Physics MRI

Overview of MRI

• 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.

Learning Outcomes

• 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.

Basics of MR Signals

• 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 and Magnetic Fields

• 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 and MRI

• 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.

Precession of Protons

• 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).

Proton Spin and Signal Generation

• 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 and Radiofrequency Excitation

• 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.

Relaxation Mechanisms

• 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).

Conclusion

• 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.