At the present of microbubbles (MBs), focused ultrasound (FUS) can locally induce inertial cavitation (IC) effect and cause turbulence of fluid. . Previous studies associated BBB opening with duty cycle and pulse repetitive frequency (PRF) [1,2]. In this study, a fast low-angle shot (FLASH) sequence was performed for in vitro experiments to observe MR signal changes during FUS transmission with various conditions of duty cycle and PRF. In addition, a preliminary test of rat experiments was conducted to demonstrate the feasibility of this technique for in vivo experiments.A single-element focused piezoelectric transducer (central frequency=1.85 MHz, diameter=10 cm, curvature=12.5 cm, Imasonic, Besancon, France) transmitted 1.5 and 0.8 MPa FUS pulses to in vitro agarose phantom with chamber diameter=6 mm. The experimental setup was shown in Fig. 1a. The MBs solutions (lipid shell with C3F8, mean diameter=1.25 µm, diluted to 0001X) [3] were injected into the phantom. The FLASH sequence (TR/TE=8/3.61 ms, pixel size=1.56x1.56x3 mm3, flip angle=20°, temporal resolution=0.8 s) was performed in a 3.0 Tesla MR scanner (Tim Trio, Siemens, Erlangen, Germany). All images were acquired at the focal plane and were perpendicular to the direction of ultrasound beams. The FUS transmitted pulses for continuous 94.4 s. The regions-of-interest (ROIs), approximately 12 pixels, were selected around the focal point for evaluating signal intensity (SI) changes (Fig. 1a). The SI within ROI was normalized to mean SI of pre-FUS: normalized SI = (SI/SI_pre-FUS)x100%. The minimal SI (Min. SI) and the full width at half minimum (FWHM) of the duration of reduced SI (FUS-late) were together to characterize the time course of each exam. Images of in vivo experiment were prescribed at central sinus of a healthy male Sprague-Dawley rat (300g), as the dotted line in Fig. 1b. The ROI was selected on the FUS focus (Fig. 1c). The FLASH parameters were as follows: pixel size=0.52x0.52x1.5 mm3, flip angle=15°, NEX=2, temporal resolution=4s. 0.02 mL solution of MBs and gadolinium contrast agent were infused into rat via carotid artery 20 s before sonication. FUS pulses (0.8MPa, PRF=1Hz, duty cycle=1%) were transmitted for continuous 93 s.Five specific statuses during the time courses of normalized SI and standard deviation (SD) were indicated as: (I) Pre-FUS, (II) Flow-related enhancement (FRE) at the beginning of FUS transmission, (III) minimal SI, (IV) latter phases of FUS transmission (FUS-late), (V) Post-FUS. In Fig. 2, at Pre- and Post-FUS, normalized SI were approximate 100%. As turning on 1.5 MPa FUS transmission, SI first increased to an extremely high value due to the FRE effect and then dropped to approximate 80-85%. With PRF=1 Hz (Fig. 2a), exams with higher duty cycle exhibited lower minimal SI: 85.7±0.6 %, 81.9±1.3 %, and 80.2±1.1 % for 2%, 5%, and 10% of duty cycle, respectively. With PRF 100 Hz (Fig. 2b), higher duty cycles displayed larger FWHM: 7.4, 8.0±0.1, and 13.2±0.1 s for 2%, 5% and 10% of duty cycle, respectively. The corresponding calculation of SD of SI within ROI were shown in Figs. 2(c,d). Figure 3 showed the mean normalized SI with conditions of 0.001X MBs, 0.8 MPa, PRF 1 Hz, 1% or 0.5% duty cycle. With applying such conditions, large FWHM of 62.23±12.7% and 87.35±2.9% s as well as distinct minimal SI of 87.54±0.3% and 87.65±2.0% were observable in 1% and 0.5% duty cycle. Regarding in vivo experiment, Fig. 4a exhibit two SI drop in normalized SI.In this study, we adopted fundamental experiments of using FLASH to monitor SI changes at the present of IC effect for in vitro gel phantom and in vivo rat experiments. Upon transmitting FUS pulses on MBs, FRE effect can be attributed to the effect of fresh spins flowing into imaging slice and produced increased SI. After that, IC effect induced disturbing flow and reduced SI as well as increased SD were exhibited to reflect the effect of intravoxel dephasing and chaotic flowing effect [4]. In Fig. 2, larger PRF led to shorter duration of SI drop, illustrating the fast disruption of MBs while transmitting FUS with higher PRF. Moreover, higher duty cycle resulted in more distinct SI changes and higher SD, suggesting the more chaotic flowing effect. For in vivo experiment, we observed two reduced SI peaks with an interval approximately equaled to the period of systemic circulation in rats of 20 s, presenting the recirculation of MBs and the two IC events. In conclusion, FLASH has been proved to be a useful technique for real-time monitoring of IC when transmitting FUS pulses to MBs for in vitro and in vivo experiments.