Forces can result from the interaction of magnetic fields. Pulsed magnetic field gradients can interact with the main magnetic field during the MRI scan, to produce acoustic noise through the gradient coil.
Magnetic fields attract ferromagnetic objects with forces, which can be a lethal danger if one is hit by an unrestrained object in flight. One could also be trapped between the magnet and a large unrestrained ferromagnetic object or the object could damage the MRI machine.
Access control and personnel awareness are the best preventions of such accidents. The attraction mechanism for ferromagnetic objects is that the magnetic field magnetizes the iron. This induced magnetization reacts with the gradient of the magnetic field to produce an attraction toward the strongest area of the field. The details of this interaction are very dependent on the shape and composition of the attracted object. There is a very rapid increase of force as one approaches a magnet. There is also a torque or twisting force on objects, e.g. a long cylinder (such as a pen or an intracranial aneurysm clip) will tend to align along the magnet's field lines. The torque increases with field strength while the attraction increases with field gradient.
Depending on the magnetic saturation of the object, attraction is roughly proportional to object mass. Motion of conducting objects in magnetic fields can induce eddy currents that can have the effect of opposing the motion.

See also Duty Cycle.

See also the related poll result: 'Most outages of your scanning system are caused by failure of'

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.

Metal detector coils can be used to find hidden ferromagnetic materials inside of clothes or the body of persons before entering a scanning room. Magnetic fields attract ferromagnetic objects with forces, which can be dangerous for humans and MRI machines.

Not necessarily a contraindication, but the examination may damage or impair it. Interference in electronic cochlear implants and ferromagnetic mechanical stapedial replacements has been reported.

Quick Overview

A disturbance of the field homogeneity, because of magnetic material (inside or outside the patient), technical problems or scanning at the edge of the field.
When images were obtained in a progression from the center to the edge of the coil, the homogeneity of the field observed by the imaged volume, changes when the distance from the center of the volume increase. The same problem appears by scanning at a distance from the isocenter in left-right direction or too large field of view.
There are different types of bad image quality, the images are noisy, distorted or the fat suppression doesn't work because of badly set shim currents.
E.g. by using an IR sequence, changes in the T1 recovery rates of the tissues are involved. The inversion time at the center of the imaged volume is appropriate to suppress fat, but at the edge of the coil the same inversion time is sufficient to suppress water. Since the inversion time is not changed, the T1 recovery rates will increase.

Take a smaller imaging volume (and for fat suppression a volume shimming), take care that the imaged region is at the center of the coil and that no magnetic material is inside the imaging volume.