An apparatus comprises: an imaging system (10) configured to acquire emission data from a cyclically varying element; a monitoring instrument (20, 22) configured to measure the cyclical varying of the cyclically varying element; and an electronic device (40) configured to locate an image feature corresponding to the cyclically varying element in the acquired emission data based on correlation of time variation of the emission data with the cyclical varying of the cyclically varying element measured by the monitoring instrument. The located image feature may be verified by: thresholding a projection image generated from the emission data to generate a mask image; identifying in the mask image one of (i) a hollow circular feature, (ii) a hollow oval feature, (iii) a circular cavity feature, and (iv) an oval cavity feature; and verifying the located image feature based on whether the identifying operation is successful