Metallic Hip Implant: Choosing a Safe and Diagnostic Protocol

Before scanning, obtain the manufacturer's MR-conditional labeling for the exact implant model: static field limit, maximum spatial gradient, maximum whole-body-averaged SAR, maximum SAR over any 10 g of tissue, and any restriction on transmit coil type (e.g., no transmit body coil above a certain SAR). Confirm the scanner can meet these conditions, especially at 3 T where RF heating risk is higher.

The reported warmth and discomfort is a safety signal, not a benign side effect to dismiss. Localized heating near an elongated conductive implant is the expected mechanism of RF-induced heating; patient-perceived warmth can indicate that SAR or the local E-field near the implant tip was high. Document the report, review the prior study's SAR and coil configuration if available, and consider:

• Scanning at 1.5 T if the implant's 3 T conditions cannot be verified or met.
• Using lower SAR sequences and avoiding long high-SAR trains near the implant.
• Providing a communication device and instructing the patient to report heat immediately.

If the condition sheet cannot be obtained and the implant's status is uncertain, treat it as MR-unsafe until proven otherwise and discuss alternatives (CT, ultrasound, or a different modality) with the referring clinician.

Metal artifact arises from susceptibility-induced field inhomogeneities (B0 distortion, signal void, pile-up) and from eddy currents in the implant. STIR is especially vulnerable because its inversion-recovery preparation is sensitive to B1/B0 errors and its fat suppression is less robust near metal.

Sequence-level mitigations:

• Increase receiver bandwidth to shorten echo spacing and reduce readout distortion; this also reduces chemical-shift artifact.
• Use a spin-echo or FSE-based fluid-sensitive sequence rather than gradient-echo, since 180 deg refocusing partially compensates static field offsets.
• Shorten echo time and reduce echo spacing to limit the time over which off-resonance accrues.
• Use MAR techniques: view-angle tilting (VAT), slice-encoding for metal artifact correction (SEMAC), or multi-acquisition variable-resonance image combination (MAVRIC), which combine multiple frequency offsets to recover signal near metal.
• Orient the frequency-encoding direction along the implant's long axis to push distortion away from the region of interest.
• Replace STIR with a Dixon-based fat-suppressed T2 if the vendor supports it, since Dixon is less sensitive to B1 inhomogeneity than STIR near metal.

For periprosthetic infection, a contrast-enhanced T1 with Dixon fat suppression plus a MAR-corrected T2 may be more diagnostic than a distorted STIR.

No. Whole-body SAR is only one of several limits. For an elongated conductive implant, the critical metric is often local SAR over any 10 g of tissue and the E-field at the implant tip, neither of which is captured by whole-body SAR. The MR-conditional labeling typically specifies a maximum local SAR (e.g., 2 W/kg over 10 g) and may restrict the transmit coil or require a specific landmark position.

Proceed only if:

• The implant's condition sheet is verified and the scanner's predicted local SAR meets the labeled limit.
• The landmark and coil configuration match the labeled conditions (e.g., body coil transmit with a receive-only array).
• The patient is monitored and can communicate.

If local SAR cannot be predicted or the labeling is unavailable, reduce SAR (lower flip angles, longer TR, fewer slices), switch to 1.5 T if labeled there, or choose a non-MR modality. Document the decision and the rationale.