MRI susceptibility artefacts related to scaphoid screws: the effect of screw type, screw orientation and imaging parameters

Managing hardware-related metal artifacts in MRI: current and evolving techniques

Magnetic resonance imaging (MRI) around metal implants has been challenging due to magnetic susceptibility differences between metal implants and adjacent tissues, resulting in image signal loss, geometric distortion, and loss of fat suppression. These artifacts can compromise the diagnostic accuracy and the evaluation of surrounding anatomical structures. As the prevalence of total joint replacements continues to increase in our aging society, there is a need for proper radiological assessment of tissues around metal implants to aid clinical decision-making in the management of post-operative complaints and complications. Various techniques for reducing metal artifacts in musculoskeletal imaging have been explored in recent years. One approach focuses on improving hardware components. High-density multi-channel radiofrequency (RF) coils, parallel imaging techniques, and gradient warping correction enable signal enhancement, image acquisition acceleration, and geometric distortion minimization. In addition, the use of susceptibility-matched implants and low-field MRI helps to reduce magnetic susceptibility differences. The second approach focuses on metal artifact reduction sequences such as view-angle tilting (VAT) and slice-encoding for metal artifact correction (SEMAC). Iterative reconstruction algorithms, deep learning approaches, and post-processing techniques are used to estimate and correct artifact-related errors in reconstructed images. This article reviews recent developments in clinically applicable metal artifact reduction techniques as well as advances in MR hardware. The review provides a better understanding of the basic principles and techniques, as well as an awareness of their limitations, allowing for a more reasoned application of these methods in clinical settings.

SEMAC + VAT for Suppression of Artifacts Induced by Dental-Implant-Supported Restorations in Magnetic Resonance Imaging

The purpose of this study was to assess the feasibility of SEMAC + VAT to reduce artifacts induced by dental implant-supported restorations, such as its impact on the image quality. Dental-implant supported restorations were installed in a dry mandible. Magnetic resonance scans were acquired on a 3-Tesla MRI system. Artifact suppression (SEMAC + VAT) was applied with different intensity modes (weak, moderate, strong). Artifacts assessment was performed by measuring the mandible volume increase in MRI images prior (reference dataset) and after installation of dental implant-supported prosthesis. Image quality was assessed by two examiners using a five-point scale. Inter-examiner concordance and correlation analysis was performed with Cronbach's alpha and Spearman's test with a significance level at p = 0.05. Mandible volume increased by 60.23% when no artifact suppression method was used. By applying SEMAC + VAT, the volume increase ranged from 17.13% (strong mode) to 32.77% (weak mode). Visualization of mandibular bone was positively correlated with SEMAC intensity degree. SEMAC + VAT reduced MRI artifacts caused by dental-implant supported restorations. A stronger suppression mode improved visualization of mandibular bone in detriment of the scanning time.

Magnetic Resonance Imaging Around Metal at 1.5 Tesla: Techniques From Basic to Advanced and Clinical Impact

ABSTRACT

During the last decade, metal artifact reduction in magnetic resonance imaging (MRI) has been an area of intensive research and substantial improvement. The demand for an excellent diagnostic MRI scan quality of tissues around metal implants is closely linked to the steadily increasing number of joint arthroplasty (especially knee and hip arthroplasties) and spinal stabilization procedures. Its unmatched soft tissue contrast and cross-sectional nature make MRI a valuable tool in early detection of frequently encountered postoperative complications, such as periprosthetic infection, material wear-induced synovitis, osteolysis, or damage of the soft tissues. However, metal-induced artifacts remain a constant challenge. Successful artifact reduction plays an important role in the diagnostic workup of patients with painful/dysfunctional arthroplasties and helps to improve patient outcome. The artifact severity depends both on the implant and the acquisition technique. The implant's material, in particular its magnetic susceptibility and electrical conductivity, its size, geometry, and orientation in the MRI magnet are critical. On the acquisition side, the magnetic field strength, the employed imaging pulse sequence, and several acquisition parameters can be optimized. As a rule of thumb, the choice of a 1.5-T over a 3.0-T magnet, a fast spin-echo sequence over a spin-echo or gradient-echo sequence, a high receive bandwidth, a small voxel size, and short tau inversion recovery-based fat suppression can mitigate the impact of metal artifacts on diagnostic image quality. However, successful imaging of large orthopedic implants (eg, arthroplasties) often requires further optimized artifact reduction methods, such as slice encoding for metal artifact correction or multiacquisition variable-resonance image combination. With these tools, MRI at 1.5 T is now widely considered the modality of choice for the clinical evaluation of patients with metal implants.

Metallic Implants in MRI - Hazards and Imaging Artifacts

BACKGROUND

 Magnetic resonance imaging (MRI) is an examination method for noninvasive soft tissue imaging without the use of ionizing radiation. Metallic implants, however, may pose a risk for the patient and often result in imaging artifacts. Due to the increasing number of implants, reducing these artifacts has become an important goal. In this review, we describe the risks associated with implants and provide the background on how metal-induced artifacts are formed. We review the literature on methods on how to reduce artifacts and summarize our findings.

METHOD

 The literature was searched using PubMed and the keywords "MRI metal artifact reduction", "metallic implants" and "MRI artefacts/artifacts".

RESULTS AND CONCLUSION

 The MRI compatibility of implants has to be evaluated individually. To reduce artifacts, two general approaches were found: a) parameter optimization in standard sequences (echo time, slice thickness, bandwidth) and b) specialized sequences, such as VAT, OMAR, WARP, SEMAC and MAVRIC. These methods reduced artifacts and improved image quality, albeit at the cost of a (sometimes significantly) prolonged scan time. New developments in accelerated imaging will likely shorten the scan time of these methods significantly, such that routine use may become feasible.

KEY POINTS

  · Metallic implants may pose a risk for patients and often cause artifacts.. · Imaging artifacts can be reduced by parameter optimization or special sequences.. · Metal artifacts are reduced with a lower TE, smaller voxel size, larger matrix, and higher bandwidth.. · SPI, STIR, VAT, SEMAC, MAVRIC, and MAVRIC-SL are specialized MR sequences that can reduce artifacts further..

CITATION FORMAT

· Peschke E, Ulloa P, Jansen O et al. Metallic Implants in MRI - Hazards and Imaging Artifacts. Fortschr Röntgenstr 2021; 193: 1285 - 1293.

MRI following scoliosis surgery? An analysis of implant heating, displacement, torque, and susceptibility artifacts

Objectives

The implant constructs used in scoliosis surgery are often long with a high screw density. Therefore, it is generally believed that magnetic resonance imaging (MRI) should not be carried out after scoliosis surgery, with the result that computed tomography is often preferred despite the ionizing radiation involved. The objective of this study was to evaluate the MRI compatibility of long pedicle-screw-rod constructs at 1.5 T and 3 T using standardized methods of the American Society for Testing and Materials (ASTM).

Methods

Constructs between 130 and 430 mm long were systematically examined according to the ASTM standards F2182 (radio frequency–induced heating), F2119 (susceptibility artifacts), F2213 (magnetically induced torque), and F2052 (magnetically induced displacement force).

Results

The maximum heating in the magnetic field was 1.3 K. Heating was significantly influenced by magnetic field strength (p < 0.001), implant length (p = 0.048), and presence of cross-links (p = 0.001). The maximum artifact width for different lengths of the anatomically bent titanium rods with CoCr alloy ranged between 14.77 ± 2.93 mm (TSE) and 17.49 ± 1.82 mm (GRE) for 1.5 T and between 23.67 ± 2.39 mm (TSE) and 27.77 ± 2.37 mm (GRE) for 3 T. TiCP and TiAl showed the smallest and CoCr and CoCr Plus the largest artifact widths. The magnetically induced torque and displacement force were negligible.

Conclusions

MRI following scoliosis surgery with long implant constructs is safe with the patient in supine position. Although susceptibility artifacts can severely limit the diagnostic value, the examination of other regions is possible.

Key Points

• Large spinal implants are not necessarily a contraindication for MRI; MR conditional status can be examined according to the ASTM standards F2182, F2119, F2213, and F2052.

• A metallic pedicle-screw-rod system could be reliably and safely examined in all combinations of length (130 to 430 mm), configuration, and material in a B₀at 1.5 T and 3 T.

• According to ASTM F2503, the examined pedicle-screw-rod system is MR conditional and especially the young patients can benefit from a non-ionizing radiation MRI examination.

Magnetic resonance imaging artifacts produced by dental implants with different geometries

OBJECTIVES

The purpose of this study was to evaluate the MRI-artifact pattern produced by titanium and zirconia dental implants with different geometries (diameter and height).

METHODS

Three titanium (Titan SLA, Straumann) and three zirconia (Pure Ceramic Implant, Straumann) dental implants differing on their design (diameter x height) were installed in porcine bone samples. Samples were scanned with a MRI (3T, T1W turbo spin echo sequence, TR/TE 25/3.5ms, voxel size 0.22×0.22×0.50 mm, scan time 11:18). Micro-CT was used as control group (80kV, 125mA, voxel size 16µm). Artifacts' distribution was measured at vestibular and lingual sites, mesial and distal sites, and at the apex. Statistical analysis was performed with Within-ANOVA (p=0.05).

RESULTS

Artifacts distribution measured 2.57 ± 1.09 mm for titanium artifacts and 0.37 ± 0.20 mm for zirconia artifacts (p<0.05). Neither the measured sites (p=0.73) nor the implant geometries (p=0.43) influenced the appearance of artifacts.

CONCLUSION

Artifacts were higher for titanium than zirconia implants. The artifacts pattern was similar for different dental implant geometries.

Comparison of Shape Memory Staple and Gelled Platelet-Rich Plasma versus Shape Memory Staple alone for the Treatment of Waist Scaphoid Nonunion: A Single-Center Experience

Purpose  The aim of this study was to analyze the results of two different methods of surgical treatment of waist scaphoid nonunions (SNUs). We retrospectively analyzed data from 87 patients referred to our department from January 2010 to December 2017 who were surgically treated for waist SNU.

Methods  The mean period of time passed from trauma was 11.2 (±5.6) months. Patients were divided into two groups based on surgical treatment received: volar exposure osteosynthesis with shape memory staple (SMS) (group A) and volar exposure osteosynthesis with SMS and gelled platelet-rich plasma (GPRP) application at the bone defect level (group B). A cast (thumb excluded) was maintained for 4 weeks. Healing was checked clinically (pain, QuickDASH [Disabilities of the Arm, Shoulder, and Hand] score, Mayo Wrist Score) and radiologically (standard X-ray).

Results  Union was achieved in 40 patients in group A (95.2%) and in all patients in group B (45 patients; 100%). A statistically significant difference was observed in the improvement of the Mayo Wrist Score, QuickDASH score, and pain (measured through the visual analog scale) after 3 months from surgery ( p  = 0.02).

Conclusion  SMS is effective in treating waist SNU at more than 6 months from trauma. GPRP application can improve bone healing and upper limb function.

Level of Evidence  This is a retrospective observational Level III study.

Imaging response assessment following stereotactic body radiotherapy for solid tumour metastases of the spine: Current challenges and future directions

Patients with metastatic disease are routinely serially imaged to assess disease burden and response to systemic and local therapies, which places ever-expanding demands on our healthcare resources. Image interpretation following stereotactic body radiotherapy (SBRT) for spine metastases can be challenging; however, appropriate and accurate assessment is critical to ensure patients are managed correctly and resources are optimised. Here, we take a critical review of the merits and pitfalls of various imaging modalities, current response assessment guidelines, and explore novel imaging approaches and the potential for radiomics to add value in imaging assessment.

Artifacts in cranial MRI caused by extracranial foreign bodies and analysis of these foreign bodies

Purpose:

The purpose of our study was to conduct a chemical analysis of extracranial foreign bodies (FBs) causing artifacts in cranial magnetic resonance imaging (MRI) and to investigate the association between chemical composition, magnetic susceptibility, and artifact size.

Materials and Methods:

A total of 12 patients were included in the study. The FBs responsible for the artifacts were visualized using cranial computed tomography (CT). Artifact-causing FBs were removed from the scalps of 10 patients and analyzed using scanning electron microscope with energy dispersive spectroscopy (SEM-EDS), X-ray diffraction spectroscopy (X-RD), and Fourier-transform infrared spectroscopy (FT-IR). The magnetic susceptibility of the samples was determined using the reference standard material MnCl₂.6H₂O. The volume of the MRI artifacts was measured in cubic centimeters (cm³).

Results:

EDS results demonstrated that the mean Fe ratio was 5.82% in the stone samples and 0.08% in the glass samples. Although no phase peaks were detected in the X-RD spectra of the glass samples, peaks of Fe₂O₃, Al₂Ca (SiO₄) were detected in the X-RD spectra of the stone samples. The FT-IR spectra revealed metal oxide peaks corresponding to Fe, Al, in the stone samples and peaks confirming Al₂SiO₅ and Na₂SiO₃ structures in the glass samples. The mean volumes of the MRI artifacts produced by the stone and glass samples were 5.9 cm³ and 2.5 cm³, respectively.

Conclusions:

Artifacts caused by extracranial FBs containing metal/metal oxide components are directly associated with their chemical composition and the artifact size are also related to element composition and magnetic susceptibility.

Imaging for Acute and Chronic Scaphoid Fractures

The scaphoid is the most commonly fractured bone in the wrist but 20% to 40% of scaphoid fractures are radiographically occult. Delayed or misdiagnosis can have significant consequences with late complications such as nonunion, malunion, or the development of avascular necrosis in the proximal pole. After initial negative radiographs, advanced cross-sectional imaging, including CT and MRI, ultimately may provide more accurate and rapid diagnosis than conventional radiography. With chronic fractures, the preferred modality depends on the clinical question. New techniques are evolving that will further advance imaging for diagnosis and treatment of scaphoid fractures.

Three-dimensional quantification of magnetic resonance imaging artifacts associated with shape factors

Differences in the volumes of artifacts caused by variously shaped titanium objects on magnetic resonance imaging (MRI) were evaluated. Spherical-, square cubic-, and regular tetrahedron-shaped isotropic, and elongated spherical-, elongated cubic-, and elongated tetrahedron-shaped anisotropic objects, with identical volumes, were prepared. Samples were placed on a nickel-doped agarose gel phantom and covered with nickel-nitrate hexahydrate solution. Three-Tesla MR images were obtained using turbo spin echo and gradient echo sequences. Areas with ±30% of the signal intensity of the standard background value were considered artifacts. Sample volumes were deducted from these volumes to calculate the total artifact volumes. Isotropic samples had similar artifact volumes. For anisotropic samples, the artifact volume increased in proportion with the normalized projection area. MRI artifact size can be reduced by high anisotropic designs, and by positioning the long axis of the metal device as parallel as possible to the magnetic field axis.

Material-Dependent Implant Artifact Reduction Using SEMAC-VAT and MAVRIC: A Prospective MRI Phantom Study

OBJECTIVE

The aim of this study was to compare the degree of artifact reduction in magnetic resonance imaging achieved with slice encoding for metal artifact correction (SEMAC) in combination with view angle tilting (VAT) and multiacquisition variable resonance image combination (MAVRIC) for standard contrast weightings and different metallic materials.

METHODS

Four identically shaped rods made of the most commonly used prosthetic materials (stainless steel, SS; titanium, Ti; cobalt-chromium-molybdenum, CoCr; and oxidized zirconium, oxZi) were scanned at 3 T. In addition to conventional fast spin-echo sequences, metal artifact reduction sequences (SEMAC-VAT and MAVRIC) with varying degrees of artifact suppression were applied at different contrast weightings (T1w, T2w, PDw). Two independent readers measured in-plane and through-plane artifacts in a standardized manner. In addition, theoretical frequency-offset and frequency-offset-gradient maps were calculated. Interobserver agreement was assessed using intraclass correlation coefficient.

RESULTS

Interobserver agreement was almost perfect (intraclass correlation coefficient, 0.86-0.99). Stainless steel caused the greatest artifacts, followed by CoCr, Ti, and oxZi regardless of the imaging sequence. While for Ti and oxZi rods scanning with weak SEMAC-VAT showed some advantage, for SS and CoCr, higher modes of SEMAC-VAT or MAVRIC were necessary to achieve artifact reduction. MAVRIC achieved better artifact reduction than SEMAC-VAT at the cost of longer acquisition times. Simulations matched well with the apparent geometry of the frequency-offset maps.

CONCLUSIONS

For Ti and oxZi implants, weak SEMAC-VAT may be preferred as it is faster and produces less artifact than conventional fast spin-echo. Medium or strong SEMAC-VAT or MAVRIC modes are necessary for significant artifact reduction for SS and CoCr implants.

KEY POINTS

In vitro stent assessment by MRI: visibility of lumen and artifacts for 27 modern stents

Purpose:

The purpose of this study was to measure artifacts and visibility of lumen for modern and most commonly used stents in vascular interventions according to a standardized test method of the American Society for Testing and Materials (ASTM).

Materials and methods:

Twenty-four peripheral self-expanding nitinol stents and three stainless steel stents with diameters between 5 and 8 mm and lengths between 30 and 250 mm from seven different manufacturers were compared on a 1.5T and a 3T magnetic resonance (MR) scanner. The visualization of lumen and artifacts was measured according to ASTM F2119 for a turbo spin echo (TSE) [repetition time(TR)/echo time (TE) 500/26 ms] and a gradient echo (GRE) (TR/TE 100/15 ms) sequence. The stents were placed parallel and perpendicular to the radio frequency field (B

Results:

There were large differences in visibility of the lumen for the stent models. The visualization of the lumen varies between 0% and 93% (perpendicular to B

Conclusion:

Reliable stent lumen visualization is possible for Misago, Supera, Tigris, and Viabahn stents, if their axis is perpendicular to B

[Magnetic resonance imaging artefacts and fixed orthodontic attachments]

OBJECTIVES

Orthodontic appliances are often removed before magnetic resonance imaging (MRI) scans because they are known to produce artefacts. The purpose of this study was to find the exact indications for removal of various fixed attachments when imaging four specific areas of the head and neck.

MATERIALS AND METHODS

Sixty patients requiring an MRI scan of the head for medical reasons volunteered for this investigation. One of four different types of fixed attachments (stainless steel brackets, titanium brackets, ceramic brackets with metal slots, and stainless steel retainers) were assigned to a patient. Each patient had two scans at 1.5 T: with an "empty wax jig" and with a wax jig including the attachment. Archwires were not investigated as they are easily removed before a scan. Two radiologists evaluated the scans of each patient and each of the four areas under investigation: maxillary sinus, oral cavity, temporomandibular joints and posterior cerebral fossa.

RESULTS

Stainless steel brackets always caused non-interpretability of all anatomic areas (100 per cent). Titanium brackets, ceramic brackets with metal slots, and stainless steel retainers caused artefact in the oral cavity only (for 20, 16.65, and 86.65 per cent of the subjects).

CONCLUSIONS

Our results show that ceramic brackets with metal slots and titanium brackets do not always have to be removed before MRI scans of the head and neck, depending on the area under investigation. Metal fixed retainers should only be removed if the oral cavity itself is under investigation. Stainless steel brackets should always be removed before MRI scans of the head and neck.

Magnetic resonance imaging susceptibility artifacts in the cervical vertebrae and spinal cord related to monocortical screw-polymethylmethacrylate implants in canine cadavers

OBJECTIVE To characterize and compare MRI susceptibility artifacts related to titanium and stainless steel monocortical screws in the cervical vertebrae and spinal cord of canine cadavers. SAMPLE 12 canine cadavers. PROCEDURES Cervical vertebrae (C4 and C5) were surgically stabilized with titanium or stainless steel monocortical screws and polymethylmethacrylate. Routine T1-weighted, T2-weighted, and short tau inversion recovery sequences were performed at 3.0 T. Magnetic susceptibility artifacts in 20 regions of interest (ROIs) across 4 contiguous vertebrae (C3 through C6) were scored by use of an established scoring system. RESULTS Artifact scores for stainless steel screws were significantly greater than scores for titanium screws at 18 of 20 ROIs. Artifact scores for titanium screws were significantly higher for spinal cord ROIs within the implanted vertebrae. Artifact scores for stainless steel screws at C3 were significantly less than at the other 3 cervical vertebrae. CONCLUSIONS AND CLINICAL RELEVANCE Evaluation of routine MRI sequences obtained at 3.0 T revealed that susceptibility artifacts related to titanium monocortical screws were considered mild and should not hinder the overall clinical assessment of the cervical vertebrae and spinal cord. However, mild focal artifacts may obscure small portions of the spinal cord or intervertebral discs immediately adjacent to titanium screws. Severe artifacts related to stainless steel screws were more likely to result in routine MRI sequences being nondiagnostic; however, artifacts may be mitigated by implant positioning.

Spinal metastases: multimodality imaging in diagnosis and stereotactic body radiation therapy planning

Due to increased effectiveness of cancer treatments and increasing survival rates, metastatic disease has become more frequent compared to the past, with the spine being the most common site of bony metastases. Diagnostic imaging is an integral part of screening, diagnosis and follow-up of spinal metastases. In this article, we review the principles of multimodality imaging for tumor detection with respect to their value for diagnosis and stereotactic body radiation therapy planning for spinal metastases. We will also review the current international consensus agreement for stereotactic body radiation therapy planning, and the role of imaging in achieving the best possible treatment plan.

A standardized evaluation of artefacts from metallic compounds during fast MR imaging

OBJECTIVES

Metallic compounds present in the oral and maxillofacial regions (OMRs) cause large artefacts during MR scanning. We quantitatively assessed these artefacts embedded within a phantom according to standards set by the American Society for Testing and Materials (ASTM).

METHODS

Seven metallic dental materials (each of which was a 10-mm³ cube embedded within a phantom) were scanned [i.e. aluminium (Al), silver alloy (Ag), type IV gold alloy (Au), gold-palladium-silver alloy (Au-Pd-Ag), titanium (Ti), nickel-chromium alloy (NC) and cobalt-chromium alloy (CC)] and compared with a reference image. Sequences included gradient echo (GRE), fast spin echo (FSE), gradient recalled acquisition in steady state (GRASS), a spoiled GRASS (SPGR), a fast SPGR (FSPGR), fast imaging employing steady state (FIESTA) and echo planar imaging (EPI; axial/sagittal planes). Artefact areas were determined according to the ASTM-F2119 standard, and artefact volumes were assessed using OsiriX MD software (Pixmeo, Geneva, Switzerland).

RESULTS

Tukey-Kramer post hoc tests were used for statistical comparisons. For most materials, scanning sequences eliciting artefact volumes in the following (ascending) order FSE-T₁/FSE-T₂ < FSPGR/SPGR < GRASS/GRE < FIESTA < EPI. For all scanning sequences, artefact volumes containing Au, Al, Ag and Au-Pd-Ag were significantly smaller than other materials (in which artefact volume size increased, respectively, from Ti < NC < CC). The artefact-specific shape (elicited by the cubic sample) depended on the scanning plane (i.e. a circular pattern for the axial plane and a "clover-like" pattern for the sagittal plane).

CONCLUSIONS

The availability of standardized information on artefact size and configuration during MRI will enhance diagnosis when faced with metallic compounds in the OMR.

In Vivo Magnetic Resonance Imaging Evaluation of Porous Tantalum Interbody Fusion Devices in a Porcine Spinal Arthrodesis Model

STUDY DESIGN

Animal experimental study.

OBJECTIVE

To investigate the use of magnetic resonance (MR) imaging in the assessment of lumbar interbody fusion using porous tantalum implants in a porcine spinal fusion model.

SUMMARY OF BACKGROUND DATA

Porous tantalum has been used successfully as a spinal interbody fusion device. However, to our knowledge, there has been no consensus on the optimal technique for evaluating spinal fusion when using porous tantalum implants.

METHODS

Twelve 12-week-old female Danish Landrace pigs underwent 3 levels of anterior lumbar interbody fusion at L2-3, L4-5 and L6-7. One level was fused using a solid porous tantalum cage with pedicle screw fixation. The other 2 levels were fused using a hollow porous tantalum ring packed with autograft and stabilized anteriorly with staples. Six months postoperatively, T1-weighted, T2-weighted, and gadolinium-DTPA contrast-enhanced MRI were obtained on a GE 1.5-T unit. After sacrifice, conventional radiograph and histological examination were carried out.

RESULTS

Eleven pigs went through the experiment without complications. On all the MR sequences, there were significant higher signal intensity bands at the vertebrae-implant interface of nonfused segments compared with that of fused segments and the vertebral bone and implants themselves (P < 0.001). There was an agreement between histological findings of fibrous tissues surrounding the implants and the high signal intensity band on T1- and T2-weighted MR images respectively (sensitivity 69.6% and 56.5%, specificity 90.7% and 95.3%), both of which were superior to conventional radiograph images (sensitivity 52.2%, specificity 97.7%), especially the T1-weighted MR images.

CONCLUSION

MR imaging could be an effective and noninvasive way to determine the fusion status of tantalum metal implants. Compared with T2-weighted MR imaging and conventional radiograph, T1-weighted spin-echo MR imaging is more sensitive and specific in detecting nonunion via the lucency between the vertebral body and tantalum metal device.

LEVEL OF EVIDENCE

N/A.

Metal artifact reduction: standard and advanced magnetic resonance and computed tomography techniques

An increasing number of joint replacements are being performed in the United States. Patients undergoing these procedures can have various complications. Imaging is one of the primary means of diagnosing these complications. Cross-sectional imaging techniques, such as computed tomography (CT) and MR imaging, are more sensitive than radiographs for evaluating complications. The use of CT and MR imaging in patients with metallic implants is limited by the presence of artifacts. This review discusses the causes of metal artifacts on MR imaging and CT, contributing factors, and conventional and novel methods to reduce the effects of these artifacts on scans.

Magnetic resonance imaging artefacts and fixed orthodontic attachments

BACKGROUND/OBJECTIVES

Orthodontic appliances are often removed before magnetic resonance imaging (MRI) scans because they are known to produce artefacts. The purpose of this study was to find the exact indications for removal of various fixed attachments when imaging four specific areas of the head and neck.

MATERIALS/METHODS

Sixty patients requiring an MRI scan of the head for medical reasons volunteered for this investigation. One of four different types of fixed attachments (stainless steel brackets, titanium brackets, ceramic brackets with metal slots, and stainless steel retainers) were assigned to a patient. Each patient had two scans at 1.5 T: with an 'empty wax jig' and with a wax jig including the attachment. Archwires were not investigated as they are easily removed before a scan. Two radiologists evaluated the scans of each patient and each of the four areas under investigation: maxillary sinus, oral cavity, temporomandibular joints, and posterior cerebral fossa.

RESULTS

Stainless steel brackets always caused non-interpretability of all anatomic areas (100 per cent). Titanium brackets, ceramic brackets with metal slots, and stainless steel retainers caused artefact in the oral cavity only (for 20, 16.65, and 86.65 per cent of the subjects).

CONCLUSIONS/IMPLICATIONS

Our results show that ceramic brackets with metal slots and titanium brackets do not always have to be removed before MRI scans of the head and neck, depending on the area under investigation. Metal fixed retainers should only be removed if the oral cavity itself is under investigation. Stainless steel brackets should always be removed before MRI scans of the head and neck.

Effects of low-dose microwave on healing of fractures with titanium alloy internal fixation: an experimental study in a rabbit model

Background

Microwave is a method for improving fracture repair. However, one of the contraindications for microwave treatment listed in the literature is surgically implanted metal plates in the treatment field. The reason is that the reflection of electromagnetic waves and the eddy current stimulated by microwave would increase the temperature of magnetic implants and cause heat damage in tissues. Comparing with traditional medical stainless steel, titanium alloy is a kind of medical implants with low magnetic permeability and electric conductivity. But the effects of microwave treatment on fracture with titanium alloy internal fixation invivo were not reported. The aim of this article was to evaluate the security and effects of microwave on healing of a fracture with titanium alloy internal fixation.

Methods

Titanium alloy internal fixation systems were implanted in New Zealand rabbits with a 3.0 mm bone defect in the middle of femur. We applied a 30-day microwave treatment (2,450MHz, 25W, 10 min per day) to the fracture 3 days after operation. Temperature changes of muscle tissues around implants were measured during the irradiation. Normalized radiographic density of the fracture gap was measured on the 10th day and 30th day of the microwave treatment. All of the animals were killed after 10 and 30 days microwave treatment with histologic and histomorphometric examinations performed on the harvested tissues.

Findings

The temperatures did not increase significantly in animals with titanium alloy implants. The security of microwave treatment was also supported by histology of muscles, nerve and bone around the implants. Radiographic assessment, histologic and histomorphometric examinations revealed significant improvement in the healing bone.

Conclusion

Our results suggest that, in the healing of fracture with titanium alloy internal fixation, a low dose of microwave treatment may be a promising method.

Reducing susceptibility artefacts in magnetic resonance images of the canine stifle following surgery for cranial cruciate ligament deficiency

BACKGROUND

Magnetic resonance (MR) images of the postoperative canine stifle are adversely affected by susceptibility artefacts associated with metallic implants.

OBJECTIVES

To determine empirically to what extent susceptibility artefacts could be reduced by modifications to MR technique.

METHODS

Three cadaveric limbs with a tibial plateau levelling osteotomy (TPLO), tibial tuberosity advancement (TTA), or extra-capsular stabilization (ECS) implant, respectively, were imaged at 1.5T. Series of proton density and T2-weighted images were acquired with different combinations of frequency-encoding gradient (FEG) direction and polarity, stifle flexion or extension, echo spacing (ES), and readout bandwidth (ROBW), and ranked. The highest rank (a rank of 1) corresponded to the smallest artefact.

RESULTS

Image ranking was affected by FEG polarity (p = 0.005), stifle flexion (p = 0.01), and ROBW (p = 0.0001). For TPLO and TTA implants, the highest ranked images were obtained with the stifle flexed, lateromedial FEG, and medial polarity for dorsal images, and craniocaudal FEG and caudal polarity for sagittal images. For the ECS implant, the highest ranked images were obtained with the stifle extended, a proximodistal FEG and proximal polarity for dorsal images, and craniocaudal FEG and cranial polarity for sagittal images.

CLINICAL SIGNIFICANCE

Susceptibility artefacts in MR images of postoperative canine stifles do not preclude clinical evaluation of joints with ECS or TTA implants.

Spinal fusion-hardware construct: Basic concepts and imaging review

The interpretation of spinal images fixed with metallic hardware forms an increasing bulk of daily practice in a busy imaging department. Radiologists are required to be familiar with the instrumentation and operative options used in spinal fixation and fusion procedures, especially in his or her institute. This is critical in evaluating the position of implants and potential complications associated with the operative approaches and spinal fixation devices used. Thus, the radiologist can play an important role in patient care and outcome. This review outlines the advantages and disadvantages of commonly used imaging methods and reports on the best yield for each modality and how to overcome the problematic issues associated with the presence of metallic hardware during imaging. Baseline radiographs are essential as they are the baseline point for evaluation of future studies should patients develop symptoms suggesting possible complications. They may justify further imaging workup with computed tomography, magnetic resonance and/or nuclear medicine studies as the evaluation of a patient with a spinal implant involves a multi-modality approach. This review describes imaging features of potential complications associated with spinal fusion surgery as well as the instrumentation used. This basic knowledge aims to help radiologists approach everyday practice in clinical imaging.

Effects of surgical implants on high-field magnetic resonance images of the normal canine stifle

To determine the effect of surgical implants on the depiction of canine stifle anatomy in magnetic resonance (MR) images, three canine cadaver limbs were imaged at 1.5 T before and after tibial plateau leveling osteotomy (TPLO), tibial tuberosity advancement (TTA), and extra-capsular stabilization (ECS), respectively. Susceptibility artifacts associated with implants were identified in MR images as a signal void and/or signal misregistration, which obscured or distorted the anatomy. Using the preoperative images as a reference, articular structures of the stifle in postoperative images were graded using an ordinal scale to describe to what degree each anatomic structure could be evaluated for clinical purposes. The TPLO implant, which contains ferromagnetic stainless steel, produced marked susceptibility artifacts that obscured or distorted most stifle anatomy. The titanium alloy TTA implants and the stainless steel crimps used for ECS produced susceptibility artifacts that mainly affected the lateral aspect of the stifle, but allowed the cruciate ligaments and medial meniscus to be evaluated satisfactorily. Susceptibility artifact was significantly less marked in images obtained using turbo spin-echo (TSE) sequences than in sequences employing spectral fat saturation. Clinical MR imaging of canine stifles containing certain metallic implants is feasible using TSE sequences without fat saturation.

Metal-related artifacts in instrumented spine. Techniques for reducing artifacts in CT and MRI: state of the art

The projectional nature of radiogram limits its amount of information about the instrumented spine. MRI and CT imaging can be more helpful, using cross-sectional view. However, the presence of metal-related artifacts at both conventional CT and MRI imaging can obscure relevant anatomy and disease. We reviewed the literature about overcoming artifacts from metallic orthopaedic implants at high-field strength MRI imaging and multi-detector CT. The evolution of multichannel CT has made available new techniques that can help minimizing the severe beam-hardening artifacts. The presence of artifacts at CT from metal hardware is related to image reconstruction algorithm (filter), tube current (in mA), X-ray kilovolt peak, pitch, hardware composition, geometry (shape), and location. MRI imaging has been used safely in patients with orthopaedic metallic implants because most of these implants do not have ferromagnetic properties and have been fixed into position. However, on MRI imaging metallic implants may produce geometric distortion, the so-called susceptibility artifact. In conclusion, although 140 kV and high milliamperage second exposures are recommended for imaging patients with hardware, caution should always be exercised, particularly in children, young adults, and patients undergoing multiple examinations. MRI artifacts can be minimized by positioning optimally and correctly the examined anatomy part with metallic implants in the magnet and by choosing fast spin-echo sequences, and in some cases also STIR sequences, with an anterior to posterior frequency-encoding direction and the smallest voxel size.

Brain and spine MRI artifacts at 3Tesla

INTRODUCTION

We illustrate here the most common MRI artifacts found on routine 3T clinical neuroradiology that can simulate pathology and interfere with diagnosis.

MATERIALS AND METHODS

Our group has worked with a 3-T Magnetom Trio (Siemens, Erlangen, Germany) system for two years, with 50% of our time devoted to clinical work and 50% dedicated to research; 65% of the clinical time is dedicated to neuroradiology (2705 patients) and the remaining time to whole-body MRI. We have detected these artifacts during our case readings and have selected the most representative of each type to illustrate here.

RESULTS

We have observed magnetic susceptibility artifacts (29%), pulsation artifacts (57%), homogeneity artifacts (3%), motion artifacts (6%), truncation artifacts (3%) and, finally, artifacts due to poor or inadequate technique in the examined region.

CONCLUSION

High-field imaging offers the benefit of a higher signal-to-noise ratio, thus making possible the options of a higher imaging matrix, thinner slices, the use of spectroscopy and diffusion tensor imaging in the routine clinical neuroradiology with a reduction in time spent. It is vital to be able to recognize these artifacts in everyday practice as they can mimic pathological appearances, thus causing diagnostic errors that could lead to unnecessary treatment. Indeed, most of these artifacts could be avoided with an adequate technique.

Overcoming artifacts from metallic orthopedic implants at high-field-strength MR imaging and multi-detector CT

At magnetic resonance (MR) imaging and multidetector computed tomography (CT), artifacts arising from metallic orthopedic hardware are an obstacle to obtaining optimal images. Although various techniques for reducing such artifacts have been developed and corroborated by previous researchers, a new era of more powerful MR imaging and multidetector CT modalities has renewed the importance of a systematic consideration of methods for artifact reduction. Knowledge of the factors that contribute to artifacts, of related theories, and of artifact reduction techniques has become mandatory for radiologists. Factors that affect artifacts on MR images include the composition of the metallic hardware, the orientation of the hardware in relation to the direction of the main magnetic field, the strength of the magnetic field, the pulse sequence type, and other MR imaging parameters (mainly voxel size, which is determined by the field of view, image matrix, section thickness, and echo train length). At multidetector CT, the factors that affect artifacts include the composition of the hardware, orientation of the hardware, acquisition parameters (peak voltage, tube charge, collimation, and acquired section thickness), and reconstruction parameters (reconstructed section thickness, reconstruction algorithm used, and whether an extended CT scale was used). A comparison of images obtained with different hardware and different acquisition and reconstruction parameters facilitates an understanding of methods for reducing or overcoming artifacts related to metallic implants.

The role of MR imaging in scaphoid disorders

The scaphoid bone of the wrist is one of the most commonly fractured bones in the body. Due to its importance in the biomechanics and functionality of the wrist, it is important to depict and characterize the type of injury. Plain radiographs and scintigraphy may fail to disclose the type and severity of the injury. In patients with normal initial plain radiographs, MR imaging can discriminate occult fractures from bone bruises and may also demonstrate ligamentous disruption. MR imaging can also discriminate the proximal pole viability versus avascular necrosis secondary to previous fracture, which is important for treatment planning. Treatment of non-united fractures with vascularized grafts can be evaluated with contrast-enhanced MR imaging. Idiopathic osteonecrosis or Preiser's disease was originally described after trauma. The non-traumatic disorders of the scaphoid include post-traumatic osteoarthritis, inflammatory bone marrow edema in patients with rheumatoid arthritis, and osteomyelitis. MR imaging is helpful in all the above disorders to demonstrate early bone marrow edema, cartilage degeneration and associated subchondral marrow changes. The most commonly found tumors in the scaphoid are usually benign and include enchondroma, osteoblastoma and osteoid osteoma. MR imaging is not mandatory for the initial diagnosis, which should be based on plain X-ray findings.

Modeling susceptibility difference artifacts produced by metallic implants in magnetic resonance imaging with point-based thin-plate spline image registration

Magnetic resonance imaging (MRI) suffers from geometric distortions arising from various sources. One such source are the non-linearities associated with the presence of metallic implants, which can profoundly distort the obtained images. These non-linearities result in pixel shifts and intensity changes in the vicinity of the implant, often precluding any meaningful assessment of the entire image. This paper presents a method for correcting these distortions based on non-rigid image registration techniques. Two images from a modelled three-dimensional (3D) grid phantom were subjected to point-based thin-plate spline registration. The reference image (without distortions) was obtained from a grid model including a spherical implant, and the corresponding test image containing the distortions was obtained using previously reported technique for spatial modelling of magnetic susceptibility artifacts. After identifying the nonrecoverable area in the distorted image, the calculated spline model was able to quantitatively account for the distortions, thus facilitating their compensation. Upon the completion of the compensation procedure, the non-recoverable area was removed from the reference image and the latter was compared to the compensated image. Quantitative assessment of the goodness of the proposed compensation technique is presented.

Experimental and computational analyses of the effects of slice distortion from a metallic sphere in an MRI phantom

Susceptibility artifacts due to metallic prostheses are a major problem in clinical magnetic resonance imaging. We theoretically and experimentally analyze slice distortion arising from susceptibility differences in a phantom consisting of a stainless steel ball bearing embedded in agarose gel. To relate the observed image artifacts to slice distortion, we simulate images produced by 2D and 3D spin-echo (SE) and a view angle tilting (VAT) sequence. Two-dimensional SE sequences suffer from extreme slice distortion when a metal prosthesis is present, unlike 3D SE sequences for which--since slices are phase-encoded--distortion of the slice profile is minimized, provided the selected slab is larger than the region of interest. In a VAT sequence, artifacts are reduced by the application of a gradient along the slice direction during readout. However, VAT does not correct for the excitation slice profile, which results in the excitation of spins outside the desired slice location and can lead to incorrect anatomical information in MR images. We propose that the best sequences for imaging in the presence of a metal prosthesis utilize 3D acquisition, with phase encoding replacing slice selection to minimize slice distortion, combined with excitation and readout gradient strengths at their maximum values.

Metal artifact reduction in musculoskeletal magnetic resonance imaging

Many strategies can be employed to reduce the size and scale of metal susceptibility artifacts in the vicinity of orthopedic hardware; factors include selection of metal hardware material, patient positioning, and MRI sequence adjustments and techniques. The adjustments to sequence parameters include high spatial resolution fast spin echo sequences with minimal interecho spacing and increased receiver bandwidth. Frequency and phase encoding gradients can be orientated so misregistration artifacts arising from the metal are directed away from areas of anticipated clinical diagnostic interest. Complications arising in the vicinity of metallic hardware, including loosening, can be assessed after implication of these metal artifact reduction techniques.

Improving geometric accuracy in the presence of susceptibility difference artifacts produced by metallic implants in magnetic resonance imaging

Geometric and intensity distortions due to the presence of metallic implants in magnetic resonance imaging impede the full exploitation of this advanced imaging modality. The aim of this study is to provide a method for (a) quantifying and (b) reducing the implant distortions in patient images. Initially, a set of reference images (without distortion) was obtained by imaging a custom-designed three-dimensional grid phantom. Corresponding test images (containing the distortion) were acquired with the same imaging parameters, after positioning a specific metallic implant in the grid phantom. After determining: 1) the nonrecoverable; 2) the distorted, but recoverable; and 3) the unaffected areas, a point-based thin-plate spline image registration algorithm was employed to align the reference and test images. The calculated transformation functions utilized to align the image pairs described the implant distortions and could therefore be used to correct any other images containing the same distortions. The results demonstrate successful correction of grid phantom images with a metallic implant. Furthermore, the calculated correction was applied to porcine thigh images bearing the same metallic implant, simulating a patient environment. Qualitative and quantitative assessments of the proposed correction method are included.

Scaphoid nonunions treated with vascularised bone grafts: MRI assessment

PURPOSE

To assess the value of MR imaging (MRI) with regard to union, graft viability and proximal pole bone marrow status, after use of vascularized bone grafts for treating scaphoid nonunions.

MATERIALS AND METHODS

Vascularized bone grafts from the distal radius were used to treat 47 scaphoid nonunions resulting from fractures or enchondromas. Clinical and imaging evaluation was used for the pre- and postoperative assessment of all patients. Apart of conventional radiographs obtained in all cases, 15 patients were also assessed postoperatively with MRI at 3 months. From these 15 patients, eight were assessed preoperatively with MRI whereas nine had serial MRI evaluations at 6 and 12 months. The clinical follow-up time of this subgroup of 15 patients ranged from 6 to 27 months.

RESULTS

All patients showed clinical signs of union within 12 weeks form the procedure and at the latest follow-up they experienced complete (10 cases) or almost complete (five cases) relief from pain. Both plain and contrast-enhanced MRI obtained at 3 months showed viability of the bone graft in all cases. At 3 months union was established with plain radiographs in 12 patients at both sides of the graft and in three patients between the graft and proximal pole. At 3 months plain MRI showed nonunion in four patients (two between graft and proximal pole, two between graft and distal pole and one at both sides of the graft) whereas contrast-enhanced MRI revealed only one case of nonunion between graft and proximal pole. Four patients were considered to have osteonecrosis of the proximal pole intraoperatively. Two of them showed necrosis of the proximal pole with preoperative and postoperative plain radiographs and three of them with plain postoperative MRI. Contrast-enhanced MRI at 3 months showed postoperative reversal of necrotic changes in all four scaphoids. MRI also revealed bone marrow oedema of the carpal bones surrounding the scaphoid in 14 cases. Serial MRI at 6 and 12 months, obtained in nine patients, revealed resolution of the bone marrow oedema of the surrounding bones and full graft incorporation in all cases.

CONCLUSION

Contrast-enhanced MRI is able to demonstrate the early union after treatment of scaphoid nonunions with vascularised bone grafts allowing thus earlier mobilisation. In addition, MRI can assess the viability of the proximal pole and the graft as well as the postoperative bone marrow oedema-like lesions of the surrounding bones.

Artifacts from dental casting alloys in magnetic resonance imaging

The potential advantage of magnetic resonance imaging (MRI) has been limited by artifacts due to the presence of metallic materials. For quantitative evaluation of the magnitude of artifacts from dental casting alloys and implant materials in MR imaging, 11 dental casting or implant materials were imaged by means of 1.5 T MRI apparatus with three different sequences. Mean and standard deviation of water signal intensity (SI) around the sample in the region of interest (1200 mm(2)) were determined, and the coefficient of variation was compared for evaluation of the homogeneity of the SI. A variety of artifacts with different magnitudes was observed. Only one of the samples, composed mainly of Pd, In, and Sb, showed no artifacts in all imaging sequences. We concluded that selection of specific dental casting alloys according to their elemental compositions could minimize the metal artifacts in MRI; however, titanium alloys currently pose a problem with respect to causing MRI artifacts.