FDMAS has been successfully used in microwave imaging for breast cancer detection. FDMAS gained its popularity due to its capability to produce results faster than any other adaptive beamforming technique such as minimum variance (MV) which requires higher computational complexity. The average computational time for single point spread function (PSF) at 40 mm depth for FDMAS is 87 times faster than MV. The new beamforming technique has been tested on PSF and cyst phantoms experimentally with the ultrasound array research platform version 2 (UARP II) using a 3-8 MHz 128 element clinical transducer. FDMAS is able to improve both imaging contrast and spatial resolution as compared to DAS. The wire phantom main lobes lateral resolution improved in FDMAS by 40.4% with square pulse excitation signal when compared to DAS. Meanwhile the contrast ratio (CR) obtained for an anechoic cyst located at 15 mm depth for PWI with DAS and FDMAS are -6.2 dB and -14.9 dB respectively. The ability to reduce noise from off axis with auto-correlation operation in FDMAS pave the way to display the B-mode image with high dynamic range. However, the contrast to noise ratio (CNR) measured at same cyst location for FDMAS give less reading compared to DAS. Nevertheless, this drawback can be compensated by applying compound plane wave imaging (CPWI) technique on FDMAS. In overall the new FDMAS beamforming technique outperforms DAS in laboratory experiments by narrowing its main lobes and increases the image contrast without sacrificing its frame rates

Alomari, Z

Cowell, DMJ

Freear, S

Harput, S

Moubark, AM

White Rose Research Online

This is a repository copy of Enhancement of contrast and resolution of B-mode plane wave imaging (PWI) with non-linear filtered delay multiply and sum (FDMAS) beamforming.White Rose Research Online URL for this paper:http://eprints.whiterose.ac.uk/110290/Version: Accepted VersionProceedings Paper:Moubark, AM, Alomari, Z, Harput, S et al. (2 more authors) (2016) Enhancement of contrast and resolution of B-mode plane wave imaging (PWI) with non-linear filtered delay multiply and sum (FDMAS) beamforming. In: IEEE International Ultrasonics Symposium, IUS. 2016 International Ultrasonics Symposium, 18-21 Sep 2016, Tours, France. IEEE . ISBN 9781467398978 https://doi.org/10.1109/ULTSYM.2016.7728678(c) 2016, IEEE. This is an author produced version of a paper published in IEEE International Ultrasonics Symposium, IUS . Personal use of this material is permitted. Permission from IEEE must be obtained for all other users, including reprinting/ republishing this material for advertising or promotional purposes, creating new collective works for resale or redistribution to servers or lists, or reuse of any copyrighted components of this work in other works. Uploaded in accordance with the publisher’s self-archiving policy.eprints@whiterose.ac.ukhttps://eprints.whiterose.ac.uk/Reuse Unless indicated otherwise, fulltext items are protected by copyright with all rights reserved. The copyright exception in section 29 of the Copyright, Designs and Patents Act 1988 allows the making of a single copy solely for the purpose of non-commercial research or private study within the limits of fair dealing. The publisher or other rights-holder may allow further reproduction and re-use of this version - refer to the White Rose Research Online record for this item. Where records identify the publisher as the copyright holder, users can verify any specific terms of use on the publisher’s website. Takedown If you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request. Enhancement of Contrast and Resolution ofB-Mode Plane Wave Imaging (PWI) withNon-Linear Filtered Delay Multiply and Sum(FDMAS) BeamformingAsraf Mohamed Moubark, Zainab Alomari, Sevan Harput, David M. J Cowell and Steven FreearUltrasound Group, School of Electronic and Electrical Engineering, University of Leeds, UK.E-mail: elamm@leeds.ac.uk and S.Freear@leeds.ac.ukAbstract—FDMAS has been successfully used in microwaveimaging for breast cancer detection. FDMAS gained its pop-ularity due to its capability to produce results faster thanany other adaptive beamforming technique such as minimumvariance (MV) which requires higher computational complexity.The average computational time for single point spread function(PSF) at 40 mm depth for FDMAS is 87 times faster than MV.The new beamforming technique has been tested on PSF andcyst phantoms experimentally with the ultrasound array researchplatform version 2 (UARP II) using a 3-8 MHz 128 elementclinical transducer. FDMAS is able to improve both imagingcontrast and spatial resolution as compared to DAS. The wirephantom main lobes lateral resolution improved in FDMAS by40.4% with square pulse excitation signal when compared toDAS. Meanwhile the contrast ratio (CR) obtained for an anechoiccyst located at 15 mm depth for PWI with DAS and FDMAS are-6.2 dB and -14.9 dB respectively. The ability to reduce noise fromoff axis with auto-correlation operation in FDMAS pave the wayto display the B-mode image with high dynamic range. However,the contrast to noise ratio (CNR) measured at same cyst locationfor FDMAS give less reading compared to DAS. Nevertheless,this drawback can be compensated by applying compound planewave imaging (CPWI) technique on FDMAS. In overall the newFDMAS beamforming technique outperforms DAS in laboratoryexperiments by narrowing its main lobes and increases the imagecontrast without sacrificing its frame rates.I. INTRODUCTIONBeamforming is a process of generating time delay that willbe applied to a set of array at each time during transmissionand reception. This is to focus and steer the beam patternat intended region of interest (ROI). The most commonlyused beamforming technique in medical ultrasound imaging isDAS technique. However, the final B-Mode image producedwith DAS technique highly influenced by off-axis noise. Toreduce the noise, apodization technique have been applied attransmitting and receiving stages. Even though this techniqueable to reduce the noise and side lobes but as trade off itincrease the main lobes size which directly reduce the imageresolution. Thus to overcome this issue, several new types ofadaptive beamforming methods such as minimum variance andeigenvector minimum variance has been introduced [1]. Thesenew beam formers able to dynamically change the receiveaperture weights based on the received signals and able toincrease the resolution and reduce side lobes but at expenses ofhigh computational complexity and time. In order to improvethe image resolution and contrast without increasing much thecomputational time, a new types of beamforming known asFDMAS which originally conceived in RADAR microwavesystem for breast cancer detection has been applied to PWIand CPWI [2]. Initial works on FDMAS carried out by [3]on linear array imaging with 2-cycle sinusoidal burst showspromising results. In this paper, we have explore the potentialof this new non-linear beamforming technique with PWI andCPWI.II. FILTERED DELAY MULTIPLY AND SUM BEAMFORMINGInitial process in FDMAS is same as DAS where each RFsignal received at ith element, Si(t) will be aligned accordingto a set of calculated delay according to following equation:τi(x, z) =zcosθ + xsinθ + W2sinθc+√z2 + (xi − x)2c(1)Where τi(x, z) is time required for the signal to reach fieldpoint located at axial and lateral location x and z respectivelyand return to ith element on the transducer, W is physicalwidth of the transducer, c is speed of sound on the mediumand finally xi is distance between ith element and centre of thetransducer, W/2, and is steering angle between transmittedsignal and face of the transducer for each plane wave. Noticethat θ will be zero for PWI. Instead of calculating timedelay for each field point separately which will cause highcomputational time, a set of time delay vector can be computedin lateral direction, zkl where k represent starting point ofimaging and l is the end point or depth of the image. Thecomputed time delay vector added to the received RF signal,Si(t) is known as aligned RF signal, Vi can be represented byfollowing equation:Vi =Si(t− τi(x, zkl)) (2)i = 1, 2, ..., 128To form single B-Mode image line, a set of N number Vineeded. However, the summation process will not take placeafter the aligned process as in DAS beamforming techniquebut each frame or a set of aligned will gone through processsimilar to auto-correlation to form each image line as givenby equation (3)RFDMAS =N−1∑iN∑msgn{Vi(t)Vm(t)} ×√|Vi(t)Vm(t)| (3)Multiplying two RF signal with same frequency will eventu-ally produce harmonics and DC components. Thus a bandpassfilter applied on RFDMAS to extract its second harmonics.More details on mathematical operation explanation on FD-MAS can be found on [3].III. EXPERIMENTSIn order to study the effectiveness of FDMAS beamformingtechnique on PWI and CPWI, several laboratory experimentshave been carried out on wire and multipurpose phantomwith UARP II [4] [5]. Our first experiment was carried outon 120µm wire phantom located from 20 mm until 60 mmdepth with 10 mm spacing between them as shown in Fig. 1.Our second experiment have been conducted on computerizedimaging reference system (CIRS) multipurpose, multi tissueultrasound phantom on hypoechoic cyst phantom.-10 0 10Lateral Distance [mm]102030405060Depth [mm]-40-35-30-25-20-15-10-50(a)                                           (b)Fig. 1. (a) Wire phantom scanned with clinical transducer inside degas waterand (b) its B-Mode model.A broad band square pulse with 50 ns duration have beenemployed on both experiments. A single B-Mode image wasformed by compounding 1, 3, 5 and 7 PWI steered from −15◦to +15◦ degree with increment of 5◦. The details for bothexperiments parameters are given in Table 1.IV. PERFORMANCE EVALUATIONIn order to evaluate the final B-Mode images qualitiesformed with DAS and FDMAS beamforming techniques,several key performance indicator haven used. The main lobesresolution of PSF was measured on wire phantom located at40 mm depth with Full width half maximum (FWHM), -6dB. While the image CR and CNR of hypoechoic cyst wascomputed on CIRS phantom located at 15 mm depth. TheTABLE IEXPERIMENTS PARAMETERSProperties Wire Phantom CIRSSpeed of Sound, m/s 1480 1520Medium Attenuation, dB/MHz/cm 0.22 0.5Transducer centre frequency, MHz 5Sampling Frequency, Tx/Rx, MHz 160/80No of Elements 128Bandwidth, % 57Excitation signals Square PulseSteering Angles -15,-10,-5,0,+5,+10,+15CR is used to express the detectability of the object contrastbetween ROI inside the cyst and its background. While CNRis used the measure the cyst contrast with speckle or noisevariation inside and outside of the cyst [6]. High CNR valuemeans cyst can be visualize easily and the acoustic noisestandard deviation is small or more uniform. Both CR andCNR equation are given by [6]CR(dB) = 20log10(µcystµback) (4)CNR(dB) = 20log10(|µcyst − µBack|√(σcyst2 + σBack2)) (5)Where µcyst and µBack are means of image intensitiesinside and outside of the cyst respectively while σcyst2 andσBack2 are their variances. CR and CNR were calculatedon the cysts by creating two different regions with the samedimensions. The first region is inside the cyst while the otherregion is located outside the cyst at the same depth. This is toensure that the attenuation caused by frequency doesn’t affectthe measurements.V. RESULTS AND DISCUSSIONThe B-Mode images of PWI on wire phantom formed withDAS and FDMAS beamforming techniques using square pulseexcitation signals is shown in Fig. 2 at 40 dB dynamic range.(a) DAS-10 0 10Lateral Distance [mm]102030405060Depth [mm](b) FDMAS-10 0 10Lateral Distance [mm]102030405060Depth [mm]-40-35-30-25-20-15-10-50Fig. 2. Wire phantom B-Mode image for (a) DAS and (b) FDMAS.It can be seen from Fig. 2(b) that the FDMAS significantlyreduce more noises in lateral direction compared to axialdirection. The lateral and axial normalized amplitude profilespresented in Fig. 3 for the wire phantoms also prove that thereduction of noises in lateral direction is more than in axialdirection.39.2 39.4 39.6 39.8 40 40.2 40.4 40.6 40.8Axial Direction [mm]-60-50-40-30-20-100Normalized Amplitude [dB](a) DASFDMAS-5 -4 -3 -2 -1 0 1 2 3 4 5Lateral Direction [mm]-60-50-40-30-20-100Normalized Amplitude [dB](b) Fig. 3. Comparison of (a) axial and (b) lateral resolution of a wire phantomlocated at 40 mm depth with DAS and FDMAS beamforming techniques.The spatial resolution at axial and lateral direction measuredusing function created by [7] at -6 dB main lobe width onwire phantom inside degas water at 40 mm depth. The resultsis presented in graphical form in Fig. 4. At 0◦ steering angle,the lateral resolution measured at main lobes for DAS andFDMAS beamforming techniques employing pulse excitationsignal are 0.49 mm and 0.41 mm respectively. The newnon-linear beamforming technique shows 16.3% improvementwhen compared to traditional linear beamforming technique.The increment in the resolution getting better as the number ofcompounding increases from one to seven angles. The lateralresolution measured with CPWI of seven angles from −15◦to +15◦ with increment of 5◦ with DAS beamforming tech-nique is 0.42 mm and 0.25 mm with FDMAS beamformingtechnique. This is 40.4% increment which is more than doublecompare to PWI.The improvement on lateral resolution is expected sincethe auto-correlation process in FDMAS take place on thatlateral direction and not in axial direction. The auto-correlationprocess produces maximum values when the same patternsignals detected and it reduces any unwanted side lobesor noises on the lateral direction. Meanwhile compounding1 2 3 4 5 6 70.20.30.40.50.6Lateral Resolution [mm](a) DASFDMAS1 2 3 4 5 6 7Number of Compounding Angles, N0.280.30.320.340.36Axial Resolution [mm](b) Fig. 4. (a) Lateral resolution and (b) axial resolution for wire phantom at 40mm depth beam formed with DAS and FDMAS techniques and compoundedwith different number of angles.process enhances the lateral resolution with FDMAS techniqueby averaging and cancelling the noises that present in lateraldirection [8]–[10]. However, the same spatial improvementcant be seen on axial direction. As for axial direction, there isno significant change between DAS and FDMAS beamformingtechniques with PWI and CPWI, N=3. The changes or minorimprovement in axial resolution with FDMAS only starting tobe visible at CPWI, N=5 and N=7. At N=5, the improvementin axial direction with FDMAS is 8.3% while with N=7the improvement is 19.4%. Complete comparison on axialresolution between DAS and FDMAS is shown in Fig. 4.The B-Mode images of PWI on 4.5 mm and 1.3 mmdiameter CIRS hypoechoic cyst at 15 mm depth is shown inFig. 5. The PWI considered as the worst case scenario producevery low quality B-mode images with DAS as shown in Fig.5(a) yet FDMAS able to identify or map the location of 1.3mm cyst as shown in Fig. 5 (b).(a) DAS, N=1-10 0 101315171921Depth [mm](b) FDMAS, N=1-10 0 101315171921(c) DAS, N=7-10 0 10Lateral distance [mm]1315171921Depth [mm](d) FDMAS, N=7-10 0 10Lateral distance [mm]1315171921  Fig. 5. B-Mode image of two hypoechoic cyst with (a) DAS, PWI, (b)FDMAS, PWI, (c) DAS, CPWI (N=7) and (d) FDMAS, CPWI (N=7) locatedat 15 mm depth with diameter of 4.5 mm and 1.3 mm displayed at 40 dBdynamic range.The CR for PWI with DAS beamforming technique is-6.2 dB while with FDMAS is -14.9 dB. Increasing thecompounding angles from one to seven eventually increasethe CR of the cystic region for DAS and FDMAS. With 7compounding angles, the CR of DAS and FDMAS improves to-10.6 dB and -15.27 dB, respectively. However, the CR valuesdifference between the two beamforming techniques decreasesas the number of compounding angles increase from one toseven. In PWI, the CR improvement is 140.3% while withCPWI the improvement is only 44%. The reduction in CRgap between DAS and FDMAS can be related to the noisecancelation through increasing number compounding anglesin DAS. Complete CR for both beamforming technique withdifferent number of compounding angles is given in Fig. 6.1 2 3 4 5 6 7-20-15-10-5CR [dB](a) DASFDMAS1 2 3 4 5 6 7Number of Compounding Angles, N-2024CNR [dB](b) Fig. 6. (a) CR and (b) CNR for 4.5 mm diameter hypoechoic cyst locatedat 15 mm depth.Another performance indicator used to evaluate FDMASbeamforming technique is CNR. The CNR values measuredfor PWI using square pulse as excitation signal is -1.3 dB and-1.2 dB for DAS and FDMAS respectively. AS the numberof compounding angles increases into seven, the CNR valuesbecome 2.3 dB and 1.53 dB for DAS and FDMAS. The CNRvalues for FDMAS generally lower when compared to but itkeep improving as number of compounding angels increasing.This is because, the auto-correlation process eliminate orreduce the small speckle or noise values that present outsidethe cyst. The area outside the cyst becomes non-uniform(high number of black and white spots) and causes highfluctuation between the speckle constructive and destructiveregion Thus it introduce extra black spots as can be seenon Fig. 5(b) and 5(d) compared to Fig. 5(a) and 5(c). Onthe other hand, DAS beamforming with CPWI reduce thespeckle variation and produce more uniform region outside thecyst. As the compounding angles increases, the CNR valuesfor DAS and FDMAS improves gradually. Complete CNRfor both beamforming techniques with different number ofcompounding angles using square pulse excitation signal isgiven in Fig. 6.VI. CONCLUSIONA new type of non-liner beamforming technique known asFDMAS has been applied to PWI and CPWI using squarepulse excitation signals. The performance parameters usedto measure the B-Mode image performance shows that FD-MAS produced better lateral resolution without any significantchanges or small improvement on axial resolution. The imagecontrast also improve due to speckle noise reduction. Com-pounding technique significantly increase the spatial resolutionon lateral direction and improve the image contrast. However,the CNR values are lower with FDMAS compared with DAS.This however can be overcome by CPWI and displaying theFDMAS at high dynamic range.REFERENCES[1] J.-F. Synnevag, A. Austeng, and S. Holm, “Benefits of minimum-variance beamforming in medical ultrasound imaging,” IEEE transac-tions on ultrasonics, ferroelectrics, and frequency control, vol. 56, no. 9,pp. 1868–1879, 2009.[2] H. B. Lim, N. T. T. Nhung, E.-P. Li, and N. D. Thang, “Confocalmicrowave imaging for breast cancer detection: Delay-multiply-and-sum image reconstruction algorithm,” IEEE Transactions on BiomedicalEngineering, vol. 55, no. 6, pp. 1697–1704, 2008.[3] G. Matrone, A. S. Savoia, G. Caliano, and G. 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Enhancement of contrast and resolution of B-mode plane wave imaging (PWI) with non-linear filtered delay multiply and sum (FDMAS) beamforming

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Enhancement of contrast and resolution of B-mode plane wave imaging (PWI) with non-linear filtered delay multiply and sum (FDMAS) beamforming

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