This study demonstrates improvement in MR image sensitivity and uniformity from a 4-channel phased array using a secondary resonator having switchable detuning circuits. The secondary resonator was located at the opposite side of the 4-channel phased array to improve sensitivity and uniformity of the acquired MR images. The numerical simulation results of |B1+| in a transmit mode showed the magnetic field uniformity would be decreased with the designed secondary resonator having no detuning circuits because of unwanted interferences between the transmit birdcage coil and the secondary resonator. Whereas, the sensitivity and uniformity of |B1-| in a receive mode were improved with the secondary resonator and a 4-channel phased array. For experimental verification, a uniform saline phantom and mice were analyzed by MR using a 7.0T small animal MRI system.