Ultrasounds & Electricity in Medicine

Ultrasounds & Electricity in Medicine
Luís Rita | 78680 | luis20dr@gmail.com
Abstract
With an increasingly higher life expectancy and demanding from the population, big efforts have
been done to improve and create new imaging techniques. This article pretends to review the
most important based in ultrasounds and in the currents generated by our own nervous system.
Simple mathematical and theoretical approaches will be also considered to easily understand the
technique behind. Applications and hazards of electric current were addressed at the end.
Keywords
Ultrasounds, Doppler, EEG, ECG, EMG, ERG, EOG, MCG, MEG, TENS, Iontophoresis
Introduction
With an increasingly higher life expectancy (“In USA the number of adults age 65–84 is expected
to double from 35 million to nearly 70 million by 2025 (…)”. [1]), the prevalence of age-associated
diseases is rising. And, consequently, the costs in health are also higher than ever (“Overall health
care expenditures in the United States reached $1.8 trillion in 2004 with almost 45 million
Americans uninsured. It is projected that health care expenditures will reach almost 20% of the
Gross Domestic Product (GDP) in less than 10 years, threatening the wellbeing of the entire
economy” [1]). Important factors like, cheaper medicines, more effective ones, medical imaging,
play an important role in cost containment.
Nowadays, a plenty of imaging techniques are available on the market. Although, some
arise big concerns about ionizing radiation exposure. The importance of ultrasounds and
electricity based methods are directly related with this. In fact, they don’t empower you with
significant better time or spatial resolution, but turn possible the detection of a large quantity of
pathologies in the safest way possible, nowadays. One can distinguish 2 big groups of medical
imaging techniques. One is constituted by those that require the emission of some external sort
of wave (sound or electromagnetic), where we can include ultrasounds. The other, comprises
only the detection of a signal the body itself generates, then no emitter is necessary, just a
receiver.

2
Medical Ultrasounds (Chosen theme to develop, as suggested)

Ultrasounds are high frequency sound waves (>20 kHz), which can be generated using
piezoelectric crystals. Usually are made of quartz and are able to convert electric impulses into
mechanical stimuli, as well as the inverse.
In spite of focusing their application in the medical domain, they are used for many
animals, replacing or complementing their vision. After emitting these sound waves, distances
between the emitter and different objects around can be determined, based on the time that the
echo took to arrive to the animal. This capability among animal kingdom is called echolocation. In
species that live in natural dark environments, this is an evolutionary advantage, since they have
an enhanced spatial orientation comparing to those that haven’t this great audition. Later, the
preys itself, started developing the same skills, restoring some fairness in competition.
In medicine, the same principle can be used. It’s now, expected the signal to get reflected
and captured by the transmitter due to the presence of organs, soft tissues, generally, living
tissue. The main purpose will be to obtain pictures of inside of our body and later to use them to
detect possible causes of pain, swelling or infection. It can also be used in helping surgeries,
diagnose heart conditions, and assess damage after a heart attack, those require a permanent
monitoring overtime, provided by sonography (an alternative term to refer to ultrasound
imaging).
Within sonography, we can identify 3 different techniques, with different purposes, than
just observe anatomically the human body.
® Colour Doppler - uses a computer to convert Doppler measurements into an array of
colors to show the speed and direction of blood flow through a blood vessel.
® Power Doppler - newer technique that is more sensitive than color Doppler and capable
of providing greater detail of blood flow, especially when blood flow is little or minimal.
Power Doppler, however, does not help the radiologist determine the direction of blood
flow, which may be important in some situations.
® Spectral Doppler - displays blood flow measurements graphically, in terms of the
distance traveled per unit of time, rather than as a color picture. It can also convert blood
flow information into a distinctive sound that can be heard with every heartbeat.
How does it work?
During Doppler ultrasound, a handheld device is passed lightly over the skin above a blood vessel.
The device is called a transducer. It sends and receives sound waves that are amplified through a
microphone. The sound waves bounce off solid objects, including blood cells. The movement

3
of blood cells causes a change in the pitch of the reflected sound
waves. This is called the Doppler effect. If there is no blood flow, the
pitch does not change. Then, pictures of these waves can be used to
quantify the blood flow through blood vessels.
Assuming radial velocity, i.e an angle of insonation (𝜃) equal to 0°,
then:
𝜑 = 2×
(
)
×2𝜋 1
𝑟 = 𝑟. − 𝑣( 𝑡 (2)
𝜔 =
56
57
=
5
57
2×
(89:;7
)
×2𝜋 = −
<=×:;
)
⟹ ∆𝑓 = −
A×:;
)
(3)
𝜆 =
𝑐
𝑓
(4)
∆𝑓 = −
2𝑓𝑣(
𝑐
(5)
We should generalize this equation, so it is possible to
predict the velocity of the bloodstream without needing so small
angles (in practice, very difficult to achieve):
∆𝑓 = −
2𝑓𝑣(
𝑐
cos 𝜃 (6)
Acoustic impedance is an important factor that should be
considered when analyzing wave reflection. The intensity of a
reflected echo is proportional to the difference in acoustic
impedances between two mediums. If two tissues have identical
acoustic impedance, no echo is generated. Interfaces between soft tissues
of similar acoustic impedances usually generate low-intensity echoes. Conversely interfaces
between soft tissue and bone or the lung generate very
strong echoes due to a large acoustic impedance gradient.

Limitations
Every substance, such as a nerves, muscles, or fat, has a
unique property called “acoustic impedance”. Acoustic
impedance is a complex concept, but basically depends on the
density of the substance and the speed of ultrasound in the same
substance. Substances with different acoustic impedances alter the course of ultrasound waves
in an important manner.
Fig. 1 – Doctor performing Doppler.
Fig. 2 – Scheme showing ultrasound
propagation.
Fig. 3 – Ultrasound features varying with
transducer frequency.

4
Some events are mainly responsible for the degradation of imaging
when using ultrasounds:
1. When an ultrasound wave tries to pass from one substance to
another substance, part of the ultrasound waves continues into
the second substance, becoming slightly bent away from their
original direction. The bending away when ultrasound passes from
one substance to another substance with a different acoustic
impedance is called refraction;
2. Some of the ultrasound waves are attenuated. That is, the body
absorbs the ultrasound energy, making the waves to vanish. These waves don’t return to
the probe and are therefore “wasted”. The more the body tissues that the ultrasound
waves have to cross, the more attenuation the waves suffer (not all tissues will attenuate
in a same way – Fig. 4). Additionally, as higher is the frequency if the ultrasound waves,
as attenuated they will get (Fig. 3). That is one reason why it is more difficult to image
deeper structures.

To prevent these events, a gel is usually applied between the probe and the human body
to reduce air presence in this interface. As well as, when imaging areas like thorax and head, it’s
important to avoid bone (skull and ribs, respectively) between the transducer and the target.

How to prepare it?
You should wear comfortable, loose-fitting clothing (although, you me be asked to wear a gown
during the procedure) for your ultrasound exam. You may need to remove all clothing and jewelry
in the area to be examined.
Specific preparation for the procedure will depend on the type of examination. For some
scans, doctors may instruct not to eat or drink for as many as 12 hours before appointment. For
others, people may be asked to drink plenty of water prior the exam and avoid urinating so that
their bladder is full enough when the scan begins.

Fig. 4 – Attenuation vs frequency in
3 different biological materials.

5
Muscular Physiology
The muscular system is the biological system of
humans that produces movement. The
muscular system, in vertebrates, is controlled
through the nervous system, although some
muscles, like cardiac muscle, can be completely
autonomous. Muscle is contractile tissue and is
derived from the mesodermal layer of
embryonic germ cells. Its function is to produce
force and cause motion, either locomotion or
movement within internal organs. Much of
muscle contraction occurs without conscious
thought and is necessary for survival, like the
contraction of the heart or peristalsis (e.g. in
esophagus), which pushes food through the
digestive system. On the other hand, voluntary
muscle contraction is used to move the body
and can be finely controlled (thanks to the
coordinated response of agonists’ and antagonists’
muscles), such as movements of the finger or gross movements executed by the biceps and
triceps.
Muscle is constituted of muscle cells, named muscle fibers. Inside these compartments
are myofibrils, which can be seen as a bunch of sarcomeres connected on their both ends.
Zooming out, it is possible to observe that muscle fibers are organized in fascicles, which are lined
by a layer named perimysium. These bundles then group together to form muscle, lined by
epimysium.
3 different types of muscles are identifiable in the human body:
® Skeletal muscle – "voluntary muscle" is connected by tendons to the bone and is used
to effect skeletal movement such as locomotion. Skeletal muscle cells are
multinucleated with the nuclei peripherally located. Skeletal muscle is called 'striated'
because of the longitudinally striped appearance under light microscopy. The main
functions of this tissue include: to provide movement (it is connected to bone through
tendons); helps maintaining a constant temperature; support of the body; protection of
internal organs and contribute to joint stability.
Fig. 5 – Skeletal muscle structure.

6
This category can be subdivided in another 3, related to paths of generating ATP, as well
as to the velocity of contraction:
• Type I – slow oxidative or "red" muscle is dense with capillaries and is rich in
mitochondria and myoglobin, giving the muscle tissue its characteristic red
color. It can carry more oxygen and sustain aerobic activity (predominant fibers
in human body, 50-55 %);
• Type IIa – slow muscle, are aerobic, rich in mitochondria and capillaries and
appears red (30-35 %);
• Type IIb – anaerobic, glycolytic, "white" muscle that is even less dense in
mitochondria and myoglobin (10-20 %).
® Cardiac muscle – "involuntary muscle" but it is striated in structure and appearance. Like
smooth muscle, cardiac muscle cells contain only one nucleus. Cardiac muscle is found
only within the heart;
® Smooth muscle – also an "involuntary muscle" consists of spindle shaped muscle cells
found within the walls of organs and structures such as the esophagus, stomach,
intestines, bronchi, uterus, ureters, bladder, and blood vessels. Smooth muscle cells
contain only one nucleus and no striations.
Muscular tension depends on the type (indirectly depending on features like the length
and the diameter of muscular cells) and number of fibers under stress. The frequency of the
nervous impulses (which arrive to the muscles) and the number of fibers per motor unit, must
also be considered.
Mechanism of Contraction [Skeletal Muscle]
A muscle contraction is the activation of the muscle fibres. This action can only be completely
defined by its tension and length variation associated to the muscle. A consequence of being
possible to exert tension and maintaining fibres length constant, and the inverse. At the level of
skeletal muscle, this happens due to motor neuron innervation. The same motor unit can excite
more than one muscle cell at a single time, thereby causing the fibres to contract all at the same
time. As the nervous impulse arrives the end of the axon, it causes the exocytosis of a
neurotransmitter called acetylcholine. This will change the electrical resting potential of under
the motor end plate. Initiating an action potential which passes in both directions along the
surface of the muscle fiber. At the opening of each transverse tubule onto the muscle fiber
surface, the action potential spreads inside the muscle fiber. And where it touches part of the
sarcoplasmic reticulum Ca2+
are released. This will cause the movement of troponin and

7
tropomyosin on their thin filaments and enable myosin molecules to grab and pull the actin
filaments. Culminating with the muscle contraction.

Medical Imaging – Based on Electric & Magnetic
Fields
EEG
Electroencephalography is a technique used to record the electric activity of the brain. It’s
possible to measure ionic current within the neurons in the brain generated by propagation of
an action potentials. N.B the signal detected is a summation of different signals generated
almost simultaneously in many neurons.
After preparing the scalp area by light abrasion (to reduce impedance due to dead skin
cells), electrodes are displaced individually over the scalp or a cap with embedded sensors
(particularly useful when a big spatial resolution is needed – more
electrodes) may be used. Most of the times this is done non-invasively,
although for some situations a stronger signal is required, then electrodes
should be set internally. The displacement of these is not random, most of
the times follows a standardized distribution recognized as International 10-
20 system (Fig. 6) in order to be easier the comparison between exams
performed in different imaging units (hospitals, clinics…). Then, circuitry of
amplification, filtering and analog to digital converters are used.
At the end, each electrode will present its own characteristic curve, which is
an overlap of many clinically important known waves:
® Delta – It tends to be the highest in amplitude and the slowest waves. It is seen
normally in adults in slow-wave sleep. It is also seen usually in babies [< 4 Hz];
® Theta - Theta is seen normally in young children. It may be seen in drowsiness or
arousal in older children and adults [4-7 Hz];
® Alpha - It emerges with closing of the eyes and with relaxation, and attenuates with eye
opening or mental exertion [7-14 Hz];
® Beta - It is seen usually on both sides in symmetrical distribution and is most evident
frontally. Beta activity is closely linked to motor behaviour and is generally attenuated
during active movements [15-30 Hz];
Fig. 6 – International 10-20 system.

8
® Gamma - Gamma rhythms are thought to represent binding of different populations of
neurons together into a network for carrying out a certain cognitive or motor function
[30-100 Hz];
® Mu - Partly overlaps with other frequencies. It reflects the synchronous firing of motor
neurons in rest state [8-13 Hz].
This technique is usually performed to detect cerebral vascular accidents, to diagnose
epilepsy, coma, brain death, among others.

9
ECG
Alike EEG, electrocardiography is used to detect
electrical activity but from the heart over a period of
time (usually 10 seconds). The electrodes used are
able to record electrical changes in the skin, caused by
depolarizing and repolarizing events during each
heartbeat.
A healthy heart will present an orderly progression of
depolarization that starts with pacemaker cells in
the sinoatrial node, spreads out through the atrium,
passes through the atrioventricular node down into
the bundle of His and into the Purkinje fibers,
spreading down and to the left throughout
the ventricles. This orderly pattern of depolarization
gives rise to the characteristic ECG tracing (Fig. 7).
The electrodes can be placed just over the heart, as well as in the limbs. The limb electrodes can
be far down on the limbs or close to the hips/shoulders as long as they are placed symmetrically
(Fig. 8).
A large number of pathologies can be diagnosed using this technique, namely: myocardial
infarction, pulmonary embolism, cardia dysrhythmias, among others.

EMG
Electromyography is designed to register the electrical activity from the skeletal muscles. The
instrument used to achieve this is called electromyograph and the record electromyogram. A
Fig. 7 – 1 period of a common ECG wave.
Fig. 8 – Common electrodes displacement.

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set of electrodes are also used at the surface or, if needed, may be necessary to insert them
across the skin (Fig. 9). N.B. It is only expected to obtain any signal when the muscle is under
contraction, other ways no electric potential from muscle fibers is expected.
After acquiring the signal, through a detailed analysis, the measured EMG signals can
be decomposed into their constituent MUAPs (motor unit action potentials). This is not easy
due to the number of motor units present, as well as variations in the location of electrodes can
completely change the shape and intensity of this signal.
In terms of diagnosis, this technique had a great impact in the diagnosis of pathologies
related to motion: amyotrophic lateral sclerosis, myasthenia gravis, and muscular dystrophy.
Biomechanics also benefited a lot from these devices, allowing researchers to project prosthesis
and evaluate motion in a non-pathologic point of view.
E.g. to improve athletes’ performance.
Usually performed in combination with EMG are
nerve conduction studies. So, it becomes possible to
measure nerve and muscle function at the same time.
This may be indicated when there is pain in the limbs,
weakness from spinal nerve compression, or concern
about some other neurologic injury or disorder. After
inserting the stimulator and the detector electrodes, the distance
between them is determined and the latency is measured (most of the times in ms).

ERG
Electroretinography is technique which records electrical activity from
the photoreceptors (rods and cones), inner retinal cells and
the ganglion cells. The electroretinogram will then show a signal with a
very low voltage, typically, measured in microvolts or nanovolts.
After dimming a dark-adapted eye with an intense stimulus, it
is expected the rod system to primarily respond. Being adapted to
light, cones and other structures will sum a small current to the
previous one, firstly detected.
Usually, an electrode is placed over the cornea and another on
the skin near to the eye (Fig. 10).
Technique usually applied by ophthalmologists or optometrists
to diagnose many diseases, among them: diabetic retinopathy and
cone dystrophy.
Fig. 9 – EMG usual setup.
Fig. 10 – ERG in progress.

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EOG
Electrooculography records the corneo-retinal standing potential that exists between the front
and the back of the human eye. The resulting registry is called electrooculogram. The main
applications implicated in this test are in ophthalmological diagnosis and in recording eye
movements. This is possible because eye acts as a dipole in which the anterior pole is positive
and the posterior pole is negative.
Consequently, in left gazes
the cornea approaches the electrode near the
outer canthus of the left eye, resulting in a
negative-trending change in the recorded
potential difference and, when looking to the
other side, positive-trending change will be
recorded. One of the possible sources of noise
is eye blinking, cluttering the signal.
In practice, this is mainly used to
assess the function of the pigment epithelium.
One of the main differences between EOG and ERG
is related to the fact that responses to individual stimulus are not measured using this
technique.

MEG
Magnetoencephalography intends to capture the magnetic field
associated to an electric field generated by an ion flux and
captured by EEG. The signal coming from each neuron is so
small, that are necessary at least 50 000 active neurons to be
detectable. Moreover, this exam must be made in a room
isolated from every possible magnetic field (including the
Earth’s magnetic field) and the magnetometers (arrays of
SQUIDs are most common magnetometers) must be very
sensitive (in order to detect fields in the order of fT).
MEG may be used to detect pathological areas on the brain,
before surgical removal.
Fig. 11 – EOG results from the left eye.
Fig. 12 – MEG simulation.

12

MCG
A recent imaging technique which consists in detecting
the magnetic field generated by the electric current
present on the heart. Using a multi-channel device, a
map of the magnetic field can be made over the torso
and, knowing the conductivity structure of the torso, it is
possible to find the source of the signal.
This technique is useful on detecting possible
sources of arrhythmia.

Evoked Potentials
Generally, evoked potential consists in electrical potential recorded through the nervous
system, after presenting a stimulus. In these tests, spontaneous potentials are not of interest
like in many other techniques seen above. 2 big groups of evoked potentials can be defined:
1. Sensory evoked potentials – are recorded after the stimulation of sense organs, from
the CNS. Examples of these are visuals, auditory and somatosensory evoked potentials;
2. Motor evoked potentials – recorded from muscles following direct stimulation of
exposed motor cortex.
These allow the diagnose of disorders of the optic nerve, to detect tumors or other
problems affecting the brain and spinal cord. They are also used to assess brain function during
coma.

Fig. 13 – Magnetic field generated by the heart.
Fig. 14 – Visual evoked potentials test.

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Harmful Effects of Electricity – Humans
Under some circumstances, electric current can be threatening for humans. The simplest way of
causing injury is through Joule effect. Just like a common flame, current can induce burns in the
human body (even beneath the skin of the victim, burning internal organs). This happens due to
the presence of a resistance and current, by applying Ohm’s law, the power dissipated across it,
can calculated using the formula: P = RI2
(W).
Another effect of exogenous electricity in the human body, the more hazardous one,
regards the nervous system. In other words, the network of special cells in the body called “nerve
cells” or “neurons” which process and conduct the all the signals responsible for regulation of
many body functions. The brain, spinal cord, and sensory/motor organs in the body function
together to allow it to sense, move, respond, think and remember.
Nerve cells communicate to each other by acting as “transducers:” creating electrical signals (very
small voltages and currents) in response to the input of neurotransmitters, and releasing it when
stimulated by electrical signals. If electric current of sufficient magnitude is conducted through a
living creature (human or otherwise), its effect will be to overlap the tiny electrical impulses
normally generated by the neurons, overloading the nervous system and preventing both reflex
and rational signals from being able to actuate muscles. Muscles triggered by an external (shock)
current will involuntarily contract, and there is nothing the victim can do about it. The worst
scenario occurs when the source is in contact with our hands. When this happens, the relevant
muscles responsible for fingers’ flexion in the forearm have a greater tendency to activate than
the others involved in their extension. This clenching action will force the hand to hold the wire
firmly, thus worsening the situation by securing excellent contact with the source. The immediate
effects in the body comprises involuntary contraction of muscles, leading to all types of
impairment on tissues/organs that actively depend on a fine control by the nervous system to
maintain their proper activity. Medically, this uncontrolled contraction is called tetanus (Fig. 15).
This is particularly important when vital organs like lungs or hearth are affected. As it’s known,
respiration (inhalation) cycles depend on a coordinated contraction of diaphragm and intercostal
Fig. 15 – Tetanus in a skeletal muscle.

14
muscles. When their activity is abruptly changed due to the passage of an electric current, the
risk of asphyxia increases. Many times, even when the intensity of the stimuli is not enough cause
tetanus, hearth can be sent in a condition called fibrillation. This organ became unable of pumping
the blood to vital organs and death becomes a reality.
Interactions between electricity and the associated magnetic fields with the human body
may also be considered hazardous, even without direct contact (Fig. 20).
Electric current doesn’t have to be necessary associated to life threatening events. In fact,
the limits of perception vary a lot accordingly to
many factor. Intensity, time of exposure,
resistance, tension, type of current (DC vs AC),
path and the contact region between the emitter
and the exposed individual.
Intensity
A summary of how current intensity relates to
human perception is depicted in Fig. 16.
Time of Exposure
The overall quantity of electric charge (𝑄 = 𝐼×𝑡)
transferred to the body is an important factor, as
it is not necessary high currents to induce e.g.
ventricular fibrillation.
Resistance
Either mucous membranes and skin offer different resistances to current conduction. Thus, it’s
expectable an electrical stimulus to be less perceived in a dry skin with high callosity, than in a
highly-hydrated surface.
Voltage
High-voltage transmission lines are commonly perceived as more dangerous than the low-voltage
analogous. Considering power dissipated is proportional to current and voltage, this leads to an
increased probability of burns.
Type of Current
Direct current (DC) is more likely to cause muscle tetanus than alternating current (AC), making
DC more likely to “freeze” a victim in a shock scenario. However, AC is more likely to cause a
victim’s heart to fibrillate, which is a more dangerous condition for the victim after the shocking
current has been halted.
Fig. 16 – Electric current vs human perception.

15
Path
Different paths will have distinct resistances associated and the
current will also affect organs differently (Fig. 17).
Contact Region
Generally, the passage of current through the human body depends
on the points where the person interacts with the electric circuit.
Looking on the circuit of Fig. 18, it’s expected not to experience any
current, once there isn’t voltage between the 2 points where the man
and the bird are contacting. On the other hand, in Fig. 19, the bird
still not feel current because it is in the same place, although the man will
get a shock. This occurs because a voltage equal to the one provided by the source is present
across the him.
Depending on the region where the person interacts with the electric
circuit, different outcomes will be present. Exemplifying, if one
interacts the positively charged wire with one hand and the negative
is in contact with his feet, then the current will travel from extremety
to the other (Fig. 21 (R)). The same happens if he/she is holding each
wire with different hands (although the path is different, Fig. 21 (L)).

Fig. 17 – Electric path.
Fig. 18 – Contact region – no shock.
Fig. 19 – Contact region – with shock.

Fig. 21 – Bipolar (L) and unipolar contact (R).
Fig. 20 – Interactions between electricity and human body with
no direct contact.

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Applications Electric Current in Medicine
Iontophoresis
Iontophoresis is a technique which uses electric current
to deliver medicines or other chemical through the skin.
It’s often considered a vaccine with no needle.
Therefore, it is possible to propel high concentrations of
charged substances, transdermally, by repulsive
electromotive forces (Fig. 22).
Some parameters, like the current type,
amplitude, the duration of the treatment should be fined
adjusted to guarantee the effectiveness of the method (Table 1).
Moreover, the substance must be displaced under the electrode
with the same charge in order the substance to diffuse. Another factor that also influences this
process is the presence of sweat gland ducts and hair follicles. Once the stratum corneum per se
couldn’t allow an effective absorption. The depth to which a substance is delivered by
iontophoresis is uncertain. Although, most studies have demonstrated penetration of 3 to 20
mm.
The ions (ionic solution) used will depend on the
therapeutic effects which are intended. The table in this
document identifies some of the more commonly
employed solutions, their use and the electrode under
which they need to be placed in order for the
iontophoretic effect to be achieved. These substances
range from tap water (with the aim of controlling
hyperhidrosis) to steroid based medicines. Regulations
concerning their use will vary from country to country
depending on prescription and therapist autonomy.
Due to the presence of a positive and negative
electrode connected to the skin, this circuit is prompt to
accumulate acidic regions in the anode (accumulation of
Cl-
, forming HCl) and basic solutions in cathode
(accumulation of Na+
, forming NaOH).
Fig. 22 – Iontophoresis stimulating
absorption across the skin.

Table 1 – Solutions being applied
accordingly to the pathology.

17
Similar techniques such as phonophoresis uses ultrasounds instead of electric current to
promote the delivery of chemicals across the skin. More on a cell basis, electroporation may be
used to induce a progressive frailty on the plasma membrane, leading it to an increased
permeability to incorporate smaller molecules, e.g. plasmids.

TENS
Transcutaneous electrical nervous stimulation is a technique based on the application of a low-
voltage current on the skin of the human body. Although, its empirical analgesic effect, it is not
proved as being effective as pain-reducer. One theory says that electrical stimulus near to the
painful region stimulates the nerves around, blocking the pain. Another, claims the technique
stimulates the release of endorphins, natural pain-killers, hence relieving the pain.
Generally, these devices are perceived as very useful for chronic pain sufferers, mainly at
the level of musculoskeletal impairness.
Usually, this technique consists in covering the
painful region with 2 or more electrodes, then applying an
electrical current with a customizable intensity, duration
and frequency.
TENS electrodes are contraindicated to application over the eyes due to risk of increasing
intraocular pressure; transcerebrally, on the front of the neck due to risk of acute hypotension
(through vasovagal response); on the chest using posterior and anterior electrodes; internally;
over wounds or broken skin areas; directly to the spinal cord and over a tumour/malignancy (in
vitro experiments showed electricity promotes cell growth).

Fig. 23 – Common configurations of TENS.

Fig. 24 –TENS common apparatus.

18
References
[1] http://www.nchc.org/facts/ cost.shtml;
[2] https://en.wikipedia.org/wiki/Ultrasound;
[3] https://www.radiologyinfo.org/en/info.cfm?pg=genus;
[4] https://en.wikipedia.org/wiki/Nerve_conduction_study;
[5] https://en.wikipedia.org/wiki/Evoked_potential;
[6] https://en.wikipedia.org/wiki/Magnetocardiography;
[7] https://en.wikipedia.org/wiki/Electrooculography;
[8] https://en.wikipedia.org/wiki/Electromyography;
[9] https://www.techtransfer.com/blog/electrical-safety-workplace/;
[10] https://en.wikipedia.org/wiki/Electrocardiography;
[11] https://en.wikipedia.org/wiki/Electroencephalography;
[12] https://www.allaboutcircuits.com/textbook/direct-current/chpt-3/shock-current-path/;
[13] http://meat.tamu.edu/ansc-307-honors/muscle-contraction/;
[14] https://www.saudecuf.pt/areas-clinicas/exames/neurologia/potenciais-evocados;
[15] https://en.wikibooks.org/wiki/Human_Physiology/The_Muscular_System;
[16] http://www.webmd.com/pain-management/tc/transcutaneous-electrical-nerve-
stimulation-tens-topic-overview;
[17] https://en.wikipedia.org/wiki/Transcutaneous_electrical_nerve_stimulation;
[18] http://www.electrotherapy.org/modality/iontophoresis;
[19] http://www.webmd.com/dvt/doppler-ultrasound#1;
[20] http://www.radartutorial.eu/11.coherent/co06.en.html;
[21] https://pt.slideshare.net/shaffar75/principles-of-doppler-ultrasound;
[22] https://news.ncsu.edu/2014/11/jing-metamaterial-2014/.

Ultrasounds & Electricity in Medicine

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