SEMAC: Slice encoding for metal artifact correction in MRI

Department of Radiology, Stanford University, Stanford, CA 94305-5488, USA.
Magnetic Resonance in Medicine (Impact Factor: 3.57). 07/2009; 62(1):66-76. DOI: 10.1002/mrm.21967
Source: PubMed


Magnetic resonance imaging (MRI) near metallic implants remains an unmet need because of severe artifacts, which mainly stem from large metal-induced field inhomogeneities. This work addresses MRI near metallic implants with an innovative imaging technique called "Slice Encoding for Metal Artifact Correction" (SEMAC). The SEMAC technique corrects metal artifacts via robust encoding of each excited slice against metal-induced field inhomogeneities. The robust slice encoding is achieved by extending a view-angle-tilting (VAT) spin-echo sequence with additional z-phase encoding. Although the VAT compensation gradient suppresses most in-plane distortions, the z-phase encoding fully resolves distorted excitation profiles that cause through-plane distortions. By positioning all spins in a region-of-interest to their actual spatial locations, the through-plane distortions can be corrected by summing up the resolved spins in each voxel. The SEMAC technique does not require additional hardware and can be deployed to the large installed base of whole-body MRI systems. The efficacy of the SEMAC technique in eliminating metal-induced distortions with feasible scan times is validated in phantom and in vivo spine and knee studies.

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Article: SEMAC: Slice encoding for metal artifact correction in MRI

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    • "An increasing trend in the frequency of spinal fusion operations is evident [1] [2], and thus imaging of prosthesis-related complications is of increasing importance. SEMAC-coded MR sequence without fat saturation has been introduced and showed clinical usefulness [4] [6] [9]. Fluid sensitive MR imaging plays an important role in postoperative evaluation. "
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    ABSTRACT: Background and Purpose Fluid-sensitive MR imaging in postoperative evaluation is important, however, metallic artifacts is inevitable. The purpose is to investigate the feasibility of fat-saturated slice encoding for metal artifact correction (SEMAC)-corrected T2-weighted magnetic resonance (MR) at 3 T in patients with spinal prostheses. Methods Following institutional review board approval, 27 SEMAC-encoded spinal MR between September 2012 and October 2013 in patients with spinal metallic prostheses were analyzed. The MR images were scanned on a 3 T MR system including SEMAC-corrected and uncorrected fast spin echo (FSE) T2-weighted MR images with fat-saturation. Two musculoskeletal radiologists compared the image sets and qualitatively analyzed the images using a five-point scale in terms of artifact reduction around the prosthesis, visualization of the prosthesis and pedicle, and intervertebral neural foramina. Quantitative assessments were performed by calculating of the ratio of signal intensity from the fixated vertebra and that from upper level vertebra. For statistical analyses, paired t-test was used. Results Fat-saturated SEMAC-corrected T2-weighted MR images enabled significantly improved metallic artifact reduction (P < 0.05). Quantitative evaluation of the signal intensity ratio of screw-fixated vertebra and upper level vertebra showed a significantly lower ratio on fat-saturated SEMAC images (P < 0.05), however, the high signal intensity of signal pile-up could be not completely corrected. Conclusion SEMAC correction in fat-suppressed T2-weighted MR images can overcome the signal loss of metallic artifacts and provide improved delineation of the pedicle screw and peri-prosthetic region. Signal pile-up, however, could not be corrected completely, therefore readers should be cautious in the evaluation of marrow around the prosthesis.
    Magnetic Resonance Imaging 10/2014; 32(8). DOI:10.1016/j.mri.2014.04.009 · 2.09 Impact Factor
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    • "Reducing metal artifacts has been discussed substantially in MRI field for a long time and several sequences have been developed to address this issue [17-20]. However, those techniques remain mostly in research studies. "
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    ABSTRACT: Background For cervical cancer patients treated with MR-guided high dose rate brachytherapy, the accuracy of radiation delivery depends on accurate localization of both tumors and the applicator, e.g. tandem and ovoid. Standard T2-weighted (T2W) MRI has good tumor-tissue contrast. However, it suffers from poor uterus-tandem contrast, which makes the tandem delineation very challenging. In this study, we evaluated the possibility of using proton density weighted (PDW) MRI to improve the definition of titanium tandems. Methods Both T2W and PDW MRI images were obtained from each cervical cancer patient. Imaging parameters were kept the same between the T2W and PDW sequences for each patient except the echo time (90 ms for T2W and 5.5 ms for PDW) and the slice thickness (0.5 cm for T2W and 0.25 cm for PDW). Uterus-tandem contrast was calculated by the equation C = (Su-St)/Su, where Su and St represented the average signal in the uterus and the tandem, respectively. The diameter of the tandem was measured 1.5 cm away from the tip of the tandem. The tandem was segmented by the histogram thresholding technique. Results PDW MRI could significantly improve the uterus-tandem contrast compared to T2W MRI (0.42±0.24 for T2W MRI, 0.77±0.14 for PDW MRI, p=0.0002). The average difference between the measured and physical diameters of the tandem was reduced from 0.20±0.15 cm by using T2W MRI to 0.10±0.11 cm by using PDW MRI (p=0.0003). The tandem segmented from the PDW image looked more uniform and complete compared to that from the T2W image. Conclusions Compared to the standard T2W MRI, PDW MRI has better uterus-tandem contrast. The information provided by PDW MRI is complementary to those provided by T2W MRI. Therefore, we recommend adding PDW MRI to the simulation protocol to assist tandem delineation process for cervical cancer patients.
    Radiation Oncology 01/2013; 8(1):16. DOI:10.1186/1748-717X-8-16 · 2.55 Impact Factor
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    • "The situation may even get worse when, in the presence of extremely large field inhomogeneities, for example, near steel implants, the maximum available or allowable RF bandwidth becomes too small to cover the full range of off-resonance frequencies. Multiband excitation techniques, like those used in dedicated techniques for imaging near implants such as slice encoding for metal artifact correction (SEMAC) [9] and multi-acquisition variableresonance image combination (MAVRIC) [10], may be invoked to overcome this problem at the cost of efficiency. Further, it should be noted that conducting structures, such as metallic implants, also give rise to RF artifacts, including flip angle errors and phase errors, which may favor FID techniques over the SE techniques used in our study [11]. "
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    ABSTRACT: Lack of spatial accuracy is a recognized problem in magnetic resonance imaging (MRI) which severely detracts from its value as a stand-alone modality for applications that put high demands on geometric fidelity, such as radiotherapy treatment planning and stereotactic neurosurgery. In this paper, we illustrate the potential and discuss the limitations of spectroscopic imaging as a tool for generating purely phase-encoded MR images and parameter maps that preserve the geometry of an object and allow localization of object features in world coordinates. Experiments were done on a clinical system with standard facilities for imaging and spectroscopy. Images were acquired with a regular spin echo sequence and a corresponding spectroscopic imaging sequence. In the latter, successive samples of the acquired echo were used for the reconstruction of a series of evenly spaced images in the time and frequency domain. Experiments were done with a spatial linearity phantom and a series of test objects representing a wide range of susceptibility- and chemical-shift-induced off-resonance conditions. In contrast to regular spin echo imaging, spectroscopic imaging was shown to be immune to off-resonance effects, such as those caused by field inhomogeneity, susceptibility, chemical shift, f(0) offset and field drift, and to yield geometrically accurate images and parameter maps that allowed object structures to be localized in world coordinates. From these illustrative examples and a discussion of the limitations of purely phase-encoded imaging techniques, it is concluded that spectroscopic imaging offers a fundamental solution to the geometric deficiencies of MRI which may evolve toward a practical solution when full advantage will be taken of current developments with regard to scan time reduction. This perspective is backed up by a demonstration of the significant scan time reduction that may be achieved by the use of compressed sensing for a simple phantom.
    Magnetic Resonance Imaging 08/2012; 31(1). DOI:10.1016/j.mri.2012.06.023 · 2.09 Impact Factor
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