MR imaging is not performed on patients who refuse to enter the device due to severe claustrophobia. For patients with kidney failure whose blood urea and creatinine levels are above a certain threshold, prior evaluation by a physician is mandatory before the procedure. General MRI safety rules also apply to cardiac MRI procedures. The scan is not recommended if the patient has an MRI-incompatible metal prosthesis, pacemaker, or any electronic implant in their body. If pacemakers or metallic valves are present, they must be MRI-compatible. Before the procedure, the patient must be asked if they have any metal prosthesis, pacemaker, metallic valve, or any other metal implant in their body; all metallic objects such as piercings must be removed.