Purpose

The purpose of this discussion is to give you a basic understanding of the underlying physics of MRI signal generation and acquisition.

Objectives

• Be able to explain how a signal is generated during an MRI study.

Signal Generation

Nuclear Spin

• Spin - the magnetic moment and angular momentum of a proton caused by electrical current on a proton.
• Lamour frequency - the rate at which charge "rotates" across a proton.
  • Atom specific
• Precession - the gyroscopic motion of a spinning object.

Spin in an external magnetic field

• The axis of precession will align with any external magnetic field.
• Parallel state – a low energy state. The axis is in alignment with the external magnetic field.
• Antiparallel state – a high energy state. The axis is in an opposing alignment (not perpendicular) with the external magnetic field.

Magnetizing a spin system

• MRI does not measure individual protons (otherwise I could do far more with it!).
• MRI measures the net magnetization (M) of matter in a defined space.
• M is a vector with two components:
  • longitudinal – parallel and antiparallel to M.
  • transverse – perpendicular to M.
• Transverse component cancels out.
• Longitudinal can be measured as the proportion of all parallel and antiparallel spins.

Excitation 

• An electromagnetic pulse is applied to matter in an external magnetic field.
• Protons absorb that energy.
• The ratio of parallel to antiparallel protons changes.
  • A 90° pulse results in a 1 to 1 ratio.
  • A 180° pulse results in a ratio favoring the antiparallel state.
• When the pulse stops, protons release electromagnetic energy (called relaxation) and return to their previous orientation.

Relaxation

• Relaxation – protons return to their original orientation after the excitation pulse has ended.
  • Transverse relaxation – the loss of magnetization perpendicular to the magnetic field.
    • T₂ decay – a time constant describing transverse relaxation related to spin-spin differences.
    • T₂* decay – a time constant describing transverse relaxation resulting from spin-spin differences and local inhomogeneities in the magnetic field.
  • Longitudinal relaxation – the return of net magnetization parallel to the magnetic field.
    • T₁ – a time constant describing the the recovery of the longitudinal component of net magnetization.
• These time constants are used to determine when an MR signal is recorded and affect image contrast.