Artifact reduction from metallic dental materials in T1‐weighted spin‐echo imaging at 3.0 tesla

Purpose:

To investigate and propose a method of artifact reduction arising from metallic dental materials by applying a slice‐encoding for metal artifact correction (SEMAC) technique on T1‐weighted spin‐echo (SE) imaging at 3 Tesla.

Materials and Methods:

The view angle tilting (VAT) technique was adapted to conventional T1‐weighted spin‐echo (SE) sequence to correct the in‐plane distortion, and the SEMAC technique was used for correcting the remaining through‐plane distortions. Fourier transform based B0 field simulations were performed to estimate the amount of field perturbation and a scout imaging method was developed which guide in selecting the number of slice‐encodings needed in SEMAC sequences. Phantoms of six different dental materials with various shapes and sizes that are used in practice (amalgam; titanium implant; gold and Ni‐Cr crowns; Ni‐Ti and stainless steel orthodontic wires) were imaged. In vivo images of two subjects were also acquired. The amounts of artifact reduction were quantified in phantom studies.

Results:

Compared with conventional SE imaging in phantom studies, in‐plane artifacts were reduced by up to 43% in the VAT SE images and 80% in the SEMAC images. Through‐plane artifacts were reduced by up to 65% in SEMAC images. In vivo SEMAC images also showed reduced artifacts.

Conclusion:

The SEMAC technique can mitigate artifact caused by metallic dental materials for T1w‐SE imaging. J. Magn. Reson. Imaging 2013;37:471–478. © 2012 Wiley Periodicals, Inc.     

Slice encoding for metal artifact correction with noise reduction

Magnetic resonance imaging (MRI) near metallic implants is often hampered by severe metal artifacts. To obtain distortion‐free MR images near metallic implants, SEMAC (Slice Encoding for Metal Artifact Correction) corrects metal artifacts via robust encoding of excited slices against metal‐induced field inhomogeneities, followed by combining the data resolved from multiple SEMAC‐encoded slices. However, as many of the resolved data elements only contain noise, SEMAC‐corrected images can suffer from relatively low signal‐to‐noise ratio. Improving the signal‐to‐noise ratio of SEMAC‐corrected images is essential to enable SEMAC in routine clinical studies. In this work, a new reconstruction procedure is proposed to reduce noise in SEMAC‐corrected images. A singular value decomposition denoising step is first applied to suppress quadrature noise in multi‐coil SEMAC‐encoded slices. Subsequently, the singular value decomposition‐denoised data are selectively included in the correction of through‐plane distortions. The experimental results demonstrate that the proposed reconstruction procedure significantly improves the SNR without compromising the correction of metal artifacts. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

New MR imaging methods for metallic implants in the knee: artifact correction and clinical impact

Purpose:

To evaluate two magnetic resonance imaging (MRI) techniques, slice encoding for metal artifact correction (SEMAC) and multiacquisition variable‐resonance image combination (MAVRIC), for their ability to correct for artifacts in postoperative knees with metal.

Materials and Methods:

A total of 25 knees were imaged in this study. Fourteen total knee replacements (TKRs) in volunteers were scanned with SEMAC, MAVRIC, and 2D fast spin‐echo (FSE) to measure artifact extent and implant rotation. The ability of the sequences to measure implant rotation and dimensions was compared in a TKR knee model. Eleven patients with a variety of metallic hardware were imaged with SEMAC and FSE to compare artifact extent and subsequent patient management was recorded.

Results:

SEMAC and MAVRIC significantly reduced artifact extent compared to FSE (P < 0.0001) and were similar to each other (P = 0.58), allowing accurate measurement of implant dimensions and rotation. The TKRs were properly aligned in the volunteers. Clinical imaging with SEMAC in symptomatic knees significantly reduced artifact (P < 0.05) and showed findings that were on the majority confirmed by subsequent noninvasive or invasive patient studies.

Conclusion:

SEMAC and MAVRIC correct for metal artifact, noninvasively providing high‐resolution images with superb bone and soft tissue contrast. J. Magn. Reson. Imaging 2011;33:1121–1127. © 2011 Wiley‐Liss, Inc.     

Accelerated slice encoding for metal artifact correction

Purpose:

To demonstrate accelerated imaging with both artifact reduction and different contrast mechanisms near metallic implants.

Materials and Methods:

Slice‐encoding for metal artifact correction (SEMAC) is a modified spin echo sequence that uses view‐angle tilting and slice‐direction phase encoding to correct both in‐plane and through‐plane artifacts. Standard spin echo trains and short‐TI inversion recovery (STIR) allow efficient PD‐weighted imaging with optional fat suppression. A completely linear reconstruction allows incorporation of parallel imaging and partial Fourier imaging. The signal‐to‐noise ratio (SNR) effects of all reconstructions were quantified in one subject. Ten subjects with different metallic implants were scanned using SEMAC protocols, all with scan times below 11 minutes, as well as with standard spin echo methods.

Results:

The SNR using standard acceleration techniques is unaffected by the linear SEMAC reconstruction. In all cases with implants, accelerated SEMAC significantly reduced artifacts compared with standard imaging techniques, with no additional artifacts from acceleration techniques. The use of different contrast mechanisms allowed differentiation of fluid from other structures in several subjects.

Conclusion:

SEMAC imaging can be combined with standard echo‐train imaging, parallel imaging, partial‐Fourier imaging, and inversion recovery techniques to offer flexible image contrast with a dramatic reduction of metal‐induced artifacts in scan times under 11 minutes. J. Magn. Reson. Imaging 2010;31:987–996. ©2010 Wiley‐Liss, Inc.     

SEMAC快速发现白芍吸收成分群的研究

目的:建立基于肠吸收技术以快速发现白芍吸收成分群的方法,为选择白芍质量控制指标提供参考.方法:采用大鼠肠外翻模型,收集3个质量浓度白芍提取物给药后不同时间的肠囊液样品,用HPLC进行检测分析,并计算其有效成分芍药内醋苷、芍药苷的累计吸收量.结果:白芍中5种主要成分均可进人肠囊,在45 min后能全部检测到;不同质量浓度白芍中芍药内酯苷、芍药苷在各肠段均为线性吸收,R2均大于0.9,符合零级吸收速率;芍药内醋苷和芍药苷在空肠、回肠中的吸收速率常数(Ka)随着白芍提取物浓度增加而增加(P<0.05),符合被动吸收.结论:基于肠吸收的质量评价指标选择方法(SEMAC)可以用于白芍吸收成分群的研究.肠囊对药物成分吸收有选择性,回肠与空肠相比可更多、更快地给出吸收成分信息,选择肠外翻60min的样品检测白芍吸收成分比较适宜.