Metal artifacts caused by gradient switching

Abstract

In metal parts, e.g., implants or instruments, eddy currents can be induced from gradient switching if positioned off-center inside the MR scanner. For the first time, a systematic analysis of related artifacts was performed. Current strength increases in conjunction with increasing size of the part, increasing electrical conductivity, distance from isocenter, and increasing gradient strengths. A xy-plane oriented copper ring (d o ‫؍‬ 20 mm, d i ‫؍‬ 15 mm, 2 mm thick) was examined at isocenter and at x ‫؍‬ 15 cm, y ‫؍‬ z ‫؍‬ 0. Comparisons of xy-, xz-, and yz-slices, recorded for both possibilities to select encoding directions, revealed effects from ramp-down of the slice-selection and ramp-up of the read-out gradient. Near the metal part, temporary inhomogeneities were superimposed to the static field and spin-dephasing signal loss resulted, despite using spin-echo technique. Artifacts depended on excitation and read-out bandwidth. For an equivalent titanium ring, conductivity related effects could not be ascertained but distinct susceptibility effects occurred. MR compatibility of implants/instruments therefore requires both low susceptibility and low conductivity. Key words: metal artifact; eddy current; implant; instrument; MRI Depending on the medical indication magnetic resonance imaging (MRI) may be performed on patients having nonferromagnetic metallic implants. MRI is also increasingly applied in radiologic interventions where metallic instruments are used.Most metals show distinctly different magnetic susceptibility compared to human tissue and perturbed B 0 homogeneity leads to typical related image artifacts. In gradientecho (GRE) imaging, signal loss due to spin dephasing dominates, whereas disturbed gradient linearity near the metal becomes visible as distortions in spin-echo (SE) images. Perturbed slice selection and perturbed spatial encoding in the read-out (RO) direction produce characteristic artifacts (1).In addition, metals can interact with the time varying magnetic fields of the MR scanner (RF magnetic field B 1 , gradient switching) due to their electrical conductivity, i.e., induction phenomena can occur. Concerning B 1 effects, the alteration of the amplitude adjacent to the metal (2,3) and shielding inside circular structures, i.e., vascular stents, has been investigated (4,5). Gradient switching effects from eddy currents, induced in conductive components of the scanner, are a well-known problem and have been intensively examined (6,7). However, apart from the very recent observations of Shenhav and Azhari (8) no examinations exist confirming artifact generation caused by gradient switching induced eddy currents in metal parts embedded in the sample, such as in the case of medical implants. A probable explanation seems to be that in vitro examination of specimens has been performed near isocenter (9,10) and that in vivo susceptibility artifacts dominate in most cases.The presented work demonstrates that gradient switching induced eddy currents can play an important role for artifact ...

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