MRI Basic Principles

MRI Basic Principles

• MRI uses magnetic fields and radio waves to create detailed images of the body
• The presentation outlines key phases and parameters in MRI
• The presenter is Hayder Jasim Taher, PhD of Medical Imaging

Outline of Presentation

• Precessional phase
• Resonance
• MR signal
• Free induction decay (FID) signal
• Pulse timing parameters

Precessional Phase

• In-phase (coherent): Hydrogen magnetic moments are aligned in the same direction at a specific moment in time.
• Out-of-phase (incoherent): Hydrogen magnetic moments are not aligned in the same direction at a specific moment in time. This misalignment/incoherence leads to a loss of signal.

Resonance

• Resonance occurs when an object is exposed to an oscillating perturbation with a frequency similar to its natural frequency
• When a nucleus is exposed to an oscillating external force/field similar to its Larmor frequency, the nucleus absorbs energy
• This absorption leads to a transition to a higher energy state

Result of Resonance

• The net magnetization vector (NMV) moves out of alignment in response to a given RF pulse
• Magnetic moments of Hydrogen nuclei move to the transverse plane

MRI Signal

• Faraday's Law: Motion + electricity = magnetism.
• Recovery and Dephasing are key aspects of signal formation. Imaging depends on differences in relaxation times.

Free Induction Decay (FID) Signal

• When the RF pulse is turned off, the net magnetization vector (NMV) tries to realign with the static magnetic field (B0)
• Hydrogen nuclei lose energy, which is called relaxation
• Some high-energy nuclei return to low-energy state, aligning with B0
• Magnetic moments of hydrogen nuclei lose coherence (dephase) concurrently
• The induced voltage is called the FID (free induction decay) signal
• The decaying voltage signal reflects transverse magnetization decrease

Pulse Timing Parameters

• Repetition Time (TR): The time interval between successive RF excitation pulses. Determines the amount of longitudinal relaxation. Measured in milliseconds (ms).
• Echo Time (TE): The time between the RF excitation pulse and the signal peak. Determines the amount of transverse relaxation. Measured in milliseconds (ms).

Image Contrast

• The image contrast is controlled by two groups of parameters:

• Extrinsic contrast parameters: These are controlled by the operator

  • Repetition Time (TR)
  • Echo Time (TE)
  • Flip Angle
  • Turbo Factor/Echo Train Length (ETL/TF)
  • Time from Inversion (TI)
  • b-value

• Intrinsic contrast mechanisms: These are not controlled by the operator

  • T1 recovery
  • T2 decay
  • Proton density
  • Flow
  • Apparent diffusion coefficient (ADC)

MRI Terms

• Dephasing: The loss of phase coherence of signals in the transverse plane
• Diffusion: Movement of molecules due to random thermal motion
• Dipole: A magnetic field with a north and south pole
• Display matrix: The total number of pixels in the image, determined by the product of phase and frequency axis
• Electromagnet: A magnet using coils of wire
• Equilibrium: Balance between opposing forces
• Excitation: Energy is transferred to nuclei to make them spin
• Echo spacing: Spacing between echoes in fast spin echo sequences
• Echo train: Series of 180-degree pulses in fast spin echo sequences
• Echo train length (ETL): Number of 180-degree pulses in fast spin echo
• Effective TE: Time between the echo and the RF pulse initiating the echo
• Electrons orbit: Electrons arranged around the nucleus
• External magnetic field (EMF): A field responsible for driving a current in a circuit

This quiz covers the foundational concepts of MRI, including key phases like precessional phase, resonance, and pulse timing parameters. It is designed to enhance understanding of how magnetic fields and radio waves are used to create images of the body. Perfect for students and professionals in medical imaging.