Graf Hansjörg, Steidle Günter, Martirosian Petros, Lauer Ulrike A, Schick Fritz
Section on Experimental Radiology, University Hospital Tübingen, Tubingen, Germany.
Magn Reson Med. 2005 Jul;54(1):231-4. doi: 10.1002/mrm.20524.
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.
在金属部件中，例如植入物或器械，如果位于磁共振成像（MR）扫描仪内的偏心位置，梯度切换会感应出涡流。首次对相关伪影进行了系统分析。电流强度会随着部件尺寸的增大、电导率的增加、与等中心距离的增大以及梯度强度的增加而增大。在等中心以及x = 15 cm、y = z = 0处检查了一个xy平面取向的铜环（外径d(o) = 20 mm，内径d(i) = 15 mm，厚2 mm）。对于选择编码方向的两种可能性所记录的xy、xz和yz切片的比较，揭示了切片选择梯度下降和读出梯度上升的影响。尽管使用了自旋回波技术，但在金属部件附近，静态场会叠加临时不均匀性，导致自旋去相位信号损失。伪影取决于激发和读出带宽。对于等效的钛环，无法确定与电导率相关的影响，但会出现明显的磁化率效应。因此，植入物/器械的MR兼容性既需要低磁化率又需要低电导率。
