Pulse Sequences in MRI Technology

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Questions and Answers

What function does the 90-degree RF pulse serve in the Spin Echo pulse sequence?

It rephases the protons.

It activates the phase encoding gradient.

It induces free induction decay.

It flips the net magnetization vector into the transverse plane. (correct)

Which of the following is not a type of pulse sequence classification mentioned?

Dual Spin Echo sequence (correct)

Echo Planar Imaging (EPI)

Spin Echo sequence (SE)

Gradient Echo sequence (GRE)

What is the name of the signal induced in the receiver coil after the 180-degree pulse in a Spin Echo sequence?

Phase Encoding Signal

Free Induction Decay (FID)

Spin Echo (correct)

Transverse Magnetization

What does TE stand for in the context of pulse sequences?

Time to Echo Signup and view all the answers

In Spin Echo sequences, what is the role of the 180-degree pulse?

To rephase the dephased protons. Signup and view all the answers

When is the slice selection gradient activated during a Spin Echo sequence?

When the 90-degree RF pulse is sent. Signup and view all the answers

Which of the following statements about the Free Induction Decay (FID) signal is true?

FID is the first signal received before rephasing. Signup and view all the answers

Which pulse sequence is primarily associated with Echo Planar Imaging (EPI)?

Gradient Echo sequences Signup and view all the answers

What is the main advantage of using a single-shot fast spin-echo sequence?

It acquires all K-Space lines in a single TR. Signup and view all the answers

In gradient echo (GRE) sequences, what primarily causes the rephasing of transverse magnetization?

Gradients, specifically the reversal of the frequency encoding gradient. Signup and view all the answers

How does T2 relaxation in GRE differ from T2 relaxation in SE sequences?

T2 relaxation in GRE is referred to as T2 star (T2*). Signup and view all the answers

What is the effect of a smaller flip angle in GRE sequences?

Allows for an early recovery of longitudinal magnetization. Signup and view all the answers

Which characteristic distinguishes spoiled or incoherent GRE sequences?

The destruction of residual transverse magnetization before the next TR. Signup and view all the answers

What happens to K-Space in a single-shot fast spin-echo sequence?

More than half is filled instantly while the remainder is mathematically calculated. Signup and view all the answers

Why is T2* (T2 star) relaxation significant in GRE sequences?

It encompasses dephasing effects from magnetic field inhomogeneity. Signup and view all the answers

What is one main difference between spin echo (SE) and gradient echo (GRE) sequences?

GRE sequences eliminate the need for RF pulses for rephasing. Signup and view all the answers

What sequence forms the basis for understanding all other MRI sequences?

Spin Echo Signup and view all the answers

Which type of weighted images are T1-weighted images primarily used for?

Showing anatomy Signup and view all the answers

What modification to the conventional SE sequence sends two 180-degree pulses after each 90-degree pulse?

Dual Spin-Echo Signup and view all the answers

How does the Turbo factor in a Fast Spin-Echo sequence influence imaging?

Decreases the scan time Signup and view all the answers

What term is used for the effective TE at which the center of K-Space is filled in a Turbo Spin-Echo sequence?

TE effective Signup and view all the answers

In the context of Spin Echo sequences, what does a longer TR in Dual Spin-Echo sequences allow?

Increased proton density weighting Signup and view all the answers

What does the term 'Turbo factor' refer to in Turbo Spin-Echo sequences?

Number of 180-degree pulses per TR Signup and view all the answers

What relationship exists between turbo factor and effective TE in Fast Spin-Echo sequences?

Short turbo factor decreases effective TE Signup and view all the answers

What term is used for echoes generated by multiple 180° pulses?

Spin Echo Echo Planar Imaging Signup and view all the answers

Which EPI sequence is known to use gradients for rephasing?

General Echo Echo Planar Imaging Signup and view all the answers

What imaging technique is specifically used for studying contrast medium uptake in lesions?

Perfusion Weighted Imaging Signup and view all the answers

In which scenario is Diffusion Weighted Imaging (DWI) primarily useful?

To differentiate between salvageable and non-salvageable brain tissue Signup and view all the answers

What is the main purpose of Magnetization Transfer (MT) contrast?

Suppress background tissue to enhance visibility of vessels Signup and view all the answers

Which technique uses coherent GRE pulse sequences to demonstrate arterial and venous flow?

Time of Flight MRA Signup and view all the answers

How do scan times of Phase Contrast MRA compare to GE pulse sequences?

They are longer Signup and view all the answers

What defines Functional MRI (fMRI)?

It involves subtraction of images acquired during rest and stimulation Signup and view all the answers

What is a characteristic feature of steady-state or coherent GRE sequences?

They allow coexistence of longitudinal and transverse magnetization. Signup and view all the answers

Which flip angle range is most commonly used to achieve a steady state in GRE sequences?

30° to 45° Signup and view all the answers

What is the effect of residual transverse magnetization in incoherent (spoiled) gradient echo sequences?

It is minimized to reduce its effect on contrast. Signup and view all the answers

What distinguishes Inversion Recovery (IR) sequences from standard GRE sequences?

They utilize a 180-degree inverting pulse prior to imaging. Signup and view all the answers

Which factor is the main determinant of contrast in Inversion Recovery (IR) sequences?

Time to invert (TI) Signup and view all the answers

Why do tissues with longer T2 values appear with high signal intensity in steady-state GRE sequences?

Because of minimal spoilage of residual magnetization. Signup and view all the answers

What is the primary recovery difference between protons in fat and water during an IR sequence?

Protons in fat recover faster than protons in water. Signup and view all the answers

What role does a shorter TR and TE play in steady-state GRE sequences?

They enable the sequences to be fast and acquired with breath-hold. Signup and view all the answers

What is the primary purpose of the 180-degree pulse in the Inversion Recovery sequence?

To completely saturate fat and water Signup and view all the answers

How does the TI value used in Inversion Recovery affect tissue suppression?

It corresponds to the relaxation time of tissues Signup and view all the answers

What is the typical TI value used in the A-STIR pulse sequence?

Around 100-200 ms Signup and view all the answers

What does B-FLAIR sequence target for suppression?

CSF containing areas Signup and view all the answers

What happens to the signal from a tissue if the TI corresponds to its recovery time at the halfway stage?

No signal is received from that tissue Signup and view all the answers

What is a characteristic of 4-Echo Planar Imaging (EPI)?

All lines of K-Space are filled in a single TR Signup and view all the answers

What is a key effect of using an inversion recovery image in MRI?

It highlights differences in T1 relaxation times Signup and view all the answers

In the context of IR sequences, what does TI stand for?

Inversion Time Signup and view all the answers

Study Notes

Pulse Sequences and Image Contrast

Pulse sequences are a series of parameters that create a complex cascade of events using RF pulses and gradients to form a magnetic resonance (MR) image.
Pulse sequences are essentially a timetable of these interactions.
The timetable includes:
- Patient's longitudinal magnetization
- Transmission of RF pulses (adjustable degrees)
- Gradient activation for localization and signal (echo) acquisition
- K-Space filling with acquired signals or echoes

Outline of Presentation

What is a pulse sequence?
Classification of pulse sequences
Spin Echo (SE) sequences
Modifications of SE sequences:
- Dual Spin-Echo
- Fast (Turbo) Spin-Echo
- Single-Shot Fast Spin-Echo

Spin Echo (SE)

SE sequences use 90° and 180° RF pulses.
The 90° pulse flips net magnetization from the Z-axis into the transverse (X-Y) plane.
Transverse magnetization precesses at Larmor frequency, creating a signal (FID).
FID is weak and prone to decay (dephasing), requiring a 180° pulse to rephas the protons.
Rephasing increases magnetization and creates a stronger signal (spin echo).
TR (Time to Repeat): Time between two 90° pulses
TE (Time to Echo): Time between the 90° pulse and echo signal reception
Slice selection gradient is turned on during RF pulse.
Phase encoding occurs between the excitation (90°) pulse and signal measurement
Frequency encoding takes place during signal measurement
SE sequences form the basis for understanding other sequences (common in all exams).
T1-weighted images are helpful for anatomical depiction.
T2-weighted images highlight edema and vascularity, showing pathology well.

Modifications of SE Sequences

Conventional SE sequences fill one line in K-Space per TR. More than one echo can be obtained per TR (180° pulses).
Dual Spin-Echo Sequence: Two 180° pulses create two echoes per TR (PD+T2 double echo sequence).
Long TR, Short TE sequence produces a proton density weighted image after the first 180° pulse.
Long TR, Long TE sequence produces T2-weighted images after the second 180° pulse.

Fast (Turbo) Spin-Echo Sequence

Turbo sequences obtain multiple echoes per TR with each 180° pulse filling a single K-Space line (rapid scan).
Turbo factor determines the number of 180° pulses after a 90° pulse.
TE increases as the number of 180° pulses increases.
Higher turbo factors maximize signal at TE effective, improving T1-weighting while reducing scanning time. Lower turbo factors increase T2-weighting, improving T2-weighting while reducing scanning time.

Single-Shot Fast Spin-Echo Sequence

Acquires all echoes in a single TR, significantly reducing scan time.
Fills roughly half a K-Space line resulting in halved scan time, with the other half calculated from the acquired information.

Gradient Echo (GRE) Sequence

No 180° pulses in GRE sequences; rephasing by gradients instead.
Flip angles are usually less than 90°.
Shorter TR values enhance scanning speed.
GRE sequences are susceptible to magnetic field inhomogeneity which results in T2* (T2 star) relaxation.

Types of GRE Sequences

Spoiled/Incoherent GRE sequences: Residual TM is nullified after each TR. Increased T1 weighting.
Steady-State/Coherent GRE sequences: Residual TM is refocused after a few TRs resulting in a steady-state magnitude of LM and TM. (favors T2 weighting).

Inversion Recovery (IR) Sequence

180° pulse is applied before the usual spin-echo/gradient echo sequence (inverting longitudinal magnetization).
This sequence saturates tissue, and recovery is dependent on T1 relaxation times, leading to differing contrast depending on tissue type.
TI (Time to Invert): time between the 180° inversion pulse and the 90° excitation pulse The inversion 180° pulse flips longitudinal magnetization (LM) along the negative Z-axis.
Tissue with shorter T1 values will recover faster and produce a brighter signal on the image.
Can be used to suppress specific tissues.

Types of IR Sequences

Short Inversion Recovery (STIR): Suppresses fat signal in images, often for better visualization of edema (tissues)
Fluid-Attenuated Inversion Recovery (FLAIR): Suppresses CSF (cerebrospinal fluid) signal, allowing better visualization of brain lesions.

4-Echo Planar Imaging (EPI)

EPI sequences fill K-space in a single TR by generating echoes with multiple 180° pulses thus increasing speed.
Spin-echo EPI(SE-EPI): involves multiple 180° pulses to generate echoes..
Gradient-echo EPI (GE-EPI) : uses gradients to rephrase, making it faster,
Single-shot EPI(SS-EPI) fills complete k-space in a single TR , further improving speed.
EPI sequences are faster than conventional SE sequences.

Examples of EPI Sequences

Perfusion-weighted imaging (PWI): Used with contrast to study tissue uptake. Helpful in abnormalities of brain, pancreas, liver and prostate.
Diffusion-weighted imaging (DWI): Show areas with restricted diffusion (often indicative of infarcted tissue)
Functional magnetic resonance imaging (fMRI): Demonstrates brain activity by detecting changes in blood flow during tasks or stimulation.
Magnetization transfer (MT) contrast:Suppresses background tissue, improving visibility of vessels and certain disease processes
Magnetic Resonance Angiography (MRA):-Shows blood vessels -Time of Flight (TOF) MRA: Uses gre sequences in demonstrating arterial and venous flow
-Phase Contrast (PC) MRA: uses gre sequences, offering excellent background suppression but with longer scan times.

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Description

Explore the intricate world of pulse sequences used in magnetic resonance imaging (MRI). This quiz covers the various types of pulse sequences including Spin Echo and its modifications, along with the elements that contribute to MR image contrast. Test your knowledge on how these parameters interact to form detailed images.