MRI Techniques: GRE and Steady State

Podcast

Play an AI-generated podcast conversation about this lesson

Questions and Answers

What occurs to the residual transverse magnetization in a Spoiled GRE?

It is enhanced, resulting in brighter images.

It is destroyed, leaving only longitudinal magnetization. (correct)

It cycles, leading to increased T2 contrast.

It is preserved and contributes to T2 weighting.

Which sequence of parameters would maximize T1 weighting in GRE imaging?

Small flip angle, intermediate TR, short TE.

Large flip angle, intermediate TR, short TE. (correct)

Large flip angle, short TR, short TE.

Small flip angle, long TR, long TE.

What disadvantage is associated with GRE images?

Increased susceptibility to magnetic field inhomogeneity. (correct)

Reduced need for echo time (TE).

Enhanced T2 weighting.

Higher signal-to-noise ratio (SNR).

Which type of GRE is also known as incoherent GRE?

Spoiled GRE. Signup and view all the answers

How can PD images be produced in Spoiled GRE?

By using a long TR with a small flip angle. Signup and view all the answers

What is a benefit of using GRE imaging?

Increases scanning speed. Signup and view all the answers

What characterizes T2* decay in GRE imaging?

Increases sensitivity to magnetic field inhomogeneity. Signup and view all the answers

What technique is employed to spoil Mxy in GRE imaging?

Phase offset in successive RF pulses. Signup and view all the answers

What is a significant drawback of applying a 180 degree refocusing pulse in Spin Echo imaging?

It increases the power deposition in the human body. Signup and view all the answers

How does the application of multiple echoes affect scanning time in Fast Spin Echo imaging?

It reduces the scanning time by the echo train length factor. Signup and view all the answers

Which combination of TR and TE maximizes T1-weighted imaging?

Short TR and short TE Signup and view all the answers

What is the echo train length (ETL) in relation to scanning time in Turbo Spin Echo imaging?

Scanning time is reduced by the factor of ETL. Signup and view all the answers

Which of the following represents the typical value for a long TR in Spin Echo imaging?

2000 ms Signup and view all the answers

What effect does the 180o pulse have on the dephasing of the magnetization vector?

It reverses the dephasing effects caused by magnetic field inhomogeneity. Signup and view all the answers

During the TE/2 time, what happens to the magnetization vector?

It begins to dephase due to T2⋆ interactions. Signup and view all the answers

How do nuclei that precess at a frequency higher than ωL behave in the rotating frame?

They rotate clockwise in the xy plane. Signup and view all the answers

What does the application of a 180o refocusing pulse do to the fan of magnetization vectors?

It rotates them 180o about the x′ axis. Signup and view all the answers

What is the phase relationship between the spin echo signal and the FID signal?

The spin echo has an opposite phase. Signup and view all the answers

Why is the amplitude of the spin echo smaller than that of the FID?

Due to irreversible spin-spin relaxation effects. Signup and view all the answers

What does the term 'dephasing' refer to in the context of magnetization vectors?

The dispersion of magnetization vectors due to field inhomogeneity. Signup and view all the answers

What initial effect does a 90o pulse have on the magnetization vector Mz?

It tips Mz away from the +z axis onto the y′ axis. Signup and view all the answers

What is the purpose of adding a rewinder gradient in a coherent GRE sequence?

To recycle residual transverse magnetization Signup and view all the answers

How does the Inversion Recovery (IR) prepared-GRE contrast mechanism primarily work?

By allowing different tissues to recover their longitudinal magnetization at varying rates Signup and view all the answers

What happens to the transverse magnetization during a long TR in a GRE sequence?

It becomes completely dephased Signup and view all the answers

What is the main characteristic of the Driven Equilibrium (DE) prepared-GRE technique?

It uses both 90° and 180° RF pulses to manipulate magnetization Signup and view all the answers

In what scenario is saturation most likely to occur during a fast GRE sequence?

When tissues have long T1 times at high Bo Signup and view all the answers

What is the effect of using variable magnetic field gradients in a GRE sequence?

It can cancel out the residual transverse magnetization Signup and view all the answers

What initiates the Spin Echo (SE) process developed by Hahn?

The application of a 90° RF pulse to flip the magnetization Signup and view all the answers

What is the main purpose of using a 180° RF pulse in the Inversion Recovery technique?

To invert the longitudinal magnetization to the –z axis Signup and view all the answers

Which characteristic is true of the coherent GRE sequences?

They rely on preserving dephased spins through the sequence Signup and view all the answers

In the DE-GRE technique, what does the TE/2 delay allow for?

Decay of tissues at different rates prior to inversion Signup and view all the answers

What happens to the resulting echoes in the Carr-Purcell sequence if the 180o refocusing pulse is not perfect?

The magnetization vector will diverge either above or below the transverse plane. Signup and view all the answers

In the Carr-Purcell Meiboom-Gill (CPMG) sequence, how does the RF carrier wave of the refocusing pulses compare to the 90o pulse?

It is phase shifted by π/2. Signup and view all the answers

Which axis do spins flip about in the Carr-Purcell Meiboom-Gill sequence?

y′ axis Signup and view all the answers

What is the main advantage of the Carr-Purcell Meiboom-Gill sequence over the standard Carr-Purcell sequence?

It ensures all spin echoes converge along y′ and have the same positive phase. Signup and view all the answers

If a 180o refocusing pulse is applied improperly, what effect does it have on the decay of the echoes?

The echo decay will not represent true T2 anymore. Signup and view all the answers

In the Carr-Purcell sequence, what separation time is denoted as TE?

Time between two refocusing 180o pulses. Signup and view all the answers

Which of the following accurately describes the behavior of echoes in the standard Carr-Purcell sequence?

Echoes alternate between -y′ and y′. Signup and view all the answers

What is the effect on magnetization if perfect 180o pulses are assumed in the CPMG sequence?

Magnetization aligns perfectly along y′. Signup and view all the answers

Which artifact can arise from cumulative errors in the 180o pulses of the Carr-Purcell sequence?

Phase cancellation of echoes. Signup and view all the answers

Study Notes

Steady State Magnetization

Residual transverse magnetization accumulates over multiple cycles until it reaches a steady state (Mss)
In spin echo (SE) sequences, TR is long enough to allow complete dephasing of spins in the x-y plane, resulting in negligible x-y magnetization at the end of each cycle

Gradient Echo (GRE) Image Contrast

GRE images can be optimized to highlight T1, T2*, or proton density (PD) by adjusting the pulse sequence parameters
T1 weighting is achieved by using a large flip angle, intermediate TR, and short TE
T2* weighting is maximized using a small flip angle, and long TR and TE
PD weighting is achieved with a small flip angle, long TR, and short TE

Advantages and Disadvantages of GRE

Advantages of GRE sequences include increased speed, sensitivity to magnetic susceptibility (useful for hemorrhage, functional imaging, and iron imaging), and capability to image flowing blood (magnetic resonance angiography, MRA)
Disadvantages include decreased signal-to-noise ratio (SNR) due to smaller flip angles and short TRs and T2* decay, increased magnetic susceptibility artifact in regions like the paranasal sinuses or abdomen, and increased sensitivity to magnetic field inhomogeneity and intra-voxel dephasing

Coherent vs. Incoherent GRE

The residual transverse magnetization can be either spoiled (destroyed) or cycled in GRE sequences
Spoiled GRE, synonymous with incoherent GRE, destroys the residual transverse magnetization, leaving only longitudinal magnetization
Non-spoiled GRE, known as coherent GRE, recycles the residual transverse magnetization, increasing its length from cycle to cycle

Incoherent (Spoiled) GRE

Spoiling in incoherent GRE eliminates residual transverse magnetization, preventing the creation of T2 or T2* effects
Long TR and large flip angles in incoherent GRE result in T1-weighted images
With long TR and small flip angles, PD-weighted images are produced

Spoiling Methods

RF spoiling offsets the phase of each successive RF pulse, causing cancellation of successive Mss vectors
Variable magnetic field gradients, known as "crushers," are added with varying strengths between cycles
Very long TRs completely dephase the Mss

Coherent GRE

In coherent GRE, residual transverse magnetization is recycled, leading to an increasing Mss
A rewinder gradient, opposite in polarity but similar in strength to the original phase encoding (PE) gradient, is used to counteract the phase shift introduced by the PE gradient
This ensures no dephasing from previous PE gradients in subsequent cycles

Fast GRE Techniques

Short TR and TE times are desirable for ultrafast GRE sequences
Saturation of tissues with long T1 times, especially at high field strengths, can significantly reduce SNR
To achieve high speed with preserving signal, two strategies are employed: Inversion Recovery (IR)-prepared GRE and Driven Equilibrium (DE)-prepared GRE

Inversion Recovery (IR)-prepared GRE

A 180-degree RF pulse inverts the magnetization to -z before the GRE sequence begins
An inversion time (TI) is introduced, allowing varying degrees of recovery back towards the +z axis for different tissues
Tissues with shorter T1 times recover faster, leading to higher longitudinal magnetization at the start of the GRE sequence
This technique introduces T1 contrast before the GRE sequence begins

Driven Equilibrium (DE)-prepared GRE

A 90-degree RF pulse flips magnetization into the x-y plane
A delay (TE/2) allows tissues to decay at different rates (T2*)
A 180-degree pulse inverts the dephased spins along the -y axis
After another TE/2 delay, the transverse vectors converge, but each tissue maintains its own vector length (tissues with shorter T2 times have smaller vectors)
Another 90-degree RF pulse flips the vectors back to the +z axis
All tissues are now aligned along +z but with different vector lengths, reflecting their T2 values
A GRE sequence with a small flip angle excites the vectors, resulting in varying Mxy components and T2-weighted contrast

Spin Echo (SE)

Introduced by Hahn in 1950, SE utilizes a 90-degree RF pulse followed by a 180-degree pulse for refocusing dephased spins
The 180-degree pulse counteracts the effects of magnetic field inhomogeneity, leaving only signal degradation due to irreversible spin-spin (T2) relaxation
The 180-degree pulse is often referred to as a "refocusing pulse"

Conventional (Hahn) SE

A 90-degree RF pulse rotates magnetization from the +z axis to the y′ axis
During TE/2, magnetic moments dephase due to spin-spin interactions and field inhomogeneity
Nuclei experiencing higher magnetic fields precess faster, while those with lower fields precess slower, resulting in a fanning out of magnetization vectors
A 180-degree refocusing pulse is applied along the x′ axis at time TE/2, rotating the fan 180 degrees around the x′ axis
The dephased vectors converge along the -y′ axis at time TE after the 90-degree pulse
The spin echo signal amplitude represents the remaining transverse magnetization after T2 relaxation, which is smaller than the initial FID signal

Carr-Purcell Sequence

Multiple 180-degree refocusing pulses separate by TE are applied along the x′ axis, creating multiple echoes
Echoes alternate between -y′ and y′, with decay reflecting true T2 if refocusing is perfect
Imperfect 180-degree pulses cumulatively cause magnetization vectors to deviate from the transverse plane

Carr-Purcell Meiboom-Gill (CPMG) SE

The RF carrier wave of the refocusing pulses is phase-shifted by π/2 relative to the 90-degree pulse in the CPMG modification
This aligns the B1 field of the refocusing pulses along y′, while the 90-degree B1 remains along x′
Spin flips occur around the y′ axis, resulting in all spin echoes converging along y′ with the same (positive) phase
Perfect 180-degree refocusing is assumed for this description, and not all echoes can be utilized in practice

Important Notes about Spin Echo

The 180-degree refocusing pulse eliminates the effect of T2*, preserving signal loss only due to irreversible T2 relaxation
Drawbacks of the 180-degree pulse include increased scanning time due to longer TE and TR, and increased power deposition in the body
Fast spin echo (FSE) or Turbo spin echo (TSE) sequences use multiple echoes to reduce scan time
Echo train length (ETL) represents the number of echoes produced, with scan time reducing by this factor: Scan time (TSE) = Scan time (SE) / ETL

Image Contrast in Conventional SE

T1 weighting is maximized using short TR and TE
T2 weighting is maximized using long TR and TE
PD weighting is achieved with long TR and short TE

Typical Values of Imaging Parameters for SE

Long TR = 2000 ms
Short TR = 300-700 ms
Long TE = 60-80 ms
Short TE = 10-25 ms

Studying That Suits You

Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

Description

This quiz explores the concepts of steady-state magnetization and gradient echo (GRE) imaging techniques. It covers the factors influencing image contrast, including T1, T2*, and proton density weighting, as well as the advantages and disadvantages of GRE methods in MRI. Test your knowledge of these advanced imaging principles!