Slice encoding for metal artifact correction with noise reduction

Comprehensive assessment of nonuniform image quality: Application to imaging near metal

PURPOSE

Comprehensive assessment of image quality requires accounting for spatial variations in (i) intensity artifact, (ii) geometric distortion, (iii) signal-to-noise ratio (SNR), and (iv) spatial resolution, among other factors. This work presents an ensemble of methods to meet this need, from phantom design to image analysis, and applies it to the scenario of imaging near metal.

METHODS

A modular phantom design employing a gyroid lattice is developed to enable the co-registered volumetric quantitation of image quality near a metallic hip implant. A method for measuring spatial resolution by means of local point spread function (PSF) estimation is presented and the relative fitness of gyroid and cubic lattices is examined. Intensity artifact, geometric distortion, and SNR maps are also computed. Results are demonstrated with 2D-FSE and MAVRIC-SL scan protocols on a 3T MRI scanner.

RESULTS

The spatial resolution method demonstrates a worst-case error of 0.17 pixels for measuring in-plane blurring up to 3 pixels (full width at half maximum). The gyroid outperforms a cubic lattice design for the local PSF estimation task. The phantom supports four configurations toggling the presence/absence of both metal and structure with good spatial correspondence for co-registered analysis of the four quality factors. The marginal scan time to evaluate one scan protocol amounts to five repetitions. The phantom design can be fabricated in 2 days at negligible material cost.

CONCLUSION

The phantom and associated analysis methods can elucidate complex image quality trade-offs involving intensity artifact, geometric distortion, SNR, and spatial resolution. The ensemble of methods is suitable for benchmarking imaging performance near metal.

Quantitative evaluation of the reduction of distortion and metallic artifacts in magnetic resonance images using the multiacquisition variable‑resonance image combination selective sequence

Magnetic resonance imaging (MRI) is superior to computed tomography (CT) in determining changes in tissue structure, such as those observed following inflammation and infection. However, when metal implants or other metal objects are present, MRI exhibits more distortion and artifacts compared with CT, which hinders the accurate measurement of the implants. A limited number of reports have examined whether the novel MRI sequence, multiacquisition variable-resonance image combination selective (MAVRIC SL), can accurately measure metal implants without distortion. Therefore, the present study aimed to demonstrate whether MAVRIC SL could accurately measure metal implants without distortion and whether the area around the metal implants could be well delineated without artifacts. An agar phantom containing a titanium alloy lumbar implant was used for the present study and was imaged using a 3.0 T MRI machine. A total of three imaging sequences, namely MAVRIC SL, CUBE and magnetic image compilation (MAGiC), were applied and the results were compared. Distortion was evaluated by measuring the screw diameter and distance between the screws multiple times in the phase and frequency directions by two different investigators. The artifact region around the implant was examined using a quantitative method following standardization of the phantom signal values. It was revealed that MAVRIC SL was a superior sequence compared with CUBE and MAGiC, as there was significantly less distortion, a lack of bias between the two different investigators and significantly reduced artifact regions. These results suggested the possibility of utilizing MAVRIC SL for follow-up to observe metal implant insertions.

Effective magnetic susceptibility of 3D-printed porous metal scaffolds

PURPOSE

3D-printed porous metal scaffolds are a promising emerging technology in orthopedic implant design. Compared to solid metal implants, porous metal implants have lower magnetic susceptibility values, which have a direct impact on imaging time and image quality. The purpose of this study is to determine the relationship between porosity and effective susceptibility through quantitative estimates informed by comparing coregistered scanned and simulated field maps.

METHODS

Five porous scaffold cylinders were designed and 3D-printed in titanium alloy (Ti-6Al-4V) with nominal porosities ranging from 60% to 90% using a cellular sheet-based gyroid design. The effective susceptibility of each cylinder was estimated by comparing acquired B₀ field maps against simulations of a solid cylinder of varying assigned magnetic susceptibility, where the orientation and volume of interest of the simulations was informed by a custom alignment phantom.

RESULTS

Magnitude images and field maps showed obvious decreases in artifact size and field inhomogeneity with increasing porosity. The effective susceptibility was found to be linearly correlated with porosity (R²  = 0.9993). The extrapolated 100% porous (no metal) magnetic susceptibility was -9.9 ppm, closely matching the expected value of pure water (-9 ppm), indicating a reliable estimation of susceptibility.

CONCLUSION

Effective susceptibility of porous metal scaffolds is linearly correlated with porosity. Highly porous implants have sufficiently low effective susceptibilities to be more amenable to routine imaging with MRI.

Using SEMAC at 3 T MR to evaluate spinal metallic implants and peripheral soft tissue lesions

We aimed to assess the usefulness of slice-encoding metal artifact correction (SEMAC) for the evaluation of spinal metallic implants and peripheral soft tissue lesions at 3T magnetic resonance.Twenty-seven patients with spinal metal implants underwent both SEMAC and high bandwidth (HiBW) based sequences scanning for reduction artifacts. The area size and maximum longitude of artifacts, the peri-prosthetic soft tissue, and metal visualization were assessed by 2 independent doctors, as well as the lesions signs were reviewed by 2 senior readers. A paired 2-tailed t-test and McNemar test were used for statistical analysis.The size of artifacts on SEMAC images decreased by 37% and 24%, and the scores are higher than that on HiBW images. T1 weighted (T1W)-SEMAC acquired the highest score in metal prosthesis visualization, while short tau inversion recovery SEMAC showed more signs of lesions than clinical HiBW group.SEMAC effectively reduces the metal artifacts and is useful for assessing soft tissue lesions.

Intensity of artefacts in cone beam CT examinations caused by titanium and glass fibre-reinforced composite implants

OBJECTIVES:

The aim was to compare titanium and glass fibre-reinforced composite (FRC) orbital floor implants using cone beam CT (CBCT). FRC implants are nonmetallic and these implants have not been analysed in CBCT images before. The purpose of this study is to compare the artefact formation of the titanium and the FRC orbital floor implants in CBCT images.

METHODS:

One commercially pure titanium and one S-glass FRC with bioactive glass particles implant were imaged with CBCT using the same imaging values (80 kV, 1 mA, FOV 60 × 60 mm). CBCT images were analysed in axial slices from three areas to determine the magnitude of the artefacts in the vicinity of the implants. Quantified results based on the gray values of images were analysed using analysis-of-variance.

RESULTS:

Compared to the reference the gray values of the titanium implant are more negative in every region of interest in all slices (p < 0.05) whereas the gray values of the FRC implant differ statistically significantly in less than half of the examined areas.

CONCLUSIONS:

The titanium implant caused artefacts in all of the analysed CBCT slices. Compared to the reference the gray values of the FRC implant changed only slightly and this feature enables to use wider imaging options postoperatively.

[Modern radiological diagnostics in spine surgery]

BACKGROUND

Radiological imaging is important in the preoperative diagnosis of many forms of spinal pathology and plays a fundamental role in the assessment of p.o. effects, which can be verified on the spinal column as well as on the surrounding soft tissues, depending on the imaging method used.

AIM

The article provides an overview of the current status and possibilities of radiological diagnostic methods for the verification of possibly recommended spine surgery in the context of degenerative, inflammatory-infectious, post-traumatic or p.o. pathologies and changes in the spine: X‑rays, computed tomography (CT), magnetic resonance imaging (MRI). The supplementary nuclear medicine procedures (scintigraphy, PET[-CT], SPECT, etc.) which may be required for special questions are not discussed.

MATERIAL AND METHODS

The merits and limitations of the techniques used in the investigation of advanced degenerative spinal pathologies and post-traumatic conditions are discussed, with multidetector CT being the focus of attention in spinal clearance for traumatic injuries. In most cases of spinal infection, MRI images, as a central diagnostic tool, show typical findings such as destruction of adjacent endplates, bone marrow and intervertebral disc abnormalities, and paravertebral or epidural abscesses. However, it is not always easy to diagnose a spinal infection, especially if atypical MR patterns of infectious spondylitis are present. Knowledge of them means misdiagnosis and improper treatment can be avoided.

RESULTS

It is shown that high-quality modern radiological examinations are essential for diagnosis and p.o. management, as these provide answers to the main questions in the treatment: Is the entity/injury stable or unstable, acute or old, benign or malign; is there a myelopathy or p.o. complication?

DISCUSSION

The main indications for p.o. diagnostic imaging, difficulties such as metal artefact formation, and potential pitfalls are analyzed. Entity-specific radiological image patterns, imaging algorithms and differential diagnostic peculiarities are presented and discussed based on current literature and selected case studies.

Spine MR images in patients with pedicle screw fixation: Comparison of conventional and SEMAC-VAT sequences at 1.5 T

BACKGROUND AND PURPOSE

Slice-encoding metal artifact corrections (SEMAC)-view-angle tilting (VAT) sequences have recently been used in clinical protocols to reduce metal artifacts in MR scans of patients with spinal instrumentation. The objective of this study was to compare the SEMAC-VAT sequence with the conventional MR sequence with a low bandwidth turbo-spin echo (TSE) in terms of image quality, visibility of periprosthetic structures, and diagnostic confidence for detection of postoperative complications in patients who underwent pedicle screw fixation at 1.5 T.

METHODS

Seventy patients who underwent pedicle screw fixation between the thoracic vertebrae and the sacrum were included in the study. The MR scans were retrospectively evaluated by two radiologists for signal-to-noise ratio of anatomical structures and size of artifacts, visibility of periprosthetic anatomical structures, and diagnostic confidence for detection of postoperative complications on conventional TSE and on SEMAC-VAT images. Paired t-tests and Wilcoxon signed-rank tests were used for comparisons, and kappa values were used for inter-observer agreement.

RESULTS

SEMAC-VAT images demonstrated significantly fewer metal artifacts, providing improved delineation of most periprosthetic anatomical structures and higher diagnostic confidence for detection of postoperative complications compared with conventional TSE images (p < 0.001). For the spinal canal, however, the visibility of anatomical structures and diagnostic confidence for detection of postoperative complications were better for conventional TSE than for SEMAC-VAT imaging (p < 0.001).

CONCLUSION

In conclusion, although SEMAC-VAT can significantly reduce metal artifact and provide improved delineation of periprosthetic anatomical structures compared to conventional TSE images, TSE is better for spinal canal evaluation. Therefore, it is important to understand the advantages and disadvantages of SEMAC-VAT and to use it properly.

Evaluation and reduction of magnetic resonance imaging artefacts induced by distinct plates for osseous fixation: an in vitro study @ 3 T

OBJECTIVES:

To analyze MRI artefacts induced at 3 T by bioresorbable, titanium (TI) and glass fibre reinforced composite (GFRC) plates for osseous reconstruction.

METHODS:

Fixation plates including bioresorbable polymers (Inion CPS, Inion Oy, Tampere, Finland; Rapidsorb, DePuy Synthes, Umkirch, Germany; Resorb X, Gebrueder KLS Martin GmbH, Tuttlingen, Germany), GFRC (Skulle Implants Oy, Turku, Finland) and TI plates of varying thickness and design (DePuy Synthes, Umkirch, Germany) were embedded in agarose gel and a 3 T MRI was performed using a standard protocol for head and neck imaging including T₁W and T₂W sequences. Additionally, different artefact reduction techniques (slice encoding for metal artefact reduction & ultrashort echo time) were used and their impact on the extent of artefacts evaluated for each material.

RESULTS:

All TI plates induced significantly more artefacts than resorbable plates in T₁W and T₂W sequences. GFRCs induced the least artefacts in both sequences. The total extent of artefacts increased with plate thickness and height. Plate thickness had no influence on the percentage of overestimation in all three dimensions. TI-induced artefacts were significantly reduced by both artefact reduction techniques.

CONCLUSIONS:

Polylactide, GFRC and magnesium plates produce less susceptibility artefacts in MRI compared to TI, while the dimensions of TI plates directly influence artefact extension. Slice encoding for metal artefact reduction and ultrashort echo time significantly reduce metal artefacts at the expense of scan time or image resolution.

MRI-only treatment planning: benefits and challenges

Over the past decade, the application of magnetic resonance imaging (MRI) has increased, and there is growing evidence to suggest that improvements in the accuracy of target delineation in MRI-guided radiation therapy may improve clinical outcomes in a variety of cancer types. However, some considerations should be recognized including patient motion during image acquisition and geometric accuracy of images. Moreover, MR-compatible immobilization devices need to be used when acquiring images in the treatment position while minimizing patient motion during the scan time. Finally, synthetic CT images (i.e. electron density maps) and digitally reconstructed radiograph images should be generated from MRI images for dose calculation and image guidance prior to treatment. A short review of the concepts and techniques that have been developed for implementation of MRI-only workflows in radiation therapy is provided in this document.

Comparisons of slice-encoding metal artifact correction and view-angle tilting magnetic resonance imaging and traditional digital radiography in evaluating chronic hip pain after total hip arthroplasty

Purpose

The aims of this study were (1) to compare the areas of metal-induced artifacts and definition of periprosthetic structures between patients scanned with the slice-encoding metal artifact correction and view-angle tilting (SEMAC-VAT) turbo-spin-echo (TSE) prototype and those scanned with the standard TSE magnetic resonance (MR) sequences and (2) to further clarify the superiority of the SEMAC-VAT MR imaging technique at detecting lesions in patients after total hip arthroplasty (THA), compared with digital radiography (DR).

Materials and methods

A total of 38 consecutive patients who underwent THA were referred to MR imaging at our institution. All patients suffered from chronic hip pain postoperatively. Twenty-three patients of the 38 were examined with a 1.5-T MR scanner using a SEMAC-VAT TSE prototype and standard TSE sequence, and the remaining 15 patients were examined with the same 1.5-T MR scanner, but using the SEMAC-VAT TSE prototype only. The traditional DR imaging was also performed for all patients. Two radiologists then independently measured the area of metal-induced artifacts and evaluated the definition of both the acetabular and femoral zones based on a three-point scale. Finally, the positive findings of chronic hip pain after THA based on SEMAC-VAT TSE MR imaging and traditional DR imaging were compared and analysed.

Results

The areas of metal-induced artifacts were significantly smaller in the SEMAC-VAT TSE sequences than those in the standard TSE sequences for both the T1-weighted (p < 0.001) and T2-weighted (p < 0.001) turbo inversion recovery magnitude images. In addition, 28 patients showed a series of positive signs in the SEMAC-VAT images that were not observed in the traditional DR images.

Conclusion

Compared with the standard TSE MR imaging, SEMAC-VAT MR imaging significantly reduces metal-induced artifacts and might successfully detect most positive signs missed in the traditional DR images.

Translational potential of this article

The main objective of this research was to show that MR sequences from the SEMAC-VAT TSE prototype provide a significant advantage at detecting lesions in patients after THA because of the excellent soft-tissue resolution of the MR imaging. SEMAC-VAT MR can evaluate chronic hip pain after THA and determine the cause, which can help the clinician decide on whether a surgical revision is needed.

Advanced MRI Techniques for the Ankle

OBJECTIVE

The purposes of this article are to present a state-of-the-art routine protocol for MRI of the ankle, to provide problem-solving tools based on specific clinical indications, and to introduce principles for the implementation of ultrashort echo time MRI of the ankle, including morphologic and quantitative assessment.

CONCLUSION

Ankle injury is common among both athletes and the general population, and MRI is the established noninvasive means of evaluation. The design of an ankle protocol depends on various factors. Higher magnetic field improves signal-to-noise ratio but increases metal artifact. Specialized imaging planes are useful but prolong acquisition times. MR neurography is useful, but metal reduction techniques are needed whenever a metal prosthesis is present. An ultrashort echo time sequence is a valuable tool for both structural and quantitative evaluation.

Advances in MRI around metal

The prevalence of orthopedic metal implants is continuously rising in the aging society. Particularly the number of joint replacements is increasing. Although satisfying long-term results are encountered, patients may suffer from complaints or complications during follow-up, and often undergo magnetic resonance imaging (MRI). Yet metal implants cause severe artifacts on MRI, resulting in signal-loss, signal-pileup, geometric distortion, and failure of fat suppression. In order to allow for adequate treatment decisions, metal artifact reduction sequences (MARS) are essential for proper radiological evaluation of postoperative findings in these patients. During recent years, developments of musculoskeletal imaging have addressed this particular technical challenge of postoperative MRI around metal. Besides implant material composition, configuration and location, selection of appropriate MRI hardware, sequences, and parameters influence artifact genesis and reduction. Application of dedicated metal artifact reduction techniques including high bandwidth optimization, view angle tilting (VAT), and the multispectral imaging techniques multiacquisition variable-resonance image combination (MAVRIC) and slice-encoding for metal artifact correction (SEMAC) may significantly reduce metal-induced artifacts, although at the expense of signal-to-noise ratio and/or acquisition time. Adding advanced image acquisition techniques such as parallel imaging, partial Fourier transformation, and advanced reconstruction techniques such as compressed sensing further improves MARS imaging in a clinically feasible scan time. This review focuses on current clinically applicable MARS techniques. Understanding of the main principles and techniques including their limitations allows a considerate application of these techniques in clinical practice. Essential orthopedic metal implants and postoperative MR findings around metal are presented and highlighted with clinical examples.

LEVEL OF EVIDENCE

4 Technical Efficacy: Stage 3 J. Magn. Reson. Imaging 2017;46:972-991.

Knee imaging: Rapid three-dimensional fast spin-echo using compressed sensing

PURPOSE

To investigate the feasibility of using compressed sensing (CS) to accelerate three-dimensional fast spin-echo (3D-FSE) imaging of the knee.

MATERIALS AND METHODS

A 3D-FSE sequence was performed at 3T with CS (CUBE-CS with 3:16-minute scan time) and without CS (CUBE with 4:44-minute scan time) twice on the knees of 10 healthy volunteers to assess signal-to-noise ratio (SNR) using the addition-subtraction method and once on the knees of 50 symptomatic patients to assess diagnostic performance. SNR of cartilage, muscle, synovial fluid, and bone marrow on CUBE and CUBE-CS images were measured in the 10 healthy volunteers. The CUBE and CUBE-CS sequences of all 50 symptomatic patients were independently reviewed twice by two musculoskeletal radiologists. The radiologists used CUBE and CUBE-CS during each individual review to determine the presence or absence of knee joint pathology. Student's t-tests were used to compare SNR values between sequences, while the kappa statistic was used to determine agreement between sequences for detecting knee joint pathology. Sensitivity and specificity of CUBE and CUBE-CS for detecting knee joint pathology was also calculated in the 18 symptomatic patients who underwent subsequent arthroscopic knee surgery.

RESULTS

CUBE and CUBE-CS had similar SNR (P = 0.15-0.67) of cartilage, muscle, synovial fluid, and bone marrow. There was near-perfect to perfect agreement between CUBE and CUBE-CS for both radiologists for detecting cartilage and bone marrow edema lesions, medial and lateral meniscus tears, anterior cruciate ligament tears, effusions, and intra-articular bodies. CUBE and CUBE-CS had similar sensitivity (75.0-100%) and specificity (87.5-100%) for detecting 60 cartilage lesions, 20 meniscus tears, four anterior cruciate ligament tears, and four intra-articular bodies confirmed at surgery.

CONCLUSION

CS provided a 30% reduction in scan time for 3D-FSE imaging of the knee without a corresponding decrease in SNR or diagnostic performance.

LEVEL OF EVIDENCE

1 J. MAGN. RESON. IMAGING 2017;45:1712-1722.

SEMAC-VAT MR Imaging Unravels Peri-instrumentation Lesions in Patients With Attendant Symptoms After Spinal Surgery

The study aimed for evaluating the diagnostic value of a 2D Turbo Spin Echo (TSE) magnetic resonance (MR) imaging sequence implanted slice-encoding metal artifact correction (SEMAC) and view-angle tilting (VAT) in patients with spinal instrumentation.Sixty-seven consecutive patients with an average age of 59.7 ± 17.8 years old (range: 32-75 years) were enrolled in this study. Both sagittal, axial T1-weighted and T2-weighted MRI images were acquired with a standard TSE sequence and a high-bandwidth TSE sequence implemented the SEMAC and VAT techniques. Three continuous sections around the instrumentation in axial and sagittal images were selected for quantitative evaluation. The measurement included cumulative areas of signal void on axial images and the length of spinal canal obscuration on sagittal images. Three radiologists independently evaluated all images blindly. The inter-observer reliability was evaluated with inter-class coefficient. We defined patients with discomfortable symptoms caused by spinal instrumentation as spinal instrumentation adverse reaction.Visualizations of all periprosthetic anatomic structures were significantly better for SEMAC-VAT compared with standard imaging. For axial images, the area of signal void at the level of the instrumentation were statistically reduced with SEMAC-VAT TSE sequences than with standard TSE sequences for T2-weighted images (9.9 ± 2.6 cm vs 29.8 ± 14.7 cm, P < 0.001). For sagittal imaging, the length of spinal canal obscuration at the level of the instrumentation was reduced from 5.2 ± 2.0 cm to 1.2 ± 0.6 cm on T2-weighted images (P < 0.001), and from 4.8 ± 2.1 cm to 1.1 ± 0.5 cm on T1-weighted images with SEMAC-VAT sequences (P < 0.001). Interobserver agreement for visualization of anatomic structures and image quality was good for both SEMAC-VAT (k = 0.77 and 0.68, respectively) and standard (k = 0.74 and 0.80, respectively) imaging. The number of abnormal findings noted on SEMAC images (59 findings) was significantly higher than detected on standard images (40 findings). The incidence rate of spinal instrumentation adverse reaction was 38.81%.MR images with SEMAC-VAT can significantly reduce metal artifacts for spinal instrumentation and improve delineation of the instrumentation and periprosthetic region. Furthermore, SEMAC-VAT technique can improve diagnostic accuracy in patients with post-instrumentation spinal diseases.

Modified wideband three-dimensional late gadolinium enhancement MRI for patients with implantable cardiac devices

PURPOSE

To study the effects of cardiac devices on three-dimensional (3D) late gadolinium enhancement (LGE) MRI and to develop a 3D LGE protocol for implantable cardioverter defibrillator (ICD) patients with reduced image artifacts.

THEORY AND METHODS

The 3D LGE sequence was modified by implementing a wideband inversion pulse, which reduces hyperintensity artifacts, and by increasing bandwidth of the excitation pulse. The modified wideband 3D LGE sequence was tested in phantoms and evaluated in six volunteers and five patients with ICDs.

RESULTS

Phantom and in vivo studies results demonstrated extended signal void and ripple artifacts in 3D LGE that were associated with ICDs. The reason for these artifacts was slab profile distortion and the subsequent aliasing in the slice-encoding direction. The modified wideband 3D LGE provided significantly reduced ripple artifacts than 3D LGE with wideband inversion only. Comparison of 3D and 2D LGE images demonstrated improved spatial resolution of the heart using 3D LGE.

CONCLUSION

Increased bandwidth of the inversion and excitation pulses can significantly reduce image artifacts associated with ICDs. Our modified wideband 3D LGE protocol can be readily used for imaging patients with ICDs given appropriate safety guidelines are followed.

Metal artefact suppression at 3 T MRI: comparison of MAVRIC-SL with conventional fast spin echo sequences in patients with Hip joint arthroplasty

OBJECTIVE

The aim of our study was to evaluate the clinical feasibility and diagnostic value of a new MRI metal artefact reduction pulse sequence called MAVRIC-SL in a 3 T MRI environment.

SUBJECTS AND METHODS

Two MAVRIC-SL sequences obtained in 61 patients with symptomatic total hip replacement were compared with standard FSE-STIR sequences optimized for imaging around metal. Artefact size was measured on the slice of greatest extent. Image quality, fat saturation, image distortion, visibility of anatomical structures, and detectability of joint abnormalities were visually assessed and graded on qualitative scales. Differences between MAVRIC-SL and FSE sequences were tested with the Wilcoxon signed-rank test.

RESULTS

MAVRIC-SL sequences at 3 T showed significantly smaller metal artefacts compared to FSE-STIR sequences (p < 0.0001). The general image quality of MAVRIC-SL sequences was reduced with regard to spatial resolution, noise and contrast (p = 0.001), and fat saturation (p < 0.0001). The reduction of artefact size and image distortion significantly improved visualization of joint anatomy (p < 0.0001) and diagnostic confidence regarding implant-associated abnormalities (p = 0.0075 to <0.0001).

CONCLUSION

Although the image quality of MAVRIC-SL sequences is limited at 3 T, its clinical application is feasible and provides important additional diagnostic information for the workup of patients with symptomatic hip replacement through substantially reduced metal artefacts.

KEY POINTS

• Metal artefacts compromise imaging of total hip replacement with MRI. • Metal artefacts are aggravated with 3 Tesla MRI. • MAVRIC-SL is a technique to suppress metal artefacts. • MAVRIC-SL effectively reduces metal artefacts at 3 Tesla and improves diagnostic quality.

Metal artefact reduction in MRI at both 1.5 and 3.0 T using slice encoding for metal artefact correction and view angle tilting

OBJECTIVE

To compare metal artefact reduction in MRI at both 3.0 T and 1.5 T using different sequence strategies.

METHODS

Metal implants of stainless steel screw and plate within agarose phantoms and tissue specimens as well as three patients with implants were imaged at both 1.5 T and 3.0 T, using view angle tilting (VAT), slice encoding for metal artefact correction with VAT (SEMAC-VAT) and conventional sequence. Artefact reduction in agarose phantoms was quantitatively assessed by artefact volume measurements. Blinded reads were conducted in tissue specimen and human imaging, with respect to artefact size, distortion, blurring and overall image quality. Wilcoxon and Friedman tests for multiple comparisons and intraclass correlation coefficient (ICC) for interobserver agreement were performed with a significant level of p < 0.05.

RESULTS

Compared with conventional sequences, SEMAC-VAT significantly reduced metal artefacts by 83% ± 9% for the screw and 89% ± 3% for the plate at 1.5 T; 72% ± 7% for the screw and 38% ± 13% for the plate at 3.0 T (p < 0.05). In qualitative analysis, SEMAC-VAT allowed for better visualization of tissue structures adjacent to the implants and produced better overall image quality with good interobserver agreement for both tissue specimen and human imaging (ICC = 0.80-0.99; p < 0.001). In addition, VAT also markedly reduced metal artefacts compared with conventional sequence, but was inferior to SEMAC-VAT.

CONCLUSION

SEMAC-VAT and VAT techniques effectively reduce artefacts from metal implants relative to conventional imaging at both 1.5 T and 3.0 T.

ADVANCES IN KNOWLEDGE

The feasibility of metal artefact reduction with SEMAC-VAT was demonstrated at 3.0-T MR. SEMAC-VAT significantly reduced metal artefacts at both 1.5 and 3.0 T. SEMAC-VAT allowed for better visualization of the tissue structures adjacent to the metal implants. SEMAC-VAT produced consistently better image quality in both tissue specimen and human imaging.

What are the advantages and disadvantages of imaging modalities to diagnose wear-related corrosion problems?

BACKGROUND

Adverse tissue reactions are known to occur after total hip arthroplasty using both conventional and metal-on-metal (MoM) bearings and after MoM hip resurfacing arthroplasty (SRA). A variety of imaging tools, including ultrasound (US), CT, and MRI, have been used to diagnose problems associated with wear after MoM hip arthroplasty and corrosion at the head-trunnion junction; however, the relative advantages and disadvantages of each remain a source of controversy.

QUESTIONS/PURPOSES

The purposes of this review were to evaluate the advantages and disadvantages of (1) US; (2) CT; and (3) MRI as diagnostic tools in the assessment of wear-related corrosion problems after hip arthroplasty.

METHODS

A systematic literature review was performed through Medline, EMBASE, Scopus CINAHL, and the Cochrane Library without time restriction using search terms related to THA, SRA, US, CT, MRI, adverse tissue reactions, and corrosion. Inclusion criteria were Level I through IV studies in the English language, whereas expert opinions and case reports were excluded. The quality of included studies was judged by their level of evidence, method of intervention allocation, outcome assessments, and followup of patients. Four hundred ninety unique results were returned and 40 articles were reviewed.

RESULTS

The prevalence of adverse local tissue reactions in both asymptomatic and symptomatic patients varies based on the method of evaluation (US, CT, MRI) and imaging protocols. US is accessible and relatively inexpensive, yet has not been used to report synovial thicknesses in the setting of wear-related corrosion. CT scans are highly sensitive and provide information regarding component positioning but are limited in providing enhanced soft tissue contrast and require ionizing radiation. MRI has shown promise in predicting both the presence and severity of adverse local tissue reactions but is more expensive.

CONCLUSIONS

All three imaging modalities have a role in the assessment of adverse local tissue reactions and tribocorrosion after total hip arthroplasty. Although US may serve as a screening technique for the detection of larger periprosthetic collections, only MRI has been shown to predict the severity of tissue destruction found at revision and correlate to the degree of tissue necrosis at histologic evaluation.

In vitro assessment of knee MRI in the presence of metal implants comparing MAVRIC-SL and conventional fast spin echo sequences at 1.5 and 3 T field strength

PURPOSE

To assess lesion detection and artifact size reduction of a multiacquisition variable-resonance image combination, slice encoding for metal artifact correction (MAVRIC-SEMAC) hybrid sequence (MAVRIC-SL) compared to standard sequences at 1.5T and 3T in porcine knee specimens with metal hardware.

MATERIALS AND METHODS

Artificial cartilage and bone lesions of defined size were created in the proximity of titanium and steel screws with 2.5 mm diameter in 12 porcine knee specimens and were imaged at 1.5T and 3T magnetic resonance imaging (MRI) with MAVRIC-SL PD and short T1 inversion recovery (STIR), standard fast spin echo (FSE) T2 PD, and STIR and fat-saturated T2 FSE sequences. Three radiologists blinded to the lesion locations assessed lesion detection rates on randomized images for each sequence using receiver operating characteristic (ROC). Artifact length and width were measured.

RESULTS

Metal artifact sizes were largest in the presence of steel screws at 3T (FSE T2 FS: 28.7 cm(2) ) and 1.5T (16.03 cm(2) ). MAVRIC-SL PD and STIR reduced artifact sizes at both 3T (1.43 cm(2) ; 2.46 cm(2) ) and 1.5T (1.16 cm(2) ; 1.59 cm(2) ) compared to FS T2 FSE sequences (27.57 cm(2) ; 13.20 cm(2) ). At 3T, ROC-derived AUC values using MAVRIC-SL sequences were significantly higher compared to standard sequences (MAVRIC-PD: 0.87, versus FSE-T2 -FS: 0.73 [P = 0.025]; MAVRIC-STIR: 0.9 vs. T2 -STIR: 0.78 [P = 0.001] and vs. FSE-T2 -FS: 0.73 [P = 0.026]). Similar values were observed at 1.5T. Comparison of 3T and 1.5T showed no significant differences (MAVRIC-SL PD: P = 0.382; MAVRIC-SL STIR: P = 0.071).

CONCLUSION

MAVRIC-SL sequences provided superior lesion detection and reduced metal artifact size at both 1.5T and 3T compared to conventionally used FSE sequences. No significant disadvantage was found comparing MAVRIC-SL at 3T and 1.5T, although metal artifacts at 3T were larger. J. Magn. Reson. Imaging 2015;41:1291-1299. © 2014 Wiley Periodicals, Inc.

Hexagonal undersampling for faster MRI near metallic implants

PURPOSE

Slice encoding for metal artifact correction acquires a three-dimensional image of each excited slice with view-angle tilting to reduce slice and readout direction artifacts respectively, but requires additional imaging time. The purpose of this study was to provide a technique for faster imaging around metallic implants by undersampling k-space.

METHODS

Assuming that areas of slice distortion are localized, hexagonal sampling can reduce imaging time by 50% compared with conventional scans. This work demonstrates this technique by comparisons of fully sampled images with undersampled images, either from simulations from fully acquired data or from data actually undersampled during acquisition, in patients and phantoms. Hexagonal sampling is also shown to be compatible with parallel imaging and partial Fourier acquisitions. Image quality was evaluated using a structural similarity (SSIM) index.

RESULTS

Images acquired with hexagonal undersampling had no visible difference in artifact suppression from fully sampled images. The SSIM index indicated high similarity to fully sampled images in all cases.

CONCLUSION

The study demonstrates the ability to reduce scan time by undersampling without compromising image quality.

Ripple artifact reduction using slice overlap in slice encoding for metal artifact correction

PURPOSE

Multispectral imaging (MSI) significantly reduces metal artifacts. Yet, especially in techniques that use gradient selection, such as slice encoding for metal artifact correction (SEMAC), a residual ripple artifact may be prominent. Here, an analysis is presented of the ripple artifact and of slice overlap as an approach to reduce the artifact.

METHODS

The ripple artifact was analyzed theoretically to clarify its cause. Slice overlap, conceptually similar to spectral bin overlap in multi-acquisition with variable resonances image combination (MAVRIC), was achieved by reducing the selection gradient and, thus, increasing the slice profile width. Time domain simulations and phantom experiments were performed to validate the analyses and proposed solution.

RESULTS

Discontinuities between slices are aggravated by signal displacement in the frequency encoding direction in areas with deviating B0. Specifically, it was demonstrated that ripple artifacts appear only where B0 varies both in-plane and through-plane. Simulations and phantom studies of metal implants confirmed the efficacy of slice overlap to reduce the artifact.

CONCLUSION

The ripple artifact is an important limitation of gradient selection based MSI techniques, and can be understood using the presented simulations. At a scan-time penalty, slice overlap effectively addressed the artifact, thereby improving image quality near metal implants.

Total knee arthroplasty MRI featuring slice-encoding for metal artifact correction: reduction of artifacts for STIR and proton density-weighted sequences

OBJECTIVE

The purpose of this article is to compare slice-encoding for metal artifact correction (SEMAC) sequences versus optimized standard MRI sequences in patients with total knee arthroplasty (TKA).

SUBJECTS AND METHODS

Forty-two patients with TKA underwent 1.5-T MRI. Sequences optimized for metal implant imaging (SEMAC) were compared with standard sequences optimized with high bandwidth for STIR and proton density (PD)-weighted images. In 29 patients, CT was available as reference standard. Signal void and insufficient fat saturation were quantified. Qualitative criteria (anatomy, distortion, blurring, and noise) were assessed on a 5-point scale (1, no artifacts; 5, severe artifacts) by two readers. Abnormal imaging findings were noted. A Student t test and a Wilcoxon signed rank test was used for statistics.

RESULTS

Signal void areas and insufficient fat saturation were smaller for the SEMAC sequences than for the optimized standard sequences (p ≤ 0.005 for all comparisons). Depiction of anatomic structures was better on STIR with SEMAC versus standard sequences optimized with high bandwidth (score range, 2.9-3.7 vs 4.2-4.9) and on PD-weighted imaging with SEMAC versus standard sequences optimized with high bandwidth (score range, 2.5-3.5 vs 3.1-3.8), which was statistically significant (p < 0.001 to p = 0.007 for different structures). Distortion and noise were lower for SEMAC than for the standard sequences (p ≤ 0.001), whereas no technique had a clear advantage for blurring. Detection of abnormal imaging findings was markedly increased for the SEMAC technique (p < 0.001) and was most pronounced for STIR images (98 and 74 findings for STIR with SEMAC for readers 1 and 2, respectively, vs 37 and 37 findings for readers 1 and 2, respectively, for STIR with standard sequences optimized with high bandwidth). Sensitivity for detection of periprosthetic osteolysis was improved for STIR with SEMAC (100% and 86% for readers 1 and 2, respectively) compared with STIR with standard sequences optimized with high bandwidth (14% and 29% for readers 1 and 2, respectively).

CONCLUSION

SEMAC sequences showed a statistically significant artifact reduction. The detection of clinically relevant findings such as periprosthetic osteolysis was markedly improved.

Reducing metallic artefacts in post-operative spinal imaging: slice encoding for metal artefact correction with dual-source parallel radiofrequency excitation MRI at 3.0 T

OBJECTIVE

To compare the effects of metal artefacts and acquisition time among slice encoding for metal artefact correction (SEMAC), SEMAC with dual-source parallel radiofrequency (SEMAC-DSPRF) transmission and fast spin echo (FSE) images using 3.0-T MRI.

METHODS

The signal-to-noise ratio (SNR) was calculated in a phantom study using a pedicle screw. A total of 16 patients who underwent spinal surgery using pedicle screws were included in the clinical study. T1 weighted FSE, SEMAC and SEMAC-DSPRF images were obtained. Four imaging findings (visibility of the dural sac, neural foramens, bone-implant interface and overall artefacts) were evaluated by using five-point scales independently by two observers. The mean scan time was recorded.

RESULTS

The mean SNR was 71.2, 25.7 and 28.4 for FSE, SEMAC and SEMAC-DSPRF images, respectively. FSE images were ranked lower than SEMAC and SEMAC-DSPRF images, and ranking of SEMAC and SEMAC-DSPRF images did not differ statistically for all four imaging findings. The mean scan time was 9 min 51 s and 6 min 31 s for SEMAC and SEMAC-DSPRF images, respectively.

CONCLUSION

SEMAC can reduce metallic artefacts and improve the visualisation of anatomical structures around metal implants. An additional DSPRF technique can reduce the acquisition time of SEMAC images without the loss of SNR and image quality.

ADVANCES IN KNOWLEDGE

This study demonstrates that the use of the DSPRF transmission technique can reduce the acquisition time of SEMAC images without loss of image quality in patients with metal implants.

Compressed-sensing multispectral imaging of the postoperative spine

PURPOSE

To apply compressed sensing (CS) to in vivo multispectral imaging (MSI), which uses additional encoding to avoid magnetic resonance imaging (MRI) artifacts near metal, and demonstrate the feasibility of CS-MSI in postoperative spinal imaging.

MATERIALS AND METHODS

Thirteen subjects referred for spinal MRI were examined using T2-weighted MSI. A CS undersampling factor was first determined using a structural similarity index as a metric for image quality. Next, these fully sampled datasets were retrospectively undersampled using a variable-density random sampling scheme and reconstructed using an iterative soft-thresholding method. The fully and undersampled images were compared using a 5-point scale. Prospectively undersampled CS-MSI data were also acquired from two subjects to ensure that the prospective random sampling did not affect the image quality.

RESULTS

A two-fold outer reduction factor was deemed feasible for the spinal datasets. CS-MSI images were shown to be equivalent or better than the original MSI images in all categories: nerve visualization: P = 0.00018; image artifact: P = 0.00031; image quality: P = 0.0030. No alteration of image quality and T2 contrast was observed from prospectively undersampled CS-MSI.

CONCLUSION

This study shows that the inherently sparse nature of MSI data allows modest undersampling followed by CS reconstruction with no loss of diagnostic quality.

Reduction of metal artifacts in patients with total hip arthroplasty with slice-encoding metal artifact correction and view-angle tilting MR imaging

PURPOSE

To compare the new "warp" sequence (slice-encoding metal artifact correction [SEMAC], view-angle tilting [VAT], and increased bandwidth) for the reduction of both through-plane and in-plane magnetic resonance (MR) artifacts with current optimized MR sequences in patients with total hip arthroplasty (THA).

MATERIALS AND METHODS

The institutional review board issued a waiver for this study. Forty patients with THA were prospectively included. SEMAC, VAT, and increased bandwidth were applied by using the warp turbo-spin-echo sequence at 1.5 T. Coronal short tau inversion-recovery (STIR)-warp and transverse T1-weighted warp (hereafter, T1-warp) images, as well as standard coronal STIR and transverse T1-weighted sequence images optimized with high bandwidth (STIR-hiBW and T1-hiBW), were acquired. Fifteen additional patients were examined to compare the T1-warp and T1-hiBW sequence with an identical matrix size. Signal void was quantified. Qualitative criteria (distinction of anatomic structures, blurring, and noise) were assessed on a five-point scale (1, no artifacts; 5, not visible due to severe artifacts) by two readers. Abnormal imaging findings were recorded. Quantitative data were analyzed with a t test and qualitative data with a Wilcoxon signed rank test.

RESULTS

Signal void around the acetabular component was smaller for STIR-warp than STIR-hiBW images (21.6 cm2 vs 42.4 cm2; P=.0001), and for T1-warp than T1-hiBW images (17.6 cm2 vs 20.2 cm2; P=.0001). Anatomic distinction was better on STIR-warp compared with STIR-hiBW images (1.9-2.8 vs 3.6-4.6; P=.0001), and on T1-warp compared with T1-hiBW images (1.3-2.8 vs 1.8-3.2; P<.002). Distortion, blurring, and noise were lower with warp sequences than with the standard sequences (P=.0001). Almost half of the abnormal imaging findings were missed on STIR-hiBW compared with STIR-warp images (55 vs 105 findings; P=.0001), while T1-hiBW was similar to T1-warp imaging (50 vs 55 findings; P=.06).

CONCLUSION

STIR-warp and T1-warp sequences were significantly better according to quantitative and qualitative image criteria, but a clinically relevant artifact reduction was only present for STIR images.