1. Generalized instrumentation system
Perceptible
output
Output
display
Signal
processing
Variable
Conversion
element
Sensor
Primary
Sensing
Measurand

2. Generalized instrumentation system
Perceptible
output
Output
display
Control
And
feedback
Signal
processing
Data
transmission
storage
Variable
Conversion
element
Sensor
Primary
Sensing
Measurand
Calibration
signal
Power
source

3. Generalized instrumentation system
Perceptible
output
Output
display
Control
And
feedback
Signal
processing
Data
transmission
storage
Variable
Conversion
element
Sensor
Primary
Sensing
Measurand
Calibration
signal
Radiation,
electric current,
or other applied
energy
Power
source
Eg. Heart beats
Figure 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 are optional for some applications.

4. Measurand: Physical quantity
Biopotential
Pressure
Flow
Dimensions (imaging)
Displacement (velocity, acceleration, force)
Impedance
Temperature
Chemical Concentration

5. Sensor and Transducer Transducer Sensor
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
displacement
electric voltage
pressure
diaphragm
Strain gage

6. Signal Conditioning Amplification Filtering Impedance matching
Analog/Digital for signal processing
Signal form (time and frequency domains)

7. Output Display Numerical Graphical Discrete or continuous Visual
Hearing

8. Auxiliary Element Calibration Signal
Control and Feedback (auto or manual)
Adjust sensor and signal conditioning

9. 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

10. 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.

11. 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 {2.434 or 2.43}
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.

12. 1.9 Generalized Static Characteristics
Parameters 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 input quantity, Gd.

13. 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.
Figure 1.3 (b) Static sensitivity: zero drift and sensitivity drift. Dotted lines indicate that zero drift and sensitivity drift can be negative.

14. Independent nonlinearity
- A% deviation of the reading
- B% deviation of the full scale x1 y1
(x1 + x2)
Linear
Linear
(y1 + y2)
system
system
and
and x2 y2
Kx1
Ky1
Linear
Linear
system
system
(a)
Least-squares
straight line
y (Output)
B% of full scale
A% of reading
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 (that is, whichever permits the larger error).
Overall tolerance band
xd (Input)
Point at which
Input Ranges (I):
Minimum resolvable input < I < normal linear operating range
A% of reading = B% of full scale
(b)

15. 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.

16. Input Impedance: 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

17. 1.10 Generalized Dynamic Characteristics
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 Functions
The output can be predicted for any input (transient, periodic, or random)

18. Frequency Transfer Function
Can be found by replacing D by j
Example:
If x(t) = Ax sin ( t)
then y(t) = |H()| Ax sin ( t + /_H())

19. Zero-Order Instrument a0 y(t) = b0 x(t)
K: static sensitivity
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.

20. First-Order Instrument
Where  is the time constant

21. First-Order Instrument
Output y(t) R + +
Slope = K
= 1
x(t) C
y(t) - -
Input x(t)
(a)
(b)
x(t)
Log
Y (jw)
scale
X (jw) 1
1.0
0.707 S L wL wS
Log scale w t
(c)
(d) f
y(t) 1 0° S
Log scale w L
0.63
- 45°
-90° S L t
Example 1.1: High-pass filter

22. Second-Order Instrument
Many medical instrument are 2nd order or higher
Operational Transfer Function
Frequency Transfer Function

23. 2nd order mechanical force-measuring Instrument
Output y ( t )
(b)
(d)
(c) 1 Ks
x(t)
y(t) yn
yn + 1
Resonance 2
Log
scale
-90°
0.5
-180°
Log scale w K
Input x(t)
Slope K = 0° wn
Y (jw)
X (jw) f
displacement
(a)
Input
Force x(t)
B = viscosity constant
Ks = spring constant
Natural freq.
Damping ratio
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  = (d) Sinusoidal steady-state frequency response,  = 2,  = 1,  = 0.5.

24. Overdamped Critically damped Underdamped Damped natural freq. y(t) 1Ks
0.5
Damped natural freq. t

25. Example 1.2: for underdamped second-order instruments, find the damping ratio from the step response
and
Logarithmic decrement

26. Time Delay System Y (jw) X (jw) Log scale K Log scale w f Log scale w0°

27. Design Criteria
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.

28. Commercial Medical Instrumentation Development Process
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)

29. Commercial Medical Instrumentation Development Process
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

30. Regulation of Medical Devices
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 Way: (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

31. Regulation of Medical Devices
Second Way of classifications: (see Table 1.2 in textbook)
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