Medical Ultrasound Power Measurement Systemusing PVDF Sensor and FPGA Technology

Imamul MuttakinFaculty of Health Science and Biomedical Engineering

Universiti Teknologi MalaysiaUTM Skudai, 81310 Johor, Malaysia

Email: imamul@ieee.orghttp://diagnostics.my/imamul

Eko SupriyantoDepartment of Clinical Science and Engineering

Faculty of Health Science and Biomedical EngineeringUniversiti Teknologi Malaysia

UTM Skudai, 81310 Johor, MalaysiaTelephone: +60-755-35273

Fax: +60-755-36222Email: eko@biomedical.utm.my

Abstract—This work deals with the development of ultrasoundpower measurement system on Field Programmable Gate Array(FPGA) platform. Polyvinylidene Fluoride (PVDF) was employedto sense medical ultrasonic signal. PVDF film’s behavior and itselectro-acoustic model were observed. Signal conditioner circuitwas then described. Next, a robust low-cost casing for PVDFsensor was built, followed by the proposal of the use of digital-system ultrasound processing algorithm. The simulated sensorprovided 2.5 MHz to 8.5 MHz response with output amplitudeof around 4 Vpp. Ultrasound analog circuits, after filtering andamplifying, provided frequency range from 1 MHz until 10 MHzwith -5 V to +5 V voltage head-rooms to offer a widebandmedical ultrasonic acceptance. Frequency from 500 kHz to10 MHz with temperature span from 10 oC to 50 oC andpower range from 1 mW/cm2 up to 10 W/cm2 (with resolution0.05 mW/cm2) had been expected by using the establishedhardware. The test result shows that the platform is able toprocess 10 us ultrasound data with 20 ns time-domain resolutionand 0.4884 mVpp magnitude resolutions. This waveform wasthen displayed in the personal computer’s (PCs) graphical userinterface (GUI) and the calculation result was displayed onliquid crystal display (LCD) via microcontroller. The wholesystem represents a novel design of low-cost ultrasound powermeasurement system with high-precision capability for medicalapplication. This may improve the existing power meters whichhave intensity resolution limitation (at best combination, of allproducts, utilize: 0.25 MHz - 10 MHz frequency coverage; 10 oCto 30 oC working temperature; 0 W/cm2 - 30 W/cm2 power range;20 mW/cm2 resolution), neither having mechanism to handle thetemperature disturbance nor possibility for further data analysis.

I. INTRODUCTION

Ultrasound machine is widely used in industrial and medicalinstitutions. With the purpose of avoiding the unwanted powerexposed on human, ultrasound power meter is employed tomeasure output power of ultrasound machine for diagnostic,therapeutic and non-destructive testing purposes. The existingultrasound power meter, however, is high-cost, low-resolutionand only for specific machine. Radiation balance methodconsists of calculation and calibration complexity while thecalorimetric produces inaccurate result compared to the stan-dard. On the other hand, application of piezoelectric sensorin hydrophone-based measurement requires advancement onprocessing device and technique.

Recent years of widespread availability of equipment stillbe acquainted with poor calibration status of physiotherapytools. Thus, it is beneficial to propose a simple and inexpensivetechnique that can be applicable both at manufacturer anduser side [1]. Overall, the most distinctive problem is lackof quickly applicable measurement methods that also cost-effective at the point of treatment. Commercially available ra-diation force with better than ±10% uncertainty of power leveltends to be expensive and needs expertise to set and operate.Those characteristics render them inappropriate for end user.Therefore, there is a necessity for novel type of measurementdevice which is compact and simple in construction, low-cost,easy and quick use, but still provide a good output of ultrasonicquantity [2] [3].

This project will develop a measurement system for novellow cost ultrasound power meter. This includes investigationof optimized signal processing hardware for ultrasound powermeter and development of signal acquisition hardware tocapture signal from PVDF sensor, and result display panel. Thealgorithm to convert ultrasound signal output to be intensitywill be explored and implemented in FPGA using VerilogHDL (Hardware Description Language).

II. ULTRASOUND POWER MEASUREMENT

There are various procedures to determine the ultrasonicoutput power underwater. They are the radiation force bal-ance technique [4], the use of piezoelectric hydrophones[5], acousto-optic [6], thermo-acoustic [7], calorimetry [8]and ultrasonic power through electro-acoustic efficiency oftransducers [9]. At megahertz frequencies, both calorimetricand radiation pressure methods are satisfactory [10].

A. Calorimetric

The calorimetric type of ultrasound power meter is measur-ing the total beam power and appointing it into fundamentalphysics quantities. The calorimetric measures ultrasound’stotal acoustic power in which the transducer’s complete beamis directed to the target. Thermocouple is the sensor usedin the calorimetric method for the absorption of the heat

produced by the ultrasound beam. The measurement is dueto the temperature as reference set value.

It is well understood by calculating temperature increaseproduced by ultrasound estimated in various condition ofexposure. The rate of heat generation per unit volume is givenby the expression:

Q = 2αITA =αpp∗

2(1)

where α is the ultrasonic amplitude absorption coefficientwhich linearly proportional to frequency; p and p* are theinstantaneous ultrasonic pressure and its complex conjugate.The p multiplied by p* is equal to the amplitude of ultrasonicpressure squared at location in the medium where Q isdetermined and temporal-average quantity is forecasted.

For a given ITA, the maximum temperature increase ∆Tmax,under the assumption that no heat is lost by conduction, con-vection, or any other heat removal processes, is approximately:

∆Tmax =Q∆t

CV(2)

where ∆t is the time duration of exposure; CV is the medium’sheat capacity per unit volume. For longer exposure, heatremoval process should be taken into consideration. Still, forshort exposure times the formula is sufficient. On the otherhand, pyroelectric based measurement concept was stated in[2] [3].

B. Radiation Force Balance

Ultrasound power meters currently available mostly rely onradiation force balance and conventionally described in IECstandard 61161 [11]. It is balancing the radiation force exertedby and ultrasound wave on a reflector with a restoring force.The restoring force may be provided by an electronic feedbacksystem. The main concept is using the reflecting rather thanabsorbing targets. The power meter measures the entire of thebeam ultrasound and the quantity pressure is the main concernparameter to carry out the measurement for the power meter.

However, the balance target’s instability may impose anerror in alignment and complex mathematical calculations.Those circumstances occur because power balance mainlyrelies on mass (m), gravity (g), and speed of sound in medium(c):

P =4m.g.c(t)

1 +R2(3)

where P is the ultrasonic power, ∆m is the deviated weightcaused by the radiation force, g is the gravity (9.78297034m/s2), c(t) is the velocity of ultrasound waves in water as afunction of the water temperature (t) and R is the reflectioncoefficient of the target.

Nevertheless, the effect of the reflection coefficient of thetarget (R) is often neglected. Furthermore, the need to turnultrasound machine “on” and “off” during measurement is lessconvenient. Accurate power measurement results could notbe obtained by simply observing the weight change (because

water continued to evaporate so the weight of the target isdecreasing) in the displayed values as these contained con-tributions from the radiation force and weight loss. Approx-imation and extrapolation methods used become additionaldifficulties. Acoustical imperfection of the target and the non-plane contour of ultrasound field will as well disturb theresult. In addition, humidity, atmospheric pressure and am-bient temperature should also be monitored during each mea-surement. Moreover, exposed weight of target might changeduring operation. Gravity apparently depends on geographicallocation while speed of sound is affected by temperature ofenvironment.

III. METHODOLOGY

The development and characterization of hydrophone powermeter for medical application have been approached formalglobal agreement on recommended method to measure thepower and intensity for the ultrasound machine. The hy-drophone made of piezoelectric polymer sensor is placedinside tank and using water as the propagation medium forultrasound sources. Heat absorbed by the PVDF sensor willbe converted to the electrical energy as additional variablefor intensity measurement. The hydrophone power meter withPVDF sensor can be utilized for wideband frequencies whichis suitable for most medical application. Water or ultrasoundgel are the most suitable material for the medium since itsresponse to ultrasound wave is almost same with human tissue.In addition, absorber is necessary to neglect any reflectedultrasound wave that will influence measurement accuracy[12].

This research output is a FPGA prototype of UltrasoundPower Meter (UPM). The device contains sensors, analogcircuit, digital circuit, personal computer (PC), and embeddedsystem implementation. It is prepared to measure 1 mW/cm2

– 10 W/cm2 power range with 0.05 mW/cm2 of minimumresolution while working frequency is 0.5 MHz up to 10MHz. Two PVDF sensors plus one temperature sensor wouldbe used. Ultrasound machine’s probe which is covered tobe tested is 2.5 cm in radius non-focused. Contact-modemeasurement would use gel as medium; while water wouldbe tanked in immerse-mode.

Moreover, for monitoring purpose, each medical device hasto display data that is user-friendly. To fulfill that need, theGraphical User Interfaces (GUI) shall also be developed onsidehardware instrument. With further help from software applica-tion, there are possibilities to do various analysis. The integritywill make the system has a wide range of acceptance forpractical implementation. An overall top system architecturediagram is shown in Fig. 1.

Figure 1: Top System Architecture Diagram

This ultrasound power measurement system consists of twogeneral parts. First is signal acquisition front-end part andsecond is signal processing back-end platform. Initially, thePVDF film behavior will be observed. Effects of distance,frequency, voltage and temperature on the received signal(voltage) is then analyzed. SPICE model of piezoelectric willbe described after. Using electro-acoustic Leach’s approach,the modeled transducer should cover resonances along medicalultrasound frequencies. Then, the generated transducer designwill be simulated with signal conditioner circuit for completeultrasound analog system. In order to enable PVDF sensorfor ultrasound power measurement, a robust low-cost casingis built. The casing shall be designed to enable optimumcapturing ultrasound power from therapeutic and diagnosticultrasound machine, minimize interference effect and noise aswell as stabilize mechanical construction of the sensor. Math-ematical correlation between ultrasound intensity and sensor’sgenerated voltage will be derived. For signal processing unit,a FPGA based ultrasound processing algorithm is proposed.This platform need to process data from two PVDF sensors.In addition, a GUI then being utilized for further analysis.

Detail at Table I describes the research procedure.

Table I: Work Breakdown

SensorModeling andCharacteriza-

tion

ReceiverCasing Design

AnalogCircuit

Development

DigitalPlatform

Sensormaterial’sbehavior

observation

Specification-based sensor

housing design

Conditionercircuit’stopology

formulation

Systemspecification

Mathematicalformula

derivation

Robust casingsketching

Modularcircuit

simulation

Algorithmdesign

Electro-acoustic

modeling

Designintegration

Transducermodel

simulation

Architecturedevelopment

Sensorassembling

Ultrasoundsystem

simulation

Functionalverification

Data convertersimulation

Interfacing

Circuitassembling

Testing

IV. SYSTEM DESIGN

Figure 2 shows the complete design of the water-tank forpower meter. The circle plate inside the tank is the PVDF layerthat has already been covered by the epoxy resin to preventit from water leakage. This PVDF sensor is attached to theBNC cable that will connect this receiver to the oscilloscope.

Figure 2: Complete Design of PVDF Sensor Water-TankCasing

Cascaded highpass filter and lowpass filter resulting wide-band bandpass filter was designed for low-frequency cut-off 1 MHz and high-frequency cut-off 10 MHz to satisfymedical ultrasound frequencies. Current feedback amplifierwas used with differential topology in order to reject noise,common mode disturbance, and as it might be high frequencyinterferences. Amplification can be set by simply adjustingratio of feedback resistor (RF ) and series resistor (RG). Outputresistor, capacitor, and voltage were prepared to be matchedfor interfacing with ADC in further data acquisition stage.

The Analog-to-Digital interface circuit was designed asfollowing Fig. 3.

Figure 3: ADC Circuit

It needs to be driven by similar differential amplifier, butwith unity gain. Common mode (CM) voltage is produced bythe ADS807E chip to be fed into LMH6551 VCM port in orderto control level of voltage being processed. At the output stage

of driver amplifier, a simple filtering should be established.This is to adjust the frequency and bandwidth which is goingto enter the data converter. It must be taken to note that analogand digital supply or ground should be well-separated so thatthe sensitive signal such as digital clock can be shielded andfree from hazards.

The algorithm of calculation involves FPGA. Signal fromtwo polymer sensors would be converted to digital form andthen buffered into memories. PIC is driving LCD to display theresult. Some arithmetic operations are then performed. Initiallystored constant values are take place as divider operand. Basedon control command, the system passed either maximum or av-erage values which occur to the register. Hence, it would forthbeing accumulated. The control command decides whether thesystem would calculate peak or average in temporal intensity.

Sequence to be processed based on the derived formulabrought to digital method is given in equation below:

ITA =1

N

N∑i=0

V (i, T, x)2

M(f)2.ρ.c(4)

where N is number of sampling, V (i, T, x) is sensor’s outputvoltage dependant on temperature and distance, M(f) issensitivity of sensor, ρ is density of medium, and c is speedof sound in medium. All denumerators are constant value.

V. IMPLEMENTATION RESULT

Hardware platform for ultrasound power measurement sys-tem is shown in Fig. 4.

Figure 4: Ultrasound Power Measurement Hardware Unit

Commercial ultrasound therapy portable GB-818 (GreenUltrasonic Science & Technology Equipment Manufacturing)was used. It has 1 MHz of frequency and 8 intensity levels withboth continuous and pulse mode. For measurement device,Ultrasound Power Meter Model UPM-DT-1 & 10AV (OhmicInstruments Co.) was prepared. Test for pulse signal at lowestlevel of ultrasound therapy on both commercial UPM and thesystem is shown in Fig. 5.

(a) Measurement with Commer-cial Ultrasound Power Meter

(b) Measurement with the System

Figure 5: Test for 1 MHz Ultrasound Pulse

Figure 6: Result on LCD

Zero temperature display at Fig 6 indicates that there areno change in temperature (4T influence which is consideredin characterization) between initial measurement and takendisplay.

The Ultrasound Power Meter UPM-DT-1 & 10AV displayedcalculated average power in term of grams and watts (custommode). Its work principle relies with a relation betweenweight measurement caused by pressure from ultrasound waveand ultrasound power. Radiation force method equalize 14.65grams of weight measurement to 1 Watt of ultrasound power.

P = 14.65W (5)

Therefore, in order to calculate the W/cm2 output, thetotal watts being read from the device should be divided bythe area of transducer. If area of transducer is larger thanmeasurement target in device (metal cone-shaped receiver), thecone’s diameter (8.2 cm) shall be used. GB-818 therapy probehas 4.7 cm of surface diameter. Taking the result, intensityvalue would be:

I =P

A=

P

πd2/4(6)

I =0.02

π(4.7)2/4= 1.153 [mW/cm2]

Meanwhile, the developed measurement system can directlycalculate the intensity without any manual conversion calcula-tion. Given the digitized ultrasound wave data obtained fromA/D converter, equation (4) was empowered. Ultrasound wavefrom therapy probe was displayed in the user interface as inFig. 7.

Figure 7: US Signal 1 MHz Level 1 Therapy Pulse at GUI

Details of unit figure are: voltage/div is 200 mV andtime/div is 40 ms. Time domain data are then calculated withadjusted (4).

ITA =1

N

N∑i=0

V [i]2

M(f)2.ρ.c

Table II below shows the result along with its variables.

Table II: Time Domain Ultrasound Power Calculation

1N

∑N

i=0V [i]2 0.979 mV

M(f) 1E-7 V/PaPressure 9.79E+3 Pa

ρ 1000 kg/m3

c 1480 m/sIntensity 64.7 W/m2

I [mW/cm2] 1.29

From method above, ultrasound intensity is obtained at 1.29mW/cm2 (after adjustment form amplifier gain compensationand sensor’s surface area 1 cm2).

VI. DISCUSSION

Between commercial UPM and the developed system cal-culation there is 12.37% difference. Still, commercial UPMcomes with result in term of power which needs to be adjustedwith probe surface area (whether the area is less or greater thantarget cone has different consequent in calculation mode) toobtain intensity value. On the contrary, the system directlygives intensity value without any need to consider probesurface area nor further user work to calculate the ultrasoundintensity.

The displayed result 1.255 was shown in LCD. The dis-play error of 2.79% caused by microcontroller limitation inacquiring signal from FPGA’s LVDS (Low Voltage Differential

Signaling) where microcontroller has TTL (Transistor Transis-tor Logic) level. It makes processing digits likely to produceslight error, especially when high precision 0.003 (mW/cm2-from ADC resolution) is desired.

If result from commercial UPM -neglecting its everydisadvantage- is referred as standard measurement, the devel-oped system introduces a maximum 18.56% error and 0.035unit of display uncertainty. According to FDA regulations[13] and [14], the measurement result error shall not exceeds20%. It concludes that the developed system has met thespecification.

VII. CONCLUSIONS

The impact of new technologies on medical care and itscosts is enormous. Concerning costs provides a powerfulincentive to look for new types of instrumentation which mayeither be less expensive than present techniques, or allowa breakthrough in accuracy, sensitivity or convenience. Thewhole system represents a novel design of low-cost ultrasoundpower measurement system with high precision capability formedical (diagnostic and therapeutic) applications. This devel-oped FPGA-based ultrasound power meter may improve theexisting power meter which has power resolution limitation,no mechanism to handle the temperature disturbance, andno possibility for further data analysis. Moreover, it shallincrease the safety of measurement using ultrasound machinefor diagnostic and therapeutic purposes.

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