MRI Basic Principles

MRI Basic Principles

• MRI uses the principles of magnetism and radio waves to produce images of the body's internal structures.
• The presentation covers precessional phase, resonance, MR signal, free induction decay (FID) signal, and pulse timing parameters.

Precessional Phase

• In phase means hydrogen magnetic moments are aligned at the same place on the precessional path at a given instant.
• Out of phase means hydrogen magnetic moments are aligned at different positions on the precessional path at a given instant.

Resonance

• Resonance occurs when an object is exposed to an oscillating perturbation with a frequency close to its natural frequency.
• When a nucleus is exposed to an external force with an oscillation similar to its Larmor frequency, the nucleus gains energy.
• The result of resonance is that the net magnetic vector moves out of alignment from B0, and the magnetic moments of H nuclei move into phase with each other.

MR Signal

• Faraday's law (motion + electricity = magnet) explains how the movement of the magnetic vector produces a voltage.
• Recovery refers to the net magnetic vector returning to its original position at 90° to B0.
• Dephasing describes the loss of coherence of signals due to uneven magnetic fields or interactions between hydrogen protons in tissue.

Free Induction Decay (FID) Signal

• When the RF excitation pulse is turned off, the net magnetic vector (NMV) tries to realign with the main magnetic field (B0).
• Hydrogen nuclei lose energy during relaxation.
• Nuclei return to their stable low-energy levels, aligning their magnetic moments in the spin-up direction.
• Dephasing occurs independently due to inhomogeneities in the main magnetic field (B0) and interactions between the hydrogen protons which leads to a decrease in the transverse magnetization and thereby the voltage in the receiver coil.

Pulse Timing Parameters

• Repetition Time (TR): Time between the application of consecutive RF pulses for the same slice.
• Echo Time (TE): Time between RF pulse and the peak signal in the receiver coil.
• TR affects the amount of longitudinal relaxation; T1 relaxation.
• TE affects the amount of transverse relaxation; T2 relaxation.

Image Contrast

• Image contrast is determined by extrinsic contrast parameters (controlled by the operator) and intrinsic contrast mechanisms (not controlled by the operator).

Extrinsic Contrast Parameters

• Repetition Time (TR): Time between successive RF pulses.
• Echo Time (TE): Time between the excitation pulse and the collection of the signal in coil.
• Flip Angle: Angle through which net magnetization is tilted by the RF pulse.
• Turbo Factor/Echo Train Length (ETL): Used in fast imaging sequences.
• Time of Inversion (TI): Time between the inversion pulse and the excitation pulse. B-value: A factor that reflects the strength and timing of the gradients used to generate diffusion-weighted images.

Intrinsic Contrast Mechanisms

• T1 Recovery
• T2 Decay
• Proton Density
• Flow
• Apparent Diffusion Coefficient (ADC): A measure of the diffusion of water molecules in tissue.

MRI Terms

• Dephasing: Loss of phase coherence.
• Diffusion: Movement of molecules due to random thermal motion.
• Dipole: Magnetic field with north and south poles.
• Display Matrix: Total number of pixels in the image.
• Electromagnet: Magnet utilizing coils of wire.
• Equilibrium: Balance between opposing forces.
• Echo Spacing: Time between echoes in fast imaging sequences.
• Excitation: Applying RF pulses to alter the energy states of nuclei.

This quiz explores the foundational principles of MRI, including magnetism and radio waves. Key concepts covered include precessional phase, resonance, MR signals, and pulse timing parameters. Test your understanding of how these elements contribute to imaging the body's internal structures.