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Unified Shimming and RF coil (UNIC) Reduces Susceptibility Artifacts in Prostate DWI in Subjects with Metallic Hip Prosthesis at 3T1Biomedical Imaging Research Institude, Cedars-Sinai Medical Center, Los Angeles, CA, United States, 2Department of Radiology, Weill Cornell Medicine, New York, NY, United States, 3Department of Bioengineering, University of California, Los Angeles, Los Angeles, CA, United States, 4Department of Imaging, Cedars-Sinai Medical Center, Los Angeles, CA, United States
Synopsis
Keywords: Prostate, Shims, Metallic Artifacts, Hip Prosthesis
Motivation: Metallic implants in the body may cause severe image artifacts.
Goal(s): To improve prostate DWI imaging in patients with metallic hip replacement using a novel B0 local shimming system.
Approach: Unified Shimming and RF coil (UNIC) arrays were used to improve local B0 shimming. Improved DWI image quality was assessed at 3T MRI on subjects with hip prostheses.
Results: UNIC arrays significantly improved B0 homogeneity and reduced DWI distortions in prostate, resulting in improved Dice and Jaccard coefficients.
Impact: UNIC coil arrays can enable 3T prostate DWI MRI for patients with metallic hip implants and provide the necessary care for the fast-growing population.
Introduction
Hip arthroplasty increased from 600,000 in 2010 to over 1,500,000 in 2020 (2.5x) and is projected to reach 3,400,000 in 2030 (5.5x)1-2. This trend is particularly significant in the context of an aging population, where the prevalence of total hip replacements (THR) and prostate cancer are higher. In MRI, susceptibility variation between metallic hip prosthesis and the surrounding tissue results in severe variation in the static magnetic field3. Multi-parametric MRI holds a pivotal role in prostate imaging, wherein Diffusion-Weighted Imaging (DWI) at 3T is regarded as the clinical standard for diagnosing and staging prostate cancer4-5. The utilization of single-shot echo-planar sequences renders DWI susceptible to B0 inhomogeneity. Particularly, because the hip joint is close to the prostate, the presence of metallic THR at 3T scanners can lead to strong image distortion and signal dropout, predominantly in the peripheral zone (PZ) where 70% of prostate cancers are situated6-8. Making 3T prostate MRI contraindicated in this fast-growing patient population with THR. Recently, our group has developed an integrated high-order local shim and RF array (UNIC), which showed promising results for alleviating B0 inhomogeneity all around the human body9-12. In this work, we extend the application of UNIC in prostates for patients with metallic THR. We compared UNIC with the conventional B0 shimming in a clinical scanner (2nd-order spherical harmonic shimming) and assessed its ability to recover the image quality of DWI.Methods
In vivo studies were conducted on 8 volunteers with metallic hip prostheses using a 3T Siemens Biograph mMR scanner. A set of UNIC coil array, integrates 12 RF receive channels and 42 shimming channels9-11, was adopted to provide local B0 shimming without compromising SNR, scan time, or SAR12. The coils were positioned such that one was located beneath the patient and the other was situated above, ensuring that the prostate region was centrally aligned with the board.Imaging acquisition involved two identical clinical prostate sequence series for each patient, as depicted in Fig. 1A. The first set utilized scanner 2nd order Sphere Harmonic (SH) shimming methods (Siemens optimized 2nd order SH shimming for abdomen region), while the second incorporated UNIC currents for shimming adjustments. Dual-echo GRE measured the magnetic field map in all scans. T1-VIBE sequences in axial and sagittal planes facilitated prostate localization, followed by T2-weighted images (T2W), which serve as the anatomical references. Prostate outlines were defined on a T2W image13. Parameters for the DWI-EPI sequence are illustrated in Fig. 1B, including three b-values (50, 800, 1400) and a DWI b=0 image.
For image analysis, T2W and B0 field maps were resampled to match the resolution of DWI images using in-house developed MATLAB scripts. Manual contours were drawn to delineate the prostate based on the T2W images. The degree of geometrical distortion (Dice and Jaccard indices) of the DWI images and ADC maps are assessed and compared between the UNIC shim and scanner shim. In addition, the prostate B0 field maps in THR patients were analyzed using numerical simulations to assess the effectiveness of the shimming methods.
Results and Discussion
The implementation of the UNIC array provides compelling evidence of its efficacy in reducing DWI distortion, particularly in regions proximate to metallic implants. Fig. 2 illuminates the superior performance of the UNIC array relative to the 2nd spherical harmonic shimming method. A juxtaposition of the B0 field map in Fig. 2—all with and without the UNIC array—highlights a significant reduction in B0 frequency variation.Fig. 3A displays reduced image distortion in both the DWI low b value and the ADC map, specifically around PZ area of the prostate. The group analysis in Fig. 3B and 3C accentuates the preeminence of the UNIC array. Notably, Dice coefficient ascends from 0.56±0.11 to 0.77±0.13, and there's a concomitant enhancement in the Jaccard coefficient, which escalates from 0.41±0.11 to 0.64±0.16. When benchmarked against the contemporary scanner shim, the advancements facilitated by the UNIC array become pronounced. These refinements are attributed to the augmented B0 homogeneity, as depicted in Fig. 3D. The interquartile range (IQR) of the B0 field map within the prostate region narrows, transitioning from 52.25±19.46Hz to a more confined 33.70±7.25Hz after UNIC shimming.
Conclusion
The unified shim-RF coil array provides a direct and effective strategy to reduce hip-implant-induced DWI artifacts during prostate imaging by improving B0 field homogeneity. This technique has the potential to facilitate clinical adoption of multiparametric MRI for subjects undergoing hip arthroplasty and enable personalized prostate cancer management for the fast-growing population.Acknowledgements
Hsin-Jung Yang, and Hui Han contributed equally to this work. This work was supported by NIH R01 HL156818; R01 NS121544; R43 NS120795.References
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