Proton density weighted spin-echo images
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Proton density (PD) weighted images are related to the number of nuclei in the area being imaged (number of hydrogen protons), as opposed to the magnetic characteristics of the hydrogen nuclei. They are produced from the first echo. Proton density weighted images result when the contribution of both T1 and T2 contrast is minimized. They have a long TR (2000+ms) to minimize T1 differences because all tissues exhibit full longitudinal relaxation before the next 90 degrees RF pulse. They have a short TE (TE1, 20ms) to minimize T2 differences. Higher proton density tissues appear brighter (CSF > fat > gray matter > white matter).
In musculoskeletal imaging, TR is more than 1000 msec and TE is less than 30 msec. It provides good anatomic detail but little overall tissue contrast. In the cervical spine, cerebrospinal fluid (CSF) has slightly higher intensity than intervertebral discs in proton density images 2.