Microwave tomography is enhanced by using an inhomogeneous background. In this paper, the measurement region is located in a circular perfect electrical conductor (PEC) chamber where a known object is placed inside the imaging domain as an inhomogeneous background. This can not only make use of the prior information about the background, but also increase the equivalent radiation source for the target detection. The Green function of a circular PEC chamber with inhomogeneous background is obtained using the method of moments. Based on the Green functions for both homogeneous and inhomogeneous background in circular PEC chamber, the properties of the radiation operators are analyzed by comparing the condition numbers and the singular value spectra. Simulations are carried out in homogeneous, lossless inhomogeneous and lossy inhomogeneous backgrounds respectively, and the relative errors are discussed. The results show that using inhomogeneous background can improve the convergence rate and accuracy, and the lossy inhomogeneous background produces better results than the lossless one. In addition, it can enhance the inversion results without changing the microwave tomography system, which can be used in the medical imaging and industrial nondestructive detection.