The principal contraindications of the MRI procedure are mostly related to the presence of metallic implants in a patient. The risks of MRI scans increase with the used field strength. In general, implants are becoming increasingly MR safe and an individual evaluation is carried out for each case.

Some patients should not be examined in MRI machines, or come closer than the 5 Gauss line to the system.

Absolute Contraindications for the MRI scan:
Patients with absolute contraindications should not be examined or only with special MRI safety precautions. Patients with an implanted cardiac pacemaker have been scanned on rare occasions, but pacemakers are generally considered an absolute contraindication. Relative contraindications may pose a relative hazard, and the type and location of an implant should be assessed prior to the MRI examination.

Relative Contraindications for the MRI scan:
Osteosynthesis material is usually anchored so well in the patients that no untoward effect will result. Another effect on metal parts in the patient's body is the heating of these parts through induction. In addition, image quality may be severely degraded. The presence of other metallic implants such as surgical clips etc. should be made known to the MRI operators. Most of these materials are non-magnetic, but if magnetic, they can pose a hazard.

See also MRI safety, Pregnancy, Claustrophobia and Tattoos.

Advantages of low field imaging are the small-sized 5 Gauss fringe field and therefore the less static magnetic field exposure for the surrounding area, as well as less contraindications causing lower risks for the MRI safety by implemented metal and magnetic devices and equipment.
Low field systems are sometimes for restricted use, e.g. dedicated extremity scanner or open MRI devices. Open MRI devices equipped with permanent magnets are well-suited for MR guided interventions because these machines combine the lower magnetic fields of this type of magnets and the better patient access of open MRI scanner.
In some cases, the contrast of different tissues is better at lower field strength, depending on their T1 or T2 relaxation times. The disadvantage of the lower signal to noise ratio are a poor resolution and a longer scan time for a good image quality.

See also Claustrophobia, Contraindications and MRI Safety.

See also the related poll result: 'In 2010 your scanner will probably work with a field strength of'

There are different types of contraindications that would prevent a person from being examined with an MRI scanner. MRI systems use strong magnetic fields that attract any ferromagnetic objects with enormous force. Caused by the potential risk of heating, produced from the radio frequency pulses during the MRI procedure, metallic objects like wires, foreign bodies and other implants needs to be checked for compatibility. High field MRI requires particular safety precautions. In addition, any device or MRI equipment that enters the magnet room has to be MR compatible. MRI examinations are safe and harmless, if these MRI risks are observed and regulations are followed.

Safety concerns in magnetic resonance imaging include:

It is important to remember when working around a superconducting magnet that the magnetic field is always on. Under usual working conditions the field is never turned off. Attention must be paid to keep all ferromagnetic items at an adequate distance from the magnet. Ferromagnetic objects which came accidentally under the influence of these strong magnets can injure or kill individuals in or nearby the magnet, or can seriously damage every hardware, the magnet itself, the cooling system, etc.. See MRI resources Accidents.
The doors leading to a magnet room should be closed at all times except when entering or exiting the room. Every person working in or entering the magnet room or adjacent rooms with a magnetic field has to be instructed about the dangers. This should include the patient, intensive-care staff, and maintenance-, service- and cleaning personnel, etc..
The 5 Gauss limit defines the 'safe' level of static magnetic field exposure. The value of the absorbed dose is fixed by the authorities to avoid heating of the patient's tissue and is defined by the specific absorption rate. Leads or wires that are used in the magnet bore during imaging procedures, should not form large-radius wire loops. Leg-to-leg and leg-to-arm skin contact should be prevented in order to avoid the risk of burning due to the generation of high current loops if the legs or arms are allowed to touch. The patient's skin should not be in contact with the inner bore of the magnet.
The outflow from cryogens like liquid helium is improbable during normal operation and not a real danger for patients.
The safety of MRI contrast agents is tested in drug trials and they have a high compatibility with very few side effects. The variations of the side effects and possible contraindications are similar to X-ray contrast medium, but very rare. In general, an adverse reaction increases with the quantity of the MRI contrast medium and also with the osmolarity of the compound.

See also 5 Gauss Fringe Field, 5 Gauss Line, Cardiac Risks, Cardiac Stent, dB/dt, Legal Requirements, Low Field MRI, Magnetohydrodynamic Effect, MR Compatibility, MR Guided Interventions, Claustrophobia, MRI Risks and Shielding.

In the last years, cardiac MRI techniques have progressively improved. No other noninvasive imaging modality provides the same degree of contrast and temporal resolution for the assessment of cardiovascular anatomy and pathology. Contraindications MRI are the same as for other magnetic resonance techniques.
The primary advantage of MRI is extremely high contrast resolution between different tissue types, including blood. Moreover, MRI is a true 3 dimensional imaging modality and images can be obtained in any oblique plane along the true cardiac axes while preserving high temporal and spatial resolution with precise demonstration of cardiac anatomy without the administration of contrast media.
Due to these properties, MRI can precisely characterize cardiac function and quantify cavity volumes, ejection fraction, and left ventricular mass. In addition, cardiac MRI has the ability to quantify flow (see flow quantification), including bulk flow in vessels, pressure gradients across stenosis, regurgitant fractions and shunt fractions. Valve morphology and area can be determined and the severity of stenosis quantified. In certain disease states, such as myocardial infarction, the contrast resolution of MRI is further improved by the addition of extrinsic contrast agents (see myocardial late enhancement).
A dedicated cardiac coil, and a field strength higher than 1 Tesla is recommended to have sufficient signal. Cardiac MRI acquires ECG gating. Cardiac gating (ECGs) obtained within the MRI scanner, can be degraded by the superimposed electrical potential of flowing blood in the magnetic field. Therefore, excellent contact between the skin and ECG leads is necessary. For male patients, the skin at the lead sites can be shaved. A good cooperation of the patient is necessary because breath holding at the end of expiration is practiced during the most sequences.

See also Displacement Encoding with Stimulated Echoes.
For Ultrasound Imaging (USI) see Cardiac Ultrasound at Medical-Ultrasound-Imaging.com.

See also the related poll results: 'In 2010 your scanner will probably work with a field strength of' and 'MRI will have replaced 50% of x-ray exams by'

During the MRI scan an augmentation of T waves is observed at fields used in standard imaging but this possible MRI side effect is completely reversible upon removal from the magnet. A field strength dependent increase in the amplitude of the ECG in rats has been observed during exposure to high homogeneous stationary magnetic fields, but this side effect is not transferable to standard imaging situations for humans.

The minimum level at which augmentation can be observed is 0.3 T and increases by higher field strength. An augmentation in T-wave amplitude can occur instantaneously and is immediately reversible after exposure to the magnetic field ceased. There should be no abnormalities in the ECG in the later follow-up. Augmentation of the signal amplitude in the T-wave segment may result from superimposed electrical potential. No circulatory alterations coincide with the ECG changes. Therefore, no biological risks are believed to be associated with them.

For more MRI safety information see also Contraindications and MRI Risks.