Magnetic Resonance Imaging (MRI) Practice Test

Contrast in diffusion-weighted imaging (DWI) depends on what?

1. The number of phase encodings

2. The degree to which molecular mobility is impaired

3. The TR time selected

4. The type of receiver coil used

The correct answer is: The degree to which molecular mobility is impaired

In diffusion-weighted imaging (DWI), the contrast is primarily influenced by the degree to which molecular mobility is impaired. DWI is a specific type of MRI that measures the diffusion of water molecules within tissue. When cellular structures restrict the movement of water molecules, a higher signal is obtained, indicating areas of reduced diffusion. This is particularly useful in assessing conditions such as stroke or tumors, where cellularity is altered. In contrast to options that involve technical parameters or hardware components, the essence of DWI lies in the biological phenomenon of diffusion—how freely or restricted the water molecules are moving in the tissue being examined. Regions where the mobility of water is diminished will appear bright (high signal intensity) on DWI, while areas where diffusion is unrestricted will have a darker appearance. Therefore, understanding the relationship between tissue characteristics and water diffusion is crucial for interpreting DWI images effectively.