Proton Density (PD) MRI

When an MRI sequence is set to produce a PD-weighted image, the tissues with a higher concentration or density of protons (hydrogen atoms) generate the strongest signals and appear the brightest in the image. The proton density-weighted sequence produces contrast mainly by minimizing the impact of T1 and T2 differences, using a long TR (2000-5000 ms) and a short TE (10-20 ms).

Proton Density MRI image appearance

The easiest way to identify PD-weighted images is to compare the fluid signal against the fat signal. Fluids typically appear as a grayish-white color, with an appearance almost similar to the fat in the body.

pathology appearance on PD images

Various pathological conditions can display distinct appearances in Proton Density (PD) MRI images due to differences in tissue composition, water content, and proton density. Here are some examples of how different pathologies might manifest on PD images:

  • Edema:

    • Areas of edema, characterized by increased fluid accumulation in tissues, may exhibit brighter appearances on PD images due to higher water content. This heightened signal can indicate inflammation or tissue injury.
  • Cysts:

    • Cysts, fluid-filled sacs, can appear as bright or light grey areas in PD images, owing to their fluid content.
  • Tumors:

    • Solid tumors can showcase varying signal intensities on PD images contingent on their composition. Tumors with a substantial cellular component might appear with intermediate signal intensity, while areas of necrosis or hemorrhage might present differently.
  • Inflammatory Conditions:

    • Inflammatory processes can lead to augmented water content and altered tissue properties. Changes related to inflammation might yield brighter signal intensities on PD images.
  • Degenerative Changes:

    • Degenerated tissues, such as cartilage in osteoarthritic joints, might display altered signal intensities on PD images due to shifts in tissue composition and water content.
  • Tendon and Ligament Pathologies:

    • Conditions affecting tendons and ligaments, like tears or inflammation, can induce shifts in tissue properties. These alterations might be discernible through modified signal intensities on PD images.
  • Synovial Pathologies:

    • Disorders impacting synovial tissue within joints, such as synovitis, could lead to changes in water content and inflammation. Such changes might culminate in signal intensity variations on PD images.
  • Bone Marrow Changes:

    • Pathological conditions in bone marrow, such as marrow infiltration due to cancer, might impact proton density and water content, resulting in signal intensity alterations.
  • Nerve Pathologies:

    • Nerve-related disorders, including nerve compression or inflammation, may induce shifts in water content and tissue properties, potentially appearing bright on PD images.

Tissues and their PD appearance

Bone marrow: – equal to or higher than that of muscle (fatty marrow is usually bright)
Fat – bright (slightly darker than the fat signal in T1 images)
Fluids – bright (darker than the fluid signal in T2 images)
White matter – darker than bright gray
Gray matter – bright gray
Moving blood- dark
Bone – dark
Air – dark


PD sagittal sequence used in knee imaging

PD sagittal image of Knee

PD coronal sequence used in knee imaging

PD coronal image of Knee