basic concepts of medical instrumentation

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Gujarat Technological UniversityTopic:2: Basic Concepts Of Medical InstrumentationSubject: BMI, Code: 2171102, A.Y : 2016-17

Compiled By: Prof G B RathodEC Dept., BVM Engineering collegeV. V Nagar, Anand, Gujarat, IndiaEmail: [email protected]

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Outlines• Generalized Medical Instrumentation systems• Alternative Operating Modes• Medical Measurement Constraints• Classifications of Biomedical Instruments• Interfering Modifying Inputs• Compensation Techniques• Design Criteria• Commercial Medical Instrumentation Development

Process. • Outcomes• References

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Generalized Medical Instrumentation systems

Fig.1.1: Generalized Medical Instrumentation System

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• Measurand• Physical quantity, property, or condition that the system

measures• Accessibility: internal, body surface, emanation from the

body, or tissue sample• Category: Bio-potential, pressure, flow, dimensions,

displacement, impedance, temperature, and chemical concentration

• Localization: organ or anatomical structure

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• Sensor• Transducer (sensor) is a device that converts one form of

energy to another (electric)• Specific• Minimization of the extracted energy• Minimally invasive• Primary sensing element and variable conversion element

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• Signal Conditioning• Amplification, filtering, impedance matching, A/C

conversion, DSP, etc.• Output Display• Visual sense• Numerical or graphical• Discrete or continuous• Permanent or temporary• Auditory sense• Tactile sense• Human factors engineering guidelines and preferred

practices for the design of medical devices (AAMI, 1993)

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• Auxiliary Elements• Calibration• Control and feedback• Storage• Transmission

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Alternative Operating Modes• Direct-Indirect Modes• Accessible (noninvasive or invasive) direct mode• Not accessible indirect mode1. Cardiac output (CO) by Fick method, dye dilution, or

thermodilution2. Morphology of internal organs by X-ray shadows3. Pulmonary volumes by thoracic impedance

plethysmography

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Alternative Operating Modes• Sampling and Continuous Modes• Frequency content of the measurand: temperature

(sampling) or ECG (continuous)• Objective of the measurement• Condition of the patient• Potential liability of the physician

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Alternative Operating Modes• Generating and Modulating Sensors• Generating sensor: measurand produces output from the

energy taken directly from• itself, photovoltaic cell• Modulating sensors: measurand alters the flow of energy

from an external source, photoconductive cell

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Alternative Operating Modes• Analog and Digital Modes• Analog: continuous in time and continuous in amplitude• Digital: discrete in time and take only a finite number of different

values1. Greater accuracy2. Repeatability3. Reliability4. Noise immunity5. No periodic calibration6. Readability (in display)• Analog sensor, indirect digital sensor, quasi-digital sensor, and

digital sensor• Data conversions: ADC and DAC

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Alternative Operating Modes• Real-Time and Delayed-Time Modes• Short processing time real-time mode• Long processing time delayed-time mode

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Medical Measurement Constraints• Typical medical parameter magnitude and frequency ranges: See Table • Small amplitude and low frequency (AF or below to dc)• Frequent inaccessibility due to the lack of proper measurand-sensor

interface• Not possible to turn it off or remove a part of it during measurements• Inherent variability with time and among subjects• Many feedback loops among physiological systems• Unknown safety level of the externally applied energy• Additional constraints of a medical equipment1. Reliability2. Easy to use3. Must withstand physical abuse and exposure to corrosive chemicals4. Minimized electric-shock hazard

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Classifications of Biomedical Instruments

• Quantity that is sensed: pressure, flow, temperature, etc.• Principle of transduction: resistive, inductive, capacitive,

ultrasonic, electrochemical,• etc.• Organ system: cardiovascular, pulmonary, nervous,

endocrine, etc.• Clinical medicine specialties: pediatrics, obstetrics,

cardiology, radiology, etc

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Interfering and Modifying Inputs• Desired input: Vecg in Fig. 1.2• Interfering input: 60-Hz noise in Fig. 1.2• Modifying input: orientation of the patient cables in Fig.

1.2

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Interfering and Modifying Inputs

Fig.1.2: Simplified ECG Recording System

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Interfering and Modifying Inputs• Two possible interfering inputs are stray magnetic fields

and capacitive coupled noise. Orientation of patient cables and changes in electrode-skin impedance are two possible modifying inputs. Z1 and Z2 represent the electrode-skin interface impedances.

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Compensation Techniques• Inherent Insensitivity• Make all components inherently sensitive only to desired

inputs• Twisting the lead wires in Fig. 1.2• Use Ag/AgCl electrode to reduce motion artifact

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Compensation Techniques• Negative Feedback• If the transfer function Gd is affected by modifying inputs,

negative feedback the output y by (Hfy) to the input xd and make Hf insensitive to modifying inputs

• Advantages: less power, more accurate and linear, less loading

• Disadvantages: possible oscillation

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Compensation Techniques• Signal Filtering• A filter is "a device or program that separates data,

signals, or material in accordance with specific criteria" (a filter usually separates signals according to their frequencies)

• Electrical, mechanical, pneumatic, thermal, or electromagnetic filters

• Filters at input, intermediate, and output stage

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Compensation Techniques• Opposing Inputs• Use additional interfering inputs to cancel undesired

output due to interfering and/or modifying inputs• Opposing inputs before or after the appearance of

undesired signals

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Design Criteria

Fig. Design Process of Medical Instruments

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Commercial Medical Instrumentation Development Process• Sources of ideas: physicians, nurses, clinical engineers,

sales personnel• Feasibility analysis and product description1. Medical need for the product2. Patient indications3. Number of patients4. Clinical specialty5. How, why, and by whom the device will be utilized

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Commercial Medical Instrumentation Development Process• Technical feasibility1. Core technology2. Suitability of existing or readily modifiable components3. Breakthroughs or inventions required4. System analysis5. Preliminary cost analysis• Marketing research and analysis for evolutionary products

(not for revolutionary products)• Fitness with company's current or planned product lines

and sales method

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Commercial Medical Instrumentation Development Process• Manufacturing feasibility analysis• Business plan1. Financial aspects2. Personnel requirements3. Competition patents4. Standards5. Manufacturing requirements6. Sales and service requirements7. Time schedule for the project

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Commercial Medical Instrumentation Development Process• Development of functional prototype1. Sensor, signal processing elements, and new or unique

components2. Animal and/or human experiments• Favorable feasibility review product specification1. Everything about 'what' is required and nothing about

'how'2. Numerical values for performance3. Internal testing requirements4. Size, weight, and color

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Commercial Medical Instrumentation Development Process• Product specification design and development

engineers1. Function partitioning into hardware and software2. Circuit diagrams, software requirements and

mechanical designs using CAD3. Simulations of functional operations4. Manufacturable prototype: test (animal and/or human),

design assurance5. Design review

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Commercial Medical Instrumentation Development Process• Test equipment design engineers• Quality assurance engineers• Manufacturing or production engineers• ECO (Engineering Change Order)• Technical support

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Outcomes• One can understand the basic/generalize medical

instrumentation system• The Operating modes and many medical measurement

constraints are also understood.• The Noise interference, compensation techniques to

remove that interference and also commercial design criteria of medical Instruments are also understood.

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References• “Medical Instrumentation Application and Design”, by John

G. Webster, 3rd Edition, Wiley Publication.• www.ejwoo.com - , Dept of Biomedical Engineering,

Kyung Hee University

http://www.ejwoo.com-/

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Thank You