Final answer:
Gradient-Echo (GRE) is a type of MRI sequence that creates signal by varying the magnetic field and utilizing the reemission of radio waves by protons. The image-weighting in an MRI is determined by the MRI sequence and timing parameters, such as echo time and repetition time.
Step-by-step explanation:
Gradient-Echo (GRE) is a type of magnetic resonance imaging (MRI) sequence that creates signal by utilizing variations in the magnetic field. In a GRE sequence, the magnetic field is varied to cause the resonant frequency of hydrogen nuclei (protons) to vary with position. Radio frequencies are then applied and absorbed by the protons, which reemit the radio waves as they return to equilibrium. The intensity of the reemitted radio waves is proportional to the concentration of the protons, as well as the chemical environment in the imaged area.
The image-weighting in an MRI is determined by several factors, including the choice of the MRI sequence and the timing parameters. Fast GRE sequences, for example, produce T1-weighted images, which provide information about the water content and fat composition of tissues. On the other hand, spin echo sequences with long echo times can produce T2-weighted images, which highlight fluid-filled regions and areas of inflammation. Additionally, contrast agents can be used to enhance the differences between tissues in the image.