TU Wien:Biomedical Sensors and Signals VO (Kaniusas)/Prüfungsfragen Ausarbeitung

Die folgende Ausarbeitung enthält alle Prüfungsfragen, schriftlich wie mündlich, die vorgekommen sind und auch von Studierenden der Nachwelt verfügbar gemacht wurden. Es gibt keine Garantie, dass die Ausarbeitung richtig ist. Sinnvoll ist es, wenn viele Studierende an ihr mitarbeiten und sie laufend aktualisiert wird.

Der aktuelle Stand der Fragen ist 2.5.2017 (die obersten Fragen sind in der Prüfung am 2.5. schriftlich oder mündlich vorgekommen und waren nicht ausgearbeitet).

Wichtige Vokabeln

Folgende Vokabel sollte gekannt werden:

• current = Strom
• voltage = Spannung
• impedance = Widerstand
• sodium = Natrium (Na+)
• potassium = Kalium (K+)
• blood vessels = Blutgefäße
• cessation = Stillstand

Schriftliche Prüfungsfragen

Relation in between P_O2 and blood oxygen saturation

Oxygen saturation (SaO2) is a measurement of the percentage of how much hemoglobin is saturated with oxygen. Oxygen is transported in the blood in two ways: oxygen dissolved in blood plasma (pO2) and oxygen bound to hemoglobin (SaO2). About 97% of oxygen is bound to hemoglobin while 3% is dissolved in plasma

Exogenous clocks

e.g. light, temperature and length of day.

Laplace law

K = P * r / h

• K... Wandspannung
• P ... Druck (unterschied)
• r ... Gefäßradius
• h ... Wanddicke des Gefäßes

ST-interval and what it means?

The ST-interval is the time between the QRS complex and the T-Wave on an ECG. It is the period between depolarisation (QRS) and repolarisation (T-Wave) of the ventricles.

If the ST-WAVE is heightened it is a sign for a myocardial infarct -> we learned that a fully polarised or an unpolarised cell has zero potential, if the ST-Wave is heightened it has potential and therefore is not fully unpolarised (which it should be since we just depolarised it with the QRS complex)

Rods & cones - which is larger, and what does it mean?

• photoreceptros
• rods are larger then cones
• rods are much more sensitive
• rods: do not detect color but are highly sensitive to light, fast, detect shifts in light
• cones: three types, endow us with color of a bright light

SLEEP APNEA. What is sleep apnea? What are the types of sleep apnea?

Sleep apnea is a sleep disorder, caused by abnormal pauses in breathing. An obstructive sleep apnea is a cessation of breathing for at least 10s. It’s caused by a physical block to airflow because of reduced muscle tone (reduzierte Muskelspannung), by high body mass or narrowing of the upper airways. It’s usually accompanied by snoring. The 3 types of sleep apnea are:

1. obstructive sleep apnea
   caused by reduced muscle tone (reduzierte Muskelspannung)
2. central sleep apnea
   caused by the CNS (central nervous system) - the brain "forgets" to breath
3. mixed apnea
   include both, obstructive and central sleep apnea

ECG. Function of ECG

the ECG measures the propagation of the electrical signal inside the heart muscles. the P-wave shows the excitation of the atria, the R-peak shows the excitation of the ventricals (they are larger than the atria, so the peak is higher), and the T-wave shows the relaxation of the ventricals. (Slides Ch4 Slide 1-20, Handwritten Page 32)

HEART RATE: How is it connected with the autonomous nervous system?

Heart Rate is mainly independent from the brain. The autonomous nervous system regulates the keyfunctions of the body e.g. heart-rate, constriction of blood vessels

HEART RATE: Describe the heart rate dependent to the breathing cycle!

Breathing in increases the heart rate, breathing out decreases the heart rate. this is because when breathing in, there is less space for the heart, stress levels increase, sympathetic system is activated.

Sleep Stages

REM (Rapid Eye Movements) = For counteracting daily live's wearing effects, dreaming etc. Implicite Memory Tasks (motor skills, cycling a bicycle)

NREM = 4 Stages, explicite memory tasks (e.g. learning vocabularies)

1. Stage: muscle movements are inhibit, partly unconcious)

2. Stage: inconscious but awakend easily

3&4 Stage: deep sleep

PROBING DEPTH. describe the probing depth

changes of optical characteristics at a depth Z within the tissue (in various layers), change the intensity by at least 5% (measured with fotodetector). it is a key issue for reflection of light.

non-linear dependance of scattering coefficient on wave length and absorption - oxygenation, wave length, volume, distance are factors.

more blood locally means higher absorption, decreasing probing depth

PENETRATION DEPTH. describe the penetration depth

depth at which Intensity (I) has fallen by 1/e (by about 35%) - key issue for transmission of light.

The light penetration depth is defined as a tissue depth at which the incident light intensity has exponentially fallen by about 63%.

Key issue for transmission of light

SALT RECEPTORS

NaCl -Depolarisation- Opening of Ca2+ channel- synaptic propagation

SKIN CURVATURE SENSOR. describe the skin curvature sensor (current or voltage better?)

• reducing bending sensitivity: use trilayer layout (Magnetic-Counter-Magnetic Layer)
• eliminate thermal/bending sensitivity: use 2 coils (outer & inner); inner will be sensitive to temperature only

bi-/trilayer is sensor element. coils around it create electrical sensor signal - needs to be extremely flat

if calibration is done too often, material develops memory effect - calibration won't work anymore

bending sensitivity is higher for thick/stiff non-magnetic (counter) layer

single layer: bending/tension has lower effect than compression - no permeability changes

Tonic-phasic receptor

quick adaption, moderately rapid - velocity detector (Meissner corpuscule)

slow adaptation - intensitiy detector (Ruffini endings)

tonic codes basic level; phasic codes change - tonic-phasic is a mixture, adapts to the signal, frequency deteriorates over time (Gewöhnungseffekt)

Structure of membrane

Lipid molecules as basic substance: hydrophobic tail, hydrophilic head - interaction with H20

• extra-cellular: polar binding between heads and water (electrostatic interaction), polar binding between heads
• intracellular: hydrophobic binding between tails

saturation of stray electric fields inside & outside

fluid double layer structure, low conductivity, high capacity (cell acts as capacitor!), affinity to positive Ions (Na+, K+)

receptors, enzymes, antigens & ion channels on outer layer

Frequency dependent sound transmission

• wavelength (space) + frequency (time) => velocity
• lower frequency sounds propagate along tissue (softer tissue resonates to lower frequency, can't align to too high freqs), higher frequency propagates along stiffer emdia e.g. bones and airways
• vessels with rising pressure inside stiffen -> pulse waves increase in frequency
• low frequency sounds: hard to locate source; high frequency sounds: dampening is higher, source must be closer to oscultation (listening) location

Ion channels

channels are selective - either for Na+ or K+ ions

• K+ channels: only for small effective diameter (K+-radius plus H2O shell)
• Na+ channels: Na+ + H2O goes through, K+ with H2O shell would be too large

Pacemakers in the heart

propagation of activation

• sinoatrial node: primary pacemaker, about 70 signals per minute
• atrioventricular node (50/minute)
• bundle of His (30/minute)

atria are electrically isolated from ventricles. pacemaker potential is overriden by sinoatrial node

Dispersion mechanisms in the tissue

permittivity, charge accumulation

• displacement polarization (atomar level): for f < 100 GHz (upper bound for atomic nucleus movement)
• gamma dispersion (molecular level): orientation polarization, f < 30 GHz (e.g. water, 15 GHz)
• with E != 0, molecules align according to E (taking into account molecule inertnes
• beta dispersion: cell membrane polarization (f < 100 MHz), strucutures consisting of multiple molecules

decrease of permittivity over frequency

larger structures need more time to align, can only handle lower frequencies! (also true for sound!)

Geometrical damping versus medium damping of acoustical sounds

• geometrical: 1/r (reverse square law)
• medium: e^(-alpha * r) with alpha = absorption coefficient
• low frequency travels along soft tissue - hard to locate source
• high frequency travels along stiff materials. damping is higher, listening/oscultation location must be close source

Bending sensitivity of bilayer skin curvature sensor

• finite length bilayer -> strongly inhomogenous stress distribution
• mechanical factor of sensitivity depends on material properties and dimensions
• sensitivity is higher with stiffer/thicker materials

Pumping of venous blood

parallel arrangement of veins and arteries -> indirect pumping of venous blood, support by venous valves

"respiratory pump": blood volume and vein cirmumference decrease during inspiration, "sucking" blood back towards heart

Electrical axis of the heart

It means direction of total dipole during QRS complex (major electrical activity); R-peaks are not concurrent in the leads.

Determination by the net area of QRS complex.

• area > 0 = heart vector in direction of lead vector
• area ~ 0 = heart vector perpendicular to lead vector
• area < 0 = heart vector opposite to lead vector.

Effect of respiration on optical biosignals

with more blood in local vessels, absorption increases, probing depth decreases (during expiration)

again: respiratory pump! less blood volume, smaller vein circumference during inspiration -> increase of transmitted light intensity during inspiration

Types of controlled channels

• voltage controlled
• transmitter controlled
• stress controlled
• temperature controlled
• ion pumps

Right leg drive

added to the ECG to reduce interferene (e.g. 50Hz interference from power lines). located as far away from heart as possible to single out interference

Relation between blood pressure and pulsetransittime (PTT)

ptt = time a pulse wave takes to travel a certain distance within a blood vessel. measure can allow deductions considering blood pressure, elasticity of vessels.

systolic-diastolic deflection!

Transmitter controlled channel

complementary conformation & charge between channel control molecule and transmitter

Blood pressure and respiration

arterial blood pressure decreases during inspiration, vagus nerve activity is impeded. "respiratory pump": less blood volume during inspiration, vein circumference decreasing

Blood pressure generally

• systolic pressure: proportional to stroke volume (SV, ~80ml) and arterial impedance (= 1/compliance), problems caused e.g. by arteriosclerosis
• diastolic pressure: proportional to peripheral resistance. problems caused by e.g. vascoconstriction (impeded blood flow -> higher blood pressure)

components of vessels controlling blood pressure:

• stiff collagen (more in distal arteries; higher stiffness -> reduced compliance, higher pressure)
• compliant elastin (more elastin in proximal arteries to make them more compliant; more elasticity -> lower pressure)
• smooth muscle

Attenuation of acoustic signals

• inner friction: due to differences in local sound particle velocity, there is friction between the moving particles
• thermal conduction: propagation linked with local variations of temperature, balancing of which withdraws energy from soundwave
• molecular relaxation: pressure translates to vibration nof molecules (with a bit of delay) at the expense of rotational energy (of atoms) and translational energies (gas pressure)

Primary sensing cells

• bare nerve endings (dendrites) as receptors, nerve axons as output
• e.g. touch, warmth, tissue damage, muscle tone
• requires a lot of activation energy.

in the human body: skin (touch) mouth & nose (smell) - with phasic receptors, only change in smell is coded

Secondary sensing cells

• special epithelium cells as receptors, synaptic cleeft with a following nerve cell as output
• e.g. sound, acceleration, light
• requires less activation energy than primary sensing cells

Osmoreceptor

regulated by volume/salt concentration

If (osmotic pressure > 0)

• H2O "flows" into cell, cell swells
• mechanical channels for K+, Cl- open
• salt concentration inside cell decreases -> H2O inflow stops
• osmotic pressure != 0

Adaptive filtering

to cancel out noise, feedback, cancellation

e.g. when monitoring cardiac activity with skin curvature sensor on the neck

Refractory period

characteristic recovery period - cell is incapable of repeating an action during this time it takes for the cell to be ready for another stimulus.

about 2ms in axons - this is the reason we have one-way propagation of signals!

temporal inactivity of Na+ channels, increased (delayed!) opening of K+ channels

Rayleigh scattering

scattering of light on molecular structures smaller than wavelength (lambda)

examples: lipid water interface of cell membranes (9 nm), water-protein periodicity of collagen fibrilles (70nm)

scattering magnitude scales with (s/lambda)^4

Frequency behaviour of accoustic signals on air-tissue interface

concentrated sources vs. distributed sources

pathway of sound depends on frequency, thus velocity depends upon frequency!

• low-freq (<300Hz): coupling from airways into mediastinum/parenchyma, airways as non-rigid tubes, adsorbing sound energy
• high-freq: sounds propagate along airways, travelling into branching structures

Relation between wavelength and scattering

depending on wavelength, light is scattered by different structures. isotropic vs anisotropic scattering

Electrical membrane properties

• low conductivity (gamma = 10^-7 S/m)
• high capacity
• affinity to positive ions (Na+, K+)
• passive channels (diffusion) vs. active channels
• potential ~70mV

Augmentation Index

increase of pressure P_ref because of reflected/backwards wave

P_ref rises with increasing distal stiffness, delta t decreases with higher distal stiffness - together, AI and myocardial coontractility rise to overcome increased P -> loard on heart increases

Excitatory Post-Synaptic Potential (EPSP)

synaptic propagation - EPSP is not action potential, but CAUSES action potential.

1. presynaptic action potential
2. opening of voltage-controlled Ca2+ channels
3. fusing of neurotransmitter-containing vesicles with the cell membrane
4. neurotransmitter diffuses across the synaptic cleft
5. neurotransmitter binds to receptors at postsynaptic membrane
6. simultaneous opening of transmitter-controlled channels for Na+ and K+
7. Na+ inflow is stronger than K+ inflow; depolarisation of the postsynaptic membrane -> EPSP (NOT action potential!)
8. equalizing current across the membrane
9. action potential to the right & left

Gamma dispersion

medium impedance, for frequency of <30GHz, e.g. molecules

molecules align because of electrical field -> orientation polarization

Mie scattering

Scattering on molecule structures >= lambda (anisotropic)

• protein aggregates, mitochondria (1 µm)
• collagen fibre bundles (3 µm)

scattering scales with magnitude s/lambda.

Acoustic signals

Heart sounds (closing of valves), lung sounds (air turbulence), snoring sounds (vibration of soft palate or uvula), apneical sounds (no breathing, followed by gasping for air)

• heart sounds: ~ 100Hz
• lung sounds: ~ 100-500Hz
• snoring sounds: up to 800Hz
• obstructive snoring: 2000Hz

Probing depth versus penetration depth

Both describe depths to which light penetrates the tissue

• penetration depth:
  • where intensity I has fallen by 1/e (by ~ 63%)
  • given by 1/µ or 1/alpha
  • key issue for transmission of light

• probing depth:
  • changes in optical properties at depth Z which yield changes in intensity I of at least 5% (measured by photodetector)
  • given by source-photodetector separation distance D and µ_a, µ_s
  • key issue for reflection of light

Composition of arteries

• elastin (compliant)
• collagen (stiff)
• smooth muscle

proximal arteries: elastin dominates

distal arteries: collagen dominates

EFPG (electric field plethysmography)

// Achtung: Das sind nur meine (eher chaotischen) Notizen, die sollten vermutlich geordnet werden, damit sinnvoll damit gelernt werden kann.

EFPG is an active approach - we apply current, and measure the medium impedance to interpret them for results

alternating current, because the electrical field of the body alternates, too.

• frequency is 20-100 kHz, because
  • below 20kHz, nerves are stimulated, and
  • over 100kHz dispersion means blood, bone and other tissues all have the same permittivity & conductivity!
• current amplitude (I) is 1mA, because
  • higher I, lower f increase pobability of neural stimulus
  • thermal effects increase with higher I
  • signal-to-noise ratio of alternating component increases with higher I
• recording with 2 vs. 4 electrodes
  • 2 electrodes also measure contact and electrode impedance, and movement artefacts occur
  • 4 electrodes show no contact impedance - if I is constant and the volt meter is ideal
• applying current instead of voltage, because
  • current allows local assessment of conductivity changes
  • current yields higher sensitivity

inhalation: more air -> more impedance -> more equipotential lines (-> higher voltage output)

systole-diastole: change of impedance/equipotential lines only around the heart (local occurrence!). heart filled with blood during diastole -> less potential difference (blood has good conductivity)

alternating component of signal: ~10%

• respiratory & cardiac activity
• displacement of organs
• displacement of liquids
• blood volume changes, flow velocity changes

higher speed of blod means higher impedance (because blood cells align along blood vessels, pathway for current is longer)

Forward vs. inverse problem

• forward problem: we want to know what the source does to the signal, e.g. impedance - this is easy, we measure it.
• inverse problem: determine what happens based on the signal, e.g. out of the ECG, determine if the heart is healthy (more complicated)

Stethoscope and frequencies

oscultation on the chest: only low frequencies can be heard, because they progress through soft tissue

membrane:

• smaller radius is better for higher frequencies (small head vs. large head)
• stiffer membrane is better for higher frequencies

bell:

• for resonance
• small bell -> smaller volume -> higher frequencies

Why do low frequencies propagate through soft tissue, but high freqs don't?

because soft tissue is too slow to align to the higher frequency. works a lot like dispersion of tissue related to current (see EFPG for details).

prettified: because the tissue is yielding, it resonates to the low frequency, but cannot resonate to high frequencies.

Salt receptors

Na+ concentration in extra cellular space increases - more Na# ions enter the cell, causing depolarisation. Ca2+ channels open, causing a synaptic signal

Sour receptors

• detection of pH-level via H+ channels, blocked K+ channels, activated Na+ channels
• low pH level (signal for spoilt food) means more H+ ions overall and inside the cell -> membrane potential increases.

Temperature-controlled channels

• warm: channels react to rising temperature (and cross-sensitivity to capsaicin ["scharf"]). more Na+ inflow, action potential ensues.
• cold: channels open for lower temperature (and cross-sensitivity to menthol). less K+ leaving the cell - action potential ensues.

Voltage-controlled channels

in resting state: extremely high electrical field, channel closed by protein.

reduction of E (electrical field) to the threshold value E_S by voltage -> opening of Na+ gate (protein re-aligns to the changed electrical field)

HRV - Heart Rate Variability

fluctuation of RR-Interval (in between 2 R-peaks on ECG, = 1/f)

sympathetic (low freqs) vs. parasympathetic (high freqs of spectral function of RR-intervals) nervous system

highly predictive value for

• survival probability after myocardiac infarction
• sleep monitoring

Cell membrane - schematic

Cells can act like a

• capacitor - a certain amount of change in the cell is necessary to achieve action potential (es braucht eine gewisse spannung, um den Kondensator zu überbrücken)
• resistor - for movement of (charged!) ions, because channels control the ion flow.

Linear vs. adaptive filtering

• applies to e.g. skin curvature sensor when monitoring cardiac activity on the neck.
• linear filtering uses a fixed cutoff for noise. adaptive filtering uses an adaptive cutoff.

Windkessel effect

• is a single compliant compartment just outside the left chamber at the beginning of the aorta.
• is an elastic reservoir, yielding space under high (blood) pressure (during systole), returning to original shape and size during low (blood) pressure (during diastole), to create a more even blood flow.
• with advancing age, arteries and the Windkessel stiffen, the effect lessens -> increased systolic blood pressure. the increased bp indicates risk for myocardiac infarction, strokes, heart failure, cardiovascular diseases generally.

Goldmann equation

used to determine the reversal potential across a cell's membrane, taking into account all of the ions that are permeant through that membrane.

Synaptic propagation

propagation from a nerve cell to another via a network of nerve cells.

Relation of r (radius of blood vessel) and p (blood pressure)

• smaller radius = higher blood pressure
• diastolic blood pressure is linked to impedance of distant vessels!
• if muscles around vessels relax and widen vessels, pressure decreases.

Sleeping stages - average duration, changes in the body

• REM (rapid eye movement): paradoxical sleep, heightened mental activity. inhibition of muscle movements, dreaming, counteracting daily life's wearing effects. heart rate and blood pressure increase. processing/saving of motoric skills memories.
• NREM: revitalization of the body. respiratory frequency decreases, breath volume increases slightly. heart rate & blood pressure decrease. processing/saving of explicite memory tasks, e.g. learning of vocabulary.
  • stage 1: beginning of sleep. slow eye movements, partial consciousness
  • stage 2: unconciousness, though awakened easily
  • stage 3+4: deep sleep

Classification primarily via EEG (electroencephalogram)

Functionality of blood oxygenation

Haemoglobin oxygen saturation (S):

• 20-80% in venous blood
• 90-100% in arterial blood

O2 is mainly carried by haemoglobin (protein molecule with embedded iron)

delivery to tissues by diffusion, driven by p_O2 difference between plasma & tissue cell

Body temperature

healthy target core temperature = 37°C

regulatory mechanisms:

• physical work, increasing core temp
  • dilation of vessels, reduced heat exchange between arteries & veins -> perspiration, normalization of core temp
• freezeng, decreasing core temp
  • constriction of vessels, muscle tremor (Zittern), normalization of core temp
• fever, increasing core temp
  • reduced heat dissipation, reduced blood circulation in the skin, vessel constriction
  • increased heat production due to shivering
  • sensation of cold
• fever, decreasing core temp
  • increased blood circulation -> perspiration -> sensation of heat

circadian variation:

• plus/minus 0.6°C is normal
• minimum at about 3 a.m.
• maximum at about 6 p.m.

Pain perception

Pain usually indicates tissue under duress or damaged tissue.

Primary sensing cells (nociceptors) respond to extremes of temperature, pressure or chemicals (using respective channels)

• A-delta nerve fibres: quick & sharp pain
• C-fibres: slow & dull pain

sensitivity of nociceptors may be increased by inflammation and duration of pain

temporal ignorance of pain because of endorphine/enkephalins (ajdusting activity of brain neurons processing pain).

Types of pain

• referred pain: pain from various organs, felt in certain skin areas (e.g. myocardial infarction in skin over heart) -> convergence of neurons
• phantom pain: severed sensory nerves or intermediate spinal nerves continue sending pain signals
• chronic pain: origin, e.g. injury, is already healed up, but pain persists because of still active neurons in the spine

nociceptors are nearly permanently activated, as well as the corresponding inhibitory systems.

Sense of hearing

• frequency of action potential up to 500Hz - but human ear hears 20-16.000Hz => no direct coding possible
• transmission of frequency information into spatial information
• High pitched sounds vibrate basilar membrane at the thinner (less compliant) end, low pitched sounds vibrate at the thicker (softer) parts.
• Cochlea is filled with liquid, has sensory hairs which register the movement of the basilar membrane, code this information into the signal.

heart vs. lung sounds - frequency & propagationHheart sounds:

• caused by closingn of valves
• concentrated source
• frequency < 100Hz

lung sounds:

• caused by air turbulence
• distributed source
• frequencies = 100-500Hz

low frequencies travel trhough tissue, high freqs along stiffer airways

damping is stronger for high frequency sounds -> oscultation location needs to be closer to the source

Different types of lung sounds

• vesicular sounds: at peripheral lung fields; air turbulences, mainly during inspiration - distributed sources
• bronchial sounds: over large airways, e.g. on the neck - turbulent airflow induces vibrations of airway walls. central source.
• continuous sounds: both healthy/normal and pathological. narrowing, constrction of airways.
• discontinuous sounds: pathological only. alveoli closed by fluids re-open with cracking sounds.
• snoring

Different types of snoring

• nasal snoring: uvula oscillating
• oral snoring: soft palate oscillating
• "normal" snoring: flow limitation due to narrowing of airways during inspiratin - reduced muscle tone because o stress, tiredness, alcohol. f < 800Hz
• obstructive snoring: narrowing & temporal occlusion of airways due to high compliance of airway walls and masses obstructing airways -> pressure outside blockage is low, airway will collapse even further. 800Hz < f < 2kHZ

Light scattering

Far more dominant than absorption. scattering increases the pathway of photons, thus increasing the probability for absorption. dispersion of light due to chaotic variation in refractive index.

Photon diffusion theory

• photon diffusion is a situation where photons travel through material without being absorbed, rather undergoing repeated scattering, changing the direction of their path.
• used to create images of brain & thorax - diffuse optical imaging

Cardiac/respiratory sensitivity with optical signals

• more blood in arteries => more absorption (expiration, systole)
• less blood => less reflection during diastole, inspiration
• veins are more compliant => higher variability of diameter and the blood filling it