Quantitative assessment of an MR technique for reducing metal artifact: application to spin-echo imaging in a phantom

Objective. To quantify image artifact reduction using a new technique (MARS – metal artifact reduction sequence) in vitro. Design. Coronal T1-weighted MR images were obtained through two metal phantoms (titanium/chromium-cobalt and stainless steel femoral prostheses) immersed in water. Comparison of artifact volume was made with images obtained using conventional and modified (MARS) T1-weighted sequences. Signal intensity values outside a range of ±40% the average signal intensity for water were considered artifact and segmented into low or high signal artifact categories. Considering the arbitrary selection of this threshold value, volumetric calculations of artifact were also evaluated at ±50%, 60%, 70%, and 80% the mean signal for water. Results. Conventional T1-weighted images produced 87% more low signal artifact and 212% more high signal artifact compared with the MARS modified T1-weighted images of the stainless steel prosthesis. Conventional T1-weighted images of the titanium prosthesis produced 84% more low signal artifact and 211% more high signal artifact than the MARS modified sequence. The level of artifact reduction was essentially uniform for the various threshold levels tested and was greatest at ±20% the global signal intensity average for water. Conclusion. The MARS technique reduces the volume of image signal artifact produced by stainless steel and titanium/chromium-cobalt femoral prostheses on T1-weighted spin-echo images in a tissue phantom model. Received: 11 April 2000 Revision requested: 22 May 2000 Revision received: 26 October 2000 Accepted: 27 November 2000     

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.     

MRI of spinal hardware: comparison of conventional T1-weighted sequence with a new metal artifact reduction sequence

Objective. This study was designed to compare diagnostic quality of MR images of patients with spinal hardware acquired using a conventional T1-weighted spin-echo sequence and a new metal artifact reduction sequence (MARS). Conclusion. The new MARS sequence effectively reduces the degree of tissue-obscuring artifact produced by spinal fixation hardware and subjectively improves image quality compared with the conventional T1-weighted spin-echo sequence. Received: 30 May 2000 Revision requested: 12 September 2000 Revision received: 27 September 2000 Accepted: 27 November 2000     

Performance of single-energy metal artifact reduction in cardiac computed tomography: A clinical and phantom study

BackgroundTo investigate the performance of a reconstruction algorithm, single-energy metal artifact reduction (SEMAR), against standard reconstruction in cardiac computed tomography (CT) studies of patients with implanted metal and in a defibrillator lead phantom.MethodsFrom a retrospective, cross-sectional clinical study with institutional review board approval of 118 patients with implanted metal, 122 cardiac CT studies from November 2009 to August 2016 performed on a 320-detector row scanner with standard and SEMAR reconstructions were included. The maximum beam hardening artifact radius, artifact attenuation variation surrounding the implanted metal, and image quality on a 4-point scale (1-no/minimal artifact to 4-severe artifact) were assessed for each reconstruction. A defibrillator lead phantom study was performed at different tube potentials and currents with both reconstruction methods. Maximum beam hardening artifact radius and average artifact attenuation variation were measured.ResultsIn the clinical study, SEMAR markedly reduced the maximum beam hardening artifact radius by 77% (standard: 14.8 mm [IQR 9.7–22.2] vs. SEMAR: 3.4 mm [IQR 2.2–7.1], p < 0.0001) and artifact attenuation variation by 51% (standard: 130.0 HU [IQR 75.9–184.4] vs. SEMAR: 64.3 HU [IQR 48.2–89.2], p < 0.0001). Image quality improved with SEMAR (standard: 3 [IQR 2–3.5] vs. SEMAR: 2 [IQR 1–2.5], p < 0.0001). The defibrillator lead phantom study confirmed these results across varying tube potentials and currents.ConclusionsSEMAR reconstruction achieved superior image quality and markedly reduced maximum beam hardening artifact radius and artifact attenuation variation compared to standard reconstruction in 122 clinical cardiac CT studies of patients with implanted metal and in a defibrillator lead phantom study.     

Metal artifact reduction sequence: early clinical applications.

Artifact arising from metal hardware remains a significant problem in orthopedic magnetic resonance imaging. The metal artifact reduction sequence (MARS) reduces the size and intensity of susceptibility artifacts from magnetic field distortion. The sequence, which is based on view angle tilting in combination with increased gradient strength, can be conveniently used in conjunction with any spin-echo sequence and requires no additional imaging time. In patients with persistent pain after femoral neck fracture, the MARS technique allows visualization of marrow adjacent to hip screws, thus enabling diagnosis or exclusion of avascular necrosis. Other applications in the hip include assessment of periprosthetic soft tissues after hip joint replacement surgery, postoperative assessment after resection of bone tumors and reconstruction, and localization of unopacified methyl methacrylate cement prior to hip arthroplasty revision surgery. In the knee, the MARS technique allows visualization of structures adjacent to implanted metal staples, pins, or screws. The technique can significantly improve visualization of periprosthetic bone and soft-tissue structures even in patients who have undergone total knee arthroplasty. In patients with spinal fixation hardware, the MARS technique frequently allows visualization of the vertebral bodies and spinal canal contents. The technique can be helpful after wrist fusion or screw fixation of scaphoid fractures.     

Fast iterative algorithm for metal artifact reduction in X-ray CT

RATIONALE AND OBJECTIVES: The reduction of metal artifacts in x-ray computed tomography (CT) has important clinical applications. An iterative method adapted from the expectation maximization (EM) formula for emission CT was shown to be effective for metal artifact reduction, but its computational speed is slow. The goal of this project was to accelerate that iterative method for metal artifact reduction. MATERIALS AND METHODS: Using the row-action/ordered-subset (EM) formula for emission CT as a basis, the authors developed a fast iterative algorithm for metal artifact reduction. In each iteration of this algorithm, both reprojection from an intermediate image and backprojection from discrepancy data are performed. RESULTS: The feasibility of the fast iterative algorithm was demonstrated in numerical and phantom experiments. In comparison with the nonaccelerated iterative algorithm, the speed of iterative metal artifact reduction is improved by an order of magnitude given image quality in terms of visual inspection, I-divergence in the projection domain, and the euclidean distance in the image domain. CONCLUSION: The fast iterative algorithm corrects intermediate reconstruction according to subsets of projections and produces satisfactory image quality at a much faster speed than the previously published iterative algorithm. This algorithm has important potential in clinical applications, such as orthopedic, oncologic, and dental imaging.     

MR imaging with metal artifact-reducing sequences and gadolinium contrast agent in a case-control study of periprosthetic abnormalities in patients with metal-on-metal hip prostheses

Objective

To apply and compare magnetic resonance imaging (MRI) metal artifact reducing sequences (MARS) including subtraction imaging after contrast application in patients with metal-on-metal (MoM) hip prostheses, investigate the prevalence and characteristics of periprosthetic abnormalities, as well as their relation with pain and risk factors.

Materials and methods

Fifty-two MoM prostheses (35 cases with pain and or risk factors, and 17 controls) in 47 patients were examined in a 1.5-T MR scanner using MARS: turbo spin echo (TSE) with high readout bandwidth with and without view angle tilting (VAT), TSE with VAT and slice encoding for metal artifact correction (SEMAC), short tau inversion recovery (STIR) with matched RF pulses, and post-contrast imaging. The relations of MRI findings to pain and risk factors were analyzed and in five revised hips findings from operation, histology, and MRI were compared.

Results

TSE VAT detected the highest number of osteolyses. Soft tissue mass, effusion, and capsular thickening were common, whereas osteolysis in acetabulum and femur were less frequent. Contrast enhancement occurred in bone, synovia, joint capsule, and the periphery of soft tissue mass. There was no significant relation between MRI findings and pain or risk factors.

Conclusions

MARS and gadolinium subtraction imaging are useful for evaluation of complications to MoM prosthesis. TSE VAT had the highest sensitivity for osteolysis. Contrast enhancement might indicate activation of aseptic lymphocyte-dominated vasculitis-associated lesion (ALVAL). Pain, small head, or steep prosthesis inclination angle are not useful predictors of periprosthetic abnormalities, and wide indications for MR follow-up are warranted.     

Metal artifact reduction image reconstruction algorithm for CT of implanted metal orthopedic devices: a work in progress

Introduction  Despite recent advances in CT technology, metal orthopedic implants continue to cause significant artifacts on many CT exams, often obscuring diagnostic information. We performed this prospective study to evaluate the effectiveness of an experimental metal artifact reduction (MAR) image reconstruction program for CT. Materials and methods  We examined image quality on CT exams performed in patients with hip arthroplasties as well as other types of implanted metal orthopedic devices. The exam raw data were reconstructed using two different methods, the standard filtered backprojection (FBP) program and the MAR program. Images were evaluated for quality of the metal–cement–bone interfaces, trabeculae ≤1 cm from the metal, trabeculae 5 cm apart from the metal, streak artifact, and overall soft tissue detail. The Wilcoxon Rank Sum test was used to compare the image scores from the large and small prostheses. Interobserver agreement was calculated. Results  When all patients were grouped together, the MAR images showed mild to moderate improvement over the FBP images. However, when the cases were divided by implant size, the MAR images consistently received higher image quality scores than the FBP images for large metal implants (total hip prostheses). For small metal implants (screws, plates, staples), conversely, the MAR images received lower image quality scores than the FBP images due to blurring artifact. The difference of image scores for the large and small implants was significant (p = 0.002). Interobserver agreement was found to be high for all measures of image quality (k > 0.9). Conclusion  The experimental MAR reconstruction algorithm significantly improved CT image quality for patients with large metal implants. However, the MAR algorithm introduced blurring artifact that reduced image quality with small metal implants.     

双能量CT虚拟单能量重建技术显示肝癌TACE术后碘油沉积的价值

目的：探讨双能量CT虚拟单能量重建技术对肝癌经导管动脉化疗栓塞（TACE）术后碘油沉积的临床诊断价值。方法：对11例肝癌TACE术后患者进行双能量腹部CT平扫。在100 kVp、140 kVp、线性融合图像（M＝0．5）和以9组虚拟单能量（40、60、80、100、120、140、160、180和190 keV）图像上,在碘油沉积最显著的层面,测量伪影区及正常肝实质区的CT值,计算图像噪声（SD）和伪影指数（AI）。对伪影大小、图像噪声、碘油形态及综合图像质量分别进行主观评分。结果：客观评估：80 keV 图像的AI 值最小;线性融合图像、80 keV 图像的噪声最小。主观评估：在140 kVp、100～190 keV图像上伪影较少;线性融合图像和80 keV 图像的噪声评分最小;碘油形态以140 kVp、线性融合图像、80及100 keV图像上显示较为清晰。整体综合评分以线性融合图像和80 keV图像最高。结论：双能量CT 虚拟单能量重建技术在肝癌TACE术后碘油沉积的显示中,可以改善图像质量,以80 keV的虚拟单能量重建图像为最佳。     

The SENSE ghost: field-of-view restrictions for SENSE imaging

PURPOSE: To describe a known (but undocumented) limitation in parallel imaging using simulation and experiment. This limitation consists of an artifact that appears when the imaging field of view (FOV) is less than the object size. This study demonstrates this artifact in the phase- and partition-encoding dimensions. MATERIALS AND METHODS: One-dimensional simulations as well as in vivo experiments were performed with FOVs greater and less than the object being imaged. Full-FOV, reduced-FOV, and SENSE reconstructions were visually compared. RESULTS: Image artifacts occurred when the final SENSE FOV was smaller than the object being imaged. This artifact, termed the SENSE ghost, was a residual fold-over/aliasing artifact. Its location was in the central portion of the image rather than at the edges of the image. CONCLUSION: This image artifact results from an FOV being smaller than the imaged object. The SENSE reconstruction cannot unfold this additional fold-over, and will place it in a predictable image location based on the SENSE reduction factor. Knowledge of this artifact is necessary when prescribing SENSE acquisitions and interpreting the resulting images.     

Using the dGEMRIC technique to evaluate cartilage health in the presence of surgical hardware at 3T: comparison of inversion recovery and saturation recovery approaches

Objective

To evaluate the effect of metal artifact reduction techniques on dGEMRIC T₁ calculation with surgical hardware present.

Materials and methods

We examined the effect of stainless-steel and titanium hardware on dGEMRIC T₁ maps. We tested two strategies to reduce metal artifact in dGEMRIC: (1) saturation recovery (SR) instead of inversion recovery (IR) and (2) applying the metal artifact reduction sequence (MARS), in a gadolinium-doped agarose gel phantom and in vivo with titanium hardware. T₁ maps were obtained using custom curve-fitting software and phantom ROIs were defined to compare conditions (metal, MARS, IR, SR).

Results

A large area of artifact appeared in phantom IR images with metal when TI?≤?700 ms. IR maps with metal had additional artifact both in vivo and in the phantom (shifted null points, increased mean T₁ (+151 % IR ROI_artifact) and decreased mean inversion efficiency (f; 0.45 ROI_artifact, versus 2 for perfect inversion)) compared to the SR maps (ROI_artifact: +13 % T₁ SR, 0.95 versus 1 for perfect excitation), however, SR produced noisier T₁ maps than IR (phantom SNR: 118 SR, 212 IR). MARS subtly reduced the extent of artifact in the phantom (IR and SR).

Conclusions

dGEMRIC measurement in the presence of surgical hardware at 3T is possible with appropriately applied strategies. Measurements may work best in the presence of titanium and are severely limited with stainless steel. For regions near hardware where IR produces large artifacts making dGEMRIC analysis impossible, SR-MARS may allow dGEMRIC measurements. The position and size of the IR artifact is variable, and must be assessed for each implant/imaging set-up.     

Temperature quantification using the proton frequency shift technique: In vitro and in vivo validation in an open 0.5 tesla interventional MR scanner during RF ablation

Open magnetic resonance (MR) scanners allow MR-guided targeting of tumors, as well as temperature monitoring of radio frequency (RF) ablation. The proton frequency shift (PFS) technique, an accurate and fast imaging method for temperature quantification, was used to synthesize thermal maps after RF ablation in an open 0.5 T MR system under ex vivo and in vivo conditions. Calibration experiments with 1.5% agarose gel yielded a chemical shift factor of 0.011 +/- 0.001 ppm/ degrees C (r2 = 0.96). Three gradient echo (GRE) pulse sequences were tested for thermal mapping by comparison with fiberoptic thermometer (Luxtron Model 760) readings. Temperature uncertainty decreased from high to low bandwidths (BW): +/-5.9 degrees C at BW = 15.6 kHz, +/-1.4 degrees C at BW = 3.9 kHz, and +/-0.8 degrees C at BW = 2.5 kHz. In vitro experiments (N = 9) in the paraspinal muscle yielded a chemical shift factor of 0.008 +/- 0.001 ppm/ degrees C. Temperature uncertainty was determined as +/-2.7 degrees C (BW = 3.9 kHz, TE = 19.3 msec). The same experiments carried out in the paraspinal muscle (N = 9) of a fully anesthetized pig resulted in a temperature uncertainty of +/-4.3 degrees C (BW = 3.9 kHz, TE = 19.3 msec), which is higher than it is in vitro conditions (P < 0.15). Quantitative temperature monitoring of RF ablation is feasible in a 0.5 T open-configured MR scanner under ex vivo and in vivo conditions using the PFS technique.     

Improved Image Quality in Head and Neck CT Using a 3D Iterative Approach to Reduce Metal Artifact

BACKGROUND AND PURPOSE:Metal artifacts from dental fillings and other devices degrade image quality and may compromise the detection and evaluation of lesions in the oral cavity and oropharynx by CT. The aim of this study was to evaluate the effect of iterative metal artifact reduction on CT of the oral cavity and oropharynx.MATERIALS AND METHODS:Data from 50 consecutive patients with metal artifacts from dental hardware were reconstructed with standard filtered back-projection, linear interpolation metal artifact reduction (LIMAR), and iterative metal artifact reduction. The image quality of sections that contained metal was analyzed for the severity of artifacts and diagnostic value.RESULTS:A total of 455 sections (mean ± standard deviation, 9.1 ± 4.1 sections per patient) contained metal and were evaluated with each reconstruction method. Sections without metal were not affected by the algorithms and demonstrated image quality identical to each other. Of these sections, 38% were considered nondiagnostic with filtered back-projection, 31% with LIMAR, and only 7% with iterative metal artifact reduction. Thirty-three percent of the sections had poor image quality with filtered back-projection, 46% with LIMAR, and 10% with iterative metal artifact reduction. Thirteen percent of the sections with filtered back-projection, 17% with LIMAR, and 22% with iterative metal artifact reduction were of moderate image quality, 16% of the sections with filtered back-projection, 5% with LIMAR, and 30% with iterative metal artifact reduction were of good image quality, and 1% of the sections with LIMAR and 31% with iterative metal artifact reduction were of excellent image quality.CONCLUSIONS:Iterative metal artifact reduction yields the highest image quality in comparison with filtered back-projection and linear interpolation metal artifact reduction in patients with metal hardware in the head and neck area.

Imaging plays a crucial role in the staging of oral cancers and is essential for determining tumor resectability, choosing suitable anatomic reconstruction, and planning radiation therapy. The imaging method of choice for evaluating the oral cavity and oropharynx is MR imaging because it provides higher soft-tissue contrast and is less susceptible to artifacts caused by dental hardware. Yet, the limited availability and higher costs of MR imaging, as well as individual patient conditions (breathing or swallowing disorders, claustrophobia, electronic implants such as pacemakers or ferromagnetic foreign bodies), make CT an important alternative option for many patients. Thus, CT is used frequently to stage or follow-up patients because of its wide availability, relatively low cost, and very short scan time. In patients with dental fillings or implants, however, image quality can be degraded by photon starvation and beam hardening.¹ Due to these artifacts, tumors may be only partially visible or completely obscured, making it challenging to define tumor extent. Moreover, streak artifacts may obscure ipsilateral or contralateral lymph node metastases, which can potentially change the therapeutic approach.The use of high-resolution kernels and extended CT-value ranges² improves image quality; evaluating the surrounding soft tissue, however, remains challenging or even impossible in many cases and can lead to missed findings. For metal artifact reduction (MAR),^3,4 sinogram in-painting methods have been proposed. Areas affected by metal artifacts are regarded as missing data and are filled in by different interpolation techniques, such as linear interpolation metal artifact reduction (LIMAR). Because LIMAR is associated with algorithm-induced artifacts, normalized MAR (NMAR) was developed, and it has demonstrated the potential to improve image quality in patients with artifacts from dental hardware and to improve the diagnostic accuracy of head and neck and of pelvic CT^5,6 while minimizing algorithm-induced artifacts.An extension of the MAR methods (ie, LIMAR and NMAR) is a frequency-split technique that also recovers noise texture and anatomic details in close proximity to metal. In a previous study of pelvic CT, this technique delineated adjacent bone and tissue next to metal implants more accurately than NMAR.⁶The aim of this study was to evaluate a novel 3D iterative approach using normalized and frequency split metal artifact reduction in clinical routine head and neck imaging. The resulting image quality was compared with that of filtered back-projection (FBP) reconstructions and LIMAR.     

Artifact and noise suppression in GRAPPA imaging using improved k-space coil calibration and variable density sampling.

A parallel imaging technique, GRAPPA (GeneRalized Auto-calibrating Partially Parallel Acquisitions), has been used to improve temporal or spatial resolution. Coil calibration in GRAPPA is performed in central k-space by fitting a target signal using its adjacent signals. Missing signals in outer k-space are reconstructed. However, coil calibration operates with signals that exhibit large amplitude variation while reconstruction is performed using signals with small amplitude variation. Different signal variations in coil calibration and reconstruction may result in residual image artifact and noise. The purpose of this work was to improve GRAPPA coil calibration and variable density (VD) sampling for suppressing residual artifact and noise. The proposed coil calibration was performed in local k-space along both the phase and frequency encoding directions. Outer k-space was acquired with two different reduction factors. Phantom data were reconstructed by both the conventional GRAPPA and the improved technique for comparison at an acceleration of two. Under the same acceleration, optimal sampling and calibration parameters were determined. An in vivo image was reconstructed in the same way using the predetermined optimal parameters. The performance of GRAPPA was improved by the localized coil calibration and VD sampling scheme.     

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.     

The effects of incomplete breath-holding on 3D MR Image Quality

The purpose of this study was to investigate how fast three-dimensional (3D) MR image quality is affected by breath-holding and to develop an optimal breath-holding strategy that minimizes artifact in the event of an incomplete breath-hold. A computer model was developed to study variable-duration breath-holds during fast 3D imaging. Modeling was validated by 3D gradient-echo imaging performed on 10 volunteers. Signal-to-noise ratio (SNR) and image blur were measured for both simulated and clinical images. Insights gained were applied to clinical 3D gadolinium-enhanced MR angiography. Breath-holding significantly improved abdominal 3D MR image quality. Most of this benefit could be achieved with a breath-hold fraction of 50% if it occurred during acquisition of central k space. Breath-holding during peripheral k-space acquisition, however, had no significant benefit. Respiratory motion artifact on fast 3D MRI occurring when a patient fails to suspend respiration for the entire scan duration can be minimized by collecting central k space first (centric acquisition) so that premature breathing affects only the acquisition of peripheral k space.     

Gemstone spectral imaging reduced artefacts from metal coils or clips after treatment of cerebral aneurysms: a retrospective study of 35 patients

Objective:

This study aimed to evaluate the effect of gemstone spectral imaging (GSI) for metal artefact reduction in cerebral artery CT angiography (CTA) after metal coils or clips treatment.

Methods:

35 patients with cerebral aneurysms were treated with metal coils or clips and underwent CTA using gemstone spectral CT between February and December 2013. The data were reconstructed into three image groups including Group A (quality check images with 140 kVp), Group B (monochromatic image sets in the range of 40–140 keV) and Group C [monochromatic image sets with metal artefacts reduction software (MARS GE Medical Systems, Waukesha, WI)]. CT attenuation value of cerebral artery, contrast-to-noise ratio (CNR), signal-to-noise ratio (SNR) and the subjective score of all images were measured and compared statistically.

Results:

CT attenuation value of cerebral artery decreased in Groups B and C as the photon energy increased. The average energy levels of 60.05 ± 5.37 and 59.93 ± 5.57 keV presented the best CNR in Groups B and C, respectively. CNR values, SNR values and the subjective scores of the image quality of the two sets were higher than those of Group A.

Conclusion:

GSI reduced metal artefact and improved the image quality of CTA after metal coils or clips treatment in patients with cerebral aneurysm. The monochromatic images at the average energy level of 60.05 ± 5.37 keV with MARS and 59.93 ± 5.57 keV without MARS were suggested to be the optimal parameters.

Advances in knowledge:

GSI could reduce metal artefact after metal coils or clips treatment in patients with cerebral aneurysm.     

CT scans through metal scanning technique versus hardware composition

Objective: Streak artifact on CT scans of metal containing areas has been a long standing problem. Although several artifact reducing methods have been used to improve image quality, most have been limited by requiring specialized equipment or lengthy complex calculations that are not automated. Others have shown that increasing the beam energy results in increased thickness of metal that may be imaged by CT without severe image degradation. We have studied the image quality of bone surrounding metal both with titanium and cobalt-chrome prostheses using various scanning techniques. Methods: In a double blind fashion, 28 radiology residents and attendings were surveyed as to the best technique for imaging bone detail surrounding metal. A series of images was arranged of an implanted titanium prosthesis, a cobalt-chrome prosthesis and a pelvis repaired with stainless steel pelvic reconstruction plates. Scans were performed using three techniques: 120 kVp, 170 mA, 2 s, 360° rotation, 140 kVp,140 mA, 3 s, 360° rotation, 140 kVp, 140 mA, 4 s, 420° rotation. Results: Titanium was superior to cobalt-chrome (p < .0001 Wilcoxon Signed Rank Test). No advantage was noted for higher kVp or increased scan arc of 420° compared to the standard 360°. Conclusion: Titanium allows improved bone detail surround the metal than CT cobalt-chrome. We have found no advantage to using either high kVp or a 420° scan are to improve the image quality of bone surrounded by metal.     

Comparison of parallel acquisition techniques generalized autocalibrating partially parallel acquisitions (GRAPPA) and modified sensitivity encoding (mSENSE) in functional MRI (fMRI) at 3T

PURPOSE: To evaluate the parallel acquisition techniques, generalized autocalibrating partially parallel acquisitions (GRAPPA) and modified sensitivity encoding (mSENSE), and determine imaging parameters maximizing sensitivity toward functional activation at 3T. MATERIALS AND METHODS: A total of eight imaging protocols with different parallel imaging techniques (GRAPPA and mSENSE) and reduction factors (R = 1, 2, 3) were compared at different matrix sizes (64 and 128) with respect to temporal noise characteristics, artifact behavior, and sensitivity toward functional activation. RESULTS: Echo planar imaging (EPI) with GRAPPA and a reduction factor of 2 revealed similar image quality and sensitivity than full k-space EPI. A higher incidence of artifacts and a marked sensitivity loss occurred at R = 3. Even though the same eight-channel head coil was used for signal detection in all experiments, GRAPPA generally showed more benign patterns of spatially-varying noise amplification, and mSENSE was also more susceptible to residual unfolding artifacts than GRAPPA. CONCLUSION: At 3T and a reduction factor of 2, parallel imaging can be used with only little penalty with regard to sensitivity. With our implementation and coil setup the performance of GRAPPA was clearly superior to mSENSE. Thus, it seems advisable to pay special attention to the employed parallel imaging method and its implementation.     

Reduction of fast spin echo cusp artifact using a slice‐tilting gradient

A “featherlike” artifact, termed a cusp artifact, is sometimes seen along the phase‐encoding direction in sagittal or coronal fast spin echo images. This artifact arises from the spins, at a location distant from the magnet isocenter, that are excited and aliased to the field of view because their precession frequency is similar to those at the isocenter. Such a situation is created due to a combination of excessive gradient nonlinearity and rapid change of the main magnetic field near the edge of the magnet where the artifact‐producing spins are located. A novel technique is proposed to reduce this artifact, in which a fast spin echo pulse sequence is modified to slightly tilt the slice selected by the radiofrequency excitation pulse away from the slice selected by the radiofrequency refocusing pulses. At the edge of the field of view, the incomplete overlap between the slices selected by the excitation and refocusing pulses effectively reduces the signals from the artifact‐prone region. In contrast, the slices overlap substantially within the field of view so that the signals are largely retained. This slice‐tilting technique has been implemented on two commercial MRI scanners operating at 3.0 T and 1.5 T, respectively, and evaluated on phantoms and human spine and extremities using clinical protocols. Both phantom and human results showed that the technique decreased the strength of the cusp artifact by at least 65% and substantially limited the spatial extent of the artifact. This technique, which can be further enhanced by a simple postprocessing step, offers significant advantages over a number of other techniques for reducing the fast spin echo cusp artifact. It can be implemented on virtually any scanner without hardware modification, complicated calibration, sophisticated image reconstruction, or patient‐handling alteration. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.