MRI - physics & instrumentations

Proton

A subatomic particle having a positive charge indential in magnitude to the negative charge of an electron and together with the neutron, a compenent of all atomic nuclei.

Vector

A symbol to describe the direction and strength of a magnetic field.

Dipole

a magnetic field characterized by its own magnetic north and south poles separated by a finite distance.

The spinning motion gives the H proton

a magnetic moment.

Neutron

a subatomic particle that is neutral there for has no charge. It is located in the nucleus and serves as a stabilizer.

Electrons

a subatomic particle that, along with protons and neutrons, make up atoms.

Flux

flow of energy; magnetic field flowing from north to south poles represented by magnetic lines of force.

Atoms

fundamental submicroscopic unit of all mass.

Electrons have

half-integral spin

Due to its spin the H nuclei's proton

has the same properties as a bar (dipole) magnet.

Field or flux lines

measure the flow and area of a magnetic field.

Unpaired orital electrons

spin in the same direction and have the tendency to line up with the magnetic field, therefore producing additional magnetic fields.

Paramagnetic

the property of substances with positive magnetic susceptibility that attract and repel magnetic lines of force.

Diamagnetic

the property of the most substances with pared orbital electrons that have opposite spinning, and their magnetic moments cancel each other out.

When a magnetic field is applied to a rotating body,

the resulting change results in precession.

Amplitude

the signal height. The greater the amplitude, the larger the number of protons and the brighter the image.

Four categories are

1. Diamagnetic 2. Paramagnetic 3. Super-paramagnetic 4. Ferromagnetic

Fleming's right hand rule

If the electron flow is in the direction of the curled fingers, the thumb will point in the direction of its magnetic north pole.

Fleming's righ hand, thumb up rule

Shows north and south poles of magnet. If the electron flow is in the counter clockwise direction, the thumb will point in the direction of its magnetic north pole. If the electron flow is in the clockwise direction, the thumb will point in the direction of its magnetic south pole.

Homogeneity

Term used to describe magnetic uniformity in the large static magnetic field.

Force

The ability to create work or change.

Ferromagnetic

The property of substances with large positive magnetic susceptibility that become Ferromagnetic substances include a number of iron and cobalt-containing matals.

20-30 ppm

The specification that is acceptable for field homogeneity of an MRI magnet for clinical scanning.

Fringe Fields

Weaker magnetic fields that are away from isocenter, in MRI outside of the scanning area.

Antenna

a device enabling the sending and or receiving of electromagnetic waves.

The proton has

an intrinisic angular momentum or spin and thus a magnetic moment.

Isocenter

area of the magnetic field where the field strength is the greatest, in MRI the center of the scan area.

These substances: gadolinium, methemoglobin, free radicals

contain one or more unpaired orbital electrons and produce marked reductions in the T1 and T2 relaxation times.

Frequency is measured in

cycles per second or hertz or megahertz.

Magnetic susceptibility

defined as the tendency of a substance to attract or repel magnetic lines of force dependent on the magnetic properties of its electrons.

The number of protons in the nucleus of an atom

determines what element it is.

Super paramagnetic materials

do not exhibit residual magnetism when the external field is removed.

Radiofrequency or RF

electromagnetic radiation lower in energy than infrared.

These substances: hemosiderin and superparmagnetic iron oxide - SPIO

have a single magnetic domain and are magnetized in a magnetic field. Their magnetic susceptibilities are much larger than those of paramagnetic substances and similar to those of ferromagnetic materials.

Faraday's Law

in a loop of conductor where an electric current is introduced and maintained, a magnetic field will be produced in the center. This is also true inversely.

The R F used in MRI is

in the form of a burst of RF energy (pulse) in the 10 MHz to 100 MHz range RF pulse, delivered to the patient by RF transmitter.

Electromagnetic energy EME

is an electric and magnetic disturbance that travels through space at the speed of light, symbolized by a sine wave that has 2 components of measurement, wavelength and frequency.

Magnetism

is described as the ability of a substance to attract iron, cobalt and nickel. It is the power of force created by a substance having the property of creating or inducing magnetic dipoles.

Lightest of all atoms

is hydrogen

The mass of a neutron

is slightly greater than the proton mass and has an enormous effect on the quantum spin value of the element.

The charge of the electron

is the basic unit of electricity.

Spin

is the property of subatomic particles that indicates the particle's angular momentum.

The strongest area of a magnetic field

is where the field or flux lines converge and is called the isocenter.

The net charge on the neutron

is zero

The atomic number

of an element denotes the number of protons in the nucleus.

The magnetic field

of any magnet has a direction, signified north and south. This property of magnet makes it a dipole.

Magnetic field a vector quantity consisting

of both a north and south pole; it exerts an induction force on ferromagnetic and paramagnetic substances.

Magnetic susceptibility

of most human body tissues are described as diamagnetic.

Spin

property of certain substances with an odd numbers of nuclei that result in an intrinsic angular momentum and magnetic moment. Each spin exits in a distinct energy state and has an identifiable spin quantum number.

The RF frequency, at the Larmor frequency,

results in rotation of the net magnetization vector and phase coherence of the nuclear spins.

Electromagnetic Wave or EM

simultaneous periodic variations of electric and megnetic properties at 90 degrees to each other.

When symbolizing the flow and area of a magnetic field

special lines are used; they are field or flux lines.

Sine Wave:

symbol used to illustrate wave length and frequency of electromagnetic energy.

When describing the strength of an MRI main magnetic field,

the S I unit most commonly used is tesla.

Angular momentum

the angle formed between a precessing object (proton) and its imaginary axis. The vector quantity given by the product of the momentum of a proton and its position.

In the absence of external forces,

the angular momentum remains constant, with the result that any rotating body tends to maintain the same axis of rotation.

Wavelength

the distance between 2 consecutive peaks and is measured in mm, cm and m.

Away from the isocenter

the field or flux lines represent the fringe fields.

Magnetic moment

the measure of the magnetic properties of an object or proton that causes them to align with the static (main) magnetic field, it is the magnetic field of a proton without any RF pulse applied.

Frequency

the number of repetitions of rhythmic periodic processes in a unit of time.

Precession

the phenomenon of a magnetic field spinning or gyrating around the axis of its own creation.

Wave

the propagation of a wavelength of energy that has the dimension of period, frequency and amplitude.

Super-paramagnetic

the property of substances with large positive magnetic suceptibility which strongly attract or repel magnetic lines of force.

An RF coil is used for

transmitting and receiving MRI signals (RF).

Substances with diamagnetic susceptibility

weakly attract or repel magnetic lines of force.