adaptive-focusingalgorithm for amicrowave planar … · commercial microwave hyperthermia...
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An Adaptive-FocusingAlgorithmfor a Microwave Planar PhasedArray Hyperthermia SystemAlan ].Fenn, Chris J. Diederich, and Paul R. Stauffer
• We have experimentally investigated the use of adaptive-focusing techniquesin the hyperthermia treatment of cancer. Gradient-search adaptive-focusingsoftware developed at Lincoln Laboratory has been implemented on acommercial microwave hyperthermia planar phased-array antenna system at theUniversity of California at San Francisco (UCSF). The system, manufactured byLabthermics Technologies, Inc., consists of 16 independent amplitude/phasecontrolled waveguide antenna elements operating at 915 MHz.
In the experiments, conducted at UCSF, a method of steepest-ascentgradient-search feedback algorithm was used to control the hyperthermia arrayphase shifters and focus the transmitted radiation beam at a hypothetical tumorsite in a muscle-equivalent sugar/saline liquid phantom. A feedback probe,embedded in the phantom, measured the resulting electric field (E-field)generated by the antenna array. The measured data indicate a significantincrease in the focal-region field strength with a rapid convergence of theadaptive-focusing algorithm in 10 to 15 iterations. From the measurements, themaximum useful heating depth in thesugarlsaline phantom is estimated for the915-MHz system at about 3 em.
ADAPTIVE ARRAY ANTENNAS are well known fortheir ability to improve, in real time, the performance of communications and radar sys
tems [1-4]. Recendy, adaptive array techniques havebeen applied in the medical field for the hyperthermia treatment of deep-seated tumors [5-12]. (For anintroduction to hyperthermia treatment, see the box,"Treating Cancer with Hyperthermia," on the following page.) With an adaptive radio frequency (RF) ormicrowave hyperthermia array, it is possible to control the electric field (E-field) automatically at multiple positions within the target body [5-12]. The Efield radiated by a hyperthermia phased array can beminimized (nulled) and maximized (focused) at desired target positions by adaptively adjusting the transmit amplifiers and phase shifters of the hyperthermiaapparatus. Multiple adaptive E-field nulls and adap-
tive focusing were experimentally demonstrated atthe State University of New York (SUNY) HealthScience Center on a modified commercial RF hyperthermia system. In the experiments, gradient-searchsoftware, developed at Lincoln Laboratory, was usedto control the generated E-field of the Sigma-60(manufactured by BSD Medical Corp. of Salt LakeCity, Utah), an annular phased-array antenna systemoperating at approximately 100 MHz [9-14]. Detailsof the SUNY experiments have been reported in anearlier issue of this journal [11].
Subsequent to the SUNY experiments, the samegradient-search algorithm was implemented on a different commercial hyperthermia system-theMicrotherm-lOOO (manufactured by LabthermicsTechnologies, Inc., of Champaign, Illinois), a planarphased-array microwave antenna system operating at
VOLUME 6, NUMBER Z, 1993 THE LINCOLN lABORATORY JOURNH 269
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TREATING CANCER WITH HYPERTHERMIA
THE TREATMENT OF malignantcumors is often a difficult task.The objective is to reduce orcompletely remove the tumormass through the use of one ormore modalities, commonly sur·gery, chemotherapy, and radiationtherapy [11. One particular meth·od used in conjunction with another modality is hyperthermia[1-6], in which a tumor is heatedto diminish it.
Hyperthermia treatment requires a controlled thermal dosedistribution. Typical localizedhyperthermia temperatures thatare necessary for the therapeutictreatment of cancer are in therange 42.5° to 45°C. (The nor·mal temperature of human tissueis 3rc.) During treatment, sur·rounding healthy tissue shouldbe kept :1.( temperamres below42.S°C. The most difficult aspectof inducing hyperthermia witheither electromagnetic or acous·tic (ultrasound) waves is producing sufficiem heating at the siteof a tumor, especially a deepseated one, without damaging anyhealthy tissue.
Fordecuomagnetic hyperthermia tre:Hmem, muhiple-antenn:Iradio frequency (RF) or microwave hyperthermia :Irrays :Irecommonly used to provide a focused main beam at the tumorsite. A focal region should be concentrated at the tumor with minimal energy delivered to the sur-
rounding normal tissue. Becausethe hyperthermia antenna beam·width is proportional to the wavelength, a small focal region suggests that the wavelength be assmall as possible. Due to propagation losses in tissue, however,the penetration depth decreaseswith increasing transmit frequency. Typically, for noninvasivephased arrays an operating frequency close to 100 MHz is rec·ommended for the heating ofdeep-seated tumors and a frequen.cy close to 900 MHz for the heat·ing ofshallow tumors.
A typical clinical RF or micro·\V:lve hyperthermia treatment con·sisrs of several [Wo~hour sessionsspread over a period of weeks.During the first hour of a session,Jnsrrumentation to monitor tern·perature and other vital signs isattached to the patient. Next. forabout 15 minutes, the hyperthermia equipmem is turned onand adjusted to achieve the desired temperature in the tumor.The adjustments to the hyperthermia equipment can consist ofchanges in the transmit power level (for single-antenna devices). orchanges in both the transmil power level and phase (for multipleantenna devices). The tumor isthen heated for approximately45 minutes by radiated e1ectromagnetlc energy to a temper:ltureof 42.5° to 45°C. Note thar, inorder to limit the overall length
of the treatment sessions to tWOhours, only a maximum of 15minutes is available for adjustingthe hyperthermia equipment to
achieve the ideal electric field (E·field) distribution for a particularpatient.
Current clinical operation ofcommercial hyperthermia ph~darray devices allows limited manual control of the array transmitantenna amplitude and phase.Although this manual trial-and·error method can achieve someimprovement in the E-field distribution, automatic adjustmenttechniques, such as those offeredby adaptive arrays, are desirablebecause of their promise of faster operation and bene.r E-fidddistributions.
Several journals have publishedspecial issues 011 the acoustic andelectromagnetic hyperthermiatreatment of cancer [6-81. andvarious articles have investigatedthe widely differing methods forimproving such treatment. For ex:Imple, many studies [7. 9-12Jhave shown thal phased arrays canbe used to produce improved therapeutic field distributions: researchers have demonstrated thatphase comrol can synthesize improved RF t:ldiation patternswithom adaptive control of thetransmit-array weights, and thattransmit-array weights (definedhere as amplitude and phase statesassoci:Iled with a transmir chan-
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• FENN, DIEDERICH, AND STAUFFERAll Ad"ptiIJt-FIK/lSlIIg Afgt'rII/'mfor (/ "1i(Tf)l<'fll~ PllI/lilr Pbmrd-Amry Hyp(r(/'tTmill SJ1um
nell can be adaptively controlled[Q maximize the tumor temperature (or microwave power de:li\l~
ered LO a tumor). while minimizing the surrounding [issuetemperarure (or microwave power delivered to the surroundingtissue). It has also ~en shownthat ulrrasound receive feedbacksensors [l3] and a pseudoinverse
EkfrrtnU1I. C.A. rein and L w: Bmy. Prindpkr
andPraCliuDfRadiarion Onl'DIooO.B.Lippincolt. Phibddphia. 19Sn.
2. A.W. Guy. ~Hi$IOry of Biological Effe<;tS:mti Medical Applicl.Iiol1$ of MiCl'()\<....,"C Encrgy,~ IEEE Trims. Mkrow.TINo" T«h. MIT·32, 1182 (Sept.1984).
3. J.O...crga=l."ThcEff(l;tof!...ocl.lHyJlCTlhami.ll Alone and in Combirulionwith Radiation on Solid Tumors. M inCUlm ThmJpy bJ HJpmhcmia aruiIUu1i4licn, Proc 2nd If/I. Symp., 2-4Junt 1997, ed. C. Strdkr (Urban &Sch"''3J'Unberg. Balrimore, 1978), pp.49-61.
4. }. Ovcrg:wd. ~ainical HypmhermiaAn Update, ~ Proc. 8th 1m. Congrm ofRmJilzrion nr-rrh 2. juL /9$1. eds.E.M. Fielden.j.E Fowler, }.H. Hendry,and D.O. &ou, p. 942.
5. S.B. Field and j.W H:md, eds., All Intl"rNilmion 10 tiN Pnuticillkpms ofClinicd H,pmJxrmilz (Taylor & Fr=cis.,london. 1990).
6. ~SpeciaI Issue on Can«r Therapy byElCO"romagnctic Hyperthermia,~ j. Micn>u.l ~160une1981).
7. ·Speciallssuc on Phased Arnys for HyperthermiaTrcl.[mcntofCancer,· IEEETmns.. Microw. TINory T«h. MlT-34(May 1986).
8. ·SpeciaI Issue on Hyperthermia andCancer Thcr.apy; IEEE Tmm. Biomcti.till. BME-31 O:m. 1984).
9. D. Sulliv:m, ~Thltt-Dimcnsional CompUler Simulation in ncq, Ikgional HypCTthermia Using the Finite-DifferenceTime-Domain Me1:hod.~ IEEE Tmm.Mirrow. Tlxory T«b. MTf-38, 204(Feb. 1990).
10. R.B. Roemer. K. Hynynen, e.johnson,and It Krc:ss, ~Fttdback Control andOplirniz:nion or Hyperthermia Heat-
pattern synthesis method can beused to geneme multiple focalpoints wirh a phased-array hyperthermia applicator [14, 151. Other studies [16, 17J and several conference articles [18-20] haveinvestigated the theoretical bene~
fit of using near-field adaptivenulling [21-24] with noninvasive
auxiliary dipole sensors to reduce
ing P;tterns: Present StatuS and FutureKecds.~ I££E Eighzh Alll/udCol/! Eng.Meti. BioL 5«.. p. 14% (Nov. 1986).
11. P: Wust.j. Nadobny. R Fdix, P: Deuflhard, A. louis, and W.john, ~Strate
gies for Optimized Application of Annular-Phased-Amy SystCn1S in Oinic:aJHyperthermia," InL}. H,pmhermilz7.157 Oan.-Feb. 1991}.
12.j.W. Srrohbehn, E.H. Curtis. K.D.P;u15en, :md D.R Lyndl, ~Optimw.
don of the Absorbed Power Distribution for an Annubr Phased-Amy Hypcnhermia System: Inr. f RmJiilrUmOnroL BioL Phys. 16, 589 (1989).
13. E.S. Ebbini. H. Wang, M. O·Donndl.and CA. Cain, "Acoustic: Fcedb3dt forHyperthermia Phased-Array AppliCl.tors: Aberration Correction. MotionCompe~tion, and Multiple Focusingin the Pre:scnce ofTISSUe Inhomogeneities,~ Proc. IEEE /99/ UlrrllSl)n. Symp.2,1343 (1991).
14. E.S. Ebbini and C.A. Cotin. -Experimental Evalu.uion of a Prototype Cylindric:aJ $a:rion Ulu250und Hypmhamia Phased-ArI'1llY AppliCl.lor," IEEETroffS,. Ulmwn. !YrrMkm: Freq. Conl7'()/38, 510 (1991).
15. E.S. Ebbini:md C.A. Cain, -Mulripl.....Focus Ultrasound Phased-Array P;ucmSynthesis: Optional Driving-Sigrul Distributions for Hyperthermia; lEE£Tmm. Ulmwn. !YmNJ«rT. Frrq. Control36, 540 (1989).
16. A.J. Fenn, ~Applic.ation of Adaptive Nulling to Electromagnelic Hy.perthermia ror Improved ThermalDose Distribution in G-mcer Therapy: Trdlllimi Heport 917. LincolnLaboratory (3 July 1991), OTICItAD-A241026.
17. A.]. Fenn and G.A. King. "An Adaptive Radio-Frequency Hyperthermia
the E-field intensity at selectedpositions in a target body whilemaintaining a desired focus at a
tumor site. In particular, multiple adaptive E~field nulls wereused to show a theoretical reduction in hot spots for a homoge~
neous elliptical phantom targetsurrounded by a water bolus andhyperthermia ring array [18-20].
Pha.s«\-Array System for ImprovedCancer Therapy: Phantom TargetMeasurements," T«hnkill Report 999.Lincoln Ltbor:ttory (19 Nov. 1993),ESC-TR-9J..299.
18. A..j. Fenn...Foo....scd Ncat-Fidd Nulling for Adaptive E1=magnetic Hyperthermia Applications, ~ /99/ frog.£larromtZgn. &s. SJmp. Prtx. Cambridge. MA, /-5j",/y 199/, p. 393-
19. A.j. Fenn, ·Adaptlvc: Hyperthermia forImproved Thermal Dose Distribution."in RmJilztion ~ilrrh; A TwemiezhCrnt:lry Pmp«rive I: CAngrm Absm:(TS,7-/1 juJ, /991, cds.j.D. ChaplTU1\,We. IXY.-cy, :md G.F. Whitmore (A<:adanie Press, Sm Diego. 1991), p.290.
20. A.J. Fenn, -Noninvasivc:Aaaprivc: Nulling for Improved Hypmhermia Thermal Dose Distribudon,- IEEE Eng.M«l. BioL Soc. InL Con! 13, 976 (31Oct.-3 Nov. 1991).
21. A.J. Fenn. -Theory and Analysis ofNeat-Fidd Adaptive: Nulling, ~ /986kiIomilT Con! on Sigmsls, Sptmu Qlld
CompulnS (ComputCT Society Press ofthe IEEE, Washington, DC, 1986), pp.105-109.
22. A..J. Fenn, "Ev:aluadon of AdaplivePhased-Amy Antcnrut Far-Field Nulling Performan« in the Near-Fidd Region," lEEETmns.. Mtnr1JilS P'roJH1g. 38,173 (1990).
23. A.J. Fenn. H.M. Aum:mn, EG. Willwerth, and J.R. Johnson, ·Focused.Near-Fidd Adaplive Nulling: opedmentailnvestig;ltion." /990 IEEE AIlfellllllS ProJH1g. 5«. IIIL Symp. Dig. I,7-11 May 1990. p. 186.
24. A.J. Fenn. "Anal)'liis or Phasc:-FocuscdNcar-FiddTesling for Mulliphasc·Ccnler AdJpdvc: Radar Syslems: IEEETm1lJ. Anullluu Propng. 40, 878 (Aug.1992).
10l~MI 6 .~MB!~ 1 1993 r~( 1IlCOli lIBO~ll0~1 JOURlll 271
..• FENN, DIEDERICH. AND STAUFFER
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915 MHz. Expcrimc=nts were conducted at the Uni·versity of California at San Francisco (UCSF) Radiation Oncology Depanmem in which the algorithm
was used for comrolling !.he hyperthermia array phaseshifters to focus the transmim:d radiation beam :11 a
hypothetical tumor site in :l sugarfsalinc liquid phantom (dielectric losses of3 dB/em). The UCSF experiments are the subject of this article.
~eT21 major differences exist bc=rween the SUNYand UCSF experimenrs. In panicuJar, the hyperthermia equipment uso:l in the two experimenrs had differem transmit frequencies (tOO MHz versus 915MHz) 2nd array geometries (annular versus planar).
BecauS(: of the difference in transmit frequencies. theSUNY experimcnts investig:ncd thc treatment ofdcepseated {Umors (depth gre:llcr [han about 10 em) whilethe UCSF experimentS wcre concerned with shallowtumors (depth less than about 3 em). In addition. theSUNY experimenrs investigated the use of adaptive
nulling as well as adaptive focusing while the UCSFexperiments considered just adaptive focusing.
Background
Microwave antenna applicators have frequently beenconsidered for inducing localized hyperrhermia 10
superficialwmors located 011 lile chest wall and headand neck regions. For single~aperrure applicators, however, the power-deposition patterns are often limitedto effective heating well within the apenure boundaries [lS-18). (Note: The power-deposilion patternsare also referred to as [he specific absorption rate(SARl, which is equal to Yza 1£1~/p, where £is the Efield, G is the elecuic.,1 conductivity of the [issue, andp is the tissue density.) With 915-MHz microwaveradiation, for example, ::t waveguide applicllor has aheating deplil limited to less Ihan 3 em and l::tteralheating dimensions of 3 [0 5 em-insufficient totreat many tumors [191. The he<tting depth is limilcd
16-channel microwave transmitter Sugar/saline liqUid phantom
FIGURE t. The Microtherm-IOOO (manufactured by Labthermics TechnologIes, Inc., of Champaign. III.), a 915MHz microwave planar phased-array hyperthermia system used in experiments at the University of Californiaat San Francisco (UCSF) Radiation Oncology Department.
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15.2cm
FIGURE 2. The Microtherm·l000 system: (8) footprint and(b) cross section 01 applicator.
••
...
Coaxialfeed lines
c
Distendablebolus
(deionized water)
K G
(.J
Waveguides(16)
o
(b)
Additionalhigh.dielectric
constantmaterial
/
,
Applicatorhousing
I~r+-r---,I-------,
30m
I
Aperturesurface
\ ....
Outerapplicatorhousing
Eu
'"
field probe used in the UCSF measurcments was :tshort dipole with a semiconduaor diode detector(51-54). (Note: Thc Microtherm-IOOO currently doesnot supply [·ficld probes to monitor clinical hy~rlhcrmia treatments; however, as the currcnt measuremems indicatc. an E-field probe could be added toIhe system 10 provide feedback signals to the adaptivealgorithm. In theory. with 16 independent transmitchannels. the [-field rndiation pattern can be controlled at up to 16 poims [in the radiation field! byllsing a feedback signal measured at each point.) In
to only about 3 em because the dielectric loss of915MHz radiation in lissue is typically 3 dB/em. Fromprevious experiments, researchers have found that apredictor of good lOCI! healing comrol is to use the50% iso~SAR coverage (laterally and with depth) ofthe tumor (20].
Several mtthods for improving the SAR heatingpatterns of single-aperture hyperthermia devicts areunder investigation. For example, microwave-absorbing saline-filled boluses can shape the hearing panernof a waveguide applicator, as has been demonstratedin Sherar et al. [211. Mechanically scanned microwave applicators have also been used to shape ,hepower-deposition pattern (19, 22, 231.
Another method of improving the SAR he'Hingpattern is to use muhiple.aperture arrays (24-431.Researchers have found that phased-array applicatorsan be used to shape the power-deposition pallernproduced by planar (19. 22. 44. 45J and conformalarrays [46,471. And stud its 10 improve penelrationdeplh with phased :trrays by COlli rolling the phase andamplitude ofeach array clement have been conducted[44,45,481,
In our experiments, we: have used the Microthenn.1000-a hyperthermia microwave system with 16alllennas (Figure I) [491. By transmitting close to thepatient with a planar array, the system is able toobtain superficial heating over large (approximately15 x 15 em) as well as small (approximately 3.5 X3.5cm) areas. depending on the array amplitude illumination function. Figure 2 shows the 15 x 15-cl11planar array wilh its 16 square waveguide eIemenlsoperating al 915 MHz. The 16 independent variablepower amplifiers drive: Ihe waveguides, and each ofthe 16 active channels has an electronically controlledvariable-phase shifter 10 focus the array. A cool~water
bolus between the: patient and the phased arrayprevents excess healing of the skin surF.1ce. The bolusis filled with circulating deionized water, which hasa very low microwave propagation loss of abom0.3 dB/cm.
Previously, UCSF had evaluated (in experimentswith phantom materials such as deionized water andmuscle-equivalent liquid, and with animals) a system·atic-se:lrch, iterative focusing algorithm for phased.array control of the Microtherrn-1000 [501. The E·
,OlUWI8 XUlurl2 IIU '~llINCOlW l1101l1011 ~OUUll 273
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Hyperthermia array
~
Receiver
Focus-",,,,r
Feedbacksignal
"'-Y'....T urnor
E·field sensor
~
R'source
Weightcommands
(amplitude/phase)
ISignal processor
Adaptive·focusingalgorithm
FIGURE J. Adaptive-focusing hyperthermia system concept.
this aniclc. we investig:nc an alternative Clndidatealgorithm for the ad:tptive focusing of the Microthcrm1000. Developed by Lincoln l...:tboratory. the computer algorithm uses a gradient search based on themaximization of the signal that is received by an Efield sensor positioned within a tumor.
Theory
Adnptiw.Focusing Hyptrtbamin SyJum COtlctpt
The concept of an adaptive.focusing hyperthermiaSYSlem is shown in r-igure 3. To generate the desiredE-ficld disrribulion with a clinical ad:tptive hyperthermia system, a field probe is positioned as closelyas possible to the lllmor site and the hypenhermi:tarray is focllsed to produce the required field intensityat that site. The probe provides feedback from whichthe: adaptive.:trray weights W
IICOln be adjusted to con
trol the: amplirudes A and phases ~ of the individualantenna elemenfs such that the energy received 3( thetumor sile is maximized. (NOIe: Although Figure 3shows only one field probe. an array ofprobes may beu=l.)
Adnptiz~Tmmmil·A"ay FontmlllliolJ
Consider a hy~rth~rmia array with N am~nna e1em~nlS (N '" 16 for the Microth~rm-l000 system).
The input signal to each of the N array elements isobtained from the amplitude and phase:.adjusted signal distributed by a power divider network. The number ofadapt ive chan ncls is assumed to be equal 10 thenumber of transmit antennas N.
Commonly, the weights are constrained to delivera required amoul1l of power to the hyperthermiaarray or the tumor, For simplicity in the experimentaladapdve-hypcflhcrmia-arr.lY comrol software, we con~
strain the weights such that
",'
where lw"l is the transmit-weight magnitude for the11th adaptive channel and Kis a constant. To generatean adaptive phase focus. a gradient-search algorithmcan be used to control the transmit weights (phaseshifters).
Cmditm·Srl1rch Algorithm
Gradient-search algorithms are commonly used inadaptive-array appliCltions, particularly when thechannel-Io-channel correlation of the antenna de·mems Clnnot be Cllculated or measured. W'ith a gradient search, only the received power at the E-ficldprobe(s) is measured and used as a feedback signal to
274 Illl-etlllll.OllICllf .IO.....l l'Ol••t, ".Itt 2 ltt3
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the algorithm. Ekc3u~ the Microthenn-l 000 systemat UCSF measures only me received E-field power. itis appropriate to consider a gradient-search algorithmfor the application.
GradienBearch algorithms control the transmitweights iter.ttivdy to maximize (focus) the microwavesignal received by the field pro~(s). Transmit-arrayphase: shifters are adaptively changed in small increments (the process is called dithering) and the received power at the pro~(s) is monitOred to determine the phase settings that will increase the powermost rapidly to a maximum. A wide variety of gradi.ent searches exiSts [55-60); in our work we have useda standard method ofsteepest ascent. The mathemati-
cal formulation for the method is straightforward155.61]; a detailed description of the formubtion inthe context of hyperthermia is contained in References 9 and 62.
Spurn Cons;d"al;ons
Figure 4 is a block diagram of an adaptive hyperthermia system controlled by a gradient-search algorithm.Transmit weights W1j' ...• w"}' ... , wNj at the jthiteration are shown at the [Op of the figure. Thetra.nsmit phased-array al1tcnna induces a voltage acrossthe terminal of the ;,h receive field-probe antenna.(Note: Figure 4 assumes that the system lIses an arrayof Naux field probes.) For any given configura.tion of
Control
nTransmit
• •• N
Transmitweights(ith state)
Transmitarray
i=1.2•...• N.U •
ith receive probe (total of N'UJ probes)
Gain -..Adjust a
Receive,compute,and control
Received power p
Feedback signal (PI)~---'--~
Computeweight search
directions
Computeweight updates
Control
Adaptive transmit weight (j + 1)th stale
FIGURE4. Block diagram of gradient-search algorithm for adaptive-focusing hyperthermia system.
,Ollfll( 6 .UU1111 1113 r~llI\COlllllalirOAl JOUlUl 275
• FENN, DIEDERICH, AND STAUFFERAll Adllf'ltl'f'-Forusmg Algorill"" for II />1,rrowll''l! PI",,,,, Pbmld-Arr"J HJprrtlH'rmlll SYS/lm
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FIGURE 6. Amplifier circuit for E-field probe.
,-----'\/W------,-,----\ +
High-resistancesensor leads
depending on rhefewer chan abour 15 iterations,gradieOl~search Slep size {j,1/J.
Materials and Methods
The hyperthermia phased~array system used in thesemeasurements is rhe Labrhermics MicrOlherm-1000planar phased-array applicator (Figure I). The systemconsists ora 15 x 15-cm planar array with a 4 x 4 gridof 16 uniformly spaced square waveguide elementsoperaring ar 915 MHz (Figure 2). Figure 5 shows aphorograph of Ihe planar phased-array applicator.Because of the water bolus covering the applicator,the acwal waveguide radialOrs or {he array are nOI
visible in dle photograph. The aperture dimensionsof the individual waveguides are 3.8 X 3.8 cm, andthe waveguides arc filled wirh a high-dielectricconstant material to provide maximum radiationar 915 MHz. The waveguides are linearly polarizedwith the dominant E-field component aligned with
they-direction (refer to Figure 2).The 16 individual waveguides are driven by 16
high-power ampliFiers with up to 35-W average powerper channel. The power applied to each element iscOlllrollable in 10% increments of the adjustable mas·ter power leveL E.1.ch of Ihe 16 active channels has anelectronically controlled variable phase shifter (8 bits,0° w 360°) for focllsing, the array. The phased~array
applicawr can be driven in incoherent mode (with 16independently operating sources with frequencies ncar915 M Hz) or coherent mode (with a single-frequencygener.uor :md independent control of the relative phaseof each aperture to within 1.5°). Invasive Luxtronfiber~optic temperature sensors [63] are included wirhthe Microtherm-l000 system to monitor the lem
pcratures obtained during tre:ltmenc.To prevent excess heating of a patient's skin sur
F.tce, the Microlherm-IOOO uses a cool-waler bolusplaced between the patient and the phased array. Thebolus consisls of deionized water, which has a very
low microwave propagalion loss, contained within ;l
flcxible membrane Ihal can be expanded outward to 6to 8 cm in from of the applicalOr fuce to improve themicrowave coupling berween Ihe applicator and tisSllC. Thc bolus rcmpcrarure is conrrollcd by circulating cooling water through a thermal-conduction plas~
ric-tubing heat exchanger located in the outer cdges
soooDiode to measure 500 nmagnitude of E-fjeJd
60 knfm
/
the transmit weights, each weight is dithered by asmall amoufll in amplirucle and phase, and rhe received power :H the irh probe is stored in the computer for calculation of rhe lora I received power fromthe array of Naux probes, the amplitude and phase
search directions [62], and rhe updated (j + Ihhlrallsmil-weighr configuration. Olle rr.msmit weightis dithered with the remaining transmit weights intheir jlh state. The process continues unci! Ihe(j + l)th weight configuration has converged. Basedon other measurements (10, 121 nOi shown here,convergence for adaptive focusing occurs typically in
FIGURE 5. The 915-MHz microwave planar phased-arrayapplicator used in the Microlherm-I(XXI.
276 1~IIl~ClllN lUllU10RI.lOUR~1l YOlUtH 6 ~UIlUR l 1993
• FENN. DIEDERIC.... AND STAUFFERA" Ad'ipliw-Focusiltg Algorithm jor 'I 1l1ilTOlII,wr PllIIUlr T'!JlIJN(-ArrilY HyprTlh~rmi'l Syslrm
16·channelphased-array antenna ---
Sugar/saline liquid ---phantom
Dipole probe
:..,;:.~-- Probe (if, Y, z)positioner
FIGURE 7. Dipole probe, probe (x, Y, z) posilioner system, and the muscle-equivalent phantom lank.
of the bolus housing. To improve the cooling capacityand transient control of the bolus in the Microtherm1000, researchers at UCSF replaced the heal exchangerinside the bolus with a closed~circllit pumping sYSlemthat circubres remperaulre-controlled deionized waler directly through the bolus compartment in serieswith a heat exchanger mounted in a temperatureregulated water bath.
The E·field probe [51, 52. 54J used in our mea~
surements was fabricated at UCSF and integratedwilh Ihe Microtherm-I 000 system. The E-field sen~
sor is a Schottky detector diode (Hewlett-PackardHP-3486 with an olller diameter of 1.9 mm) with thediode leads arranged to form a dipole a/Henna of I emlengrh. The sensor is coupled to an amplifier circuit(Figure 6) via high-resislance lead material to minimiz.e perturbation of the fields. (Notc: Manuf.1cruredby Holaday Industries. the lead malerial has an outerdiameter of 1.2 mm, including the outer insulation.)The rectified DC OutpUt voltage is a fUllction of theE-ficld squared in the orienrarioll of the dipole leads.Figure 7 shows the dipole probe hardware and (x. y. z)positioner. A 12~bit analog-to-digital (AID) convertersamples Ihe E~ficld probe signal.
In our experi melltS, we used a liquid Illuscle-equiva-
lem phantom (sugar and salt dissolved in deioniz.edwater) to approximate human tissue. The electricalproperties of the phantom (a ~ 1.38 Siemens/m andrdative permittivity £t = 54.7 (641) were similar to
that of high-water-colllent Ilssue at 915 MHz (a =
1.28 Siemens/m :md £, = 51.0 [65J). The liquid phan~
tom was contained within a 50 X 50 X 22-cm-deepPlexiglas tank, which is shown in Figure 7.
The UCSF hyperthermia array is controlled andmonitored by an MS-DOS-hased personal compurersysrem with software that implements the gradienrsearch adaplive-focusing algorithm. The software, developed by Lincoln Laboratory and integr::ued byUCSF with the l..abrhcrmics SySlem, allows the operalOr to run an adaptive-focusing algorithm chat usesthe OUtput power of the E·fidd probe as a feedbacksignal during rhe gradienr se:lrch. The adaptive~focus
ing algorithm uses only phase control 10 maximizethe E-rield at the probe's position.
Measured Results
R~vi~w of£arliu Investigations at UCSF
Because of inter-element mlltual~collpling effeclS inthe phased array and because of the variation that can
~OlUW( 5 Ulll!l ,. liU ,~[ lINCQL~ 1I8Q~UQRI JQUUIl 2n
• FENN, DIEDEkJCH, AND sr,\UI'FmtAll Affllplil't-Focming Algoridmifir " M,rrtJU'II/V PI,III"r I'IJ1lsr,I-AmIJ HJprrtlJrrmltl 5pum
17 ,----.-----..,."'qr--,--;rr--,----,
13
9
E~~
5~•~
"
(.)
7 ,-------,----....---,------,
5
3
-5
0.1
ox-distance (em)
5
(b)
10
FIGURES. Measured E-field radiation pattern fort he Microtherm-1000 planarphased array focused at (a) z = 7em in thedeionized-water phantom (propagation loss =0.3 dB/em), resulting in a well-focused beam; and at (bl z :::4 em in the muscle-equivalent liquid phantom (propagation loss =3 dB/em),resulting in a beam that is not centered at the desired location. In theexperiment, the UCSF systematic iterative-search algorithm was used tofocus the array. (Note: The radiation pattern contours are shown in 10%power intervals and the location of the focusing probe is indicated with a red"+"sign.)
be expected in phamom m:uerials and humans, it wasnot possible to select II priori the correct phases to
foclis rhe Microtherrn-IOOO 149J. [n addition, research was hampered by an inability to calibrate orpreset the MicrOlherm- JOOO's individual phase shiftersaccurately 10 absolute values. Anempts at using theoretically selected ph:tses th:\{ were applied to eachclement produced, at best, marginal results in a water
ph:lI11om.To compensate for these problems and to accom
modate expected tissue heterogeneities in vivo, a phaseselection, or optimiz:nion. scheme was developed at
278 THlllNCOlN UIORIIORI JOU~NIL 'OlUlI15 _UWIII' 1993
UCSF to select the phasing of each transmit channelireralively for the maximization of the measured SARat the desired location. A computer/soFtware systemlinked to the Microtherm~IOOO turned on individual
c1emclHs of the phased arr.ty in a predetermined se·quence :lnd adjusted the phasing iter:ni'lcly until amaximulll SAR was obtained.
In the process, clemelH F (see Figure 2) is turnedon ;It :t 0° phase with :til other clements turned off.
Next. element K is turned on and adjusted iter.tri'ldyin phase (from 0° to 360° in J00 increments) toobt:tillthe maximum SAR reading. ElemenrG is then
• FENN. DIEOEIUCII. AND SfAUFI'ERAll AdJ'pn.,.FonnJJlg Algoml"" fi"" Mirroll'l1l~Plil/lltr PI",~".ArraJ H..rrrrtINTn"" SJ5um
Pig thighmuscle
Multisensortemperature probe (16)
Applicator
Trial focus points
• 2.Scm deep
• 4.Scm deep
in Figure 10(3) indic:ues a maxImum penetrationdepth of aboUl 1 cm (for a 50% SAR) on the centralaxis. This result is consistent with a microwave propa~
gation loss of3 dB/cOl. Figures IO(b) and IO(c) showthe measured SAR palterns for a focus at z'" 2.5 and4.5 em, respectively. For the focus at z '" 2.5 cm, themeasured ma.-cimum penetration depth (on the principal axis) is 2.3 em, and the beun peak is at z'" 1 cm.For the focus a[ z = 4.5 em, the penetration depth isonly 1.9 em. and the beam~ is at the surface ofthe target.
Intuitively. one would expect the Z= 4.5-cm focusto penemue more deeply than the z '" 2.5-cm focus,but instead the measurements indic:ne a reduction inpenetration depth when the focus is increased beyondz'" 2.5 cm. This discrepancy was the principal reasonIh:tt UCSF was interested in comparing the Lincoln
FIGURE 9. UCSF e~perimenta' test setup tor in vivo mea·surements 01 a pig thigh (propagalion loss = 3 dB/em).Note thai temperature measurements are taken at numerous sites by 16 multisensor temperature probes spaced1 em apart. Each 01 the 16 probes measures the temperature at 7 diHerent vertical locations spaced 1 em apart.Thus temperalures within the pig thigh are measured on al-cm x l-em grid at 112 sItes.
rurned on and similarly adjusted until a new maximum SAR reading is obt:lined. This process cominues for the remaining e1emen15 in the following sequence: J, B, 0, . C, E, L. H, I, A, I~ M, and D.During this systematic iterarive·~arch process. thepreviously tested elements are kept on at their opri.mi7..ed values while the n~t element in the sequenceis turned on and ph~ adjuSted to maximize theSAR. The entire procedure, from the adjwting ofdemen! F through 0, takes approximately 1 min tocomplete. Experimen15 wing this phasing approachhave bttn conducted with different marerials; deionized-water, tissue-equivalem liquid phantoms, and ap;g<h;gh [50J.
With the UCSF systematic iterative-search algorithm, it was possible to focus the Microtherm-lOOO:n depths of 7 em in the deionized·w~uer phantom, asshown in Figure 8(a). (Note: The measured E-fieldradiation pattern COntours in the figure are given in10% power imervals.) Clearly, maximum radialionoccurs in the deionized water in the viCinity of thedesired focus. Based on the 50% SAR value, [hemaximum penetration depth is 15 em in the deionized·water medium. Using the same algorithm, similar focusing attempts in a muscle·cquivalem phantom(Figure 8(bJ) did nOI generale a useful focused beamat deplh but produced, in essence, a collimated beamwith a beam peak effectively .11 the surface of thephantom. The data in Figure 8(b) indicate a maximum penetration depth of 3 cm. based on the 50%contour, which reaches z '" 3 cm.
The microwave propagation loss in deionized water is 0.3 dB/cm. whereas the loss in the muscle·equivalent phantom is dose to 3 dB/cm. Clearly thehigh propagation loss in muscle tissue contributedgreatly to the system's inability to focus at the desireddepth. Nevertheless, a 3-Clll penetration depth is stillpotentially useful for certain shallow tumors.
The efficacy of the UCSF systematic iterative~search
algorithm was also evaluated with a pig thigh in whichan implamed SAR sensor was used [0 oplimize thephase. Figure 9 shows the in vivo experiment.:ll ~tupwith trial focus posilions (at z '" 2.5 and 4.5 cm)indicted by the shaded circles located on the centralaxis. In the incoherent mode with the 16 waveguidesoperating indepcndend)" [he measured SAR pattern
'l'lll..f' U ....U1 IttJ 1~lllltllUlll'OI"'OIIJOIIlIl 279
, .
• FENN, OIEDER1Cl-I, ANI) STAUfFERAll AdJlptn~-ForrlsillgAlgtJTirhmfo,,, /l1irroll'fl/" PltJmlT I'IJllud-Aml) 1l,J>rrtlJrnw{/ S,iU/Il
FIGURE 10. Measured specific absorption rate (SAR) forthe Microtherm-l000 planar phased array illuminating thepig thigh of Figure9: (a) incoherent mode with all elementsincoherently driven, (b) focused at z = 2.5 cm, and (c)focused at z = 4.5 cm. For parts band c, the UCSF systematic iterative-search algorithm was used to focus the array.(Note: SAR Is defined here as c!:lT/t!J, where c is the spe~
cific heat, and t!.T is the change in temperature over thetime interval t!J =30 sec. The location of the focusingprobe is indicated with a red "+" sign. )
New MtllSlfrtlflmfs at UCSF
UCSF and Lincoln Laboratory began collaboratingin June 1992 to implement an adaptive-focusing,gradienHearch algorithm with the Microtherm-I 000.Experiments were performed wi[h the deionized-water and the musc1e~eqtli~lentliquid phantoms. Phasefocusing was attempted III the central axis of theMicrotherm applicator at depths of6 and 8 cm from
rhe lowest ridge on the applicator housing with anaddirional 3~cm path length in deionized water from
the apertures to this point.Measured E-freld pacterns (Figure 12) with the
focus set at 8 em indicate that both optimi"l.:ltion
Labor,Hory gradiclH-search algorithm wilh the systcmalic iterative-search algorithm that had beell used.
In Figure 11, the measured SAR is compared withthe temperature distribution (or the 2.5~cm~focus case
(Figure IO\bJ), and the twO sets of data are consistent.That is, the peak temperature value occurs near thepeak of the SAR distribution. (Note: SAR is definedhere as cA TIAI. where c is the specific heal, and ATis[he change in temperature over the time interval 6.1.)
From the results shown in Figure 8(a), we con~
eluded th;H the systemaric irer.nive-search algorithmcould be used to focus the Microtherm-IOOO in a10w~loss deionized-water phantom. With the musclephalllolll and pig thigh, however, the algorithm didnOl result in useful focuses (although it did illustrate apossible improvement in penetration depth). Thusone question that remained concerned the effectiveness of the systematic iterative-search algorithm. In
particular, we were eager to investigate whether benerfocusing in a lossy medium could be obtained with anadaptive oprimi7A1rion algorithm, as had been considered previously [7,10, 12J.
32
2 3
2 3
I I
-
~
I
a
a
I
-1
-1
I
+
-1 ax-distance (em)
(0)
x-distance (em)
I')
x-distance (em)Ib)
-2
-2
-2
-3
-3
-3
I
O·~--""""""O.4
~.2~02-----1--::
-
5
6
5
a
6
280 !HI LI~COl~ lUOUIO~' JOUUIl VOlUMt6 MUMUR 2, 1993
• FENN. DI£DERIC.... AND STAUFFERAll Ad'lpull(-FtKusillJ, Algorithm for II Micro/IN/I., PftlllJlr Phasrd-ArraJ f/Jputlunnill SJStt'1II
o
6(.) (b)
5
E 4q~
!!-£ 3C.•~
" 2 0.6
x-distance (em) x-distance (em)
FIGURE 11. Comparison of (a) measured SAR and (b) temperature distributions (given in degrees centigrade for steady-state conditions aher 10 min) for the Microtherm-1000 system focused at 2.5 em in the pigthigh of Figure 9. In the experiment, the UCSF systematic iterative-search algorithm was used to focus theMicrotherm-l000. (Note: SAR is defined here ascAT/M, wherec is the specific heat, and AT is the change intemperature overthetime interval At = 30 sec. The location of the focusing probe is indicated with a red "+"sign.)
schemes-the UCSF systematic iterarive~search fo
cusing algorithm and the Lincoln Laboratoryadaptive gradient-search focusing algorithm-work similarly. Both algorithms result in well-defined focusedbeams :11 the desired depths. The current measurelllents do not consider field mappings in other vertical and horii'-Omal planes. Such additional mappings:lre necessary to characterize the locations and m:lgnimdes of the E-field siddobes and possible grating
lobes.It is interesting to nOte that the radiation patterns
in Figure 12 exhibit a z.-dependellt ripple with spacing approximately equal to 1.8 cm. We suspected thatthis ripple was caused by a standing-wave patterncre:necl by incident and renectcd waves in the phantom water tank. By assuming a dieleCtric constant of80 and an electrical conductiviry of 0.19 Siemens/m,we calculated the wavelength of915-MHz microwaveradiation in deionized water as 3.6 cm. Thus theripple spacing was equal to one-half the wavelength,as was expected with a standing-wave pattern.
Measurements in the muscle-equivalent phantom
were performed at a depth of 4 cm for three cases ofamplitude illumination: equal illumination applied at
100% of the selected power level of 5 W (Figure 13),adjllstcd uniform illumination with premeasured SARamplitudes for each element (Figure 14) [49], andinverse~tapered illumination with the application ofdouble the power to the outer elements (Figure 15).These radi:uion-parrcrn measurements arc similar tothose obtained in earlier studies with the UCSF systematic itcrativc~search algorithm [50]. Note that (hereare only minor differences between lhe three radia
tion patterns of Figures 13 through 15. It is very likelythat the four-element group at [he center of the arrayis the dominant contriblllor to the radiation patternshape.
Lastly, Figure [6 shows the convergence of thegradient-search steepest-ascent algorithm for the caseof the 4-cm focal depth. The measured E-field at thefOClIS has been ploned on the y-axis in power indecibels by computing 1Olog(p), where p is the value(measured power) of the AID convener. In the figure,
the focal-probe power increases by 17 dB over the 15iterations with the data indicating convergcnce inabour 7 iterations.
In the experiments. each phase shifter was controlled by an 8-bit D/A convener that had 256 states
'OlUIiE 6. NUWBIR 7. 19U 1"1 tlNCOlN lJlORUOU JOUhll 281
• FENN. DIFDERICH. A,"l'D SfAUFFERIt" AJapuw-ForwIIlf Allomh",fOr u M,rrtlU.'flIV PiJlllnr Phouni-Am'J HJpmJonmu, Spum
16
11
E
'"~0.•~" 6
1
-10 -5 o,-distance (cm)
(.j
5 10
105o,-distance (em)
(b)
-51-10
16
11
E
'"~0.•~,
\J- 6
0
FIGURE 12. Comparison 01 measured two-dimensional radiation pallerns of theMierotherm-1000 with focal depth of 8 cm in deionized-water phantom: (a) UCSF sys
lematic iterative-search focusing algorithm and (b) lincoln laboratory adaptive gradient-search focusing algorithm. (Note: The radiation patterns are shown in 10% powerlevels.)
282 I~I -eDt_LUNI"n Hllhll 'f'It11l(' '11111., 1993
• FENN, DIEDERICtI, AND STAUFFERA" NWflllY-F(I('IlJUlK A'Koml.",JPr" M,nrlu'/l/Jt' r"lIInr P"il~d.Arr".r H..y/'",IJtTm,Il Sp","
100 100 100 100
y
100 100 100 1001-
100 100 100 100
100 100 100 100
(.)
7
~5 0.1~
<i• 3~
.:.1-10 -, , 10
x-distance (em)(b)
FIGURE 13. Effect of array illuminatIon on radiation panern for the case of Uniform illumination: (a) array illumination (inpercent of the selected power level of 5 W) and (b) resulting two·dimenSlonal radiation pattern (in 10% power levels) of theMic,otherm·tOOO using Uncoln Laboratoryadaptlve-focuslOg algorithm with 4-em focal depth in a muscle-equivalent liquidphantom. Notethat the power level of each waveguide Isattoo% relative to each other. The location olthe focusing probe isindicated with a red "+" sign.
10,ox-distance (cm)
(b)
-51-10
7,-----,----,---,----,
(.)
50 60 20 60
y100 SO 70 100,-100 90 60 SO
60 60 30 SO
FIGURE 14. Effect 01 array illumination on radiation pattern for the case 01 adjusted uniform illumination: (a) array illumination (in percent of the selected power level of 5 W) and (b) resulting two-dimensional radiation pallern (in lOot. power levels)of the Microtherm-1000 using lincoln Laboratory adaptive·focusing algorithm with 4-cm focal depth in a muscle-equivalentliquid phantom. Note that the power level 01 each waveguide has been adjusted to provide equal amplitude as measured ata receive probe located at the 4-cm deplh. The location of the focusing probe is indicated with a red "+" sign.
10,ox-distance (cm)
(b)
-51-10
7,------,-----,----.-----,
(.)
50 60 20 60
Y100 40 40 100
:'-100 50 30 SO
60 60 30 SO
FIGURE t5. Effect of array illumination on radiation pallern for the case of inverse-tapered illumination: (a) array illumination (in percent of the selected power level of 5 W) and (b) resulting two-dimensional radiation pallern (in 10% power levels)of the Mlcrotherm-tOOO using Lincoln laboratory adaptive-focusing algorithm with 4-cm focal depth in a muscle-equivalentliquid phantom. The power level 01 the outer ring 01 waveguides has been adjusted to provide twice the power of the innerring of elements as measured at a receive probe located at the 4-cm depth. The location of the locusing probe is indicatedwith a red "+" sign.
"11111 , 1U1l11t' '"3 I'lll.COlI llWlllDn .IllnUI 283
• FENN, OIEOElUCH, AND STAUfFl-;RAll Ad'lplil.~-FfKlIJillg Algorithm fur tJ Mirroll''If>r Plm"lr PllIIud-Army HYf'Cl'/hamia SF",m
3<l I I
iii
'" 20 - -•,u.E
/" I. -"••0"'-
• '- -
I I-10 • 5 I. 15
Iteration number
FIGURE 16. Measured E-fieJd focused at z =4 em as a function ofadaptive phase-focusing gradient-search iteration number for a muscleequivalent liquid phantom. The power level at the focus increases by17 dB as a result of the Lincoln Laboratory adaptive gradient-searchalgorithm.
covering a range from 0° to 360°. We chose a maximum Step size!:l.4J equal co five D/A states, which wefelt would guaramee convergence and stability of thegradient~search algorithm. Each iteration rook lessthan I min to execute; thus the array was focusedautomatically in less than 10 min, which should be ashon enough time for patient ther:tpy. We did notinvestigate varying the step size tlrp, which would haveaffeC!ed [he rate of convergence.
The results from these investigations indicate thatthe low number (16) of array antenna clements, the
high anenuation of the signal from the outer ele~
menrs in the array. and the irregular beam patterns ofrhe individual elemems prevent the Micrmherm-l 000from producing a well-defined focus in lossy muscletissue al any appreciable depth beyond 3 cm. BecausetwO proven types of phase~oplimiz..ltion routinesthe systemalic ilCrativc-search algorithm and the gra~
dient-search algorithm-have yielded similar results.we are confident that the measured data truly depicllhe best possible performance of the Microtherm
1000 system.The usc of beam shaping and different illumina
tion smllegies to improve the penetradon depth couldbe investigated in the fUlme, but significant improvement is not cxpected. Instead, a better strategy for
improving the effective penelration depth might be[he use of a curved phased-array applicator with theradiating antenna e1emenu pointed directly towardthe focus {Q provide nearly equivalent path lengthsro the focal point from each radiating element. Thistype of array geometry will be investigated infuture measurements.
Conclusion
The power-deposition capabilities ofa microwave plaIlar phased-array hyperthermia system using an adaptive-focusing algorithm have been characterized experimentally at the University of California at SanFrancisco. The measurements shown in this articledemonstrate that an adaptive feedback gradient-searchfoclIsing compUler algorithm developed at LincolnLaboratory can be used to control the E-field radia~
tion pattern of the MicrOlherm-1 000. a commercial
hyperthermia system consisting of a planar phasedarray with 16 antennas operating at 915 MHz. In OLlrexperiments, the algorithm was used {Q focus the EIIdd (i.e., maximi7.e the power deposition) of the
phased array at a desired target position.The adaptive algorithm uses a gradient-search feed
back technique (method of steepest ascent). In thealgorithm, transmil-weight phase dithering is used
• FENN, DU'D1:R1CI-I. "NO ST"UFJ:ERAll Ad"pril~-FomJillg Algorithm fir fl MUnJw,/vr I'umar 1'I"md-ArrflJ HJ~rlhrrmia Splt'lII
along widl E-field probe power measuremClllS at thedesired target position to calculate the requiredgradient-search directions for sequentially and iteratively focusing all antenna element phase shifters ofthe array. The algorithm produces t'\vo-dimensionalradiation pauerns thai arc similar 10 those producedby a systematic iterative-search algorithm that hadbeen demonsrrated previously at UCSF.
E-field focusing is intended to maximize the mi
crowave power delivered 10 a tumor site relative 10 thepower deposiled in surrounding normal tissues. Thedata presented in this 3.rtide suggest that this goal canbe achieved for shallow tumors (i.e., tumors less than3 em beneath the skin surface) with a microwaveplanar phased-array hyperthermia system using adaptive focusing. Measurements have proven that a planar phased army at 915 MHz c.:m provide a usefulfocus in lossy muscle tissue (dielectric loss of 3 dBIem) at a maximum depth of 3 em.
With the hardware and software modificationsimplemented at UCSF, the 16-channel 915-MHzMicrotherm-IOOO system can now serve as a testbed
for new types of adaptive phased-array applicalOrs.Fumre measurementS may investigate other types ofapplicators, for example, a new noninvasive adaptivephased array of monopole antennas operating at915 MHz (recently designed and fabricated at Lincoln Laboratory [66]). Other non-planar array microwave applicators arc under development that shouldprovide much needed geomerric flexibility for conforming (Q contoured body surfaces. Conformablespiral microslTip antennas [461 have been used successfully for superficial hearing with noncoherent arrays, bUI (hese armys will very likely prove difficult tofocus a[ depth because of the radiated electric field,which is circularly polarized and has a somewhal
higher normal field component. An alternative designofa lightweight, conformable microwave antenna hasbeen repoTted ill Reference 67. Although this antennaalso has a complex radialed field with an electric fieldoriented radially across a circumferential gap and asignificanr normal field component. this multi-element array has demonstrated phase focusing:H depthin preliminary experiments at UCSF [681. Perhapsmost suitable for usc as a multi-element conformable
array applicator for phase-focused heating at depth is
the Current Sheet Applicator design [44,69], whichhas a linearly polarized elecrric field oriented tangentialtO the body surface for the simplified phasing ofadjacent elements.
Acknowledgments
The authors are grateful for their technical discussions with Dr. Everette C. Burdene, formerly ofL3bthermics Technologies. Inc., and currently with
Dornier Medical Systems.This work was sponsored by {he Depamnem of
[he Air Force and the N:l.[ional Insrilutes of Heallh.
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45. H.R. UoderworK!, A.F. l'et<'i"S<ll1, amI RL M~gin, ~EJeclri,
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YOtUll1 6 NUW81~ 2 U!lJ tHE LlNC01NlllORUORI JOURNIL 287
• FENN. DIEDERICH. ANI) STAUFFERAll Adnptil~Fo{l4SlI/X Algorilhm ftr (/ MirrowlIl'r Plal/Ilr PJmsrd-ATTIIJ Hyprlb..rmill SJSI<"III
..
ALAN J. FliNN
received the following dc:grtt.'iin eleclnal engineering: a B.S.in 1974 from the University oflJJinois in Chiago and all M.S.and Ph.D. in 1976 and 1978,rcspteli ...ely. from Ohio Sr:neVniversicy in Columbus.
From 1974 to 1978, Alan~ a graduate r=rch associate al the Ohio State: Vniversicy8ecuoScienee Labor-nory,whe:re: he: did rc:sc::lrc:h inphasc:d-arr.ty antenna analysis.He later beame: a senior enginc:<:r at Manin Mariett;l Aerospace in Denver. where: he~involved in br~doond antennarc:scarc:h and developmem from1978 to 1981. He joined theSLafT of Lincoln labor-nory in1981 and~ a member of the:Space Radar Technology Groupfrom 1982 to 1991, where hisrcscarc:h was in phased-atr.lYantenna dcsign, adaptive-arraynear-field tcsling, and anu:nnaand r:ldar el'O$S-scetion measure:ments. Since 1990 he hasbeen eonduCling rest;:lrc:h in theapplication of adaptive-nullingtechniques to radio frequency(RF) hype:nhermia tre;ltnlenl.He is currently an =istantleader in the RF TechnologyGroup, where he is investigating optoelectronic imegr:lted-
circuit phased-arr.ty amennasfor mobile S:llellilt communicalions applications.
In 1990 Alan w.IS acorceipitm of the IEEE Ante:nnas & Propagation Sodtly'sH.A. Whc:<:Ie:r ApplicationsPrizt raptr Award for "l'h.lS<'dArray Antenna Calibr:ltion andPautrn Prtdielion Using Mutual Coupling Mtasure:mtnts,'a paptr Ihat he: coauthored forthe 1£££ Trr1>lSilrriMS 1)1/ Amm1JJlS mull'ropng,ll;Q1/. Ht Sl'rvedas an associate: editor in tht are:aof adaptive arrays for the: 1£££1h11lSIUl;Q'/S fin AlIfrl1>las andPropaglltio" from 1989 to
1991. He is a senior member ofthe IEEE and a member of du::North American HyperthermiaSociety. and has bttn appoimcJ{O a five-year t('fm of membership in the: Institute for Systenuand Components of the8ec,romagneda ACldcmy.
He is the author or coaUlhor of more Ihan 20 articlesin his fidd and has presentedmore than 20 P.1pers al nationaland international societymc:<:tings.
CIIRIS J. DI£OERtCtl
received a B.S. degree inbioeuginccring from the University of California, SanDjll,'O, in 1984, and an M.S.and a Ph.D. degree in electricalengineering from the VnivcrsiryofAriwnain 1986 and1990, resp«:livdy. Ht is an;!.'\Sistant adjunct profo:ssor atlhe UniversityofDJifornia,San Fr.tneisco, RadiationOncology Department, where:his focus ofrc:serrch has Ix-tnon the design and developmentofhypenhermia devices andother thermal therapy ddiVl:ryS)'Slcms. Chris is a member ofthe Nonh Americlll Hyperthermia Socicry.
PAUl R. STAUI'I'I'.R
n~ctived a BoA. <Iegrcc in physics from the College ofWOOSterin 1975 and an M.S. dq;rc:<: indectrial cngineering from theVni...el$ity ofAriwna in 1979.He is an :tSSOCiate adjunctprofessor at the University ofC:llifornia, San Francisco,Radiation Oncology Depan~
menl. where his focus of r('~
search has been on thc engineering dcvelopmellt of RF,microw:we, and uhl':lSOundhyperthermia technologies. Hehas receivN a clinical engincc:ringcerlification from thellllernaliollal CertificationCommission and a mediClIphysies cerrificatinn in hyperthermia from the Amer;c:tnBo.1rd of MediClll'hysia. He isa mcmbe:rof the North Am('rie:tn Hypenhennia Society.
288 rH( lINeOL' t.llO~1101Y JOU~~.t ~lltUIII 6. ~UIlII'11U3