What is the optimal flip angle range for PD weighting in GRE sequences?

The optimal flip angle range for proton density (PD) weighting in gradient echo (GRE) sequences is within the range of 5-20 degrees. This lower flip angle is effective in maximizing the contribution of the proton density signal while minimizing T1 effects, which can be significant at higher flip angles. When using a lower flip angle, the images produced highlight the density of protons in the tissue being imaged, allowing for enhanced contrast based primarily on the number of hydrogen nuclei in the area of interest. This is particularly useful in applications where distinguishing tissue types based on their water content is necessary, such as in musculoskeletal imaging or assessing lesions in the body.

Higher flip angles tend to increase susceptibility to T1 effects, which can alter image contrast in ways that might not be desired for PD-weighted sequences. Therefore, maintaining flip angles in the 5-20 degree range ensures the emphasis is placed on the inherent density of protons, making it the correct choice for achieving optimal fat and fluid contrast in PD-weighted imaging.