Delay and Sum

Evaluation of a hemi-spherical wideband

antenna array for breast cancer imaging

M. Klemm,1 I. J. Craddock,1 A. Preece,1 J. Leendertz,1 and R. Benjamin2

Received 10 December 2007; revised 11 June 2008; accepted 1 October 2008; published 19 December 2008.

[1] Using similar techniques to ground penetrating radars, microwave detection of breasttumors is a potential nonionizing and noninvasive alternative to traditional body-imagingtechniques. In order to develop an imaging system, the team at Bristol have beenworking on a number of antenna array prototypes, based around a stacked-patch element,starting with simple pairs of elements and progressing to fully populated planar arrays.As the system commences human subject trials, a curved breast phantom has beendeveloped along with an approximately hemi-spherical conformal array. This contributionwill present details of the conformal array design and initial results from this uniqueexperimental imaging system as applied to an anatomically shaped breast phantom.

Citation: Klemm, M., I. J. Craddock, A. Preece, J. Leendertz, and R. Benjamin (2008), Evaluation of a hemi-spherical

wideband antenna array for breast cancer imaging, Radio Sci., 43, RS6S06, doi:10.1029/2007RS003807.

1. Introduction

[2] Breast cancer is the most common cancer inwomen. X-ray mammography is currently the mosteffective detection technique; however, it suffers fromrelatively high missed- and false-detection rates, involvesuncomfortable compression of the breast and also entailsexposure to ionizing radiation. Microwave detection ofbreast tumors is a potential nonionizing alternative beinginvestigated by a number of groups [Hagness et al.,1998; Fear et al., 2003; Fear, 2005; Bialkowski and Wee,2007]. In these microwave-based systems, in a similarfashion to ground penetrating radars, microwaves aretransmitted from an antenna or antenna array, and thereceived signals, which contain reflections from tumors,are recorded and analyzed.[3] This contribution presents details of the conformal

radar-based breast cancer detection system. Unlike theother published work on the subject, the developedexperimental radar system operates in a multistatic mode,originally proposed for breast cancer and land minedetection by [Benjamin, 1996]. Compared to the mono-static approaches, a multistatic approach with a fullypopulated antenna array enables far more data to begathered.

[4] To date most of the work on the breast cancerdetection have been based on the computer simulations[Kosmas et al., 2004; Abas et al., 2007]. There have beenonly a few experimental breast-imaging radar systemsreported in the open literature [Craddock et al., 2005; Silland Fear, 2005]. In this paper we present for the firsttime an assessment of techniques to de-embed the tumorresponse from real experimental data.

2. Development of an Experimental System

2.1. Antenna Design

[5] A prerequisite for all microwave imaging systemsis a suitable antenna array. Initial work concentrated ondeveloping a simple but low-profile and wide-bandantenna that would cover the 4–10 GHz frequencyrange. An aperture stacked-patch antenna was designedfor this purpose. The antenna used herein is a modifiedversion of the antenna presented in [Nilavalan et al.,2007], where it was employed in a planar array for breastimaging.[6] For the conformal array, the antenna was rede-

signed. The final antenna design in presented in Figure 1.The antenna cross-section, dielectric materials and sizeof individual patches were kept the same as in [Nilavalanet al., 2007]. Only the ground plane size (and hence afeeding line substrate) was substantially reduced to 28 �17 mm2. Dimensions of two dielectric substrates wherepatches are printed are 17 � 17 mm2. Additionally, as welearned from the experience of using the planar array[Craddock et al., 2005] it is better to shield the antennafrom the surrounding environment, therefore we added a

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cavity at the back of the antenna. The cavity has a planarinner dimensions of 18 � 11 mm2 and is 12 mm long. Toabsorb the back radiation of the antenna and avoid anyresonances the cavity was lined with a broad-bandabsorbing material (Eccosorb FGM-40 from Emmerson& Cumming).[7] In Figure 2 we present the measured antenna input

match (S11), which shows that the antenna is matched(S11 < �5 dB) between 4 and 10 GHz. The goodtransient performance in this frequency range is visiblewhen looking at the simulated (FDTD) transmit transferfunction of the antenna shown in Figure 3. The 10 dB

bandwidth of the transmit transfer function is about 7GHz (from 3.5 to 10.5 GHz).[8] Additionally, we performed a transmissionmeasure-

ment between two antennas (face to face, 10 cm separa-tion) immersed in a lossy matching liquid (described inthe following paragraph). As an input pulse we chose thewaveform presented in Figure 4, which covers a fre-quency range between 4 and 9 GHz on a �3 dB level. Asdescribed by Hines and Stinehelfer [1974], this type ofpulse is suitable for time domain analysis of microwavesystemswhen performingmeasurements in the frequency-domain. The resulting pulse transmitted between our

Figure 1. Cavity backed aperture stacked-patch antenna for breast cancer detection.

Figure 2. Measured S11 (input match) characteristic of the antenna.

RS6S06 KLEMM ET AL.: BREAST CANCER IMAGING

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antennas and its spectrum is shown in Figure 5. Thetransmitted pulse is clearly longer than the input pulse,due to the antenna’s response but also due to a lossy anddispersive immersion medium. Comparing the simulatedtransfer function of the single antenna (see Figure 4) withthe measured two-antenna transfer function (see Figure 5),we can clearly see the effect of the lossy medium. The10 dB bandwidth of the measured transfer function wasreduced to 3 GHz (3.5–6.5 GHz), for the 10 cm distancebetween antennas. Obviously, as the distance betweenantennas with change, also the transfer function will havedifferent 10 dB bandwidth and also its maximum valuewill be at slightly different frequency. From these trans-mission experiments it is apparent that dispersive lossesof the normal breast tissue will have a biggest impact onlimiting the achievable temporal resolution.

2.2. Conformal Symmetrical Antenna Array Design

[9] Given the effort in designing and constructing aconformal hemi-spherical array, the intention from theoutset was to design not only an array for laboratory useon a realistic, curved phantom, but also one that wouldserve as an initial clinical prototype. Approximately 20female volunteers came forward from the University andthe fit between their breasts and various plastic sphericalsections was assessed with them lying in a prone position- the prone (face-down) position being felt to offer thebest chance of the breast forming a gently and uniformlycurved shape. Following this assessment, the dimensionsof the array were input into a 3-D CAD model, alongwith the antenna elements and all supporting metalwork.[10] The resulting antenna array is formed around

lower part of a 78 mm-radius sphere, in four rows offour antennas. The side view of the array and breast

Figure 3. Simulated (FDTD) transmit transfer function (at boresite) of the antenna.

Figure 4. Synthetic pulse used as an antenna excitation: (left) time domain waveform and (right)spectrum.


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model is shown in Figure 6. The arrangement of antennaelements (top view) seen in Figure 7 gives enoughclearance for the cables and connectors, which passbetween the elements of adjacent rows. The partlyconstructed array is shown in Figure 8.[11] The clear advantage of the designed curved array

is its conformity to the breast’s shape, providing a goodbreast coverage by antennas radiation patterns. The realaperture array, together with the switching network, givea fast data acquisition. In a single scan there is nomechanical scanning involved, saving a lot of measure-ment time (it is important for measurement with realpatients). Another benefit of this radar system is the factthat it operates in a multistatic mode, what gives a fargreater spatial diversity compared with the monostaticoperation. The main disadvantage of the presented realaperture radar system is the antenna coupling, as well asadditional reflection from mechanical parts of the array.We will describe later in the paper how to deal with theseundesired signals.

2.3. Three-Dimensional Physical Breast Phantom

[12] For experimental testing we developed appropri-ate materials and a 3-D breast phantom. As shown inFigure 6, during the measurements the antennas areimmersed in a matching liquid, to reduce reflectionsfrom the skin and for a more compact antenna design.We decided that this matching liquid would be the sameas the material simulating properties of a normal breast-fat, mainly for practical reasons (only one liquid requiredin manufacturing). The developed matching and normalbreast tissue equivalent liquid [Leendertz et al., 2003]has a relative dielectric constant of about 9.5 andattenuation of 1.2 dB/cm at 6 GHz. This material is alsodispersive (see Craddock et al. [2005] for its frequency-dependent characteristics).

[13] Next, a curved skin phantom was developed. Theskin layer is 2 mm thick, it is a part of a 58 mm-radiushemi-sphere. When the skin phantom is fitted into thearray, as shown in Figure 9, it lies 20 mm above theantenna elements. This standoff between antennas andbreast provides a reasonable coverage of a breast by anantenna radiation pattern. The electrical parameters ofthe skin layer were chosen again according to thepreviously published data: the material is dispersiveand at 6 GHz it has a relative dielectric constant of 30and attenuation of 16 dB/cm.[14] After fitting the skin phantom into antenna array, a

plastic tank is connected to the array as presented inFigure 10. The tank is then filled with the normal breast-fat equivalent liquid (the same as matching liquid).[15] A tumor phantom material with a relative dielec-

tric constant close to 50 and conductivity 7 S/m (at 6 GHz)was also developed. This gives a contrast betweendielectric properties of breast fat and tumor phantom

Figure 5. Two antenna transfer function (along boreside direction). Antennas were immersed inthe lossy matching liquid, 10 cm distance.

Figure 6. Hemi-spherical antenna array and breastphantom configuration: side view.


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Figure 7. Positions of antenna elements in the array, looking from the top.

Figure 8. Partly constructed real antenna array.


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materials of around 1:5. Recently published data inLazebnik et al. [2007], based on a large clinical study,however suggest that the contrast between healthy andmalignant breast tissues might be lower, at least in somewomen.

3. Focusing Algorithms

[16] To obtain the 3-D image of the scattered energy,we employ postreception synthetic focusing. We employa modified version of a classical delay-and-sum (DAS)beamforming [Benjamin et al., 2001], briefly describedbelow.

3.1. Preprocessing: Equalization

[17] Before applying the focusing algorithm we haveto perform a preprocessing step. This process aims atequalization of scattered tumor responses for differentantenna pairs. Ideal preprocessing would result in allreceived pulses being of the same shape, amplitude andperfectly time-aligned. In our preprocessing the follow-ing steps are performed: (1) extraction of the tumorresponse from measured data, (2) equalization of tissuelosses, and (3) equalization of radial spread of thespherical wavefront.[18] In the work reported herein there will be frequency-

dependence both of the tissue losses and of the radiationpatterns of the antennas, however for simplicity we donot attempt to correct for these frequency-dependenciesin our processing.

3.2. Modified Delay-and-Sum Algorithm

[19] The modified DAS algorithm uses an additionalweighting factor QF (quality factor), compared to thestandard DAS. QF can be interpreted as a quality factorof the coherent focusing algorithm.

[20] The characteristic equation of the improved DASalgorithm is expressed as:

Fe x; y; zð Þ ¼QF x; y; zð Þ �Z t

0

�XMi¼1

wi x; y; zð Þ � yi t � Ti x; y; zð Þð Þ !2

dt

ð1Þ

where M = N(N-1)/2 (N is the number of antennas in thearray), wi is the location dependent weight calculatedduring preprocessing (steps 2 and 3 of the preproces-sing), yi is the measured radar signal and Ti is the timedelay. The time delay Ti for a given transmitting andreceiving antenna is calculated based on the antenna’sposition, position of the focal point r = (x, y, z) as well asan estimate of average wave propagation speed, which inour processing is simply assumed to be constant acrossthe band (though in our experiments it will not be). Aswe recently presented in Klemm et al. [2008], by usingthis additional weighting factor QF, quality of images issignificantly improved due to the clutter reductioncapability of the new algorithm.

4. Array Evaluation Results and Imaging

Results

4.1. Tumor Response De-embedding Techniques

[21] Before applying the preprocessing step and thefocusing algorithm, the tumor response must be extractedfrom measured data. Measured data contain the tumorresponse, as well as additional undesired signals (antennacoupling, reflections from the skin, reflections from

Figure 9. Skin phantom fitted into the antenna array.

Figure 10. Antenna array together with the tank.


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mechanical parts of the array). To subtract all theunwanted signals, we employ two techniques: either(1) background subtraction, or (2) rotation subtraction.[22] In the background subtraction method the tumor

response is extracted using two measurements, with andwithout the tumor present in the breast. This method iseffective and useful in evaluating the array imagingproperties, as well as in array calibration, but cannotpossibly be used with real patients.[23] The second method, rotation subtraction, also

employs two measurements, but with the tumor presentin the breast in both cases. The first measurement isperformed with the array in a given position, then thearray is rotated (in a horizontal plane, around its centralvertical axis) and a second measurement is recorded.Because the rotation subtraction method does not requirea background measurement, it can potentially be used inrealistic scenarios with breast cancer patients.[24] To obtain the relevant measured data for back-

ground and rotation subtraction methods, the followingmeasurement steps were performed:[25] 1. First, in a given array position (AP1), a back-

ground measurement BAP1 is taken. There is no tumorpresent in the breast phantom.[26] Then, as the array stays in the same position AP1,

we introduce a tumor into the breast phantom at a certainlocation and a measurement TAP1 is recorded.[27] 3. Next, keeping the breast phantom with tumor

fixed we rotate the array to a second position AP2. Afterrotation we perform a second measurement of the breastwith tumor (TAP2).

[28] 4. Finally, while the array is in position AP2, thetumor is removed from the breast phantom and thesecond background measurement BAP2 is obtained.[29] These four data sets provide a basis for obtaining

three focused images of the phantom: two images usingthe impractical but ideal background subtraction methodand one using the more practical rotation subtractionmethod. The tumor responses TR for respective imagesare obtained using the following operations on therecorded time domain signals (not on the images): (1)Image 1: TRI1 = TAP1 � BAP1; (2) Image 2: TRI2 = TAP2� TAP1; (3) Image 3: TRI3 =TAP2 � BAP2.

4.2. Comparison of Tumor Response ExtractionMethods: Evaluation of Experimental Radar Signals

[30] In this paragraph we compare the tumor responseextraction techniques described above, by investigatingindividual multistatic radar signals. Additionally, weshow how we minimize effects of antenna mutualcouplings.[31] We compare effectiveness of two extraction tech-

niques by looking at respective raw measured multistaticradar signals. As an example, we chose a pulse trans-mitted between antennas 1 and 2 (see Figure 8 forantenna positions), where we can also observe the effectof mutual coupling between antennas. In Figure 11 wecan see two radar signals, without and with the tumorinside the breast phantom, used in background subtrac-tion method. As described in the previous paragraph, bysubtracting these two signals we can easily extract tumorresponse from measured signals. This method however is

Figure 11. Comparison of two radar signals (raw measured data) used in background subtractiontechnique. Both signals are for the same multistatic path (transmission between antennas 1 and 2, asshown in Figure 8), with and without the tumor inside breast phantom.


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not clinically useful, because the background measure-ment (without tumor inside a breast) will not be availablewith real cancer patient. In the figure we indicated adifferent parts of the radar signal. As this is a signaltransmitted between two adjacent antennas in the array, afirst signal received in a direct signal coupled from thetransmitting antenna. Next, we can identify a signal

reflected from the skin and then the tumor response.As we can see, the tumor response is very small andhidden within the late-time portion of the received signal.The tumor response can only be extracted by subtractingthe background signal (shown later in a paper inFigure 13). Other parts of the signal (antenna coupling

Figure 12. Comparison of two radar signals (raw measured data) used in rotation subtractiontechnique. Both signals are for the same multistatic path (transmission between antennas 1 and 2, asshown in Figure 8), but at two array positions: AP1 and AP2.

Figure 13. Comparison of extracted tumor response when using background subtraction androtation subtraction methods.


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and skin reflection) are almost identical for cases withand without the tumor.[32] Next, in Figure 12 we can see radar signals used in

the second tumor extraction technique: rotation subtrac-tion. Signals are shown for the same multistatic channelas used in background subtraction. Both signals fromFigure 12 contain tumor response, they were obtainedwhen array was in two different positions due to arrayrotation (AP1 and AP2, as described in previous para-graph). Again we can distinguish signals originatingfrom antenna coupling, skin reflection and the tumor.[33] In Figure 13 we present signals obtained from

background and rotation subtraction methods. We cansee that the ‘ideal’ background subtraction method (solid

line) provides a good tumor response, with very smalldistortions. The antenna coupling and skin reflectionsignals were nicely canceled. Significantly more distor-tions can be seen in the rotation subtraction signal(dashed line). The tumor response was reasonably recov-ered from measured data. However, signals at otherranges also appeared. Especially the skin reflectionsignal is visible. It has actually higher magnitude thatthe tumor response. The remaining of the sin reflectionsignal is due to measurement imperfections during arrayrotation. Because in raw measured data the skin reflec-tion signal is few orders of magnitude stronger thantumor backscattered response, even small changes indistance between antennas and the skin layer, or changes

Figure 14. Experimental imaging and antenna array evaluation results for 8 mm spherical tumorlocated at position P1: x = 10, y = 0, z = �20: (a) 3-D focused image, (b) 2-D image through thehorizontal plane z = �15, (c) channel data at the location of the detected tumor (x = 9, y = �3, z =�15), (d) as Figure 14c but normalized. Images were obtained using background subtraction forarray position AP1. Two-dimensional contour plot shows signal energy on a linear scale,normalized to maximum in the 3-D volume, values below 0.1 rendered as white.


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in a skin thickness will result in skin reflection signal attwo array positions not being the same. This can beobserved in Figure 12 on raw signals (before applyingsubtraction; only small difference in amplitudes for bothskin reflection signals, hardly visible). It is worth adding,that the antenna coupling signal was also well canceledusing rotation subtraction technique.[34] The problem of remaining signals when using

rotation subtraction although clearly visible on individualmultistatic radar signals, is not very critical overall.These clutter signals will usually add incoherently duringfocusing processing. But the tumor response will becombined coherently, as presented in a next section. Inconclusion, we can clearly see the difference betweenideal (not practically useful) background subtraction andthe practically useful rotation subtraction method.

4.3. Comparison of Tumor Response ExtractionMethods: Experimental Imaging Results

[35] In this section we compare the techniques pre-sented above for tumor response extraction and theirimpact on imaging quality. Using the system described insection 2, we performed a number of measurements oftumors in different locations within the breast phantom.[36] We present an example of a 8 mm (diameter)

spherical tumor phantom at two different locations in thebreast phantom. For each location, we compared thethree focused images obtained using the tumor responseextraction methods described above. Additionally, weevaluated the quality of coherent radar operation byexamining the radar signals obtained after focusing atthe focal location where the tumor was detected.

Figure 15. Experimental imaging and antenna array evaluation results for 8 mm spherical tumorlocated at position P1: x = 10, y = 0, z = �20: (a) 3-D focused image, (b) 2-D image through thehorizontal plane z = �15, (c and d) channel data at the location of the detected tumor (x = 9, y = 0,z = �15). Images obtained using background subtraction for array position AP2.


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4.3.1. Results for Tumor Location P1: x = 10, y = 0,z = 220 mm[37] Below we present experimental tumor detection

results for a 8 mm spherical tumor phantom located inthe breast at a position P1: x = 10, y = 0, z = �20 (allpositions quoted in mm). Following the measurementprocedure described in section 4.1, three focused imagesof the detected tumor were created, two using backgroundsubtraction method and one using rotation subtraction.[38] In Figure 14 we present results for the background

subtraction method with an array in position AP1 (asdescribed in section 4.1). Figure 14a shows a three-dimensional (3-D) focused energy image (�3 dBenergy contour), with the tumor detected at position:x = 9, y = �3, z = �15. The skin is also shown in the

image (in gray). There is a slight shift in the location ofthe detected tumor, but otherwise the image is clear andof good quality (there is no clutter present in the image).The small spatial offset is most likely due to nonideallycoherent summation of received pulse (which differslightly in shape and duration).[39] When looking at the horizontal plane where tumor

was detected (z = �15), shown in Figure 14b, we caneasily identify the tumor. Moreover, there is no visibleclutter in this 2-D image, demonstrating the good qualityof our imaging process. Two-dimensional contour plotsshow signal energy on a linear scale, normalized tomaximum in the 3-D volume, values below 0.1 renderedas white.

Figure 16. Experimental imaging and antenna array evaluation results for 8 mm spherical tumorlocated at position P1: x = 10, y = 0, z = �20: (a) 3-D focused image, (b) 2-D image through thehorizontal plane z = �15, (c) channel data at the location of the detected tumor (x = 9, y = �3, z =�21), (d) as Figure 16c but normalized. Images obtained using rotation subtraction for arraypositions AP1 and AP2.


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[40] To further evaluate the quality of our results, weinvestigated the coherence quality of our radar system.As described in section 3.2, using a synthetic postrecep-tion focusing algorithm, by proper time-shift and align-ment of received signals, coherent operation of the radarshould be achieved. Therefore, at the focal locationwhere tumor is located, after ideal focusing and prepro-cessing of all radar channels the received pulses shouldhave the same amplitude and be time-aligned. In realitywe cannot expect all pulses to be the same, because wedo not account for frequency-dependent factors (e.g.antenna characteristics, losses). Also, the tumor scatter-ing is angle-dependent, resulting in slightly differentpulse shapes received by different antennas. However,we do expect the pulses to be well time-aligned.

[41] In Figures 14c and 14d we can see the arraychannel data at the focal point where the tumor wasdetected (x = 9, y = �3, z = �15). Figure 14c shows thesignal with absolute amplitude values, to investigatetime-alignment but also the amplitude spread of thesignals after preprocessing. The figure shows about800 time samples (x axis) of the received channel data(with 16 antennas we record 120 channels, each dis-played as a line parallel to the x axis). The black lines onthe figure represent the extent of the integration windowt used in our modified DAS algorithm. We can observethat pulses have a similar amplitude across all channels,demonstrating their good equalization. In Figure 14d thesame data are presented but with the amplitudes normal-ized to the maximum in each channel, we can see more

Figure 17. Experimental imaging and antenna array evaluation results for 8 mm spherical tumorlocated at position P2: x = 0, y = 30, z = �20: (a) 3-D focused image, (b) 2-D image throughthe horizontal plane z = �15, (c) channel data at the location of the detected tumor (x = 6, y = 24,z = �12), (d) as Figure 17c but normalized. Images obtained using background subtraction forarray position AP1.


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easily the good time-alignment of the received pulses.The slight differences in time-shifts at individual chan-nels are most likely due to the effects discussed above.[42] In Figure 15 we present imaging results and

channel data for the second background measurement,after the array was rotated by 10 degrees to position AP2.We can observe in the focused images (3-D in Figure 15aand 2-D in Figure 15b), that the tumor phantom is againsuccessfully detected. However, due to the array rotationit is now seen at a slightly different location (x = 9, y = 0,z = �15) by the array. The channel data presented inFigures 15c and 15d show again that good coherent radaroperation is achieved.[43] Finally, in Figure 16, results are shown for the case

when rotation subtraction method was used to de-embed

the tumor response from raw measured data. Focusedenergy images (Figures 16a and 16b for 3-D and 2-Dresults, respectively) show that although tumor was againdetected without any problems, detection quality isslightly degraded. Beside the tumor response, there isalso some clutter present in images. The reason for thiscan be seen by looking at channel data in Figures 16cand 16d. Here the received channel data have a muchlarger amplitude spread, compared with results for back-ground subtraction. Moreover, the coherence of pulses isalso not as good.[44] The degraded performance can be explained by

differences in time-shifts for different array channels, dueto the difference in the physical displacement of eachantenna during rotation: when rotating the array around

Figure 18. Experimental imaging and antenna array evaluation results for 8 mm spherical tumorlocated at position P2: x = 0, y = 30, z = �20: (a) 3-D focused image, (b) 2-D image through thehorizontal plane z = �15, (c) channel data at the location of the detected tumor (x = �3, y = 24,z = �12), (d) as Figure 18c but normalized. Images obtained using background subtraction forarray position AP2.


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its center, antennas close to the center of rotationexperience smaller displacement than antennas locatedfurther away. Nevertheless, this rotation subtraction stillprovides performance sufficient to detect small tumors inthe phantom.4.3.2. Results for Tumor Location P2: x = 0, y = 30,z = 220 mm[45] We present here results similar to those shown in

the previous subsection, but for a tumor located at adifferent position: P2: x = 0, y = 30, z = �20. An 8 mmspherical tumor was again used in the measurements.[46] In Figures 17 and 18 we present results for two

background subtraction measurements, with the array inpositionAP1 andAP2. The arraywas rotated by 10 degrees,as before. By looking at focused images in Figures 17a

and 17b for position AP1, and Figures 18a and 18b forposition AP2, we can observe that tumor was in bothcases easily detected. However, the difference in thelocation of the detected tumor is larger than for the firsttumor location, as would be expected (due to largerphysical displacement for the same angle of rotation).The tumor was detected at position: x = 6, y = 24, z =�12for AP1, and position: x = �3, y = 24, z = �12 for AP2.Focused images are clear with no clutter visible. Thisgood performance is again confirmed by the channel data(Figures 17c and 17d and Figures 18c and 18d) at thefocal points where the tumor was detected: signals arerelatively well-aligned and with comparable amplitudes.[47] When using rotation subtraction, the tumor was

also detected, as seen in the focused images Figures 19a

Figure 19. Experimental imaging and antenna array evaluation results for 8 mm spherical tumorlocated at position P2: x = 0, y = 30, z = �20: (a) 3-D focused image, (b) 2-D image through thehorizontal plane z = �15, (c) channel data at the location of the detected tumor (x = �3, y = 27,z = �12), (d) as Figure 19c but normalized. Images obtained using rotation subtraction forarray positions AP1 and AP2.


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and 19b. Although a small amount of clutter exists inthese images, their overall quality is more than satisfac-tory. The channel data in Figures 19c and 19d showagain that the signals vary more in amplitude afterrotation subtraction, compared to the ideal backgroundsubtraction, and they are also less aligned. But the goodquality of focused images shows that this effect ofsmaller coherence is equally translated onto the entirefocusing domain. This is an encouraging conclusion ofour evaluations, since possibly only the rotation subtrac-tion method could be used in realistic breast cancerdetection scenarios.

5. Conclusions and Future Work

[48] In this contribution we presented a microwavesystem for breast cancer detection, employing a confor-mal antenna array. The antenna elements populate theinside of a section of a hemisphere, this being a suitablegeometry for clinical application. A 3-D physical breastphantom was also presented including a geometricallyand electrically realistic skin (this being the dominantsource of clutter).[49] The new investigations presented herein have

focused on the evaluation of two methods of de-embedding tumor response from raw measured data.We compared quality of detection as well as coherencequality of these methods. Results show that both de-embedding techniques provide good quality images andthere is no difficulty in detecting 8 mm spherical tumor atdifferent locations within the breast phantom. Howeverusing a rotational subtraction the coherent radar opera-tion is degraded, compared to the ideal (but unachiev-able) case of background subtraction.[50] This effect is due to the different physical dis-

placement of each antennas after array rotation arisingfrom the different antenna locations in the array. Webelieve that this effect can be lessened to a certain degreeand this is the subject of ongoing research.

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tion, IEEE Trans. Microwave Theory Tech., 52(8), Part 2,

1890–1897.

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deband microwave dielectric properties of normal breast tis-

sue obtained from reduction surgeries, Phys. Med. Biol., 52,

2637–2656.

Leendertz, J., A. Preece, R. Nilavalan, I. J. Craddock, and

R. Benjamin (2003), A liquid phantom medium for micro-

wave breast imaging, paper presented at 6th International

Congress of the European Bioelectromagnetics Association,

Budapest, Hungary, Nov.

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R. Benjamin (2007), Wideband microstrip patch antenna

design for breast cancer tumour detection, IET Microwaves

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for breast cancer detection - Experimental investigation of

simple tumor models, IEEE Trans. Microwave Theory Tech.,

53(11), 3312–3319.

��R. Benjamin, 13 Bellhouse Walk, Kingsweston, Bristol, UK.

I. J. Craddock, M. Klemm, J. Leendertz, and A. Preece,

Centre for Communications Research, Department of Electrical

and Electronic Engineering, University of Bristol, Merchant

Venturers Building, Woodland Road, Bristol BS8 1UB, UK.

([email protected])


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Delay and Sum