A transmitter unit with an attachable 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, 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, in order to transfer measured physiological data to the external device. The attachable energy source can be a battery pack or battery holder provided with connecting electrical connecting surfaces. Preferably, the connection is protected from penetrating fluids by a protective seal.