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1、Chapter 1-WebsterBasic Concepts of Medical InstrumentationFigure 1.1 The sensor converts energy or information from the measurand to another form (usually electric). This signal is the processed and displayed so that humans can perceive the information. Elements and connections shown by dashed lines
2、 are optional for some applications.PerceptibleoutputOutputdisplayControlAndfeedbackSignalprocessingDatatransmissionDatastorageVariableConversionelementSensorPrimarySensingelementMeasurandCalibrationsignalRadiation,electric current,or other appliedenergyPowersourceGeneralized instrumentation system
3、Measurand: Physical quantityBiopotentialPressureFlowDimensions (imaging)Displacement (velocity, acceleration, force)ImpedanceTemperatureChemical ConcentrationSensor and TransducerTransducerConverts one form of energy to anotherSensorConverts a physical measurand to an electrical outputInterface with
4、 living systemMinimize the energy extractedMinimally invasivediaphragmStrain gagepressuredisplacementelectric voltageSignal ConditioningAmplificationFilteringImpedance matchingAnalog/Digital for signal processingSignal form (time and frequency domains)Output DisplayNumericalGraphicalDiscrete or cont
5、inuousVisualHearingAuxiliary ElementCalibration SignalControl and Feedback (auto or manual)Adjust sensor and signal conditioning1.3 Alternative Operational ModesDirect Mode: Measurand is readily accessibleTemperatureHeart BeatIndirect Mode: desired measurand is measured by measuring accessible measu
6、rand.Morphology of internal organ: X-ray shadowsVolume of blood pumped per minute by the heart: respiration and blood gas concentrationPulmonary volumes: variation in thoracic impedance1.3 Sampling and Continuous ModesSampling and collecting data will depend on the following:The rate of change in th
7、e measurandCondition of the patientGenerating and Modulating SensorsGenerating sensors produce their outputs from energy taken from measurand (Photovoltaic cell)Modulating Sensors uses the measurand to alter the flow of energy from an external source (Photoconductive cell)Analog and Digital ModesRea
8、l-Time and Delayed-Time Modes1.4 Medical Measurement ConstraintsMagnitude and frequency range of medical measurand are very lowProper measurand-sensor interface cannot be obtainedMedical variables are seldom deterministicExternal energy must be minimized to avoid any damageEquipment must be reliable
9、 1.5 Classification of Medical InstrumentQuantity that is sensedpressure, flow, tempPrinciple of transductionresistive, capacitive, electrochemical, ultrasoundOrgan systemcardiovascular, pulmonary, nervousMedicine specialtiespediatrics, cardiology, radiology1.6 Interfering and Modifying InputsDesire
10、d Inputs: measurands that the instrument is designed to isolate.Interfering Inputs: quantities that unintentionally affect the instrument as a consequence of the principles used to acquire and process the desired inputs.Modifying Inputs: undesired quantities that indirectly affect the output by alte
11、ring the performance of the instrument itself. Figure 1.2 Simplified electrocardiographic recording system Two possible interfering inputs are stray magnetic fields and capacitively coupled noise. Orientation of patient cables and changes in electrode-skin impedance are two possible modifying inputs
12、. Z1 and Z2 represent the electrode-skin interface impedances.Electrodes60-Hzac magneticfieldDisplacementcurrentsDifferentialamplifier+-+Vcc-VccZ1Zbody Z2vovecg1.6 Interfering and Modifying InputsDesired input: Electrocardiographic voltage VecgInterfering input: voltage due to 60-Hz1.7 Compensation
13、TechniquesTo eliminate interfering and modifying input:1.Alter the design of essential instrument components to be less sensitive to interference. (preferred)2.Adding new components designed to offset the undesired inputs.1.7 Compensation TechniquesInherent InsensitiveNegative Feedback to minimize G
14、d which is effected by the modifying inputs(xd Hfy)Gd = y(1.1)xdGd = y(1 + HfGd)(1.2) (1.3)Signal Filtering (electric, mechanical, magnetic)Opposing InputsCompensation Techniques- ExampleAn amplifier with gain 10 that has 20% fluctuation due to temperature and environmental change. How will compensa
15、te the system to minimize the fluctuation?1.8 BiostatisticsApplications of Statistics to medical data-Design experiment-Clinical Study: summarize, explore, analyze-Draw inference from data: estimation, hypothesis-Evaluate diagnostic procedures: assist clinical decision makingMedical Research Studies
16、- Observational: Characteristics of patients are observed and recorded-Case-series: describe characteristic of group-Case-control: observe group that have some disease-Cross-sectional: Analyze characteristics of patients-Cohort: determine if a particular characteristic is a precursor for a disease.-
17、Experimental Intervention: Effect of a medical procedure or treatment is investigated-Controlled: Comparing outcomes to drug and placebo-Uncontrolled: No placebo and no comparison-Concurrent controls: patient are selected the same way and for the same time.-Double-blindStatistical MeasurementsMeasur
18、es of the mean and central tendency-Mean-Median: Middle value (used for skewed data)-Mode: is the observation that occurs most frequently-Geometric Mean: used with data on a logarithmic scaleStatistical MeasurementsMeasure of spread or dispersion of dataRange: Difference between the largest and smal
19、lest observationStandard deviation: is a measure of the spread of data about the meanFor symmetric distribution 75% of the data lies between (mean - 2s) and (mean + 2s)Coefficient of variation: standardize the variation to compare data measured in different scales.Statistical MeasurementsPercentile:
20、 gives the percentage of a distribution that is less than or equal to the percentile number.Standard error of the mean (SEM): Express the variability to be expected among the mean in future samples.Correlation Coefficient r: is a measure of a linear relationship between numerical variables x and y f
21、or paired observationsMethods for inference about a value in a population of subjects from a set of observations.Estimation and confidence of interval: are used to estimate specific parameters such as the mean and the variance.Hypothesis testing and P-value: reveals whether the sample gives enough e
22、vidence for us to reject the null hypothesis. P-value indicates how often the observed difference would occur by chance alone. Methods for measuring the accuracy of a diagnostic procedureSensitivity of a test: Probability of its yielding positive results in patients who actually have the disease.Spe
23、cificity of a test: Probability of its yielding negative results in patients who do not have the diseasePrior Probability: the prevalence of the condition prior to the test. Characteristics of Instrument PerformanceTwo classes of characteristics are used to evaluated and compare new instrumentStatic
24、 Characteristics: describe the performance for dc or very low frequency input.Dynamic Characteristics: describe the performance for ac and high frequency input.1.9 Generalized Static CharacteristicsParameters used to evaluate medical instrument:Accuracy: The difference between the true value and the
25、 measured value divided by the true valuePrecision: The number of distinguishable alternatives from which a given results is selected 2.434 or 2.43Resolution: The smallest increment quantity that can be measured with certaintyReproducibility: The ability to give the same output for equal inputs appl
26、ied over some period of time.1.9 Generalized Static CharacteristicsParameters used to evaluate medical instrument:Statistical Control: Systematic errors or bias are tolerable or can be removed by calibration.Statistical Sensitivity: the ratio of the incremental output quantity to the incremental inp
27、ut quantity, Gd.Finding static sensitivity Gd using line equation with the minimal sum of the squared difference between data points and the line Figure 1.3 (b) Static sensitivity: zero drift and sensitivity drift. Dotted lines indicate that zero drift and sensitivity drift can be negative.Zero Drif
28、t: all output values increase or decrease by the same amount due to manufacturing misalignment, variation in ambient temperature, vibration,. Sensitivity Drift: Output change in proportion to the magnitude of the input. Change in the slope of the calibration curve. Figure 1.4 (a) Basic definition of
29、 linearity for a system or element. The same linear system or element is shown four times for different inputs. (b) A graphical illustration of independent nonlinearity equals A% of the reading, or B% of full scale, whichever is greater (that is, whichever permits the larger error). xd (Input)B% of
30、full scaleA% of readingOverall tolerance bandLeast-squaresstraight line(a)(b)Point at whichA% of reading = B% of full scale y (Output)x1(x1 + x2)y1x2Kx1Ky1y2LinearsystemLinearsystemLinearsystemLinearsystemandand(y1 + y2)LinearityIndependent nonlinearity - A% deviation of the reading - B% deviation of the full scaleInput Ranges (I): Minimum resolvable input I 30 days.Custom: Devices not available to other licensed and not in finished formInvestigational: Unapproved devices undergoing clinical investigationTransitional: devices that were regulated as drugs and now defined as medical devices
