ORT21BMI

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ORT21BMI

• Week 4--Generation and detection of biomedical
  signals

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Clinical context

• Youre about to record your first ERG
• Youve followed all the safety tips from last
  week
• You open the supply drawer
• There are electrodes of all sizes and shapes
• Some are tiny needles, some are disks, some have
  suction cups!
• How do you choose which one to use?

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Summary of clinical context

• Where does the ERG come from?
• Where on the body can you measure it?
• What do you measure it with?
• How does electrode selection placement affect
  the result?
• When you measure an ERG, is that all youre
  recording? What else contributes to the final
  signal?

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Learning goals

• Where are the different types of biomedical
  signals generated?
• 1) Internally generated physiological signals
• 2) externally generated diagnostic inputs
• What sorts of electrodes and transducers do we
  use to detect biopotentials and apply diagnostic
  signals?
• 1) Electrodes
• 2) Ultrasound and radiological signals

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Why examine biomedical signals?

• Even the best clinical exam isnt always enough
  to assess a patient
• In our first hypothetical patients, examination
  of the fundus is important but may not be enough
  to make the definitive diagnosis
• Physiological recordings can tell us how
  something works as well as how it looks
• Imaging allows us to examine internal body
  structures

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Physiological electromagnetic signals

• These are the underlying focus of much of this
  subject
• All sensory, motor and homeostatic functions use
  electrical transmission of information
• Monitoring this electrical activity can provide
  valuable clinical information
• What are some of the challenges involved?

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Electrophysiological issues

• 1) In lab animals, you can stick an electrode
  directly into their brains or nerves. Patients
  generally dont allow this. So what?
• This means recording from the outside of the
  skull, which is a pretty good insulator
• 2) The skulls also not round, it varies in
  thickness and it has holes in it
• 3) Neurones dont act alone--generally thousands
  act together, spread widely in space and time

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Problems!

• Lets say youve got 64 electrodes connected
  across your patients scalp
• You can present a stimulus to your patient and
  record some sort of response from his brain at
  each electrode
• How do you know where the signals come from?
• You may recall solving systems of equations in
  several unknowns back in algebra (if you took it)
• Isnt this something similar?

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No!

• The problem here is that you know neither how
  many sources there are nor where they are
• Without that information, you cant set up a
  unique set of equations to solve
• In fact, there are an infinite number of possible
  source distributions that will give you the same
  external measurements
• This is known as the inverse problem it dates
  back to Helmholtz. Its cardiologys problem, too

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A way out!

• If we can make some assumptions about where the
  signals are coming from in the patients brain,
  we can calculate the strength and timing of the
  voltages arising at each of these locations and
  check the results of these assumed sources
  against the measured results
• Now we just have to come up with some good
  assumptions...

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Getting around the inverse problem

• Use as many scalp electrodes as possible
• Dont assume that the brain, dura and skull are
  concentric spheres use realistically shaped
  models
• Represent the conductivity of the skull, dura,
  scalp, muscles, etc at each point
• Model the patients brain with an MRI-generated
  version of the real thing

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Then what?

• Based on a priori information about what sources
  youd expect to be active during your test task,
  assume that they can be represented as tiny
  batteries with a plus and minus end these are
  called dipole sources
• Having made that assumption, calculate what
  dipole voltages, timing and orientations would be
  needed to best fit the results you obtained
  clinically

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Some examples
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Limitations

• The patients brain may be atypical
• You may not have good data about where the
  dipoles should be located, if the task is novel
• Complex, more cognitive tasks may activate
  unexpected areas
• The patient may respond abnormally--thats
  probably why theyre a patient!
• More than a few dipoles are too hard to use
• Dipoles may poorly model real brain activity
• A broad region, not just a focal area, may be
  active at once

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Summing up where we are

• The nervous systems electrical activity can be a
  valuable ally in assessing patients
• Some measures (eg, flash ERG) are
  straightforward others (eg, correlates of
  cognition) extremely complex
• The capability of clinical instrumentation is
  growing rapidly, but its limitations must never
  be forgotten if the results are to be used
  appropriately

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Other bodily electrical activity

• Dont forget the muscles!
• Muscles contract in response to neural activity
• The contraction of each fibre itself generates
  electrical activity
• These can be monitored and can be clinically
  valuable--this is called electromyography (EMG)
• Whats a drawback to EMG recording in an
  ophthalmic setting?

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EMGs

• As with nerves, you can either record
  noninvasively from the outside from many fibres
  at once or invasively from single fibres.
• Again, as ease goes up, precision goes down
• Whats a major use of EMG in ophthalmic practice?
• Hint it involves a bacterial toxin

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For completeness...

• Remember last years electromagnetic waves in
  optics?
• The electrical activity in the brain means
  theres simultaneous magnetic activity

• Measuring magnetic fields isnt as easy as
  recording electrical voltages, but it can be done
• Many of the same issues arise as for electrical
  source identification, but this research may
  eventually show up in clinics

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Externally generated diagnostic signals

• Most of these are familiar already
• Radiological--the X-ray
• The denser the tissue, the more X-rays are
  absorbed
• Plain X-rays are of limited relevance in
  ophthalmology, but in the guise of CAT scanning
  theyre extremely important
• PET scanning also makes use of radiation
• Magnetic field
• Magnetic resonance imaging, both structural and
  functional--relies on strong magnetic fields and
  their interaction with hydrogen atoms in tissue
• Localised magnetic fields can also be applied to
  disrupt brain activity--transcranial magnetic
  stimulation

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Ultrasound--the most familiar

• If you havent used this one yet, you will
• Sound waves bounce off surfaces
• Bats have used this phenomenon for millions of
  years to find their prey in the dark
• Ophthalmologists began using it in 1956
• Extremely high frequency sound waves (10 MHz)
  are generated by oscillating crystals
• A simple reflection time measurement can tell you
  how far away something is (eg, the front and rear
  surfaces of the lens, the retina)

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Ultrasound, contd

• This simple distance measure is the A-scan

• If you sweep the acoustic beam across a
  structure, you produce a 2-dimensional image--a
  cross section of the object. This is the B-scan

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Intra-vascular images of a coronary artery

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Ultrasound, summary

• A relatively simple, inexpensive and safe
  technique for measuring distances between tissue
  structures or making images of internal
  structures
• Limited in resolution, accessibility
• The brain would be a challenge, wouldnt it?

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Electrodes for signal detection

• Why is this an issue? If you measure the
  condition of a battery with a voltmeter, do you
  stop and think about the connections?
• Has anyone here ever measured a voltage?
• Whats different about measuring voltages from
  patients?
• What carries current in a battery or piece of
  electronic equipment?
• What carries current in your body?

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The problem

• In organisms, current flows in the body make use
  of ionic conduction, where positive and negative
  ions are the charge carriers
• What are the likely ions in us?
• In most electronic and electrical equipment,
  current is carried by free electrons

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Making the transition

• The electrode is where the change occurs from
  ionic to electronic conduction
• The chemical implications of this are one reason
  why you dont simply measure an ERG by sticking a
  voltmeter in your patients eye
• There are other reasons, of course!
• If the wrong electrode is chosen, results may be
  useless

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Some chemistry...

• You may remember from chemistry class (or the
  discussion of Galvanis frogs) that if two
  dissimilar metals are inserted into an
  electrolyte solution, a potential difference will
  exist between them
• This is how you can run a digital clock with a
  lemon
• This is how batteries work
• The materials differ in electrode potential thus
  one gives up electrons, the other accepts them

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At the border...

• Heres another view of what happens when a
  current flows from an electrode rightward into an
  electrolyte
• Notice here that both types of charge carrier are
  at work (C cation, A anion). They keep the
  charge neutral.
• In the electrode, only electrons move, opposite
  to current flow
• In the electrolyte, both positive cations and
  negative anions are moving

current
Excellent reference http//engr.smu.edu/cd/EE53
40/lect14/sld005.htm
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Still at the border...

• Cations form when their element is oxidised and
  gives up an electron, thus becoming positive
• The cations can also be reduced by combining with
  an electron and becoming neutral again
• Different materials show varying degrees of
  enthusiasm (and directionality) for these
  reactions
• This determines their electrode or half-cell
  potential

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Electrode potentials

• This is another version of the table in your
  manual
• Materials on top corrode or oxidise most readily
  those on the bottom, the least
• Do you think of aluminium as reactive?

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At the boundaries

• What happens when electrode and electrolyte meet?
• It depends on the electrode material used
• If its something like silver or gold, a charged
  layer builds up where it meets the electrolyte

• This is a polarisable electrode. It acts like a
  capacitor
• Think about what sorts of signals this would be
  good for, and which it wouldnt
• What about other sorts of important signals?

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Non-polarisable electrodes

• What if the ionic layer build-up were countered?
• The commonest electrode to overcome this is the
  silver-silver chloride (Ag-AgCl) electrode
• If this electrode is in an electrolyte containing
  much Cl-, the AgCl in the electrode can prevent
  the formation of the double layer seen previously

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Ag-AgCls virtues

• Stable
• Non-polarisable
• Non-toxic
• Not too expensive (not cheap, though)
• How does it compare with other materials?

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Drift--nice and stable
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Inherent noise--quiet
Ag-AgCl Copper Stainless steel
Quiet Noisy
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Recovery time after current pulse
Recovery to 50 mV from baseline in 0.2
ms Recovery to 50 mV from baseline in 3.5s (18x
longer than Ag-AgCl) Recovery to 50 mV from
baseline in 120 sec (600x longer than Ag-AgCl)
Ag-AgCl
Copper
Stainless steel
Current pulse
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What else matters?

• Electrode paste (usually a NaCl aqueous gel)
• It provides the electrolyte
• It keeps the skin moist
• It minimises the effects of mechanical
  disturbance
• Keep in mind that if the relationship between
  electrode and skin is altered, the charge
  distribution will also change, leading to a
  change of signal

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An electrode and its equivalent circuit
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Stainless steel electrodes

• If Ag-AgCl is so great, why use anything else?
• Sometimes electrodes dont sit on the
  body--theyre stuck into it
• What are hypodermic syringes made of?
• Not a good electrode, but makes holes well
• Whats an ophthalmic use of this?

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Some electrode types
Needle pitch 50 x 100 microns