Physical Address

304 North Cardinal St.
Dorchester Center, MA 02124

Metal Artifact


KEY FACTS

Terminology

  • CT: Beam-hardening artifact and blooming artifact

  • MR: Magnetic susceptibility artifact

  • Image degradation related to metal prostheses/implants

Imaging

  • CT: Artifact from metal hardware related to image reconstruction algorithm, tube current, x-ray kilovolt peak, pitch, hardware composition, shape, & location

    • Beam-hardening artifact is dark banding between dense objects, such as bone

    • Bloom artifacts result of partial volume effects or areas of photon starvation propagated by high-density structures, such as metal, within scanned object

    • Materials with lower x-ray attenuation coefficients produce less artifactual distortions

      • –

        Plastic (best) < titanium < < tantalum < stainless steel < cobalt chrome (worst)

    • Metal composition, mass, orientation + position of implant are important factors that determine magnitude of image artifact

  • MR susceptibility artifact due to geometric distortion + signal loss secondary to dephasing

  • MR methods to minimize metal artifact

    • Fast spin-echo > conventional spin-echo > gradient-echo

    • Larger field of view

    • Smaller voxel size

    • Increase transmit and receive bandwidth

    • Frequency encoding direction along long axis of hardware

    • Lower magnet field strength

    • STIR sequences are alternative method of fat suppression and less dependent on homogeneity of main magnetic field

Plain film shows right lateral fusion extending from the L2-L5 with vertebral body screws and longitudinal rods, interbody graft material, and lower lumbar posterior pedicle screw fixation from L4-S1 .

MR study in the same patient shows very little obscuration of the central canal by artifact, despite the amount of metal present. Metal artifact is present from lateral fusion and posterior fixation . Solid fusion with incorporated graft is noted by fatty disc space signal.

Coronal CT reconstruction illustrates bloom hardening artifact . Beam artifacts are a result of partial volume effects or underranging caused by areas of photon starvation propagated by high-density structures, such as metal, within the scanned object. When subtle, these artifacts appear as shading and, when severe, as high-intensity streaks and areas of photon starvation.

Axial NECT shows star artifact along the margins of the right corporal screw, which has migrated out .

TERMINOLOGY

Definitions

  • Magnetic susceptibility

    • Partial magnetization of material in presence of applied external magnetic field

    • Nonferromagnetic metals may produce local electrical currents induced by changing scanner magnetic field

    • Tissues with greatly different magnetic susceptibilities in uniform magnetic field lead to difference of susceptibilities, causing distortion in magnetic field → distortion on MR

    • Magnetic susceptibility artifact consists of 2 additive components

You're Reading a Preview

Become a Clinical Tree membership for Full access and enjoy Unlimited articles

Become membership

If you are a member. Log in here

Related Posts