Quantification and minimization of magnetic susceptibility artifacts on GRE images

Use of Ultra-Short Echo Time MRI to Improve Temporal Bone Imaging

OBJECTIVE

The short T2 nature of cortical bone causes it to appear similar to air on MR, forcing clinicians to rely on computed tomography imaging, with its attendant ionizing radiation exposure, to define temporal bone structures. Through the use of novel MR sequences with ultra-short echo times (UTE), short T2 structures are now able to be visualized, allowing for improved understanding of anatomical relationships.

METHODS

Eight patients (50% female) undergoing MR imaging of the skull base for diagnostic purposes (62.5% for vestibular schwannoma surveillance) at a tertiary care center were enrolled to evaluate the safety and efficacy of UTE imaging. CT scans were completed in 37.5% of the patients as part of their workup and used for comparison purposes. The repetition time, short echo time, and long echo time for the UTE sequence were 11, 0.032, and 2.2 msec, respectively.

RESULTS

The protocol added 6 min to the total scanning time, and all patients tolerated the sequence without issue. The ossicles, mastoid air cells, antrum, and epitympanum were able to be seen and had a high Dice similarity coefficient when compared to CT (>0.5). UTE allowed for clear delineation of all segments of the facial nerve with a signal-to-noise ratio of 35 (although the BRAVO sequences had a superior ratio of 140). Vestibular schwannomas were able to be distinguished from normal brain parenchyma.

CONCLUSIONS

UTE is safe and effective for visualizing anatomic structures not normally seen on traditional MRI, potentially allowing for improved surgical planning in patients.

LEVEL OF EVIDENCE

III Laryngoscope, 2024.

Cerebral Microvascular Perfusion Assessed in Elderly Adults by Spin-Echo Dynamic Susceptibility Contrast MRI at 7 Tesla

Perfusion measures of the total vasculature are commonly derived with gradient-echo (GE) dynamic susceptibility contrast (DSC) MR images, which are acquired during the early passes of a contrast agent. Alternatively, spin-echo (SE) DSC can be used to achieve specific sensitivity to the capillary signal. For an improved contrast-to-noise ratio, ultra-high-field MRI makes this technique more appealing to study cerebral microvascular physiology. Therefore, this study assessed the applicability of SE-DSC MRI at 7 T. Forty-one elderly adults underwent 7 T MRI using a multi-slice SE-EPI DSC sequence. The cerebral blood volume (CBV) and cerebral blood flow (CBF) were determined in the cortical grey matter (CGM) and white matter (WM) and compared to values from the literature. The relation of CBV and CBF with age and sex was investigated. Higher CBV and CBF values were found in CGM compared to WM, whereby the CGM-to-WM ratios depended on the amount of largest vessels excluded from the analysis. CBF was negatively associated with age in the CGM, while no significant association was found with CBV. Both CBV and CBF were higher in women compared to men in both CGM and WM. The current study verifies the possibility of quantifying cerebral microvascular perfusion with SE-DSC MRI at 7 T.

Framewise multi-echo distortion correction for superior functional MRI

Functional MRI (fMRI) data are severely distorted by magnetic field (B0) inhomogeneities which currently must be corrected using separately acquired field map data. However, changes in the head position of a scanning participant across fMRI frames can cause changes in the B0 field, preventing accurate correction of geometric distortions. Additionally, field maps can be corrupted by movement during their acquisition, preventing distortion correction altogether. In this study, we use phase information from multi-echo (ME) fMRI data to dynamically sample distortion due to fluctuating B0 field inhomogeneity across frames by acquiring multiple echoes during a single EPI readout. Our distortion correction approach, MEDIC (Multi-Echo DIstortion Correction), accurately estimates B0 related distortions for each frame of multi-echo fMRI data. Here, we demonstrate that MEDIC's framewise distortion correction produces improved alignment to anatomy and decreases the impact of head motion on resting-state functional connectivity (RSFC) maps, in higher motion data, when compared to the prior gold standard approach (i.e., TOPUP). Enhanced framewise distortion correction with MEDIC, without the requirement for field map collection, furthers the advantage of multi-echo over single-echo fMRI.

Geometric distortions in clinical MRI sequences for radiotherapy: insights gained from a multicenter investigation

BACKGROUND

As magnetic resonance imaging (MRI) becomes increasingly integrated into radiotherapy (RT) for enhanced treatment planning and adaptation, the inherent geometric distortion in acquired MR images pose a potential challenge to treatment accuracy. This study aimed to evaluate the geometric distortion levels in the clinical MRI protocols used across Danish RT centers and discuss influence of specific sequence parameters. Based on the variety in geometric performance across centers, we assess if harmonization of MRI sequences is a relevant measure.

MATERIALS AND METHODS

Nine centers participated with 12 MRI scanners and MRI-Linacs (MRL). Using a travelling phantom approach, a reference MRI sequence was used to assess variation in baseline distortion level between scanners. The phantom was also scanned with local clinical MRI sequences for brain, head/neck (H/N), abdomen, and pelvis. The influence of echo time, receiver bandwidth, image weighting, and 2D/3D acquisition was investigated.

RESULTS

We found a large variation in geometric accuracy across 93 clinical sequences examined, exceeding the baseline variation found between MRI scanners (σ = 0.22 mm), except for abdominal sequences where the variation was lower. Brain and abdominal sequences showed lowest distortion levels ([0.22, 2.26] mm), and a large variation in performance was found for H/N and pelvic sequences ([0.19, 4.07] mm). Post hoc analyses revealed that distortion levels decreased with increasing bandwidth and a less clear increase in distortion levels with increasing echo time. 3D MRI sequences had lower distortion levels than 2D (median of 1.10 and 2.10 mm, respectively), and in DWI sequences, the echo-planar imaging read-out resulted in highest distortion levels.

CONCLUSION

There is a large variation in the geometric distortion levels of clinical MRI sequences across Danish RT centers, and between anatomical sites. The large variation observed makes harmonization of MRI sequences across institutions and adoption of practices from well-performing anatomical sites, a relevant measure within RT.

Numerical approach to investigate MR imaging artifacts from orthopedic implants at different field strengths according to ASTM F2119

OBJECTIVE

This study presents an extended evaluation of a numerical approach to simulate artifacts of metallic implants in an MR environment.

METHODS

The numerical approach is validated by comparing the artifact shape of the simulations and measurements of two metallic orthopedic implants at three different field strengths (1.5 T, 3 T, and 7 T). Furthermore, this study presents three additional use cases of the numerical simulation. The first one shows how numerical simulations can improve the artifact size evaluation according to ASTM F2119. The second use case quantifies the influence of different imaging parameters (TE and bandwidth) on the artifact size. Finally, the third use case shows the potential of performing human model artifact simulations.

RESULTS

The numerical simulation approach shows a dice similarity coefficient of 0.74 between simulated and measured artifact sizes of metallic implants. The alternative artifact size calculation method presented in this study shows that the artifact size of the ASTM-based method is up to 50% smaller for complex shaped implants compared to the numerical-based approach.

CONCLUSION

In conclusion, the numerical approach could be used in the future to extend MR safety testing according to a revision of the ASTM F2119 standard and for design optimization during the development process of implants.

Fiber-orientation independent component of R2* obtained from single-orientation MRI measurements in simulations and a post-mortem human optic chiasm

The effective transverse relaxation rate (R2*) is sensitive to the microstructure of the human brain like the g-ratio which characterises the relative myelination of axons. However, the fibre-orientation dependence of R2* degrades its reproducibility and any microstructural derivative measure. To estimate its orientation-independent part (R2,iso*) from single multi-echo gradient-recalled-echo (meGRE) measurements at arbitrary orientations, a second-order polynomial in time model (hereafter M2) can be used. Its linear time-dependent parameter, β1, can be biophysically related to R2,iso* when neglecting the myelin water (MW) signal in the hollow cylinder fibre model (HCFM). Here, we examined the performance of M2 using experimental and simulated data with variable g-ratio and fibre dispersion. We found that the fitted β1 can estimate R2,iso* using meGRE with long maximum-echo time (TEmax ≈ 54 ms), but not accurately captures its microscopic dependence on the g-ratio (error 84%). We proposed a new heuristic expression for β1 that reduced the error to 12% for ex vivo compartmental R2 values. Using the new expression, we could estimate an MW fraction of 0.14 for fibres with negligible dispersion in a fixed human optic chiasm for the ex vivo compartmental R2 values but not for the in vivo values. M2 and the HCFM-based simulations failed to explain the measured R2*-orientation-dependence around the magic angle for a typical in vivo meGRE protocol (with TEmax ≈ 18 ms). In conclusion, further validation and the development of movement-robust in vivo meGRE protocols with TEmax ≈ 54 ms are required before M2 can be used to estimate R2,iso* in subjects.

The impact of iron deposition on the fear circuit of the brain in patients with Parkinson's disease and anxiety

Objective

Anxiety is one of the most common psychiatric symptoms of Parkinson's disease (PD), and brain iron deposition is considered to be one of the pathological mechanisms of PD. The objective of this study was to explore alterations in brain iron deposition in PD patients with anxiety compared to PD patients without anxiety, especially in the fear circuit.

Methods

Sixteen PD patients with anxiety, 23 PD patients without anxiety, and 26 healthy elderly controls were enrolled prospectively. All subjects underwent neuropsychological assessments and brain magnetic resonance imaging (MRI) examinations. Voxel-based morphometry (VBM) was used to study morphological brain differences between the groups. Quantitative susceptibility mapping (QSM), an MRI technique capable of quantifying susceptibility changes in brain tissue, was used to compare susceptibility changes in the whole brain among the three groups. The correlations between brain susceptibility changes and anxiety scores quantified using the Hamilton Anxiety Rating Scale (HAMA) were compared and analyzed.

Results

PD patients with anxiety had a longer duration of PD and higher HAMA scores than PD patients without anxiety. No morphological brain differences were observed between the groups. In contrast, voxel-based and ROI-based QSM analyses showed that PD patients with anxiety had significantly increased QSM values in the medial prefrontal cortex, anterior cingulate cortex, hippocampus, precuneus, and angular cortex. Furthermore, the QSM values of some of these brain regions were positively correlated with the HAMA scores (medial prefrontal cortex: r = 0.255, p = 0.04; anterior cingulate cortex: r = 0.381, p < 0.01; hippocampus: r = 0.496, p < 0.01).

Conclusion

Our findings support the idea that anxiety in PD is associated with iron burden in the brain fear circuit, providing a possible new approach to explaining the potential neural mechanism of anxiety in PD.

Susceptibility-weighted imaging in intracranial hemorrhage: not all bleeds are black

To correctly recognize intracranial hemorrhage (ICH) and differentiate it from other lesions, knowledge of the imaging characteristics of an ICH on susceptibility weighted imaging (SWI) is essential. It is a common misconception that blood is always black on SWI, and it is important to realize that hemorrhage has a variable appearance in different stages on SWI. Furthermore, the presence of a low signal on SWI does not equal the presence of blood products.

In this review, the appearance of ICH on SWI during all its stages and common other causes of a low signal on SWI are further discussed and illustrated.

Optimised passive marker device visibility and automatic marker detection for 3-T MRI-guided endovascular interventions: a pulsatile flow phantom study

BACKGROUND

Passive paramagnetic markers on magnetic resonance imaging (MRI)-compatible endovascular devices induce susceptibility artifacts, enabling MRI-visibility and real-time MRI-guidance. Optimised visibility is crucial for automatic detection and device tracking but depends on MRI technical parameters and marker characteristics. We assessed marker visibility and automatic detection robustness for varying MRI parameters and marker characteristics in a pulsatile flow phantom.

METHODS

Guidewires with varying iron(II,III) oxide nanoparticle (IONP) concentration markers were imaged using gradient-echo (GRE) and balanced steady-state free precession (bSSFP) sequences at 3 T. Furthermore, echo time (TE), slice thickness (ST) and phase encoding direction (PED) were varied. Artifact width was measured and contrast-to-noise ratios were calculated. Marker visibility and image quality were scored by two MRI interventional radiologists. Additionally, a deep learning model for automatic marker detection was trained and the effects of the parameters on detection performance were evaluated. Two-tailed Wilcoxon signed-rank tests were used (significance level, p < 0.05).

RESULTS

Medan artifact width (IQR) was larger in bSSFP compared to GRE images (12.7 mm (11.0-15.2) versus 8.4 mm (6.5-11.0)) (p < 0.001) and showed a positive relation with TE and IONP concentration. Switching PED and doubling ST had limited effect on artifact width. Image quality assessment scores were higher for GRE compared to bSSFP images. The deep learning model automatically detected the markers. However, the model performance was reduced after adjusting PED, TE, and IONP concentration.

CONCLUSION

Marker visibility was sufficient and a large range of artifact sizes was generated by adjusting TE and IONP concentration. Deep learning-based marker detection was feasible but performance decreased for altered MR parameters. These factors should be considered to optimise device visibility and ensure reliable automatic marker detectability in MRI-guided endovascular interventions.

Quantification and 3D localization of magnetically navigated superparamagnetic particles using MRI in phantom and swine chemoembolization models

OBJECTIVE

Superparamagnetic nanoparticles (SPIONs) can be combined with tumor chemoembolization agents to form magnetic drug-eluting beads (MDEBs), which are navigated magnetically in the MRI scanner through the vascular system. We aim to develop a method to accurately quantify and localize these particles and to validate the method in phantoms and swine models.

METHODS

MDEBs were made of Fe₃O₄ SPIONs. After injected known numbers of MDEBs, susceptibility artifacts in three-dimensional (3D) volumetric interpolated breath-hold examination (VIBE) sequences were acquired in glass and Polyvinyl alcohol (PVA) phantoms, and two living swine. Image processing of VIBE images provided the volume relationship between MDEBs and their artifact at different VIBE acquisitions and post-processing parameters. Simulated hepatic-artery embolization was performed in vivo with an MRI-conditional magnetic-injection system, using the volume relationship to locate and quantify MDEB distribution.

RESULTS

Individual MDEBs were spatially identified, and their artifacts quantified, showing no correlation with magnetic-field orientation or sequence bandwidth, but exhibiting a relationship with echo time and providing a linear volume relationship. Two MDEB aggregates were magnetically steered into desired liver regions while the other 19 had no steering, and 25 aggregates were injected into another swine without steering. The MDEBs were spatially identified and the volume relationship showed accuracy in assessing the number of the MDEBs, with small errors (8.8%).

CONCLUSION AND SIGNIFICANCE

MDEBs were able to be steered into desired body regions and then localized using 3D VIBE sequences. The resulting volume relationship was linear, robust, and allowed for quantitative analysis of the MDEB distribution.

Clinical Feasibility of Multi-Acquisition Variable-Resonance Image Combination-Based T2 Mapping near Hip Arthroplasty

Background: Hip arthroplasty is increasingly prevalent, and early detection of complications can improve outcomes. Quantitative magnetic resonance imaging (qMRI) methods using multi-acquisition variable-resonance image combination (MAVRIC) may allow for the assessment of soft tissues in close proximity to hip arthroplasty devices. Question/Purposes: We sought to determine the clinical feasibility of MAVRIC-based T2 mapping as a qMRI approach for assessing synovial reactions in patients with a hip arthroplasty device. We hypothesized that there would be differences in T2 metrics by synovial type, clinical impression, and clinical findings related to synovitis. Methods: We conducted a cross-sectional study of 141 subjects with 171 hip arthroplasties with greater than 1 year post-implantation. We enrolled subjects who had had a primary total hip arthroplasty or hip resurfacing arthroplasty between May 2019 and March 2020, excluding those with a revision hip arthroplasty and those with standard safety contraindications for receiving an MRI. Institutional standard 2D fast spin echo (FSE), short-tau inversion recovery (STIR), and susceptibility-reduced MAVRIC morphological MR images were acquired for each hip and followed by a dual-echo acquisition MAVRIC T2 mapping sequence. Results: While 131 subjects (81%) were classified as having a "normal" synovial reaction, significantly longer T2 values were found for fluid synovial reactions compared with mixed reactions. In addition, subjects with synovial dehiscence and decompression present had T2 prolongation. Larger synovial volumes were found in subjects with low-signal intensity deposits. Conclusions: MAVRIC-based T2 mapping is clinically feasible and there are significant quantitative differences based on type of synovial reaction. Patients undergoing hip arthroscopy revision surgery will warrant comparison of T2 values with direct histologic assessment of a tissue sample obtained intraoperatively. The approach used in this study may be used for a quantitative evaluation and monitoring of soft tissues around metal implants.

IPEM topical report: guidance on the use of MRI for external beam radiotherapy treatment planning. Phys Med Biol 2021;66:055025. [PMID: 33450742 DOI: 10.1088/1361-6560/abdc30]

This document gives guidance for multidisciplinary teams within institutions setting up and using an MRI-guided radiotherapy (RT) treatment planning service. It has been written by a multidisciplinary working group from the Institute of Physics and Engineering in Medicine (IPEM). Guidance has come from the experience of the institutions represented in the IPEM working group, in consultation with other institutions, and where appropriate references are given for any relevant legislation, other guidance documentation and information in the literature. Guidance is only given for MRI acquired for external beam RT treatment planning in a CT-based workflow, i.e. when MRI is acquired and registered to CT with the purpose of aiding delineation of target or organ at risk volumes. MRI use for treatment response assessment, MRI-only RT and other RT treatment types such as brachytherapy and gamma radiosurgery are not considered within the scope of this document. The aim was to produce guidance that will be useful for institutions who are setting up and using a dedicated MR scanner for RT (referred to as an MR-sim) and those who will have limited time on an MR scanner potentially managed outside of the RT department, often by radiology. Although not specifically covered in this document, there is an increase in the use of hybrid MRI-linac systems worldwide and brief comments are included to highlight any crossover with the early implementation of this technology. In this document, advice is given on introducing a RT workload onto a non-RT-dedicated MR scanner, as well as planning for installation of an MR scanner dedicated for RT. Next, practical guidance is given on the following, in the context of RT planning: training and education for all staff working in and around an MR scanner; RT patient set-up on an MR scanner; MRI sequence optimisation for RT purposes; commissioning and quality assurance (QA) to be performed on an MR scanner; and MRI to CT registration, including commissioning and QA.

Quality of Prostate MRI: Is the PI-RADS Standard Sufficient?

RATIONALE AND OBJECTIVE

The Prostate Imaging Reporting and Data System version 2 (PI-RADSv2) published a set of minimum technical standards (MTS) to improve image quality and reduce variability in multiparametric prostate MRI. The effect of PIRADSv2 MTS on image quality has not been validated. We aimed to determine whether adherence to PI-RADSv2 MTS improves study adequacy and perceived quality.

MATERIALS AND METHODS

Sixty-two prostate MRI examinations including T2 weighted (T2W) and diffusion weighted image (DWI) consecutively referred to our center from 62 different institutions within a 12-month period (September 2017 to September 2018) were included. Six readers assessed images as adequate or inadequate for use in PCa detection and a numerical image quality ranking was given using a 1-5 scale. The PI-RADSv2 MTS were synthesized into sets of seven and 10 rules for T2W and DWI, respectively. Image adherence was assessed using Digital Imaging and Communications in Medicine (DICOM) metadata. Statistical analysis of survey results and image adherence was performed based on reader quality scoring (Kendall Rank tau-b) and reader adequate scoring (Wilcoxon test for association) for T2 and DWI quality assessment.

RESULTS

Out of 62 images, 52 (83%) T2W and 38 (61%) DWIs were rated to be adequate by a majority of readers. Reader adequacy scores showed no significant association with adherence to PI-RADSv2. There was a weak (tau-b = 0.22) but significant (p value = 0.01) correlation between adherence to PIRADSv2 MTS and image quality for T2W. Studies following all PI-RADSv2 T2W rules achieved a higher median average quality score (3.58 for 7/7 vs. 3.0 for <7/7, p = 0.012). No statistical relationship with PI-RADSv2 MTS adherence and DWI quality was found.

CONCLUSION

Among 62 sites performing prostate MRI, few were considered of high quality, but the majority were considered adequate. DWI showed considerably lower rates of adequate studies in the sample. Adherence to PI-RADSv2 MTS did not increase the likelihood of having a qualitatively adequate T2W or DWI.

Needle artifact characteristics and insertion accuracy using a 1.2T open MRI scanner: A phantom study

PURPOSE

To evaluate the characteristics of needle artifacts and the accuracy of needle insertion using a 1.2 Tesla open magnetic resonance imaging (MRI) system in a phantom.

MATERIALS AND METHODS

First, the apparent width of the needle on the MRI and the needle tip position error of 16- and 18-gauge MRI-compatible introducer needles and a 17-gauge cryoneedle were examined with different needle angles (0°, 30°, 45°, 60°, and 90°) to the main magnetic field (B0), sequence types (balanced steady-state acquisition with rewound gradient echo [BASG] and T2-weighted fast spin echo [FSE] sequence), and frequency encoding directions. Second, the accuracy of needle insertion was evaluated after 10 MRI fluoroscopy-guided insertions in a phantom.

RESULTS

The apparent needle widths was larger when the angle of the needle axis relative to B0 was larger. The needles appeared larger on BASG than on T2-weighted FSE images, with the largest apparent widths of 16-, 17-, and 18-gauge needles of 14.3, 11.6, and 11.0mm, respectively. The apparent needle tip position was always more distal than the actual position on BASG images, with the largest longitudinal error of 4.0mm. Meanwhile, the 16- and 18-gauge needle tips appeared more proximal on T2-weighted FSE images with right-to-left frequency encoding direction. The mean accuracy of MRI fluoroscopy-guided needle insertion was 3.1mm.

CONCLUSION

These experiments clarify the characteristics of needle artifacts in a 1.2 Tesla open MRI. With this system, the MRI fluoroscopy-guided needle insertion demonstrated an acceptable accuracy for clinical use.

Development and evaluation of a deep learning based artificial intelligence for automatic identification of gold fiducial markers in an MRI-only prostate radiotherapy workflow

Identification of prostate gold fiducial markers in magnetic resonance imaging (MRI) images is challenging when CT images are not available, due to misclassifications from intra-prostatic calcifications. It is also a time consuming task and automated identification methods have been suggested as an improvement for both objectives. Multi-echo gradient echo (MEGRE) images have been utilized for manual fiducial identification with 100% detection accuracy. The aim is therefore to develop an automatic deep learning based method for fiducial identification in MRI images intended for MRI-only prostate radiotherapy. MEGRE images from 326 prostate cancer patients with fiducials were acquired on a 3T MRI, post-processed with N4 bias correction, and the fiducial center of mass (CoM) was identified. A 9 mm radius sphere was created around the CoM as ground truth. A deep learning HighRes3DNet model for semantic segmentation was trained using image augmentation. The model was applied to 39 MRI-only patients and 3D probability maps for fiducial location and segmentation were produced and spatially smoothed. In each of the three largest probability peaks, a 9 mm radius sphere was defined. Detection sensitivity and geometric accuracy was assessed. To raise awareness of potential false findings a 'BeAware' score was developed, calculated from the total number and quality of the probability peaks. All datasets, annotations and source code used were made publicly available. The detection sensitivity for all fiducials were 97.4%. Thirty-six out of thirty-nine patients had all fiducial markers correctly identified. All three failed patients generated a user notification using the BeAware score. The mean absolute difference between the detected fiducial and ground truth CoM was 0.7 ± 0.9 [0 3.1] mm. A deep learning method for automatic fiducial identification in MRI images was developed and evaluated with state-of-the-art results. The BeAware score has the potential to notify the user regarding patients where the proposed method is uncertain.

Critical appraisal of the International Prophylaxis Study Group magnetic resonance image scale for evaluating haemophilic arthropathy

A goal of the International Prophylaxis Study Group (IPSG) is to provide an accurate instrument to measure MRI-based disease severity of haemophilic arthropathy at various time points, so that longitudinal changes in disease severity can be identified to support decisions on treatment management. We review and discuss in this paper the evaluative purpose of the IPSG MRI scale in relation to its development and validation processes so far. We also critically appraise the validity, reliability and responsiveness of using the IPSG MRI scale in different clinical and research settings, and whenever applicable, compare these clinimetric properties of the IPSG MRI scale with those of its precursors, the compatible additive and progressive MRI scales.

Quantification of fiducial marker visibility for MRI-only prostate radiotherapy simulation

To objectively compare the suitability of MRI pulse sequences and commercially available fiducial markers (FMs) for MRI-only prostate radiotherapy simulation. Most FMs appear as small signal voids in MRI images making them difficult to differentiate from tissue heterogeneities such as calcifications. In this study we use quantitative metrics to objectively evaluate the visibility of FMs in 27 patients and an anthropomorphic phantom with a variety of standard clinical MRI pulse sequences and commercially available FMs. FM visibility was quantified using the local contrast-to-noise-ratio (lCNR), the difference between the 80th and 20th percentile iso-intensity FM volumes (V _fall) and the largest iso-intensity volume that can be distinguished from background: apparent-marker-volume (AMV). A larger lCNR and AMV, and smaller V _fall represents a more easily identifiable FM. The number of non-marker objects visualized by each pulse sequence was calculated using FM-derived template-matching. The FM-based target-registration-error (TRE) between each MRI and the planning-CT image was calculated. Fiducial marker visibility was rated by two medical physicists with over three years of experience examining MRI-only prostate simulation images. The rater's classification accuracy was quantified using the F ₁ score, which is the harmonic mean of the rater's precision and recall. These quantitative metrics and human observer ratings were used to evaluate FM identifiability in images from nine subtypes of T ₁-weighted, T ₂-weighted and gradient echo (GRE) pulse sequences in a 27-patient study. A phantom study was conducted to quantify the visibility of 8 commercially available FMs. In the patient study, the largest mean lCNR and AMV and, smallest normalized V _fall were produced by the 3.0 T multiple-echo GRE pulse sequence (T ₁-VIBE, 2° flip angle, 1.23 ms and 2.45 ms echo-times). This pulse sequence produced no false marker detections and TREs less than 2 mm in the left-right, anterior-posterior and cranial-caudal directions, respectively. Human observers rated the 1.23 ms echo-time GRE images with the best average marker visibility score of 100% and an F ₁ score of 1. In the phantom study, the Gold-Anchor GA-200X-20-B (deployed in a folded configuration) produced the largest sequence averaged lCNR and AMV measurements at 16.1 and 16.7 mm³, respectively. Using quantitative visibility and distinguishability metrics and human observer ratings, the patient study demonstrated that multiple-echo GRE images produced the best gold FM visibility and distinguishability. The phantom study demonstrated that markers manufactured from platinum or iron-doped gold quantitatively produced superior visibility compared to their pure gold counterparts.

Magnetic Susceptibility in Normal Brains of Young Adults Based on Quantitative Susceptibility Mapping

OBJECTIVES

To explore the changes of brain susceptibility of different sides and genders in healthy young adults using quantitative susceptibility mapping (QSM).

METHODS

Totally 42 healthy young right-handed adults underwent conventional brain magnetic resonance imaging and QSM scans, and the susceptibility maps were obtained by image post-processing software. Then the regions-of-interest (ROI) of bilateral frontal gray matter (FGM), frontal white matter (FWM), caudate (CA), globus pallidus (GP), putamen (PU), thalamus (TH), substantia nigra (SN), red nucleus (RN), dentate nucleus (DN), pons (PO), and corpus callosum (CC) were manually drawn to obtain magnetic susceptibility on the susceptibility maps. The magnetic susceptibility of each ROI was compared between 2 sides and genders by Wilcoxon rank sum test.

RESULTS

Magnetic susceptibility of bilateral ROI was the highest in GP, followed by SN, and the lowest in FWM. No statistically significant difference was found in susceptibility of bilateral FGM, FWM, CA, GP, PU, TH, SN, RN, DN, PO, or CC. Magnetic susceptibility in CA significantly different genders.

CONCLUSION

Brain magnetic susceptibility measured by QSM can be used to quantitatively assess brain iron concentrations.

Quantification of metal-induced susceptibility artifacts associated with ultrahigh-field magnetic resonance imaging of spinal implants

Reports on spinal-implant metallic artifacts in 7-T magnetic resonance imaging (MRI) are lacking. Thus, we investigated the magnitude of metal artifacts derived from spinal implants in 7-T MRI and analyzed the differences obtained with spinal rods manufactured from pure titanium, titanium alloy, and cobalt-chrome (5.5-mm and 6.0-mm diameters and 50-mm length). Following the American Society for Testing and Materials guidelines, we measured the artifact size and artifact volume ratio of each rod during image acquisition using 7-T MRI scanners with three-dimensional (3D) T1-weighted and 3D T2* spoiled gradient echo (GRE), 3D T2-weighted fast spin echo, zero echo time (ZTE), and diffusion-weighted imaging sequences. Pure titanium and titanium alloy rods yielded significantly smaller artifacts than did cobalt-chrome rods, with no significant difference between pure titanium and titanium alloy rods. The artifact sizes of the 5.5-mm and 6.0-mm diameter rods were similar. The artifact magnitude increased in the following sequence order: ZTE, 3D T2 fast spin echo, 3D T1 spoiled GRE, 3D T2* spoiled GRE, and diffusion-weighted imaging. Artifacts obtained using the spin echo method were smaller than those obtained with the GRE method. Because the echo time in ZTE is extremely short, the occurrence of artifacts because of image distortion and signal loss caused by differences in magnetic susceptibility is minimal, resulting in the smallest artifacts. ZTE can be a clinically useful method for the postoperative evaluation of patients after instrumentation surgery, even with 7-T MRI.

Functional MRI Safety and Artifacts during Deep Brain Stimulation: Experience in 102 Patients

BackgroundWith growing numbers of patients receiving deep brain stimulation (DBS), radiologists are encountering these neuromodulation devices at an increasing rate. Current MRI safety guidelines, however, limit MRI access in these patients.PurposeTo describe an MRI (1.5 T and 3 T) experience and safety profile in a large cohort of participants with active DBS systems and characterize the hardware-related artifacts on images from functional MRI.Materials and MethodsIn this prospective study, study participants receiving active DBS underwent 1.5- or 3-T MRI (T1-weighted imaging and gradient-recalled echo [GRE]-echo-planar imaging [EPI]) between June 2017 and October 2018. Short- and long-term adverse events were tracked. The authors quantified DBS hardware-related artifacts on images from GRE-EPI (functional MRI) at the cranial coil wire and electrode contacts. Segmented artifacts were then transformed into standard space to define the brain areas affected by signal loss. Two-sample t tests were used to assess the difference in artifact size between 1.5- and 3-T MRI.ResultsA total of 102 participants (mean age ± standard deviation, 60 years ± 11; 65 men) were evaluated. No MRI-related short- and long-term adverse events or acute changes were observed. DBS artifacts were most prominent near the electrode contacts and over the frontoparietal cortical area where the redundancy of the extension wire is placed subcutaneously. The mean electrode contact artifact diameter was 9.3 mm ± 1.6, and 1.9% ± 0.8 of the brain was obscured by the coil artifact. The coil artifacts were larger at 3 T than at 1.5 T, obscuring 2.1% ± 0.7 and 1.4% ± 0.7 of intracranial volume, respectively (P < .001). The superficial frontoparietal cortex and deep structures neighboring the electrode contacts were most commonly obscured.ConclusionWith a priori local safety testing, patients receiving deep brain stimulation may safely undergo 1.5- and 3-T MRI. Deep brain stimulation hardware-related artifacts only affect a small proportion of the brain.© RSNA, 2019Online supplemental material is available for this article.See also the editorial by Martin in this issue.

Unexpectedly Smaller Artifacts of 3.0-T Magnetic Resonance Imaging than 1.5 T: Recommendation of 3.0-T Scanners for Patients with Magnet-Resistant Adjustable Ventriculoperitoneal Shunt Devices

BACKGROUND

Magnetic resonance imaging (MRI) artifacts of adjustable shunt devices are thought to be similar to metal clip artifacts, in that they are larger with higher field strength scanners. We have published several reports about the artifacts of new MRI-resistant adjustable shunt devices, and we found a case in which a 3.0-T scanner showed smaller artifacts than the 1.5-T scanner. We aimed to clarify whether this claim is true or not.

METHODS

Under permission of our institutional Ethical Committee, 2 volunteers underwent imaging studies using 3.0-T and 1.5-T scanners from GE, Siemens, and Philips. Four MRI-resistant adjustable shunt devices-proGAV2.0 (Miethke), Codman Certas Plus (Johnson & Johnson), Polaris (Sophysa), and Strata MR valve (Medtronic)-were fixed on the left temporal scalp. Routine MRI images, including T1-and T2-weighted imaging, fluid-attenuated inversion recovery, diffusion-weighted imaging (DWI), and magnetic resonance angiography (MRA), were obtained. We also compared artifacts between a 3.0-T scanner and a-1.5 T scanner in 4 patients.

RESULTS

The 3.0 T-scanners showed smaller artifacts than the 1.5-T scanners on DWI and MRA images for all shunt devices and scanners. In the other sequences, the results depended on the MRI scanner manufacturer; however, the GE 3.0-T scanner showed smaller artifacts in every sequence. This was also true in the 4 clinical cases.

CONCLUSIONS

A 3.0-T scanner is recommended over a 1.5-T scanner for patients with MRI-resistant adjustable shunt devices in the diagnosis of acute ischemic condition or when using GE scanners.

Using C-Arm X-ray images from marker insertion to confirm the gold fiducial marker identification in an MRI-only prostate radiotherapy workflow

Prostate cancer radiotherapy workflows, solely based on magnetic resonance imaging (MRI), are now in clinical use. In these workflows, intraprostatic gold fiducial markers (GFM) show similar signal behavior as calcifications and bleeding in T2‐weighted MRI‐images. Accurate GFM identification in MRI‐only radiotherapy workflows is therefore a major challenge. C‐arm X‐ray images (CkV‐images), acquired at GFM implantation, could provide GFM position information and be used to confirm correct identification in T2‐weighted MRI‐images. This would require negligible GFM migration between implantation and MRI‐imaging. Marker migration was therefore investigated. The aim of this study was to show the feasibility of using CkV‐images to confirm GFM identification in an MRI‐only prostate radiotherapy workflow. An anterior‐posterior digitally reconstructed radiograph (DRR)‐image and a mirrored posterior‐anterior CkV‐image were acquired two weeks apart for 16 patients in an MRI‐only radiotherapy workflow. The DRR‐image originated from synthetic CT‐images (created from MRI‐images). A common image geometry was defined between the DRR‐ and CkV‐image for each patient. A rigid registration between the GFM center of mass (CoM) coordinates was performed and the distance between each of the GFM in the DRR‐ and registered CkV‐image was calculated. The same methodology was used to assess GFM migration for 31 patients in a CT‐based radiotherapy workflow. The distance calculated was considered a measure of GFM migration. A statistical test was performed to assess any difference between the cohorts. The mean absolute distance difference for the GFM CoM between the DRR‐ and CkV‐image in the MRI‐only cohort was 1.7 ± 1.4 mm. The mean GFM migration was 1.2 ± 0.7 mm. No significant difference between the measured total distances of the two cohorts could be detected (P = 0.37). This demonstrated that, a C‐Arm X‐ray image acquired from the GFM implantation procedure could be used to confirm GFM identification from MRI‐images. GFM migration was present but did not constitute a problem.

Susceptibility Vessel Sign and Cardioembolic Etiology in the THRACE Trial

PURPOSE

The susceptibility vessel sign (SVS) has been described on gradient echo (GRE) magnetic resonance imaging (MRI) in acute ischemic stroke patients by large vessel occlusion. The presence of SVS (SVS+) was associated with treatment outcome and stroke etiology with conflicting results. Based on multicenter data from the THRombectomie des Artères CErebrales (THRACE) study, we aimed to determine if the association between SVS and cardioembolic etiology (CE) was independent of GRE sequence parameters.

MATERIAL AND METHODS

Patients with a pretreatment brain GRE sequence were identified. Logistic regression tested the association between SVS+, CE, time from onset to imaging and GRE sequence parameters (e.g. echo time, voxel size, field strength). We calculated the sensitivity, specificity, positive and negative predictive values (PPV and NPV) for the SVS to predict a stroke from a CE.

RESULTS

An SVS+ was observed in 237 out of 287 (83%) patients. In the univariate analysis, there was a significant association between SVS+ and a CE with an odds ratio (OR) and 95% confidence interval (95% CI) of 2.10 (1.02-4.29), respectively (p = 0.04) but not with GRE sequence parameters. In multivariate analysis, there was an independent relationship between SVS+ and CE (OR [95% CI]: 2.14 [1.02-4.45], p = 0.04). Sensitivity and specificity of SVS+ to predict CE were 0.89 and 0.21, respectively. The PPV and NPV of SVS+ were 0.44 and 0.78, respectively.

CONCLUSION

The presence of SVS is associated to CE, independent of GRE sequence parameters. While the specificity and the PPV of the sign were low, CE seems less likely in the absence of an SVS.

Limitations of Resting-State Functional MR Imaging in the Setting of Focal Brain Lesions

Methods of image acquisition and analysis for resting-state functional MR imaging (rsfMR imaging) are still evolving. Neurovascular uncoupling and susceptibility artifact are important confounds of rsfMR imaging in the setting of focal brain lesions such as brain tumors. This article reviews the detection of these confounds using rsfMR imaging metrics in the setting of focal brain lesions. In the near future, with the wide range of ongoing research in rsfMR imaging, these issues likely will be overcome and will open new windows into brain function and connectivity.

Imaging Sinonasal disease with MRI: Providing insight over and above CT

This article illustrates and discusses the applications and value of magnetic resonance imaging (MRI) in the evaluation of sinonasal disease. There are several clinical scenarios where MRI can add value over conventional computed tomography (CT) evaluation of the sinonasal spaces. Specifically, MRI can provide insight through better depiction of the anatomy of certain sinonasal sub-sites including the olfactory structures. It can aid in evaluating anosmia, sinusitis (fungal sinusitis and complications), benign and malignant lesions, CSF leaks and pathology extending into sinonasal spaces.

In vitro artifact assessment of an MR-compatible, microwave antenna device for percutaneous tumor ablation with fluoroscopic MRI-sequences

OBJECTIVE

To evaluate artifact configuration and diameters of a magnetic resonance (MR) compatible microwave (MW) applicator using near-realtime MR-fluoroscopic sequences for percutaneous tumor ablation procedures.

MATERIAL AND METHODS

Two MW applicators (14 G and 16 G) were tested in an ex-vivo phantom at 1.5 T with two 3 D fluoroscopic sequences: T1-weighted spoiled Gradient Echo (GRE) and T1/T2-weighted Steady State Free Precession (SSFP) sequence. Applicator orientation to main magnetic field (B₀), slice orientation and phase encoding direction (PED) were systematically varied. The influence of these variables was assessed with ANOVA and post-hoc testing.

RESULTS

The artifact was homogenous along the whole length of both antennas with all tested parameters. The tip artifact diameter of the 16 G antenna measured 6.9 ± 1.0 mm, the shaft artifact diameter 8.6 ± 1.2 mm and the Tip Location Error (TLE) was 1.5 ± 1.2 mm.The tip artifact diameter of the 14 G antenna measured 7.7 ± 1.2 mm, the shaft artifact diameter 9.6 ± 1.5 mm and TLE was 1.6 ± 1.2 mm. Orientation to B₀ had no statistically significant influence on tip artifact diameters (16 G: p = .55; 14 G: p = .07) or TLE (16 G: p = .93; 14 G: p = .26). GRE sequences slightly overestimated the antenna length with TLE(16 G) = 2.6 ± 0.5 mm and TLE(14 G) = 2.7 ± 0.7 mm.

CONCLUSIONS

The MR-compatible MW applicator's artifact seems adequate with an acceptable TLE for safe applicator positioning during near-realtime fluoroscopic MR-guidance.

Prospective analysis of in vivo landmark point-based MRI geometric distortion in head and neck cancer patients scanned in immobilized radiation treatment position: Results of a prospective quality assurance protocol

Purpose

Uncertainties related to geometric distortion are a major obstacle for effectively utilizing MRI in radiation oncology. We aim to quantify the geometric distortion in patient images by comparing their in-treatment position MRIs with the corresponding planning CTs, using CT as the non-distorted gold standard.

Methods

Twenty-one head and neck cancer patients were imaged with MRI as part of a prospective Institutional Review Board approved study. MR images were acquired with a T2 SE sequence (0.5 × 0.5 × 2.5 mm voxel size) in the same immobilization position as in the CTs. MRI to CT rigid registration was then done and geometric distortion comparison was assessed by measuring the corresponding anatomical landmarks on both the MRI and the CT images. Several landmark measurements were obtained including; skin to skin (STS), bone to bone, and soft tissue to soft tissue at specific levels in horizontal and vertical planes of both scans. Inter-observer variability was assessed and interclass correlation (ICC) was calculated.

Results

A total of 430 landmark measurements were obtained. The median distortion for all landmarks in all scans was 1.06 mm (IQR 0.6–1.98). For each patient 48% of the measurements were done in the right-left direction and 52% were done in the anteroposterior direction. The measured geometric distortion was not statistically different in the right-left direction compared to the anteroposterior direction (1.5 ± 1.6 vs. 1.6 ± 1.7 mm, respectively, p = 0.4). The magnitude of distortion was higher in the STS peripheral landmarks compared to the more central landmarks (2.0 ± 1.9 vs. 1.2 ± 1.3 mm, p < 0.0001). The mean distortion measured by observer one was not significantly different compared to observer 2, 3, and 4 (1.05, 1.23, 1.06 and 1.05 mm, respectively, p = 0.4) with ICC = 0.84.

Conclusion

MRI geometric distortions were quantified in radiotherapy planning applications with a clinically insignificant error of less than 2 mm compared to the gold standard CT.

Evaluating the effect of rectal distension on prostate multiparametric MRI image quality

PURPOSE

To evaluate the effect of rectal distension on the quality of anatomical and functional prostate multiparametric (mp) MRI.

MATERIALS AND METHODS

Multiparametric (mp) 3T-MRI images of 173 patients were independently evaluated by two radiologists in this retrospective study. Planimetry rectal volumes were derived and a subjective assessment of rectal distension was made using a 5-point Likert scale (1=no stool/gas, 5=large amount of stool/gas). Image quality of diffusion-weighted imaging (DWI) was evaluated using a 5-point Likert scale. DWI was further scored for distortion and artefact. T2W images were evaluated for image sharpness and the presence of motion artefact. The stability of the dynamic contrast-enhancement acquisition was assessed by recording the number of corrupt data points during the wash-out phase.

RESULTS

There was a strong correlation between subjective scoring of rectal loading and objectively measured rectal volume (r=0.82), p<0.001. A significant correlation was shown between increased rectal distension and both reduced DW image quality (r=-0.628, p<0.001), and increased DW image distortion (r=0.814, p<0.001). There was also a significant trend for rectal distension to increase artefact at DWI (r=0.154, p=0.042). Increased rectal distension led to increased motion artefact on T2 (p=0.0096), but did not have a significant effect on T2-sharpness (p=0.0638). There was no relationship between rectal distension and DCE image quality (p=0.693). 63 patients underwent lesion-targeted biopsy post MRI, there was a trend to higher positive predictive values in patients with minor rectal distension (34/38, 89.5%) compared to those with moderate/marked distension (18/25, 72%), p=0.09.

CONCLUSION

Rectal distension has a significant negative effect on the quality of both T2W and DW images. Consideration should therefore be given to bowel preparation prior to prostate mpMRI to optimise image quality.

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.

MRI compatibility of several early transition metal based alloys and its influencing factors

Magnetic resonance imaging (MRI) compatibility of three early transition metal (ETM) based alloys was assessed in vitro with agarose gel as a phantom, including Zr-20Nb, near-equiatomic (TiZrNbTa)₉₀ Mo₁₀ and Nb-60Ta-2Zr, together with pure tantalum and L605 Co-Cr alloy for comparison. The artifact extent in the MR image was quantitatively characterized according to the maximum area of 2D images and the total volume in reconstructed 3D images with a series of slices under acquisition by fast spin echo (FSE) sequence and gradient echo (GRE) sequence. It was indicated that the artifacts extent of L605 Co-Cr alloy with a higher magnetic susceptibility (χ_v ) was approximately 3-fold greater than that of the ETM-based alloys with χ_v in the range of 160-250 ppm. In the ETM group, the MRI compatibility of the materials can be ranked in a sequence of Zr-20Nb, pure tantalum, (TiZrNbTa)₉₀ Mo₁₀ and Nb-60Ta-2Zr. In addition, using a rabbit cadaver with the implanted tube specimens as a model for ex vivo assessment, it was confirmed that the artifact severity of Nb-60Ta-2Zr alloy is significantly reduced in comparison with the L605 alloy. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 377-385, 2018.

Imaging near orthopedic hardware

Over one million total joint replacement surgeries were performed in the US in 2013 alone, and this number is expected to more than double by 2030. Traditional imaging techniques for postoperative evaluation of implanted devices, such as radiography, computerized tomography, or ultrasound, utilize ionizing radiation, suffer from beam hardening artifact, or lack the inherent high contrast necessary to adequately evaluate soft tissues around the implants, respectively. Magnetic resonance imaging (MRI), due to its ability to generate multiplanar, high-contrast images without the use of ionizing radiation is ideal for evaluating periprosthetic soft tissues but has traditionally suffered from in-plane and through-plane data misregistration due to the magnetic susceptibility of implanted materials. A recent renaissance in the interest of imaging near arthroplasty and implanted orthopedic hardware has led to the development of new techniques that help to mitigate the effects of magnetic susceptibility. This article describes the challenges of performing imaging near implanted orthopedic hardware, how to generate clinically interpretable images when imaging near implanted devices, and how the images may be interpreted for clinical use. We will also describe current developments of utilizing MRI to evaluate implanted orthopedic hardware.

LEVEL OF EVIDENCE

3 Technical Efficacy: Stage 2 J. MAGN. RESON. IMAGING 2017;46:24-39.

Magnetic susceptibility, artifact volume in MRI, and tensile properties of swaged Zr-Ag composites for biomedical applications

Zr-Ag composites were fabricated to decrease the magnetic susceptibility by compensating for the magnetic susceptibility of their components. The Zr-Ag composites with a different Zr-Ag ratio were swaged, and their magnetic susceptibility, artifact volume, and mechanical properties were evaluated by magnetic balance, three-dimensional (3-D) artifact rendering, and a tensile test, respectively. These properties were correlated with the volume fraction of Ag using the linear rule of mixture. We successfully obtained the swaged Zr-Ag composites up to the reduction ratio of 96% for Zr-4, 16, 36, 64Ag and 86% for Zr-81Ag. However, the volume fraction of Ag after swaging tended to be lower than that before swaging, especially for Ag-rich Zr-Ag composites. The magnetic susceptibility of the composites linearly decreased with the increasing volume fraction of Ag. No artifact could be estimated with the Ag volume fraction in the range from 93.7% to 95.4% in three conditions. Young's modulus, ultimate tensile strength (UTS), and 0.2% yield strength of Zr-Ag composites showed slightly lower values compared to the estimated values using a linear rule of mixture. The decrease in magnetic susceptibility of Zr and Ag by alloying or combining would contribute to the decrease of the Ag fraction, leading to the improvement of mechanical properties.

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.

Magnetic Resonance Imaging versus Multislice Computed Tomography of Thoracic Aortic Endografts

Purpose:

To compare the potential of magnetic resonance imaging (MRI) to multislice computed tomography (CT) for evaluating stent-graft placement in the thoracic aorta.

Methods:

Susceptibility artifacts in 2 different stent-graft systems (Talent and Excluder) were evaluated in vitro in 2 angulations (straight and 33° curved) using 3 different MRI gradient echo sequences (True FISP, 2-dimensional FLASH, and 3-dimensional Turbo FLASH). The size of the stent-related artifact was measured, and the relative stent lumen was calculated. In vivo stent demarcation, stent patency, and additional findings were determined in 13 patients (3 Talent, 9 Excluder, and 1 combined) and compared to CT findings.

Results:

In vitro, both endograft systems proved to be MR compatible, with the relative stent lumen value ranging from 82% to 100% in the straight configuration; in a curved model, the relative stent lumen value ranged from 56% to 92% with the 3D Turbo FLASH sequence, which provided the smallest susceptibility artifacts. The Excluder endoprosthesis caused significant signal inhomogeneity within the stent in a curved configuration. In vivo, MRI and multislice CT showed similar results, with CT imaging slightly superior in stent demarcation and MRI better in demonstrating thrombus. CT beam hardening artifacts were pronounced in the Talent system, while the Excluder device caused significant signal inhomogeneity within the stent on magnetic resonance angiography.

Conclusions:

Multislice CT and contrast-enhanced MRI are fast, reliable means of providing all relevant information for surveillance of fully MR-compatible stent-grafts in the thoracic aorta.

UTE-ΔR2 -ΔR2 * combined MR whole-brain angiogram using dual-contrast superparamagnetic iron oxide nanoparticles

The ability to visualize whole-brain vasculature is important for quantitative in vivo investigation of vascular malfunctions in cerebral small vessel diseases, including cancer, stroke and neurodegeneration. Transverse relaxation-based ΔR2 and ΔR2 * MR angiography (MRA) provides improved vessel-tissue contrast in animal deep brain with the aid of intravascular contrast agents; however, it is susceptible to orientation dependence, air-tissue interface artifacts and vessel size overestimation. Dual-mode MRA acquisition with superparamagnetic iron oxide nanoparticles (SPION) provides a unique opportunity to systematically compare and synergistically combine both longitudinal (R1 ) and transverse (ΔR2 and ΔR2 *) relaxation-based MRA. Through Monte Carlo (MC) simulation and MRA experiments in normal and tumor-bearing animals with intravascular SPION, we show that ultrashort TE (UTE) MRA acquires well-defined vascularization on the brain surface, minimizing air-tissue artifacts, and combined ΔR2 and ΔR2 * MRA simultaneously improves the sensitivity to intracortical penetrating vessels and reduces vessel size overestimation. Consequently, UTE-ΔR2 -ΔR2 * combined MRA complements the shortcomings of individual angiograms and provides a strategy to synergistically merge longitudinal and transverse relaxation effects to generate more robust in vivo whole-brain micro-MRA. Copyright © 2016 John Wiley & Sons, Ltd.

The Contribution of Common Surgically Implanted Hardware to Functional MR Imaging Artifacts

BACKGROUND AND PURPOSE

Blood oxygenation level-dependent MR imaging is increasingly used clinically to noninvasively assess cerebrovascular reactivity and/or language and motor function. However, many patients have metallic implants, which will induce susceptibility artifacts, rendering the functional information uninformative. Here, we calculate and interpret blood oxygenation level-dependent MR imaging artifact impact arising from surgically implanted hardware.

MATERIALS AND METHODS

A retrospective analysis of all blood oxygenation level-dependent MRIs (n = 343; B0 = 3T; TE = 35 ms; gradient echo EPI) acquired clinically (year range = 2006-2014) at our hospital was performed. Blood oxygenation level-dependent MRIs were most commonly prescribed for patients with cerebrovascular disease (n = 80) or patients undergoing language or motor localization (n = 263). Artifact volume (cubic centimeters) and its impact on clinical interpretation were determined by a board-certified neuroradiologist.

RESULTS

Mean artifact volume associated with intracranial hardware was 4.3 ± 3.2 cm(3) (range = 1.1-9.4 cm(3)). The mean artifact volume from extracranial hardware in patients with cerebrovascular disease was 28.4 ± 14.0 cm(3) (range = 6.1-61.7 cm(3)), and in patients with noncerebrovascular disease undergoing visual or motor functional mapping, it was 39.9 (3)± 27.0 cm(3) (range = 6.9-77.1 cm(3)). The mean artifact volume for ventriculoperitoneal shunts was 95.7 ± 39.3 cm(3) (range = 64.0-139.6 cm(3)). Artifacts had no-to-mild effects on clinical interpretability in all patients with intracranial implants. Extracranial hardware artifacts had no-to-moderate impact on clinical interpretability, with the exception of 1 patient with 12 KLS-Martin maxDrive screws with severe artifacts precluding clinical interpretation. All examined ventriculoperitoneal shunts resulted in moderate-to-severe artifacts, limiting clinical interpretation.

CONCLUSIONS

Blood oxygenation level-dependent MR imaging yields interpretable functional maps in most patients beyond a small (30-40 cm(3)) artifact surrounding the hardware. Exceptions were ventriculoperitoneal shunts, particularly those with programmable valves and siphon gauges, and large numbers of KLS-Martin maxDrive screws.

[Options for the reduction of magnetic susceptibility artifacts caused by implanted microchips in 0.5 Tesla magnetic resonance imaging]

OBJECTIVE

Microchips contain ferromagnetic materials, which lead to severe focal image interferences when performing magnetic resonance imaging (MRI). Very small animals are particularly prone to these susceptibility artifacts, which may hinder analysis of the neck-region MRI image. We investigated the impact of sequence type on the artifact's size and determined the optimal imaging parameters to minimize these artifacts. Furthermore, the minimum distance between the microchip and the spinal canal required to assess the spinal structures should be determined.

MATERIAL AND METHODS

Investigations were performed on the cadavers of 26 cats and two dogs using a low-field MRI System (field strength 0.5 Tesla). To quantify susceptibility artifacts, several sequence types (spin echo, turbo-spin echo (TSE), gradient echo) and imaging parameters (echo time (TE), voxel volume, frequency direction) were systematically varied. Additionally, computed tomography imaging was performed to determine the distance between the microchip and the spinal canal.

RESULTS

The size of the artifact was smallest with T1-weighted TSE sequences. A short TE (10 ms) and a small voxel size (acquisition matrix 256 x 256 pixels, field of view 160 mm, slice thickness 2 mm) significantly reduced artifact size. Furthermore, it could be shown that by changing the frequency- and phase-encoding direction, the shape and orientation of the maximum dimension of the artifact could be influenced. Even when using an optimized T1-weighted TSE sequence, it was impossible to evaluate the spinal cord when the distance between the microchip and the center of the spinal canal was < 19  mm.

CONCLUSION AND CLINICAL RELEVANCE

In MR studies of the cervical spine of small dogs and cats, microchips can cause severe susceptibility artifacts. Because of the small distance between the microchip and the spinal structures, spinal evaluation may be limited or impossible. The investigations demonstrated that the adjustment of sequence parameters helps to significantly minimize artifact size and shape. The greatest reduction in artifact size was achieved by using a T1-weighted TSE sequence.

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSION

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

ADVANCES IN KNOWLEDGE

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

Three-dimensional quantification of susceptibility artifacts from various metals in magnetic resonance images

Susceptibility artifacts generated in magnetic resonance (MR) images were quantitatively evaluated for various metals using a three-dimensional (3-D) artifact rendering to demonstrate the correlation between magnetic susceptibility and artifact volume. Ten metals (stainless steel, Co-Cr alloy, Nb, Ti, Zr, Mo, Al, Sn, Cu and Ag) were prepared, and their magnetic susceptibilities measured using a magnetic balance. Each metal was embedded in a Ni-doped agarose gel phantom and the MR images of the metal-containing phantoms were taken using 1.5 and 3.0 T MR scanners under both fast spin echo and gradient echo conditions. 3-D renderings of the artifacts were constructed from the images and the artifact volumes were calculated for each metal. The artifact volumes of metals decreased with decreasing magnetic susceptibility, with the exception of Ag. Although Sn possesses the lowest absolute magnetic susceptibility (1.8×10(-6)), the artifact volume from Cu (-7.8×10(-6)) was smaller than that of Sn. This is because the magnetic susceptibility of Cu was close to that of the agarose gel phantom (-7.3×10(-6)). Since the difference in magnetic susceptibility between the agarose and Sn is close to that between the agarose and Ag (-17.5×10(-6)), their artifact volumes were almost the same, although they formed artifacts that were reversed in all three dimensions.

Blood oxygen level dependent magnetization transfer (BOLDMT) effect

A few studies have reported that magnetization transfer (MT) -preparation interacts with blood oxygen level dependent (BOLD) contrast used for functional magnetic resonance imaging (MRI). However, the mechanism is still not well established. This study shows that blood oxygenation level itself affects MT contrast. MT ratio (MTR) decreases with increased blood oxygenation, which is demonstrated by ex vivo and in vivo experiments. Oxygenated blood shows less MTR contrast compared to deoxygenated blood sample; and higher levels of oxygen inhalation decrease tissue MTR in vivo especially in brain tumor region. The percentage reduction of MTR due to hyperoxia inhalation, referred to as the blood oxygen dependent magnetization transfer (BOLDMT) effect, correlates well with tissue oxygen extraction, which is highest in well-vascularized tumor rim, followed by inner tumor, gray matter (GM), and white matter (WM) normal tissue. Simulations and experiments demonstrate that BOLDMT effect induced with hyperoxia inhalation may be generated by decreased tissue T (1) due to increased O(2) dissolution and increased tissue T (2) due to reduced deoxyhemoglobin (dHb) concentration. Compared to regular T (2)* weighted BOLD contrast, BOLDMT has higher insensitivity to B (0) inhomogeneities. BOLDMT may potentially serve as a reliable and novel biomarker for tumor oxygen extraction.

MRI-based biodistribution assessment of holmium-166 poly(L-lactic acid) microspheres after radioembolisation

Objectives

To demonstrate the feasibility of MRI-based assessment of the intrahepatic Ho-PLLA-MS biodistribution after radioembolisation in order to estimate the absorbed radiation dose.

Methods

Fifteen patients were treated with holmium-166 (¹⁶⁶Ho) poly(L-lactic acid)-loaded microspheres (Ho-PLLA-MS, mean 484 mg; range 408–593 mg) in a phase I study. Multi-echo gradient-echo MR images were acquired from which R₂^* maps were constructed. The amount of Ho-PLLA-MS in the liver was determined by using the relaxivity r₂^* of the Ho-PLLA-MS and compared with the administered amount. Quantitative single photon emission computed tomography (SPECT) was used for comparison with MRI regarding the whole liver absorbed radiation dose.

Results

R₂^* maps visualised the deposition of Ho-PLLA-MS with great detail. The mean total amount of Ho-PLLA-MS detected in the liver based on MRI was 431 mg (range 236–666 mg) or 89 ± 19 % of the delivered amount (correlation coefficient r = 0.7; P < 0.01). A good correlation was found between the whole liver mean absorbed radiation dose as assessed by MRI and SPECT (correlation coefficient r = 0.927; P < 0.001).

Conclusion

MRI-based dosimetry for holmium-166 radioembolisation is feasible. Biodistribution is visualised with great detail and quantitative measurements are possible.

Key Points

• Radioembolisation is increasingly used for treating unresectable primary or metastatic liver tumours.

• MRI-based intrahepatic microsphere biodistribution assessment is feasible after holmium-166 radioembolisation.

• MRI enables quantification of holmium-166 microspheres in liver in a short imaging time.

• MRI can estimate the whole liver absorbed radiation dose following holmium-166 radioembolisation.

In vivo magnetic resonance microscopy of Drosophilae at 9.4 T

In preclinical research, genetic studies have made considerable progress as a result of the development of transgenic animal models of human diseases. Consequently, there is now a need for higher resolution MRI to provide finer details for studies of small animals (rats, mice) or very small animals (insects). One way to address this issue is to work with high-magnetic-field spectrometers (dedicated to small animal imaging) with strong magnetic field gradients. It is also necessary to develop a complete methodology (transmit/receive coil, pulse sequence, fixing system, air supply, anesthesia capabilities, etc.). In this study, we developed noninvasive protocols, both in vitro and in vivo (from coil construction to image generation), for drosophila MRI at 9.4 T. The 10 10 80-μm resolution makes it possible to visualize whole drosophila (head, thorax, abdomen) and internal organs (ovaries, longitudinal and transverse muscles, bowel, proboscis, antennae and optical lobes). We also provide some results obtained with a Drosophila model of muscle degeneration. This opens the way for new applications of structural genetic modification studies using MRI of drosophila.

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.