.2179 2.8614 2.5050 2.1486 1.7921 1.4357 1.0793 .72286 .36643 0.0100
Z
Activated Antenna
With Reflector
c
F max
r (t ) Sr (t )dt
(2) X
We examined the different antenna pairs which were presented in Figs. 7 and 8. We derive Sr(t) by calculating the inverse Fourier transform of the multiplication of the pulse spectrum of (1) by the measured S21 of the antenna pairs. The fidelity factors calculated from (2) are given in Tables II and III, showing single-polarization antennas perform better than dualpolarization antennas in terms of fidelity. A. Figure of Merit (FoM) To compare the designed antennas in this application, we propose a Figure of Merit (FoM). Our FoM is based on parameters that play an important role in the performance of the wireless links: 1) Pe: power efficiency (%), and 2) F: fidelity factor. The FoM is defined as FoM = Pe × F × 1000
(3)
In Table II and III, FoM is calculated for the links presented in Figs. 7 and 8. The results show that the single-polarization antennas have a higher FoM. The reason for not including the variation of group delays in the FoM is that the fidelity includes both distortions that are caused by i) the phase and ii) the amplitude of the frequency response of the wireless links. VI. IMPROVING PENETRATION OF PROPAGATED ELECTROMAGNETIC WAVES INSIDE BREAST AND MAXIMUM ALLOWED TRANSMITTED POWER
In this section, we show that by using a reflector for the antenna arrays, it is possible to improve penetration of propagated electromagnetic waves into the breast from the wearable
0
25
50 (mm)
Y
X
X
0
25
50 (mm)
Y
(c) Fig. 9. The real part of the Poynting vector propagating in the Z-X plane for antenna 4: without reflector (left side) and with reflector (right side) for a) different configurations for reflector, b) monopole for configuration 1, and c) spiral antenna arrays for configuration 1; color map applies to all cases. TABLE IV MAXIMUM AVERAGED TRANSMITTED POWER FOR MONOPOLE AND SPIRAL ANTENNAS IN VARIOUS ARRAY POSITIONS
Antenna
Monopole Monopole with reflector (mW) (mW) (configuration 1)
Spiral (mW)
Spiral with reflector (mW) (configuration 1)
Ant. 1
4.1
4
3.2
3.1
Ant. 2
4.1
4.1
3.2
3.1
Ant. 3
4.0
4.0
3.2
3.1
Ant. 4
3.9
4
3.1
3.1
antennas, which causes an improvement in power efficiency of the links between the antennas, and thus potentially better image resolution. We also calculate the maximum power that is allowed to be sent by antennas in different positions in the array shown in Fig. 5, with and without use of a reflector. A. Improving Penetration of Propagated Electromagnetic Waves Electromagnetic waves are used to transport information through a wireless medium or a guiding structure, from one point to the other. The quantity used to describe the power
0018-9294 (c) 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TBME.2015.2434956, IEEE Transactions on Biomedical Engineering
> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < density associated with an electromagnetic wave is the time average Poynting vector (average power density) [26]: 1 (4) Wav (x, y, z) E H 2 where E and H are the peak values of instantaneous electricfield intensity (V/m) and instantaneous magnetic-field intensity (A/m), respectively. The real part of (1) is averaged over the propagated power density, and the imaginary part represents the reactive (stored) power density associated with the electromagnetic fields. The real part is responsible for delivering power from one antenna to another antenna. Fig. 9 shows the real part of the Poynting vector when antenna 4 is propagating in the Z-X plane for monopole and spiral antenna arrays. This figure shows that by using reflector behind the antenna arrays, it is possible to improve the penetration of propagated electromagnetic inside the breast. The reflector should be placed far enough so that it does not affect the S11 of the antennas. By placing the reflector further than 1 cm from the antennas, S11 remains unchanged. HFSS results show that for the spiral antenna using the reflector, the real part of the Poynting vector in the position of (x=0, y=0, and z=0) improves by a factor of 3.3 for configuration 1 and 3.7 with configuration 2, and similarly for the monopole, by a factor of 2.6 for configuration 1 and 3.8 for configuration 2. This significant penetration of electromagnetic waves inside the breast improves the power efficiency of the link and the ability to detect the presence of a tumor. When using a reflector, the isolation between closest antennas is still more than 25 dB. The isolation for closest antennas for both cases without reflector and with reflector is more than 25 dB. As we explained in appendix III, we believe the reflectors would not make this imaging scenario more complicated. B. Maximum Allowed Transmitted Power by the Antennas Average Specific Absorption Rate (ASAR) describes the electromagnetic energy that is absorbed in biological tissues and is a critical parameter for assessing the tissue-safety of wireless communications in bio applications. The peak 1-g ASAR spatial distribution versus frequency is simulated in HFSS for all four positions of the antennas inside the array for the monopole and spiral antennas, with and without use of a reflector. The American National Standards Institute (ANSI) limitations specify a maximum peak 1-g ASAR of 1.6 W/kg [27]. Based on the maximum simulated ASAR of tissues in HFSS, the maximum allowed transmitted power from each position of the antennas in the array is calculated. The results are presented in Table IV. Sending more power can damage the biological tissues. The HFSS results show that most radiated power is absorbed by the skin which is in contact with the antenna arrays. From Table IV, the maximum averaged transmitted power for the monopole and the spiral antenna array are around 4 mW and 3.1 mW, respectively. We observe that the reflector does not significantly change this power, and neither does changing the antenna position.
8
and dual-polarization antennas on a flexible substrate for breast cancer detection operating over 2-4GHz frequency bands. The new array improves on previous microwave radar imaging systems in that it is highly flexible, cost-effective to fabricate, and light-weight. Simulations were carried out with HFSS, exploiting a layered (inhomogeneous) model with different dielectric constants and loss tangents to capture the effect of surrounding tissues. To verify the validity of our model and the antenna design procedure, the fabricated arrays were measured on a phantom that is representative of actual biological tissues. Measurements confirmed that the proposed antenna achieved our design goals, validating our antenna design methodology and our biological tissue modeling. Finally, it has been shown that by using a reflector for the arrays, penetration of the propagated electromagnetic waves can be significantly improved. For both arrays, we determined the maximum power allowed to be transmitted from the wearable antenna, by taking into account the limitation imposed by the ANSI. For our future work, we will investigate the performance of our wearable arrays (single and dual polarizations) on patients, and determine which antenna is the most practical for our application. We will then integrate the wearable arrays into a bra-like prototype for improved microwave breast imaging. APPENDIX I A home use breast cancers detection system would be most advantageous for women who have already been identified as being at high risk of developing breast cancer. Their breast health could be checked very frequently, and if there are abnormalities they can be identified at an early stage. A home use detection system could also be used for monitoring treatment progress. Statistics show between 1/7 to 1/9 women will develop breast cancer in the United States [28]. If a cancer is detected in early stage, there is a 99% survival rate. If it has spread to distant areas of the body, the survival rate drops to only 24%. But currently, only 61% of breast cancer cases are diagnosed at the early stage [28]. Home use detection could foster earlier detection enabling more successful treatment and higher likelihood of survival. Currently breast cancer is detected with mammograms. Mammography is usually done 1-2 times per year in the US, but only for older women/ high risk women [28]. For fast growing cancers, the one year interval is ineffective. A home use detection system would allow frequent monitoring that could be gathered in a larger database of past breast scans to determine tumor growth patterns in the patient. Adoption will depend heavily on availability of a cost-effective device. We are working towards a custom cancer detection system, with equipment on-chip or on-board. APPENDIX II
VII. CONCLUSION We presented a methodology for designing wearable single-
The geometrical parameters of the antennas are defined in Fig. 10 and presented in table V.
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This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TBME.2015.2434956, IEEE Transactions on Biomedical Engineering
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REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < B. S-parameter Results Fig. 6-a shows the measured S11 of the single-polarization and Fig. 6-b shows the measured S11 of the dual-polarization antennas for different positions of the antennas in the array as shown in Fig. 1. S11 results show good impedance matching (|S11| below -10 dB) across the 2-4 GHz frequency range for each position. Results demonstrate the flexible antenna is insensitive to bending, and thus is a good candidate for use in the array configuration. The amplitude and phase of S21 (transfer function) was measured for several different antenna pairs within the monopole antenna array. More specifically, the S21 between antenna 3 of array 1 and antennas 1 to 4 of arrays 2 and 3 (array layouts and antenna numbering as shown in Fig. 5) is shown in Fig. 7. The measured S21 results between antenna 3 of array 1 and antennas 1 to 4 of arrays 2 and 3 for single arm spiral antenna array are plotted in Fig. 8. From the S21 results, we conclude that antennas that are further apart communicate with higher insertion loss. Additionally, because the biological tissues have higher loss in higher frequencies, the S21 results follow this behavior (less insertion loss in lower frequency). From the measured S-parameters, it is clear that there are different channel transfer functions between each transmitreceive antenna pair.
6
TABLE II ANTENNA PERFORMANCE FOR MONOPOLE ANTENNA ARRAY Pairs Ant. 3 to
Fidelity
Power Efficiency (%)
Group Delay Variation (ps)
FoM
Ant. 1
0.94
.001
6.2
0.94
Ant. 2
0.87
.0009
11.4
0.78
Ant. 3
0.92
.003
9.1
2.7
Ant. 4
0.97
.013
7.1
12.6
Ant. 1
0.67
.0004
7
0.27
Ant. 2
0.88
.00093
9.7
0.81
Ant. 3
0.86
.0014
10
1.2
Ant. 4
0.95
.01
10.6
9.5
Array 2
Array 3
V. ANTENNA PERFORMANCE In breast cancer applications, the close proximity of the transmitter, receiver antennas, and biological tissues leads to complex propagation and antenna response that is dependent on the electrical properties of the biological tissues. In this section, we discuss the efficiency of antennas from a wireless link viewpoint. We find the received power when a specific signal is transmitted for each antenna. Furthermore, we investigate fidelity, variation of group delay vs. frequency range of interest, and a Figure-of-Merit (FoM) for this application. A. Power Efficiency and Group Delay We investigate power efficiency and group delay of the different antenna pairs via simulation by using the measured result in Figs. 7 and 8 as the received power when our antenna is used with a common UWB pulse shape. The excitation pulse consists of a Gaussian-modulated sinusoidal waveform mathematically described by [25] V(t) = sin[2πf0 (t − t 0 )] × e
(t−t0 )2 − 2τ2
TABLE III ANTENNA PERFORMANCE FOR SPIRAL ANTENNA ARRAY Pairs Ant. 3 to
Power Efficiency (%)
Group Delay Variation (ps)
FoM
Ant. 1
0.85
.00035
12.5
0.3
Ant. 2
0.8
.00034
20
0.27
Ant. 3
0.92
.00054
Ant. 4
0 .84
.002
4.6
1.7
Ant. 1
0.88
.0003
12.5
0.26
Ant. 2
0.7
.0012
13
0.84
Ant. 3
0.61
.00032
19.5
0.2
Ant. 4
0.68
.0001
Array 2
(1)
where f0 = 3 GHz, τ = 450 ps and t0 = 0 s. This pulse has a frequency spectrum centered at 3 GHz and a 40 dB bandwidth from 2 to 4 GHz. MATLABTM was used to characterize the performance of the wireless link between the antenna pairs using the measured S-parameters of each TX and RX antenna-pair. To calculate the received signal, the spectrum of the pulse is multiplied by S21. Table II and III show the portion of the total power which is received (power efficiency) by the RX antennas. The power level varies with antenna pair, due to the different frequency
Fidelity
6.9
0.5
Array 3
11.6
0.07
response associated with the path of each individual antenna pair. In Table II and III, the variation of group delay is calculated for different antenna pairs based on the measured phase of S21
0018-9294 (c) 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TBME.2015.2434956, IEEE Transactions on Biomedical Engineering
> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < in the Figs. 7 and 8. As shown, different antenna pairs have different variations and create different distortions on the transmitted pulse.
Reflector
Reflector
Antenna Array
Antenna Array
c
c
Configuration 1
Configuration 2
(a) Z
Activated Antenna
Activated Antenna
Reflector
Skin
1 cm
Fat
X
0
25
50 (mm)
Y
Z
With Reflector
Ante nna
Without Reflector
Muscle
X
Gland
B. Fidelity Factor of the Wireless Links between the Antenna Pairs System performance is optimized when the transmitted waveform is received with no distortion. Simple design goals for amplitude and group delay attempt to minimize the distortion. Unlike narrowband antennas, UWB antennas can significantly alter transmitted pulses, due to considerably different behavior across the wide frequency band [25]. The fidelity factor of the wireless link captures the similarity between the ideal expected output waveform of an antenna and the actual radiated waveform. The fidelity factor is defined as the maximum cross-correlation between the ideal and the actual radiated waveforms when both waveforms are normalized by their energies [25]. Fidelity varies between “0 and 1”, with “1” representing the minimum and “0” representing the maximum distortion introduced by the antenna. Let r(t) be the received pulse for an ideal link for a given excitation pulse shape. Let Sr(t) be the actual received waveform for that excitation pulse shape. The fidelity is defined as the following, assuming r(t) and Sr(t) have normalized energies [25]
7
X
0
25
50 (mm)
Y
(b) Poynting[W/m^2]
Z Activated Antenna
Without Reflector
5.0000 4.6436 4.2871 3.9307 3.5743 3.2179 2.8614 2.5050 2.1486 1.7921 1.4357 1.0793 .72286 .36643 0.0100
Z
Activated Antenna
With Reflector
c
F max
r (t ) Sr (t )dt
(2) X
We examined the different antenna pairs which were presented in Figs. 7 and 8. We derive Sr(t) by calculating the inverse Fourier transform of the multiplication of the pulse spectrum of (1) by the measured S21 of the antenna pairs. The fidelity factors calculated from (2) are given in Tables II and III, showing single-polarization antennas perform better than dualpolarization antennas in terms of fidelity. A. Figure of Merit (FoM) To compare the designed antennas in this application, we propose a Figure of Merit (FoM). Our FoM is based on parameters that play an important role in the performance of the wireless links: 1) Pe: power efficiency (%), and 2) F: fidelity factor. The FoM is defined as FoM = Pe × F × 1000
(3)
In Table II and III, FoM is calculated for the links presented in Figs. 7 and 8. The results show that the single-polarization antennas have a higher FoM. The reason for not including the variation of group delays in the FoM is that the fidelity includes both distortions that are caused by i) the phase and ii) the amplitude of the frequency response of the wireless links. VI. IMPROVING PENETRATION OF PROPAGATED ELECTROMAGNETIC WAVES INSIDE BREAST AND MAXIMUM ALLOWED TRANSMITTED POWER
In this section, we show that by using a reflector for the antenna arrays, it is possible to improve penetration of propagated electromagnetic waves into the breast from the wearable
0
25
50 (mm)
Y
X
X
0
25
50 (mm)
Y
(c) Fig. 9. The real part of the Poynting vector propagating in the Z-X plane for antenna 4: without reflector (left side) and with reflector (right side) for a) different configurations for reflector, b) monopole for configuration 1, and c) spiral antenna arrays for configuration 1; color map applies to all cases. TABLE IV MAXIMUM AVERAGED TRANSMITTED POWER FOR MONOPOLE AND SPIRAL ANTENNAS IN VARIOUS ARRAY POSITIONS
Antenna
Monopole Monopole with reflector (mW) (mW) (configuration 1)
Spiral (mW)
Spiral with reflector (mW) (configuration 1)
Ant. 1
4.1
4
3.2
3.1
Ant. 2
4.1
4.1
3.2
3.1
Ant. 3
4.0
4.0
3.2
3.1
Ant. 4
3.9
4
3.1
3.1
antennas, which causes an improvement in power efficiency of the links between the antennas, and thus potentially better image resolution. We also calculate the maximum power that is allowed to be sent by antennas in different positions in the array shown in Fig. 5, with and without use of a reflector. A. Improving Penetration of Propagated Electromagnetic Waves Electromagnetic waves are used to transport information through a wireless medium or a guiding structure, from one point to the other. The quantity used to describe the power
0018-9294 (c) 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TBME.2015.2434956, IEEE Transactions on Biomedical Engineering
> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < density associated with an electromagnetic wave is the time average Poynting vector (average power density) [26]: 1 (4) Wav (x, y, z) E H 2 where E and H are the peak values of instantaneous electricfield intensity (V/m) and instantaneous magnetic-field intensity (A/m), respectively. The real part of (1) is averaged over the propagated power density, and the imaginary part represents the reactive (stored) power density associated with the electromagnetic fields. The real part is responsible for delivering power from one antenna to another antenna. Fig. 9 shows the real part of the Poynting vector when antenna 4 is propagating in the Z-X plane for monopole and spiral antenna arrays. This figure shows that by using reflector behind the antenna arrays, it is possible to improve the penetration of propagated electromagnetic inside the breast. The reflector should be placed far enough so that it does not affect the S11 of the antennas. By placing the reflector further than 1 cm from the antennas, S11 remains unchanged. HFSS results show that for the spiral antenna using the reflector, the real part of the Poynting vector in the position of (x=0, y=0, and z=0) improves by a factor of 3.3 for configuration 1 and 3.7 with configuration 2, and similarly for the monopole, by a factor of 2.6 for configuration 1 and 3.8 for configuration 2. This significant penetration of electromagnetic waves inside the breast improves the power efficiency of the link and the ability to detect the presence of a tumor. When using a reflector, the isolation between closest antennas is still more than 25 dB. The isolation for closest antennas for both cases without reflector and with reflector is more than 25 dB. As we explained in appendix III, we believe the reflectors would not make this imaging scenario more complicated. B. Maximum Allowed Transmitted Power by the Antennas Average Specific Absorption Rate (ASAR) describes the electromagnetic energy that is absorbed in biological tissues and is a critical parameter for assessing the tissue-safety of wireless communications in bio applications. The peak 1-g ASAR spatial distribution versus frequency is simulated in HFSS for all four positions of the antennas inside the array for the monopole and spiral antennas, with and without use of a reflector. The American National Standards Institute (ANSI) limitations specify a maximum peak 1-g ASAR of 1.6 W/kg [27]. Based on the maximum simulated ASAR of tissues in HFSS, the maximum allowed transmitted power from each position of the antennas in the array is calculated. The results are presented in Table IV. Sending more power can damage the biological tissues. The HFSS results show that most radiated power is absorbed by the skin which is in contact with the antenna arrays. From Table IV, the maximum averaged transmitted power for the monopole and the spiral antenna array are around 4 mW and 3.1 mW, respectively. We observe that the reflector does not significantly change this power, and neither does changing the antenna position.
8
and dual-polarization antennas on a flexible substrate for breast cancer detection operating over 2-4GHz frequency bands. The new array improves on previous microwave radar imaging systems in that it is highly flexible, cost-effective to fabricate, and light-weight. Simulations were carried out with HFSS, exploiting a layered (inhomogeneous) model with different dielectric constants and loss tangents to capture the effect of surrounding tissues. To verify the validity of our model and the antenna design procedure, the fabricated arrays were measured on a phantom that is representative of actual biological tissues. Measurements confirmed that the proposed antenna achieved our design goals, validating our antenna design methodology and our biological tissue modeling. Finally, it has been shown that by using a reflector for the arrays, penetration of the propagated electromagnetic waves can be significantly improved. For both arrays, we determined the maximum power allowed to be transmitted from the wearable antenna, by taking into account the limitation imposed by the ANSI. For our future work, we will investigate the performance of our wearable arrays (single and dual polarizations) on patients, and determine which antenna is the most practical for our application. We will then integrate the wearable arrays into a bra-like prototype for improved microwave breast imaging. APPENDIX I A home use breast cancers detection system would be most advantageous for women who have already been identified as being at high risk of developing breast cancer. Their breast health could be checked very frequently, and if there are abnormalities they can be identified at an early stage. A home use detection system could also be used for monitoring treatment progress. Statistics show between 1/7 to 1/9 women will develop breast cancer in the United States [28]. If a cancer is detected in early stage, there is a 99% survival rate. If it has spread to distant areas of the body, the survival rate drops to only 24%. But currently, only 61% of breast cancer cases are diagnosed at the early stage [28]. Home use detection could foster earlier detection enabling more successful treatment and higher likelihood of survival. Currently breast cancer is detected with mammograms. Mammography is usually done 1-2 times per year in the US, but only for older women/ high risk women [28]. For fast growing cancers, the one year interval is ineffective. A home use detection system would allow frequent monitoring that could be gathered in a larger database of past breast scans to determine tumor growth patterns in the patient. Adoption will depend heavily on availability of a cost-effective device. We are working towards a custom cancer detection system, with equipment on-chip or on-board. APPENDIX II
VII. CONCLUSION We presented a methodology for designing wearable single-
The geometrical parameters of the antennas are defined in Fig. 10 and presented in table V.
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This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TBME.2015.2434956, IEEE Transactions on Biomedical Engineering
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