Rapid submillimeter QSM and R2* mapping using interleaved multishot 3D-EPI at 7 and 3 Tesla.

PURPOSE: To explore the high signal-to-noise ratio (SNR) efficiency of interleaved multishot 3D-EPI with standard image reconstruction for fast and robust high-resolution whole-brain quantitative susceptibility (QSM) and R 2 ∗ $$ {R}_2^{\ast } $$ mapping at 7 and 3T. METHODS: Single- and multi-TE segmented 3D-EPI is combined with conventional CAIPIRINHA undersampling for up to 72-fold effective gradient echo (GRE) imaging acceleration. Across multiple averages, scan parameters are varied (e.g., dual-polarity frequency-encoding) to additionally correct for B 0 $$ {\mathrm{B}}_0 $$ -induced artifacts, geometric distortions and motion retrospectively. A comparison to established GRE protocols is made. Resolutions range from 1.4 mm isotropic (1 multi-TE average in 36 s) up to 0.4 mm isotropic (2 single-TE averages in approximately 6 min) with whole-head coverage. RESULTS: Only 1-4 averages are needed for sufficient SNR with 3D-EPI, depending on resolution and field strength. Fast scanning and small voxels together with retrospective corrections result in substantially reduced image artifacts, which improves susceptibility and R 2 ∗ $$ {R}_2^{\ast } $$ mapping. Additionally, much finer details are obtained in susceptibility-weighted image projections through significantly reduced partial voluming. CONCLUSION: Using interleaved multishot 3D-EPI, single-TE and multi-TE data can readily be acquired 10 times faster than with conventional, accelerated GRE imaging. Even 0.4 mm isotropic whole-head QSM within 6 min becomes feasible at 7T. At 3T, motion-robust 0.8 mm isotropic whole-brain QSM and R 2 ∗ $$ {R}_2^{\ast } $$ mapping with no apparent distortion in less than 7 min becomes clinically feasible. Stronger gradient systems may allow for even higher effective acceleration rates through larger EPI factors while maintaining optimal contrast.

Age-related differences of cerebellar cortex and nuclei: MRI findings in healthy controls and its application to spinocerebellar ataxia (SCA6) patients.

Understanding cerebellar alterations due to healthy aging provides a reference point against which pathological findings in late-onset disease, for example spinocerebellar ataxia type 6 (SCA6), can be contrasted. In the present study, we investigated the impact of aging on the cerebellar nuclei and cerebellar cortex in 109 healthy controls (age range: 16 - 78 years) using 3 Tesla magnetic resonance imaging (MRI). Findings were compared with 25 SCA6 patients (age range: 38 - 78 years). A subset of 16 SCA6 (included: 14) patients and 50 controls (included: 45) received an additional MRI scan at 7 Tesla and were re-scanned after one year. MRI included T1-weighted, T2-weighted FLAIR, and multi-echo T2*-weighted imaging. The T2*-weighted phase images were converted to quantitative susceptibility maps (QSM). Since the cerebellar nuclei are characterized by elevated iron content with respect to their surroundings, two independent raters manually outlined them on the susceptibility maps. T1-weighted images acquired at 3T were utilized to automatically identify the cerebellar gray matter (GM) volume. Linear correlations revealed significant atrophy of the cerebellum due to tissue loss of cerebellar cortical GM in healthy controls with increasing age. Reduction of the cerebellar GM was substantially stronger in SCA6 patients. The volume of the dentate nuclei did not exhibit a significant relationship with age, at least in the age range between 18 and 78 years, whereas mean susceptibilities of the dentate nuclei increased with age. As previously shown, the dentate nuclei volumes were smaller and magnetic susceptibilities were lower in SCA6 patients compared to age- and sex-matched controls. The significant dentate volume loss in SCA6 patients could also be confirmed with 7T MRI. Linear mixed effects models and individual paired t-tests accounting for multiple comparisons revealed no statistical significant change in volume and susceptibility of the dentate nuclei after one year in neither patients nor controls. Importantly, dentate volumes were more sensitive to differentiate between SCA6 (Cohen's d = 3.02) and matched controls than the cerebellar cortex volume (d = 2.04). In addition to age-related decline of the cerebellar cortex and atrophy in SCA6 patients, age-related increase of susceptibility of the dentate nuclei was found in controls, whereas dentate volume and susceptibility was significantly decreased in SCA6 patients. Because no significant changes of any of these parameters was found at follow-up, these measures do not allow to monitor disease progression at short intervals.

Perfusion and T2 Relaxation Time as Predictors of Severity and Outcome in Sepsis-Associated Acute Kidney Injury: A Preclinical MRI Study.

BACKGROUND: Preventing sepsis-associated acute kidney injury (S-AKI) can be challenging because it develops rapidly and is often asymptomatic. Probability assessment of disease progression for therapeutic follow-up and outcome are important to intervene and prevent further damage. PURPOSE: To establish a noninvasive multiparametric MRI (mpMRI) tool, including T1 , T2 , and perfusion mapping, for probability assessment of the outcome of S-AKI. STUDY TYPE: Preclinical randomized prospective study. ANIMAL MODEL: One hundred and forty adult female SD rats (65 control and 75 sepsis). FIELD STRENGTH/SEQUENCE: 9.4T; T1 and perfusion map (FAIR-EPI) and T2 map (multiecho RARE). ASSESSMENT: Experiment 1: To identify renal injury in relation to sepsis severity, serum creatinine levels were determined (31 control and 35 sepsis). Experiment 2: Animals underwent mpMRI (T1 , T2 , perfusion) 18 hours postsepsis. A subgroup of animals was immediately sacrificed for histology examination (nine control and seven sepsis). Result of mpMRI in follow-up subgroup (25 control and 33 sepsis) was used to predict survival outcomes at 96 hours. STATISTICAL TESTS: Mann-Whitney U test, Spearman/Pearson correlation (r), P < 0.05 was considered statistically significant. RESULTS: Severely ill septic animals exhibited significantly increased serum creatinine levels compared to controls (70 ± 30 vs. 34 ± 9 µmol/L, P < 0.0001). Cortical perfusion (480 ± 80 vs. 330 ± 140 mL/100 g tissue/min, P < 0.005), and cortical and medullary T2 relaxation time constants were significantly reduced compared to controls (41 ± 4 vs. 37 ± 5 msec in cortex, P < 0.05, 52 ± 7 vs. 45 ± 6 msec in medulla, P < 0.05). The combination of cortical T2 relaxation time constants and perfusion results at 18 hours could predict survival outcomes at 96 hours with high sensitivity (80%) and specificity (73%) (area under curve of ROC = 0.8, Jmax = 0.52). DATA CONCLUSION: This preclinical study suggests combined T2 relaxation time and perfusion mapping as first line diagnostic tool for treatment planning. LEVEL OF EVIDENCE: 2 TECHNICAL EFFICACY STAGE: 2.

The differential association between local neurotransmitter levels and whole-brain resting-state functional connectivity in two distinct cingulate cortex subregions.

We examined the association between rsFC and local neurotransmitter levels in the pregenual anterior cingulate cortex (pgACC) and the anterior mid-cingulate cortex (aMCC) by varying rsFC-strengths at the whole-brain level. Our results showed region-dependent directionality of associations in the investigated ACC subdivisions.

High-resolution MRI of the human palatine tonsil and its schematic anatomic 3D reconstruction.

The palatine tonsils form an important part of the human immune system. Together with the other lymphoid tonsils of Waldeyer's tonsillar ring, they act as the first line of defense against ingested or inhaled pathogens. Although histologically stained sections of the palatine tonsil are widely available, they represent the tissue only in two dimensions and do not provide reference to three-dimensional space. Such a representation of a tonsillar specimen based on imaging data as a 3D anatomical reconstruction is lacking both in scientific publications and especially in textbooks. As a first step in this direction, the objective of the present work was to image a resected tonsil specimen with high spatial resolution in a 9.4 T small-bore pre-clinical MRI and to combine these data with data from the completely sectioned and H&E stained same palatine tonsil. Based on the information from both image modalities, a 3D anatomical sketch was drawn by a scientific graphic artist. In perspective, such studies could help to overcome the difficulty of capturing the spatial extent and arrangement of anatomical structures from 2D images and to establish a link between three-dimensional anatomical preparations and two-dimensional sections or illustrations, as they have been found so far in common textbooks and anatomical atlases.

In vitro measurements of radiation exposure with different modalities (computed tomography, cone beam computed tomography) for imaging the petrous bone with a pediatric anthropomorphic phantom.

BACKGROUND: Various imaging modalities, such as multi-detector computed tomography (CT) and cone beam CT are commonly used in infants for the diagnosis of hearing loss and surgical planning of implantation hearing aid devices, with differing results. OBJECTIVE: We compared three different imaging modalities available in our institution, including a high-class CT scanner, a mid-class CT scanner and an angiography system with a cone beam CT option, for image quality and radiation exposure in a phantom study. MATERIALS AND METHODS: While scanning an anthropomorphic phantom imitating a 1-year-old child with vendor-provided routine protocols, organ doses, surface doses and effective doses were determined for these three modalities with thermoluminescent dosimeters. The image quality was evaluated using the signal difference to noise ratio (SDNR) and the spatial resolution of a line-pair insert in the phantom head. The dose efficiency, defined as the ratio of SDNR and effective dose, was also compared. RESULTS: The organ and surface doses were lowest with the high-class CT protocol, but the image quality was the worst. Image quality was best with the cone beam CT protocol, which, however, had the highest radiation exposure in this study, whereas the mid-class CT was in between. CONCLUSION: Based on our results, high-end CT should be used for surgical planning because it has the lowest dose, while the image quality is still sufficient for this purpose. However, if highest image quality is needed and required, e.g., by ENT surgeons, the other modalities should be considered.

In vivo assessment of anisotropy of apparent magnetic susceptibility in white matter from a single orientation acquisition.

Multiple studies have reported a significant dependence of the effective transverse relaxation rate constant (R2*) and the phase of gradient-echo based (GRE) signal on the orientation of white matter fibres in the human brain. It has also been hypothesized that magnetic susceptibility, as obtained by single-orientation quantitative susceptibility mapping (QSM), exhibits such a dependence. In this study, we investigated this hypothesized relationship in a cohort of healthy volunteers. We show that R2* follows the predicted orientation dependence consistently across white matter regions, whereas the apparent magnetic susceptibility is related differently to fibre orientation across the brain and often in a complex non-monotonic manner. In addition, we explored the effect of fractional anisotropy measured by diffusion-weighted MRI on the strength of the orientation dependence and observed only a limited influence in many regions. However, with careful consideration of such an impact and the limitations imposed by the ill-posed nature of the dipole inversion process, it is possible to study magnetic susceptibility anisotropy in specific brain regions with a single orientation acquisition.

Experience-dependent structural plasticity in the adult brain: How the learning brain grows.

Volumetric magnetic resonance imaging studies have shown that intense learning can be associated with grey matter volume increases in the adult brain. The underlying mechanisms are poorly understood. Here we used monocular deprivation in rats to analyze the mechanisms underlying use-dependent grey matter increases. Optometry for quantification of visual acuity was combined with volumetric magnetic resonance imaging and microscopic techniques in longitudinal and cross-sectional studies. We found an increased spatial vision of the open eye which was associated with a transient increase in the volumes of the contralateral visual and lateral entorhinal cortex. In these brain areas dendrites of neurons elongated, and there was a strong increase in the number of spines, the targets of synapses, which was followed by spine maturation and partial pruning. Astrocytes displayed a transient pronounced swelling and underwent a reorganization of their processes. The use-dependent increase in grey matter corresponded predominantly to the swelling of the astrocytes. Experience-dependent increase in brain grey matter volume indicates a gain of structure plasticity with both synaptic and astrocyte remodeling.

Interrelations between dopamine and serotonin producing sites and regions of the default mode network.

Recent functional magnetic resonance imaging (fMRI) studies showed that blood oxygenation level-dependent (BOLD) signal fluctuations in the default mode network (DMN) are functionally tightly connected to those in monoaminergic nuclei, producing dopamine (DA), and serotonin (5-HT) transmitters, in the midbrain/brainstem. We combined accelerated fMRI acquisition with spectral Granger causality and coherence analysis to investigate causal relationships between these areas. Both methods independently lead to similar results and confirm the existence of a top-down information flow in the resting-state condition, where activity in core DMN areas influences activity in the neuromodulatory centers producing DA/5-HT. We found that latencies range from milliseconds to seconds with high inter-subject variability, likely attributable to the resting condition. Our novel findings provide new insights into the functional organization of the human brain.

Immersion of Achilles tendon in phosphate-buffered saline influences T1 and T2 * relaxation times: An ex vivo study.

Robust mapping of relaxation parameters in ex vivo tissues is based on hydration and therefore requires control of the tissue treatment to ensure tissue integrity and consistent measurement conditions over long periods of time. One way to maintain the hydration of ex vivo tendon tissue is to immerse the samples in a buffer solution. To this end, various buffer solutions have been proposed; however, many appear to influence the tissue relaxation times, especially with prolonged exposure. In this work, ovine Achilles tendon tissue was used as a model to investigate the effect of immersion in phosphate-buffered saline (PBS) and the effects on the T1 and T2* relaxation times. Ex vivo samples were measured at 0 (baseline), 30 and 67 hours after immersion in PBS. Ultrashort echo time (UTE) imaging was performed using variable flip angle and echo train-shifted multi-echo imaging for T1 and T2* estimation, respectively. Compared with baseline, both T1 and T2* relaxation time constants increased significantly after 30 hours of immersion. T2* continued to show a significant increase between 30 and 67 hours. Both T1 and T2* tended to approach saturation at 67 hours. These results exemplify the relevance of stringently controlled tissue preparation and preservation techniques, both before and during MRI experiments.

A new framework for assessing subject-specific whole brain circulation and perfusion using MRI-based measurements and a multi-scale continuous flow model.

A large variety of severe medical conditions involve alterations in microvascular circulation. Hence, measurements or simulation of circulation and perfusion has considerable clinical value and can be used for diagnostics, evaluation of treatment efficacy, and for surgical planning. However, the accuracy of traditional tracer kinetic one-compartment models is limited due to scale dependency. As a remedy, we propose a scale invariant mathematical framework for simulating whole brain perfusion. The suggested framework is based on a segmentation of anatomical geometry down to imaging voxel resolution. Large vessels in the arterial and venous network are identified from time-of-flight (ToF) and quantitative susceptibility mapping (QSM). Macro-scale flow in the large-vessel-network is accurately modelled using the Hagen-Poiseuille equation, whereas capillary flow is treated as two-compartment porous media flow. Macro-scale flow is coupled with micro-scale flow by a spatially distributing support function in the terminal endings. Perfusion is defined as the transition of fluid from the arterial to the venous compartment. We demonstrate a whole brain simulation of tracer propagation on a realistic geometric model of the human brain, where the model comprises distinct areas of grey and white matter, as well as large vessels in the arterial and venous vascular network. Our proposed framework is an accurate and viable alternative to traditional compartment models, with high relevance for simulation of brain perfusion and also for restoration of field parameters in clinical brain perfusion applications.

The relationship between heart rate and functional connectivity of brain regions involved in autonomic control.

The peripheral autonomic nervous system (ANS) adjusts the heart rate (HR) to intrinsic and extrinsic demands. It is controlled by a group of functionally connected brain regions assembling the so-called central autonomic network (CAN). More specifically, forebrain cortical regions, limbic and brainstem structures within the CAN have been identified as important components of circuits involved in HR regulation. The present study aimed to investigate whether functional connectivity (FC) between these regions varies in subjects with different heart rates. Thus, 84 healthy subjects were separated according to their HR in slow, medium and fast. We observed a direct association between HR and FC in CAN regions, where stronger FC was related to slower HR. This relationship, however, is non-linear, follows an exponential course and is not restricted to CAN areas only. The network-based analysis (NBS) using time series from 262 independent anatomical ROIs revealed significantly increased functional connectivity in subjects with slow HR compared to subjects with fast HR mainly in regions being part of the salience network, but also of the default-mode network. We additionally simulated the effect of aliasing on the functional connectivity using several TRs and heart rates to exclude the possibility that FC differences might be due to different aliasing effects in the data. The result of the simulation indicated that aliasing cannot explain our findings. Thus, present results imply a functionally meaningful coupling between FC and HR that need to be accounted for in future studies. Moreover, given the established link between HR and emotional, cognitive and social processes, present findings may also be considered to explain individual differences in brain activation or connectivity when using corresponding paradigms in the MR scanner to investigate such processes.

Assessment of MR imaging during one-lung flooding in a large animal model.

OBJECTIVE: Magnetic resonance imaging (MRI) of the lung remains challenging due to the low tissue density, susceptibility artefacts, unfavourable relaxation times and motion. Previously, we demonstrated in vivo that one-lung flooding (OLF) with saline is a viable and safe approach. This study investigates the feasibility of OLF in an MRI environment and evaluates the flooding process on MR images. METHODS: OLF of the left lung was performed on five animals using a porcine model. Before, during and after OLF, standard T2w and T1w spin-echo (SE) and gradient-echo (GRE) sequences were applied at 3 T. RESULTS: The procedure was successfully performed in all animals. On T1w MRI, the flooded lung appeared homogenous and isointense with muscle tissue. On T2w images, vascular structures were highly hypointense, while the bronchi were clearly demarcated with hypointense wall and hyperintense lumen. The anatomical demarcation of the flooded lung from the surrounding organs was superior on T2w images. No outflow effects were seen, and no respiration triggering was required. DISCUSSION: OLF can be safely performed in an MR scanner with highly detailed visualization of the pulmonary structures on T2w images. The method provides new approaches to MRI-based image-guided pulmonary interventions using the presented experimental model.

Evaluation of liver tissue by ultrasound elastography and clinical parameters in children with multiple blood cell transfusions.

BACKGROUND: Children receiving multiple blood cell transfusions are prone to iron overload and successive tissue damage in liver parenchyma, making noninvasive screening options desirable. Ultrasound (US) elastography using acoustic radiation force impulse (ARFI) imaging enables evaluation of liver parenchyma stiffness, and MRI allows for quantification of liver iron concentration. OBJECTIVE: The objective was to correlate US elastography with MRI in children who had undergone bone marrow transplantation and to evaluate the modification of liver tissue with US in combination with clinical parameters at follow-up. MATERIALS AND METHODS: ARFI, T2*-weighted MRI and a clinical score (HepScore, based on parameters of liver function) were performed in 45 patients (24 male; mean age 9.7 years) before and 100 days and 365 days after transplantation. All received multiple blood transfusions (mean number 22.2 up until 1 year after transplantation). We correlated US findings and HepScore with MRI findings. RESULTS: We observed signs of iron accumulation in 29/45 (64.4%) patients on MRI (T2*<10 ms) and 15/45 (33.3%) showed increased tissue stiffness (ARFI>5.5 kPa). Correlation of elastography and MRI was not significant (P=0.57; n=51 matched measurements). Comparing US elastography with HepScore in receiver operating characteristic (ROC) curve analysis indicated a cut-off for affected parenchyma if HepScore was >5 points (sensitivity 67%, specificity 68%). Simultaneous increases of both indicated tissue alteration. CONCLUSION: Combining US and HepScore enabled detection of liver tissue alteration through iron overload, but we found no direct significant effect of estimated iron from MRI on ARFI imaging.

The influence of brain iron and myelin on magnetic susceptibility and effective transverse relaxation - A biochemical and histological validation study.

Quantitative susceptibility mapping (QSM) and effective transverse relaxation rate (R2*) mapping are both highly sensitive to variations in brain iron content. Clinical Magnetic Resonance Imaging (MRI) studies report changes of susceptibilities and relaxation rates in various neurological diseases which are often equated with changes in regional brain iron content. However, these mentioned metrics lack specificity for iron, since they are also influenced by the presence of myelin. In this study, we assessed the extent to which QSM and R2* reflect iron concentration as well as histological iron and myelin intensities. Six unfixed human post-mortem brains were imaged in situ with a 7 T MRI scanner. After formalin fixation, the brains were sliced axially and punched. 671 tissue punches were subjected to ferrozine iron quantification. Subsequently, brain slices were embedded in paraffin, and histological double-hemispheric axial brain slices were stained for Luxol fast blue (myelin) and diaminobenzidine (DAB)-enhanced Turnbull blue (iron). 3331 regions of interest (ROIs) were drawn on the histological stainings to assess myelin and iron intensities, which were compared with MRI data in corresponding ROIs. QSM more closely reflected quantitative ferrozine iron values (r = 0.755 vs. 0.738), whereas R2* correlated better with iron staining intensities (r = 0.619 vs. 0.445). Myelin intensities correlated negatively with QSM (r = -0.352), indicating a diamagnetic effect of myelin on susceptibility. Myelin intensities were higher in the thalamus than in the basal ganglia. A significant relationship was nonetheless observed between quantitative iron values and QSM, confirming the applicability of the latter in this brain region for iron quantification.

Comparison of metabolic adaptations between endurance- and sprint-trained athletes after an exhaustive exercise in two different calf muscles using a multi-slice 31 P-MR spectroscopic sequence.

Measurements of exercise-induced metabolic changes, such as oxygen consumption, carbon dioxide exhalation or lactate concentration, are important indicators for assessing the current performance level of athletes in training science. With exercise-limiting metabolic processes occurring in loaded muscles, 31 P-MRS represents a particularly powerful modality to identify and analyze corresponding training-induced alterations. Against this background, the current study aimed to analyze metabolic adaptations after an exhaustive exercise in two calf muscles (m. soleus - SOL - and m. gastrocnemius medialis - GM) of sprinters and endurance athletes by using localized dynamic 31 P-MRS. In addition, the respiratory parameters VO2 and VCO2 , as well as blood lactate concentrations, were monitored simultaneously to assess the effects of local metabolic adjustments in the loaded muscles on global physiological parameters. Besides noting obvious differences between the SOL and the GM muscles, we were also able to identify distinct physiological strategies in dealing with the exhaustive exercise by recruiting two athlete groups with opposing metabolic profiles. Endurance athletes tended to use the aerobic pathway in the metabolism of glucose, whereas sprinters produced a significantly higher peak concentration of lactate. These global findings go along with locally measured differences, especially in the main performer GM, with sprinters revealing a higher degree of acidification at the end of exercise (pH 6.29 ± 0.20 vs. 6.57 ± 0.21). Endurance athletes were able to partially recover their PCr stores during the exhaustive exercise and seemed to distribute their metabolic activity more consistently over both investigated muscles. In contrast, sprinters mainly stressed Type II muscle fibers, which corresponds more to their training orientation preferring the glycolytic energy supply pathway. In conclusion, we were able to analyze the relation between specific local metabolic processes in loaded muscles and typical global adaptation parameters, conventionally used to monitor the training status of athletes, in two cohorts with different sports orientations.

GlucoCEST magnetic resonance imaging in vivo may be diagnostic of acute renal allograft rejection.

Acute cellular renal allograft rejection (AR) frequently occurs after kidney transplantations. It is a sterile T-cell mediated inflammation leading to increased local glucose metabolism. Here we demonstrate in an allogeneic model of Brown Norway rat kidneys transplanted into uninephrectomized Lewis rats the successful implementation of the recently developed glucose chemical exchange saturation transfer (glucoCEST) magnetic resonance imaging. This technique is a novel method to assess and differentiate AR. Renal allografts undergoing AR showed significantly increased glucoCEST contrast ratios of cortex to medulla of 1.61 compared to healthy controls (1.02), syngeneic Lewis kidney to Lewis rat transplants without rejection (0.92), kidneys with ischemia reperfusion injury (0.99) and kidneys affected by cyclosporine A toxicity (1.10). Receiver operating characteristic curve analysis showed an area under the curve value of 0.92, and the glucoCEST contrast ratio predicted AR with a sensitivity of 100% and a specificity of 69% at a threshold level over 1.08. In defined animal models of kidney injuries, the glucoCEST contrast ratios of cortex to medulla correlated positively with mRNA expression levels of T-cell markers (CD3, CD4, CD8a/b), but did not correlate to impaired renal perfusion. Thus, the glucoCEST parameter may be valuable for the assessment and follow up treatment of AR.

The pH heterogeneity in human calf muscle during neuromuscular electrical stimulation.

PURPOSE: The aim of the study was to examine pH heterogeneity during fatigue induced by neuromuscular electrical stimulation (NMES) using phosphorus magnetic resonance spectroscopy (31 P-MRS). It is hypothesized that three pH components would occur in the 31 P-MRS during fatigue, representing three fiber types. METHODS: The medial gastrocnemius of eight subjects was stimulated within a 3-Tesla whole body MRI scanner. The maximal force during stimulation (Fstim ) was examined by a pressure sensor. Phosphocreatine (PCr), adenosintriphosphate, inorganic phosphate (Pi), and the corresponding pH were estimated by a nonvolume-selective 31 P-MRS using a small loop coil at rest and during fatigue. RESULTS: During fatigue, Fstim and PCr decreased to 27% and 33% of their initial levels, respectively. In all cases, the Pi peak increased when NMES was started and split into three different peaks. Based on the single Pi peaks during fatigue, an alkaline (6.76 ± 0.08), a medium (6.40 ± 0.06), and an acidic (6.09 ± 0.05) pH component were observed compared to the pH (7.02 ± 0.02) at rest. CONCLUSION: It is suggested that NMES is able to induce pH heterogeneity in the medial gastrocnemius, and that the single Pi peaks represent the different muscle fiber types of the skeletal muscle. Magn Reson Med 77:2097-2106, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Overview of quantitative susceptibility mapping.

Magnetic susceptibility describes the magnetizability of a material to an applied magnetic field and represents an important parameter in the field of MRI. With the recently introduced method of quantitative susceptibility mapping (QSM) and its conceptual extension to susceptibility tensor imaging (STI), the non-invasive assessment of this important physical quantity has become possible with MRI. Both methods solve the ill-posed inverse problem to determine the magnetic susceptibility from local magnetic fields. Whilst QSM allows the extraction of the spatial distribution of the bulk magnetic susceptibility from a single measurement, STI enables the quantification of magnetic susceptibility anisotropy, but requires multiple measurements with different orientations of the object relative to the main static magnetic field. In this review, we briefly recapitulate the fundamental theoretical foundation of QSM and STI, as well as computational strategies for the characterization of magnetic susceptibility with MRI phase data. In the second part, we provide an overview of current methodological and clinical applications of QSM with a focus on brain imaging. Copyright © 2016 John Wiley & Sons, Ltd.

Methods for the computation of templates from quantitative magnetic susceptibility maps (QSM): Toward improved atlas- and voxel-based analyses (VBA).

PURPOSE: To develop and assess a method for the creation of templates for voxel-based analysis (VBA) and atlas-based approaches using quantitative magnetic susceptibility mapping (QSM). MATERIALS AND METHODS: We studied four strategies for the creation of magnetic susceptibility brain templates, derived as successive extensions of the conventional template generation (CONV) based on only T1 -weighted (T1 w) images. One method that used only T1 w images involved a minor improvement of CONV (U-CONV). One method used only magnetic susceptibility maps as input for template generation (DIRECT), and the other two used a linear combination of susceptibility and T1 w images (HYBRID) and an algorithm that directly used both image modalities (MULTI), respectively. The strategies were evaluated in a group of N = 10 healthy human subjects and semiquantitatively assessed by three experienced raters. Template quality was compared statistically via worth estimates (WEs) obtained with a log-linear Bradley-Terry model. RESULTS: The overall quality of the templates was better for strategies including both susceptibility and T1 w contrast (MULTI: WE = 0.62; HYBRID: WE = 0.21), but the best method depended on the anatomical region of interest. While methods using only one modality resulted in lower WEs, lowest overall WEs were obtained when only T1 w images were used (DIRECT: WE = 0.12; U-CONV: WE = 0.05). CONCLUSION: Template generation strategies that employ only magnetic susceptibility contrast or both magnetic susceptibility and T1 w contrast produce templates with the highest quality. The optimal approach depends on the anatomical structures of interest. The established approach of using only T1 w images (CONV) results in reduced image quality compared to all other approaches studied. LEVEL OF EVIDENCE: 2 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2017;46:1474-1484.