Magnetic Resonance Imaging (MRI) Board Practice Exam

Metallic hardware creates local magnetic field distortions that cause susceptibility artifacts. The amount of artifact a sequence shows depends on how it handles these inhomogeneities.

Gradient-echo sequences rely on T2*-weighting and do not include a 180-degree refocusing pulse. Because there’s no mechanism to rephase spins locally dephased by the metal-induced field distortions, signal loss and geometric distortion are amplified near implants. This makes gradient-echo the least desirable choice when metal hardware is present.

Spin-echo sequences, on the other hand, include a 180-degree refocusing pulse that rephases spins and mitigates dephasing from static field inhomogeneities, markedly reducing artifacts near metal.

Inversion-recovery sequences can be implemented with spin-echo readouts or other approaches that preserve some of that artifact resistance, so they tend to be less problematic than plain gradient-echo.

Echo-planar imaging is typically gradient-echo based and therefore also very sensitive to susceptibility effects, but the fundamental reason gradient-echo is least suitable is its lack of refocusing and reliance on T2*-type decay.

So, in the presence of implanted metallic hardware, avoiding gradient-echo sequences helps minimize artifacts, while spin-echo or SE-based approaches are preferred to reduce distortion and signal loss.