In this paper, a compact multi-static microwave imaging system using a novel design of high-fidelity ultrawideband (UWB) slot antenna is employed to improve the accuracy of tumor detection inside full head phantom. In order to create a circular array based microwave imaging system 18 elements of the proposed UWB slot antennas are simulated in CST medium around the phantom inside a designed matching medium. The proposed slot antenna consists of a square radiating patch with a microstrip feed-line on one side, a rectangular slot and a defected ground plane by a pair of Cshaped slots on the other side, which provides a wide usable fractional bandwidth of more than 100% (3.19-10.73 GHz). The inserted C-shaped slots on the ground plane's corners can provide an additional current path, hence additional resonance is excited and much wider impedance bandwidth is produced. In order to demonstrate the usefulness of the proposed antenna for microwave imaging system, the fidelity analysis for angles up to 90 away from bore-sight radiated pulses is presented (fidelity factor > 80%). In addition, a novel hierarchical calibration method is employed to improve the accuracy reconstructed image results. This calibration includes all delays of multi-static antenna array to put more energy at coherence reflected signal integration. Hence, stronger signals are available to achieve higher accuracy for precise spatial localization. In the proposed image, reconstruction method a confocal image-reconstructing algorithm based on back-projection method has been employed. Simulated results are presented to validate the effectiveness of the proposed method for precisely localizing small targets.