What is the purpose of applying two equal gradient pulses in diffusion-weighted imaging?

The purpose of applying two equal gradient pulses in diffusion-weighted imaging is to differentiate the diffusion rates of protons. This technique relies on measuring how much water molecules in the tissue can move, or diffuse, within a certain time frame.

When the two gradient pulses are applied, the first pulse dephases the spins of the protons, and the second pulse rephases them. If the protons have moved during the time between the two pulses, their phase will not fully realign, resulting in a decrease in signal intensity. This decrease is quantified and reflects how freely water can move within the tissue, allowing for the assessment of diffusion rates.

Different tissues and pathology will exhibit varying diffusion characteristics, which is essential for identifying conditions like strokes, tumors, or demyelination. Therefore, this method allows for enhanced contrast in images depending on tissue type and its microenvironment, making it crucial for diagnosis and analysis in MRI exams.