Assessment of muscle oxygenation with balanced SSFP: a quantitative signal analysis

PURPOSE: To investigate the feasibility of balanced steady-state free precession (b-SSFP) for blood oxygenation level-dependent (BOLD) MRI during a short-term ischemia/reactive hyperemia (RH) experiment on human calf muscles. MATERIALS AND METHODS: To investigate contributions to the b-SSFP signal during an RH experiment, the relaxation times T(1), T(2), and T(2) (*) were quantified in an interleaved fashion. Data from soleus, gastrocnemius, and tibialis muscle groups of five healthy subjects were evaluated. RESULTS: During ischemia a decreased b-SSFP signal amplitude as well as a decrease in T(2), T(2) (*), and the initial intensity I(0) was observed. RH provoked an overshoot of T(2), T(2) (*), and the b-SSFP signal. No paradigm-related changes in T(1) were observed. Comparing the evolution of transverse relaxation times, initial intensity, and b-SSFP signal amplitude, we concluded that the measured b-SSFP signal in muscle tissue is not only determined by T(2) variations but also significantly influenced by I(0) changes. These I(0) changes are attributed to spin density variations since inflow effects were suppressed by saturation bands. CONCLUSION: b-SSFP signal changes during a RH paradigm cannot unambiguously be assigned to oxygenation changes. Therefore, care has to be taken with their interpretation.