The purpose of this work is to analyze BOLD-induced signal changes in bSSFP acquisitions at 9.4T, and compare these results to conventional GE-EPI. The results are based on Monte Carlo simulations and measurements from micro spheres mimicking a neurovascular network.Up to date fMRI using GE-EPI sequences have been the primary imaging tool to measure the neurovascular hemodynamic changes associated with neuronal activity¹. The sensitivity of GE-EPI and SE-EPI has been analyzed in detail using Monte Carlo simulations and measurements on appropriate phantoms, and in animals and humans^2,3,4. In a recent paper the high quality and stability of BOLD imaging using bSSFP at 9.4T has been demonstrated⁵. Figure 1 shows a representative example of these measurements for bSSFP, GE-EPI and SE-EPI with corresponding signal change time courses. In this abstract we analyze the sensitivity of bSSFP to oxygenation changes, and compare the results to GE-EPI and SE-EPI. Our analysis is based on Monte Carlo simulations using spheres with different diameter and concentration as a model for the neurovascular network. In addition, we performed measurements on a phantom with micro spheres applying bSSFP, GE-EPI and SE-EPI. Furthermore, we have analyzed the influence of the repetition time and flip angle of bSSFP on BOLD-induced signal changes.We created a 3D 2-compartment model filled with a volume fraction of 2%,3% and 4% of random position of spheres. A susceptibility map correlated with the model was calculated with the FPM model⁶ using the main magnetic field, susceptibility differences between compartments, oxygenation and hematocrit as parameters. Diffusion was simulated using 10000 random walkers and the phase gain was recorded in each time step of 20 ms. GRE, SE and bSSFP were simulated. We build a spherical phantom containing of 19 test tubes of 1 cm inner diameter. Samples contain Dy-DTPA with concentrations of 3 and 5 mM, precision polystyrene microspheres and a reference sample. The phantom was placed in the isocenter of a custom-built head coil⁷ (16 transmit / 31 receive channels). Several experiments with different parameters were performed (bSSFP: TR = 5ms, FA = 5, 10, 15, 20, 25, 30 deg; TR = 4, 6, 8 ms, FA = 15 deg; GRE EPI: TE= 10, 20, 30 ms). The sequence used for human brain imaging at 9.4T shown in Fig. 1 are: bSSFP: TR/TE = 5/2.5ms, FA = 15 deg; GE-EPI: TE = 18 ms, SE-EPI: TE = 36 ms, 1 mm isotropic resolution for all measurements.For bSSFP, TR and flip angles have been varied between 2-8 ms and 10-70 degrees, respectively. Simulated and measured signal changes are on-resonance, i.e. at the center of the passband of bSSFP. Figure 2 shows simulated signal changes for different flip angles as a function of the sphere size for a) TR= 2 ms and b) TR = 8 ms. Gradient echo and spin echo simulations are shown in Fig. 2 c) and d), respectively, for different echo times and as a function of sphere size. Both bSSFP and gradient echo signal changes are of similar range. Figure 3 shows results obtained from the measurements with micro spheres in Dy-DTPA solution for bSSFP and gradient echo for different flip angles and echo times as a function of sphere size. These measurements confirm the comparable sensitivity of bSSFP and GRE to microscopic susceptibility at 9.4TOur simulations and measurements on spheres dissolved in Dy-DTPA solution confirm the comparable sensitivity of bSSFP and GRE to BOLD-related effects for sphere (vessel) sizes around 1-3 mm. For sphere sized above 5 mm GRE shows a much stronger sensitivity to BOLD than bSSFP. Therefore, the about three-fold higher signal change for GGE-EPI compared to bSSFP in human data as shown in Fig. 1 can be explained by the massive contribution of larger vessels. We thus conclude that in the range of small vessels bSSFP is equally sensitive to GRE, with the advantage to be more sensitive to this small vessel regime. Therefore, using bSSFP to detect BOLD changes offers similar sensitivity as GRE but in addition is more selective to the small vasculature, which is probably closer to the neuronal event. The simulated and measured results shown in Fig. 2 and 3 also allow for a further improvement of the sensitivity of bSSFP to the BOLD effect, i.e. to go for high flip angles and longer TR. A significant advantage of bSSFP over spin echoes (both are sensitive to the smaller vessels) is the much lower SAR demand of bSSFP compared to spin echoes. Thus, whole brain coverage with bSSFP is easily possible with bSSFP at 9.4T.