Gradient Echo Pulse Sequences Review

Vocabulary flashcards covering key terms and definitions related to Gradient Echo Pulse Sequences from the lecture notes.

Gradient Echo Pulse Sequences

MRI sequences that use variable RF excitation pulse flip angles and gradients to rephase magnetic moments, resulting in shorter TRs and sequence duration compared to Spin Echo.

Gradient Dephasing

The process where coherent Net Magnetization Vectors (NMV) alter their precession frequency due to an applied gradient, causing some to speed up and some to slow down.

Spoilers (gradients)

Gradients applied to dephase coherent Net Magnetization Vectors.

Gradient Spoiling

The process of dephasing coherent Net Magnetization Vectors using applied gradients.

Gradient Rephasing

The process of applying a gradient of opposite polarity to a dephased system of Net Magnetization Vectors, causing them to speed up or slow down until they are rephased and induce a signal.

Gradient Echo (signal)

The signal induced when Net Magnetization Vectors are rephased by an applied gradient.

Rewinders

Gradients applied to rephase Net Magnetization Vectors.

Weighting (in gradient-echo sequences)

The process in gradient-echo pulse sequences affected by extrinsic parameters (TR, TE, flip angle), steady state, and residual transverse magnetization.

T2* contrast

Contrast in gradient echo sequences controlled by the TE, reflecting T2 decay along with magnetic field inhomogeneities (which are not compensated for).

T1 weighting (Gradient Echo)

A weighting strategy in gradient echo sequences where differences in T1 recovery times of tissues are maximized, and differences in T2* decay times are minimized.

T2* weighting (Gradient Echo)

A weighting strategy in gradient echo sequences where differences in T2* decay times of tissues are maximized, and differences in T1 decay times are minimized.

PD weighting (Gradient Echo)

A weighting strategy in gradient echo sequences where differences in both T1 and T2* processes are minimized.

Steady State (MRI)

A condition achieved in MRI sequences when the Repetition Time (TR) is shorter than the T1 and T2 relaxation times of tissues, leading to the co-existence of longitudinal and transverse magnetization after a few successive RF pulses.

Residual Transverse Magnetization

The transverse component of magnetization that does not decay completely between successive TRs in a pulse sequence and builds up, enhancing T2 contrast.

Stimulated Echo (in GE sequences)

An echo produced by any two RF pulses, where the second RF pulse re-phases the FID and any residual magnetization present. GE sequences contain data from FIDs and stimulated echoes.

Coherent Gradient Echo

A gradient echo sequence that uses a variable flip angle excitation and a gradient re-phasing pulse, maintains a steady state, and keeps residual magnetization coherent by rewinding the phase encoding gradient after readout.

Gradient Moment Rephasing

A technique used in Coherent Gradient Echo sequences to accentuate T2* weighting and reduce flow artifacts.

Coherent Gradient Echo uses

Used to produce rapid T2* weighted images, acquire angiograms, myelograms, or arthrograms, determine vessel patency, and for single breath-hold sequences.

Incoherent Gradient Echo

A gradient echo sequence where only the transverse magnetization from the previous excitation is used, and residual transverse magnetization is spoiled (either by RF spoiling or gradient spoiling) to enable T1 contrast to dominate.

RF Spoiling

A method of spoiling transverse magnetization in incoherent gradient echo sequences where the receiver coil only digitizes frequencies from echoes just created by the excitation pulse.

Gradient Spoiling (Incoherent GE)

A method of spoiling transverse magnetization in incoherent gradient echo sequences where slice select, phase encoding, and frequency encoding gradients de-phase residual magnetization to reduce T2* or T2 effects.

Steady State Free Precession (SSFP)

A gradient echo sequence designed to achieve more true T2 weighting by digitizing only stimulated echoes and repositioning them, as opposed to conventional GE where TE is often too short for T2 measurement.

Actual TE (SSFP)

In Steady State Free Precession (SSFP) sequences, the time between the echo and the next excitation pulse.

Effective TE (SSFP)

In Steady State Free Precession (SSFP) sequences, the time from the echo to the excitation pulse that created its FID, calculated as (2 × TR) − Actual TE.

Steady State Free Precession uses

Used to acquire images demonstrating true T2 weighting, particularly for brain and joints with both 2D and 3D volumetric acquisitions.

Balanced Gradient Echo

A modified coherent gradient echo sequence using a balanced gradient system to correct for phase errors in flowing blood and CSF, and an alternating RF excitation scheme to enhance steady state effects, resulting in higher SNR, CNR, and fewer flow artifacts.

Balanced Gradient Echo uses

Initially developed for heart and great vessels imaging, now mainly used in spinal imaging (especially cervical spine and internal auditory meatus) due to reduced CSF flow artifacts.

Fast Gradient Echo

Very fast pulse sequences that acquire volume in a single breath-hold by employing coherent or incoherent gradient echo sequences with significantly reduced TE and TR, reading only a portion of the RF pulse and echo (partial echo), and using extra pulses for pre-magnetization.

Partial Echo (Fast Gradient Echo)

A technique in Fast Gradient Echo sequences where only a portion of the RF pulse and a proportion of the echo is read, contributing to shorter scan times.

Pre-magnetization (Fast Gradient Echo)

The process of applying extra RF pulses (e.g., 180° or 90°/180°/90°) before a Fast Gradient Echo pulse sequence begins to enhance T1 contrast, null signal, or produce T2 contrast by driving coherent transverse magnetization into the longitudinal plane.

Driven Equilibrium (DRIVE)

A pre-magnetization technique in Fast Gradient Echo sequences using 90°/180°/90° pulses to produce T2 contrast by creating transverse magnetization, re-phasing it, and then driving the coherent transverse magnetization into the longitudinal plane.

Echo Planar Imaging (EPI)

The fastest Gradient Echo sequence, where all lines of K-space are filled from a single echo train within one TR, requiring rapid switching of readout and phase encoding gradients.

Echo Planar Imaging Spin Echo (EPI SE)

An Echo Planar Imaging sequence that uses a 90° excitation pulse followed by a 180° re-phasing pulse, and then an EPI readout of gradient echoes.

Echo Planar Imaging uses

Used to freeze motion, visualize physiological processes such as perfusion and blood oxygenation (BOLD), and image the heart and coronary vessels.

GRASE (GRadient And Spin Echo)

A pulse sequence that is an effective compromise between Spin Echo (longer scan times, better image quality, higher RF deposition) and Gradient Echo, combining aspects of both.