What is a gradient recalled echo pulse sequence?
A GRE pulse sequence begins with a <90-degree RF pulse, followed by application of a magnetic field gradient, leading to formation of an MRI signal called a gradient recalled echo. The pulse sequence is then repeated until enough gradient recalled echoes have been acquired to create MR images. The TR and TE are short, leading to rapid acquisition times. As such, this pulse sequence is commonly used to obtain precontrast and postcontrast T1-weighted images.
In addition, in-phase T1-weighted GRE images (where signals from fat and water protons in the same voxel add to each other) are used to detect iron deposition, metal, and gas in tissues as manifested by a loss of signal intensity relative to out-of-phase T1-weighted images, whereas out-of-phase T1-weighted GRE images (where signals from fat and water protons in the same voxel cancel each other) are used to detect presence of microscopic lipid within tissues as manifested by a loss of signal intensity relative to in-phase T1-weighted images.