A pacemaker is a device for internal or external battery-operated cardiac pacing to overcome cardiac arrhythmias or heart block. All implanted electronic devices are susceptible to the electromagnetic fields used in magnetic resonance imaging. Therefore, the main magnetic field, the gradient field, and the radio frequency (RF) field are potential hazards for cardiac pacemaker patients.
The pacemaker's susceptibility to static field and its critical role in life support have warranted special consideration. The static magnetic field applies force to magnetic materials. This force and torque effects rise linearly with the field strength of the MRI machines. Both, RF fields and pulsed gradients can induce voltages in circuits or on the pacing lead, which will heat up the tissue around e.g. the lead tip, with a potential risk of thermal injury.
Regulations for pacemakers provide that they have to switch to the magnet mode in static magnetic fields above 1.0 mT. In MR imaging, the gradient and RF fields may mimic signals from the heart with inhibition or fast pacing of the heart. In the magnet mode, most of the current pacemakers will pace with a fix pulse rate because they do not accept the heartsignals. However, the state of an implanted pacemaker will be unpredictable inside a strong magnetic field. Transcutaneous controller adjustment of pacing rate is a feature of many units. Some achieve this control using switches activated by the external application of a magnet to open/close the switch. Others use rotation of an external magnet to turn internal controls. The fringe field around the MRI magnet can activate such switches or controls. Such activations are a safety risk.
Areas with fields higher than 0.5 mT (5 Gauss Limit) commonly have restricted access and/or are posted as a safety risk to persons with pacemakers.

A Cardiac pacemaker is because the risks, under normal circumstances an absolute contraindication for MRI procedures.
Nevertheless, with special precaution the risks can be lowered. Reprogramming the pacemaker to an asynchronous mode with fix pacing rate or turning off will reduce the risk of fast pacing or inhibition. Reducing the SAR value reduces the potential MRI risks of heating. For MRI scans of the head and the lower extremities, tissue heating also seems to be a smaller problem. If a transmit receive coil is used to scan the head or the feet, the cardiac pacemaker is outside the sending coil and possible heating is very limited.

Open MRI scanners have been developed for people who are anxious or obese or for examination of small parts of the body, such as the extremities (knee, shoulder). In addition, some systems offer imaging in different positions and sequences of movements. The basic technology of an open MRI machine is similar to that of a traditional MRI device. The major difference for the patient is that instead of lying in a narrow tunnel, the imaging table has more space around the body so that the magnet does not completely surround the person being tested.
Types of constructions:
Difficulties with a traditional MRI scan include claustrophobia and patient size or, for health related reasons, patients who are not able to receive this type of diagnostic test. The MRI unit is a limited space, and some patients may be too large to fit in a narrow tunnel. In addition, weight limits can restrict the use of some scanners. The open MRI magnet has become the best option for those patients.
All of the highest resolution MRI scanners are tunnels and tend to accentuate the claustrophobic reaction. While patients may find the open MRI scanners easier to tolerate, some machines use a lower field magnet and generates lower image quality or have longer scan time. The better performance of an advanced open MRI scanner allows good image quality caused by the higher signal to noise ratio with maximum patient comfort.

See also Claustrophobia, MRI scan and Knee MRI.