MRI Basic Principles
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Questions and Answers

What does the apparent diffusion coefficient (ADC) measure?

  • The magnitude of diffusion (correct)
  • The strength of the magnetic field
  • The number of electrons orbiting the nucleus
  • The spacing between echoes in FSE

    • What is dephasing in MRI?

  • The enhancement of signal coherence
  • The loss of phase coherence of signals (correct)
  • The reduction of signal strength
  • The stabilization of resonance frequency

    • What role does an electromagnet play in MRI?

  • It generates static magnetic fields for imaging (correct)
  • It allows for phase coherence in signals
  • It enhances thermal motion of molecules
  • It produces resonance frequencies in nuclei

    • Which option best defines effective TE in MRI?

    <p>Time between the RF pulse and the first echo</p> Signup and view all the answers

    What does the echo train length (ETL) refer to?

    <p>The number of 180° RF pulses in a sequence</p> Signup and view all the answers

    What is the term for the induction of decaying voltage in MRI?

    <p>Free Induction Decay (FID)</p> Signup and view all the answers

    How is the Repetition Time (TR) defined in MRI?

    <p>Time from one RF pulse application to the next</p> Signup and view all the answers

    What does the Echo Time (TE) measure in MRI?

    <p>Time from RF pulse to signal collection peak</p> Signup and view all the answers

    Which of the following is NOT an extrinsic contrast parameter?

    <p>T1 recovery</p> Signup and view all the answers

    What impact does the TR have on MRI imaging?

    <p>It influences the duration of longitudinal relaxation.</p> Signup and view all the answers

    Which intrinsic contrast mechanism relates to the decay of magnetization?

    <p>T2 Decay</p> Signup and view all the answers

    Which parameter reflects the strength and timing of gradients in diffusion-weighted imaging?

    <p>'b' value</p> Signup and view all the answers

    The time from the application of one RF pulse to the next RF pulse is specifically known as what?

    <p>Repetition Time (TR)</p> Signup and view all the answers

    What occurs during resonance in magnetic moments of hydrogen nuclei?

    <p>They gain energy from an external oscillating force.</p> Signup and view all the answers

    What is the primary effect observed when the RF excitation pulse is turned off?

    <p>The NMV realigns with B0.</p> Signup and view all the answers

    What is meant by the precessional phase of hydrogen nuclei?

    <p>Magnetic moments are at different positions on the precessional path.</p> Signup and view all the answers

    Which statement best describes the process of relaxation in MRI?

    <p>Hydrogen nuclei emit energy and align with low-energy states.</p> Signup and view all the answers

    What primarily governs the natural frequency of a nucleus in MRI?

    <p>The strength of the magnetic field.</p> Signup and view all the answers

    What happens during dephasing of magnetic moments in MRI?

    <p>Magnetic moments lose coherency and fall out of phase.</p> Signup and view all the answers

    What term describes the signal produced when the RF excitation pulse is switched off?

    <p>Free Induction Decay (FID) signal.</p> Signup and view all the answers

    What is the relationship between motion and electricity as described in the context of MRI?

    <p>Moving magnetic moments can generate electric signals.</p> Signup and view all the answers

    Study Notes

    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

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    Description

    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.

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