An example system includes a distortion reference sensor (DRS) device attachable to a surface adjacent a region of interest within the patient. The DRS device includes at least two sensor coils arranged at a predetermined angle relative to each other and at a predetermined spatial position in a three-dimensional space relative to each other. The system also includes a computing device to determine a sensed angle and a sensed spatial position of the at least two sensor coils in the three-dimensional coordinate system in response to application of an electromagnetic field. Electromagnetic field distortion is indicated based on a difference between the predetermined angle and the sensed angle and/or the predetermined spatial position and the sensed spatial position exceeding an expected difference value.