The large magnetic susceptibility differences that occur between metallic implants and tissue generates severe B₀ inhomogeneities. This leads to several challenges including excitation of the entire off-resonance spectrum, distortion in the frequency encoding direction, and severe intra-voxel dephasing. 3D-Multispectral Imaging (3D-MSI) techniques such as MAVRIC (1) and SEMAC (2) have substantially reduced metal related artifacts. However, these techniques are limited by distortions related to frequency encoding (3,4) that manifest as signal loss or signal pile-up artifacts. The purpose of this work was to develop a clinically feasible, fully phase encoded method to image near metallic implants.In this work we propose an ultra-short echo time (UTE) acquisition with broadband excitation of the entire off-resonance spectrum in a single acquisition, by using low flip angles and non-selective RF pulses. Large in-plane distortions caused by the combination of frequency encoding and broadband excitation were avoided by using single point imaging (SPI) while clinically feasible acquisition times were obtained through 3D undersampling schemes. Figure 1 describes the proposed UTE-SPI pulse sequence where excitation occurs with the gradients on at a maximum gradient amplitude (“G_RF”) to minimize unwanted slice selectivity and signal excitation loss near the implant (5). Immediately after excitation, gradients are ramped.

All acquisitions were performed at 3T (MR 750, GE Healthcare, Waukesha. WI) using a 16-channel wrap coil (NeoCoil, Pewaukee, WO). All UTE-SPI datasets were reconstructed using a 3D GRAPPA (6) reconstruction. UTE-SPI acquisitions of a total hip replacement phantom composed of a Co/Cr/Mo alloy head and titanium stem (Alliance X-Series, Biomet Orthopedics, Inc., Warsaw, IN) were acquired using three different RF pulses: 8μs (G_RF=30%, TE=270μs), 24μs (G_RF=10%, TE=310μs) and 100μs (G_RF=0%, TE=336μs). The following imaging parameters were held constant over all acquisitions: R=2x2x1, matrix=224x148x80, TR=1.5ms, FA=2°, Field of View=224x170x80mm, BW=±250kHz, G_max=45mT/m, acquisition time =9.5min.

UTE-SPI (8μs and 24μs RF pulses), T₁-weighted MAVRIC, and 2D-FSE acquisitions were acquired of a volunteer with a total knee replacement. UTE-SPI imaging parameters were: R=2x2x1, TR=1.5ms, TE=366μs, matrix=224x170x40, G_RF=0%, G_max=45mT/m, BW=±250kHz, Field of View=224x170x180mm, FA=2°/6° (8μs/24μs), acquisition time=5:32min. T₁-weighted MAVRIC imaging parameters were: R=2x2, TR=1245ms, TE=6.7ms, FA=90°, ETL=24, matrix=224x170x40, Field of View=224x170x180mm, acquisition time=5:22min. 2D-FSE imaging parameters were: TR=4000ms, TE=8.8ms, FA=111°, ETL=24, matrix=256x256x30, Field of View=224x224mm, slice thickness=4mm, slice spacing=1mm, NEX=2.

Figure 2 demonstrates that short RF pulses allow for improved spectral coverage and smaller minimum TEs (through the use of higher G_RF) allowing imaging of signal close to the implant. Improved T₁ weighting can be achieved using longer pulses with larger maximum attainable flip angles (Figure 3). A comparison of the 24μs (FA=6°) UTE-SPI to T₁-weighted MAVRIC with matching resolution showed both acquisitions had similar contrast and acquisition efficiency. Blue arrows highlight signal pile-up and ripple artifacts seen in the MAVRIC image. UTE-SPI techniques also open the potential to image short T₂^* signal such as the plastic liners of implants. Future work will examine the feasibility of imaging these liners.3D undersampled UTE-SPI acquisitions with broadband excitation can generate distortion free images near metallic implants in clinically feasible acquisition times. The trade-offs of shorter and longer RF pulses were demonstrated in a total hip replacement phantom and a volunteer with a total knee replacement. Shorter RF pulses captured signal closer to the implant by improving spectral coverage and allowing shorter TEs while longer pulses improve T₁ weighting through the larger maximum attainable flip angles.