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Biomedical Instrumentation
Winter 1393
Bonab University
Introduction
Course information
Lecturer: Dr. Fariborz Rahimi• Email: [email protected]
Prerequisites:• Electronic Measurements
Recommended Books and Notes:• J.G. Webster, “Medical Instrumentation Application and Design”, John Wiley & Sons, 2010
• J. Aston, “Principles of Biomedical Instrumentation and Measurement”, Merrill Publishing Company, 1990.
• J.D. Enderle, J.D.Bronzino, “Introduction to Biomedical Engineering”, Wiley, 3rd Ed. 2008
Tentative Grading:• Project (including in-class presentation) 35%
• Oral Presentation in class 20%
• Review paper (2-3 pages, IEEE conference format) 15%
• Final Exam 65%
2
Intro
The main Course book
3
J.G. Webster, “Medical
Instrumentation Application and
Design”, John Wiley & Sons, 4th
ED., 2010
Describes:
-principles
-applications
-design
Medical instruments commonly
Used in hospitals
Just fundamentals (details
change with time)
Intro
About John G Webster’s book
4
Intro
سرفصل مصوب وزارت
5
Intro
Examples: Cochlear Implant
6
Intro
• A surgically implanted electronic device that provides a sense of sound to a person who is profoundly deaf
• The quality of sound is different from natural hearing, with less sound information
• Each sensory fiber of the cochlear nerve handles a specific frequency (electively sensitive to a very narrow
frequency band) stimulate all fibers
Examples: Advances in Vision (Retinal Stimulation)
7
Intro
• A retinal implant is a biomedical implant
technology currently being developed
• Meant to partially restore useful vision to
people who have lost their vision due to
degenerative eye conditions
• Provide the user with low resolution images
by electrically stimulating surviving retinal
cells
• Sufficient for restoring specific visual
abilities, such as light perception and
object recognition
Examples: Mini Gastric Imaging
• It is considered to be a very safe method to determine an unknown cause of a gastrointestinal bleed
• to examine parts of the gastrointestinal tract that cannot be seen with other types of endoscopy
• capsule usually passes through feces within 24–48 hours
8
Intro
A success story: The AutoAnalyzer (Technicon, 35 years)
• New medical instrument: invention-prototype-development-clinical testing-regulatory approval-manufacturing-marketing-sale of new instrument…
• An automated analyzer using a flow technique called continuous flow analysis (CFA)
• The design is based on separating a continuously flowing stream with air bubbles
• A continuous stream of material is divided by air bubbles into discrete segments in which chemical reactions occur
• Was used: determine levels of albumin, alkaline phosphatase, blood urea nitrogen, bilirubin, calcium, cholesterol,… but now is replaced by discrete systems
• Now mainly in industrial processes: Water, soil extracts
9
CH-1
Generalized Medical instrumentation system
10
Figure 1.1 The sensor converts energy or information from the measurand
to another form (usually electric). This signal is then processed and
displayed so that humans can perceive the information. Elements and
connections shown by dashed lines are optional for some applications.
Perceptible
outputOutput
display
Control
And
feedback
Signal
processing
Data
transmissionData
storage
Variable
Conversion
element
Sensor
Primary
Sensing
element
Measurand
Calibration
signal
Radiation,
electric current,
or other applied
energy
Power
source
CH-1
Medical System
Measurand (quantity the system measures): Physical quantity
• Measurand accessibility:• Internal (blood pressure), on the body surface
(electrocardiogram potential), emanate from body
(infrared radiation), derived from a sample (blood, biopsy)
• Biopotential
• Pressure
• Flow
• Dimensions (imaging)
• Displacement (velocity, acceleration, force)
• Impedance
• Temperature
• Chemical Concentration
11
CH-1
Medical System
Sensor and Transducer
• Transducer• Converts one form of energy to another
• Sensor• Converts a physical measurand to an electrical output
• Interface with living system
• Minimize the energy extracted
• Minimally invasive
12
diaphragm Strain gagepressure
displacement electric voltage
CH-1
Pulse Oximetry
Medical System
Signal Conditioning
• Amplification
• Filtering
• Impedance matching
• Analog/Digital for signal processing
• Signal form (time and frequency domains)
13
CH-1
Medical System
Output Display
• Numerical
• Graphical
• Discrete or continuous
• Visual
• Hearing
14
CH-1
Beeps
Medical System
Auxiliary Element
• Calibration Signal (as early in signal processing chain
as possible)
• Control and Feedback (auto or manual)• Adjust sensor and signal conditioning
15
CH-1
Medical System
1.3 Alternative Operational Modes
• Direct Mode: Measurand is readily accessible• Temperature
• Heart Beat
• Indirect Mode: desired measurand is measured by measuring accessible measurand.• Morphology of internal organ: X-ray shadows
• Volume of blood pumped per minute by the heart: respiration and blood gas concentration
• Pulmonary volumes: variation in thoracic impedance
(Breathing out = Low impedance)
16
CH-1
Medical System
1.3 Sampling and Continuous Modes
• Sampling and collecting data will depend on the following:• The rate of change in the measurand (temp., ion concentration = slow
sampling vs. ECG or respiratory gas continuous)
• Condition of the patient
• Generating and Modulating Sensors• Generating 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 Modes
• Real-Time and Delayed-Time Modes
17
CH-1
Medical System
1.4 Medical Measurement Constraints
• Magnitude and frequency range of medical measurand are very low
• Proper measurand-sensor interface cannot be obtained (without damage)
• Medical variables are seldom deterministic
• External energy must be minimized to avoid any damage
• Equipment must be reliable
18
CH-1Medical System
Ballistocardiograph
19
A person lies down on a flat board set on rollers. A laser beam is
directed at a tiny mirror positioned on one of the rollers. The laser
beam is projected onto the ceiling or wall. The beating of the
person's heart causes a slight movement in the body as indicated by
the laser. This upward movement of the body is due to the 3rd Law
reaction force of the blood being pumped to the lower body. The
left ventricle of the heart squeezes blood upward into the aorta
shown below. At the peak of the contraction, about 80 grams of
blood is moving upward at 30 cm/s. The aorta does a U-turn forcing
most of the blood to flow down to the lower body. The aorta and
body force the blood down and in turn the body is forced up. The
amount is too small to be seen by eye but can be seen when
"amplified" by the laser-mirror arrangement used in the
demonstration. It can also be seen when standing quietly on a
weight scale if the scale is sensitive enough and the vibration is not
damped by the scale mechanism. Your weight decreases slightly
when the blood slams into the top of the aorta.
CH-1
Medical System
1.5 Classification of Medical Instrument
• Quantity that is sensed• pressure, flow, temp
• Principle of transduction• resistive, capacitive, electrochemical, ultrasound
• Organ system• Cardiovascular
• Pulmonary
• Nervous
• Medicine specialties• pediatrics, cardiology, radiology
20
CH-1Medical System
1.6 Interfering and Modifying Inputs
• Desired 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 altering the performance of the instrument itself.
21
CH-1
Effect of a burst or ESD (Electrostatic discharge)
disturbance on an electronic board.http://incompliancemag.com/article/emc-design-in-the-ic-
environment-with-respect-to-esd-and-burst/
Interference
1.6 Interfering and Modifying Inputs
22
Electrodes
60-Hz
ac magnetic
field
Displacement
currents
Differential
amplifier
+
-
+Vcc
-Vcc
Z1
ZbodyZ2
vo
vecg
Desired input: Electrocardiographic voltage Vecg
Interfering input: voltage due to 60-Hz
Figure 1.2 Simplified electrocardiographic recording system Two possible interferinginputs are stray magnetic fields and capacitively coupled noise. Orientation of patient cables and changes in electrode-skin impedance are two possible modifying inputs. Z1 and Z2represent the electrode-skin interface impedances.
CH-1
Interference
1.7 Compensation Techniques
To 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.
23
CH-1
The four
electromagnetic
interference (EMI)
coupling modes
Interference
1.7 Compensation Techniques
•Inherent Insensitive (twist electrode wires in ECG)
•Negative Feedback to minimize Gd 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)• At the input, output, inside the device (many designers use non-electric at input)
•Opposing Inputs (additional interfering inputs to cancel undesired)
24
d
df
d
1x
GH
Gy
CH-1
Interference
Compensation Techniques- Example
An amplifier with gain 10 that has 20% fluctuation due to temperature and environmental change. How to compensate the system to minimize the fluctuation?
• Solution: (say, for when gain decreases by 20%)• Use a thermistor (temperature dependent resistor)
• Adjust characteristics of active
system elements
(say, amplification factor)
25
CH-1
Interference
1.8 Biostatistics
• Applications of Statistics to medical data
- Design experiment
- Clinical Study: summarize, explore, analyze
- Draw inference from data: estimation, hypothesis
- Evaluate diagnostic procedures: assist clinical decision making
26
CH-1
Biostatistics
Medical Research Studies
• - 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 (1 particular time)
- Cohort: determine if a particular characteristic is a precursor for a disease.
- 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-blind: Patients random to treatments and investigator does not know which
27
CH-1
Biostatistics
Statistical Measurements
• Measures 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 scale
28
CH-1
nnXXXXGM 321
n
XX
i
Biostatistics
Statistical Measurements
Measure of spread or dispersion of data• Range: Difference between the largest
and smallest observation
• Standard deviation: is a measure of the
spread of data about the mean
• For 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.
29
CH-1
1
2
-
-
n
XXs
i
%100
X
sCV
Biostatistics
Statistical Measurements
• Percentile: 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 for paired observations
30
CH-1
--
--
22
YYXX
YYXXr
ii
ii
Biostatistics
Methods for inference
Methods for inference about a value in a population of subjects from a set of observations.
• Estimation and confidence 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 evidence for us to reject the null hypothesis. P-value indicates how often the observed difference would occur by chance alone.
31
CH-1
Biostatistics
Methods for measuring the accuracy of a diagnostic procedure
•Sensitivity of a test:
Probability of its yielding positive results in patients who actually have the disease.
•Specificity of a test:
Probability of its yielding negative results in patients who do not have the disease
•Prior Probability:
the prevalence of the condition prior to the test.
32
CH-1
Biostatistics
Characteristics of Instrument Performance
• Two classes of characteristics are used to evaluated and compare new instrument
• Static Characteristics:
describe the performance for dc or very low frequency input.
• Dynamic Characteristics:
describe the performance for ac and high frequency input.
33
CH-1
Characteristic
1.9 Generalized Static Characteristics
Parameters used to evaluate medical instrument:
• Accuracy:
The difference between the true value and the measured value divided by the true value
• Precision:
The number of distinguishable alternatives from which a given results is selected {2.434v or 2.43v}
• Resolution:
The smallest increment quantity that can be measured with certainty
• Reproducibility:
The ability to give the same output for equal inputs applied over some period of time.
34
CH-1Static Char.
1.9 Generalized Static Characteristics
Parameters used to evaluate medical instrument:
• Statistical Control:
Accuracy is meaningful if all environmental factors are known Ensures:Systematic errors or bias are tolerable or can be removed by calibration.
Systematic error / bias can be removed by calibration / correction factors , but random variation more difficult
• Statistical Sensitivity:
Static calibration = hold all inputs constant except one incrementally increase that input
The ratio of the incremental output quantity to the incremental input quantity, Gd.
35
CH-1
Static Char.
Finding static sensitivity Gd using line equation with the minimal sum of the squared difference between data points and the line
36
CH-1
-
-
2
d
2
d
dd
xxn
yxyxnm
-
-
2
d
2
d
dd
2
d
xxn
xyxxyb
bmxy d
n: Total number
of points
Static Char.
1.9 Generalized Static Characteristics
37
CH-1
Figure 1.3 (b) Static sensitivity: zero drift and sensitivity drift. Dotted lines indicate that zero drift and sensitivity drift can be negative.
Zero Drift: 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.
Static Char.
38
CH-1
Figure 1.4 (a) Basic definition of 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 (whichever permits the largererror). xd (Input)
B% of full scale
A% of reading
Overall tolerance band
Least-squares
straight line
(b)
Point at whichA% of reading = B% of full scale
y (Output)
(a)
x1 (x1 + x2)y1
x2 Kx1 Ky1y2Linearsystem
Linearsystem
Linearsystem
Linearsystem
and and
(y1 + y2)
Linearity
Independent nonlinearity- A% deviation of the reading
- B% deviation of the full scale
Input Ranges (I): Minimum resolvable input < I < normal linear operating range
Static Char.
Example
A linear system described by the following equation y=2x+3. Find the overall tolerance band for the system if the input range is 0 to 10 and its independent nonlinearity is 0.5% deviation of the full scale and 1.5% deviation of the reading.
39
CH-1
y
x
3
0 10
0.5% FSD = .0523
1.5% Rdng = .15
Static Char.
Input Impedance
40
CH-1
variableflow
iableeffort var
d2
d1 X
XZ x
2
d2
2
d1d2d1 XZ
Z
XXXP x
x
• Disturb the quantity being measured.
• Xd1 : desired input (voltage, force, pressure)
• Xd2 : implicit input (current, velocity, flow)
• P = Xd1.Xd2 :Power transferred across the tissue-sensor interface
• Generalized input impedance Zx
•Goal: Minimize P, when measuring effort variable Xd1, by
maximizing Zx which in return will minimize the flow
variable Xd2.
•Loading effect is minimized when source impedance Zs is
much smaller then the Zx
Static Char.
1.10 Generalized Dynamic Characteristics
41
CH-1
)()( 0101 txbdt
dxb
dt
xdbtya
dt
dya
dt
yda
m
m
mn
n
n
)()( 0101 txbDbDbtyaDaDa m
m
n
n
01
01
)(
)(
aDaDa
bDbDb
Dx
Dyn
n
m
m
Most medical instrument process signals that are functions
of time. The input x(t) is related to the output y(t) by
ai and bi depend on the physical and electrical parameters
of the system.
Transfer FunctionsThe output can be predicted for any input (transient,
periodic, or random)
Dynamic Char.
Frequency Transfer FunctionCan be found by replacing D by j
42
CH-1
01
01
)(
)(
aDaDa
bDbDb
Dx
Dyn
n
m
m
01
01
)()(
)()(
)(
)()(
ajωajωa
bjωbjωb
jωX
jωYjH
n
n
m
m
Example:If x(t) = Ax sin ( t)
then y(t) = |H()| Ax sin ( t + /_H())
Dynamic Char.
Zero-Order Instrument
43
CH-1
Figure 1.5 (a) A linear potentiometer, an example of a zero-order system. (b) Linear static characteristic for this system. (c) Step response is proportional to input. (d) Sinusoidal frequency response is constant with zero phase shift.
a0 y(t) = b0 x(t)
Ka
b
jX
jωY
Dx
Dy
0
0
)(
)(
)(
)(
K: static sensitivity
Dynamic Char.
First-Order Instrument
44
CH-1
)()()(
001 txbtyadt
tdya
)()(1τD tKxty
τD
K
Dx
Dy
1)(
)(
1τ/arctan1
τ1
22ω
τω
K
jωX
jωY
jω
K
jωX
jωY
-
0
0
0
1
a
bK
a
a Where is the time constant
/1 teKty --
Dynamic Char.
First-Order Instrument
45
CH-1
t
1
(c)
(a)
C
+
-
+
-
y(t)
Output y(t)
Input x(t)
Slope = K = 1
(b)
Y (j
X (j
Log
scale
1.0
0.707
Log scale
(d)
0°
- 45°
-90°
Log scale
t
1
0.63
LS
L
S
SL
L
S
x(t)
x(t)
y(t)
R
τD
K
Dx
Dy
1)(
)(
/1 teKty --
Example 1.1:
Low-pass filter
)()()(
txtydt
tdyRC
1)(1 txKRC
Dynamic Char.
Second-Order Instrument Many medical instrument are 2nd order or higher
46
CH-1
txbtya
dt
tdya
dt
tyda 0012
2
2 tKxtyω
ζD
ω
D
nn
1
22
2
unitsinput by defined unitsoutput y,sensitivit static0
0 a
bK
rad/s frequency, natural undamped 2
0 a
aωn
essdimensionl ratio, damping 2
ζ20
1 aa
a
12
2
2
nn ω
ζD
ω
D
K
Dx
DyOperational Transfer Function
ωωωω
ζ
ωωζωω
K
jωX
jωY
ωζjωωjω
K
jωX
jωY
nnnn
nn
//
2arctan
/4/1
1/2/
22222
2
-
-
Frequency Transfer Function
Dynamic Char.
2nd order mechanical force-measuring Instrument
47
CH-1
Figure 1.7 (a) Force-measuring spring scale, an example of a second-order instrument. (b) Static sensitivity. (c) Step response for overdamped case = 2, critically damped case = 1, underdamped case = 0.5. (d) Sinusoidal steady-state frequency response, = 2, = 1, = 0.5.
Output y(t)
(b)
(d)(c)
1
Ks
x(t)
y(t)yn yn + 1
Resonance
2
Logscale
1
2
-90°
0.51
2 -180°
1
0.5
0.5
Log scale
Log scale
K1
t
t
Input x(t)
Slope K =1
Ks
0°
n
n
Y (j
X (j
Output
displacement
(a)
Input
Force x(t)0
y(t)
2
2
dt
tydMtyK
dt
tdyBtx s --
sKK /1
M
Kω s
n
MK
Bζ
s2
B = viscosity constant
Ks = spring constant
Natural freq.
Damping ratio
Dynamic Char.
Overdamped
48
CH-1
KKeζ
ζζKe
ζ
ζζty
tωζζtωζζ nn
-
--
-
--
---
-- 1
2
21
2
2 22
12
1
12
1
:1ζ
KKetωty tω
nn - -1
:1ζ
2
2
2
1arcsin
1sin1
ζ
KtωζKζ
ety n
tζωn
-
--
--
Underdamped
Critically damped
1
Ks
y(t)
0.5
t
21 - nd Damped natural freq.
:1ζDynamic Char.
Example 1.2: for underdamped second-order instruments, find the damping ratio from the step response
49
CH-1
21
2/3
ζω
πt
n
n
-
-
2
1
1
2/7
ζω
πt
n
n
-
-
21
2
22
22
1
1
2ln
1
2exp
1
2/7exp
1
1
2/3exp
1
ζ
πζ
y
y
ζ
πζ
ζω
πζω
ζ
K
ζω
πζω
ζ
K
y
y
n
n
n
n
n
n
n
n
-
-
-
--
-
-
--
-
224
πζ
and
Logarithmic decrement
KtωζKζ
ety n
tζωn
--
--
2
21sin
1
Dynamic Char.
Time Delay System
50
CH-1
dτtKxty -dτt
djωKejωX
jωY -
Logscale
Log scale
Log scale
K
0°
Y (j
X (j
dτ
Output is exactly as input,
only delayed
Dynamic Char.
Design Criteria
51
CH-1
Figure 1.8 Design process for medical instruments
Choice and design of instruments are affected by signal factors, and also by environmental,
medical, and economic factors.
Device Design
Commercial Medical Instrumentation Development Process
52
CH-1
•Ideas: come from people working in the health care
•Detailed evaluation and signed disclosure
•Feasibility analysis and product description
•Medical need
•Technical feasibility
•Brief business plan (financial, sales, patents, standards, competition)
•Product Specification (interface, size, weight, color)
“What” is required but nothing about “how”
•Design and development (software and hardware)
Device Design
Commercial Medical Instrumentation Development Process
53
CH-1
•Prototype development
•Testing on animals or human subjects
•Final design review (test results for, specifications, subject feedback, cost)
•Production (packaging, manual and documents)
•Technical support
Device Design
Regulation of Medical Devices
54
CH-1
Medical devices is “any item promoted for a medical
purpose that does not rely on chemical action to achieve its
intended effect”
2 Ways for Medical Devices Classification
First Method: (based on potential hazards)
Class I: general controls
Class II: performance standards
Class III: premarketing approval
Second Method: (see Table 1.2 in textbook)
preamendment, postamendment, substantially equivalent,
implant, custom, investigational, transitional
Device Design
Regulation of Medical Devices
55
CH-1
Second Way of classifications: ( Table 1.2 )
Preamendment: Devices on the market before 5/28/1976
Postamendment: Devices on the market after 5/28/1976
Substantially equivalent: Equivalent to preamendment devices
Implant: devices inserted in human body and intended to remain there for >30 days.
Custom: Devices not available to other licensed and not in finished form
Investigational: Unapproved devices undergoing clinical investigation
Transitional: devices that were regulated as drugs and now defined as
medical devices
Device Design