We propose and demonstrate a miniature microwave device capable of detecting wide-band ultrasound without the use of piezo materials. By the nature of the transduction mechanism, the scheme naturally implements orthogonal frequency division multiplexing (OFDM): each device can be assigned its own microwave band, mitigating electrical cross-talk between adjacent sensor elements for future imaging array systems. Furthermore, the entire array could be addressed with a single electrical connection, since addressing is in the frequency, rather than spatial, domain. We anticipate that the convenience afforded by this fully electronic platform will enable its use as a low cost and versatile solution in cases where high sensitivity is required over a large portion of the acoustic spectrum

Alles, EJ

Desjardins, AE

Renaud, CC

Robbins, TS

Seddon, JP

Zhang, EZ

UCL Discovery

Microwave Oscillator Ultrasound ReceiversTom S Robbins∗, James P Seddon∗, Erwin J Alles†,Edward Z Zhang†, Adrien E Desjardins† and Cyril C Renaud∗∗ Department of Electronic and Electrical Engineering,University College LondonLondon, United KingdomEmail: thomas.robbins.15@ucl.ac.uk† Department of Medical Physics and Biomedical Engineering,University College London,London, United KingdomAbstract—We propose and demonstrate a miniature mi-crowave device capable of detecting wide-band ultrasoundwithout the use of piezo materials. By the nature of thetransduction mechanism, the scheme naturally implements or-thogonal frequency division multiplexing (OFDM): each devicecan be assigned its own microwave band, mitigating electricalcross-talk between adjacent sensor elements for future imagingarray systems. Furthermore, the entire array could be addressedwith a single electrical connection, since addressing is in thefrequency, rather than spatial, domain. We anticipate thatthe convenience afforded by this fully electronic platform willenable its use as a low cost and versatile solution in cases wherehigh sensitivity is required over a large portion of the acousticspectrum.Index Terms—ultrasound, microwaves, sensors, RFI. IntroductionUltrasound imaging techniques which use short, broad-band pulses—such as laser generated ultrasound [1] andphotoacoustics [2]—require receivers which are sensitiveacross a wide acoustic spectrum [3]. Currently, piezoce-ramic transducers are often used in these applications:due to the high acoustic impedance of piezoceramics, theyresonate according to their machined dimensions whencoupled to low Young’s modulus media such as biologicaltissue or water. However, this makes them highly sensitiveonly to signals around their resonant frequency. Wide-band receivers such as PVDF hydrophones, which featurepiezo crystals embedded in a polymer matrix, offer betterbroadband performance, although at the expense of peaksensitivity [4]. Furthermore, scaling down elements madefrom these materials in order to increase their acousticangle of acceptance and allow higher resolution fieldsampling causes a corresponding reduction in sensitivity[5], limiting their performance for high contrast imaging.We propose a new type of electronic ultrasound re-ceiver whose operation is inspired by the interferometrictechniques used in optical ultrasound sensors [6], butimplemented in the microwave frequency band insteadThis work was supported by EPSRC Centre for Doctoral Train-ing (CDT) in Integrated Photonic and Electronic Systems (IPES)(EP/L015455/1)of the near infra-red or visible. Unlike capacitive micro-machined ultrasound transducers, it requires no MEMSfabrication processes to produce [7], and the flexibilityoffered by its simple construction makes it possible toincorporate acoustic lensing, as commonly found in piezotransducers. At the same time, it benefits from all theadvantages of a fully electronic sensor, such as low cost,low power and easy multiplexing. Since each sensing ele-ment can carry the ultrasound signal on its own microwaveband, the electrical cross-talk inherent in other arraysis eliminated [8]. Finally, since the sensing mechanismis based on an electromagnetic resonance, the effectivelength of interaction between the ultrasound field and thesensor is scale-invariant, meaning the element size canbe reduced without loss of sensitivity. We present heresome preliminary results from a prototype single-elementdevice developed and manufactured entirely within ourown labs. We demonstrate successful detection of a 5MHzultrasound pulse, estimate the device sensitivity, anddescribe potential areas for improvement.II. Theory and PrincipleThe device comprises a pressure-sensitive microwaveresonator, depicted schematically in Figure 1. It operatesin analogy to the Fabry-Perot optical devices, only insteadof the phase change being produced by an optical cavitywhich changes in length [3], we have a microwave res-onator whose resonant frequency is perturbed by pressurechanges. In either case, the resonance shifts, the corre-sponding amplitude envelope can be detected, and fromthat an ultrasound signal measured.The resonator is excited near its fundamental mode, atfrequency ω0. As the boundary conditions of the resonatorare changed, the amplitude of the signal is modulated. Foran exemplary 1D system, this can be represented byE = A cos(ω0t), (1)where E is the electric field component of the microwaveoscillation, ω0 is its angular frequency, and t is time. Theamplitude A is modulated by the ultrasound signal withfrequency ωu and amplitude Au, givingE = Au cos(ωut) cos(ω0t)= Au cos[(ω0 + ωu)t]/2 +Au cos[(ω0 − ωu)t]/2.(2)I.e., the ultrasound signal can be detected on thesidebands of the high-frequency signal at ω0 + ωu andω0 − ωu. For a broadband ultrasound pulse, the signalwould be represented by the summation of the continuumof frequencies {ωu} with amplitudes given by their Fouriercomponents {Au}.III. MethodsWe aimed to create a device with a sub-3GHz resonancein as small a footprint as possible, allowing it to beoperated using widely available microwave sources whilsthaving a small acoustic aperture. A prototype single-element device with a resonance around 1.8GHz wasdesigned and simulated using the CST Microwave Studiosoftware package. After fabrication—details of which areomitted here due to intellectual property constraints—the resonance was characterised using a network analyser(EC5071C ENA Series Network Analyser, Aligent Tech-nologies) to measure the transmission (S21) and reflection(S11) spectra. The position of the resonance was found tobe within reasonable error of the FDTD simulation (within2%). After this initial characterisation, the device was setup in the configuration illustrated by Figure 2. To enhancethe sensitivity, a homodyne detector was implementedusing a power splitter (ANRITSU K241B), phase shifter(ARRA 9426R) and frequency mixer (Mini-Circuits ZX05-C42). The output from this was passed through a low-pass filter (passband from DC to 80MHz) and then anMicrowave resonatorOscillator fundamentalSide bands:ultrasound signalFrequencyUltrasound signalFig. 1. The microwave resonator, represented by a box, is perturbedby an ultrasound signal. The fundamental mode of the resonator,represented by the blue standing wave in the box and the largecentral peak in the spectrum, is modulated by the changing boundaryconditions of the resonator, resulting in the depicted sidebands whichcarry the ultrasound signal.ultrasound receiver with a high gain amplifier (Olympus5072PR). We then coupled a piezo ultrasound transducer(Harisonic 13-0504-R) to the top side of the device. Ablock of pure gel wax [9] was used as a spacer betweenthe device and the transducer. The piezo transducer wasdriven by a train of pulses at its resonant frequency(5MHz) from an ultrasound pulse generator and receiver(Olympus 5077PR).A high-frequency microwave source (Keysight N9310ARF Signal Generator) was tuned to near the quadrature(point of greatest slope) of the device’s electromagneticresonance and connected to the system. This tuning wasachieved by running the signal from the device through apower detector (Pasternack PE8014) and taking voltagereadings whilst changing the frequency of the RF source.To detect ultrasound, an oscilloscope (Agilent Technolo-gies DSO-X 3024A) was externally triggered by the ultra-sound pulse generator and traces for both the transducerand receiver were displayed. Since the ultrasound pulsereflects off the sensor back to the transducer, we observetwo pulses generated by the pulse transceiver: an initialpulse representing the ultrasound being generated at thetransducer interface at time t = 0; and a second pulseafter the time taken to make a round trip, t = T . Hencethe response of the device can be seen as the pulse onthe second trace half-way between the two pulses fromthe transceiver, at time t = T/2 (see Figure 3). Pressingthe transducer closer to the receiver and deforming thegel wax block changes the ultrasound path length T . Asa final check, we confirmed that even when displacingthe transducer in this way, the signal at the receiver didindeed remain at T/2. In this way we eliminated anydoubt regarding the origin of the electrical signal fromthe receiver.IV. Results and DiscussionThe transmission spectrum of the device is shown inFigure 4. The device simulations predict a resonance Q-factor of 8000. However, in practice, manufacturing defectsand material losses result in a measured Q-factor of 40.The homodyne configuration shown in Figure 2 enhancesthe sensitivity of the system, although at the expense ofincreased signal loss.From a semi-empirical model, we determined the sen-sitivity of the resonance to perturbations of its boundaryconditions to be 0.57MHz/µm. By varying the frequencyof the RF source and measuring the output voltage, wefound the system had a peak sensitivity of 0.6mV/MHzat the point of largest gradient near the microwaveresonance of the device. Using a model derived from[3], the deformation due to ultrasound was calculated as0.2 µm/MPa. Multiplying these three figures gives an esti-mate for the device sensitivity: 0.07mV/MPa. The noiseof the system, including the 60 dB amplifier, was measuredexperimentally as 390mV/√200 MHz peak-to-peak noise,RF Source Pulse generator/receiverPiezo US transducerMixerPhase shifter Low-pass filterOscilloscopeResonatorAmplifierFig. 2. Block diagram of the system for testing the ultrasound receivertherefore we estimate the noise equivalent pressure (NEP)detectable by our system to be 5.7 kPa√200 MHz.In practice, we were able to observe 5MHz pulsesgenerated by the piezo transducer driven at 400Vp−pat an SNR of about 2 (over a 200MHz measurementbandwidth, see Figure 5). Some degradation of the signalcan be accounted for by the large losses introduced bythe homodyne system, and also in acoustic losses notaccounted for in our model. Sources of noise in the systeminclude background electromagnetic interference, phasenoise from the RF source, and noise introduced by theamplifier and mixer.Integrating the sensing system onto a single PCB wouldreduce noise and losses by removing most of the RF coaxialconnections and shortening transmission lines. It wouldalso significantly reduce the cost and size of the system.Once integrated in such a way, it would be possible toproduce a compact array of sensing elements, with minimalelectrical cross-talk, and a single electrical lead and read-out pathway. In the present design, the sensing elementfootprint is about 13mm2. However, with a ten-foldincrease in microwave frequency (still within the widelyattainable frequency band of sub-20GHz) and hence ten-fold reduction in length-scale, the footprint reduces to un-der 1mm2. Further improvements to the resonator designcould bring this value even lower. Finally, we note that thefabrication method of the receivers could potentially beadapted for non-planar or even flexible arrays or elements,enabling various acoustic lensing possibilities.V. ConclusionAn EM-resonance-based microwave ultrasound detectorcould enable wide-band ultrasound detection at sensi-tivities not possible with existing PVDF sensors. For afuture imaging system, the possibility of assigning eachsensing element its own RF band could result in reducedelectrical cross-talk between sensing elements. With im-provements, such as replacing lab-bench equipment withintegrated circuits, noise equivalent pressure ratings below400Pa√MHz could be realised.AcknowledgementsTSR thanks Efthymios Maneas for supplying the gel-wax material, and James Guggenheim and Paul Beard forproviding helpful discussions and encouragement. He alsothanks Martin Scott and John Langdon for their technicalcontributions.References[1] R. J. Colchester, C. A. Mosse, D. S. Bhachu, J. C.Bear, C. J. Carmalt, I. P. Parkin, B. E. Treeby,I. Papakonstantinou, and A. E. Desjardins, “Laser-generated ultrasound with optical fibres using func-tionalised carbon nanotube composite coatings,” Ap-plied Physics Letters, vol. 104, no. 17, p. 173 502,Apr. 2014, issn: 0003-6951, 1077-3118. doi: 10.1063/1.4873678. [Online]. Available: http://scitation.aip.org/content/aip/journal/apl/104/17/10.1063/1.4873678(visited on 11/20/2015).[2] P. Beard, “Biomedical photoacoustic imaging,” en,Interface Focus, vol. 1, no. 4, pp. 602–631, Aug.2011, issn: 2042-8898, 2042-8901. doi: 10 . 1098 /rsfs . 2011 . 0028. [Online]. Available: http : / / rsfs .royalsocietypublishing.org/content/1/4/602 (visitedon 12/16/2015).[3] P. C. Beard and T. N. Mills, “Extrinsic optical-fiberultrasound sensor using a thin polymer film as alow-finesse Fabry–Perot interferometer,” EN, AppliedOptics, vol. 35, no. 4, pp. 663–675, Feb. 1996, issn:2155-3165. doi: 10 . 1364 /AO . 35 . 000663. [Online].Available: https://www.osapublishing.org/abstract.cfm?uri=ao-35-4-663 (visited on 12/15/2017).[4] M. D. Sherar and F. S. Foster, “The design andfabrication of high frequency poly(vinylidene fluoride)transducers,” Ultrasonic Imaging, vol. 11, no. 2,pp. 75–94, Apr. 1989, issn: 0161-7346. doi: 10.1016/0161 - 7346(89 ) 90001 - 1. [Online]. Available: http :/ / www . sciencedirect . com / science / article / pii /0161734689900011 (visited on 02/26/2018).[5] J. A. Guggenheim, J. Li, T. J. Allen, R. J. Colch-ester, S. Noimark, O. Ogunlade, I. P. Parkin, I.Papakonstantinou, A. E. Desjardins, E. Z. Zhang, andP. C. Beard, “Ultrasensitive plano-concave opticalmicroresonators for ultrasound sensing,” En, NaturePhotonics, vol. 11, no. 11, p. 714, Oct. 2017, issn:1749-4893. doi: 10.1038/s41566-017-0027-x. [Online].Available: https://www.nature.com/articles/s41566-017-0027-x (visited on 11/03/2017).[6] P. C. Beard and T. N. Mills, “Miniature optical fibreultrasonic hydrophone using a Fabry-Perot polymerfilm interferometer,” Electronics Letters, vol. 33,no. 9, pp. 801–803, Apr. 1997, issn: 0013-5194. doi:10.1049/el:19970545.[7] A. Caronti, G. Caliano, R. Carotenuto, A. Savoia, M.Pappalardo, E. Cianci, and V. Foglietti, “Capacitivemicromachined ultrasonic transducer (CMUT) ar-rays for medical imaging,” Microelectronics Journal,vol. 37, no. 8, pp. 770–777, Aug. 2006, issn: 0026-2692.doi: 10.1016/j.mejo.2005.10.012. [Online]. Available:http://www.sciencedirect.com/science/article/pii/S0026269205003812 (visited on 04/09/2018).[8] S. Zhou and J. A. Hossack, “Reducing inter-elementacoustic crosstalk in capacitive micromachined ultra-sound transducers,” IEEE Transactions on Ultrason-ics, Ferroelectrics, and Frequency Control, vol. 54,no. 6, pp. 1217–1228, Jun. 2007, issn: 0885-3010. doi:10.1109/TUFFC.2007.375.Piezo US transducerGel waxblockReceivert = 0Pulse sentt = T/2Pulse detectedt = TReflected pulsereturnsFig. 3. To demonstrate the device, the signals from the piezotransducer and the receiver are separated in the time domain. Thered trace represents the signal from the transducer, and the bluetrace the signal from the receiver1.5 1.6 1.7 1.8 1.9 2.0Frequency [GHz]252015105S21 [dB]Fig. 4. The transmission spectrum (S21) of the device, showing aresonance at 1.8GHzFig. 5. Oscilloscope trace of the received ultrasound signal (yellow)and the pulse/echo response from the transducer (green)[9] E. Maneas, W. Xia, O. Ogunlade, M. Fonseca, D. I.Nikitichev, A. L. David, S. J. West, S. Ourselin,J. C. Hebden, T. Vercauteren, and A. E. Desjardins,“Gel wax-based tissue-mimicking phantoms for mul-tispectral photoacoustic imaging,” EN, BiomedicalOptics Express, vol. 9, no. 3, pp. 1151–1163, Mar.2018, issn: 2156-7085. doi: 10.1364/BOE.9.001151.[Online]. Available: https://www.osapublishing.org/boe / abstract . cfm?uri=boe - 9 - 3 - 1151 (visited on10/04/2018).

Microwave Oscillator Ultrasound Receivers

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Microwave Oscillator Ultrasound Receivers

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