MRI (Magnetic Resonance Imaging) is supported by numerous Nobel Prizes (6 to 12) across physics, chemistry, and medicine,
Significant discoveries leading to the development of MR scanners.
Invention of MRI awarded the Nobel Prize in Physiology or Medicine shared by 2 scientists.
The comprehensive list of learning outcomes for the MRI lectures serves as a guide.
Understanding each concept will help in crafting an articulate description of MRI.
Key Topics:
Origin of MR signals in the patient's body.
Relationships between electromagnetism and MRI.
Properties of Electromagnetic Waves:
Combination of electric and magnetic fields results in electromagnetic waves:
Radio frequency, crucial for MRI, is a lower energy part of the spectrum.
Ampere's Law:
Current through a wire induces a magnetic field.
Protons generate a magnetic field due to their charge and spin.
Key consequence: Protons can create an artificially strong magnetic field via electromagnets.
Unit of Magnetic Field: Tesla (T), with Earth's magnetic field strength estimated between 0.07 to 0.10 T.
NMR (Nuclear Magnetic Resonance):
Various applications, including spectroscopy, a Nobel Prize-winning discovery in chemistry.
Difference between NMR and MRI:
NMR focuses on signal strength post-radio frequency pulse.
MRI examines how quickly these signals decay based on tissue properties.
Protons behave as tiny magnets, aligning with external magnetic fields.
Spin Orientation:
Protons can either be aligned with the magnetic field (spin up) or against it (spin down).
Spin up is a lower energy state; spin down is higher.
Precession:
Protons wobble around the magnetic field direction due to torque.
Larmor Frequency: Rate of precession is proportional to the strength of the magnetic field, determined by the gyromagnetic ratio (42.58 MHz/T).
Hydrogen atoms (protons) in the body provide signals for MRI.
In a magnetic field, more protons will be in the lower energy configuration (spin up) than higher.
For every million protons: 7 are typically in the spin up state.
Longitudinal Magnetization (BL): Projection of proton spins along the z-axis in the direction of the magnetic field.
When RF pulses are applied:
Longitudinal magnetization decreases to zero during a 90-degree RF pulse, flipping into transverse magnetization (BT).
Precession of spins aligns in phase after excitation, creating a measurable signal.
When RF pulse is switched off:
Longitudinal Magnetization (BL) starts to recover, while Transverse Magnetization (BT) decays.
T1 Relaxation: Recovery of longitudinal magnetization, involves spin-lattice relaxation (exchange of thermal energy with the surroundings).
Takes approximately 0.3 to 2 seconds, depending on tissue.
T2 Relaxation: Decay of transverse magnetization, involves spin-spin relaxation (interaction between protons), measured in milliseconds (30 to 150 ms).
This lecture sets the foundation for understanding MRI's principles, physics, and its applications in medical imaging. Further exploration of T1 and T2 relaxation processes will continue in subsequent lectures.