Magnetic resonance imaging (MRI) techniques can usually be classified into spin-echo (SE) and gradient-echo (GRE) pulse sequences. In this presentation, the basic physical principles of GRE imaging, as well as different mechanisms to generate image contrast will be explained. Differences between SE and GRE MRI will be discussed. Additionally, the influence of different pulse sequence parameters (e.g. echo time, repetition time, flip angle; as well as spoiling techniques and preparation pulses) on the image contrast will be covered. Clinical applications of GRE imaging techniques will be shown exemplarily.
Objectives
Introduction
Magnetic resonance imaging (MRI) techniques can usually be classified into spin-echo (SE) and gradient-echo (GRE) pulse sequences. However, also hybrids such as the GRASE (gradient and spin echo) technique exist (1). The physical principles and clinical applications of GRE imaging have been reviewed in several articles (e.g. by Markl and Leupold (2) and Hargreaves (3)).
In SE pulse sequences, refocusing RF pulses are applied that lead to a re-phasing of the magnetization even in the presence of static magnetic field inhomogeneities. Refocusing RF pulses generate spin-echoes (4) and result in robust imaging techniques.
In GRE techniques, refocusing pulses are lacking. As a consequence, shorter echo (TE) and repetition times (TR) can used. Thus, 3D imaging or other applications where fast data acquisition is required are common applications of GRE imaging. In addition, the lack of refocusing RF pulses in GRE imaging leads to a lower specific absorption rate (SAR), which can be beneficial in high (B0 = 3T) and ultra-high field (B0 ≥ 7T) MRI. Disadvantages of GRE imaging are that these techniques are more prone to off-resonance artifacts. However, the latter can also be exploited for diagnostic purposes, since signal extinctions can also be caused by micro-bleedings. This effect is utilized in T2*- and susceptibility weighted imaging (5).
Signal decay in GRE imaging is usually described by the T2*-relaxation time, whereas in SE imaging, the decay of the transverse magnetization is described by the T2-time. T2* relaxation contains effects of dephasing caused by static inhomogeneities (T2’) and time-varying field fluctuations (T2-relaxation). The relationship between T2 and T2* is described by equation 1. The T2’-decay originates from temporally constant (“static”) magnetic field inhomogeneities. These inhomogeneities can be caused by local differences of the magnetic tissue properties. Dephasing caused by T2’-decay can be fully reversed in SE pulse sequences but is not compensated in GRE imaging. Thus, T2* is always shorter than T2.
$$\frac{1}{T_2^*}=\frac{1}{T_2}+\frac{1}{T_2'} (equation 1) $$
Contrast Mechanisms in GRE Imaging
Image contrast in GRE imaging can be generated by a number of mechanisms and techniques. These techniques include the adaption of the basic pulse sequence parameters TE, TR and flip angle, the application of spoiling techniques and contrast preparation.
TE, TR and flip angle: Whereas 90° excitation pulses are applied in SE pulse sequences, the flip angles in GRE pulse sequences are usually much smaller than 90°. For example, for a given TR (e.g. 50 ms) and short TE (e.g. 3 ms), image contrast can be changed from a spin-density weighted contrast to a heavily T1-weighted image by solely increasing the flip angle. In this case, small flip angles (e.g. 5°) lead to a spin-density-weighted contrast, whereas a lager flip angle (e.g. > 20°) generates a T1-weighted image.
Spoiling techniques: The term “spoiling” describes different methods (RF-spoiling or gradient spoiling) that can be applied to cancel (or “to destroy”) transverse magnetization at the end of each TR interval. A combination of gradient and RF spoiling leads to the FLASH-technique (6) (also known as spoiled-GRE (SPGR), T1-Fast-field-echo (FFE)). FLASH imaging enables pure T1 contrast and is often used for 3D contrast-enhanced MRI.
In balanced steady-state free precession (balanced SSFP) (7), no spoiling is performed and all gradients are fully rewound. In balanced SSFP, image contrast is based on T2/T1 and the signal-to-noise ratio (SNR) is usually superior to the SNR of spoiled imaging techniques. This comes at the expense of a higher sensitivity to off-resonance effects. An application of the balanced SSFP technique is rapid morphological imaging, especially of the beating heart.
Additional Modification of Image Contrast: Preparation pulses can be used to further optimize the contrast for certain applications. For example, magnetization preparation by an 180° inversion pulse followed by an delay time and a rapid 3D GRE readout is used in magnetization-prepared rapid gradient-echo imaging (3D MP-RAGE) to optimize image contrast (8). 3D MP-RAGE belongs to the most popular sequences for high-resolution T1-weighted brain MRI. Applications include voxel-based morphometry and contrast-enhanced imaging.