There has been significant advancement in the ability to perform MR imaging around metallic prostheses^1-3. Accurate modeling of the large field perturbations induced by the metal is essential for the design and optimization of MRI acquisition and reconstruction strategies near metallic implants⁴. These field perturbations are affected by composition (magnetic susceptibility of implant), geometry, orientation relative to the main magnetic field, and field strength. While the magnetic susceptibility values of pure metals are well understood⁵, manufacturers do not provide susceptibility values for specific commercial implantable alloys. The purpose of this work was to estimate the magnetic susceptibility of commonly implanted metal alloys by measuring the magnetic moment across a range of clinical field strengths (0-5 Tesla) using a SQUID (Superconducting QUantum Interference Device) magnetometer.

Representative samples of popular total hip and knee metallic prostheses (Figure 1, Table 1) were obtained from an orthopedic manufacturer (Zimmer Biomet, Warsaw, ID) and sent to the National High Magnetic Field Laboratory (Tallahassee, FL). Sample preparation included cutting to a desired shape (~6x6x6 mm3) by electrical discharge machining (samples 1-5) or by milling (sample 6) and washing with dilute acid to remove any ferromagnetic contamination introduced during the cutting process. Samples 1-5 were treated in a 5% HCl solution for 1 hour to remove any residue. Sample 5 was additionally treated for 24 hours. Sample 6 was washed with ethanol. The rough and porous surface of Samples 2 and 3 required multiple washing and drying cycles to achieve a steady mass over time (washing: deionized water, 2 hours; drying: dry box, 1 week). Sample masses were recorded before and after the washing process and no significant changes were observed.

The measurements of the magnetic moments were conducted using a Quantum Design, MPMS-5 SQUID magnetometer. The magnetic moment of each sample was measured at 310 K (normal body temperature) as the field strength swept from 0 to 5 Tesla and back down to -0.2 Tesla. Measurements were then normalized by the respective masses. Mass susceptibility was taken as the slope of the averages of the linear fit done for both up and down sweeps. The following equation allowed conversion of mass susceptibility (χ_g) to volume ppm susceptibility: χ_ppm = χ_g * ρ * 4π * 10⁶ (multiplied by: ρ (density) for volume susceptibility, 4π for SI quantity, 10⁶ for ppm).

The magnetic moments of the measured alloys and plastic demonstrated a linear dependence up to 5 T (Figure 2). The magnetization of the Co-Cr-Mo alloys compared to the Ti-6Al-4V alloys were approximately double. Sample 1, 2, 3, 4, and 5 demonstrated paramagnetism (positive linear) while Sample 6 (UHMWPE) demonstrated diamagnetism (negative linear). The magnetic susceptibility values for the samples are shown in Table 1.This work estimated the magnetic susceptibility of several commonly implanted metallic alloys across a range of field strengths using a SQUID magnetometer. The reported susceptibility values were noted to differ with values used previously in the literature that were obtained by estimation with MRI^6-7. Furthermore, a commonly made assumption regarding the linearity of magnetization across clinically relevant field strengths was experimentally validated.