42 Aero Camino, Goleta, CA 93117 ECG TIMING DELAY Tel (805) 685-0066 | Fax (805) 685-0067 [email protected] | www.biopac.com 10.15.2014 Application Note 281: ECG Timing Delay Associated with Amplifier Filter Selections This application note is concerned with the nature of ECG100C and ECG100C-MRI timing delays. In particular, the focus is on ECG data collected in fMRI or MRI for the express purpose of triggering magnet scans during imaging. This procedure is important when attempting to image physiological processes in synchronization with the heart. Typically, ECG data is collected via the ECG100C-MRI and this data is piped out of an MP System, to drive a Digital Trigger Unit (DTU300), which in turn drives the magnet scanning process, via magnet scan trigger input. This entire process occurs in real time, not subject to any computer processing delay The signal sequence would appear as follows: ECG100C-MRI  UIM100C  DTU300  MAGNET SCAN TRIGGER INPUT Figure 1: Data Collected with ECG100C-MRI in a 1.5T MRI Figure 2: Close-up view of ECG Channels at point of EPI Start Note that EPI starts at roughly 41.40 seconds into the recording. Note that magnetohydrodynamic (MHD) effects are apparent on the ECG data, as a consequence of the heart’s mechanical action in a 1.5T static magnetic field. Furthermore, as typical for ECG recording in the MRI or fMRI, artifact is evident as a consequence of EPI generated magnetic field gradient changes during imaging. These gradient changes induce voltages in the body of the subject and are recorded in superposition with the subject’s ECG. These artifacts can be greatly reduced by       using the ECG100C-MRI MRI Smart Amplifier using the respective cable/filter set (MECMRI-BIOP) placing ECG leads close to the heart observing good electrode attachment methods, including scrubbing skin with prep gel (ELPREP) keeping carbon composition leads short (LEAD108B preferred) checking electrode impedances - under 5 kΩ (EL-CHECK) Figure 2 Channel 1 (blue) is the raw ECG100C-MRI data, Channel 2 (green) is the Channel 1 data processed by the ECG100C-MRI amplifier module filter selection of 150 Hz LP, and Channel 3 (cyan) is the Channel 1 data processed by the module filter selection of 35 Hz LPN. Note that the bottom two waveforms are slightly delayed with respect to the raw ECG data collected in the magnet. The 150 Hz LP filter introduces a delay of 1.5 ms and the 35 Hz LPN filter introduces a delay of 6.5 ms. Accordingly, if the magnet scan is triggered when either of these amplifier module filters is selected then the scan will be delayed, by this specified amount, in relation to the actual R-wave peak timing. ECG Timing Delay in MRI or fMRI BIOPAC Systems, Inc. This data demonstrates that it is practical to collect ECG data in the fMRI or MRI and use that real-time data to reliably trigger a magnet scan. Although generated magnet artifact does not materially impact the ability of the ECG100C-MRI amplifier’s output to drive the DTU300 (in this data set), note that a synchronized scan will also be completely devoid of artifact before and after the scan period. Accordingly, so long as blanking is employed on the DTU300, any artifact generated in the ECG data will not affect DTU300 triggering operation, despite possibly egregious artifact in the collected ECG data. Figure 3: Close-up view of ECG Channels overlapped Figure 4: Close-up view of ECG Channels with Triggers Figure 4 indicates the associated trigger timing relationships depending on the ECG100C-MRI amplifier module’s filter switch options of 150 Hz LP or 35 Hz LPN. Note that depending on the value of the trigger threshold, the respective trigger position is considerably affected. In this case, the trigger threshold on the DTU300 was set to 1 mV. The 4th channel (brown) is the generated trigger associated with the module’s 150 Hz LP filter setting. The 5th channel (red) is the generated trigger associated with the module’s 35 Hz LPN filter setting. The DTU300 will output TTL compatible (0-5 V) trigger output levels to control the fMRI or MRI magnet scanning process. Related reading: Nijm, G.M.; Swiryn, S.; Larson, A.C.; Sahakian, A.V., “Characterization of the Magnetohydrodynamic Effect as a Signal from the Surface Electrocardiogram during Cardiac Magnetic Resonance Imaging,” Computers in Cardiology, 2006 , vol., no., pp.269,272, 17-20 Sept. 2006 www.biopac.com Page 2 of 2