BAYESIAN BELIEF NETWORKS AS A DIAGNOSTIC TOOL FOR PULMONARY EMBOLISM

BAYESIAN󰀀BELIEF󰀀NETWORKS󰀀󰀀

AS󰀀A󰀀DIAGNOSTIC󰀀TOOL󰀀FOR󰀀PULMONARY󰀀EMBOLISM󰀀

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Rohit󰀀Bodas1,󰀀Shailesh󰀀Deshpande1,󰀀Gursel󰀀Serpen1,󰀀E.󰀀Ishmael󰀀Parsai2,󰀀󰀀

Robert󰀀J.󰀀Coombs3󰀀and󰀀Lee󰀀S.󰀀Woldenberg3󰀀1Electrical󰀀Engineering󰀀&󰀀Computer󰀀Science,󰀀The󰀀University󰀀of󰀀Toledo󰀀2Department󰀀of󰀀Radiation󰀀Therapy,󰀀Medical󰀀College󰀀of󰀀Ohio󰀀3Department󰀀of󰀀Radiology,󰀀Medical󰀀College󰀀of󰀀Ohio󰀀

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ABSTRACT󰀀

A󰀀Bayesian󰀀Belief󰀀Network󰀀is󰀀employed󰀀to󰀀interpret󰀀the󰀀perfusion-ventilation󰀀lung󰀀scans󰀀along󰀀

with󰀀correlated󰀀chest󰀀x-rays󰀀in󰀀order󰀀to󰀀assist󰀀in󰀀diagnosis󰀀of󰀀pulmonary󰀀embolism.󰀀󰀀A󰀀rule󰀀base󰀀

is󰀀applied󰀀to󰀀interpret󰀀the󰀀images󰀀for󰀀the󰀀possibility󰀀of󰀀pulmonary󰀀embolism.󰀀The󰀀rule-base󰀀is󰀀

formulated󰀀using󰀀the󰀀modified󰀀PIOPED󰀀criteria󰀀and󰀀probabilities󰀀are󰀀assigned󰀀to󰀀different󰀀

hypotheses󰀀based󰀀on󰀀the󰀀patient󰀀data󰀀collected󰀀by󰀀the󰀀PIOPED󰀀investigators.󰀀These󰀀form󰀀the󰀀

prior󰀀probabilities󰀀for󰀀the󰀀hypothesis󰀀that󰀀pulmonary󰀀embolism󰀀is󰀀present󰀀in󰀀a󰀀patient.󰀀Based󰀀on󰀀

these󰀀probabilities,󰀀an󰀀inference󰀀is󰀀drawn󰀀in󰀀terms󰀀of󰀀probability󰀀value󰀀regarding󰀀the󰀀degree󰀀of󰀀

pulmonary󰀀embolism󰀀in󰀀a󰀀given󰀀patient.󰀀󰀀󰀀Testing󰀀results󰀀indicated󰀀that󰀀the󰀀Bayesian󰀀Belief󰀀

Network󰀀was󰀀able󰀀to󰀀implement󰀀the󰀀PIOPED󰀀criteria󰀀in󰀀interpreting󰀀the󰀀lung󰀀scans󰀀for󰀀diagnosis󰀀

of󰀀pulmonary󰀀embolism.󰀀󰀀The󰀀proposed󰀀probabilistic󰀀reasoning󰀀system󰀀aims󰀀to󰀀reduce󰀀inter-

observer󰀀variability󰀀in󰀀interpretation󰀀of󰀀lung󰀀scans󰀀and󰀀assist󰀀experienced󰀀as󰀀well󰀀as󰀀

inexperienced󰀀observers󰀀in󰀀drawing󰀀an󰀀inference󰀀regarding󰀀the󰀀presence󰀀of󰀀pulmonary󰀀embolism󰀀

in󰀀a󰀀given󰀀patient.󰀀󰀀󰀀

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INTRODUCTION󰀀

Pulmonary󰀀embolism󰀀(PE)󰀀has󰀀been󰀀and󰀀continues󰀀to󰀀be󰀀a󰀀major󰀀health󰀀problem󰀀with󰀀considerable󰀀

controversy󰀀surrounding󰀀the󰀀diagnostic󰀀approaches󰀀and󰀀their󰀀interpretation.󰀀It󰀀has󰀀been󰀀estimated󰀀that󰀀

over󰀀50,000󰀀people󰀀die󰀀annually󰀀from󰀀this󰀀disease󰀀in󰀀the󰀀United󰀀States󰀀without󰀀being󰀀diagnosed󰀀[NIH󰀀

Online󰀀Statement,󰀀1999].󰀀󰀀The󰀀high󰀀costs󰀀associated󰀀with󰀀the󰀀definitive󰀀diagnostic󰀀procedure󰀀of󰀀

pulmonary󰀀angiography󰀀and󰀀the󰀀nonspecificity󰀀of󰀀clinical󰀀signs󰀀and󰀀symptoms󰀀contribute󰀀greatly󰀀to󰀀

diagnostic󰀀problems󰀀of󰀀PE.󰀀󰀀Most󰀀pulmonary󰀀medicine󰀀physicians󰀀and󰀀angiographers󰀀agree󰀀that󰀀a󰀀

perfusion/ventilation󰀀study󰀀showing󰀀mismatched󰀀abnormalities󰀀is󰀀the󰀀most󰀀reliable󰀀non-intrusive󰀀

procedure󰀀to󰀀diagnose󰀀PE󰀀[Mettler󰀀et.󰀀al.,󰀀1991].󰀀

Pulmonary󰀀perfusion󰀀imaging󰀀is󰀀based󰀀on󰀀the󰀀principle󰀀of󰀀capillary󰀀blockade.󰀀󰀀Particles󰀀larger󰀀than󰀀

the󰀀size󰀀of󰀀the󰀀smallest󰀀capillaries󰀀are󰀀trapped󰀀in󰀀the󰀀first󰀀capillary󰀀bed󰀀they󰀀reach󰀀after󰀀peripheral󰀀

intravenous󰀀injection.󰀀󰀀Labeled󰀀particles󰀀are󰀀injected󰀀intravenously󰀀and󰀀trapped󰀀in󰀀the󰀀capillary󰀀bed󰀀of󰀀

the󰀀lungs.󰀀󰀀The󰀀distribution󰀀of󰀀these󰀀particles󰀀in󰀀the󰀀lung󰀀capillaries󰀀gives󰀀a󰀀true󰀀reflection󰀀of󰀀the󰀀

distribution󰀀of󰀀the󰀀pulmonary󰀀artery󰀀blood󰀀flow󰀀in󰀀the󰀀lungs.󰀀󰀀Tc-99m,󰀀Macro󰀀Aggrevated󰀀Albumin,󰀀is󰀀

mostly󰀀used󰀀in󰀀perfusion󰀀scanning.󰀀󰀀Ventilation󰀀images󰀀describe󰀀the󰀀regional󰀀patterns󰀀of󰀀washin󰀀and󰀀

washout󰀀of󰀀materials󰀀inspired󰀀in󰀀the󰀀lungs.󰀀󰀀Radioactive󰀀substances󰀀(Xe-133)󰀀follow󰀀the󰀀behavior󰀀of󰀀

ventilation󰀀tracks󰀀and󰀀segmental󰀀ventilation󰀀of󰀀the󰀀lungs.󰀀󰀀The󰀀perfusion󰀀scans󰀀and󰀀the󰀀ventilation󰀀

scans󰀀are󰀀complementary󰀀of󰀀one󰀀another󰀀and󰀀present󰀀the󰀀entire󰀀picture󰀀of󰀀blood󰀀flow󰀀as󰀀well󰀀as󰀀air󰀀

flow󰀀within󰀀the󰀀lungs.󰀀

Therefore󰀀in󰀀case󰀀of󰀀PE,󰀀defects󰀀will󰀀be󰀀observed󰀀on󰀀the󰀀perfusion󰀀scans󰀀but󰀀the󰀀ventilation󰀀scans󰀀

will󰀀not󰀀have󰀀the󰀀same󰀀defects.󰀀A󰀀mismatch󰀀is󰀀required󰀀to󰀀show󰀀presence󰀀of󰀀PE󰀀[Biello󰀀et.󰀀al.,󰀀1979].󰀀󰀀

In󰀀case󰀀the󰀀defects󰀀overlap,󰀀it󰀀can󰀀be󰀀interpreted󰀀that󰀀possibility󰀀of󰀀PE󰀀is󰀀low󰀀and󰀀perfusion󰀀defects󰀀

exist󰀀due󰀀to󰀀causes󰀀other󰀀than󰀀PE.󰀀󰀀󰀀󰀀

The󰀀proposed󰀀approach󰀀employs󰀀Bayesian󰀀Belief󰀀Networks󰀀(BBN)󰀀[Heckerman󰀀et.󰀀al.,󰀀1995]󰀀to󰀀

interpret󰀀lung󰀀scans󰀀(perfusion,󰀀ventilation󰀀and󰀀chest󰀀x-rays)󰀀in󰀀order󰀀to󰀀assist󰀀in󰀀diagnosis󰀀of󰀀PE.󰀀󰀀A󰀀rule󰀀base󰀀is󰀀applied󰀀to󰀀interpret󰀀the󰀀images󰀀for󰀀the󰀀possibility󰀀of󰀀pulmonary󰀀embolism.󰀀The󰀀rule󰀀base󰀀

is󰀀derived󰀀using󰀀the󰀀modified󰀀PIOPED󰀀criteria󰀀and󰀀probabilities󰀀are󰀀assigned󰀀to󰀀different󰀀hypotheses󰀀

based󰀀on󰀀the󰀀patient󰀀data󰀀collected󰀀by󰀀the󰀀PIOPED󰀀investigators󰀀[1990].󰀀These󰀀form󰀀the󰀀prior󰀀