Chapter 1

Measurement SystemsMeasurement Systems

Table 1.1 Biomedical engineers work in a variety of fields.

•Bioinstrumentation

•Biomaterials

•Biomechanics

•Biosignals

•Biosystems

•Biotransport

•Cellular engineering

•Clinical engineering

•Tissue engineering

•Rehabilitation engineering

Table 1.2 Biomedical engineers work in a variety of disciplines. One example of instrumentation is listed for each discipline.

•Agriculture - Soil monitoring

•Botany - Measurements of metabolism

•Genetics - Human genome project

•Medicine - Anesthesiology

•Microbiology - Tissue analysis

•Pharmacology - Chemical reaction monitoring

•Veterinary science - Neutering of animals

•Zoology - Organ modeling

Table 1.3 Biomedical engineers may work in a variety of environments.

•IndustryIndustry

•GovernmentGovernment

•Clinical InstitutionsClinical Institutions

•Academic ResearchAcademic Research

Problemstatem ent

R eviewprior work

S tatehypothesis

Performexperim ents

D esign furtherexperim ents

Analyzedata

F ina lconclusions

M oreexperim entsnecessary

Problemsolved

Figure 1.1 In the scientific method, a hypothesis is tested by experiment to determine its validity.

C hiefcom pla int

O bta inh is tory

L is t thed ifferentia ld iagnosis

Exam inationand tests

Select furthertests

U se datato narrow the

d iagnosis

F ina ld iagnosis

M ore thanone like ly

O nly onelike ly

T reatm entand

evaluation

Figure 1.2 The physician obtains the history, examines the patient, performs tests to determine the diagnosis and prescribes treatment.

Sensor

D atacom m unication

D atad isp lays

E ffector

M easurand

S ignalcondition ing

S ignalprocessing

D atastorage

Feedback

O utputs

Figure 1.3 A typical measurement system uses sensors to measure the variable, has signal processing and display, and may provide feedback.

Instrum entPatient

Instrum entPatient

C lin ic ian

Figure 1.4 (a) Without the clinician, the patient may be operating in an ineffective closed loop system. (b) The clinician provides knowledge to provide an effective closed loop system.

(a) (b)

Instrum entPatientC lin ic ian

Abnorm alreadings

Figure 1.5 In some situations, a patient may monitor vital signs and notify a clinician if abnormalities occur.

MeasurementMeasurement RangeRange Frequency, HzFrequency, Hz MethodMethod

Blood flowBlood flow 1 to 300 mL/s1 to 300 mL/s 0 to 200 to 20 Electromagnetic or ultrasonicElectromagnetic or ultrasonic

Blood pressureBlood pressure 0 to 400 mmHg0 to 400 mmHg 0 to 500 to 50 Cuff or strain gageCuff or strain gage

Cardiac outputCardiac output 4 to 25 L/min4 to 25 L/min 0 to 200 to 20 Fick, dye dilutionFick, dye dilution

ElectrocardiographyElectrocardiography 0.5 to 4 mV0.5 to 4 mV 0.05 to 1500.05 to 150 Skin electrodesSkin electrodes

ElectroencephalographyElectroencephalography 5 to 300 5 to 300 VV 0.5 to 150 0.5 to 150 Scalp electrodesScalp electrodes

ElectromyographyElectromyography 0.1 to 5 mV0.1 to 5 mV 0 to 100000 to 10000 Needle electrodesNeedle electrodes

ElectroretinographyElectroretinography 0 to 900 0 to 900 V V 0 to 500 to 50 Contact lens electrodesContact lens electrodes

pHpH 3 to 13 pH units3 to 13 pH units 0 to 10 to 1 pH electrodepH electrode

ppCOCO22 40 to 100 mmHg40 to 100 mmHg 0 to 20 to 2 ppCOCO22 electrode electrode

ppOO22 30 to 100 mmHg30 to 100 mmHg 0 to 20 to 2 ppOO22 electrode electrode

PneumotachographyPneumotachography 0 to 600 L/min0 to 600 L/min 0 to 400 to 40 PneumotachometerPneumotachometer

Respiratory rateRespiratory rate 2 to 50 2 to 50 breaths/minbreaths/min 0.1 to 100.1 to 10 ImpedanceImpedance

TemperatureTemperature 32 to 40 °C32 to 40 °C 0 to 0.10 to 0.1 ThermistorThermistor

Table 1.4 Common medical measurands.

SpecificationSpecification ValueValue

Pressure rangePressure range ––30 to +300 mmHg30 to +300 mmHg

Overpressure without damageOverpressure without damage ––400 to +4000 mmHg400 to +4000 mmHg

Maximum unbalanceMaximum unbalance ±75 mmHg±75 mmHg

Linearity and hysteresisLinearity and hysteresis ± 2% of reading or ± 1 mmHg± 2% of reading or ± 1 mmHg

Risk current at 120 VRisk current at 120 V 10 10 AA

Defibrillator withstandDefibrillator withstand 360 J into 50 360 J into 50

Table 1.5 Sensor specifications for a blood pressure sensor are determined by a committee composed of individuals from academia, industry, hospitals, and government.

Sensors ignal

M easurand

Figure 1.6 A hysteresis loop. The output curve obtained when increasing the measurand is different from the output obtained when decreasing the measurand.

Sensors ignal

M easurand

Sensors ignal

M easurand

Figure 1.7 (a) A low-sensitivity sensor has low gain. (b) A high sensitivity sensor has high gain.

(a) (b)

T im e

Am

plitu

de

T im e

Am

plitu

de

(a) (b)

Figure 1.8 (a) Analog signals can have any amplitude value. (b) Digital signals have a limited number of amplitude values.

SpecificationSpecification ValueValue

Input signal dynamic rangeInput signal dynamic range ±5 mV±5 mV

Dc offset voltageDc offset voltage ±300 mV±300 mV

Slew rateSlew rate 320 mV/s320 mV/s

Frequency responseFrequency response 0.05 to 150 Hz0.05 to 150 Hz

Input impedance at 10 HzInput impedance at 10 Hz 2.5 M2.5 M

Dc lead currentDc lead current 0.1 0.1

Return time after lead switchReturn time after lead switch 1 s1 s

Overload voltage without damageOverload voltage without damage 5000 V5000 V

Risk current at 120 VRisk current at 120 V 10 10

Table 1.6 Specification values for an electrocardiograph are agreed upon by a committee.

Figure 1.9 (a) An input signal which exceeds the dynamic range. (b) The resulting amplified signal is saturated at 1 V.

(a)

(b)

(a)

T im e

Am plitude

D c o ffset(b)

Figure 1.10 (a) An input signal without dc offset. (b) An input signal with dc offset.

0.05 H z 150 H z

Frequency

A m plitude

1.0

0.1

Figure 1.11 Frequency response of the electrocardiograph.

O utput

Input

O utput

Input

(a) (b)

Figure 1.12 (a) A linear system fits the equation y = mx + b. Note that all variables are italic. (b) A nonlinear system does not fit a straight line.

T im e

Am

plitu

de

T im e

Am

plitu

de

(a) (b)

Figure 1.13 (a) Continuous signals have values at every instant of time. (b) Discrete-time signals are sampled periodically and do not provide values between these sampling times.

Laboratory testLaboratory test Typical valueTypical value

HemoglobinHemoglobin 13.5 to 18 g/dL13.5 to 18 g/dL

HematocritHematocrit 40 to 54%40 to 54%

Erythrocyte countErythrocyte count 4.6 to 6.2 4.6 to 6.2 10 1066/ / LL

Leukocyte countLeukocyte count 4500 to 11000/ 4500 to 11000/ LL

Differential countDifferential count

Neutrophil 35 to 71%Neutrophil 35 to 71%

Band 0 to 6%Band 0 to 6%

Lymphocyte 1 to 10%Lymphocyte 1 to 10%

Monocyte 1 to 10%Monocyte 1 to 10%

Eosinophil 0 to 4%Eosinophil 0 to 4%

Basophil 0 to 2%Basophil 0 to 2%

Table 1.7 Complete blood count for a male subject.

(a) (b)

Figure 1.14 (a) Signals without noise are uncorrupted. (b) Interference superimposed on signals causes error. Frequency filters can be used to reduce noise and interference.

(a)

Figure 1.15 (a) Original waveform. (b) An interfering input may shift the baseline. (c) A modifying input may change the gain.

(b) (c)

(a) (b)

Figure 1.16 Data points with (a) low precision and (b) high precision.

Figure 1.17 Data points with (a) low accuracy and (b) high accuracy.

(a) (b)

O utput

Input

O utput

Input

(a) (b)

Figure 1.18 (a) The one-point calibration may miss nonlinearity. (b) The two-point calibration may also miss nonlinearity.

Figure 1.19 For the normal distribution, 68% of the data lies within ±1 standard deviation. By measuring samples and averaging, we obtain the estimated mean , which has a smaller standard deviation sx. is the tail probability that xs does not differ from by more than .

Frequency

P opula tion standarddevia tion s

E stim ated m ean xsstandard devia tion s x

x + s- sM ean

1–

)–( 2

n

xxs i

n

xx i

s

s

2)(

22 2--

Xe

Xf

0.2

0.1

0

0 1 2 3 4 5

p

K

x

x

x x

x

x

Figure 1.20 A typical Poisson distribution for m = 3.

ConclusionConclusion Real situationReal situation

HH00 true true HHaa true true

Accept Accept HH00 Correct decisionCorrect decision Type II error, Type II error, pp = =

Reject Reject HH00 Type I error, Type I error, pp = = Correct decisionCorrect decision

Table 1.8 The four outcomes of hypothesis testing.

Test resultTest result Has condition?Has condition?

NoNo YesYes

NegativeNegative True negative (TN)True negative (TN) False negative (FN)False negative (FN)

PostitivePostitive False positive (FP)False positive (FP) True positive (TP)True positive (TP)

Table 1.9 Equivalent table of Table 1.8 for results relating to a condition or disease.

N orm alpopulation

D iseasedpopulation

T ruenegative

Falsepositive, p =

T ruepositive

T hreshold

Falsenegative, p =

Figure 1.21 The test result threshold is set to minimize false positives and false negatives.