To obtain reliable in vivo human head images with the simultaneous transmit and receive system using SWIFT (SWeep Imaging with Fourier Transformation) in continuous mode 1,2.The development of methods to allow simultaneous RF excitation and signal acquisition has potential to significantly benefit MR imaging, particularly for reducing RF power requirements and for imaging fast relaxing spins. Ideally such capability would provide an optimal usage of RF transmitter peak power (lowest peak power and low SAR3) and optimal signal sampling (high SNR). However, the practical realization of such a system is challenging due to a large RF power difference between Tx and Rx channels working at the same frequency and subsequently the difficulties to obtain the necessary Tx/Rx isolation. The isolation required to avoid saturation of the receiver can be achieved by compensation of leakage signal by slightly mismatched RF coils1,2 or by destructive interference decoupling using the extra Tx coil4. Both methods allowed the acquisition of ex vivo images. However, in vivo applications using these methods have not been reported because the methods are sensitive to variation of the coil loading conditions during MRI examinations of living subjects. Here, we propose the simultaneous Tx/Rx system with highly minimized effects from variation of coil loading, which allowed us to acquire the first in vivo images with continuous SWIFT at 4T. The proposed system applied general techniques used in wireless communication (Fig.1) to cancel out the mutual coupled Tx leakage signal5-7. The system uses sampled and manipulated Tx signal (SC(G,P)) to compensate the leakage (Sleak) in Rx signal8. The critical part in our system is the RF power sampling block, which extracts the signal Sc being proportional to S*Tx signal. Thus, variation in loading conditions affecting the S*Tx signal is reflected also in compensating signal, and therefore, the system minimizes the effect from the variations of coil loading. The gain and phase of the sampled Tx signal are optimized in the coil-independent active module to maximize the Tx/Rx isolation without deformation of the quality of the coil. Considering the above-mentioned aspects, the passive (RF power sampling) and active feedback module have been designed and fabricated as shown in fig.2. The passive module uses two couplers with the high directivity and some passive components to adjust proportionality to the reflected signal9. The active module consists of a power combiner, gain controller (RVA-3000, Mini-circuits, NY), phase controller (SPHSA-251, Mini-circuit, NY), and a low noise amplifier (ZX60-P103LN, Mini-circuit, NY). The passive module was optimized for each tested coils (TEM, birdcage, and loop) and the active module is remotely controlled from the MR control room. Images were acquired using a MRI scanner equipped with a 4T (human) magnet and a Varian DirectDrive console. Acquisition parameters: sweep frequency span = 32.5 kHz, 128000 views (spokes in k-space), 256 complex points per view, the diameter of field of view was 44 cm, isotropic resolution of 1.7 mm, and total acquisition time 10 min. The residual leakage signal was subtracted before image processing using the algorithm described previously2. The isolation versus loading variation is shown in Fig.3 (top), which was obtained with a TEM volume coil connected to the conventional hybrid coupler or the high directivity coupler. This result shows the load-insensitivity of the power sampling block (Fig.1) in this work. Fig.3 (bottom) represents an example of the measured isolation performances with and without the proposed compensating system for 4T application (fLarmor = 169.3 MHz) with a loop coil (radius: 9 cm). Stable isolation less than -60 db is obtained and it is enough to keep the signal below the saturation threshold of the receiver system. Fig.4 represents MR images acquired with the proposed system and the continuous SWIFT using the ultra-low RF peak power of 50 mW. This work describes the first MR imaging of the human head with a simultaneous Tx/Rx feedback system and continuous SWIFT. Stable and high isolation have been accomplished with insensitivity to loading variations taking into account many factors: coil’s properties, mutual coupling factors, transmission method (transceiver or transmit/receive separate mode), the number of coil channel, etc. In our opinion, this simultaneous Tx/Rx feedback system is more straightforward as compared to other known methods, for example, the method using frequency modulation10. To improve the reliability of the system, automatic isolation control will be applied for the next step. Due to RF power distribution in time, the continuous SWIFT uses low RF peak power. Therefore, such system might be useful for the next-generation integrated and portable MR scanners.