Proton density (PD) measurements indicate the concentration of MR-visible water protons contributing to the MRI signal. As spins in water protons produce the signal, PD underlies any MRI measurement.
In the human brain, PD measurement can be interpreted as water content, a basic biophysical property that relates to the neuronal tissue well-being. It has been shown that maturation and ageing involve changes in brain water content. In addition, several neurological disorders (multiple sclerosis, brain tumours, stroke, etc.) exhibit increase in water content due to inflammation or oedema.
PD mapping is based on the estimation of the equilibrium magnetisation (M0) through T2* extrapolation, variable flip angle or multi-compartment T2 methods. It requires removal of signal biases such as receive coil sensitivity and calibration to external or internal water reference.
As water content governs MR signal intensity, its influence is always implicitly present. In this sense, all MR images have intrinsic PD weighting. Thus, any biophysical modelling of MR parameters (MT, T1, T2, diffusion, etc.) will benefit from knowledge of the water content generating the signal.
This chapter introduces the biophysical interpretation of PD and outlines the different methods for measuring PD. It then describes the applications of PD mapping and its relevance to other MR measurements.