physics 2 week 2

Pulse Sequence

An MRI pulse sequence is the specific pattern of RF pulses, gradient pulses, and timing parameters (TR, TE, etc.) used to create an MRI image.

Study Protocol

A complete MRI exam made up of multiple pulse sequences performed in a specific order.

Difference Between Pulse Sequence and Study Protocol

A pulse sequence creates one image type, while a study protocol is the collection of pulse sequences used during an MRI exam.

Basic Spin Echo (SE) Sequence

A Spin Echo sequence uses one 90° excitation pulse followed by one 180° refocusing pulse to produce one spin echo.

90° RF Pulse

Excites hydrogen nuclei by tipping longitudinal magnetization into the transverse plane.

180° RF Pulse

Refocuses dephasing spins to create a spin echo and reduce magnetic field inhomogeneity effects.

TR (Repetition Time)

The time between successive 90° excitation pulses. Short TR produces more T1 weighting.

TE (Echo Time)

The time from the 90° pulse to the center of the echo. Long TE produces more T2 weighting.

ETL (Echo Train Length)

The number of echoes collected after one excitation pulse during a Fast Spin Echo sequence. (Echos are 180% refocusing pulses)

Image Contrast- is mainly controlled by what?

Image contrast is primarily controlled by TR, TE, and Effective TE.

Spin Echo (SE)

Uses one 90° pulse and one 180° refocusing pulse. One phase-encoding line is filled per TR.

Fast Spin Echo (FSE/TSE)

Uses multiple 180° refocusing pulses to collect multiple echoes during one TR, greatly reducing scan time.

Turbo Spin Echo (TSE)

Another name for Fast Spin Echo (FSE).

Single Shot Fast Spin Echo (SSFSE)

Fills all k-space lines for one slice in a single TR using a very long ETL.

Driven Equilibrium (DRIVE)

Applies a reverse flip pulse after the echo train to restore longitudinal magnetization and increase fluid signal with shorter TRs.

How ETL Affects Scan Time

Increasing ETL decreases scan time because more phase-encoding lines are filled during each TR.

Scan Time Formula (SE)

Scan Time = TR × NEX × Phase Matrix.

Scan Time Formula (FSE)

Scan Time = (TR × NEX × Phase Matrix) ÷ ETL.

How ETL Affects Slice Count

Increasing ETL decreases the number of slices that can be acquired in one TR.

Advantages of FSE

Much faster scan times, especially for T2-weighted imaging. Time savings can be traded for higher resolution or increased NEX.

Disadvantages of FSE

Higher SAR, increased image blurring with long ETLs, and brighter fat on T2-weighted images.

SAR in FSE

SAR increases because multiple 180° refocusing pulses deposit more RF energy into the patient.

Image Blur in FSE

Long ETLs produce image blur because later echoes have lower SNR and different contrast.

Fat Signal in FSE

Repeated 180° pulses reduce J-coupling, causing fat to remain brighter on T2 FSE images.

Effective TE

The echo placed in the center of k-space that determines the image contrast.

Center of K-Space

Contains low spatial frequency information that determines image contrast and overall brightness.

Outer K-Space

Contains high spatial frequency information responsible for edge detail and spatial resolution.

Shallow Phase-Encoding Gradients

Fill the center of k-space and determine tissue contrast.

Steep Phase-Encoding Gradients

Fill the outer k-space and determine spatial resolution.

Phase Encoding

Each phase-encoding step fills one line of k-space.

Increasing TR

Produces longer scan times and decreases T1 weighting.

Decreasing TR

Produces more T1 weighting and shorter scan times.

Increasing TE

Produces greater T2 weighting.

Decreasing TE

Produces less T2 weighting.

Increasing NEX

Increases SNR but also increases scan time.

Increasing Phase Matrix

Increases spatial resolution but decreases SNR and increases scan time.

Increasing FOV

Increases SNR but decreases spatial resolution.

Decreasing FOV

Improves spatial resolution but decreases SNR.

Increasing Slice Thickness

Increases SNR but decreases spatial resolution.

Decreasing Slice Thickness

Improves spatial resolution but decreases SNR.

Increasing Bandwidth

Decreases SNR but shortens sampling time and echo spacing.

Decreasing Bandwidth

Increases SNR but lengthens sampling time.

Partial Fourier

Collects slightly more than half of k-space and mathematically reconstructs the remaining data to reduce scan time.

Why Partial Fourier Works

It uses k-space symmetry to estimate the missing data.

Fat Suppression in FSE

Fat suppression improves contrast because fat appears relatively bright on T2 FSE images.

Chemical Fat Saturation

Uses the chemical shift difference between fat and water to selectively suppress fat signal.

Inversion Recovery (IR)

Begins with a 180° inversion pulse before the Spin Echo sequence to suppress specific tissues.

STIR

Short Tau Inversion Recovery suppresses fat using a short inversion time (TI).

FLAIR

Fluid Attenuated Inversion Recovery suppresses CSF using a long inversion time (TI).

Why FSE is Used with STIR and FLAIR

FSE shortens the long scan times associated with inversion recovery sequences.

Short TR

Produces greater T1 weighting.

Long TR

Produces less T1 weighting and allows more T2 or PD weighting.

Short TE

Produces less T2 weighting.

Long TE

Produces greater T2 weighting.

Increasing ETL

Improves scan efficiency by collecting more echoes per TR but increases SAR and image blur.

FSE vs SSFSE

FSE fills k-space over multiple TR periods, while SSFSE fills the entire k-space for one slice in a single TR.

FSE vs Driven Equilibrium

FSE reduces scan time using multiple refocusing pulses, while Driven Equilibrium restores longitudinal magnetization after the echo train to improve fluid signal.

SSFSE vs Driven Equilibrium

SSFSE acquires an entire image in one TR, while Driven Equilibrium improves fluid brightness by restoring longitudinal magnetization.