醫(yī)學儀器的基本概念

MedicalInstrumentation--ApplicationandDesign

BasicConceptsof

MedicalInstrumentation

GeneralizedMedicalInstrumentationSystemMeasurandBiopotentialpressureFlowdimensions(imaging)displacement(velocity,accelerationandforce)ImpedancetemperatureChemicalconcentrationGeneralizedMedicalInstrumentationSystemSensorsSensor:convertsaphysicalmeasurandtoanelectricoutputTransducer:convertsoneformofenergytoanotherIngeneral,sensor=transducer+variableconversionelementVariable-conversionelementsneedexternalelectricpowertoobtainasensoroutputGeneralizedMedicalInstrumentationSystemSuchas:Adiaphragmconvertspressuretodisplacement(transducer)Astraingageconvertsdisplacementtoanelectricvoltage(variable-conversionelement)GeneralizedMedicalInstrumentationSystemSignalconditioningAmplifyandfilterthesignalMatchtheimpedanceofthesensortothedisplayAnalogtodigitalconvertsSignalfiltering:----reduceundesirablesensorsignals----convertinformationfromthetimedomaintothefrequencydomain----averagerepetitivesignalstoreducenoiseGeneralizedMedicalInstrumentationSystemOutputdisplayNumericalorgraphicalDiscreteorcontinuousPermanentortemporaryGeneralizedMedicalInstrumentationSystemAuxiliaryelementCalibrationsignalControlandfeedback(automaticormanual)DatastorageDatetransmissionRadiationelectriccurrentorotherappliedenergyAlternativeOperationalModesDirect-indirectmodesDirectmode:

measurandcanbeinterfaceddirectlytosensor.(ECG,bodytemperature)Indirectmode:

measurandisnotaccessible.Useanothermeasurandthatbearsaknownrelationtothedesiredmeasurand.

AlternativeOperationalModesExample:CardiacoutputdeterminedfrommeasurementsofrespirationandbloodgasconcentrationMorphologyofinternalorgansdeterminedfromX-rayshadowsAlternativeOperationalModesSamplingandcontinuousmodesSamplingmodes:BodytemperaturechangessoslowlythatitmaybesampledinfrequentlyContinuousmodes:ECGorrespiratorygasflowrequirescontinuousmonitoringAlternativeOperationalModesGeneratingandModulatingsensorsGeneratingsensors:producesignaloutputfromenergytakendirectlyfromthemeasurand.(photovoltaiccell)Modulatingsensors:usethemeasurandtoaltertheflowofenergyfromanexternalsourcethataffectsthesensoroutput(example:photoconductivecell)AlternativeOperationalModesAnalogandDigitalmodesAnalogmodes:continuousmeasurementsinanytimeDigitalmodes:discreteandtakeononlyafinitenumberofdifferentvaluesAdvantages:greateraccuracy,repeatability,immunitytonoiseAlternativeOperationalModesReal-timeandDelayed-timemodesReal-timemode:Sensormustacquiresignalsinrealtimeasthesignalsoccur.(ECG)Delayed-timemode:Severaldaysmayberequiredbeforeanoutputisobtained.(cellcultures)MedicalmeasurementconstraintsThemedicalinstrumentationisdesignedtomeasurevariousmedicalandphysiologicalparametersMeasurementrangeandfrequencyrangearetwomajorfactors.(Tab1.1)Manycrucialvariablesinlivingsystemsareinaccessible(cardiacoutput)MedicalmeasurementconstraintsParameterorMeasuringTechniquePrincipalMeasurementRangeofParameterSignalFrequencyRange,HZStandardSensororMethodBloodpressure,arterialDirectindirect10-400mmHg25-400mmHgDc-50Dc-60Strain-gagemanometerCuff,auscultationElectrocardiography(ECG)0.5-4mV0.01-250SkinelectrodeElectroencephalography(EEG)5-300uVDc-150ScalpelectrodesElectromyography(EMG)0.1-5mVDC-10,000NeedleelectrodesRespiratoryrate2-50breaths/min0.1-10Straingageonchestimpedance.nasalthermistorTemperatureofbody32-40℃90-104℉Dc-0.1Thermistor,thermocoupleMedicalmeasurementconstraintsVariablesmeasuredareseldomdeterministic,varywithtime(bodytemperature,bloodpressure)Manymedicalmeasurementsvarywidelyamongnormalpatient(bloodpressure,heartrate)ManyfeedbackloopsexistamongphysiologicalsystemsandmanyoftheinterrelationshipsarepoorlyunderstoodMedicalmeasurementconstraintsDependeitheronsomeformofenergybeingappliedtothelivingtissueoronsomeenergybeingappliedasanincidentalconsequenceofsensoroperation(example:X-ray,UltrasonicimagingtechniquesandelectromagneticorDopplerultrasonicbloodflow-metersdependonexternallyappliedenergy)MedicalmeasurementconstraintsSomeimportantadditionalconstraints:Reliable,easytooperate,withstandingphysicalabuseandexposuretocorrosivechemicalsTominimizeelectric-shockhazardsToconsiderthesafetyofpatientsandmedicalpersonnelClassificationsofBiomedicalInstrumentsAccordingtothequantity:Pressure,flow,temperature...AccordingtotheprincipleoftransductionResistive,inductivecapacitive,ultrasonicelectrochemical...ClassificationsofBiomedicalInstrumentsAccordingtoeachorgansystemCardiovascular,pulmonary,nervous,endocrine...AccordingtotheclinicalmedicinespecialtiesPediatrics,obstetrics,cardiologyradiology...InterferingandModifyinginputsInterferinginputs:affecttheinstrumentasaconsequenceoftheprinciplesusedtoacquireandprocessthedesiredinputsModifyinginputs:UndesiredquantitiesthatindirectlyaffecttheoutputbyalteringtheperformanceoftheinstrumentitselfInterferingandModifyinginputsSomeundesirablequantitiescanactasbothamodifyinginputandaninterferinginput.Figure1.2SimplifiedelectrocardiographyrecordingsystemInterferingandModifyinginputsThedesiredinputistheECGvoltageVecgTwopossibleinterferinginputs:straymagneticfieldsandcapacitivelycouplednoiseTwopossiblemodifyinginputs:OrientationofpatientcablesChangesinelectrode-skinimpedanceCompensationTechniquesSeveralcompensationmethodsforeliminatingtheeffectsofinterferingandmodifyinginputsInherentInsensitivity:ifallinstrumentcomponentsareinherentlysensitiveonlytodesiredinputs,theninterferingandmodifyinginputshavenoeffect.CompensationTechniquesNegativefeedback:Xd:inputY:outputGd:amplificationHi:feedbackelementCompensationTechniquesNegativefeedbackCompensationTechniquesIf>>1then

OnlythefeedbackelementHideterminestheoutputforagiveninput

CompensationTechniquesSignalfilteringadeviceorprogramthatseparatesdata,signalsormaterialinaccordancewithspecifiedcriteria.Filteringmaybeinserted:attheinstrumentinputatsomepointwithintheinstrumentattheoutputoftheinstrumentCompensationTechniquesOpposinginputsAdditionalinterferinginputscanbeusedtocancelundesiredoutputcomponentsCommercialmedicalinstrumentationdevelopmentprocessManyfactorsaffectthedesignofbiomedicalinstrumentsSignalfactorEnvironmentalfactorMedicalfactorEconomicfactorCommercialmedicalinstrumentationdevelopmentprocessMeasurandSignalfactorsEnvironmentalfactorsMedicalfactorsEconomicfactorsSensitivityRangeDifferentialorabsoluteinputInputimpedanceTransientandfrequencyresponseAccuracylinearityReliabilitySpecificitySignal-to-noiseratioStability;temperature,humidityPressure,acceleration,shockVibration,radiationPowerrequirementsMountingsize,shapeInvasiveornon-invasiveTissue-sensorinterfacerequirementsMaterialtoxicityElectricsafetyRadiationandheatdissipationPatientdiscomfortCostAvailabilityWarrantyConsumablerequirementsCompatibilitywithexistingequipmentInitialinstrumentdesignPrototypetestsFinalinstrumentdesignFDABMDapprovalProductionCommercialmedicalinstrumentationdevelopmentprocessIdeas:physicians,nurses,clinicalengineers,andsalespersonnelaregoodsourcesofideasAnalysis:marketingsearchandanalysisTechn