* MR Data Acquisition & Pulse Sequences *

What is a pulse sequence in MRI?

A grouping of two or more RF pulses used to disturb and manipulate the net magnetization of protons and spatially encode MR signal information.

What are the main types of pulse sequences in MRI?

Spin Echo (SE), Inversion Recovery (IR), Gradient Echo (GRE), and Echo Planar Imaging (EPI).

What does SE stand for in MRI?

Spin Echo.

What does IR stand for in MRI?

Inversion Recovery.

What does GRE stand for in MRI?

Gradient Recall Echo.

What does EPI stand for in MRI?

Echo Planar Imaging.

What is a pulse diagram in MRI?

A visual representation showing what occurs during a pulse sequence.

What does RFt stand for on a pulse diagram?

Transmitted radiofrequency (RF) signal.

What does RFs stand for on a pulse diagram?

Signal received back from the patient.

What does the slice select gradient do?

Determines which slice of anatomy the RF pulse will excite.

What is the phase encoding gradient?

A gradient that varies the phase of spins to spatially encode information along one axis.

What is the frequency encoding gradient?

A gradient applied during signal readout that encodes the frequency of spins for spatial information.

What is phase coherence in MRI?

The state in which rotating protons move in phase or in unison.

What are the three dephasing mechanisms?

Magnetic field inhomogeneities, magnetic susceptibility, and chemical shift.

How does chemical shift cause artifacts?

Fat and water precess at slightly different frequencies, causing misregistration on images.

How much slower does fat precess compared to water at 1.5T?

Approximately 220 Hz slower.

What is TR in MRI?

Time to Repetition — the time from one RF pulse to the next.

What is TE in MRI?

Time to Echo — the time between the RF pulse and the peak of the echo signal.

What is the most common pulse sequence used in MRI?

Spin Echo (SE).

What RF pulse combination identifies a Spin Echo sequence?

An initial 90° pulse followed by a 180° pulse.

What is the function of the 180° pulse in a Spin Echo?

It refocuses dephased spins caused by magnetic field inhomogeneities.

What type of image does Spin Echo produce?

True T1 and T2 weighted images with minimal dephasing artifacts.

What is a disadvantage of Spin Echo?

Long scan times.

What is the advantage of Spin Echo?

Produces high-quality, true T1 and T2 images without dephasing artifacts.

What does FSE stand for?

Fast Spin Echo.

What is the key feature of a Fast Spin Echo sequence?

Collects multiple echoes per TR period using multiple 180° refocusing pulses.

What is Echo Train Length (ETL)?

The number of echoes collected per TR period in a Fast Spin Echo.

What happens to scan time as ETL increases?

Scan time decreases because more echoes are collected per TR.

What is a disadvantage of using long ETL in FSE?

Increased tissue heating and brighter fat on T2-weighted images.

Why does fat appear bright on T2 FSE images?

Multiple 180° pulses maintain fat signal, reducing its normal T2 decay.

What is a practical ETL range for FSE?

8, 16, 32, or 64 echoes per TR.

What is parallel imaging?

Use of multiple coils or channels to collect k-space data lines in a single TR period.

What are examples of parallel imaging systems?

ASSET, SENSE, and iPAT.

What is the acceleration factor in parallel imaging?

The number of coils/channels used to accelerate data acquisition.

What are the advantages of parallel imaging?

Reduced scan time, improved resolution, and less heat deposition in tissue.

What is GRE in MRI?

Gradient Echo — a fast imaging technique using flip angles less than 90°.

What symbol is used to represent flip angle?

α (alpha).

Why are GRE scans faster than SE?

Lower flip angles shorten TR and TE, reducing total scan time.

What is a disadvantage of GRE?

No compensation for dephasing; not true T1 or T2 images.

What type of image contrast is produced by GRE?

T2* weighting — includes dephasing effects from field inhomogeneities.

Why are GRE sequences not true T2?

They include additional dephasing, resulting in T2* contrast.

What are spoilers in GRE?

Gradients that dephase residual transverse magnetization to prevent carry-over into the next TR.

What is Steady State Free Precession (SSFP)?

A GRE variant that keeps some transverse magnetization and uses a longer T2 time.

What is Coherent Gradient Echo?

A GRE sequence (GRASS, FISP) that keeps residual transverse magnetization in phase between TR periods.

What is the purpose of rewinding gradients in Coherent GRE?

Reverses phase encoding gradients to maintain coherence.

What is a Spoiled Gradient Echo?

An incoherent GRE sequence (SPGR, FLASH) that eliminates residual transverse magnetization before the next TR.

How is residual transverse magnetization destroyed in SPGR?

By gradient spoiling using slice select, phase, or frequency gradients.

What is a Fast Gradient Echo sequence?

A GRE variant that collects multiple images quickly using a short TE; often done in a single breath-hold.

What flip angles are used in GRE sequences?

Less than 90 degrees (commonly 15°, 45°, or 60°).

What is Inversion Recovery (IR)?

A sequence beginning with a 180° inversion pulse followed by a Spin Echo sequence.

What is TI in MRI?

Time to Inversion — the delay between the 180° and 90° pulses in an IR sequence.

What happens during TI?

Protons recover longitudinal magnetization toward equilibrium before the 90° excitation.

What are two main types of inversion recovery sequences?

STIR and FLAIR.

What does STIR stand for?

Short Tau Inversion Recovery.

What is the TI range for STIR?

100-175 ms.

What is the purpose of STIR?

To suppress fat signal on T1-weighted images.

Why is STIR useful?

It allows abnormalities surrounded by fat to be seen more clearly.

What imaging area uses STIR most often?

Musculoskeletal imaging.

What does FLAIR stand for?

Fluid Attenuated Inversion Recovery.

What is the TI range for FLAIR?

1700-2200 ms.

What is the purpose of FLAIR?

To suppress CSF signal on T2-weighted images so brain pathology appears bright.

What imaging regions use FLAIR most often?

Brain and spine.

Why does CSF appear dark on FLAIR?

Because the inversion time is set when CSF is at its null point, preventing transverse magnetization.

What is Echo Planar Imaging (EPI)?

The fastest MRI sequence that collects multiple lines of k-space per TR using rapid gradient switching.

What are the types of EPI sequences?

SE-EPI, GRE-EPI, and EPI-FLAIR.

What are EPI sequences used for?

Functional MRI (fMRI), cardiac imaging, and vascular studies.

What are disadvantages of EPI?

Increased acoustic noise and nerve stimulation from rapid gradient changes.

How can EPI be identified on a pulse diagram?

Constant equal and opposite gradients with an echo collected after each gradient.

Which pulse sequence produces the fastest scan times overall?

Echo Planar Imaging (EPI).

Which pulse sequence produces the most accurate T1 and T2 images?

Spin Echo (SE).

Which pulse sequence introduces the most heat to patients?

Fast Spin Echo (FSE) with high ETL.

Which pulse sequence is most prone to artifacts from dephasing?

Gradient Echo (GRE).

What determines whether a pulse sequence produces T1 or T2 contrast?

TR and TE values.

What are typical TR and TE values for T1-weighted sequences?

Short TR and short TE.

What are typical TR and TE values for T2-weighted sequences?

Long TR and long TE.

Which MRI sequence is best for stroke imaging?

Echo Planar Imaging (EPI).