Final answer:
To emphasize T2 contrast in MRI, use a long TR and long TE, while PD contrast is best achieved with a long TR and short TE.
Step-by-step explanation:
In Magnetic Resonance Imaging (MRI), the parameters Time of Repetition (TR) and Time to Echo (TE) are critical for emphasizing T2 and Proton Density (PD) contrast. To enhance T2 contrast, one would use a long TR with a long TE, as T2 relaxation reflects the rate at which spinning protons lose phase coherence due to interactions among themselves after the external magnetic field is turned off.
Conversely, to emphasize PD contrast, a long TR with a short TE is used because it allows for the measurement of the signal that is directly proportional to the number of protons in the tissue, independent of the T1 or T2 decay processes.