The Action Potential
Recording an AP requires the isolation
of a single cell.
Microelectrodes (with tips a few μm across) are used to stimulate
and record the
response. A
typical AP is 2-4ms long with an amplitude of about 100Mv
The Electroencephalogram EEG
EEG is the graphical representation of
the electrical activity of the brain
Very commonly used to diagnose certain neurological disorders,
such as epilepsy
More recently, also investigated whether it can detect various
forms of dementia
or schizophrenia
EEG is the specific recording obtained using the scalp
electrodes from the
surface of the skull
During surgery, electrodes may also be placed directly on the
cortex. The
resulting signal is then electrocorticogram (ECoG).
Just like ECG, EEG is also obtained using several different
electrodes places on
different regions
of the head / brain
The Event Related
Potentials –
ERPs
ERPs are really EEGs obtained under a
specific protocol that requires the
patient to response to certain stimuli –
hence event related potentials.
Also called evoked potentials these signals can be used to
diagnose certain
neurological disorders such as dementia,
and they can also be used as a liedetector
• The oddball paradigm
• The
guilty knowledge test
Electroretinogram ERG
The ERG is the record of the retinal
action currents produced by the retina in
response to a light stimulus.
It measures the electrical responses of the light-sensitive cells
(such as rods and cones).
The stimuli are often a series of light
flashes or rotating patterns
The ERG is recorded using contact lens electrode that the subject
wears while watching
the stimuli.
Phonocardiogram
– PCG
The PCG is the graphic record of the
heart sounds and murmurs. It is thus a
mechanical / audio signal, rather than
an electrical signal
Can be easily heard using a stethoscope
Or can be converted into an electrical signal using a transducer
Typically used to determine the disorders related to the heart
valve, since their
routine opening and closing create the
well-known sounds.
• S1 sounds: First heart sounds –
ventricular contractions move blood into atria closing
of the AV (mitral and tricuspid) valves,
then semilunar valves open and blood ejected
out of ventricles – immediately follows
the QRS complex
• S2 sounds: Second heart sounds –
Closure of semilunar (aortic and pulmonary) valves
• Any unexpected sound may indicate a
malfunctioning valve that causes the blood flow
into / out of a
chamber when it should not. Also called heart murmurs.
Define ultrasound
•
Mechanical
waves in different modalities (longitudinal/lateral) à needs medium to be propagated (solid, liquid,
gas)
•
>
20 kHz
•
Continuous/pulsed
•
Spherical/planar/narrow
beam/surface wave/Lamb-wave
Physical phenomena behind ultrasound measurements
Transmission
•
•
reflection
•
transit
time
•
differences
in propagation velocities
•
returns
to transit time
•
doppler-shift
in frequency
•
flow
velocity
•
change
of acoustic impedance
•
comparing
to reference
•
interference
of ultrasound waves (holography)
•
interaction
of ultrasound and light (photoacousticz)
•
ultrasound
needs medium for propagation à it doesn’t propagate in vacuum
•
because
mechanical waves need moving massunits and spring forces between them
•
in
acoustic emission the medium creates ultrasound (for example, during pressure
changes), which is received by sensors
•
pulsed
mode more common than continuous
•
continuous
reguires separate transducers for transmitting and receicving
•
in
pulsed mode an ultrasound burst is sent to the object and the same transducer
is switched to listen echoes
•
standing
wave problem
•
in
us-therapy pulsed mode gives more effective care without too much heating
The Doppler Equation describes the
relationship of the Doppler frequency shift to target velocity.
The frequency difference is equal to the
reflected frequency (FR) minus the originating frequency
(FT). If the resulting frequency is
higher, then there is a positive Doppler shift and the object is
moving toward the transducer, but if the
resulting frequency is lower, there is a negative Doppler
shift and it is moving away from the
transducer. In its simplest form it would be calculated as if
the ultrasound was parallel to the target’s direction, as shown
in diagram A below.
However, this would be a rare occurrence
in clinical practice, because the transducer is rarely
pointed head on to a blood vessel. In
real life, the ultrasound waves would approach the target
at an angle, called the Doppler angle (
). On the following page, diagram B shows the Doppler
equation used in general clinical situations, which includes the
Doppler angle.
The Doppler Angle
The ultrasound beam usually approaches
the moving target at an angle called the
Doppler angle ( ). This reduces the frequency shift in proportion to the
cosine of this
angle. If this angle is known then the
flow velocity can be calculated. The equation used
is:
The Doppler Equation
≡ Doppler shift frequency (the
difference between the transmitted and
received frequencies)
≡ transmitted frequency
≡ reflected frequency
V ≡ velocity of the blood flow towards the transducer
C ≡ velocity of sound in tissue
θ ≡ the angle between the sound beam and the direction of moving
blood
Where:
The Doppler angle ( ) is also known as
the angle of insonation. It is estimated by
the sonographer by a process known as
angle correction, which involves aligning an
indicator on the duplex image along the
longitudinal axis of the vessel.
There are a few considerations that
affect the performance of a Doppler
examination that are inherent in the
Doppler equation, which are:
– The cosine of 90° is zero, so if the
ultrasound beam is perpendicular to the
direction of blood flow, there will be
no Doppler shift and it will appear as if
there is no flow in the vessel.
– Appropriate estimation of the angle of
insonation, or angle correction, is
essential for the accurate determination
of Doppler shift and blood flow velocity.
The angle of insonation should also be
less than 60° at all times, since the
cosine function has a steeper curve
above this angle, and errors in angle
correction will be magnified.
The simplest Doppler devices use
continuous wave (CW Doppler), rather than the pulsed
wave used in more complex devices. CW
Doppler uses two transducers (or a dual element
transducer) that transmit and receive
ultrasound continuously. The transmit and receive
beams overlap in a Doppler sample volume
some distance from the transducer face, as
shown in the diagram below.
volume) is the region of transmitting
and receiving beam overlap (shaded region).
Because there is continuous transducer
transmission and reception, echoes from all
depths within the area arrive at the
transducer simultaneously.
So although CW Doppler can determine the
direction of flow, it cannot discriminate
the different depths where the motion
originates. The usefulness of CW Doppler
devices is limited, but they are used
clinically to confirm blood flow in superficial
vessels, as they are good at detecting
low velocities. As they are easily portable, this
can be done at the bedside or in the
operating room. Most other clinical applications
require pulsed wave Doppler.
Pulsed Wave Doppler (PW Doppler)https://naveenkumaropm.wixsite.com/technewstamil
