On MRI, the T2* (T2-star) effect differs from T2 relaxation in that T2* decay also includes signal loss due to:
- A Spin-spin interactions between neighboring nuclei
- B Local magnetic field inhomogeneities from tissue susceptibility differences ✓
- C Longitudinal relaxation of the magnetization vector
- D RF pulse refocusing of spin dephasing
Correct answer: B. Local magnetic field inhomogeneities from tissue susceptibility differences
Explanation
T2* decay = T2 (true spin-spin relaxation due to spin-spin interactions) + additional dephasing due to local magnetic field inhomogeneities caused by differences in magnetic susceptibility between tissues (air-tissue interfaces, iron deposits, hemosiderin, deoxyhemoglobin, calcification). T2* is measured with gradient-echo sequences which do not use 180° refocusing pulses. A spin-echo sequence with a 180° refocusing pulse corrects for T2* effects, measuring only T2. T2* is the basis for fMRI BOLD contrast and susceptibility-weighted imaging.
Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.
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