In-phase and quadrature-phase imbalance (IQI) that is caused by direct-conversion transceivers is investigated in the context of cognitive radio (CR) systems based on energy detection. More precisely, in a cooperative CR sensing framework, we derive the optimal weighting vector for combining efficiently signals received at each CR terminal in the presence of IQI at the analog front-ends of both primary and secondary users. The deflection criterion is optimized so as to derive the linear coefficients used at the fusion center of the CR system. Numerical results are provided to analyze the effect of IQI on primary transmitter and secondary receiver.