For the most part, the combined EEG-fMRI data collection is now treated as a solved problem, and commercial devices are available from major manufacturers (e.g., Electrical Geodesics, Inc.; NeuroScan/Compumedics, Inc.; Brain Products; Advanced Neuro Technology), but issues remain. There are significant residual artifacts in the EEG that occur with each heartbeat. The traces in the EEG that record this are often referred to as a, "Ballistocardiogram (BCG)," because of their presumed origin in the motion of the EEG leads in the magnetic field that occurs with each heartbeat. In practice, the causes of this artifact are not well proven and may be the results of factors such as induced electrical fields with the movement of blood, etc.