Single Point Imaging with Broadband Excitations and Ultra-Short Echo Times for Imaging near Metallic Implants
Curtis Wiens1, Nathan S. Artz1,2, Hyungseok Jang1, Alan McMillan1, Kevin Koch3, and Scott B. Reeder1,4,5,6,7

1Radiology, University of Wisconsin, Madison, WI, United States, 2Diagnostic Imaging, St. Jude Children's Research Hospital, Memphis, TN, United States, 3Biophysics and Radiology, Medical College of Wisconsin, Milwaukee, WI, United States, 4Medical Physics, University of Wisconsin, Madison, WI, United States, 5Biomedical Engineering, University of Wisconsin, Madison, WI, United States, 6Medicine, University of Wisconsin, Madison, WI, United States, 7Emergency Medicine, University of Wisconsin, Madison, WI, United States

Synopsis

Large magnetic susceptibility differences between metallic implants and tissue generate severe B0 inhomogeneities that present several challenges for MR: excitation of the entire off-resonance spectrum, distortion in the frequency encoding direction, and severe intra-voxel dephasing. In this work we propose an ultra-short echo time acquisition with broadband excitation. Single point encoding was use to avoid in-plane distortions while 3D undersampling facilitated clinically feasible acquisition times. The effects of RF pulse duration on signal loss near the implant and T1 weighting were evaluated in a total hip replacement phantom and a volunteer with a total knee replacement.

Introduction

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.

Theory

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 (“GRF”) to minimize unwanted slice selectivity and signal excitation loss near the implant (5). Immediately after excitation, gradients are ramped.

Methods

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 (GRF=30%, TE=270μs), 24μs (GRF=10%, TE=310μs) and 100μs (GRF=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, Gmax=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, GRF=0%, Gmax=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.

Results and Discussion

Figure 2 demonstrates that short RF pulses allow for improved spectral coverage and smaller minimum TEs (through the use of higher GRF) 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.

Conclusions

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.

Acknowledgements

The authors acknowledge the support of NSERC, NIH (UL1TR00427) and GE Healthcare.

References

1. Koch KM, Lorbiecki JE, Hinks RS, King KF. A multispectral three-dimensional acquisition technique for imaging near metal implants. Magn. Reson. Med. 2009;61:381–390.

2. Lu W, Pauly KB, Gold GE,et al. SEMAC: Slice encoding for metal artifact correction in MRI. Magn. Reson. Med. 2009;62:66–76.

3. Koch KM, King KF, Carl M, Hargreaves BA. Imaging near metal: The impact of extreme static local field gradients on frequency encoding processes. Magn. Reson. Med. 2013:71: 2024–2034.

4. Smith MR, Artz NS, Wiens C, et al. Characterizing the limits of MRI near metallic prostheses. Magn. Reson. Med. 2014: doi: 10.1002/mrm.25540.

5. Jang H, Wiens CN, McMillan AB. Ramped hybrid encoding for improved ultrashort echo time imaging. Magn. Reson. Med. 2015: doi: 10.1002/mrm.25977.

6. Griswold MA, Jakob PM, Heidemann RM, et al. Generalized autocalibrating partially parallel acquisitions (GRAPPA). Magn. Reson. Med. 2002;47:1202–1210.

Figures

Figure 1: The pulse sequence of the purposed Ultra-short TE Single Point Imaging (UTE-SPI) technique.

Figure 2: Shorter RF pulses have improved spectral coverage and shorter TEs (larger GRF) allowing signal closer to the implant to be captured. UTE-SPI acquisitions (coronal and axial reformats) are shown of a hip replacement phantom using different RF pulses: 8μs (GRF=30%, TE=270µs), 24μs (GRF=10%, TE=310µs), and 100μs (GRF=0%, TE=336µs).

Figure 3: Longer RF pulses allow for improved T1 weighting through larger maximum attainable flip angles. UTE-SPI acquisitions using an 8µs (FA=2°) and 24μs (FA=6°) RF pulses and clinically used 2D-FSE and T1 weighted MAVRIC are shown of a volunteer with a Co/Cr/Mo alloy total knee replacement.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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