Quick Overview

Materials with magnetic susceptibility cause this artifact. There are in general three kinds of materials with magnetic susceptibility: ferromagnetic materials (iron, nickel etc.) with a strong influence and paramagnetic/diamagnetic (aluminium, platinum etc./gold, water, most organic compounds etc.) materials with a minimal/non influence on magnetic fields. In MRI, susceptibility artifacts are caused for example by medical devices in or near the magnetic field or by implants of the patient. These materials with magnetic susceptibility distort the linear magnetic field gradients, which results in bright areas (misregistered signals) and dark areas (no signal) nearby the magnetic material.

Use a spin echo or a fast spin echo sequence, because gradient echo sequences are more sensitve to susceptibility artifacts. A high bandwidth (small water fat shift) and a short echo time help also to reduce this artifact.
In some cases it is even beneficial to use a gradient echo sequence, e.g. a cavernom contains some iron-rich haemosiderin, which also causes a signal void on gradient echo sequences and for this purpose increases the diagnostic image quality.

Quick Overview

Ferromagnetic metal will cause a magnetic field inhomogeneity, which in turn causes a local signal void, often accompanied by an area of high signal intensity, as well as a distortion of the image. They create their own magnetic field and dramatically alter precession frequencies of protons in the adjacent tissues. Tissues adjacent to ferromagnetic components become influenced by the induced magnetic field of the metal hardware rather than the parent field and, therefore, either fail to precess or do so at a different frequency and hence do not generate useful signal. Two components contribute to susceptibility artifact, induced magnetism in the ferromagnetic component itself and induced magnetism in protons adjacent to the component.
Artifacts from metal may have varied appearances on MRI scans due to different type of metal or configuration of the piece of metal. The biocompatibility of metallic alloys, stainless steel, cobalt chrome and titanium alloy is based on the presence of a constituent element within the alloy that has the ability to form an adherent oxide coating that is stable, chemically inert and hence biocompatible. In relation to imaging titanium alloys are less ferromagnetic than both cobalt and stainless steel, induce less susceptibility artifact and result in less marked image degradation.

Remove the metal when possible or take a not so sensitive sequence (a SE or another sequence with a rephasing 180° pulse).

See also Susceptibility Artifact.

Categories of negative oral contrast agents:
Negative oral contrast media are usually based on superparamagnetic particles and act by inducing local field inhomogeneities, which results in shortening of both T1 and T2 relaxation times. Superparamagnetic contrast agents have predominant T2 weighted effects. Biphasic contrast media are agents that have different signal intensities on different sequences, depending on the concentration at which they are used.
Suitable materials for oral contrast agents should have little or no absorption by the stomach or intestines, complete excretion, no motion or susceptibility artifacts, affordability, and uniform marking of the gastrointestinal tract. Benefits of negative oral contrast agents are the reduction of ghosting artifacts caused by the lack of signal. Superparamagnetic iron oxides produce also in low concentrations a noticeable signal loss; but can generate susceptibility artifacts especially in gradient echo sequences. Perfluorochemicals do not dilute in the bowel because they are not miscible with water.
High cost, poor availability, and limited evaluations of side effects are possible disadvantages.
Negative oral contrast agents are used e.g., in MRCP, where the ingestion of 600-900 ml of SPIO cancels out the signal intensity of the lumen (in addition after the injection of a gadolinium-based contrast medium, the enhancement of the inflammatory tissues is clearer seen), and in MR abdominal imaging of Crohn's disease in combination with mannitol.

Blueberry or pineapple juices are useable for examinations of the pancreas (MRCP, upper abdominal imaging) as cheep contrast agents, because of the content of magnetic substances (e.g. manganese).

See also Ferristene, Ferumoxsil, Oral Magnetic Particles, Gastrointestinal Imaging.

Burst pulse sequences are fast imaging sequences capable of image acquisition in less than 100 ms. Basically a train of low flip angle pulses generates a long train of echoes. The complete sequence is performed with the application of a constant read gradient. Phase encoding may be implemented using short phase encoding gradients between echoes.
The advantage of this sequence type is that it is less demanding on gradient speed than other fast techniques (e.g. echo planar imaging EPI) and it produces images, which are substantially free of susceptibility artifacts.
The disadvantage is that the technique is less sensitive than competing methods.

The MRI safety of cardiac stents is dependent of the material, the examined part of the body and the used field strength. A susceptibility artifact is expected also in low magnetic fields, but less.

Most of the used materials are non-magnetic, for this case there is no risk for movement caused through the magnetic field. If the cardiac stent is outside the region of the radio frequency pulse, also the risk of e.g. heating is low.