On a T2-weighted MRI sequence, free fluid (such as CSF or urine in the bladder) appears as bright (high signal intensity), while fat appears intermediate, and rapidly flowing blood appears dark (signal void). What property of free fluid accounts for its high T2 signal?

• A Free fluid has a short T1 relaxation time resulting in rapid longitudinal magnetisation recovery
• B Free fluid has a long T2 relaxation time due to unbound water molecules with high molecular mobility ✓
• C Free fluid produces Compton scatter interactions that increase signal
• D Free fluid contains paramagnetic ions that shorten the T2 relaxation time

Explanation

T2 relaxation (transverse relaxation) reflects the rate at which spinning protons lose phase coherence after the RF pulse. In free fluids, water molecules are unbound and move rapidly and freely (high molecular mobility, low viscosity), resulting in very slow loss of phase coherence and therefore a long T2 relaxation time. On T2-weighted sequences, tissues with long T2 values retain transverse magnetisation longer and produce high signal intensity. In contrast, solid tissues with restricted molecular motion have shorter T2 values and appear darker. Fat has intermediate T2 signal; paramagnetic substances such as gadolinium or haemosiderin actually shorten T2.

Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.