Intravascular BOLD signal characterization of balanced SSFP experiments in human blood at high to ultrahigh fields.

PURPOSE: To investigate the intravascular contribution to the overall balanced SSFP (bSSFP) BOLD effect in human blood at high to ultrahigh field strengths (3 T, 9.4 T, and 14.1 T). METHODS: Venous blood prepared at two different oxygenation levels (deoxygenated: Y ≈ 71%, oxygenated: Y ≈ 94%) was measured with phase-cycled bSSFP for varying TRs/flip angles at 3 T, 9.4 T, and 14.1 T. The oxygen sensitivity was analyzed by intrinsic MIRACLE (motion-insensitive rapid configuration relaxometry)-R2 estimation and passband signal differences. The intravascular BOLD-related signal change was extracted from the measured data for microvasculature and macrovasculature, and compared with the extravascular contribution obtained by Monte Carlo simulations. RESULTS: The MIRACLE-R2 values showed a characteristic increase with longer TRs in deoxygenated blood, corroborating that SE-R2 data cannot be used to assess the intravascular bSSFP BOLD effect. Passband bSSFP signal differences measured at optimal flip angles of 30° at 3 T and 20° at 9.4 T/14.1 T revealed considerable relative intravascular contributions of 95%/70% at 3 T, 74%/43% at 9.4 T, 66%/46% at 14.1 T for TR = 5 ms, and 90%/65% at 3 T, 36%/27% at 9.4 T, 13%/15% at 14.1 T for TR = 10 ms in macrovascular/microvascular regimes. CONCLUSION: The results indicate that intravascular effects have to be considered to better understand the origin of bSSFP BOLD contrast in functional MRI experiments, especially at short TRs. The MIRACLE-R2 method demonstrated the ability to quantify the apparent decrease in R2 due to rapid RF refocusing.