Metal artifacts caused by gradient switching.
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.
Graf
Hansjörg
H
Section on Experimental Radiology, University Hospital Tübingen, Tubingen, Germany. hansjoerg.graf@med.uni-tuebingen.de
Steidle
Günter
G
Martirosian
Petros
P
Lauer
Ulrike A
UA
Schick
Fritz
F
D016477:Artifacts; D007089:Image Enhancement / Q000379:methods*; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000295:instrumentation / Q000379:methods*; D008670:Metals; D019047:Phantoms, Imaging; D019736:Prostheses and Implants; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D012815:Signal Processing, Computer-Assisted
D008670:Metals