Magnetism in MRI

The first documented human MR image utilized a 15 MHz RF system.
This system functioned as a 0.3 Tesla system.
- Calculation: $15 ext{ MHz} ÷ 42.57 = 0.352 ext{Tesla}$
- Precessional frequency is defined as the resonant frequency, represented mathematically as the product of the magnetic field and the gyromagnetic ratio.

Unit Relationships:
- 1 Newton per ampere-meter = 1 Tesla [T]
- 1 Coulomb = 6.24 x 10^18 elementary charges (e-)
- 1 Coulomb/second = 1 Ampere

Hydrogen is the principal nucleus in MR imaging, primarily due to its abundance in the body and presence in water and fat.
Chemical Shift:
- The difference in chemical shift is approximately 3.5 parts per million (ppm).
- At 1 Tesla, this corresponds to a frequency difference of about 147 Hz between fat and water.
- For 1.5 Tesla: 220 Hz difference
- For 3 Tesla: 440 Hz difference

Expressed in parts per million (ppm).
Conversion standards:
- 10,000 Gauss = 1 Tesla
- A system operating at 30,000 Gauss equates to a field strength of 3 Tesla.

Superconducting Magnet Systems:
- Most common due to high field strength and superior imaging capabilities.
- The main magnetic field runs parallel to the long axis of B0.
Resistive Magnets:
- Can be quickly deactivated in emergencies.
Permanent Magnets:
- Composed of blocks of ferromagnetic plates; relatively heavy to generate magnetic fields.

The force of attraction to a magnetic field depends on:
- The specific properties of the ferromagnetic material.
- The mass of the ferromagnetic object.
- The strength of the magnetic field (measured in Tesla).
Non-Ferrous Substances:
- Examples include wood, plastic, titanium, copper.

Magnetic Susceptibility:
- Defines how much a material becomes magnetized in an external magnetic field.
Magnetic Permeability:
- Refers to how efficiently a material attracts imaginary lines of the magnetic field.

Iron may be used in walls around MR imaging rooms to shield against external magnetic field effects.
The use of iron, known for its permeability, prevents the magnetic field from extending into areas where individuals with metal implants or pacemakers could be affected.

The excess of protons in a lower energy state results in net longitudinal magnetization due to thermodynamical equilibrium.
Protons align parallel to the external magnetic field, reducing their energy level.
Terms:
- Protons aligned parallel = lower energy state.
- Protons aligned anti-parallel are referred to as "spin down" (high energy spins).
Definition:
- Longitudinal Magnetization (or Net Magnetization Vector) is the excess number of hydrogen protons aligned with the static magnetic field.
Attribute of Magnetic Vector:
- Possesses magnitude (strength) and direction.
- The net magnetization vector (NMV) aligned with the magnetic field direction lies along the longitudinal axis.

Produced from an RF pulse that excites proton magnetization.
RF Excitation Process:
- Moves net magnetization from the Z axis into a transverse plane.
- Stimulates protons into parallel or anti-parallel alignment and causes them to be in phase. - Relaxation begins upon RF deactivation.

Dephasing:
- Occurs right after RF pulse application, leading to phase differences among precessing spins and decay of transverse magnetization.
Transverse Magnetization:
- Remains non-zero immediately after the 90° RF pulse application.
Signal Acquisition in MRI:
- Signal data is derived from energy emitted by patient tissues post-RF excitation pulses.

Defined as the time it takes for 63% of longitudinal magnetization to recover.
Recovery process related to nuclei transferring energy to surrounding tissues, referred to as spin lattice relaxation.
Also known as Z axis regrowth (recovery along the longitudinal magnetization).

Defined as the time required for 63% of transverse magnetization to decay, equating to spins de-phasing to 37% of their initial value.
Spin-Spin Relaxation:
- Refers to the return of transverse magnetization to equilibrium (zero).