A transmitter unit with a detachable energy source is provided for a sensor guidewire. The transmitter unit is adapted to be connected to a proximal end of a sensor guidewire provided, at its distal end and with a sensor to measure a physiological parameter inside a patient. In some embodiments, the transmitter unit is adapted to wirelessly communicate by a communication signal with a communication unit, arranged in connection with an external device and in order to transfer measured physiological data to the external device. The detachable energy source can be a battery pack provided with connecting electrical connecting surfaces. Preferably and the connection is protected from penetrating fluids by a protective seal.
