Title: BioMed Instrumentation

Q: What is sensitivity, specifity and accuracy?
A: Sensitivity se=TP/(TP+FN)
= about smallest rate that can be detected
High, if threshold is high, but then specifity low
Specifity sp=TN/(TN+FP)
= only detecting the investigated quntity
High, if threshold is low, but then sensitivity low
Accuracy ac=(TP+TN)/(P+N)
TP=true positive etc.
Threshold= Triggerlevel (if value is above Th, data is collected)

Q: What is a biomedical device for?
A: Diagnosis:
Identifying a medical condition by signs or symptoms (biosignals)
Functional/Clinical evaluation, Bioimaging
Therapy:
Treatment by generating biosignals
(Non-) invasive treatment, surgery

Q: Name requirements for biomedical devices
A: Wearability (comfort, weight)
Durability (robust to environment)
Usability (battery, user-friendly)
Maintainability (software updates)
Performance (data rate, cost)
Functionality
Data processing (data security, resolution, limits, format, reliababilty)

Q: Explain the whole body impedance.
A: Depends on measuring points
Skin behavious capacitive
Dry, intact skin --> body impedance of 2kΩ
Wet, injured skin -->1kΩ

Q: Describe the risk of an external current flowing through the body
A: Increased risk of cardiac fibrillation, if external current flows through heart
F.e. measuring between left hand and breast (should be avoided)
Depends on path, amplitude and frequency
Let-go current for one path:
3.5mA maximum leakage current as safety for devices like dishwasher
Also depends on the phase of the heart: most vulnerable in T-wave

Q: How can a patient be protected against unwanted currents?
A: 1. Isolation to the device (not allwalys realizable)
2. Current limiting resistor and voltage limiting diodes (very easy)
3. Optical isolation with light transmission
4. Transformer isolation or capacitive coupling (only AC paths)

Q: What is done to provide hardware safety?
A: - Power supply & Biosignal isolation
- Minimize leakage current
- RF shielding (avoid interference)
- reliable GND (safety for operator)
Metal chassis must be grounded --> currents flows to Gnd if isolation fails

Q: Describe an instrumental amplifier and CMRR
A: Differential amplifier with high CMRR (common mode rejection rate) and input buffers
CMRR to get rid of DC component --> ability to obtain differnce signal while rejecting noise in dB
High CMRR important, if signal changes are small compared to the offset.
CMRR = 20*log(G_differential / G_common_mode)
Real value around 100dB

Q: Describe the demodulation of a signal
A: Phase sensitive due to feedback (lock-in technique)
Phase insensitive:

Q: Explain the moving window approach
A: Used to increase ADC resolution (f.e. from 12 to 16bits)
Feedback with DAC to analyze the true range of the current window
-->always best possible resolution, because it is dynamic

Q: Explain AC offset compensation
A: To keep amplitude of reference signal constant
Substracting the controlled ref. Signal

Q: Sketch the tissue/skin model
A: Skin and tissue acts ohmsch-capacitive

Q: Name and explain types of electrodes
A: - perfectly polarizable electrodes (behavious capacitive)
For stimulation, only a displacement current (Pt electrode)
- perfectly non-polarizable electrode (like resistor in working range)
For recording, current passes freely across the resistor (Ag/AgCl electrode)

Q: Describe the metal eletrolyte interface
A: Free electrodes remain in metall while Atom is oxidized into electrolyte. This exodized Atom is then neutralized with the e- --> a layer free of charg is formed
By combining two metall/electrolyte interfaces as one anode and one cathode, non-polarizable electrode is formed

Q: Name requirements and properties of electrodes
A: - Motion artefacts
As a outcome of a potential difference
- Biocompatibility
Au, Pt long-term biocompatible as needles
AgCl weakly toxic
- Stability
small electrochemical noise, short stabilization time
- Length and time scale
To achieve good cell attachment

Q: Which noise sources disturb bio signals? Explain coupling modes and noise reduction methods.
A: - Power line/ other biosignals interference
- Electrode contact noise
- Motion artefacts
- Data collection / Signal processing noise
Capacitive coupling
Inductive coupling
--> active / passive grounding

Q: Sketch the power line interference model.
A: High Cb for improved grounding
Low ΔZe (unbalanced impedance) to avoid noise related U on Zd
High Ziso for improved isolation
CMMR ~ 100dB
C in pF range
Z from Ω to MΩ

Q: Why do we need shielding? Explain different methods.
A: Otherwise very sensitive to disturbing noise (noise coupling)
Passive shield: shield connected to GND (noise flows in shield to GND)
Disadvantages: reduced measured voltage (voltage divider with shield) and additional a time constant due to resistive-cpacitive behaviour of shield
Active shielding: Signal is coupled to shield threw active amplifier to decrease leakage current

Q: What is bionic design? Name the most important laws of bionic designs.
A: Integration instead of additional devices
Optimization of whole system not only parts
Multifunctional
Adaptive to environment
Effective energy use
Recycleable
...
Car shape inpired by fish body
Hydrophobic surface like flower leaves --> self cleaning

Q: What is multiparametic sensing?
A: A single sensing plattform to analyze multiple parameters.
Combination of optimized sensors and application regions and signal processing to get alot of infortmation out of the data

Q: What should be considered, when designing a biomedical sensor? Sketch the general procedure for data aquisition.
A: Body output impedance hard to define
High input impedance to decrease disturbance
Low sensor output impedance
Disturbing factors like body temperature and movement

Q: How can differentiated between different parameters like ECG, EEG, EMG,..?
A: Similar measuremen methods with different electrodes
Frequency areas of these parameters overlap
--> just a FT is not enough
Signal-Range method used to also analyse amplitude

Q: Sketch the basic set up of an Electrocardiography (ECG). How is it realized clincally and which sources of disturbance are relevant?
A: Buffer after electrodes for active shielding to reduce capacitive coupling
Right leg drive as active grounding to reduce common mode signals
Multiple electrode setup
Three derivations for 2 hands plus right leg drive
Six derivations with additional electrodes on upper body
Capacitive coupling by power lines
--> right leg drive to avoid additional current in body, current is bypassed to ground
Inductive coupling by nearby magnetic fields
--> avoid big areas between electrode cables (twist cables) --> small flux density

Q: What can be measured with an Electromyography (EMG)?
A: Neural conduction velocity
Stimulation of a motor nerve at two point and measuring the response of the muscle to the signal --> Lower velocity, if nerve is degenerated
Stimulation of sensory nerve to measure sensory threshold

Q: How can blood pressure be measured?
A: - Invasive
Catheter is inserted and filled by momentary blood flow of human (continious measured)
-Non-invasive:
1. Korotkoff sounds
Cuff is inflated on upper arm to temporary stop blood flow, microphone is placed above brachial atery, after pressure is reduced, a sound (Korotkoff sound, related to systolic pressure) is generated by turbolent flow. The moment the sound disappears relates to the diastolic pressure. The time span corresponds to the blood pressure.
2. Differential measurement
Also with inflated cuff; comparision of systolic and diastolic pressure
Automated measuring system with sensors in the cuff
3. Oscillometric method
Measuring the vibration of the walls due to turbolent flow after releasing the pressure in the cuff.
4. Tonometry
Continous measuring by a pressure sensor over the radialis atery
But high application pressure on body and uncomfortable

Q: What is measured with Oxymetry and how is it done?
A: Non-invasive method to measure oxygen saturation level of the blood
Optical readout of light which is transmitted and reflected through oxygen saturated hemoglobin.
2 wavelengths used to eliminated background light (one in IR one is background light) these are substracted (only IR measurement remains)
AC amplitude only 5-10% of DC amplitude --> filtered to remove DC part

Q: What is spirometry, what can be measured?
A: To measure lung function, by measuring pressure drop over a resistance
Result described in Flow-Volume Loop
Result is very sensitive to external factors --> has to be calibrated before measuring (to temperature and humidity f.e.)

Q: What is adaptive therapy?
A: Therapeutic stimulus adjusted to biosignals of a patient.
For example: pain relieve with muscle stimulation according to the heart rate in feedback loop.

Q: How can hearing be impaired? How can it be treated?
A: Troubles in
1. Sound conduction (vibrations do not reach inner ear)
2. Loudness (increased mechanical impedance)
3. Discrimination ability (loss in sensory cells)
Microphones as sensors
Loudspeakers or bone vibration as actors (depends if nerve or sensory cells are damaged)
Split into multiple frequency bands to process high and low frequencies individually
Signal is mapped to 22 naturaly arranged frequencies in the human hearing sepctrum

Q: What is Diathermy? How do the device work?
A: Generating heat in the body by currents, to stimulate the circulation, relieve pain, destroy unhealthy tissue,...
Types of currents:
1. Cunductance currents (movement of free charges (ions in tissue) E field applied --> collisions (that realese heat))
2. Displacement currents (reorientating with E field of bound carges (water dipole molecules) --> friction (and microheating))
--> LF --> deep penetration less focus, other way round at HF
(Displacement currents main effect on LF and 1. Type at HF)
Examples:
1. Short wave diathermy (vibration of ions, no nerve stimulation, body as part of the circuit)
40-45°C --> narrow window for tumorcell killing)
Condenser method: deep penetration at low frequencies, electrodes are placed above skin, fat is heated more than muscle, few MHz
Induction method: superficial penetration with high frequencies, muscle is heated more than fat, 13 or 27MHz
2. Micro wave diathermy (vibration of water molecules, dipole rotation, 2.45GHz)
3. Ultrasonic diathermy (mechanic micromassage and heating --> relaxing, heat developed on bone, 1MHz)
4. Surgical diathermy (cells evaporate or burn due to high local current density --> cutting without application of force)

Q: How can ventilation be supported?
A: 1. Negative pressure ventilators (pressure in chamber oscillates between atm and below that --> lung expression)
Controlled/forced ventilation for suppression
2. Positive pressure ventilators (air blown into lung to expansion)
Three methods: time, volume(dangerous cause high volume peak) and pressure(gentle way) cycling (very time sensitive process)
3. Continous positive air pressure ventilator
Supports breathing for example over night (keeps airway open)

Q: What are Lithotriptors?
A: Apllication of acustic show waves to split urinary stones
--> plasma explosion from high voltage spark

Q: Which forms of pacemakers are used?
A: Electric impulse applied to heart (one or both chambers)
Asynchron: individual pace is ignored, osillator is always on
Synchron: QRS cycle detection and on demand pulse generation

Q: What is a full bridge rectifier for?
A: Conversion of AC to DC

Q: Sketch a basic differential amplifier
A: Multiplies the difference between inputs with gain
Used as base for more complex amplifiers

Sketch a (non) inverting amplifier circuit.

Q: Sketch and explain the differential amplifier
A: Amplifies U1-U2
180° phase shift at output
Gain factor is determined by R1&R2

Q: Sketch a measurement/instrumental amplifier.
A: Gain set by R2
High CMRR
High Zin at both inputs
Has often input buffers --> no input impedance matching
Low DC offset
Low drift and noise
Ud=0, output at max supply +/-9V f.e.

Q: Sketch a typical electrocardiogram (ECG) and explain.
A: =el. Signal of cardiac muscle polarization
P... depolarization of the atria (upper chambers of heart) --> they contract to push blood into the ventricles
QRS... depolarization of ventricles (lower chambers) --> pump blood out of the heart
T... repolarization of the ventricles
Slight increase in heart rate during inspiration, because Flow is constant and if the volume changes, the heart rate has to too. Q = V * f (Flow=Volume*heart rate)
stroke volume decreases due to constriction of space while inhaling --> f increases

Q: What is the signal to noise ratio?
A: Compares the level of the signal to level of the noise in dB
SNR=20*log(U_sig/U_noise)

Q: What are switching methods in prosthetic? What is done to avoid them?
A: Used to switch between different movements like grabbing, rotation, opening/closing the hand
Contraction control (COCO): two electrodes attached to flexor and extensor muscle are used to switch between activities if a fist is formed shortly
Avoided with:
4-ch approach
Pattern recognition
Targeted muscle reinnervation: transfering intact muscle nerves stump into the prosthetic.

Q: What is a Phonocardiogram (PCO)? How is it related to the ECG?
A: =recording of the sounds made by the heart (=vibrations)
4 different sounds in total
First(during R spike) and second(T wave) by closing of the valves (these are clearer and louder)
Third(between T&P) and fourth(between P&Q) caused by turbulences
Amplitudes on the PCO depend mainly on frequency, because the tissue absorbes them very different
During inspiration the signals are more attenuated