Medical Instrumentation 4-7

final test

EEG (Electroencephalogram) use

to detect brain electrical activity

typical EEG frequencies

Gamma (stimulated) 22-30Hz
Beta (awake/alert) 13-22Hz
Alpha (relaxed) 8-13Hz
Theta (asleep) 4-8Hz
Delta (deep sleep) 0.5-4Hz

Phonocardiogram use

records heart sounds
sounds of contraction, relaxation, valves and blood flow

Diseases tested for with phonocardiogram

Split sound

Murmurs

Split sound (phonocardiogram)

S1 is split, mitral and tricuspid valve closes asynchronously

Murmurs (phonocardiogram)

caused by stenoses and regurgitation at aortic, pulmonary and mitral valve

S1 of phonocardiogram (time and frequency)

t: 20-100ms

f: 30-100Hz

S2 of phonocardiogram (time and frequency)

t: 25-50ms

f: >100Hz

electromyography use

detect electrical signals used to contract muscles

evaluates the health of muscle and nerve cells

defibrillator use

applies external charge to effectively stop that heart, for SA node to trigger normal conduction cycle

defibrillator paddles size

8-10cm in diameter

formula for energy stored after charging defibrillator

w = ½CV²

what is the point of inductor in discharge circuit

to obtain desired discharge waveform

AED (automatic external defibrillator) use

visually and vocally instructs operators, measures ECG and can provide defibrillation if needed

ICD (implantable cardio-verter defibrillator) use

implanted into the chest to give time continuous ECG readings, and can deliver defibrillation if needed

Pacemaker use

regulates heart rhythm and rate

Different kinds of pacemakers

asynchronous

synchronous

asynchronous pacemaker

functions regardless of patient’s natural rhythm

synchronous pacemaker

detects ECG and applies stimulus pulse in response to feedback

battery for pacemaker

normally lithium iodide cell

energy = rating (A-H) x voltage

can last as long as 12 years

haemodialysis machine use

to remove metabolic waste products from blood of kidney failure or uremic patients

basic working principles of haemodialysis

osmosis: movement of water through a semi-permeable membrane to the more concentrated side

diffusion: movement of solute through a semi-permeable membrane to the more diluted side

ultra-filtration: bulk flow of fluid by hydrostatic pressure through a semi-permeable membrane

how does haemodialysis work

uses dialysate (same concentration as healthy blood) on one side and patients blood on the other side of the membrane

after prolonged exchange across membrane, patient’s blood plasma achieves same concentration as dialysate, hence becoming healthy

various substances in dialysate can be chosen to achieve required speed of exchange

how does peritoneal dialysis occur

dialysate is pumped into peritoneal cavity to slosh about

visceral peritoneum and parietal peritoneum allows for exchange of molecules between dialysate and capillaries behind membranes

after some time dialysate is drained

ventilator use

provides artificial ventilation for patients with reduced of failed breathing, and achieves and maintains desired arterial partial pressure of CO2 (PaCO2) and arterial partial pressure of O2 (PaO2)

can be used in anesthesia as well

kinds of ventilators

positive pressure ventilator

negative pressure ventilator

what is negative pressure ventilator

simulates negative pressure from diaphragm contraction during inspiration

what is positive pressure ventilator

applies positive pressure into airways

inspiration phase:

inspiratory flow delivery system delivers positive pressure, exhalation control system is closed

exhalation phase:

inspiratory flow delivery system is closed, exhalation control system opens to allow airway pressure to balance with atmospheric pressure

modes of positive pressure ventilator

mandatory

spontaneous

mandatory mode of positive pressure ventilator

for patients with no independent control over breathing

controls all breathing parameters

spontaneous mode of positive pressure ventilator

for recovering patients with limited independent ability to breathe

responds to patient’s effort to breathe independently

what does an x ray machine consist of

high voltage generator

• generates high voltages to emit electrons by thermionic emission

x ray tube

• produces x rays when electrons hit metal on anode

collimator

• restricts x rays with its aperture

intensifying screen and film

• primary radiation strike screen phosphor, resulting in light being exposed on the film

grid

• traps secondary radiation

CT (computer tomography) scan use

generates anatomical images that are digitally reconstructed

main components of CT

x ray tube

detector

computer system

gantry and patient table

how does a CT work

source and detector pair get rotated at a small angle, and an x ray is taken

repeats until full 180°

image is reconstructed using x ray from detectors and computers

MRI (magnetic resonance imaging) use

visualizes detailed internal structures

main components of MRI

main magnet

gradient coils

radiofrequency coils

computer system

PROS of MRI

non ionizing rays (unlike radiation)

can build image in any plane (unlike CT)

very low incidents of side effects

provides good contrast between different soft tissues (especially useful for brain muscle, heart and cancer)

how does an MRI work

uses 3 magnetic fields (very strong magnetic field, gradient magnetic field & radiofrequency field)

strong magnets generate magnetic fields to align atoms in the body

radio waves disrupt that alignment

radio waves stop and atoms return to their original position, emitting signals

signals are used to create MRI images

Ultrasound frequency band

uses ultrasounds from 1MHz to 15MHz

main components of Ultrasound

transducer/prob

processing unit

display

how an ultrasound works

piezoelectric effect is applied to crystals with dipoles

changing polarity of electrode plates beside the piezoelectric crystal causes its molecules to constantly twist to align with the produced magnetic field, causing expansion and contraction, making mechanical vibrations (sound)

ultrasonic beams enter the body and gets echoed eventually

piezoelectric crystal receives returning echo and deforms, producing a voltage

processing unit processes signal for display

impedance formula for ultrasound

Z=ρc

Z is impedance

ρ is density

c is acoustic velocity

reflection coefficient and impedance formula for ultrasound

Rcoeff = (Z2-Z1)/(Z2+Z1)

pressure and reflection coefficient formula for ultrasound

Pr = Rcoeff x Pi

P is pressure

4 major sections of medical labs

chemistry

haemotology

microbiology

blood bank

what does chemistry sections of medical labs do

perform analysis on blood, urine, cerebrospinal fluid and other fluids

what does haemotology sections of medical labs do

measures (electrical and optical) blood and its elements

what does microbiology sections of medical labs do

studies various body tissue and fluids

uses devices to automatically monitor status of blood cultures

uses devices to semi-automatically monitor sensitivity of microorganisms to antibiotics

what does blood banks sections of medical labs do

not much electronic instruments used, except for automatic classification of blood group (ABO)

shelve lives of blood and temperature storage in blood banks

whole blood ~35 days (1-6°C)

RBC ~10 years (-80°C)

platelets 5 days (20-24°C)

blood components and their measurements

plasma - 55%

WBC and platelets (leukocytes and thrombocytes) - <1%

RBC (erythrocytes) - 45%

functions of blood

transport dissolved substances

regulate pH, ions and temperature

restricts blood loss

defends against toxins and pathogens

Erythrocytes (RBC)

uses haemoglobin to transport O2 and CO2

normal range

• males: 4.6-6.2 ×10⁶ /µL

• females: 4.2-5.4 ×10⁶ /µL

size: 8×3 microns

haemoglobin

binds to oxygen and some carbon dioxide

normal range

• male: 13.5-18g/dL

• female: 12-16g/dL

leukocytes (WBC)

helps to defend against infections

normal range

• male & female: 4500-11000 /µL

size: 10 µm diameter

disease

• numbers will increase

• immature and malignant WBCs may also appear

platelets (thrombocytes)

clots bleeding

normal range

• male & female: 140,000-400,000 /µL

size: 2-3 µm diameter

blood plasma consists of:

proteins: organic repair substances

nutrients: energy storing substances

regulatory and protective substances

electrolytes: acid, base for nerve impulse transportation

metabolic waste substances: urea, uric acid and CO2

spectrophotometer use

determines the level of absorption/emission of photons at different wavelengths to find presence and quantity of specific substances

components of spectrophotometer

light source

wavelength selector

cuvette

detector and readout

light source of spectrophotometer

UV —————— Visible —————— Infrared ———>

λ<380nm 380-750nm 750-2000nm

wavelength selector of spectrophotometer

isolates desired λ

2 classes

• filters (non-dispersive)

• monochromators (dispersive)

filters for wavelength selector

selectively absorbs/transmits wave lengths

e.g. blue absorbs red (high λ) and transmits blue to green (lower λ)

consists of one of more layers to achieve low/high/band pass

monochromator for wavelength selector

prism

• glass (λ>350nm)/quartz (λ<350nm UV)

• shorter λ gets more bent than longer λ

diffraction grating

• disperses light as a function of wavelength through interference

cuvette of spectrophotometer

contains sample solution

Beer’s law for light intensity of spectrophotometer

It = Ii ×10^(⁻µCL)

µ is extinction coefficient

C is concentration of material

L is length of material

Beer’s law of absorption of light of spectrophotometer

A(λ) = lg(Ii/It) = µCL

A(λ) is absorption of light

flame atomic spectrophotometer

detects traces of pure metals

sample feeds into nebulizer with flame, which replaces cuvette and is now before wavelength selector

uses hollow cathode lamp

how does flame atomic spectrophotometer work

flame provides thermal energy to dissociate atoms from chemical bonds, atoms then emit light when returning back to ground state

hollow cathode tube: produces light of characteristic wavelength of atoms of interest

amount of absorption is proportional to the amount of atoms present

automated cell counters principles

electrical impedance principle

light scattering principle

electrical impedance principle of automated cell counter (coulter principle)

change in resistance/impedance every time a cell passes through an aperture of similar size

change in impedance is directly proportional to cell volume

light scattering principle of automated cell counter

light produced by light source gets deflected when cell passes through

deflection is detected by the photodetector

gives number of cells present in sample’s volume

pH formula

pH is determined by concentration of H⁺ ions

pH = -lg[H⁺]

normal range of blood pH

pH 7.38 - 7.44

how does the pH electrodes work

glass electrodes generate electric potential when solutions of different pH are placed on 2 sides of membrane

H⁺ ions from the sample solution diffuse past the ion selective membrane into filling solution, until charge is balanced

voltage across membrane formula

v = k(C₂ - C₁)

glucose

main circulating carbohydrate in the body

insulin promotes absorption of glucose into skeletal muscles and adipose

O₂ electrode of blood glucose sensor

current flows as a function of O₂ concentration

platinum cathode: reduction

O₂ + 2H₂O + 4e⁻ → 2H₂O₂ + 4e⁻ → 4OH⁻

Ag/AgCl anode: oxidation

4Ag + 4Cl⁻ → 4AgCl + 4e⁻

enzymatic reaction of glucose oxidase

glucose + O₂ ←→ gluconic acid + H₂O₂

how does an enzymatic glucose sensor work

one O₂ electrode is placed into gel without oxidase (I1) and another into gel with oxidase (I2), and their difference ΔI = I1 - I2 is directly proportional to amount of glucose present

major problems with enzymatic glucose sensor

instability in immobilized enzyme

fouling of membrane surface in physiological conditions

most operate effectively for a short period of time only

more highly selective membranes must be developed

pulse oximetry use

accesses tissue oxygenation, SaO₂

measured during anesthesia, pulmonary functions test, intensive care and oral surgery

one Hb can bind to 4 oxygen molecules, a full 4 is considered saturated

SaO₂ formula for pulse oximetry

SaO₂ = (O₂Hb)/(O₂Hb + Hb)

pulse oximetry transducer

940nm infrared light

660nm red light

(the 2 are alternately switched on and interleaved)

photodetector

absorption of Hb formula

A(λ) = Aₒ₂ₕ₆(λ) + Aₕ₆(λ)

kinds of absorption observed in pulse oximetry

dc - constant absorption (bone, tendon, tissue, venous blood)

ac - variable absorption due to arterial pressure changes (arterial blood)

modulation ratio of red and infrared light

R = (acλred / dcλred) / (acλir / dcλir)

R is negative proportional to SaO₂

Instrumental amplifier with gain formula

Vo = (1+2R₂/R₁)(Rբ/R₃)(V₊ᵥₑ - V₋ᵥₑ)

What is the high impedance of a buffer amplifier used for

Since the buffer has a very high impedance, it takes up negligible amounts of current from the input, hence not affecting the signal.

What is common mode rejection ratio

Differential amplifier removes common mode signals (noise).

Since both inputs have the common noise, the noise will be removed after passing through the differential amplifier, as they are common between them.

Cut off frequency formula

1/2πRC

Which part of this active band pass filter is high and low cut off

high pass frequency: 1/2πR₂C₂

low pass frequency: 1/2πR₁C₁

How to know if its high or low pass filter

If parallel to capacitor: low pass filter

If parallel to resistor: high pass filter

time constant for charging defibrillator formula

T = (Rin + Rex) (C)