Characterizing the limits of MRI near metallic prostheses

Abstract

Purpose: To characterize the fundamental limits of MRI near metallic implants on RF excitation, frequency encoding, and chemical shift-encoding water-fat separation.

Methods: Multicomponent three-dimensional (3D) digital models of a total hip and a total knee replacement were used to construct material-specific susceptibility maps. The fundamental limits and spatial relationship of imaging near metallic prostheses were investigated as a function of distance from the prosthetic surface by calculating 3D field map perturbations using a well-validated k-space based dipole kernel.

Results: Regions limited by the bandwidth of RF excitation overlap substantially with those fundamentally limited by frequency encoding. Rapid breakdown of water-fat separation occurs once the intravoxel off-resonance exceeds ∼6 ppm over a full range of fat fractions (0%-100%) and SNR (5-100).

Conclusion: Current 3D multispectral imaging methods would not benefit greatly from exciting spins beyond ±12 kHz despite the presence of signal that lies outside of this range from tissue directly adjacent to the metallic implants. Methods such as phase encoding in all three spatial dimensions are necessary to spatially resolve spins beyond an excitation bandwidth of ±12 kHz. The approach described in this study provides a benchmark for the capabilities of current imaging techniques to guide development of new MRI methods for imaging near metal.

Keywords: B0 inhomogeneity; magnetic resonance imaging; metallic implants; off-resonance; prostheses; water-fat separation.

Figure 1

Digital models obtained from manufacturer displayed to visualize the susceptibility values used for each component and also the cut-planes used below for subsequent analysis. Each subcomponent was assigned a material specific susceptibility value. The combination of susceptibility maps produced an overall susceptibility distribution that was used to generate the metal-induced field-maps. The total hip implant consisted of a femoral stem (silver, titanium alloy, Ti-6Al-4V, 182 ppm), femoral head (green, CoCrMo, 1300 ppm), plastic liner (blue, ultra high molecular weight polyethylene, 9 ppm), and acetabular cup (yellow, CoCrMo, 1300 ppm). The total knee implant consisted of a tibial component (cyan and blue, titanium alloy, Ti-6Al-4V, 182 ppm), plastic liner (yellow, ultra high molecular weight polyethylene, 9 ppm), and femoral component (pink, CoCrMo, 1300 ppm).

Figure 2

The magnetization of CoCrMo is linear up to 3T based on acquired RF bands at 1.5T (±2000 Hz) and 3.0T (±4000 Hz) with full width half max of 1.5 kHz and 3.0 kHz bandwidth respectively. The bands appear in the same spatial location as demonstrated by the line plots of the normalized signal and agree well with the simulated field-map using a susceptibility value of 1300 ppm.

Figure 3

B₀ field-maps demonstrate the spatial distribution and magnitude of the off-resonance induced by a total hip implant at 1.5T (top row) and 3T (bottom row). Off-resonance beyond ±12 kHz occurs greater than 5 mm away at 1.5T and 10 mm at 3T from the total hip replacement. Off-resonance beyond ±12 kHz occurs up to 5 mm away at 1.5T and up to 10 mm at 3T from the total knee replacement. Upper and lower limits of the color-map (black) were intentionally chosen to depict the off-resonance that occurs beyond the excitation bandwidth of ±12 kHz, which is the excitation bandwidth typically used by 3D-MSI methods at both 1.5T and 3T. White, dotted contour lines represent labeled iso-distances (mm) from the implant surface.

Figure 4

Line plots from regions adjacent to the implant are helpful to visualize the severity of the off-resonance directly adjacent to the implants. Plots represent off-resonance at 1.5T (dotted) and 3.0T (solid) as a function of distance for three different lines perpendicular to the implant surface as shown in the subplots. The lines in the left line-plots represent off-resonance superior to the acetabular cup (black), lateral to the femoral head (light gray), and superior/lateral to the femoral neck (dark gray) of the total hip implant. The lines in the right plot represent off-resonance anterior to the femoral trochlea (black), superior to the posterior femoral condyles (light gray), and posterior to the posterior femoral condyles (dark gray) of the total knee implant.

Figure 5

The gradient in the readout direction of the metal-induced B₀ field inhomogeneity demonstrates regions limited by frequency encoding artifact. Upper and lower limits of the color-map (black) were chosen to depict the local gradients exceeding a readout gradient of 781 Hz/pixel where significant frequency encoding distortion artifacts would appear. White, dotted contour lines represent labeled iso-distances (mm) from the implant surface.

Figure 6

Frequency encoding direction influences the regions of signal distortion for 3D-MSI techniques. Gradient maps are 3D and must be analyzed in multiple planes and orientations to depict this signal loss. Results are shown for 1.5T.

Figure 7

a) Spectral broadening of water and fat peaks due to an intra-voxel off-resonance of 1 ppm, 3.4 ppm and 6.5 ppm. b) Simulations to determine the bias in fat-fraction estimation due to the effects of spectral broadening on water-fat separation. The bias in the estimate fat fraction was plotted against intra-voxel off-resonance for fat fractions ranging from 0 to 100% and SNR values ranging from 5 to 100.

Figure 8

Intra-voxel dephasing due to magnetic field inhomogeneities leads to break-down of chemical shift encoded water-fat separation directly adjacent to metallic prostheses in areas where the gradient of the inhomogeneity is most severe. Based on the results shown in figure 7, upper limits of the color-map (black) were chosen to depict regions where the local intra-voxel off-resonance exceeds 6 ppm (383 Hz at 1.5T, 766 Hz at 3T), the point at which intra-voxel off-resonance causes water-fat separation to break down for SNR = 20. White, dotted contour lines represent labeled iso-distances (mm) from the implant surface.

Figure 9

Combined results for RF excitation, frequency encoding, and water-fat separation near metal shown to synthesize which factor(s) are most limiting. There is close overlap with regions limited RF excitation and frequency encoding related artifacts. This implies minimal benefit to exciting a wider bandwidth of off-resonance spins for methods such as 3D-MSI that use frequency encoding. Water-fat separation, however, fails farther from the implant, indicating that it is a more limiting challenge than either RF excitation or distortion from frequency encoding artifact due to magnetic field inhomogeneities. Contour lines represent the extent of signal excitation and artifact free imaging using an RF excitation of ±12 kHz, a readout gradient of 781 Hz/pixel, and a 6 ppm intra-voxel off-resonance threshold for water-fat separation. Gridlines are spaced 10 mm apart.