In simultaneous transmission and reception (STAR), injection of noise and spurs stemming from transmit chain is a problem as well as the leak power. Investigation of noise and spurs are important in order to determine minimum necessary isolation to achieve similar noise levels compared to conventional imaging. Noise sources during STAR are investigated and compared with experiments. It has been shown that using an active cancellation method where dominant noise source is transmit chain, with combination of passive cancellation can reduce noise and spurs. Necessary isolation is shown to depend on input noise and total gain in transmit chain.