Graduate Echo Pulse Sequences

GRADIENT-ECHO PULSE SEQUENCES

Introduction

  • This chapter focuses on the mechanisms, uses, and parameters of common gradient-echo pulse sequences.

  • Includes advantages and disadvantages of these sequences.

  • A table (Table 4.1) compares common acronyms for gradient-echo sequences across main manufacturers.

  • Parameters depend on field strength and individual systems; suitable for most clinical imaging.

Learning Objectives

After reading this chapter, you will be able to:

  1. Explain how gradient-echo sequences differ from spin-echo.

  2. Describe how gradient-echoes are created.

  3. Analyze the steady state and its importance in gradient-echo sequences.

  4. Understand the mechanisms of common gradient-echo pulse sequences.

  5. Apply knowledge to understand the creation of images of different weighting using gradient-echo sequences.

Gradient-Echo Pulse Sequences Overview

Key Concepts

  • Gradient-echo vs. Spin-echo Differences:

    • Uses variable RF excitation pulse flip angles.

    • Utilizes gradients rather than RF pulses to rephase magnetic moments of hydrogen nuclei, enabling shorter TRs and scan times.

Definition of Terms

  • TR (Repetition Time): Time between successive RF excitations.

  • TE (Echo Time): Time from the RF pulse to the peak of the echo.

  • NMV (Net Magnetization Vector): The vector sum of the magnetic moments.

Mechanisms in Gradient-Echo Sequences

Variable Flip Angle

  • Gradient-echo sequence uses a variable RF excitation pulse.

  • Typically uses flip angles of less than 90°, causing a decrease in the time taken for full T1 recovery, thereby allowing shorter TRs, resulting in shorter scan times.

Gradient Rephasing

  • After the RF pulse is removed, an FID occurs due to magnetic field inhomogeneities (T2* decay).

  • RF pulses in spin-echo sequences can rephase magnetic moments, while gradient-echo sequences utilize gradients to achieve rephasing.

  • Gradient Spoiling: Gradients are applied to dephase moments by modifying the field strength, causing dephasing based on position along the gradient axis.

    • Spoiler Gradients: Gradients that dephase magnetic moments.

    • Rewinders: Gradients that rephase magnetic moments.

Weighting in Gradient-Echo Pulses

Mechanisms of Weighting

  1. Extrinsic Parameters: TR, TE, and flip angle affect image contrast.

  2. Steady State: A stable condition where energy input matches energy output, affecting overall image quality.

  3. Residual Transverse Magnetization: Coexists with longitudinal magnetization as transverse magnetization does not fully decay before the next excitation pulse.

Influences on Weighting

  • T1 Weighting: Short TR and larger flip angles maximize T1 contrast, minimizing T2* effects by keeping TE short.

  • T2* Weighting: Long TE to allow T2* decay, small flip angles and longer TR to permit full recovery of longitudinal magnetization.

  • Proton Density (PD) Weighting: Short TE to minimize T2* effects, long TR and small flip angle to allow recovery of longitudinal magnetization.

Learning Tools for Understanding Weighting Mechanisms

  • Analogy of adjusting heat in cooking to represent adjusting parameters affecting contrasts (e.g., increase/decrease with TR and TE).

Major Gradient-Echo Pulse Sequences

Coherent or Rewound Gradient-Echo

  • Mechanism: Uses variable flip angle, maintains coherency through rewinding phase-encoding gradient.

  • Uses: T2*-weighted images, often quick scans.

  • Parameters: Flip angle 30°–45°, TR 20–50 ms, Long TE 10–15 ms.

Incoherent or Spoiled Gradient-Echo

  • Mechanism: Starts with a variable flip angle and uses gradient to create gradient echoes that limit the effect of residual transverse magnetization.

  • Uses: Mainly T1 and PD weighting due to suppression of residual magnetization effects; suitable after gadolinium contrast enhancement.

  • Parameters: Flip angle 30°–45°, TR 20–50 ms, short TE 5–10 ms.

Reverse-Echo Gradient-Echo

  • Mechanism: Uses rewinding gradients to produce a gradient-echo with a longer effective TE for improved T2 weighting.

  • Uses: Particularly effective in the brain and joints; useful for true T2-weighting.

  • Parameters: Flip angle 30°–45°, short TR 20–50 ms, actual TE as short as possible to maximize T2 contrast.

Balanced Gradient-Echo

  • Mechanism: A balanced gradient scheme correcting for flow errors and enhancing steady state effects.

  • Uses: Imaging of the heart and vessels, spinal imaging.

  • Parameters: Variable flip angle, TR <10 ms, TE 5–10 ms.

Fast Gradient-Echo

  • Mechanism: Uses rapid acquisition techniques that allow volume to be acquired in a single breath-hold with lower TE.

  • Uses: Essential in dynamic imaging tasks where temporal resolution is crucial, especially after contrast administration.

Echo Planar Imaging (EPI)

  • Mechanism: Rapid acquisition technique encompassing multiple RF pulses followed by series of gradient echoes, achievable through gradient-echo or spin-echo methods.

  • Uses: Functional MR imaging including dynamics of perfusion and blood oxygenation.

Conclusion

  • Recognizing the differences amongst gradient-echo pulse sequences is vital for clinical applications and tailoring imaging protocols for optimal results.

  • Selection of appropriate extrinsic parameters influences image quality and contrast, aiding in diagnosis.