emerging imaging sensor technologies from aerospace to healthcare

8/3/2019 Emerging Imaging Sensor Technologies From Aerospace to Healthcare

1/4

George C. Giakos

ignificant growth in medicine, biology, aerospace, re- thetic aperture sonar (SAS) for ocean acoustic tomographymote sensing, meteorology, oceanography, and envi- (OAT), have been developed durmg the last decade Reliableronmental concerns, as well as military and detection, trackmg, and motion estimation of moving objects

industrial applications, has stimulated the rapid are key parameters for these applications Currently, a chal-progress in imagmg technologies durmg the last

decade. Overall, the new technologies will merge, and areexpected to play an even larger role in these fields In par-ticular, the front-end electronics, for example, the detectoror sensor, data acqu isition electronics, and processmg orpostprocessing hardware and software are the primary fo-cus of any imaging system. But, the fmal image quality de-pends on a variety of parameters directly related to eachstage of the imagmg systemRadar target detection, classification, imaging, andidentification, remote sensing and radar meteorology;and machine vision, for marine, ground, air, andspace-born military and civilian applications can all bedone through synthetic aperture radars (SARs) SAR sys-tems generate sequential images of swath areas obtainedthrough inverse scattering of microwaves or lasers trans-mitted through a single moving radar apertu re In SARradars, an improved azimuth resolution of the outp utimages can be achieved through aperture synthesis in thedirection of flight. Conventional SARs measure the mag-nitude of the backscattered signals from a given location forspecific transmission of polarization configurations. On theother hand, a SAR operating on polarimetric principles mea-sures both the magnitude and phase of any receive-transmitpolarizations. In fact, radar polarimetry is an importan t tool inmodern electromagnetic sensor technology that provides im-proved target detection and discrimination[l], 21.

Similarly, potential imaging techniques for oceanographicengineering applications, such as sector-scanning sonars,la-ser-illuminated underwater video imaging systems, and syn-

lenging problem to the scientific community is the develop-ment of potential battlefield imaging sensors, which willprovide necessary surveillance data capable of ensuring con-tinuous monitoring of the battlefield parameters.

The whole concept can be realized through the synergisticrelationship between SAR and Moving Target Indicator Ra-dars (MTIs) 121. A diagram of this novel battlefield imagingtechnique is shown in Fig.1.This synergistic technique relieson the use of the SAR, which combines high -resolution imag-ing of stationary objects, with MTI radar , which has the abil-ity to detect and locate moving targets. Such "wide-area-

16 IEEE lnstrumen ation & Measurement Magazine1094-6969/98/$10.0001998IEEE

December 1998


2/4

Fig. 7. Shown here is battlefield im aging via a synergistic relationship between SAR and MTI.

surveillance sensors would cover regions larger than 400km, with a revisit time of less that an hour. As a result, theU.S. has begun developing two unmanned aerial vehicles(UAVs) which will operate a t high altitude aswide-area-surveillance platforms. Obviously, the use ofmore than one UAV reduces th e surveillance revisit time rel-ative to the transporter-erector-launchers (TELS).

Interestingly enough, the likelihood ofa TEL detection, clas-sification, and attack increases significantly with the mix ofSAR and MTI. Consequently, new radar and sensor technolo-gies based on lightwave, electro-optical, and acoustical princi-ples will be developed, and are expected to playa key role infuture confrontations. Furthermore, the implementation of theSAR with automated target recognition algorithms andfalse-alarm mitigation techniques, as wellas the developmentof two-dimensional inverse SAR (EAR) algorithms, will im-prove the performance of the SAR and MTI, respectively.

In the healthcare industry, efforts are underway to developX-ray digital imaging technology, which uses high-efficiencyelectronic sensors in combination with advanced computing,known as digital radiography. Digital radiography has manyadvantages over conventional radiography such as high dy-namic range, fast image acquisition and display, digital archiv-ing and retr ieval systems, teleradiology, display of storedimages without degradation, extended capabilities of dataanalysis and image processing, and reduced patient dose. Sev-eral detectors have been proposed for digital radiography, al-though there is no single technology that addresses all theissues for op timal imaging.

The Mu ltimedia Imaging DetectorRecently, the principles ofa novel multimedia imaging detectorfor medical imaging, with specific emphasis on dual-energy ra-diography have been introduced [3].This particular device oper-ates on gaseous solid-state ionization principles. Dual-energyimaging has attracted a number of investigators with their main

emphases on mammography and chestradiography [4]. Dual-energy imagingmvolves the use of two X-ray Images,one produced from a high-energy poly-chromatic spectra and another from alow-energy polychromatic spectra. Aweighted subtraction of these two im-ages produces a digital irnage, whicheliminates interferingbackground struc-tures. By simpllfyLng the backgroundstructure in this way, an increase in thedetectability or conspicuityof the tar-get structure is obtained.

The proposed detector technologymay have significant applications inmedical, industrial, and aerospace im-aging. The multimedia detectors com-bine the high-energy absorptionefficiency of the so lid ionization detec-tors, with the high spatial resolution

resulting from the fine microstrip collector size and high gain[ 5 ] .Therefore, a good spatial and contrast resolution, ow radia-tion dose, results. In addition, there isa greater degree of free-dom in designing and optimizing a dual-energy system.Specifically, the proposed detector consists of th ree elements,namely, a front detector element, an inactive midfilter segment,and a rear detector element. It can be operated under apixelated, slotted-scanned geometry ora strip-beam scanninggeometry. The front detector element,a gas microsirip detector[5], produces the digital low-energy photon image ,and he reardetector element, a semiconductor detector, i.e., a Cd,.,ZqTedetector [6], produces the digital high-energy photon image.

Gas-microstrip detectors [5] are very promising, ul-tra-high-resolution, high-internal-gain, and low-noise devices,originally proposed a few years ago. Later, they were developedfor imaging applications with emphasis on aerospace researchand high-energy physics. The fabrication of microstrip detectorsapplies photolithographic techniques commonly u!jed to makemaskplates for the semiconductor ndustry . Asa result, this tech-nology replaces anode-cathode wires with ultra-fine layers ofconductivestrips, arranged in an anode-cathodepatitern on insu-lating or partially insulating glass substrate. The high degree ofaccuracy that can be achieved with the photolithographic ech-niques ensures the microstrip detectors high gain uniformityover large areas. Advantages of gas-microstrip detectors ncludehigh spatial and contrast resolution, as well as excellentmechan-ical stability. The high resolution results from the fine collectorsize, high gain, and moderate operating gas pressures.

On the other hand, high absorption efficiency may be achievedby utilizinga high-density,high atomic number Cti,.,ZqTe [6]substrate as the rear detector element. Currently, high qualityCd,.,Zn,Te semiconductor crystals have been ~ O V M sing theHigh-pressureBridgeman (HPB) echnique.Specificailly, by alloy-ing CdTe withZn the bulk resistivityof thisnew semiconductor sapproximately 1011 cm. This high resistivity is due to the wide

December 1998 IEEE Instrumentation& Measurement Magazine 17


3/4

Scan Direction-rcElectric SemiconductorField Lines ,-etector Volume

-- BiasElectrode

' Anode' MicrostripCathode(CollectionElectrode)Fig,2. Th e experimental slot-scanned mu ltimedia detector for dual-energyimaging, of th e University of Akron Imagin g Devices, Detectors and SensorsLaboratory, Department of Biomedical Engineering.band gap of tlus ternary semiconductor.As a result, low leakagecurrents, and consequently, ow noise characteristics are seen.

Cd,.,ZnxTe detectors have a high stopp ing power duetotheir high mass density (5.8g/cm3 ) and effective atomic num-ber of 49.6 (Cd0.9:48,ZnO.l: 0, Te:52).This would allow for adecreased detector thickness, and consequently, mproved spa-tial resolution. Currently , Cd,.,Zn,Te detectors appear to be ex-cellent candidates for digital radiography. Overall, the primaryadvantages of these solid-state ionization detectors are: effi-cient radiation absorption, good linearity, high stability, highsensitivity, and wide dynamic range.

There are several advantages to using the multimedia detec-tor. For instance, a large signal amplihcation n the front detectorelement s obtained, due to thegasamplificationpropertiesof themicrostrip. Therefore, images can be obtained at a low radiationdose, while at relatively low operatinggas pressures. Tlus detec-tor technology also promises a good spatial resolution, due to theul tra-he structure of the microstrip substrate, he high scatter re-jection of the slot scanning beam detector geometry, as well as re-duced space-chargeeffects due to the rapid ion collectiontime. Inaddition, high versatihty arises from the optimization of the frontdetector element upon suitably chosen operating gas pressureand ga s mixture. In addition, the use of a Cd,.,ZnxTe semicon-ductor as a rear detector element, would allow the fabrication ofhigh-energy,absorption efficient,tlvndetector substrates.Finally,enhanced image quality will result if the systemis implementedwith kmestatic and time delay integration(TDI)principles.

A hagram of the experimental, single-element, slotted-scannedmultimedia detector,isshown in Fig.2.A large field-of-viewproto-type of h s technology should include a pixelated ormultdine-stripped real collector element for slot or multilinescanned-beam imaging, respectively. Incident X-rays, spend part oftheir energy in the xenon gas detector volume, for an active depthof interaction of 1 cm. The other part, through the interaction,isspent in the solid-state detector volume, producing, in both cases,ion-electron pairs, and holes-electrons pairs, respectively (Fig.2).An applied electric field imparts a constantdr& velocity to these

charges,dnving them toward their respective signal collectors. Theprimary electrons produced bydirectX-ray ionization of the gasdrift toward the microstrip plate. Wlen they reach the microstripsubstrate, he electrons move toward the positive strip. There, the>7experience an avalanche amplification, due to the high fieldstrength caused by the quasi-dipole, anode-cathodeconfiguration.The ions are collected rapidly on the adjacent cathode,p i n g ise tothe detected image signal.

The signal-to-noise atio (S/N), is plottedas a function of thesquare root current (mA) setting, for the front (gas microstrip)and rear detector element (Cd,-,Zn,Te) ,at 100 kVp, and timeexposure of 1s, at 1atmosphere of xenon (Fig.3). n this experi-ment, the applied microstrip anode bias voltage and the ap-plied bias voltage on the semiconductor were set at400 and 10 0V, respectively.TheS/N of the rear detecto r element as a func-tion of the square root current (mA) setting, either in anempty-gas vessel (air) or after X-rays have already spent part oftheir energy in a xenon-filled detector volumeis plotted in Fig.4. As seen in both Figs. 3 and 4, a high S/N s obtained.

In addition, the S/N data can be fit toa straight line whenplotted against the square rootof the tube current. Interestinglyenough, the S/N ratios reported here, as well as in previousworks, rely on row data.As a result, the real potential of the de-tector is underestimated. In general, a multifold S/N ratio ir-crease may result by using normalized values, with respect toreference data. Work is currently underway to explore the im-aging potential and applications of this technology.The resultsare part of a preliminary evaluation of the test detector for fu-ture dual-energy studies, as part of the development of aslot-scanned, dual-energy digital radiographic system.Molecular BioelectromagneticsLenses aimed at focusing the radiation from primary sourcesinto desired directions (Le.; the parabolic reflector), have beenused for almost one-half century for remote and deep-spacecommunications, with particular emphasis on radioastrono-my and radar technology. One example is the extension of thesolid lens concept to microwaves through the use of "artificialdielectrics." Notably, these artificial-dielectric enses comprisedifferently shaped particles such as metal spheres, disks,

1 300,250

20 0I5010050

I6 a 10 12 14 16

Square Root of Tube Current [mA'"]Fig.3. Signal-to-noise versus square root current (mA) setting, for the front(gas microstrip) and rear detector element (Cdl-xZnxTe), at 100 kVp. and timeexposure of 1 s, at 1 atmosphere of xenon

18 IEEE lnstrumeniafion& Measurement Magazine December 1998


4/4

30Square Root of Tube Current [mA2] I

Fig, 4. Signal-to-noise ratio of the rear detector element (Cdt-xZnxTe) versussquare root current (mA) setting, either in an empty-gas vessel (air), or after X-rays have already spent part of their energy in a xenon-filled detector volume. Ittakes place at 1 and 2 atmospheres of xenon, fo r an active depth of interaction of1 cm, at 100 kVp, and time exposure of 1 s.strips , or rods embedded in a dielectric material. If the parti-cles deviate from the spherical symmetry, they act as a smallelectricdipole. Here, they contribute to the total displacement,and thus, to an effective dielectric constant.It is worth noting that the presence of electric dipoles increasesthe local electrical field, giving rise to enhanced focusing proper-ties. Expanding that concept to the study of tissue pathology andthe diagnosisand treatment of diseases, t seems that the exploita-tion of a non-toxic polar molecule, combined with non-ionizingelectromagnetic imaging techniques, might prove a challengingtask to the engineeringcommunity.

Future Trends and ApplicationsAnother area of interest is the application of microwaves formedical imaging. Additionally, there is the exploitation of tech-niques aimed at the maximizing of the extracted imaging infor-mation. It is well-known hat microwave imaging is molecularly,rather than atomically based interactions of radiation, whencompared to X-ray imaging. As a result, active microwave imageformation is characterizedby the use of microwave sources o in-terrogate tissue electrical properties, and thereby retrieve thephysiological statusof the biological system. Limitationsof mi-crowave imaging arise due to the limited spatial resolution,which is lower when compared to X-rays.

On the other hand, the electrical properties of biologicaltissue offer higher contrast resolution when compared withX-rays or ultrasound waves. Therefore, n imaging, modalitiesconsisting of concomitant use of microwaves and X-rays orgamma rays, are proposed. In the future, this hybrid modalitymay be proved very appealing in the medical imaging arena.The use of inverse-scattering techniques with radar polarityanalysis would further strengthen the imaging capabilitiesofthe proposed technology, although much work remains to bedone. Finally, the embodiment of the tissue of polar dopants,may also enhance key microwave imaging parameters such ascontrast and spatial resolution.

Also challenging is the use of gas-microstrip detectors formedical imaging applications. Originally, gas microstrip detec-tors were developed for use in high-energy physics, X-ray as-tronomy, and synchrotron radiation facilities.The application ofgas microstrips into medical detector technology,is expected tobreak down the technologicalbarriers in several areas of contin-uous and discrete imaging applications.

Furthermore, the introduction of multimedia detectors formedical imaging, is appealing in a wide array of industrial andaerospace applications. For instance, multimedia detectorsmay be app lied in dual-energy X-ray imaging for use in avia-tion security.In this way, they could be used to discriminate be-tween organic and inorganic material as well asin industrialcomputed tomography (CT).Specifically, he front detector ele-ment, a gas microstrip detector, would produce the digital,low-energy photon image and the rear detector element, asemiconductor detector ( a Cdl-xZnxTe detector) would pro-duce the digital, high-energy photon image.ConclusionThe identification and transfer of imaging solutions into dif-ferent technological areas will create potential advanced solu-tions in different domains of science and technology. Newimaging technologies will merge, and are expected to play anever-expanding role in the civilian and military applicationsof the nex t century.AcknowledgmentsThis work was supported partially by a Faculty Research En-hancement Grant awarded by the Institute of Biomedical En-gineering Research (IBER).The author wishes to thank SamirChowdhury for his fine measurements.References[ l ]D. Giuli, Polarization Diversity in Rad ar, Proc. IEEE, vol. 74, no .

2, pp . 245-269,1986.[ 2 ]W.M. Boerner, Polarization Dependence in ElectromagneticInverse Problems, I E E E Transactions on Antenn as and Propagation,vol. 29, no. 2, pp . 262-271, 1981.

[3]G.C. Giakos, A Slot-Scanned Detector Operating on Gas-SolidState Imag ing Principles, IEEE Instrumentation and MeasurementTechnology C onference, St. Paul, MN, May 18-21,1998, PYOC.ol . 1,pp. 352-357,1998.

[4] R.J. Endo rf, S. Kulatunga, D.C. Spelic, S.R. Thomas, F.A.DiBianca, H.D. Zeman, a nd G.C. Giakos, PreliminaryPerformance Characteristics of a D ual-Energy KCD, Int. Soc.Opt. En8 I S P I E ) , vol. 2432, pp . 607-615,1995.

[5] A. Oed, Position-sensitive Detector with Microstrip Anode forElectron Mu ltiplication with Gases, Nuclear Instruments andMethods in Physics Research, vol. A263, pp. 351-359,1988.

[6]G . C. Giakos, B. Pillai, S. Vedantham, S. Chowdhury, A.Dasgupta, D.B. Richardson, P. Ghotra,R.J. Endo rf, A . Passalaqua,and W.J. Davros, Optimization of Cdl-xZn xTe Detectors forDigital R adiography, Jotivnal of X-ray Science and Technology, pp .37-49,1997.

December 1998 IEEE Instrumentation& Measurement Magazine 19

emerging imaging sensor technologies from aerospace to healthcare

Documents