An important aspect of MRI imaging in patients with CHD is related to image quality when performing cardiac MR scans. In patients with implantable devices, metal susceptibility artifacts may potentially limit the image quality especially in higher magnetic fields such as 3-T magnets. Metal artifacts due to magnetic susceptibility effect or induction of eddy currents by radio-frequency pulses appear as bands with increased or decreased signal in the vicinity of the leads or generators. On delayed post-contrast images, artifactual increased signal may be confused with hyperenhancing myocardial scars [24]. In our experience, these artifacts can be reduced by selecting imaging planes perpendicular to the plane of the device generator, shortening the echo time, and using spin echo and fast spin echo sequences (Fig. 31.2). In general, compared to balanced steady-state free-precession (SSFP) sequence, spoiled fast gradient echo sequence demonstrates fewer artifacts from metallic devices. This is likely related to shorter echo times used in the later sequence. Pulver and colleagues have reported that in their experience, in patients with CHD, all MRIs yielded diagnostic quality images with no evidence of major artifacts despite having predominantly epicardial leads [33]. They used routine MR sequences for their cardiac cases. Mild atrial lead artifact was seen in SSFP cine imaging, but double inversion recovery black blood series were essentially artifact-free. Notably, artifacts from leads and pacemaker generators are minimal in size. Moreover, left-sided ICD generators are the primary cause of image distortion in device recipients [34].

Given the increased clinical need for MRI scans in patients with implantable cardiac device, implantable cardiac device manufacturers have developed devices that are specifically designed to be safe in the MRI environment. These “MRI-conditional” devices pose no known hazards when MRI is performed using a specific device programming and detailed monitoring conditions [35]. Recently, the United States Food and Drug Administration (FDA) approved the Revo MRI SureScan Pacing System (Medtronic, Inc., Minneapolis, MN), one of the MRI-conditional pacing systems [36]. Additional MRI-conditional devices are available outside of the United States. These devices include the Accent MRI (St Jude Medical, St. Paul, MN) [37], Evia and Estella (Biotronik, Berlin, Germany) [38], as well as Lumax 740 (Biotronik, Berlin, Germany) [38]. Of note, these devices are approved for MRI only when attached to specific leads (e.g., CapSureFix (Medtronic, Inc., Minneapolis, MN), Tendril MRI (St Jude Medical, St. Paul, MN), and Linoxsmart (Biotronik, Berlin, Germany)) and when the MRI field strength is limited to 1.5 T.

Several modifications were made in these devices in order to make them “MRI conditional.” Due to manufacturer proprietary designs, not all changes are disclosed. However, these changes include design features to minimize the use of ferromagnetic materials. Changes in the lead conductor design and filtering are used to alleviate heating. The use of a Hall sensor rather than a reed switch results in more predictable pacemaker performance within the MRI environment [39]. Other changes include modules to simplify MRI safe device programming. For example, in the Revo MRI SureScan Pacing System (Medtronic, Inc., Minneapolis, MN), the MRI mode streamlines the choice between asynchronous and non-stimulation modes, increases pacing output to 5.0 V at 1.0 ms during MRI, and enables MRI mode programming only after system integrity has been checked and approved.

The first feasibility study for such an MRI-conditional device included 107 patients. During this study, patients underwent implantation of the EnRhythm MRI-conditional device or a conventional dual-chamber device. The authors reported no complications during the follow-up period [40]. They observed, however, a trend towards lower successful cephalic vein access when using MRI dedicated leads compared to conventional leads (60.0 % vs. 68.4 %, respectively). Also, subclavian vein access was used more frequently in patients with MRI-conditional leads (40.0 % vs. 31.6 %, respectively, p = NS). Finally, there was no difference in fluoroscopy time (6.0 ± 3.6 min vs. 6.6 ± 3.8 min), overall procedure times (71.7 ± 27.6 min vs. 76.9 ± 30.3 min), or duration of hospitalization. This study suggested that the EnRhythm MRI-compatible pacemaker is safe and could be used for cardiac pacing. Following this study, a larger-scale randomized prospective multicenter study was performed. The study included 464 patients and recruited both pacemaker-dependent and non-pacemaker-dependent patients [39]. Twelve weeks following the procedure, 211 patients underwent MRI examination using a 1.5-T scanner. Other technical parameters included a maximum specific absorption rate of 2 W/kg and a maximum gradient slew rate of 200 T/m/s. During these scans, the isocenter was limited to extra-thoracic region (above the superior surface of C1 vertebra and below the inferior surface of the body of T12). Device interrogation was performed before MRI, immediately after MRI, 1 week after MRI, and at 1 month after MRI. No events were reported during MRI examinations, and no device-related adverse events were attributable to MRI. There were no differences between the study groups with regard to pacemaker measurements including capture threshold, sensitivity, and impedances. Importantly, since this study was limited to specific isocenter locations, the results cannot be generalized to assess safety of this MRI-conditional system with MRI isocenter in the thorax (between C1 and T12 vertebra). This study led to the approval of the Revo MRI Pacemaker System with 5086 MRI CapSureFix MRI Pacing Leads (Medtronic) and the SureScan Software (Medtronic) as “MRI conditional” by the United States FDA [41].