Development of a lateral migration radiography image generation and object recognition system

107 Pages·1997·3.5 MB·English

by  Wehlburg, Joseph Cornelis

DEVELOPMENTOFALATERALMIGRATIONRADIOGRAPHY
IMAGEGENERATIONANDOBJECTRECOGNITIONSYSTEM
By
JOSEPHCORNELISWEHLBURG
ADISSERTATIONPRESENTEDTOTHEGRADUATESCHOOL
OFTHEUNIVERSITYOFFLORIDAINPARTIALFULFILLMENT
OFTHEREQUIREMENTSFORTHEDEGREEOF
DOCTOROFPHILOSOPHY
UNIVERSITYOFFLORIDA
1997
Thisdissertationisdedicatedtomyparents Dr. andMrs. Wehlburg,mywife
ChristineandmydaughterRachel.
ACKNOWLEDGMENTS
The author expresses his appreciation for the guidance and financial support
providedbyhisacademicadvisor,Dr.AlanJacobs. Theauthorisadditionallyindebted
totherestofhisPh.D.committee,Dr.EdwardDugan,Dr.SamimAnghaie,Dr.Andrew
LaineandDr.MartinValaforthesupportandencouragement. Theauthorwouldalso
liketothankDanEkdahlforallhishelpwiththeelectronics TheauthorthankstheU.S.
ArmyCECOMNightVisionandElectronicSensorDirectorateforprovidingfinancial
supportforconductingthisresearch.
TheauthorexpressesspecialthankstoDr.ShyamKeshavmurthyforallthehelp
thathegaveduringthecourseoftheproject.
Finally,theauthorwouldliketoacknowledgethatthisdissertationwouldnot
have been completed without his wife, Christine, and all her editorial guidance,
understandingandsupport.
iii
TABLEOFCONTENTS
page
ACKNOWLEDGMENTS
iii
ABSTRACT vi
CHAPTERS
INTRODUCTION
1 1
2 CONFIGURATIONFORTHELATERALMIGRATION
RADIOGRAPHYSYSTEM 6
TheX-rayMachine 6
TheSoilbox 10
TheMotionControlandDataAcquisitionSystem 10
TheDetectorSystem 13
PreviousDetectorDesigns 13
NewDetectorDesign 16
DetectorSizeVariation 18
CurrentDetectorConfiguration 22
SignalProcessing 22
3 RESULTSANDDISCUSSION 26
ImageProcessing 26
Filtering 26
SurfaceFeatureIdentification 26
BuriedFeatureIdentification 32
DepthofBurial 37
DetectorHeightVariations 40
SurfaceVariations 47
ResolutionImprovement 50
PatternRecognition 57
4 CONCLUSIONS 67
iv
APPENDICES
A CODEFORTHEMOTIONCONTROLANDDATA
ACQUISITIONSYSTEM 72
B SURFACEFEATUREDETECTIONCODE 78
C BURIEDFEATUREDETECTIONCODE 85
DPATTERNRECOGNITIONCODE 93
LISTOFREFERENCES 96
BIOGRAPHICALSKETCH 98
v
AbstractofDissertationPresentedtotheGraduateSchool
oftheUniversityofFloridainPartialFulfillmentofthe
RequirementsfortheDegreeofDoctorofPhilosophy
DEVELOPMENTOFALATERALMIGRATIONRADIOGRAPHYIMAGE
GENERATIONANDOBJECTRECOGNITIONSYSTEM
By
JosephCornellsWehlburg
May1997
Chairman: AlanM.Jacobs
MajorDepartment: NuclearandRadiologicalEngineeringSciences
Compton Backscatter Imaging (CBI) has always been impeded by inefficient
sensingofinformationcarryingphotons,andextensivestructurednoiseduetoobject
surfacefeaturesandheterogeneity. Inthisresearchproject,anewvariantofCBI,which
substantiallyresolvesbothimpediments,issuggested,developedandrigorouslytestedby
application to a difficult imaging problem. The new approach is termed Lateral
MigrationRadiography(LMR)whichaptlydescribesthespecificphotonhistoryprocess
givingrisetoresultingimagecontrast.
Thephotonsemployedinthisresearchareconventionallygeneratedxrays. A
pencil x-ray beam with a typical filtered-bremsstrahlung photon energy spectrum is
perpendicularlyincidentupon,andsystematicallyrasteredover,theobjecttobeimaged.
Efficientsensingofinformation-carryingphotonsisachievedbyemployinglarge-area
detectorswithsensitiveplanesperpendiculartotheincidentbeam. Ageometricarrayof
vi
a group of such detectors along with varying degrees of detector collimation to
discriminatesingly-scatteredfrommultiple-scattered,detectedxraysisdeveloped. The
directoutputofthedetector-arraycomponents isalgebraicallycombinedtoeliminate
imagecloakingbysurfacefeaturesandheterogeneity. Imagecontrastisgeneratedbythe
variation of x-ray interaction probabilities in the internal details relative to the
surroundingmaterial. ThesemajorimprovementstoconventionalCBIhaveallowedthe
detectionofinternalswithclaritysuchthatrecognitionoftheinternalfeaturesviathe
imagedetailsispossibleincaseswhereordinaryCBIcannotevendetectthepresenceof
theinternalstructure.
The test application is the detection and recognition of all-plastic antitank
landminesburiedinsoilatdepthsofuptothreeinches. Inthemilitaryapplicationof
clearing 12 inchdiametermines from 14-foot-wide tank-lanes, the spatial resolution
requirementofoneinchandthespeedof3to5mphoverroughterrainandclutteredsoil
surfaces presents a formidable detection and recognition problem. This application
requiresspecialx-raygenerators,whichareunderdevelopmentelsewhere. Thisresearch
projectclearlydemonstratesthat,givenasatisfactoryx-raygenerator,LMRprovidesa
viablelandmineimageformationandrecognitiontechnique. Potentialapplicationofthe
new LMR technique to general non-destructive inspection (NDI) problems are also
discussed.
vii
CHAPTER
1
INTRODUCTION
LateralMigrationRadiography(LMR)1,2,3'4'5 isaformofComptonBackscatter
Imaging(CBI). Comptonscatteringoccurswhenaphotoninteractswithanelectronand
transfersenergytotheelectronbykinematiclaws.6 Forthematerialsthatareofinterest
for the example studied, landmine detection, the Compton effect dominates. The
backscattermodalityhastheadvantageofnotrequiringadetectorbehindtheobjectbeing
imaged,allowingnon-intrusiveimaging. Somecurrentlymarketedsystemsthatemploy
thebackscattermodalityareComptonx-raybackscatterinspectionsystem(ComScan)7
andDynamicRadiography.8
OneofthelimitsofCBIsystemsisinefficientdetectionofinformationcarrying
photons. ThedetectionabilityofCBIsystemsisalsoimpairedbythepresenceofsurface
featuresandheterogeneitywhichcausestructurednoisetoappearintheimages. This
researchfocusedondevelopingasystemwhichwouldremovemanyofthelimitations
fromtheCBIsystem. TheproblemsinherentinCBIsystemsthatwereaddressedare:
separationofsurfaceandsubsurfacefeatures,detectionefficiency,anddetector-to-target
distancevariations(detectorheightvariation). Othertopicsthatwereinvestigatedduring
thisresearchare:imageresolution,detectordesign,andpatternrecognition.
Lateral migration for this project is the motion ofphotons transverse to the
incidentbeamthroughobjects. Photonsthatundergolateralmigrationandaredetected
mustexperiencemorethanonecollision. Inordertodetectphotonsthatundergolateral
1
2
migration,collimateddetectorsmustbeused. Thecollimatorspreventphotonsthathave
onlyundergoneonecollisionfromreachingthedetectorsshieldedbythecollimators.
Conventional CBIsystemsusedonlytwocollimateddetectors.9 Sincephotonsthathave
experienced lateral migrations containinformationaboutobjectsbelowthe surface it
seemsthatonlycollimateddetectorswouldberequiredtoidentifysubsurfacefeatures.
Theproblemwiththisconclusionisthephoton'spathisinfluencedbysurfacefeaturesthe
photonpassedthrough. Thismeans thatthe collimated detectors' response contains
informationaboutthesurfaceofanobjectmixedinwiththesubsurfaceinformation. In
orderto separate surface from subsurface feature information, uncollimateddetectors
wereadded. Photonsthathitthesurfaceandscatterbackcanbeefficientlydetectedbyan
uncollimateddetectorplacednearthex-raysource. Thiscombinationofcollimatedand
uncollimateddetectorsallowssubsurfaceobjectstobeseparatedfromsurfaceobjects.
In the energy range of 10 keV to 10 MeV there are three possible photon
interactions: Compton scatter (incoherent scatter), the photoelectric effect, and pair
production.6 TheenergyrangeinwhichtheLMRsystemoperates,whenappliedtothe
problemthatwasusedtoevaluateit,landminedetection,iswellbelowthe1.02MeVthat
isrequiredforpairproduction. This makestheonly interactions ofinterestforthis
applicationComptonscatteringandthephotoelectriceffect.
TheLMRsystemrespondstochangesintheComptonscatteringcrosssection(as)
andthephotoelectric effectcross section(aa)inthetargetmedium. To identifythe
behavioroftheLMRsystemallthephotonscreatedbyinteractionsmustbeaccounted
for,evenonesthatoccuraftertheprimaryeffectsoftheinteraction. Afteraphotoelectric
3
interactionaphotonisemittedwhenanelectronisabsorbedbackintothecreatedvacancy
(phosphorescence). Thephosphorescencephotonhasaverysmallprobabilityofreaching
thedetector,sothephotoelectriceffectactsasanabsorptionprocessforLMRsystems.
Table1givesalistofphosphorescence(K-edge)energiesforsomeelementsthatareof
interesttothelandminedetectionsystem. Typicalenergiesthatcanreachthedetectorare
ontheorderof20keVorgreaterforthecurrentsystemasappliedtolandminedetection.
Table1. Phosphorescenceenergyforsomeoftheelementsofinterestinlandmine
detection
Element H C N O Si Fe
Z 1.00 6.00 7.00 8.00 14.00 26.00
K-edgeenergy 0.01 0.28 0.40 0.53 1.84 7.11
keV
Source:Attix
Lateral migration occurs in all objects, surface orburied, high or lowas/cra.
Lateralmigrationonlyplaysasignificantrolewhenmaterialshaveareasonablydifferent
as/o-arelativetothesurroundingmediaandareimagedwithcollimateddetectors. An
objectwithahighas/aaallowsmorephotonstosurviveiftheytravelthroughtheobject
insteadofthroughthesurroundingmedia. Themigrationeffectyieldsanincreaseinthe
collimateddetectorresponsewhenthe source is overaburiedobjectthatallowsthe
photontohaveagreaterdistanceoftravelintheobjectthaninthesurroundingmedium.
Table2showscalculatedMonteCarlon-particle(MCNP)simulationresultsfor
theenergydepositionofdifferentcollisioncomponentsforthecollimateddetectors.