ULTRASOUND SIGNAL PROCESSING FOR AUTOMATED MEDICALMONITORING

Cara A CampbellCollege of William and MaryAdvisor: Dr. Mark Hinders

Abstract

Ultrasound is a safe, non-invasive technique that can be used in various medical ap-plications to improve patient quality of life. In this paper we discuss the current statusof two ultrasound-based medical projects. The techniques we are developing can in thefuture be applied to medical monitoring in rural or space settings. Urinary incontinenceaffects a large percentage of the population. We are developing a wearable ultrasounddevice that can measure bladder fullness and alert the wearer or caretaker of the need tovoid. We have collected ultrasound data from a phantom bladder and are planning in thenear future to collect data from human subjects. The data will be used to develop algo-rithms that can detect bladder fullness in real-time. Increased embolic load to the brainis a concern in high-altitude flight, deep water diving, and open heart surgery. Acousticradiation force can be used to push emboli out of the blood flow path. We are currentlydeveloping simulations to accurately model acoustic radiation force on spherical emboliin a viscous fluid. These models will be used to help establish an experimental techniquefor efficient emboli removal.

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Introduction

Automated medical monitoring using ul-trasound signal processing can be used in sit-uations where a caretaker or doctor is un-available. My research involves creating sig-nal processing algorithms to make automatedmonitoring possible. In this paper I will dis-cuss the need for automated medical moni-toring and then I will describe two projectsthat I currently am working on: removal ofmicroemboli from the blood stream and blad-der fullness monitoring.Telemedicine:

Telemedicine systems around the world areimproving patient care and virtual accessto medical specialists. The Indian SpaceResearch Organization has connected nearly80 rural hospitals to 22 specialty hospitalsthrough satellites, allowing more than 25,000patients in rural areas to receive teleconsulta-tions [1]. In rural Germany, specialists haveused videoconference systems in 15 minuteteleconsultations to examine 153 patients andtheir CT scans [2]. In the United States sur-geons have used telemedicine to allow distantexpert surgeons to view ultrasound imagesand advise on-site surgeons in real time [3].These results have shown that telemedicineis a promising method in evaluating patienthealth and that patients are open to the ideaof telemedicine.Space Medicine:

NASA regards medical illness and traumaas high risk for potential impact on missionand crew, especially for extended durationmissions to the moon, Mars, and beyond [4].Ultrasound is the only medical imaging tech-nology currently available for medical diagno-sis in space and there are no plans to imple-ment other medical imaging devices in space[5] because ultrasound has advantages overother imaging techniques such as X-ray or

MRI. It is not affected by the space environ-ment and does not expose the crew to radi-ation, scans can easily be performed by non-physicians, and modern ultrasound systemsare small, very light-weight, and real time.Figure 1 shows a multipurpose HDI-5000Ultrasound System (ATL/Philips, Bothwell,WA) which has been used aboard the ISS toexamine the ocular system, shoulder, and ab-domen. Crew members were given approxi-mately 3-6 hours of training with the ultra-sound apparatus. The crew member perform-ing the ultrasound was guided through theprocedure remotely by a physician at missioncontrol, with only a 2 second delay in commu-nication. In all of the trials it was found thatnon-physician crew members were capable ofacquiring high quality ultrasound images thatcould be used for diagnosis [6], [7].

Figure 1: Commander Gennady Palalka per-forms an ultrasound scan of the shoulder ofMike Fincke aboard the ISS using the HDI-5000 Ultrasound system [6]

Although ultrasound has been demon-strated in space, remote guiding by an ul-trasound expert may not work beyond lowearth orbit. For example, it can take 4 to20 minutes for communication to travel fromthe Earth to Mars (depending on where theplanets are in their orbit). Real-time remote

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guidance of medical ultrasound is not realisticat these distances. As manned space explo-ration advances beyond low earth orbit it willbe necessary to develop much better systemsto monitor crew health. Ultrasound systemsmust be developed that do not require an ex-pert to guide imaging. Our research focuseson computer processing of ultrasound signalsto create artificial intelligence algorithms thatcan interpret data without human experts.By processing ultrasound signals in this waythe computer can yield the relevant medicalinformation. We are currently working on ul-trasound signal processing for various appli-cations, as discussed below.

Bladder Monitor

The goal of this project is to develop anultrasound device that can measure bladderfullness, based on a NASA Langley tech-nology [8] but updated with new generationelectronics and signal processing algorithms .The bladder distention monitor uses a singlebroad beam ultrasonic transducer that sendsand receives signals. The monitor will be de-signed to be worn underneath the clothingagainst the skin and is non-invasive. The sig-nals are processed and related to bladder dis-tention via optimized real-time algorithms.The monitor can be worn by those sufferingfrom urinary incontinence (UI) and will use adistinct tone to warn the wearer of the needto urinate10-15 minutes before bladder con-traction.

UI affects approximately 25 million adultsin the United States [9]. It is experienced byaround 53 percent of homebound adults age65 and older and is often a key factor in tran-sitioning to long-term care [10]. UI can alsobe a deciding factor in the level independenceavailable to mentally handicapped individu-als. Approximately 37 percent of mentallychallenged children have difficulty developing

toileting skills by adulthood [11]. Further-more, in recent studies it was estimated thatthe annual cost of managing UI through long-term care is around 19.5 billion dollars [12].The ultrasound bladder monitor will be anew, improved and cost-effective method formonitoring UI.

Figure 2: Initial prototype of the bladder dis-tention monitor and an ultrasound image ofthe human urinary bladder [13].

The signal processing aspect of this projectrequires creating an algorithm to directly re-late the ultrasound signal to bladder disten-tion. The design of the device will allow atransducer to send pulses of an ultrasonicbeam into the bladder. The ultrasound waveinteracts with the bladder wall and is re-flected back to the transducer. The returningsignal is detected and the echoes will show apattern that is related to the movement ofthe bladder as it expands. This system willnot create an image from the ultrasound sig-nal, but rather, the waveform echo signal willbe processed to represent a level of bladderfullness.

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Figure 3: Phantom during ultrasonic scan.

We have already completed preliminary ul-trasonic scans and data analysis using a phan-tom pelvis and bladder (shown in figure 3).The results of these scans showed that thebladder walls can be seen in the ultrasoundecho signal, as expected. The next step in myresearch is to collect and analyze ultrasounddata from human subjects.

Gaseous Microemboli

The relationship between increased em-bolic load to the brain and neurocognitivedeficits are well documented, and are a con-cern in high-altitude flight, space travel, deepwater diving, and open heart surgery. Arte-rial line filters are now used to stop emboli inextracorporeal circuits from passing back intothe bloodstream. However, small emboli andsometimes large emboli pass through thesefilters; especially when the filters are over-loaded [14]. It is important to monitor emboliload pre-filter because a warning of increasedload allows the medical team to eliminate em-boli sources. The EDACTM QUANTIFIER(Luna Innovations Inc., Roanoke VA, USA)uses broadband ultrasound pulses to detect

and track emboli [15]. The EDAC uses mo-tion tracking algorithms identify the signalsof individual emboli. In addition, the EDAChas been proven to accurately estimate thesize of emboli using the backscatter echoesfrom emboli [16].

In this project, we are extending theEDACTM Quantifier’s capabilities by addingthe ability to remove gaseous microembolifrom the extracorporeal circuit. It is there-fore necessary to precisely know the behaviorof radiation force as a function of ultrasoundfrequency in order to optimize the removalprocess. For example, if there are fairly broadresonance peaks, it may be possible to in-crease the magnitude of radiation force whilekeeping frequency low. In the section belowI will briefly discuss the equations describingacoustic radiation force.Acoustic Radiation Force:

Acoustic radiation force is the force ex-erted upon an object by an incident soundwave. Over the past century numerous au-thors have calculated acoustic radiation force.There are various approaches for calculatingthe acoustic radiation force exerted by an in-cident plane progressive wave on a sphere im-mersed in a fluid. The derivation of radiationforce is briefly discussed below.

We begin by writing conservation of massand conservation of momentum in a fluid as[16]

∂ρ

∂t= −∇ · (ρ~v)

∂(ρ~v)

∂t= ∇σ − ρ(~v · ∇)~v − ~v(∇ · ρ~v)

(1)

where ρ is density, σ is the stress tensor and~v is velocity. Conservation of momentum was

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written using the substantive derivative

ρ∂(~v)

∂t=d~v

dt− ρ(~v · ∇)~v

= ∇σ − ρ(~v · ∇)~v .(2)

In its general form, the stress tensor can bewritten as

σik =− pδik + η

(∂vi

∂xk

+∂vk

∂xi

− 2

3

∂vj

∂xj

δik

)+ ξ

∂vj

∂xj

δik

(3)

where p is pressure, η is viscosity, and ξ isbulk viscosity [16].

Next we need to write an expression forvelocity. We begin by writing the linearizedvector wave equation

(∇2 +K2

)~v −

(1− K2

k2

)∇(∇ · ~v) = 0 (4)

in which ~v is the perturbation in the fluid ve-locity due to the acoustic field, K is the trans-verse wavenumber and k is the longitudinalwavenumber. Via Helmholtz decompositionwe can write velocity in terms of a scalar anda vector potential

~v = ∇φ+∇× ~Ψ (5)

where φ is the scalar velocity potential and Ψis the vector velocity potential. The velocitypotentials satisfy the following conditions

(∇2+k2)φ = 0 (∇2+K2)~Ψ = 0 . (6)

Finally, we write an equation for force act-

ing on a volume in a fluid:

F =

∮σ dA

=

∫∇σ dV .

(7)

In the case of a freely suspended fluid orsolid elastic sphere in an inviscid fluid weset viscosity and bulk viscosity equal zero inequation (3) so that σik = −pδik and radia-tion force becomes [17], [18].

F =

∫−pδik dA

= −2πρ1|A|2∞∑

n=0

(n+ 1)(αn + αn+1+

2αnαn+1 + 2βnβn+1)

(8)

where A is the incident wave amplitude and

αn =−G2

n

G2n +H2

n

, (9)

βn =−GnHn

G2n +H2

n

, (10)

in which

Gn =(Ln − n)jn(k1a)+ (k1a)jn+1(k1a) ,

(11)

Hn =(Ln − n)nn(k1a)+ (k1a)nn+1(k1a) .

(12)

where the compressional wavenumber in thesurrounding fluid is k1 = ω/c1 , a is the sphereradius, and nn(x) is the spherical Bessel functionof the 2nd kind. Ln is shown below for a fluidsphere:

Ln =ρ1

ρ2

(k2a)[njn(k2a)− (k2a)jn+1(k2a)]jn(k2a)

(13)

where ρ2 is the density of the sphere, k2 is thecompressional wavenumber in the sphere.

In the case of a solid elastic sphere shear waves

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are created inside the sphere, and the functionLn is equal to

Ln =12ρ1

ρ2

(K2a)2(An −Bn)(Dn − En)

(14)

in which

An =njn(k2a)− (k2a)jn+1(k2a)

(n− 1)jn(k2a)− (k2a)jn+1(k2a), (15)

Bn =2n(n + 1)jn(K2a)

[2n2 − (K2a)2 − 2]jn(K2a) + 2(K2a)jn+1(K2a), (16)

Dn =[(K2a)2/2− n(n− 1)]jn(k2a)− 2(k2a)jn+1(k2a)

(n− 1)jn(k2a)− (k2a)jn+1(k2a),

(17)

En =2n(n + 1)[(1− n)jn(K2a) + (K2a)jn+1(K2a)]

[2n2 − (K2a)2 − 2]jn(K2a) + 2(K2a)jn+1(K2a). (18)

In these equations the longitudinal and trans-verse wavenumbers in the sphere are

k2a =c1c2

(k1a) K2a =c1C2

(k1a) (19)

where c2 is the compressional sound velocityin the sphere and C2 is the shear wave speedin the sphere.

When the problem is expanded to includeviscosity in the surrounding fluid, the com-plexity of the radiation force equation in-creases dramatically. Due to the complica-tion of the expression, in this paper we willonly state the starting point below [19]:

F = 〈∫σ1nda〉+

∫〈σ2〉nds (20)

where σ1 is the first order stress term, σ2 isthe second order stress term, and the brack-ets denote the time average of the functionenclosed. By plugging small variations in ρ,p, and v, up to second order, into the Navier-Stokes equation and the conservation of massequation, equation 20 becomes:

F = 〈∫

(σ2 − ρov1l v

1k)nda〉 . (21)

We are currently numerically implementingboth the inviscid and viscous equations usingMatlab. Preliminary results are shown below.

Figures 4 - 5 show radiation force versuska for the materials that are relevant to CPBcircuits, namely air and lipid emboli in blood.During bypass surgery the body is cooled toapproximately 28◦C. The material propertiesused for these plots correspond to this tem-perature.

Figure 4: Radiation force vs ka for an airbubble in blood. The solid line with dottederror bar lines is radiation force found usingthe inviscid model. The dashed line with tra-ditional error bars is the result found usingthe viscous model. The error bars are basedon the uncertainty in the material properties.

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Figure 5: Radiation force vs ka for a lipidsphere in blood. The curvy solid line withdotted error bar lines is radiation force foundusing the inviscid model. The smooth dashedline with traditional error bars is the resultfound using the viscous model. The errorbars are based on the uncertainty in the ma-terial properties.

Figure 6: Material Properties. [20], [21], [22],[23], [24]

As seen in the figures above, when vis-cosity of the scatterer is small both meth-ods give very similar results. As expected,when viscosity of the scatterer increases thesetwo methods no longer yield the same re-sults. Furthermore, these preliminary radia-

tion force plots show that there are no broadresonance peaks that can be used to get alarge force at a low frequency.

Conclusion

The primary issue in telemedicine is that itstill requires lengthy consultations by special-ists, which are expensive enough that manypeople in rural areas cannot afford them.The type of algorithms that we are devel-oping for the projects described above canmake health care more affordable and acces-sible. Rather than proposing to solve thegeneral problem of computerized ultrasoundinterpretation, we have described two cur-rent projects which we have had underwayat William and Mary recently. These collab-orations all involve industrial partners withthe goal of commercializing the ultrasoundsystems described, as well as clinical part-ners where the human patient testing is done.Our research focuses on those aspects of theresearch related to ultrasound signal process-ing algorithm development necessary to au-tomate monitoring. In particular, the goal ofmy research is to accurately model the inter-action of ultrasound with anatomical struc-tures of interest and to develop signal pro-cessing algorithms to extract features from ul-trasound RF waveforms. To accomplish thisgoal I am collaborating with clinical partnersto obtain human and phantom data sets of ul-trasound signals. I am analyzing these signalsand comparing them to mathematical mod-els created using Matlab. Furthermore, I willuse the data from patients and phantoms todetermine key features in creating the algo-rithms. These techniques are often commonfrom one application to the next, and will be-gin to lay the groundwork for self-diagnosingultrasound systems on spacecraft. In the nearterm our results will benefit under served pa-

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tient populations in blighted rural areas andunderdeveloped countries.

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