MRI Techniques and Pulse Sequences

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Questions and Answers

What characteristic of T2-weighted images allows them to demonstrate pathology effectively?

They utilize only one echo per TR.

They are more sensitive to proton density.

Diseased tissues are more edematous and/or vascular. (correct)

They require longer TR values.

How many echoes does the DUAL SPIN-ECHO sequence obtain per TR?

Two echoes (correct)

Three echoes

Four echoes

One echo

What is the effect of a long turbo factor in the FAST (TURBO) SPIN-ECHO sequence?

Has no impact on the quality of the image.

Increases scan time and decreases T1 weighting. (correct)

Decreases effective TE and increases T1 weighting.

Increases the number of echoes per TR.

What is the primary purpose of the 180-degree rephasing pulses in the FAST (TURBO) SPIN-ECHO sequence?

To allow for multiple echoes to fill K-Space. Signup and view all the answers

What is the term used to describe the number of 180-degree pulses sent after each 90-degree pulse in the FAST sequence?

Echo train length Signup and view all the answers

In the SINGLE-SHOT FAST SPIN-ECHO Sequence, how are echoes acquired?

In a single TR Signup and view all the answers

How does a short turbo factor affect effective TE in the FAST sequence?

Decreases effective TE and increases T1 weighting. Signup and view all the answers

What type of imaging do T1-weighted images primarily demonstrate?

Anatomical structures Signup and view all the answers

What characterizes a pulse sequence in MR imaging?

It involves the interaction of RF pulses and magnetic gradients. Signup and view all the answers

Which components are involved in the time chart of a pulse sequence?

Patient's longitudinal magnetization, RF pulses, Gradient activations. Signup and view all the answers

What is the purpose of the 180-degree pulse in the Spin Echo sequence?

To bring protons back into phase after dephasing. Signup and view all the answers

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

Gradient Echo (GRE) sequences. Signup and view all the answers

What is meant by TR in a Spin Echo sequence?

Time elapsed between two successive 90-degree pulses. Signup and view all the answers

Which pulse sequence serves as the foundation for understanding other sequences?

Spin Echo (SE) sequence. Signup and view all the answers

What is FID in the context of MRI?

The initial weak signal produced from transverse magnetization. Signup and view all the answers

In practice, which pulse sequence is primarily applied with inversion recovery?

Spin Echo sequence. Signup and view all the answers

What is a defining characteristic of the Gradient Echo (GRE) sequence compared to the Spin Echo (SE) sequence?

GRE achieves rephasing through gradients instead of pulses. Signup and view all the answers

What characteristic of the flip angle in GRE sequences usually leads to reduced scanning time?

It is usually less than 90 degrees. Signup and view all the answers

What term describes the relaxation caused by dephasing due to magnetic field inhomogeneity in GRE sequences?

T2* relaxation Signup and view all the answers

How do spoiled or incoherent GRE sequences manage the residual transverse magnetization?

They completely destroy it after signal reception. Signup and view all the answers

What is the key aspect of the steady state for Coherent GRE sequences?

The TR remains shorter than both T1 and T2 times. Signup and view all the answers

What major advantage does half-Fourier transformation provide in the SINGLE-SHOT FAST SPIN-ECHO sequence?

It enables reducing scan time by mathematically calculating part of K-Space. Signup and view all the answers

What is the effect of not having a 180-degree pulse in a GRE sequence?

It leads to an inability to compensate for dephasing effects. Signup and view all the answers

Which of the following is NOT a feature of the Gradient Echo (GRE) Sequence?

Requires T1 recovery time to be longer than T2. Signup and view all the answers

What is the primary purpose of diffusion weighted imaging (DWI) in brain stroke assessment?

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

Which technique is primarily used to increase the conspicuity of vessels in MRI?

Magnetization Transfer (MT) Contrast Signup and view all the answers

In Time of Flight MRA (TOF-MRA), what combination of techniques is utilized to enhance image quality?

Coherent GRE pulse sequences with TR and flip angle combinations. Signup and view all the answers

What type of MR imaging technique is fMRI categorized as?

Dynamic imaging technique. Signup and view all the answers

Which statement accurately describes the outcome of using Phase Contrast MRA (PC-MRA)?

It typically incurs longer scan times than other MRA techniques. Signup and view all the answers

What happens to the magnetization at the halfway stage during recovery after a 180-degree inversion pulse?

Magnetization reaches zero level. Signup and view all the answers

What is the primary purpose of the STIR pulse sequence?

To suppress the signal from fat tissues. Signup and view all the answers

What TI value is generally used for the STIR sequence?

Around 100-200 ms. Signup and view all the answers

What is the main function of the FLAIR sequence?

To suppress fluid signals in the imaging data. Signup and view all the answers

Which EPI sequence is specifically faster due to its use of gradients for rephasing?

GE-EPI. Signup and view all the answers

What characterizes Single Shot EPI (SS-EPI)?

Filling all lines of K-Space in a single TR. Signup and view all the answers

What purpose does Perfusion Weighted Imaging (PWI) serve?

To analyze perfusion dynamics in specific organs. Signup and view all the answers

Which statement about EPI sequences is correct?

GE-EPI and SS-EPI are generally faster than SE-EPI. Signup and view all the answers

What flip angle range is generally favored for achieving a steady state in SS sequences?

30° to 45° Signup and view all the answers

What is a characteristic feature of SS sequences?

Very short TR and TE times Signup and view all the answers

Which sequence utilizes a variable flip angle excitation pulse to create images?

Incoherent (Spoiled) Gradient Echo Signup and view all the answers

What is the primary purpose of using an inversion 180-degree pulse in IR sequences?

To saturate all the tissues Signup and view all the answers

What does the term T1 weighting in imaging refer to?

The contrast difference created by longitudinal relaxation Signup and view all the answers

Which of the following factors contributes to different recovery rates of longitudinal magnetization in IR sequences?

T1 values of the tissues Signup and view all the answers

Which statement about IR images is accurate?

They are heavily T1-weighted with significant contrast differences. Signup and view all the answers

In Gradient Echo (GRE) sequences, what is the effect of spoiling residual transverse magnetization?

It minimizes its effect on image contrast. Signup and view all the answers

Study Notes

Pulse Sequences and Image Contrast

Pulse sequences are a series of parameters that create an MRI image.
Pulse sequences comprise RF pulses and gradients.
A pulse sequence is a time chart reflecting:
- Patient's net longitudinal magnetization.
- RF pulse transmission (e.g., 90°, 180°).
- Gradient activation (X, Y, Z) for localization.
- Signal (echo) acquisition.
- K-Space filling.

Steps in Image Acquisition

The patient enters the MRI machine.
A pulse sequence is applied.
Data is acquired.
Acquired echoes are stored in K-Space.
The MRI image is generated.

Pulse Sequence Classifications

Pulse sequences are broadly categorized into spin-echo and gradient-echo sequences.
Inversion recovery and echo planar imaging (EPI) can be applied to both types theoretically.
Practically, inversion recovery is used with spin-echo sequences, and EPI is used with gradient-echo.
Four common pulse sequence types are:
- Spin-echo (SE).
- Gradient echo (GRE).
- Inversion recovery (IR).
- Echo planar imaging (EPI).

Spin Echo (SE)

Consists of 90° and 180° RF pulses.
The 90° pulse flips net magnetization into the transverse plane.
Free induction decay (FID) is a weak initial signal.
The 180° pulse rephases the dephasing magnetization, creating a spin echo.
TR: Time to repeat (time between 90° pulses).
TE: Time to echo (time between 90° pulse and echo signal).
Slice selection gradient, phase encoding gradient, and frequency encoding gradient are used for localization.

Spin Echo (SE) Modifications

A-Dual Spin Echo: Two 180° pulses are used, obtaining two echoes per TR.
Produces both proton density (PD) and T2-weighted images.
B-FAST (Turbo) Spin Echo: Multiple 180° pulses are used.
- More echoes are created per TR.
- Reduces scan time.
- Turbo factor refers to number of 180° pulses.
- Higher turbo factor means shorter scan time, with T1 weighting potentially increasing.
C-Single-Shot Fast Spin Echo:
- Fills K-Space in a single TR.
- Reduces scan time drastically.
- Calculates half of K-Space data, using half-Fourier transformation.

Gradient Echo (GRE)

No 180° pulse.
Rephasing of transverse magnetization (TM) achieved using gradients.
Flip angle is typically smaller than 90°.
Scan time is shorter compared to spin echo, due to reduced TR.
Possible variants include:
- Steady-state (SS) or Coherent.
- Incoherent (Spoiled).

Inversion Recovery (IR)

An 180° pulse is applied before the usual spin-echo or gradient-echo sequence.
This pulse flips the longitudinal magnetization to the negative side of the z-axis.
The time between the 180° and 90° pulses is called TI (Time to invert)
Tissues with different T1 values recover at different rates.
Different types of IR sequences vary based on their TI value to perform specific tissue suppression.
- STIR (Short Inversion Recovery).
- FLAIR (Fluid-Attenuated Inversion Recovery)

Echo Planar Imaging (EPI)

Fills multiple K-space lines in a single TR.
Results in a significant reduction in scan time.
Can be spin echo (SE) based (using multiple 180° pulses) or Gradient echo (GE) based using gradients for rephasing.

Applications of EPI Sequences

Perfusion Weighted Imaging (PWI): Studies contrast uptake in lesions (e.g., brain, liver, pancreas).
Diffusion Weighted Imaging (DWI): Measures water diffusion to identify restricted diffusion, usefule in stroke.
Functional MRI (fMRI): Tracks brain activity during stimulus.
Magnetization Transfer (MT): Suppresses background tissue, increasing contrast of vessels.
Magnetic Resonance Angiography (MRA): Visualizes blood vessels with high signal, differing from surrounding tissue based on flow.
- Time of Flight (TOF): Detects moving blood with GRE.
- Phase Contrast (PC): Measures blood flow using coherent GRE sequences.

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Description

This quiz explores various aspects of MRI techniques, focusing on T2-weighted images, pulse sequences, and echo acquisition methods. Test your knowledge on the principles behind FAST (TURBO) SPIN-ECHO sequences and the significance of different parameters in MRI imaging. Ideal for students and professionals in radiology and medical imaging fields.