Magnetism in MRI

The first documented human MR image was obtained on a

15 MHz RF system (0.3T)

MHz to Tesla

MHz/42.57 = T

precessional frequency

the resonant frequency, and is equal to the product of the magnetic field and the gyromagnetic ratio

Larmor equation for precessional frequency

ω0=λ* B0

42.57 MHz per Tesla

The Larmor frequency for Hydrogen (H) at 0.5 Tesla

21.28 Mhz

The resonance frequency in a 1.5 Tesla magnet

63.86 Mhz

The resonance frequency in a 3 Tesla magnet

127.71 Mhz

1 Tesla =

1 Newton per ampere-meter

1 Coulomb=

1 Coulomb = 6.24 X 10^18 electrons

1 amp =

1 Coulomb/second

the principal nucleus utilized in MR imaging and why

hydrogen because of its abundance in the human body, a primary element in both water and fat, two tissue contrasts in MRI.

difference in chemical shift

3.5 parts-per-million (ppm)

frequency difference between fat and water at 1 Tesla, 1.5T, and 3T

147 Hz, 220 Hz, 440 Hz

Magnetic field inhomogeneity is expressed in

parts per million (ppm)

10,000 Gauss =

1 tesla

Faraday’s law of induction states

that a changing magnetic field will induce a voltage

most commonly used magnet systems and why

Superconducting magnet systems, due to the high field strength and imaging capabilities

The main magnetic field is parallel to

the long axis of B0

quickest magnets to shut off

Resistive magnets

permanent magnets

blocks of ferromagnetic plates used to generate a magnetic field, and are typically heavy

Ferromagnetic

iron and iron-like substances that can generate a relatively strong magnetic field, Ferromagnetic substances are attracted to the magnetic field

examples of ferromagnetic substances

Stainless Steel, iron, nickel, etc.

dangers of ferromagnetic substances

Ferromagnetic substances are attracted to the magnetic field, can have a missile/torpedo like effect, placing patients, visitors and employees in potential danger

paramagentic

have a very weak magnetic field

examples of paramagnetic

Gadolinium is used as a contrast material because it is paramagnetic.

diamagnetic

Have no magnetic field

examples of diamagnetic

wood, plastic, titanium and copper

The force that an object can be attracted to the magnetic field is reliant upon

The specific ferromagnetic nature/properties of the material
The mass of the projectile
The field strength (Tesla) of the MR system

Magnetic susceptibility

the extent to which a material or tissue becomes magnetized in an external magnetic field.

what happens when iron is placed in an electromagnet

it becomes magnetized. the influence of the electromagnet causes iron to become a strong permanent magnet.

Magnetic permeability

how well a material attracts the imaginary lines of the magnetic field

concept of how magnetic shielding works in MR rooms

iron is permeable and attracts the imaginary lines of the magnetic field. Therefore, the iron shielding does not allow the magnetic field to extend out into the adjacent corridor where patients may be affected.

Magnetic field gradients are

static

net longitudinal magnetization in thermodynamical equilibrium is due to

a tiny excess of protons in the lower energy state. This gives a net polarization parallel to the external field.

protons that align themselves with the direction of the static magnetic field (B0) exist in a _______ energy state than those anti-parallel to the magnetic field.

lower

Hydrogen protons in the anti-parallel state are referred to as

spin down and/or high energy spins

Longitudinal magnetization (or Net Magnetization Vector)

defined as the excess number of hydrogen protons aligned with the static magnetic field

A magnetic vector possesses

attributes of both magnitude (strength) and direction.

The net magnetization vector (NMV), when aligned with

the magnetic field direction, is aligned along the longitudinal axis

A Free Induction Decay (FID) is produced from

a RF pulse.

Free Induction Decay

RF Excitation is

the process of moving the net magnetization away from the Z axis

process of RF excitation

The RF pulse stimulates the protons into parallel or anti-parallel alignment

RF stimulates the protons to be in phase

Relaxation begins after the RF is turned off

what happens Immediately following the application of the 90° RF pulse

the precessing protons begin to precess in phase and tip into the transverse plane

Dephasing

occurs after initial RF application, causing phase differences to appear between precessing spins, resulting in decay of transverse magnetization.

transverse magnetization immediately following 90 degree RF pulse

the transverse magnetization is Non-zero.

MRI obtains signal information via

the energy emitted from the patient's tissues following RF excitation pulses

In one T1 relaxation time ______ of the longitudinal magnetization has recovered.

63%

T1 Recovery

T1 relaxation occurs when

nuclei give up their energy to the surrounding tissue, which is known as spin lattice relaxation

spin-lattice relaxation

T1 Relaxation

T1 is synonymous with

the nuclei’ regrowth along longitudinal magnetization, and is also referred to as Z axis regrowth.

Z axis regrowth

the nuclei’ regrowth along longitudinal magnetization (T1)

T1 relaxation time

the recovery of 63% longitudinal magnetization of a specific tissue’s original magnetization

Spin-lattice relaxation

The return of the longitudinal magnetization to its equilibrium value along the Z axis (T1 recovery)

T2 Decay

T2 relaxation time

the time it takes for 63% of the transverse magnetization to decay, or the time it takes the spins to de-phase to 37% of their original value

Spin-spin relaxation

The return of the transverse magnetization to its equilibrium value, zero (T2 decay)

The MRI system component that produces the magnetization of proton spins (alignment)

main magnet

The MRI system component that provides the ability to perform spatial encoding

gradient encoding