PD weighted image
The proton density (PD) weighted image visualizes the number of protons per volume. To achieve this, both the T1 and T2 components are switched off as fully as possible (for the sake of convenience, technical background is not discussed in this module). Tissues with few protons have low signal intensity, tissues with many protons have high signal intensity. Fat has a relatively high signal intensity, however, not as high as in a T1 weighted image. Fluid has an intermediate signal intensity rather than the high signal intensity as in a T2 weighted image. A PD weighted image is used among other things to evaluate meniscal tears in the knee (fig. 17).
Additionally, a PD sequence can be useful in e.g. brain MRI to evaluate gray/white matter pathology. Explanation: as opposed to a T2 weighted image, a PD clearly distinguishes between gray and white matter (gray matter has a higher signal intensity than white matter). The distinction between CSF and pathology is difficult on a T2 weighted image; both have a high signal. The contrast between CSF (intermediate signal intensity) and pathology (high signal intensity) will be better on a PD weighted image.
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Figure 17. PD weighted image in sagittal direction of the knee in two different patients (at the level of the medial meniscus). Left shows an intact meniscus, at right there is a tear in the posterior horn of the medial meniscus. Note also that fluid (hydrops & Baker’s cyst) have intermediate signal intensity on the PD.
