MRI Pulse Sequences Research Articles

Review of Quantitative Knee Articular Cartilage MR Imaging.

Osteoarthritis (OA) is one of the most prevalent disorders in today’s society, resulting in significant socio-economic costs and morbidity. MRI is widely used as a non-invasive imaging tool for OA of the knee. However, conventional knee MRI has limitations to detect subtle early cartilage degeneration before morphological changes are visually apparent. Novel MRI pulse sequences for cartilage assessment have recently received increased attention due to newly developed compositional MRI techniques, including: T2 mapping, T1rho mapping, delayed gadolinium-enhanced MRI of cartilage (dGEMRIC), sodium MRI, diffusion-weighted imaging (DWI)/ diffusion tensor imaging (DTI), ultrashort TE (uTE), and glycosaminoglycan specific chemical exchange saturation transfer (gagCEST) imaging. In this article, we will first review these quantitative assessments. Then, we will discuss the variations of quantitative values of knee articular cartilage with cartilage layer (depth)- and angle (regional)-dependent approaches. Multiple MRI sequence techniques can discern qualitative differences in knee cartilage. Normal articular hyaline cartilage has a zonal variation in T2 relaxation times with increasing T2 values from the subchondral bone to the articular surface. T1rho values were also higher in the superficial layer than in the deep layer in most locations in the medial and lateral femoral condyles, including the weight-bearing portion. Magic angle effect on T2 mapping is clearly observed in the both medial and lateral femoral condyles, especially within the deep layers. One of the limitations for clinical use of these compositional assessments is a long scan time. Recent new approaches with compressed sensing (CS) and MR fingerprinting (MRF) have potential to provide accurate and fast quantitative cartilage assessments.

MRI pulse sequence integration for deep-learning-based brain metastases segmentation.

Magnetic resonance (MR) imaging is an essential diagnostic tool in clinical medicine. Recently, a variety of deep-learning methods have been applied to segmentation tasks in medical images, with promising results for computer-aided diagnosis. For MR images, effectively integrating different pulse sequences is important to optimize performance. However, the best way to integrate different pulse sequences remains unclear. In addition, networks trained with a certain subset of pulse sequences as input are unable to perform when given a subset of those pulse sequences. In this study, we evaluate multiple architectural features and characterize their effects in the task of metastasis segmentation while creating a method to robustly train a network to be able to work given any strict subset of the pulse sequences available during training. We use a 2.5D DeepLabv3 segmentation network to segment metastases lesions on brain MR's with four pulse sequence inputs. To study how we can best integrate MR pulse sequences for this task, we consider (1) different pulse sequence integration schemas, combining our features at early, middle, and late points within a deep network, (2) different modes of weight sharing for parallel network branches, and (3) a novel integration level dropout layer, which will allow the networks to be robust to performing inference on input with only a subset of pulse sequences available at the training. We find that levels of integration and modes of weight sharing that favor low variance work best in our regime of small amounts of training data (n=100). By adding an input-level dropout layer, we could preserve the overall performance of these networks while allowing for inference on inputs with missing pulse sequences. We illustrate not only the generalizability of the network but also the utility of this robustness when applying the trained model to data from a different center, which does not use the same pulse sequences. Finally, we apply network visualization methods to better understand which input features are most important for network performance. Together, these results provide a framework for building networks with enhanced robustness to missing data while maintaining comparable performance in medical imaging applications.

Physical Reintroduction to MRI: Complete Understanding of MRI from Bloch Equations

Recent MRI technology and pulse sequences have become increasingly complex and difficult to understand. However, the Bloch equations, which are the basic principle of MRI, have remained unchanged since it was proposed in 1973. Here we have presented a method to numerically solve the Bloch equations to understand the principle of MRI. This method enables us to reproduce complex pulse sequences on a computer and to understand various phenomena related to pulse sequences.

MRI-Based Radiomics Nomogram as a Potential Biomarker to Predict the EGFR Mutations in Exon 19 and 21 Based on Thoracic Spinal Metastases in Lung Adenocarcinoma

Preoperative identifications of epidermal growth factor receptor (EGFR) mutation subtypes based on the MRI image of spinal metastases are needed to provide individualized therapy, but has not been previously investigated. This study aims to develop and evaluate an MRI-based radiomics nomogram for differentiating the exon 19 and 21 in EGFR mutation from spinal bone metastases in patients with primary lung adenocarcinoma. A total of 76 patients underwent T1-weighted and T2-weighted fat-suppressed MRI scans were enrolled in this study, 38 were positive for EGFR mutation in exon 19 and 38 were in exon 21.MRI imaging features were extracted and selected from each MRI pulse sequence, and used to form the radiomics signature. A radiomics nomogram was developed integrating the radiomics signature and important clinical factors with receiver operating characteristic, calibration and decision curve analysis to assess the nomogram. Clinical characteristics were analyzed with Mann-Whitney U and Chi-Square tests to identify the most important factors. A total of 6 features were selected as the most discriminative predictors from the two MRI pulse sequences. The nomogram integrating the combined radiomics signature, age and CEA level generated good prediction performance in the training (AUCs, nomogram vs. combined radiomics signature vs. clinical model, 0.90 vs. 0.87 vs. 0.59) and validation (AUCs, nomogram vs. combined radiomics signature vs. clinical model, 0.88 vs. 0.86 vs. 0.72) cohort. DCA analysis confirmed the potential clinical utility of the nomogram. This study demonstrated that MRI features from spinal bone metastases can be used to prognosticate EGFR mutation subtypes in exon 19 and 21. The developed pre-treatment nomogram can potentially guide treatments for lung adenocarcinoma patients.

LIMITED INTER-RATER RELIABILITY IN ANTEROLATERAL LIGAMENT DETECTION IN PEDIATRIC KNEE MRIS

Background:Despite the rapidly expanding anterolateral ligament (ALL) literature base, little has focused on the reliability of its detection on MRI. This is especially important in the pediatric population where the scarcity of pediatric cadaveric specimens hinders the accurate estimation of its prevalence.Purpose:The purpose of this study was to assess the visibility of ALL on common MRI pulse sequences and to calculate the inter-rater reliability among multiple expert readers in a cohort of normal pediatric knee MRIs.Methods:A series of 3T pediatric knee MRIs obtained from 8/2018-4/2019 were reviewed to identify patients without significant knee pathology. Knee laterality, age, gender, and regional physeal status were also recorded. One orthopedic surgeon, 1 pediatric musculoskeletal (MSK) radiologist, and 1 adult MSK radiologist, independently reviewed each MRI pulse sequence (T1-weighted, PD-weighted, and PD-weighted fat-suppressed images) separately, to determine the presence or absence of the 3 parts of the ALL (femoral, tibial, and meniscal proper). Inter-rater reliability was calculated using Fleiss’ Kappa (K).Results:Inclusion criteria were met for 75 patient knees with a mean age of 12.7±3.42y (3-19) and a greater proportion of females (57.3%). The distribution of regional physeal status was: Open-48.0%, Closing-29.3%, and Closed-22.7%. Inter-rater reliability was slight to fair (K: 0.141-0.340) for visualizing any specific ALL component on a unique MRI sequence (Table 1). For visualizing a specific attachment on any MRI sequence, there was slight to moderate agreement (K: 0.018-0.411). Visualizing any ALL attachment on any MRI sequence also yielded slight agreement (K: 0.190). Inter-rater reliability between specialities (i.e. Ortho vs. MSK Radiology) did not differ substantially from group findings. No single study sequence demonstrated enhanced reliability for all ALL components.Conclusion:The reliability of ALL detection on MRI across multiple clinicians was slight to moderate for all components in this cohort of normal 3T pediatric knee MRIs. No single sequence was consistently better for ALL detection agreement. This is in contrast with previous work showing substantial agreement among raters and describing PD-weighted imaging as preferable for viewing the ALL. Experience in musculoskeletal imaging did not confer improved agreement. Establishing diagnostic reliability should be a prerequisite for use of an imaging finding in the research or clinical setting. Existing literature drawing conclusions from ALL findings on MRI may need to be interpreted with caution if reliability testing is not described.

Quantitative assessment of acquisition imaging parameters on MRI radiomics features: a prospective anthropomorphic phantom study using a 3D-T2W-TSE sequence for MR-guided-radiotherapy.

MRI pulse sequences and imaging parameters substantially influence the variation of MRI radiomics features, thus impose a critical challenge on MRI radiomics reproducibility and reliability. This study aims to prospectively investigate the impact of various imaging parameters on MRI radiomics features in a 3D T2-weighted (T2W) turbo-spin-echo (TSE) pulse sequence for MR-guided-radiotherapy (MRgRT). An anthropomorphic phantom was scanned using a 3D-T2W-TSE MRgRT sequence at 1.5T under a variety of acquisition imaging parameter changes. T1 and T2 relaxation times of the phantom were also measured. 93 first-order and texture radiomics features in the original and 14 transformed images, yielding 1,395 features in total, were extracted from 10 volumes-of-interest (VOIs). The percentage deviation (d%) of radiomics feature values from the baseline values and intra-class correlation coefficient (ICC) with the baseline were calculated. Robust radiomics features were identified based on the excellent agreement of radiomics feature values with the baseline, i.e., the averaged d% <5% and ICC >0.90 in all VOIs for all imaging parameter variations. The radiomics feature values changed considerably but to different degrees with different imaging parameter adjustments, in the ten VOIs. The deviation d% ranged from 0.02% to 321.3%, with a mean of 12.5% averaged for all original features in all ten VOIs. First-order and GLCM features were generally more robust to imaging parameters than other features in the original images. There were also significantly different radiomics feature values (ANOVA, P<0.001) between the original and the transformed images, exhibiting quite different robustness to imaging parameters. 330 out of 1395 features (23.7%) robust to imaging parameters were identified. GLCM and GLSZM features had the most (42.5%, 153/360) and least (3.8%, 9/240) robust features in the original and transformed images, respectively. This study helps better understand the quantitative dependence of radiomics feature values on imaging parameters in a 3D-T2W-TSE sequence for MRgRT. Imaging parameter heterogeneity should be considered as a significant source of radiomics variability and uncertainty, which must be well harmonized for reliable clinical use. The identified robust features to imaging parameters are helpful for the pre-selection of radiomics features for reliable radiomics modeling.

HYFI: Hybrid filling of the dead-time gap for faster zero echo time imaging.

The aim of this work was to improve the SNR efficiency of zero echo time (ZTE) MRI pulse sequences for faster imaging of short‐T 2 components at large dead‐time gaps. ZTE MRI with hybrid filling (HYFI) is a strategy for retrieving inner k‐space data missed during the dead‐time gaps arising from radio‐frequency excitation and switching in ZTE imaging. It performs hybrid filling of the inner k‐space with a small single‐point‐imaging core surrounded by a stack of shells acquired on radial readouts in an onion‐like fashion. The exposition of this concept is followed by translation into guidelines for parameter choice and implementation details. The imaging properties and performance of HYFI are studied in simulations as well as phantom, in vitro and in vivo imaging, with an emphasis on comparison with the pointwise encoding time reduction with radial acquisition (PETRA) technique. Simulations predict higher SNR efficiency for HYFI compared with PETRA at preserved image quality, with the advantage increasing with the size of the k‐space gap. These results are confirmed by imaging experiments with gap sizes of 25 to 50 Nyquist dwells, in which scan times for similar image quality could be reduced by 25% to 60%. The HYFI technique provides both high SNR efficiency and image quality, thus outperforming previously known ZTE‐based pulse sequences, in particular for large k‐space gaps. Promising applications include direct imaging of ultrashort‐ components, such as the myelin bilayer or collagen, T 2 mapping of ultrafast relaxing signals, and ZTE imaging with reduced chemical shift artifacts.

Correlation between contrast enhanced plaques and plaque diffusion restriction and their signal intensities in FLAIR images in patients who admitted with acute symptoms of multiple sclerosis

IntroductionOne of the most common demyelinating disorders is Multiple Sclerosis (MS), which can lead to extensive disability in patients. Appearance of active lesions can be an important sign of disease development. The correlation between the restriction of DWI signal, contrast-enhancement, and demyelinated plaque's signals were evaluated in this study. Material and method34 MS patients with 1043 MS-plaques who had acute attacks took part in this study. Three MRI pulse-sequences, FLAIR, DWI, and post-contrast enhanced T1 weighted, were compared. Using the signal intensity of MS-lesions and normal brain tissue in FLAIR images, lesion/noise ratio (LNR) and lesion/white-matter ratio (LWR) were calculated. Sensitivity and specificity of LNR and LWR were estimated to determine a cut off value. Results7.86% of MS-plaques were enhanced in T1 contrast-enhanced MRI. 2.4% showed DWI restriction. 0.77% showed both diffusion-restriction and T1 contrast-enhancement, 1.63% indicated diffusion-restriction but no contrast-enhancement and 7.09% showed contrast-enhancement but no diffusion-restriction. Among diffusion-restricted plaques, 32% were enhanced in T1-weighted post-contrast enhanced images and it should be noted that diffusion-restriction was seen in 9.76% of enhanced plaques. The sensitivity and specificity of the DWI restriction and its effectiveness in detecting active-plaques were calculated as 12.31% and 98.26%, respectively. The accuracy of this method in detecting the active MS-plaques estimated as 92.91%. The cut-off value for LWR and LNR was between 0.9–1 and 24–28. ConclusionBy calculating LNR and LWR, the FLAIR images can be evaluated to detect new MS-plaques. LWR had higher sensitivity and specificity in comparison to LNR so it is a better index in the detection of active-plaques. Although post contrast T1 is the gold standard for evaluating active plaques, in the case of a contraindication to gadolinium, DWI can offer supplemental information on the diffusion restriction of MS plaques in a non-contrast protocol.

Edema Progression during MRI-Guided Glioblastoma Radiotherapy

There are limited studies evaluating brain changes on MRI during the standard six weeks of concurrent chemotherapy and radiotherapy (RT) for glioma. Existing studies suggest that MRI during chemoRT could be useful to determine response to treatment. At our institution, we have been treating select gliomas with one of the first commercially available 0.35T MRI-RT systems. The system includes one clinically useable pulse sequence called balanced steady state free precession (bSSFP), which produces a highly T2-weighted 3D image of the brain. The bSSFP MRIs are obtained daily for treatment setup and have demonstrated changes occurring in and around gliomas during patients’ chemoradiotherapy course. Our objective is to quantify the frequency, magnitude, and time course of these changes. Under a prospective registry protocol, we reviewed 14 patients at our institution who were treated for high grade glioma with chemoradiation, 60 Gy in 30 fractions plus temozolomide, on our institution’s MRI-RT system. We contoured the T2-weighted hyper-intense volumes from the MRI-RT set-up scans for all 30 fractions of treatment for each patient and then calculated the contoured MRI volumes. MRI-RT set-up scans during the last week of treatment were compared to the patients’ diagnostic MRIs obtained 3-4 weeks post-treatment completion to determine whether there were similar findings. 5 of 14 (36%) patients demonstrated greater than a 10% increase in T2-weighted MRI volume during the course of their treatment. Alternatively, 3 of 14 (21%) patients demonstrated greater than a 10% decrease in T2-weighted MRI volume during treatment. In 3 of the patients with volume growth, changes predominantly were observed during weeks 5 and 6 of treatment. The bSSFP images obtained during the last week of treatment demonstrated similar abnormalities compared to the diagnostic FLAIR MRIs obtained 3-4 weeks post-treatment. Patients who had volume expansion during treatment that resolved on serial follow-up MRIs were counted as pseudoprogressors. The time to true progression in the cohort was 19 months. Median follow-up was 14.7 months. 36% of high-grade glioma patients had growth of T2-weighted MRI volume during chemoradiotherapy, and these changes were similar to those observed on post-treatment diagnostic scans. Tumor volume expansion could be due to pseudoprogression, true progression, or changes in edema. Our future work aims to differentiate these possibilities with additional patients, follow-up, and MRI pulse sequences on the MRI-RT system. Groups using limited CTV margins for treatment planning should be aware that MRI volumes could significantly change during radiotherapy. Patients demonstrated T2-weighted MRI volume increase during weeks 5-6 of treatment, suggesting that studies using diagnostic MRI scans mid-treatment (week 3-4) may not fully capture the extent of progression.

Towards Real 3D Patient Specific QA for Mono-Isocentric Multi-Target Stereotactic Radiosurgery Involving Non-Coplanar Beams

Patient specific QA based on measurements has thus far involved recalculation of the dose on a phantom where a phantom measured dose is compared with phantom planned dose. Strictly speaking this is plan-specific rather than patient-specific QA. With the advent of 3D printers, it is possible to generate a phantom detector that closely matches in composition and geometry the patient, therefore specific to that patient. In this case, there is no need for a dose recalculation on the phantom and the real patient planned dose distribution can be directly compared to the phantom measured dose. This real 3D patient specific QA was achieved for a complex mono-isocentric case involving multiple targets and non-coplanar beams. The QA was based on a previously treated brain patient with seven (7) metastasis, average planning target volume 7.2cc ± 14.1cc (range 0.2cc to 32.4cc), single isocenter and 5 non-coplanar arcs. The average tumor distance from isocenter was 53.8 ± 15.9mm, range (26.4mm to 73.0mm). An anonymized CT data set of the patient was sent to the 3D printer to generate the phantom. In creating the phantom, the fluid and air-filled areas of the head were filled with gel dosimeter. The phantom was then setup in the treatment room just as one would the patient and a CBCT acquired to correct for setup in all translations and rotations. We underscore that the reference images of the real patients were used for the CBCT registration setup. Upon satisfactory setup, we then proceeded to beam delivery. A specialized T2-weighted MRI pulse sequence was implemented on our Siemens Magnetom Aera 1.5T unit to acquire scans of the phantom pre and post irradiation. An assessment of the CBCT of the phantom with the patient’s reference CT image showed very good match. The setup errors based on auto registration just before beam on were 0.0mm, 0.2mm, and 0.2mm in the anterior/posterior (A/P), superior/inferior (S/I), and left/right (L/R) directions respectively and 0.1, 0.1, and 0.0 degrees in yaw, pitch and roll respectively. A 3D gamma analysis considering all 7 PTVs showed a passing rate of 92.7% and 94.8% for a gamma criterion of 5%/1mm and 3%/2mm respectively. The DVH plots showed good agreement, for example mean PTV dose averaging among all 7 targets was 99.6±0.8% versus 99.4±1.3% for real patient plan and measurements respectively. The corresponding values for PTV D95 were 81.6±4.0% versus 79.9±5.2%. The utility of real 3D patient specific QA was presented on a complex case involving multiple targets and non-coplanar beams. The ability to directly measure dose in 3D and relate to the real patient anatomy means one can evaluate additional aspects of the plan such as DVH. The interest in this QA method is expected to increase as technology continues to evolve and 3D printers become readily available in radiotherapy centers.

Compressed sensing with signal averaging for improved sensitivity and motion artifact reduction in fluorine-19 MRI.

Fluorine-19 (19 F) MRI of injected perfluorocarbon emulsions (PFCs) allows for the non-invasive quantification of inflammation and cell tracking, but suffers from a low signal-to-noise ratio and extended scan time. To address this limitation, we tested the hypotheses that a 19 F MRI pulse sequence that combines a specific undersampling regime with signal averaging has both increased sensitivity and robustness against motion artifacts compared with a non-averaged fully sampled pulse sequence, when both datasets are reconstructed with compressed sensing. As a proof of principle, numerical simulations and phantom experiments were performed on selected variable ranges to characterize the point spread function of undersampling patterns, as well as the vulnerability to noise of undersampling and reconstruction parameters with paired numbers of x signal averages and acceleration factor x (NAx-AFx). The numerical simulations demonstrated that a probability density function that uses 25% of the samples to fully sample the k-space central area allowed for an optimal balance between limited blurring and artifact incoherence. At all investigated noise levels, the Dice similarity coefficient (DSC) strongly depended on the regularization parameters and acceleration factor. In phantoms, the motion robustness of an NA8-AF8 undersampling pattern versus NA1-AF1 was evaluated with simulated and real motion patterns. Differences were assessed with the DSC, which was consistently higher for the NA8-AF8 compared with the NA1-AF1 strategy, for both simulated and real cyclic motion patterns (P < 0.001). Both strategies were validated in vivo in mice (n = 2) injected with perfluoropolyether. Here, the images displayed a sharper delineation of the liver with the NA8-AF8 strategy than with the NA1-AF1 strategy. In conclusion, we validated the hypotheses that in 19 F MRI the combination of undersampling and averaging improves both the sensitivity and the robustness against motion artifacts.

Edema Progression during MRI-Guided Glioblastoma Radiotherapy

There are limited studies evaluating brain changes on MRI during the six weeks of concurrent chemotherapy and radiotherapy (RT) for glioma. Existing studies suggest that MRI during chemoradiotherapy could be useful to determine early response to treatment. At our center, we have been treating selected gliomas with one of the first commercially available 0.35T MRI-RT systems. The system includes one clinically useable pulse sequence, balanced steady state free precession (bSSFP), which obtains a 1.5 mm isotropic highly T2-weighted 3D image volume through the brain with minimal distortions. The bSSFP MRIs are obtained daily for treatment setup, and we have seen changes occurring in and around gliomas during chemoradiotherapy. Our objective is to quantify the frequency, magnitude, and time course of these changes. Under a prospective registry protocol, we reviewed 14 patients at our institution who were treated for high grade glioma with chemoradiation 60 Gy in 30 fractions plus temozolomide on our MRI-RT system. We contoured the T2-weighted hyperintense volumes from the MRI-RT bSSFP images for all 30 treatment fractions for each patient and measured the volumes of those MRIs. The MRIs from fraction 30 were compared to the diagnostic MRIs obtained 3-4 weeks post-treatment to determine whether there were similar findings. 5 of 14 (36%) patients demonstrated greater than a 10% increase in tumor volume during the course of their treatment. In 3 of these patients with growth, changes predominantly were observed during week 5 and 6 of treatment. One patient’s non-enhancing unresected tumor decreased 24% in volume during the course of treatment. The bSSFP images obtained at the end of treatment demonstrated similar FLAIR abnormalities compared to diagnostic MRIs 3-4 weeks post treatment completion. Patients who had volume expansion during treatment that resolved on serial follow-up MRIs were counted as pseudoprogression, and excluding pseudoprogressors, the progression free survival in the cohort was 19 months. Median follow-up was 14.7 months. 36% of high grade glioma patients had growth of T2-weighted volume during chemoradiotherapy, and these changes were similar to those observed on post-treatment diagnostic scans. Tumor volume expansion could be due to pseudoprogression, true progression, or solely due to changes in edema. Our future work aims to differentiate these possibilities with additional patients and MRI pulse sequences. Changes were greatest during weeks 5-6 of the treatment course, suggesting that studies using diagnostic MRI scans during RT should be obtained late in the treatment course.

Mapping oscillating magnetic fields around rechargeable batteries

Power storage devices such as batteries are a crucial part of modern technology. The development and use of batteries has accelerated in the past decades, yet there are only a few techniques that allow gathering vital information from battery cells in a nonivasive fashion. A widely used technique to investigate batteries is electrical impedance spectroscopy (EIS), which provides information on how the impedance of a cell changes as a function of the frequency of applied alternating currents. Building on recent developments of inside-out MRI (ioMRI), a technique is presented here which produces spatially-resolved maps of the oscillating magnetic fields originating from the alternating electrical currents distributed within a cell. The technique works by using an MRI pulse sequence synchronized with a gated alternating current applied to the cell terminals. The approach is benchmarked with a current-carrying wire coil, and demonstrated with commercial and prototype lithium-ion cells. Marked changes in the fields are observed for different cell types.

MRI and tractography techniques to localize the ventral intermediate nucleus and dentatorubrothalamic tract for deep brain stimulation and MR-guided focused ultrasound: a narrative review and update.

The thalamic ventral intermediate nucleus (VIM) can be targeted for treatment of tremor by several procedures, including deep brain stimulation (DBS) and, more recently, MR-guided focused ultrasound (MRgFUS). To date, such targeting has relied predominantly on coordinate-based or atlas-based techniques rather than directly targeting the VIM based on imaging features. While general regional differences of features within the thalamus and some related white matter tracts can be distinguished with conventional imaging techniques, internal nuclei such as the VIM are not discretely visualized. Advanced imaging methods such as quantitative susceptibility mapping (QSM) and fast gray matter acquisition T1 inversion recovery (FGATIR) MRI and high-field MRI pulse sequences that improve the ability to image the VIM region are emerging but have not yet been shown to have reliability and accuracy to serve as the primary method of VIM targeting. Currently, the most promising imaging approach to directly identify the VIM region for clinical purposes is MR diffusion tractography.In this review and update, the capabilities and limitations of conventional and emerging advanced methods for evaluation of internal thalamic anatomy are briefly reviewed. The basic principles of tractography most relevant to VIM targeting are provided for familiarization. Next, the key literature to date addressing applications of DTI and tractography for DBS and MRgFUS is summarized, emphasizing use of direct targeting. This literature includes 1-tract (dentatorubrothalamic tract [DRT]), 2-tract (pyramidal and somatosensory), and 3-tract (DRT, pyramidal, and somatosensory) approaches to VIM region localization through tractography.The authors introduce a 3-tract technique used at their institution, illustrating the oblique curved course of the DRT within the inferior thalamus as well as the orientation and relationship of the white matter tracts in the axial plane. The utility of this 3-tract tractography approach to facilitate VIM localization is illustrated with case examples of variable VIM location, targeting superior to the anterior commissure-posterior commissure plane, and treatment in the setting of pathologic derangement of thalamic anatomy. Finally, concepts demonstrated with these case examples and from the prior literature are synthesized to highlight several potential advantages of tractography for VIM region targeting.

Three-Dimensional UTE MR Imaging: Twelve-Month Analysis of ACL Autograft Remodeling

Objectives:Post-reconstruction imaging of the anterior cruciate ligament (ACL) is needed to non-invasively assess in vivo graft maturity before release to play. The purpose of this study was to evaluate autograft remodeling up to 12 months after ACL reconstruction using 3D ultra-short time to echo (UTE) T2* MRI. We hypothesized that the T2* values of ACL autografts would progressively change over time, initially resembling the values for the native contralateral graft source and gradually approaching that of the intact contralateral ACL.Methods:After IRB approval, 12 patients (ages 14-45 years) who underwent primary ACL reconstruction (ACLR) with semitendinosus (SemiT) or bone-patellar tendon-bone (BTB) autograft were enrolled. Patients with a history of prior injury or surgery to either knee were excluded. Subjects returned for UTE MRIs at 1, 3, 6, 9, and 12 months after ACLR. Imaging at 1-month included the contralateral knee. All scans were performed on a Philips Ingenia 3T system. MRI pulse sequences included high-resolution 3D T2 (slice thickness: 0.6mm, TR: 18.7ms; TE: 11.5ms) and a four-echo T2 UTE (slice thickness: 1mm, TR: 20ms; TE: 0.3, 3.3, 6.3, and 9.3ms). Using high-resolution 3D T2 sequences at 1 month, all slices containing the intra-articular ACL were segmented semi-automatically to generate volumetric regions of interest (ROIs) (Materialise, Inc.; Leuven, Belgium). ROIs were divided into proximal/distal and core/peripheral sub-ROIs using standardized methods. Each ROI was co-registered voxel-by-voxel to T2* maps also obtained at 1 month. These T2* maps were then co-registered with those of subsequent time points. The segmentation process was repeated for the 1-, 3-, and 6-month time points by a second reviewer for inter-observer reliability (κ). Statistical differences among the ROIs and sub-ROIs of ACL autografts were assessed with repeated-measures ANOVA and two-tailed non-parametric t-tests. P<0.05 represented statistical significance.Results:Twelve subjects were enrolled in this prospective study, and after 2 subjects withdrew, 10 subjects were included in the analysis (n=7 SemiT; n=3 BTB). Nine of 10 subjects attended all postoperative imaging sessions at the time of this writing. Inter-observer reliability for T2* values was found to be excellent (κ=0.832; 95% CI [0.70-0.91]; p<0.001). Average T2* relaxation times increased from 3.2ms (standard deviation [SD], 1.0) at 1 month, to 4.8ms (SD, 1.8) at 6 months, and 4.1ms (SD, 1.6) at 12 months (Figures 1 and 2), with statistically significant differences between the intact ACL and 1-month time point (p=0.01), and between the 1- and 6-month time points (p=0.004). The ratio of T2*[inj]/T2*[intact] increased from 59.2% at 1 month to 79.8% at 12 months for SemiT autografts, and from 86.9% at 1 month to 97.2% at 12 months for BTB autografts (p=0.001). Significant differences were found between the following pairs of sub-ROIs when combining all time points: proximal/distal, proximal-peripheral/proximal-core, distal-peripheral/distal-core (p<0.02 each).Conclusions:ACL autografts exhibited progressively increasing T2* values and T2*[inj]/T2*[intact] ratios up to 12 months after ACLR, and these changes appear to be region-dependent. UTE T2* MR imaging can provide unique insights into the condition of remodeling ACL grafts, and may improve our ability to non-invasively assess graft maturity before allowing patients to resume high-intensity activities.

MR-u: Material Characterization Using 3D Displacement-Encoded Magnetic Resonance and the Virtual Fields Method

Experimental, fully three-dimensional mechanical characterization of opaque materials with arbitrary geometries undergoing finite deformations is generally challenging. We present a promising experimental method and processing pipeline for acquiring and processing full-field displacements and using them toward inverse characterization using the Virtual Fields Method (VFM), a combination we term MR-u. Silicone of varying crosslinker concentrations and geometries is used as the sample platform. Samples are stretched cyclically to finite deformations inside a 7T MRI machine. Synchronously, a custom MRI pulse sequence encodes the local displacement in the phase of the MR image. Numerical differentiation of phase maps yields strains. We present a custom image processing scheme for this numerical differentiation of MRI phase-fields akin to convolution kernels, as well as considerations for gradient set calibration for data fidelity. The VFM is used to successfully determine hyperelastic material properties, and we establish best practice regarding virtual field selection via equalization.

Optimization methods for magnetic resonance imaging gradient waveform design.

The development and implementation of novel MRI pulse sequences remains challenging and laborious. Gradient waveforms are typically designed using a combination of analytical and ad hoc methods to construct each gradient waveform axis independently. This strategy makes coding the pulse sequence complicated, in addition to being time inefficient. As a consequence, nearly all commercial MRI pulse sequences fail to maximize use of the available gradient hardware or efficiently mitigate physiological effects. This results in expensive MRI systems that underperform relative to their inherent hardware capabilities. To address this problem, a software solution is proposed that incorporates numerical optimization methods into MRI pulse sequence programming. Examples are shown for rotational variant vs. invariant waveform designs, acceleration nulled velocity encoding gradients, and mitigation of peripheral nerve stimulation for diffusion encoding. The application of optimization methods to MRI pulse sequence design incorporates gradient hardware limits and the prescribed MRI protocol parameters (e.g. field-of-view, resolution, gradient moments, and/or b-value) to simultaneously construct time-optimal gradient waveforms. In many cases, the resulting constrained gradient waveform design problem is convex and can be solved on-the-fly on the MRI scanner. The result is a set of multi-axis time-optimal gradient waveforms that satisfy the design constraints, thereby increasing SNR-efficiency. These optimization methods can also be used to mitigate imaging artifacts (e.g. eddy currents) or account for peripheral nerve stimulation. The result of the optimization method is to enable easier pulse sequence gradient waveform design and permit on-the-fly implementation for a range of MRI pulse sequences.

SimPET: a Preclinical PET Insert for Simultaneous PET/MR Imaging.

SimPET/M7 system is a small-animal dedicated simultaneous positron emission tomography and magnetic resonance imaging (PET/MRI) scanner. The SimPET insert has been upgraded from its prototype with a focus on count rate performance and sensitivity. The M7 scanner is a 1-T permanent magnet-based compact MRI system without any cryogens. Here, we present performance evaluation results of SimPET along with the results of mutual interference evaluation and simultaneously acquired PET/MR imaging. Following NEMA NU 4-2008 standard, we evaluated the performance of the SimPET system. The M7 MRI compatibility of SimPET was also assessed by analyzing MRI images of a uniform phantom under different PET conditions and PET count rates with different MRI pulse sequences. Mouse imaging was performed including a whole-body 18F-NaF PET scan and a simultaneous PET/MRI scan with 64Cu-NOTA-ironoxide. The spatial resolution at center based on 3D OSEM without and with warm background was 0.7mm and 1.45mm, respectively. Peak sensitivity was 4.21% (energy window = 250-750keV). The peak noise equivalent count rate with the same energy window was 151kcps at 38.4MBq. The uniformity was 4.42%, and the spillover ratios in water- and air-filled chambers were 14.6% and 12.7%, respectively. In the hot rod phantom image, 0.75-mm-diameter rods were distinguishable. There were no remarkable differences in the SNR and uniformity of MRI images and PET count rates with different PET conditions and MRI pulse sequences. In the whole-body 18F-NaF PET images, fine skeletal structures were well resolved. In the simultaneous PET/MRI study with 64Cu-NOTA-ironoxide, both PET and MRI signals changed before and after injection of the dual-modal imaging probe, which was evident with the exact spatiotemporal correlation. We demonstrated that the SimPET scanner has a high count rate performance and excellent spatial resolution. The combined SimPET/M7 enabled simultaneous PET/MR imaging studies with no remarkable mutual interference between the two imaging modalities.

Towards HCP-Style macaque connectomes: 24-Channel 3T multi-array coil, MRI sequences and preprocessing

Macaque monkeys are an important animal model where invasive investigations can lead to a better understanding of the cortical organization of primates including humans. However, the tools and methods for noninvasive image acquisition (e.g. MRI RF coils and pulse sequence protocols) and image data preprocessing have lagged behind those developed for humans. To resolve the structural and functional characteristics of the smaller macaque brain, high spatial, temporal, and angular resolutions combined with high signal-to-noise ratio are required to ensure good image quality. To address these challenges, we developed a macaque 24-channel receive coil for 3-T MRI with parallel imaging capabilities. This coil enables adaptation of the Human Connectome Project (HCP) image acquisition protocols to the in-vivo macaque brain. In addition, we adapted HCP preprocessing methods to the macaque brain, including spatial minimal preprocessing of structural, functional MRI (fMRI), and diffusion MRI (dMRI). The coil provides the necessary high signal-to-noise ratio and high efficiency in data acquisition, allowing four- and five-fold accelerations for dMRI and fMRI. Automated FreeSurfer segmentation of cortex, reconstruction of cortical surface, removal of artefacts and nuisance signals in fMRI, and distortion correction of dMRI all performed well, and the overall quality of basic neurobiological measures was comparable with those for the HCP. Analyses of functional connectivity in fMRI revealed high sensitivity as compared with those from publicly shared datasets. Tractography-based connectivity estimates correlated with tracer connectivity similarly to that achieved using ex-vivo dMRI. The resulting HCP-style in vivo macaque MRI data show considerable promise for analyzing cortical architecture and functional and structural connectivity using advanced methods that have previously only been available in studies of the human brain.

The Relationship between MRI Radiofrequency Energy and Function of Nonconditional Implanted Cardiac Devices: A Prospective Evaluation.

Background The risks associated with MRI in individuals who have implanted cardiac devices are thought to arise from the interaction between the implanted device and static, gradient, and radiofrequency magnetic fields. Purpose To determine the relationship between the peak whole-body averaged specific absorption rate (SAR) and change in magnetic field per unit time (dB/dt), maximum specific energy dose, imaging region, and implanted cardiac device characteristics and their function in patients undergoing MRI. Materials and Methods This prospective observational cohort study was conducted from October 16, 2003, to January 22, 2015 (https://ClinicalTrials.gov, NCT01130896). Any individual with an implanted cardiac device who was referred for MRI was included. Clinical MRI protocols without SAR restriction were used. Exclusion criteria were newly implanted leads, abandoned or epicardial leads, and dependence on a pacemaker with an implantable cardioverter defibrillator without asynchronous pacing capability. For each MRI pulse sequence, the calculated whole-body values for SAR, dB/dt, and scan duration were collected. Atrial and ventricular sensing, lead impedance, and capture threshold were evaluated before and immediately after (within 10 minutes) completion of each MRI examination. Generalized estimating equations with Gaussian family, identity link, and an exchangeable working correlation matrix were used for statistical analysis. Results A total of 2028 MRI examinations were performed in 1464 study participants with 2755 device leads (mean age, 67 years ± 15 [standard deviation]; 930 men [64%]). There was no evidence of an association between radiofrequency energy deposition, dB/dt, or scan duration and changes in device parameters. Thoracic MRI was associated with decreased battery voltage immediately after MRI (β = -0.008 V, P < .001). Additionally, right ventricular (RV) lead length was associated with decreased RV sensing (β = -0.012 mV, P = .05) and reduced RV capture threshold (β = -0.002 V, P < .01) immediately after MRI. Conclusion There was no evidence of an association between MRI parameters that characterize patient exposure to radiofrequency energy and changes in device and lead parameters immediately after MRI. Nevertheless, device interrogation before and after MRI remains mandatory due to the potential for device reset and changes in lead or generator parameters. © RSNA, 2020 See also the editorial by Shellock in this issue.