In a PD weighted image, what must be the appropriate settings for TE and TR?

In a Proton Density (PD) weighted image, the primary goal is to emphasize the differences in the density of protons within the tissues, which is influenced by the amount of water present in different materials. To achieve this PD weighting, it's essential to have a short echo time (TE) and a long repetition time (TR).

Using a short TE allows for the capture of the signal before significant T2 decay occurs. This means that the image can effectively reflect the density of protons rather than the relaxation properties associated with T2. Conversely, a long TR helps to ensure that the protons in the tissues have sufficient time to fully relax back to equilibrium between successive excitations, maximizing the amount of signal retrieved and making the differences in proton density more pronounced on the resultant image.

Thus, having a short TE keeps the signal strong and primarily proton-density weighted, while a long TR allows for better signal intensity by giving the tissue sufficient time to recover, ensuring that the imaging reflects primarily the density of protons rather than other relaxation times.