Abstract: An apparatus includes a longitudinal member having a proximal end and a distal end. The longitudinal member is configured to be located near a tissue region in a body of a patient. A measuring device is configured and sized to be located proximal to the distal end of the longitudinal member. The measuring device includes a magnetic sensor configured to measure biomagnetism and output magnetic flux data. A signal processing device is coupled to the magnetic sensor and configured to convert the output magnetic flux data to a digital representation of the output magnetic flux data. A method of measuring electrical activity using the apparatus is also disclosed.