What characterizes metal artefacts in MRI images?

Metal artefacts in MRI images are primarily characterized by areas of zero signal surrounded by high-intensity rims. This occurs due to the presence of metallic objects, which distort the magnetic field and affect the way protons in the surrounding tissue align and return signals. The interactions between the metal and the MRI's magnetic field result in significant signal loss in the area directly adjacent to the metal, leading to the typical pattern observed. The high-intensity rims can arise from the partial volume effects and signal from the surrounding tissues that are not affected to the same extent. Understanding this phenomenon is crucial for radiologists and technologists to assess images accurately and to recognize potential distortions caused by metallic implants or other objects.