Magnetic Resonance Imaging (MRI)

Leacture (4)

Dr.khitam Y. Elwasife

MRI=Magnetic Resonance Imaging

Allows the clinician to see high quality images of the

inside of the body:

• Brain

• Heart

• Lungs

• Spine

• Knees

• Etc.

MRI machines look like a large block with a

tube running through the middle of the

machine, called the bore of the magnet.

The bore is where the patient is located for the

duration of the scan.

The MRI machine picks points in the patients body, decides what type of

tissue the points define, then compiles the points into 2 dimensional and

3 dimensional images.

Once the 3 dimensional image is created, the MRI

machine creates a model of the tissue. This allows

the clinician to diagnose without the use of surgery.

The magnet strength is measured in units of Tesla or

Gauss (1 Tesla = 10,000 Gauss).

Today’s MRI machines have magnets with strengths

from 5000 to 20,000 Gauss.

To give the strength of these magnets, the earth’s

magnetic field is about .5 Gauss, making the MRI

machine 10,000 to 30,000 times stronger.

Magnetic resonance imaging (MRI) is a spectroscopic imaging technique used in medical settings to produce images of the inside of the human body.

MRI is based on the principles of nuclear magnetic resonance (NMR), which is a spectroscopic technique used to obtain microscopic chemical and physical data about molecules

MRI

• The magnetic resonance imaging is accomplished through the absorption and

emission of energy of the radio frequency (RF) range of the electromagnetic

spectrum.

The Components

A magnet which produces a very powerful uniform magnetic field.

Gradient Magnets which are much lower in strength.

Equipment to transmit radio frequency (RF).

A very powerful computer system, which translates the signals transmitted by the

coils.

to stimulate the protons in the atoms of the elements present in the body to

release a signal, and then picked up and determine its location in the body and

displayed on the gradation of gray colors refers to the strength of the signal, and

the gradient is different tissues in the body.

- More of these elements is the stimulation of hydrogen so as to its presence in

abundance in living organisms and the presence of a proton & one in the atomic

nucleus, giving it more power than the rest of the items on the issuance of the

signals used in magnetic resonance imaging.

2- MRIThere are many different types today many ideas for magnetic resonance

devices , in general, there are three main types of magnetic resonance imaging

devices ( Permanent Resistant And anti- resistance)

- MRI generally contain part gives a strong magnetic field & part issued radio

waves to stimulate the protons and captures the incoming signal and a portion of

them & tiered system .

- The survey , which is used in medical fields will cost a million dollars per Tesla

and several hundreds of thousands of dollars are spent annually on maintenance .

- Used computers mainly in MRI scans and programs developed effectively help to

give the best results.

1- The idea of magnetic resonance

1- The resistive magnet has many coils of wire that wrap around the bore, through

which electrical currents are passed, creating a magnetic field. This particular magnet

requires a large amount of electricity to run, but are quite cheap to produce.

2- The permanent magnet is one that delivers a magnetic field, which is always on at

full strength and therefore, does not require electricity. The cost to run the machine is

low due to the constant magnetic force. However, the major drawback of these magnets

is the weight in relation to the magnetic field they produce.

types of magnetic resonance imagin devices

3- The superconducting magnets are very similar to the design of the

resistive magnets, in that they too have coils through which electricity is

passed creating a magnetic field. However, the major difference between the

resistive magnet and the superconducting magnet is the fact that the coils are

constantly bathed in liquid helium at -452.4ºC. This cold temperature causes

the resistance of the wire to be near zero, therefore reducing the electrical

requirement of the system. All of these factors allow for the machine to

remain a manageable size, have the ability to create high quality images, and

still operate at a reasonable cost. The superconducting magnet is the most

commonly used in machines today, giving the highest quality images of all

three magnet types.

The device consists of an electromagnet huge spiral for the formation of a

magnetic field around the patient produces a magnetic field of 2 Tesla ,

equivalent to 20,000 Gauss .

-This area makes hydrogen atoms Taatmgnt and moving all of its Part to

magnetic north Vtaatouhd in one direction .

- After that displays the body to radiation Mveaih lead to increased capacity of

the atoms and the piece will be tacking a certain degree , leaving us an iota of

every million corn is the process of magnetic resonance imaging , a large

number of atoms is sufficient for the emergence of a clear picture of the part to

be photographed and sending much of the energy reverse .

-This energy is the inverse of the receiving device are calculated and composed

in the form of a photo This picture shows the intensity of the hydrogen in every

region of the body. Using this image doctors can discover a lot of diseases .

-

The physics of magnetic resonance

When provoked atoms in the body are protons movement with & against the

direction of the magnetic field the main growing protons approval of the thrust

for protons anti small amount but it is very important to get the picture later ,

and disturbed these protons especially radio waves altering the status of the

vertical to the horizontal , but what would soon return to equilibrium situation ,

but for its return to put the equilibrium position.

is for the purpose of diagnostic imaging such as veins and arteries , or

photographing neurological changes in the brain , and magnetic resonance

imaging is better to clarify the types of tissues and body fluids , and is also used

for planning treatment plans based on radiation therapy. Before MRI screening

should review medical history and ensure fully the absence of previous surgery

or accidents have led to the presence of metals in the body such as shrapnel ,

and be sure that cross- examination of public -ray & routine patient passing

through the metal detector . The patient often gives a special dye is injected

into the body in order to increase the contrast and clarify parts converged

The use of magnetic resonance

Composed image of magnetic resonance imaging of multiple columns and rows are

called in English matrix, each column description contains the boxes called pixel,.

-the distribution of the signals captured from the body on these boxes so arranged in

order of the body, and this mechanism relies on a graded gives each segment of the

body force specific signal , and the signal strength captured give color to grayscale ,

consists us a picture magnetic resonance image grayscale . - Clear your equation is :

The number of squares per cm = 1 / box size

- The contrast in the image depends on the horizontal and vertical timings and proton

density and called ( internal influences ) , the echo time and repetition are considered (

external influences ) .

Magnetic resonance image

- More MRIs consists of two dimensions, each dimension is divided into a network of

rectangular elements called sham (pixels) pixels.

The intensity of each pixel in the image depends on the strength of magnetic

resonance wave emitted by the region that they contain.

The size of the image depends on the number Baksalat, and Medm images consist of

265 Peixalat vertically and horizontally Peixalat 256.

Clarity (IMAGE RESOLUTION)

MRI’s of the heart can be done to look at many different areas including: vessels,

chambers, and valves.

The MRI can detect problems associated with different heart

diseases including plaque build up and other blockages in blood

vessels due to coronary artery disease or heart attacks.

MRI’s of the brain can evaluate how the brain is working,

whether normal or abnormal.

Brain MRI’s can show damage resulting from different

problems such as: damage due to stroke,

abnormalities associated with dementia and/or

Alzheimer’s, seizures, and tumors.

fMRI are done prior to brain surgery, to give a map of

the brain, and help plan the procedure.

MRI’s can be done on the knee to evaluate damage to the meniscus,

ligaments, and tendons.

Tears in the ligaments are given a grade 1-3 depending on their

severity:

1-fluid around the ligament

2-fluid around the ligament with partial disruption of the ligament fibers

3-complete disruption of the ligament fibers

a primary magnet: creates the magnetic field by coiling electrical wire and running a current through the wire

gradient magnets: allow for the magnetic field to be altered and allow image slices of the body to be created.

a coil: emits the radiofrequency pulse allowing for the alignment of the protons.

An MRI consists of:

Once the contrast dye has been injected, the patient enters the bore of the MRI

machine on their back lying on a special table.

The patient will enter the machine head first or feet first, depending on the area to

be scanned. Once the target is centered, the scan can begin

.

•The scan can last anywhere from 20-30 minutes.

•The patient has a coil that is placed in the target area, to be scanned.

•A radio frequency is passed through the coils that excites the

hydrogen protons in the target area.

•The gradient magnets are then activated in the main magnet and

alter the magnetic field in the area that is being scanned.

•

•The patient must hold completely still in order to get a high quality

image. (This is hard for patients with claustrophobia)

•The radio frequency is then turned-off and the hydrogen protons

slowly begin to return to their natural state.

The atoms that compose the human body have a property known as spin (a fundamental property of all atoms in nature like mass or charge).

Spin can be thought of as a small magnetic field and can be given a + or – sign and a mathematical value of multiples of ½.

Components of an atom such as protons, electrons and neutrons all have spin.

Spin:

Protons and neutron spins are known as nuclear spins.

An unpaired component has a spin of ½ and two particles with opposite spins cancel one another.

• In NMR it is the unpaired nuclear spins that produce a signal in a magnetic field.

Human body is mainly composed of fat and water, which makes the human body composed of about 63% hydrogen.

Why Are Protons Important to MRI?

positively charged

spin about a central axis

a moving (spinning) charge creates a magnetic field.

When placed in a large magnetic field, hydrogen atoms have a strong tendency to align in the direction of the magnetic filed

Inside the bore of the scanner, the magnetic field runs down the center of the tube in which the patient is placed, so the hydrogen protons will line up in either the direction of the feet or the head.

•The majority will cancel each other, but the net number of protons is sufficient to produce an image.

Energy Absorption:

The MRI machine applies radio frequency (RF) pulse that

is specific to hydrogen.

• The RF pulses are applied through a coil that is specific to

the part of the body being scanned.

The gradient magnets are rapidly turned on and off which alters the main magnetic field.

The pulse directed to a specific area of the body causes the protons to absorb energy and spin in different direction, which is known as resonance

• Frequency (Hz) of energy absorption depends on strength of external magnetic field.

Fig: 2. A) The protons spinning in the nature, without an external strong field. The directions of spins are random and cancel out each other. The net magnetization is nearly 0. B) In the presence of a large external magnetic field Bo the spins align themselves either against (low energy state) or along (low energy state). There is a slight abundance of spins aligned in the low energy state.

Fig:1 A) The top spinning in the earth's

gravity. The gravity tries to pull it down

but it stays upright due to its fast rotation.

B) A charge spinning in the magnetic

field Bo.

Spin

Spin is a fundamental property of nature like electrical charge

or mass. Spin comes in multiples of 1/2 and can be + or -.

Protons, electrons, and neutrons possess spin. Individual

unpaired electrons, protons, and neutrons each possesses

a spin of 1/2.

In the deuterium atom ( 2H ), with one unpaired electron, one

unpaired proton, and one unpaired neutron, the total

electronic spin = 1/2 and the total nuclear spin = 1.

Two or more particles with spins having opposite signs can pair

up to eliminate the observable manifestations of spin. An

example is helium. In nuclear magnetic resonance, it is

unpaired nuclear spins that are of importance.

= electron

= neutron

= proton Properties of Spin

When placed in a magnetic field of strength B, a particle with a net spin can absorb a

photon, of frequency . The frequency depends on the gyromagnetic ratio, of the

particle

The magnetic field runs down the center of the patient, causing the slowing hydrogen

protons to line-up

.

The protons either align themselves pointed towards the head or the feet of the patient,

and most cancel each other out.

The protons that are not cancelled create a signal and are the ones responsible for the

image.

The contrast dye is what makes the target area stand out and show any irregularities

that are present.

The dye blocks the X-Ray photons from reaching the film, showing different densities

in the tissue.

The tissue is classified as normal or abnormal based on its response to the magnetic

field

The tissues with the help of the magnetic field send a signal to the computer.

The different signals are sent and modified into images that the clinician can evaluate,

and label as normal or abnormal. If the tissue is considered abnormal, the clinician can

often detect the abnormality, and monitor progress and treatment of the abnormality.

The MRI has allowed clinicians to treat, monitor, and learn about many different

diseases and problems. As well as, to learn how the body functions, normally, without

needing to resort to more invasive methods like surgery.

relaxation

When the RF pulse is turned off the hydrogen protons slowly return to their

natural alignment within the magnetic field and release their excess stored

energy. This is known as relaxation.

What happens to the released energy?

Released as heat

OR

Exchanged and absorbed by other protons

OR

Released as Radio Waves.

MagLab - MRI Tutorial - National High Magnetic Field ...(show)

It is important to describe NMR on a microscopic scale. A macroscopic picture is more

convenient. The first step in developing the macroscopic picture is to define the spin

packet. A spin packet is a group of spins experiencing the same magnetic field

strength. In this example, the spins within each grid section represent a spin packet.

At any instant in time, the magnetic field due to the spins in each spin packet can be

represented by a magnetization vector. The size of each vector is proportional to (N+

- N-).

The vector sum of the magnetization vectors from all of the spin packets is the net

magnetization. In order to describe pulsed NMR is necessary from here on to talk in

terms of the net magnetization, the external magnetic field and the net magnetization

vector at equilibrium are both along the Z axis

Spin Packets

For every unit volume of tissue, there is a number of cells, these cells contain water molecules, each water molecule contain one oxygen and two hydrogen atoms., Each hydrogen atom contains one proton in its nucleus. Different tissues thus produce different images based on the amount of their hydrogen atoms producing a signal

T1 Processes

At equilibrium, the net magnetization vector lies along the direction of the applied

magnetic field Bo and is called the equilibrium magnetization Mo. In this

configuration, the Z component of magnetization MZ equals Mo. MZ is referred to

as the longitudinal magnetization. There is no transverse (MX or MY)

magnetization here. It is possible to change the net magnetization by exposing the

nuclear spin system to energy of a frequency equal to the energy difference

between the spin states. If enough energy is put into the system, it is possible to

saturate the spin system and make MZ=0. The time constant which describes how

MZ returns to its equilibrium value is called the spin lattice relaxation time (T1). The

equation governing this behavior as a function of the time t after its displacement

is:

Mz = Mo ( 1 - e-t/T1 )

T1 is therefore defined as the time required to change the Z component of

magnetization by a factor of e.

Precession

If the net magnetization is placed in the XY plane it will rotate about the Z axis at a

frequency equal to the frequency of the photon which would cause a transition between

the two energy levels of the spin. This frequency is called the Larmor frequency.

T1 and T2 time constants:

Consider the net magnetisation vector of a group of spins. At thermal equilibrium (M = M0),

it is aligned with the external magnetic field, B0. Now consider the rotation of the net

magnetisation vector by 90° (π/2 radians). The net magnetisation will precess around

the external magnetic field direction (diagrams are drawn in the rotating frame of

precession). Eventually, the net magnetisation vector will return to its thermal

equilibrium position. The T1 and T2 time constants (measured in milliseconds) describe

how this happens; the T1 time constant describes the recovery of the Mz component of

the net magnetisation vector, and the T2 time constant describes the decay of the

Mxy component of the net magnetisation vector

(M does not simply "rotate" back to M0, because the T1 and T2 processes are

separate, and the T1 and T2 times for a tissue type are not the same.

The T2 time is related to the effect of nuclear spins on each other. This may

sound alot like the T1 process described above, but it is slightly different. The

spin-spin interation purely refers to the loss of phase 6as they interact with

each other via their own oscillating magnetic fields. (Phase coherence means

spins are all precessing together.) The slight changes in magnetic field which

a proton experiences causes its Larmor frequency to change. As a result, the

precession of spins moves out of phase and the overall net magnetisation is

reduced. This T2 "relaxation" can occur without loss of energy to the spin

system (spins going out of phase only), and it can occur withloss of energy to

the spin system at the same time (which is T1 relaxation, see above).

Larmor precession is the precession of the magnetic

moments of electrons move with magnetic moments .

The magnetic field exerts a torque on the magnetic moment,

Where is the torque, is the magnetic dipole moment, J is the angular

momentum vector, B is the external magnetic field, and is the gyromagnetic

ratio which gives the proportionality constant between the magnetic moment

and the angular momentum.

Remark :How do protons interact with a magnetic field?

Moving (spinning) charged particle generates its own little magnetic field

Such particles will tend to line up with external magnetic field lines (think of iron filings around a magnet)

Spinning particles with mass have angular momentum

Angular momentum resists attempts to change the spin orientation (think of a gyroscope)

The angular momentum vector precesses about the external field axis with an angular

frequency known as the Larmor frequency,

where is the Larmor frequency, m is mass, e is charge, and B is applied field. For a given

nucleus, the g-factor includes the effects of the spin of the nucleons as well as their orbital

angular momentum and the coupling between the two. Because the nucleus is so complicated,

g factors are very difficult to calculate, but they have been measured to high precision for most

nuclei.

is the gyromagnetic ratio. [the ratio of the magnetic moment of a spinning

charged particle to its angular momentum]

Many factors contribute to MR imaging

-Quantum properties of nuclear spins

-Radio frequency (RF) excitation properties

-Tissue relaxation properties

-Magnetic field strength and gradients

-Timing of gradients, RF pulses, and signal detection

1) Put subject in big magnetic field

2) Transmit radio waves into subject [2~10 ms]

3) Turn off radio wave transmitter

4) Receive radio waves re-transmitted by subject0

5) Convert measured RF data to image

Procedure of MRI

There is electric charge

on the surface of the proton,

thus creating a small current

loop and generating magnetic

moment .

The proton also has mass which

generates an angular

momentum when it is spinning.

What kinds of nuclei can be used for NMR?

Nucleus needs to have 2 properties:

Spin

charge

Nuclei are made of protons and neutrons

Both have spin ½

Protons have charge

Pairs of spins tend to cancel, so only atoms with an odd number of protons or neutrons have spin

Hydrogen atom is the only major species that is MR sensitive

Hydrogen is the most abundant atom in the body

The majority of hydrogen is in water (H2O)

Essentially all MRI is hydrogen (proton) imaging

MRI

X-Ray, CT

Electromagnetic Radiation Energy

X-RAYS

X-rays were discovered in 1895 by the German physicist Wilhelm Roentgen

X-rays are produced by bombarding a metal target (copper, tungsten, and molybdenum are

common) with energetic electrons having energies of 50 to 100 keV

X-rays extremely penetrating type of Radiation (electromagnetic)

The minimum continuous x-ray wavelength, λmin, is found to be independent of target

composition and depends only on the tube voltage, V.

all of the incident electron’s kinetic energy is converted to electromagnetic energy in the form of

a single x-rays photon. For this case we have

where V is the x-ray tube voltage

X-rays are produced by bombarding a metal with

energetic electrons having energies of

50 to 100 kV

MRI uses a combination of Magnetic and

Electromagnetic Fields

NMR measures the net magnetization of atomic nuclei in the presence of magnetic fields

Magnetization can be manipulated by changing the magnetic field environment (static, gradient, and RF fields)

Static magnetic fields don’t change (< 0.1 ppm / hr):

The main field is static and (nearly) homogeneous

RF (radio frequency) fields are electromagnetic fields that oscillate at radio frequencies (tens of millions of times per second)

Gradient magnetic fields change gradually over space and can change quickly over time (thousands of times per second)

Radio Frequency Fields

• RF electromagnetic fields are used to manipulate the

magnetization of specific types of atoms

• This is because some atomic nuclei are sensitive to magnetic

fields and their magnetic properties are tuned to particular RF

frequencies

• Externally applied RF waves can be transmitted into a subject

to perturb those nuclei

• Perturbed nuclei will generate RF signals at the same

frequency – these can be detected coming out of the subject

Radio waves are one form of electromagnetic radiation

Electromagnetic radiation has a dual nature:

In some cases, it behaves as waves

In other cases, it behaves as particles (photons)

For radio frequencies the wave model is generally more

appropriate

Electromagnetic waves can be generated by many means, but all

them involve the movement of electrical charges

Electromagnetic Spectrum

Electromagnetic Waves

Electromagnetic transmissions move in

space as Transverse waves

Waves are characterized by frequency and

wavelength:

v f

An electromagnetic wave propagating through space consists of electric and magnetic fields, perpendicular both to each other and to the direction of travel of the wave

The relationship between electric and magnetic field intensities is analogous to the relation between voltage and current in circuits

This relationship is expressed by:

H

EZ

Electric and Magnetic Fields

Power Density

Power density in space is the amount of power that flows through each square meter of a surface perpendicular to the direction of travel

Z

EPD

2

The simplest source of electromagnetic waves would be a point in space, with waves radiating equally in all directions. This is called an isotropic radiator

A wavefront that has a surface on which all the waves are the same phase would be a sphere

Plane and Spherical Waves

Radio waves propagate through free space in a straight line with a velocity of the

speed of light (300,000,000 m/s)

There is no loss of energy in free space, but there is attenuation due to the spreading

of the waves

Attenuation of Free Space An isotropic radiator would produce spherical waves . The power density of an isotropic radiator is simply be the total power divided by the surface area of the sphere, according to the square-law:

PD Pt

4r2

Reflection, Refraction, and Diffraction

These three properties are shared by light and radio waves

For both reflection and refraction, it is assumed that the surfaces involved are much larger than the

wavelength; if not, diffraction will occur

Reflection of waves from a smooth surface (specular reflection) results in the angle of reflection being equal to the angle of incidence

refraction :A transition from one medium to another results

in the bending of radio waves, just as it does with light

Snell’s Law governs the behavior of

electromagnetic waves being refracted:

n1sin1 n2 sin2

diffraction is more apparent when the object has sharp edges, that is when the dimensions are small in comparison to the wavelength

Ground-Wave Propagation

Most of the time, radio waves are not quite in free space

Terrestrial propagation modes include:

Line-of-sight propagation

Space-wave propagation

Ground waves

Sky waves

Long-range communication in the high-frequency band is possible because of refraction in a region of the upper atmosphere called the ionosphere

The ionosphere is divided into three regions known as the D, E, and F layers

Ionization is different at different heights above the earth and is affected by time of day and solar activity

Ionospheric Propagation

Line-of-Sight Propagation Signals in the VHF and higher range are not usually returned to

earth by the ionosphere

Most terrestrial communication at these frequencies uses direct radiation from the transmitter to

the receiver This type of propagation is referred to as space-wave, line-of-sight, or

tropospheric propagation

Line-of-Sight Propagation

Carriers and Sidebands

Radio transmission involves putting audio frequency information on a much higher frequency electromagnetic wave called a carrier wave. The process of superimposing the "electrical image" of the sound information on the carrier wave is called modulation, and there are two commonly used schemes: amplitude modulation (AM) and frequency modulation (FM). Either form of modulation produces frequencies which are the sum and the difference of the carrier and modulation frequencies - these frequencies are sometimes called sidebands.

Because of the existence of the sidebands, the frequency range

or bandwidth necessary for radio transmission depends on the range of

modulating frequencies.

The Carrier In an AM-modulated radio signal, a base signal, called the carrier, is continuously broadcast. The two modulating signals are called the sidebands. Any audio that you hear on an AM broadcast station is from the two sidebands. When the radio station is not transmitting any sound, you can still hear that a signal is present; that is the carrier. These two modulating (audio) sidebands are located on either side of the carrier signal--one just above the other just below. As a result, the sideband located just above the carrier frequency is called the upper sideband and that which is located just below the carrier frequency is called the lower sideband.

The carrier and sideband

The Sidebands and SSB a sideband is a band of frequencies higher than or lower than the carrier frequency,

containing power as a result of the modulation process. it was much more common in the bands to transmit using one of the sidebands,

which is known as single sideband (SSB). Single sideband transmissions can consist

of either the lower sideband (LSB) or the upper sideband (USB). If you listen to an

SSB signal on an AM modulation receiver, the voices are altered and sound a lot like

cartoon ducks. As a result, you must have a special SSB receiver to listen to these

transmissions

Audible Sound

Usually "sound" is used to mean sound which can be perceived by the human ear,

i.e., "sound" refers to audible sound unless otherwise classified. A reasonably

standard definition of audible sound is that it is a pressure wave with frequency

between 20 Hz and 20,000 Hz and with an intensity above the standard threshold

of hearing. Since the ear is surrounded by air, or perhaps under water, the sound

waves are constrained to be longitudinal waves. Normal ranges of sound

pressureand sound intensity may also be specified.

Frequency:20 Hz - 20,000 Hz(corresponds with pitch)Intensity:10-12 - 10 watts/m2(0 to

130 decibels)Pressure:2 x 10-5 - 60 Newtons/m22 x 10-10 - .0006 atmospheresFor

an air temperature of 20°C where the sound speed is 344 m/s, the audible sound

waves have wavelengths from 0.0172 m (0.68 inches) to 17.2 meters (56.4 feet).

Frequency: 20 Hz - 20,000 Hz (corresponds

with pitch)

Intensity: 10-12 - 10 watts/m2 (0 to 130 decibels)

Pressure: 2 x 10-5 - 60

Newtons/m2

2 x 10-10 -

.0006 atmospheres

AM Radio

When information from an AM radio station, the electrical image of the sound (taken from

a microphone or other program source) is used to modulate the amplitude of the

carrier wave transmitted from the broadcast antenna of the radio station. This is in

contrast to FM radio where the signal is used to modulate the frequency of the

carrier. The AM band of the Electromagnetic spectrum is between 535 KHz and 1605

kHz

FM Radio

The FM band of the electromagnetic spectrum is between 88 MHz and 108 MHz and

the carrier waves for individual stations are separated by 200 kHz for a maximum

of 100 stations. This separation of the stations is much wider than that for AM

stations, allowing the broadcast of a wider frequency band for higher music

broadcast. It also permits the use of sub-carriers which make possible the

broadcast of FM Stereo signals. FM radio uses the electrical

image of a sound source to modulate the frequency of a carrier wave.

Broadcast station

Broadcasting is the distribution of audio and video content to a

dispersed audience via any audio or visual mass communications medium, but

usually one using electromagnetic radiation (radio waves. The receiving parties

may include the general public or a relatively large subset thereof.

Broadcasting has been used for purposes of private recreation

Bandwidth for Communication

When you quote a frequency for a radio station,

you generally quote the frequency of the carrier .

But when you superimpose a signal on the carrier

by AM or FM , you produce sidebands at the

sum and difference of the carrier frequency fC

and modulation frequency fM. This means

that the transmitted signal is spread out in

frequency over a bandwidth which is twice the highest frequency in the signal.

Bandwidths are assigned for all types of broadcast communication and this imposes a

maximum signal frequency which may be transmitted.

Radio Frequency Bands

Because of the division of the FM band for the transmission of FM stereo, the

frequency limit for music transmission is at 15 kHz. This allows high fidelity

signal transmission. The operational bandwidth is limited to 150 kHz, with 25

kHz on each side of that for gaurd bands. Actually FM stereo covers 106 kHz

of that

AM radio is limited to 5000 Hz

maximum frequency by the width of

the AM bands.

Sky Waves

You could also receive sky waves. Sky waves travel toward the sky, rather

than hang out on the ground. You would not be able to hear the sky-wave

signals, except for the ionosphere. The ionosphere is many miles above the

earth, where the air is "thin"--containing few molecules. Here, the ionosphere

is bombarded by x-rays, ultraviolet rays, and other forms of high-frequency

radiation. The energy from the sun ionizes this layer by stripping electrons

from the atoms.

When a sky-wave signal reaches the ionosphere, it will either pass through it or

the layer will refract the signal, bending it back to earth. The signal can be

heard in that area where the signal reaches the earth, but depending on a

number of variables, there might be an area where no signal from that

particular transmitter is audible between the ground wave and where the sky

wave landed. This area is the skip zone. After the sky-wave signal release

from the earth, it will return toward the sky again.

In radio communication , skywave or skip refers to the propagation of radio

waves reflected or refracted back toward Earth from the ionosphere , an electrically

charged layer of the upper atmosphere . Since it is not limited by the curvature of

the Earth, skywave propagation can be used to communication .

Skipping Around the World

Again, the signal will be refracted by the ionosphere and return to the earth. If

the HF signals all bent and bounced off the ionosphere with no loss in signal

strength, HF stations around the world would be heard across the earth with

perfect signals (something like if a "super ball" was sent bouncing in a

frictionless room). Whenever radio signals are refracted by the ionosphere or

bounce from the earth, some of the energy is changed into heat, causing

absorption of the signal. As a result, the signal at the first skip is stronger than

the signal at the second skip, and so on. After several skips, typical HF

signals will dissipate.

The skip and ground waves can be remarkably close together. It is not unusual

for one station to receive a booming signal while a nearby station cannot hear

a trace of the sending station even though using a better receiver with a better

antenna. The first station was receiving either the ground wave or the first

skip and the other station was located somewhere between these two

Sky Wave Propagation

– Signal reflected from ionized layer of upper atmosphere back down to earth, which can

travel a number of hops, back and forth between ionosphere and earth’s surface.

– HF band with intermediate frequency range: 3MHz ~ 30MHz.

– e.g: International broadcast.