MRI of hip prostheses using single-point methods: in vitro studies towards the artifact-free imaging of individuals with metal implants

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.

Reducing MRI susceptibility artefacts in implants using additively manufactured porous Ti-6Al-4V structures

Magnetic Resonance Imaging (MRI) is critical in diagnosing post-operative complications following implant surgery and imaging anatomy adjacent to implants. Increasing field strengths and use of gradient-echo sequences have highlighted difficulties from susceptibility artefacts in scan data. Artefacts manifest around metal implants, including those made from titanium alloys, making detection of complications (e.g. bleeding, infection) difficult and hindering imaging of surrounding structures such as the brain or inner ear. Existing research focusses on post-processing and unorthodox scan sequences to better capture data around these devices. This study proposes a complementary up-stream design approach using lightweight structures produced via additive manufacturing (AM). Strategic implant mass reduction presents a potential tool in managing artefacts. Uniform specimens of Ti-6Al-4V structures, including lattices, were produced using the AM process, selective laser melting, with various unit cell designs and relative densities (3.1%-96.7%). Samples, submerged in water, were imaged in a 3T MRI system using clinically relevant sequences. Artefacts were quantified by image analysis revealing a strong linear relationship (RR² = 0.99) between severity and relative sample density. Likewise, distortion due to slice selection errors showed a squared relationship (RR² = 0.92) with sample density. Unique artefact features were identified surrounding honeycomb samples suggesting a complex relationship exists for larger unit cells. To demonstrate clinical utility, a honeycomb design was applied to a representative cranioplasty. Analysis revealed 10% artefact reduction compared to traditional solid material illustrating the feasibility of this approach. This study provides a basis to strategically design implants to reduce MRI artefacts and improve post-operative diagnosis capability. STATEMENT OF SIGNIFICANCE: MRI susceptibility artefacts surrounding metal implants present a clinical challenge for the diagnosis of post-operative complications relating to the implant itself or underlying anatomy. In this study for the first time we demonstrate that additive manufacturing may be exploited to create lattice structures that predictably reduce MRI image artefact severity surrounding titanium alloy implants. Specifically, a direct correlation of artefact severity, both total signal loss and distortion, with the relative material density of these functionalised materials has been demonstrated within clinically relevant MRI sequences. This approach opens the door for strategic implant design, utilising this structurally functionalised material, that may improve post-operative patient outcomes and compliments existing efforts in this area which focus on data acquisition and post-processing methods.

Improving MRI's slice selectivity in the presence of strong, metal-derived inhomogeneities

PURPOSE

To develop schemes that deliver faithful 2D slices near field heterogeneities of the kind arising from non-ferromagnetic metal implants, with reduced artifacts and shorter scan times.

METHODS

An excitation scheme relying on cross-term spatio-temporal encoding (xSPEN) was used as basis for developing the new inhomogeneity-insensitive, slice-selective pulse scheme. The resulting Fully refOCUSED cross-term SPatiotemporal ENcoding (FOCUSED-xSPEN) approach involved four adiabatic sweeps. The method was evaluated in silico, in vitro and in vivo using mice models, and compared against a number of existing and of novel alternatives based on both conventional and swept RF pulses, including an analogous method based on LASER's selectivity spatial selectivity.

RESULTS

Calculations and experiments confirmed that multi-sweep derivatives of xSPEN and LASER can deliver localized excitation profiles, centered at the intended positions and endowed with enhanced immunity to B₀ and B₁ distortions. This, however, is achieved at the expense of higher SAR than non-swept counterparts. Furthermore, single-shot FOCUSED-xSPEN and LASER profiles covered limited off-resonance ranges. This could be extended to bands covering arbitrary off-resonance values with uniform slice widths, by looping the experiments over a number of scans possessing suitable transmission and reception offsets.

CONCLUSIONS

A series of novel approaches were introduced to select slices near metals, delivering robustness against B_o and B₁⁺ field inhomogeneities.

Quasi-Random Single-Point Imaging Using Low-Discrepancy $k$ -Space Sampling

Magnetic resonance imaging of short relaxation time spin systems has been a widely discussed topic with serious clinical applications and led to the emergence of fast imaging ultra-short echo-time sequences. Nevertheless, these sequences suffer from image blurring, due to the related sampling point spread function and are highly prone to imaging artefacts arising from, e.g., chemical shifts or magnetic susceptibilities. In this paper, we present a concept of spherical quasi-random single-point imaging. The approach is highly accelerateable, due to intrinsic undersampling properties and capable of strong metal artefact suppression. Imaging acceleration is achieved by sampling of quasi-random points in -space, based on a low-discrepancy sequence, and a combination with non-linear optimization reconstruction techniques [compressed sensing (CS)]. The presented low-discrepancy trajectory shows ideal noise like undersampling properties for the combination with CS, leading to denoised images with excellent metal artefact reduction. Using eightfold undersampling, acquisition time of a few minutes can be achieved for volume acquisitions.

Fully phase-encoded MRI near metallic implants using ultrashort echo times and broadband excitation

PURPOSE

To develop a fully phase-encoded MRI method for distortion-free imaging near metallic implants, in clinically feasible acquisition times.

THEORY AND METHODS

An accelerated 3D fully phase-encoded acquisition with broadband excitation and ultrashort echo times is presented, which uses a broadband radiofrequency pulse to excite the entire off-resonance induced by the metallic implant. Furthermore, fully phase-encoded imaging is used to prevent distortions caused by frequency encoding, and to obtain ultrashort echo times for rapidly decaying signal.

RESULTS

Phantom and in vivo acquisitions were used to describe the relationship among excitation bandwidth, signal loss near metallic implants, and T₁ weighting. Shorter radiofrequency pulses captured signal closer to the implant by improving spectral coverage and allowing shorter echo times, whereas longer pulses improved T₁ weighting through larger maximum attainable flip angles. Comparisons of fully phase-encoded acquisition with broadband excitation and ultrashort echo times to T₁ -weighted multi-acquisition with variable resonance image combination selective were performed in phantoms and subjects with metallic knee and hip prostheses. These acquisitions had similar contrast and acquisition efficiency.

CONCLUSIONS

Accelerated fully phase-encoded acquisitions with ultrashort echo times and broadband excitation can generate distortion free images near metallic implants in clinically feasible acquisition times. Magn Reson Med 79:2156-2163, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

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.

Improved field-mapping and artifact correction in multispectral imaging

PURPOSE

To develop a method for improved B₀ field-map estimation, deblurring, and image combination for multispectral imaging near metal.

METHODS

A goodness-of-fit field-map estimation technique is proposed that uses only the multispectral imaging (MSI) data to estimate the field map. Using the improved field map, a novel deblurring technique is proposed that also employs a new image combination scheme to reduce the effects of noise and other residual MSI artifacts. The proposed field-map estimation and deblurring techniques are compared to the current methods in phantoms and/or in vivo from subjects with knee, hip, and spinal metallic implants.

RESULTS

Phantom experiments validate that the goodness-of-fit field-map estimation is less sensitive to noise and bias than the conventional center-of-mass technique, which reduces distortion in the deblurring methods. The new deblurring approach also is substantially less sensitive to noise and distortion than the current deblurring method, as demonstrated in phantoms and in vivo, and is able to find a good tradeoff between deblurring and distortion.

CONCLUSION

The proposed methods not only enable field-mapping with reduced noise sensitivity but are able to create deblurred images with less distortion and better signal-to-noise ratio with no additional scan time, thereby enabling improved visualization of underlying anatomy near metallic implants. Magn Reson Med 78:2022-2034, 2017. © 2017 International Society for Magnetic Resonance in Medicine.

3D single point imaging with compressed sensing provides high temporal resolution R 2* mapping for in vivo preclinical applications

OBJECTIVE

Purely phase-encoded techniques such as single point imaging (SPI) are generally unsuitable for in vivo imaging due to lengthy acquisition times. Reconstruction of highly undersampled data using compressed sensing allows SPI data to be quickly obtained from animal models, enabling applications in preclinical cellular and molecular imaging.

MATERIALS AND METHODS

TurboSPI is a multi-echo single point technique that acquires hundreds of images with microsecond spacing, enabling high temporal resolution relaxometry of large-R ₂* systems such as iron-loaded cells. TurboSPI acquisitions can be pseudo-randomly undersampled in all three dimensions to increase artifact incoherence, and can provide prior information to improve reconstruction. We evaluated the performance of CS-TurboSPI in phantoms, a rat ex vivo, and a mouse in vivo.

RESULTS

An algorithm for iterative reconstruction of TurboSPI relaxometry time courses does not affect image quality or R ₂* mapping in vitro at acceleration factors up to 10. Imaging ex vivo is possible at similar acceleration factors, and in vivo imaging is demonstrated at an acceleration factor of 8, such that acquisition time is under 1 h.

CONCLUSIONS

Accelerated TurboSPI enables preclinical R ₂* mapping without loss of data quality, and may show increased specificity to iron oxide compared to other sequences.

Externally calibrated parallel imaging for 3D multispectral imaging near metallic implants using broadband ultrashort echo time imaging

PURPOSE

To develop an externally calibrated parallel imaging technique for three-dimensional multispectral imaging (3D-MSI) in the presence of metallic implants.

THEORY AND METHODS

A fast, ultrashort echo time (UTE) calibration acquisition is proposed to enable externally calibrated parallel imaging techniques near metallic implants. The proposed calibration acquisition uses a broadband radiofrequency (RF) pulse to excite the off-resonance induced by the metallic implant, fully phase-encoded imaging to prevent in-plane distortions, and UTE to capture rapidly decaying signal. The performance of the externally calibrated parallel imaging reconstructions was assessed using phantoms and in vivo examples.

RESULTS

Phantom and in vivo comparisons to self-calibrated parallel imaging acquisitions show that significant reductions in acquisition times can be achieved using externally calibrated parallel imaging with comparable image quality. Acquisition time reductions are particularly large for fully phase-encoded methods such as spectrally resolved fully phase-encoded three-dimensional (3D) fast spin-echo (SR-FPE), in which scan time reductions of up to 8 min were obtained.

CONCLUSION

A fully phase-encoded acquisition with broadband excitation and UTE enabled externally calibrated parallel imaging for 3D-MSI, eliminating the need for repeated calibration regions at each frequency offset. Significant reductions in acquisition time can be achieved, particularly for fully phase-encoded methods like SR-FPE. Magn Reson Med 77:2303-2309, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Polarized multichannel transmit MRI to reduce shading near metal implants

PURPOSE

To investigate the benefit of a two-channel transmit system on shading close to total hip replacements and other elongated metal structures in parallel to the magnet bore.

METHODS

An analytical model comprising a water cylinder and a metal rod is introduced to describe the B1 effects close to elongated metal structures and it is verified. The dependence of the optimal polarization, which induces minimum shading, on the position of the metal is analyzed. Furthermore, the optimal polarization for two patients is determined both on the basis of the model and experimentally and its benefit compared with circular polarization is investigated.

RESULTS

The cylindrical model approximates the modification of the B1 field due to the metal well, and the optimal polarization strongly depends on the position of the rod. In vivo, shading can be ameliorated by the use of the optimal polarization; for total hip replacements with shafts of titanium, this polarization can be determined on the basis of both the analytical model and the experimental data.

CONCLUSION

Parallel transmission offers the possibility of a substantial reduction of shading close to long metal structures in parallel to the magnet bore.

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.

New opportunities for quantitative and time efficient 3D MRI of liquid and solid electrochemical cell components: Sectoral Fast Spin Echo and SPRITE

The ability to image electrochemical processes in situ using nuclear magnetic resonance imaging (MRI) offers exciting possibilities for understanding and optimizing materials in batteries, fuel cells and supercapacitors. In these applications, however, the quality of the MRI measurement is inherently limited by the presence of conductive elements in the cell or device. To overcome related difficulties, optimal methodologies have to be employed. We show that time-efficient three dimensional (3D) imaging of liquid and solid lithium battery components can be performed by Sectoral Fast Spin Echo and Single Point Imaging with T1 Enhancement (SPRITE), respectively. The former method is based on the generalized phase encoding concept employed in clinical MRI, which we have adapted and optimized for materials science and electrochemistry applications. Hard radio frequency pulses, short echo spacing and centrically ordered sectoral phase encoding ensure accurate and time-efficient full volume imaging. Mapping of density, diffusivity and relaxation time constants in metal-containing liquid electrolytes is demonstrated. 1, 2 and 3D SPRITE approaches show strong potential for rapid high resolution (7)Li MRI of lithium electrode components.

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.

Magnetic resonance imaging (MRI) artefacts in hip prostheses: a comparison of different prosthetic compositions

PURPOSE

The purpose of the study was to compare the artefacts produced by different hip prostheses on magnetic resonance imaging (MRI).

MATERIALS AND METHODS

An identical MRI protocol was used to perform a quali-quantitative in vitro evaluation of artefacts caused by different hip prosthetic materials at different field strengths: prosthesis number 1, composed of cobalt-chrome-molybdenum (head and stem); prosthesis number 2, composed of ceramic (head) and titanium (stem); prosthesis number 3, composed of cobalt-chrome (head) and titanium (stem). All prostheses were imaged with both a clinical 1 Tesla (T) (Signa Horizon, General Electrics) and 1.5 T (Achieva, Philips) MRI system, using spin echo (SE) and gradient echo (GRE) sequences: sagittal T1 SE, coronal T2 fast SE (FSE), axial T1 SE, axial T2 FSE, sagittal T2 GRE, axial T2* GRE, coronal T1 GRE, axial T1 GRE. The artefacts produced by each prosthesis were assessed in each sequence at the different field strengths, by measuring the two longest diameters of the artefact in each section and sequence and comparing them to the actual diameters so as to obtain a ratio expressing the effective degree of artefact.

RESULTS

Cobalt-chrome produced the largest artefacts both in SE (1.73 at 1 T and 2.37 at 1.5 T) and GRE sequences (2.8 at 1 T and 3.06 at 1.5 T) followed by titanium (SE, 1.6 at 1 T, 2.13 at 1.5 T; GRE, 2 at 1 T, 2.94 at 1.5 T) and cobalt-chrome-molybdenum (SE, 1.51 at 1 T, 1.67 at 1.5 T; GRE, 2.13 at 1 T and 2.48 at 1.5 T); ceramic produced the smallest artefacts in all sequences (SE, 1.0 at 1 T and 1.18 at 1.5 T; GRE, 1.3 at 1 T and 1.22 at 1.5T). Increasing the magnetic field strength, titanium showed the greatest variations in artefact size, and ceramic the smallest ones.

CONCLUSIONS

The composition of prosthetic implants is decisive in determining the quality of MR imaging.

Improved late gadolinium enhancement MR imaging for patients with implanted cardiac devices

PURPOSE

To propose and test a modified wideband late gadolinium enhancement (LGE) magnetic resonance (MR) imaging technique to overcome hyperintensity image artifacts caused by implanted cardiac devices.

MATERIALS AND METHODS

Written informed consent was obtained from all participants, and the HIPAA-compliant study protocol was approved by the institutional review board. Studies in phantoms and in a healthy volunteer were performed to test the hypothesis that the hyperintensity artifacts that are typically observed on LGE images in patients with implanted cardiac devices are caused by insufficient inversion of the affected myocardial signal. The conventional LGE MR imaging pulse sequence was modified by replacing the nonselective inversion pulse with a wideband inversion pulse. The modified LGE sequence, along with the conventional LGE sequence, was evaluated in 12 patients with implantable cardioverter defibrillators (ICDs) who were referred for cardiac MR imaging.

RESULTS

The ICD causes 2-6 kHz in frequency shift at locations 5-10 cm away from the device. This off-resonance falls outside the typical spectral bandwidth of the nonselective inversion pulse used in conventional LGE, which results in the hyperintensity artifact. In 10 of the 12 patients, the conventional LGE technique produced severe, uninterpretable hyperintensity artifacts in the anterior and lateral portions of the left ventricular wall. These artifacts were eliminated with use of the wideband LGE sequence, thereby enabling confident evaluation of myocardial viability.

CONCLUSION

The modified wideband LGE MR imaging technique eliminates the hyperintensity artifacts seen in patients with cardiac devices. The technique may enable LGE MR imaging in patients with cardiac devices, in whom LGE MR imaging otherwise could not be used for diagnosis.

Theoretical study of evaluation method for MRI metal artifact

In dental field, the effect of the Magnetic Resonance Imaging (MRI) artifact generated by the magnetic metal is a significant problem. The MRI metal artifact occurs when using magnetic attachment and the keeper of the ferromagnetic substance remains implanted in the mouth as the MR image is taken. In this study, we theoretically evaluated and analyzed the artifact of MR images caused by the keeper based on the actual principle of MRI by means of simulation. As a result we were able to recognize the changes and distortion in the signal strength of the output image. We found that our results of output images and previously reported results of actual measurement are very similar. MRI artifact caused by dental magnetic metal showed that it can be reported by theoretical simulation.

Quantifying massive allograft healing of the canine femur in vivo and ex vivo: a pilot study

BACKGROUND

Allograft integration in segmental osseous defects is unpredictable. Imaging techniques have not been applied to investigate angiogenesis and bone formation during allograft healing in a large-animal model.

QUESTIONS/PURPOSES

We used dynamic contrast-enhanced (DCE)-MRI and cone beam (CB)-CT to quantify vascularity and bone volume in a canine femoral allograft model and determined their relationship with biomechanical testing and histomorphometry.

METHODS

Femoral ostectomy was performed in three dogs and reconstructed with a 5-cm allograft and compression plate. At 0.5, 3, and 6 months, we performed DCE-MRI to quantify vascular permeability (Ktrans) and perfused fraction and CB-CT to quantify bone volume. We also performed posteuthanasia torsional testing and dynamic histomorphometry of the grafted and nonoperated femurs.

RESULTS

DCE-MRI confirmed the avascular nature of allograft healing (perfused fraction, 2.08%-3.25%). CB-CT demonstrated new bone formation at 3 months (26.2, 3.7, and 2.2 cm(3)) at the graft-host junctions, which remodeled down at 6 months (14.0, 2.2, and 2.0 cm(3)). The increased bone volume in one subject was confirmed with elevated Ktrans (0.22) at 3 months. CB-CT-identified remodeled bone at 6 months was corroborated by histomorphometry. Allografted femurs recovered only 40% of their strength at 6 months.

CONCLUSIONS

CB-CT and DCE-MRI can discriminate differences in angiogenesis and bone formation in the canine allograft model, which has potential to detect a small (32%) drug or device effect on biomechanical healing with only five animals per group.

On the utility of spectroscopic imaging as a tool for generating geometrically accurate MR images and parameter maps in the presence of field inhomogeneities and chemical shift effects

Lack of spatial accuracy is a recognized problem in magnetic resonance imaging (MRI) which severely detracts from its value as a stand-alone modality for applications that put high demands on geometric fidelity, such as radiotherapy treatment planning and stereotactic neurosurgery. In this paper, we illustrate the potential and discuss the limitations of spectroscopic imaging as a tool for generating purely phase-encoded MR images and parameter maps that preserve the geometry of an object and allow localization of object features in world coordinates. Experiments were done on a clinical system with standard facilities for imaging and spectroscopy. Images were acquired with a regular spin echo sequence and a corresponding spectroscopic imaging sequence. In the latter, successive samples of the acquired echo were used for the reconstruction of a series of evenly spaced images in the time and frequency domain. Experiments were done with a spatial linearity phantom and a series of test objects representing a wide range of susceptibility- and chemical-shift-induced off-resonance conditions. In contrast to regular spin echo imaging, spectroscopic imaging was shown to be immune to off-resonance effects, such as those caused by field inhomogeneity, susceptibility, chemical shift, f(0) offset and field drift, and to yield geometrically accurate images and parameter maps that allowed object structures to be localized in world coordinates. From these illustrative examples and a discussion of the limitations of purely phase-encoded imaging techniques, it is concluded that spectroscopic imaging offers a fundamental solution to the geometric deficiencies of MRI which may evolve toward a practical solution when full advantage will be taken of current developments with regard to scan time reduction. This perspective is backed up by a demonstration of the significant scan time reduction that may be achieved by the use of compressed sensing for a simple phantom.

Magnetic resonance imaging of the postoperative hip

Magnetic resonance imaging (MRI) is ideally suited to imaging the patient with painful hip arthroplasty due to its superior soft tissue contrast, multiplanar capabilities, and lack of ionizing radiation. MRI is the most accurate imaging modality in the assessment of periprosthetic osteolysis and wear-induced synovitis, and can also assess regional tendons and neurovascular structures. This article discusses the technical aspects of MRI around metallic implants as well as the appearance of potential complications following hip arthroplasty, including osteolysis, wear-induced synovitis, infection, hemarthrosis, fracture, loosening, component displacement, heterotopic ossification, tendinopathy, and neurovascular impingement. The specific complication of metal hypersensitivity following metal-on-metal prostheses is reviewed.

MR imaging near metal with undersampled 3D radial UTE-MAVRIC sequences

Recently developed techniques such as the multiple acquisition with variable resonance image combination and slice encoding for metal artifact correction techniques have improved the ability of clinical magnetic resonance scanners to image near metal implants. These sequences are based on fast spin echo sequences which preclude detection of short T(2) tissues such as tendons, ligaments, and cortical bone. Ultrashort echo time sequences have the potential to detect signals from these tissues. In this study, we investigate the potential of combining ultrashort echo time with multiple acquisition with variable resonance image combination to image short T(2) musculoskeletal tissues adjacent to metallic implants. Different radio frequency excitation pulse types and spectral binning strategies were studied. We found that ultrashort echo time-multiple acquisition with variable resonance image combination sequences were able to significantly reduce typical artifacts near metal, as well as detect very short T(2) signals that are usually not visualized using clinical pulse sequences.

Towards inherently distortion-free MR images for image-guided radiotherapy on an MRI accelerator

In MR-guided interventions, it is mandatory to establish a solid relationship between the imaging coordinate system and world coordinates. This is particularly important in image-guided radiotherapy (IGRT) on an MRI accelerator, as the interaction of matter with γ-radiation cannot be visualized. In conventional acquisitions, off-resonance effects cause discrepancies between coordinate systems. We propose to mitigate this by using only phase encoding and to reduce the longer acquisitions by under-sampling and regularized reconstruction. To illustrate the performance of this acquisition in the presence of off-resonance phenomena, phantom and in vivo images are acquired using spin-echo (SE) and purely phase-encoded sequences. Data are retrospectively under-sampled and reconstructed iteratively. We observe accurate geometries in purely phase-encoded images for all cases, whereas SE images of the same phantoms display image distortions. Regularized reconstruction yields accurate phantom images under high acceleration factors. In vivo images were reconstructed faithfully while using acceleration factors up to 4. With the proposed technique, inherently undistorted images with one-to-one correspondence to world coordinates can be obtained. It is a valuable tool in geometry quality assurance, treatment planning and online image guidance. Under-sampled acquisition combined with regularized reconstruction can be used to accelerate the acquisition while retaining geometrical accuracy.

Peripheral nerve surgery: the role of high-resolution MR neurography

High-resolution MRN is becoming increasingly available due to recent technical advancements, including higher magnetic field strengths (eg, 3T), 3D image acquisition, evolution of novel fat-suppression methods, and improved coil design. This review describes the MRN techniques for obtaining high-quality images of the peripheral nerves and their small branches and imaging findings in normal as well as injured nerves with relevant intraoperative correlations. Various microsurgical techniques in peripheral nerves, such as neurolysis, nerve repairs by using nerve grafts, and conduits are discussed, and MRN findings of surgically treated nerves are demonstrated.

MRI after arthroplasty: comparison of MAVRIC and conventional fast spin-echo techniques

OBJECTIVE

The goal of this study was to evaluate the quality of images obtained with a prototype imaging technique, multiacquisition variable-resonance image combination (MAVRIC), compared with fast spin-echo (FSE) images in the evaluation of patients who have undergone hip, shoulder, or knee arthroplasty.

MATERIALS AND METHODS

MRI with metal-artifact reduction FSE and MAVRIC sequences was performed in the care of 122 patients who had undergone 74 hip, 27 shoulder, and 21 knee arthroplasties. The FSE and MAVRIC images were subjectively graded for visualization of the synovium, prosthesis-bone interface, and hip abductors or supraspinatus tendon. The presence of synovitis, osteolysis, or supraspinatus tendon tear was recorded.

RESULTS

Visualization of the synovium was significantly better on MAVRIC images than on FSE images of the hip (p < 0.0001), shoulder (p < 0.01), and knee (p < 0.01). Synovitis was detected only on the MAVRIC images of nine subjects (12%) who had undergone hip arthroplasty and five subjects (18%) who had undergone shoulder arthroplasty. Visualization of the periprosthetic bone was significantly better on MAVRIC images of the hip (p < 0.0001), shoulder (p < 0.0001), and knee (p < 0.01). Osteolysis was detected only on the MAVRIC images of 12 subjects (16%) who had undergone hip arthroplasty, six (22%) who had undergone shoulder arthroplasty, and five (24%) who had undergone knee arthroplasty. Visualization of the supraspinatus tendon was significantly better on MAVRIC images (p < 0.0001). Supraspinatus tendon tears in 12 subjects (44%) were detected only on MAVRIC images.

CONCLUSION

MAVRIC complements the information on FSE images after arthroplasty and is a useful additional sequence, particularly when there is concern about synovitis, periprosthetic osteolysis, or the presence of a supraspinatus tendon tear.

Improvements in MR imaging of solids through gradient waveform optimization

Magnetic resonance imaging (MRI) is known to provide a useful approach for the exploration of the chemistry and dynamics of a wide range of soft condensed materials. However, its application to solids has been limited to those materials with relatively narrow resonances. The time needed to obtain an image of a solid with a given resolution and signal-to-noise ratio (SNR) is directly proportional to the line width of the resonance. For MRI to become practical for the imaging of solids it will have to rely on the development and use of MR sequences that avoid the issues raised by line broadening of the resonance. In this paper we review the latest contributions towards MR imaging of solids from our laboratory, in particular, applications using optimized gradient waveforms. Acoustic noise reduction and SNR improvement obtained with modifications of the standard single-point imaging sequence are presented and discussed using examples.

Imaging near metal with a MAVRIC-SEMAC hybrid

The recently developed multi-acquisition with variable resonance image combination (MAVRIC) and slice-encoding metal artifact correction (SEMAC) techniques can significantly reduce image artifacts commonly encountered near embedded metal hardware. These artifact reductions are enabled by applying alternative spectral and spatial-encoding schemes to conventional spin-echo imaging techniques. Here, the MAVRIC and SEMAC concepts are connected and discussed. The development of a hybrid technique that utilizes strengths of both methods is then introduced. The presented technique is shown capable of producing minimal artifact, high-resolution images near total joint replacements in a clinical setting.

Magnetic resonance imaging near metal implants

The desire to apply magnetic resonance imaging (MRI) techniques in the vicinity of embedded metallic hardware is increasing. The soft-tissue contrast available with MR techniques is advantageous in diagnosing complications near an increasing variety of MR-safe metallic hardware. Near such hardware, the spatial encoding mechanisms utilized in conventional MRI methods are often severely compromised. Mitigating these encoding difficulties has been the focus of numerous research investigations over the past two decades. Such approaches include view-angle tilting, short echo-time projection reconstruction acquisitions, single-point imaging, prepolarized MRI, and postprocessing image correction. Various technical advances have also enabled the recent development of two alternative approaches that have shown promising clinical potential. Here, the physical principals and proposed solutions to the problem of MRI near embedded metal are discussed.

Three-dimensional image-based planning for cervix brachytherapy with bilateral hip prostheses: a solution using MVCT with helical tomotherapy

PURPOSE

We present a method of three-dimensional image-based planning for cervix high-dose-rate (HDR) brachytherapy for patients with bilateral metal hip prostheses using megavoltage computed tomography (MVCT) imaging.

METHODS AND MATERIALS

Two patients with bilateral metal hip prostheses were treated with our standard HDR brachytherapy fractionation and critical structure tolerance limits for cervical cancer. MVCT imaging was used for treatment planning because of artifacts present in kilovoltage computed tomography (kVCT), which did not allow visualization of the organs of interest.

RESULTS

The MVCT images provided adequate contrast to allow the contouring of organs at risk and the digitization of HDR applicators. HDR brachytherapy treatment planning was successfully accomplished based on MVCT images for 2 patients with bilateral metal hip prostheses.

CONCLUSIONS

Using MVCT imaging eliminated streak artifacts, which improved the image quality for treatment planning. MVCT offers an option for three-dimensional planning for cervix brachytherapy in patients with bilateral hip prostheses.

SEMAC: Slice Encoding for Metal Artifact Correction in MRI

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

A multispectral three-dimensional acquisition technique for imaging near metal implants

Metallic implants used in bone and joint arthroplasty induce severe spatial perturbations to the B0 magnetic field used for high-field clinical magnetic resonance. These perturbations distort slice-selection and frequency encoding processes applied in conventional two-dimensional MRI techniques and hinder the diagnosis of complications from arthroplasty. Here, a method is presented whereby multiple three-dimensional fast-spin-echo images are collected using discrete offsets in RF transmission and reception frequency. It is demonstrated that this multi acquisition variable-resonance image combination technique can be used to generate a composite image that is devoid of slice-plane distortion and possesses greatly reduced distortions in the readout direction, even in the immediate vicinity of metallic implants.

Optimal k-space sampling for single point imaging of transient systems

A novel approach for sampling k-space in a pure phase encoding imaging sequence is presented using the Single Point Imaging (SPI) technique. The sequence is optimised with respect to the achievable Signal-to-Noise ratio (SNR) for a given time interval via selective sparse k-space sampling, dictated by prior knowledge of the overall object of interest's shape. This allows dynamic processes featuring short T(2)( *) NMR signal to be more readily followed, in our case the absorption of moisture by a cereal-based wafer material. Further improvements in image quality are also shown via the use of complete sampling of k-space at the start or end of the series of imaging experiments; followed by subsequent use of this data for un-sampled k-space points as opposed to zero filling.

Quantification of massive allograft healing with dynamic contrast enhanced-MRI and cone beam-CT: a pilot study

Although massive allografts are widely used for reconstruction of critical defects in long bones caused by tumor or trauma, many will have inadequate long-term outcomes. Toward a tissue engineering solution to this problem, we developed experimental stem cell and gene therapy adjuvants that induce angiogenesis, osteogenesis, and remodeling of the structural allografts. We present data from pilot studies to show the utility of dynamic contrast enhanced MRI (DCE-MRI) to quantify vascularity after femoral osteotomy in a canine femur model and cone beam CT (CB-CT) to quantify bone volume in a patient after composite prosthetic-allograft reconstructive surgery. The results demonstrate our ability to suppress the artifacts generated by the metal implants required to secure massive allografts such that precise quantification of cortical bone revascularization (>10-fold increase at 3 weeks postoperatively) and new bone formation (accurate to about 193 mum(3)) around the graft can be performed longitudinally via DCE-MRI and CB-CT, respectively.

Single-point imaging with a variable phase encoding interval

A modified single-point imaging (SPI) technique using a variable phase encoding interval is proposed. This method is based on the minimization of the phase encoding interval for further signal-to-noise ratio (SNR) optimization. This is particularly beneficial when the maximum gradient amplitude limits an optimal phase encoding interval, and the resulting SNR suffers from T(2)-related signal attenuation. Theoretical calculation of the SNR and simulation of the point spread function (PSF) for the different experimental parameters are presented. Experiments using a rubber sample (T(2)* approximately 73 micros) and a tooth (bi-exponential relaxation: T(2,1)*=111 micros and T(2,1)*=872 micros) showed a significant increase in SNR (>3 and >2, respectively) when compared with images acquired with conventional SPI.

Prepolarized magnetic resonance imaging around metal orthopedic implants

A prepolarized MRI (PMRI) scanner was used to image near metal implants in agar gel phantoms and in in vivo human wrists. Comparison images were made on 1.5- and 0.5-T conventional whole-body systems. The PMRI experiments were performed in a smaller bore system tailored to extremity imaging with a prepolarization magnetic field of 0.4 T and a readout magnetic field of 27-54 mT (1.1-2.2 MHz). Scan parameters were chosen with equal readout gradient strength over a given field of view and matrix size to allow unbiased evaluation of the benefits of lower readout frequency. Results exhibit substantial reduction in metal susceptibility artifacts under PMRI versus conventional scanners. A new artifact quantification technique is also presented, and phantom results confirm that susceptibility artifacts improve as expected with decreasing readout magnetic field using PMRI. This proof-of-concept study demonstrates that prepolarized techniques have the potential to provide diagnostic cross-sectional images for postoperative evaluation of patients with metal implants.

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.