Magnetic Resonance Overview

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Questions and Answers

Which of the following factors are beyond the control of the operator in MRI?

Excitation

Flow (correct)

Echo spacing

T1 recovery (correct)

What does the term 'dephasing' refer to in MRI?

The process of enhancing image resolution

The tightening of the magnetic field

The loss of phase coherence of signals (correct)

The initiation of energy transfer to nuclei

Which term describes the measure of diffusion of water molecules in tissue during MRI imaging?

Equilibrium

Proton density

Apparent diffusion coefficient (ADC) (correct)

External magnetic field

What characterizes an electromagnet?

It uses coils of wire and an iron core Signup and view all the answers

Which statement accurately defines 'echo train length (ETL)' in MRI?

The series of 180° RF pulses and resultant echoes Signup and view all the answers

What occurs during resonance in MRI?

The nucleus gains energy from an external source. Signup and view all the answers

What does precessional phase refer to?

Magnetic moments can be in various positions along the precessional path. Signup and view all the answers

What effect does the RF excitation pulse have on hydrogen nuclei?

It provides energy, pushing them to a higher energy state. Signup and view all the answers

What is the free induction decay (FID) signal primarily caused by?

Energy loss from high-energy nuclei. Signup and view all the answers

Which phenomenon reflects Faraday's law in MRI?

Motion and electricity produce a magnetic effect. Signup and view all the answers

What happens to the NMV after the RF pulse is switched off?

It tries to realign with B0 while undergoing relaxation. Signup and view all the answers

How does dephasing occur in hydrogen nuclei?

Via interactions with the surrounding magnetic field. Signup and view all the answers

What occurs as relaxation happens in the magnetic moments of hydrogen?

Some high-energy nuclei return to a low-energy state. Signup and view all the answers

What does the free induction decay (FID) signal represent in the imaging process?

The decaying voltage induced in the receiver coil. Signup and view all the answers

What effect does a longer repetition time (TR) have on T1 relaxation?

It increases the amount of T1 relaxation. Signup and view all the answers

How does echo time (TE) affect T2 relaxation?

Longer TE allows more T2 relaxation to occur. Signup and view all the answers

Which of the following is NOT considered an extrinsic contrast parameter?

T1 relaxation time Signup and view all the answers

What does the term 'b' value signify in MRI imaging?

A factor reflecting strength and timing of diffusion gradients. Signup and view all the answers

Which parameter determines both the TR and the amount of longitudinal relaxation?

Repetition time (TR) Signup and view all the answers

What is the relationship between echo time (TE) and the signal peak in MRI?

TE determines the timing of the signal peak collection. Signup and view all the answers

Which one of the following parameters is influenced by the operator during MRI imaging?

Repetition time (TR) Signup and view all the answers

Which of the following describes a state of balance between opposing forces in MRI?

Equilibrium Signup and view all the answers

What is the primary characteristic of an electromagnet?

It produces a magnetic field using coils of wire. Signup and view all the answers

Which term refers to the loss of phase coherence of signals within the transverse plane?

Dephasing Signup and view all the answers

How is the display matrix in MRI determined?

By the product of phase and frequency axes. Signup and view all the answers

What does the apparent diffusion coefficient (ADC) measure in MRI?

The magnitude of diffusion of water molecules. Signup and view all the answers

What does the TR (Repetition Time) directly impact in an MRI procedure?

Amount of T1 relaxation Signup and view all the answers

How is the TE (Echo Time) defined in an MRI process?

Time from RF excitation pulse to the peak of signal received Signup and view all the answers

What phenomenon does the free induction decay (FID) signal illustrate in MRI?

Induction of a current in the receiver coil Signup and view all the answers

Which extrinsic parameter affects the angle through which the NMV is moved during RF excitation?

Flip Angle Signup and view all the answers

What role do the extrinsic contrast parameters serve in the MRI process?

Control the imaging conditions set by the operator Signup and view all the answers

What does the 'b' value signify in MRI imaging?

A factor reflecting gradient strength and timing Signup and view all the answers

What occurs during the free induction decay (FID) signal creation process?

Induced voltage decreases as transverse magnetization decreases Signup and view all the answers

What effect does the Turbo-Factor or Echo Train Length (ETL) have in MRI?

Affects the duration of signal reception and image acquisition Signup and view all the answers

What phenomenon occurs when an object is exposed to an oscillating perturbation with a frequency close to its natural frequency of oscillation?

Resonance Signup and view all the answers

What term describes the process by which hydrogen nuclei align their magnetic moments after losing energy from the RF excitation pulse?

Relaxation Signup and view all the answers

What happens to the NMV when the RF excitation pulse is switched off?

It moves away from B0 Signup and view all the answers

Which of these contributes to the loss of coherency in the hydrogen nuclei's magnetic moments?

Inhomogeneities in B0 field Signup and view all the answers

What equilibrium state is achieved when the NMV returns to align with B0?

Relaxation equilibrium Signup and view all the answers

Which term refers to the gradual loss of energy by hydrogen nuclei after RF excitation?

Relaxation Signup and view all the answers

What effect does the external RF pulse have on the magnetic moments of hydrogen nuclei during resonance?

Energizes and aligns them Signup and view all the answers

What is indicated by the term 'Larmor frequency' in relation to magnetic moments?

Natural frequency of oscillation Signup and view all the answers

What happens to the magnetic moments of hydrogen nuclei during resonance?

They gain energy from the external oscillating force. Signup and view all the answers

Which phase of the NMV indicates that the magnetic moments of hydrogen are at the same place on the precessional path?

Aligned phase Signup and view all the answers

What is the primary result of free induction decay (FID) in MRI?

Hydrogen nuclei lose energy and relax. Signup and view all the answers

What causes dephasing in hydrogen nuclei during MRI?

Inhomogeneities in the B0 field and spin interactions Signup and view all the answers

What is the process by which hydrogen nuclei return to align with B0 after an RF excitation pulse?

Relaxation Signup and view all the answers

What does Faraday's law illustrate in the context of MRI?

Induction of current due to magnetic motion Signup and view all the answers

What is the consequence of the NMV moving out of alignment with B0?

Potential loss of signal Signup and view all the answers

What phenomenon occurs when the RF excitation pulse is switched off?

Dephasing increases. Signup and view all the answers

What is the time interval known as TR in MRI?

Time between RF excitation pulses Signup and view all the answers

What effect does a longer TE have on the MRI signal?

Causes greater decay of transverse magnetization Signup and view all the answers

Which parameter is affected by the flip angle in MRI?

The angle of the NMV after excitation Signup and view all the answers

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

The amount of signal decay before measurement Signup and view all the answers

How does TR directly affect MRI imaging?

It determines the interval of the RF pulse sequence Signup and view all the answers

Which of the following represents an extrinsic contrast parameter?

Repetition time (TR) Signup and view all the answers

What does the 'b' value reflect in diffusion-weighted MRI?

The timing of gradient application Signup and view all the answers

What is the purpose of adjusting flip angle in MRI?

To manipulate the amount of signal acquired Signup and view all the answers

Which of the following factors is NOT related to the intrinsic contrast mechanism in MRI?

Resonance frequency Signup and view all the answers

What does the term 'echo spacing' refer to in MRI?

The time interval between echoes in an echo train Signup and view all the answers

Which description accurately characterizes the 'apparent diffusion coefficient (ADC)' in MRI?

Assesses the magnitude of diffusion of water molecules in tissues Signup and view all the answers

What does the term 'dephasing' describe in the context of MRI?

Loss of phase coherence of signals within the transverse plane Signup and view all the answers

What is the primary function of the 'echo train' in a fast spin echo sequence?

To generate a series of rephasing pulses and echoes Signup and view all the answers

Study Notes

Precessional Phase

Precessional phase means that the magnetic moments of hydrogen are at the same place on the precessional path at a moment in time.
Precessional phase means that the magnetic moments of hydrogen are at different places on the precessional path at a moment in time.

Resonance

Resonance is the phenomenon when an object is exposed to an oscillating perturbation that has a frequency close to its own natural frequency of oscillation.
When a nucleus is exposed to an external force with an oscillation similar to the natural frequency of its magnetic moment (Larmor Frequency), the nucleus gains energy from the external source.
The result of resonance is that the NMV moves out of alignment away from B0, and the magnetic moments of H nuclei move into phase with each other.

MR Signal

Faraday’s Law: Motion + Electricity = Magnet
Recovery and Dephasing cause changes in the MR signal

Free Induction Decay (FID) Signal

When the RF excitation pulse is switched off, the NMV is only influenced by B0, and it tries to realign with it.
Hydrogen nuclei lose energy given to them by the RF excitation pulse.
The process by which hydrogen loses energy is called relaxation.
Relaxation allows the NMV to return to realign with B0 because some of the high-energy nuclei return to the low-energy population, aligning their magnetic moments in the spin-up direction.
Magnetic moments of hydrogen lose coherency due to dephasing, caused by inhomogeneities in the B0 field and interactions between spins in the patient’s tissue.
The magnitude of transverse coherent magnetization decreases, as the magnitude of the voltage induced in the receiver coil decreases.
The induction of decaying voltage is called the free induction decay (FID) signal.
Spins freely precess influenced only by B0, signal decays with time, and magnetic moments of the spins induce a current in the receiver coil.

Pulse Timing Parameters

TR is the time from the application of one RF excitation pulse to the application of the next, measured in milliseconds.
TR determines the amount of longitudinal relaxation between the end of one RF excitation pulse and the application of the next.
TR determines the amount of T1 relaxation that has occurred when signal is read.
TE is the time from the application of the RF excitation pulse to the peak of signal induced in the receiver coil, also measured in milliseconds.
TE determines how much decay of transverse magnetization occurs.
TE controls the amount of T2 relaxation that has occurred when signal is read.

Image Contrast

Image contrast is controlled by two parameters:
- Extrinsic Contrast Parameters
- Intrinsic Contrast Mechanisms

Extrinsic Contrast Parameters

Extrinsic Contrast Parameters are controlled by the system operator, which include:
- Repetition Time (TR): Time between one RF pulse and the next, measured in milliseconds. It affects the length of the relaxation period.
- Echo Time (TE): Time between an RF excitation pulse and the collection of the signal, measured in milliseconds. It affects the length of the relaxation period after the removal of an RF excitation pulse.
- Flip Angle: Angle through which the NMV is rotated by an RF excitation pulse.
- Turbo-Factor or Echo Train Length (ETL/TF).
- Time From Inversion (TI)
- ‘b’ value: reflects the strength and timing of the gradients used to generate diffusion-weighted images.

Intrinsic Contrast Mechanisms

Intrinsic Contrast Mechanics are not controlled by the operator, which include:
- T1 Recovery
- T2 Decay
- Proton Density
- Flow
- Apparent Diffusion Coefficient (ADC): measures the magnitude of diffusion (of water molecules) within tissue.

MRI Terms

Dephasing: Loss of phase coherence of signals within the transverse plane.
Diffusion: Movement of molecules due to random thermal motion.
Dipole: Magnet characterized by a North and South magnetic pole, separated by a finite distance.
Display Matrix: Total number of pixels, determined by the product of phase and frequency axis.
Electromagnet: Magnet that utilizes coils of wire, typically wound around an iron core. When current flows through the coil it becomes magnetized.
Equilibrium: State of balance between opposing forces.
Excitation: Transferring energy into the spinning nuclei via radiofrequency pulse(s), putting the nuclei into a higher energy state.
Echo Spacing: Spacing between each echo in FSE.
Echo Train: Series of 180° rephasing pulse and echoes in a fast spin echo pulse sequence.
Echo Train Length (ETL): Number of 180° RF pulses and or turbo factor resultant echoes is a fast spin echo sequence.
Effective TE: Time between the echo and the RF pulse that initiated it in SSFP and FSE sequences.
Electrons Orbit: The nucleus in distinct shells and are negatively charged.
External Magnetic Field (EMF): Drives a current in a circuit and is the result of a changing magnetic field inducing an electric field.

Precessional Phase

Occurs when the hydrogen nuclei are at the same location on the precessional path at a given moment in time.

Resonance

Happens when an object is exposed to an oscillating force that has a frequency close to its natural frequency of oscillation.
A key consequence of resonance is that the magnetic moments of the hydrogen nuclei move into phase with each other.

MRI Signal

The MRI signal is generated using Faraday's law, which states that motion and electricity together create a magnetic field.
This signal can be influenced by factors such as recovery (the return of the magnetic moments to their original alignment) and dephasing (the loss of phase coherence between the magnetic moments).

Free Induction Decay (FID) Signal

This signal is produced when the RF excitation pulse is turned off, allowing the magnetic moments to realign with the main magnetic field (B0).
The process by which hydrogen nuclei lose the energy absorbed from the RF excitation pulse is called relaxation.
The FID signal decays over time due to dephasing and relaxation.
Dephasing occurs because of inhomogeneities in the B0 field, resulting in the magnetic moments losing their coherent precession, thereby reducing the signal amplitude.

Pulse Timing Parameters

Repetition Time (TR): The time between the application of one RF excitation pulse and the next. It is measured in milliseconds (ms) and influences the amount of T1 relaxation that occurs before the next pulse.
Echo Time (TE): The time between the application of the RF excitation pulse and the peak of the signal induced in the receiver coil. It is measured in milliseconds (ms) and determines the amount of T2 relaxation that has occurred when the signal is read.

Extrinsic Contrast Parameters

These are parameters controlled by the operator and include:
- Repetition Time (TR)
- Echo Time (TE)
- Flip Angle: The angle to which the net magnetization vector (NMV) is tilted by the RF pulse.
- Turbo-factor or Echo Train Length (ETL/TF): Determines the number of echoes acquired per excitation pulse in a fast spin echo sequence.
- Time from Inversion (TI): In inversion recovery sequences, it is the time between the inversion pulse and the excitation pulse, influencing contrast based on T1 recovery.
- ‘b’ value: Reflects the strength and timing of the gradients used to generate diffusion-weighted images.

Intrinsic Contrast Mechanism

These are parameters that are not directly controlled by the operator and include:
- T1 Recovery: The rate at which longitudinal magnetization returns to equilibrium.
- T2 Decay: The rate at which transverse magnetization decays.
- Proton Density: The concentration of hydrogen nuclei in a given tissue.
- Flow: The movement of blood or other fluids within the body.
- Apparent Diffusion Coefficient (ADC): A measure of the magnitude of diffusion (of water molecules) within tissue.

Key MRI Terms

Dephasing: The loss of phase coherence of signals within the transverse plane.
Diffusion: The movement of molecules due to random thermal motion.
Dipole: A magnetic field characterized by its own north and south magnetic poles.
Display Matrix: The total number of pixels in an image, determined by the product of its phase and frequency axes.
Electromagnet: A magnet that uses coils of wire to generate a magnetic field.
Equilibrium: A state of balance between opposing forces.
Excitation: Transferring energy to nuclei using radiofrequency pulses, causing them to transition to a higher energy state and creating a net transverse magnetization that can be detected.
Echo Spacing: The spacing between each echo in a fast spin echo pulse sequence.
Echo Train: A series of 180° rephasing pulses and echoes in a fast spin echo pulse sequence.
Echo Train Length (ETL): The number of 180° RF pulses and or turbo factor resultant echoes in a fast spin echo sequence.
Effective TE: The time between the echo and the RF pulse that initiated it in SSFP and FSE sequences.
Electrons orbit: Electrons revolving around the nucleus in distinct shells, carrying a negative charge.
External Magnetic Field (EMF): A magnetic field that drives a current in a circuit, produced by a changing magnetic field inducing an electric field.

MRI Principles

Precessional Phase: Describes the synchronized spin of hydrogen nuclei within a magnetic field.
Resonance: Occurs when an object's natural oscillation frequency matches an external force's frequency. In MRI, this involves hydrogen nuclei absorbing energy from radio waves.
MR Signal: Based on Faraday's Law, movement of magnetic moments (from excited hydrogen nuclei) generates an electrical signal.
Free Induction Decay (FID) Signal: Occurs after the radiofrequency pulse is turned off. Hydrogen nuclei, now influenced only by the main magnetic field, lose energy and realign.
Pulse Timing Parameters:
- Repetition Time (TR): Determines the time between RF pulses. It influences how much longitudinal relaxation occurs, which is important for T1-weighted images.
- Echo Time (TE): The time between the RF pulse and the peak of the signal. Influenced by T2 relaxation, impacting T2-weighted images.

Extrinsic Contrast Parameters

Repetition Time (TR): The time between RF pulses, affecting the amount of T1 relaxation.
Echo Time (TE): The time between the RF pulse and the signal peak, influencing T2 relaxation.
Flip Angle: Determines the angle of the magnetization vector after receiving the RF pulse.
Turbo Factor/Echo Train Length (TF/ETL): Used in fast spin-echo sequences to accelerate signal acquisition.
Time from Inversion (TI): Used in inversion recovery sequences to suppress specific tissues.
'b' Value: Measures the strength and timing of diffusion gradients, crucial for diffusion-weighted images.

Intrinsic Contrast Mechanisms

T1 Recovery: The rate at which longitudinal magnetization re-aligns with the main magnetic field.
T2 Decay: The rate at which transverse magnetization decays.
Proton Density: The number of hydrogen protons in a tissue, influencing signal intensity.
Flow: Motion of blood and fluids in tissue, which can influence signal strength and artifacts.
Apparent Diffusion Coefficient (ADC): Measures the rate of water molecule movement in tissue.

Additional MRI Terms

Dephasing: Loss of phase coherence among signals within the transverse plane.
Diffusion: Random thermal motion of molecules within tissue.
Dipole: A magnetic field with distinct north and south poles.
Display Matrix: Determines the number of pixels in an image, influenced by phase and frequency dimensions.
Electromagnet: A magnet generated by current flowing through a coil of wire. Used in MRI machines.
Equilibrium: A state of balance between opposing forces or influences.
Excitation: Applying energy to the nuclei via radiofrequency pulses, causing them to move to a higher energy state.
Echo Spacing: The spacing between echoes in fast spin-echo sequences.
Echo Train: Series of 180° rephasing pulses and resulting echoes in fast spin-echo sequences.
Echo Train Length (ETL): Number of rephasing pulses and echoes in a fast spin-echo sequence.
Effective TE: The time between an echo and the RF pulse that initiated it, used in specific pulse sequences.
Electrons Orbit: The movement of electrons around a nucleus in specific shells, contributing to the overall magnetic properties of an atom.
External Magnetic Field (EMF): A changing magnetic field that induces an electric field.

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Description

This quiz covers key concepts in magnetic resonance, including precessional phase, resonance phenomena, and the MR signal. Understand how magnetic moments interact and the principles guiding Free Induction Decay (FID) signals. Test your knowledge on these fundamental topics in physics and MRI technology.