Magnetic Susceptibility Contrast Research Articles

Diffusion kurtosis imaging for different brain masses characterization

BackgroundDiffusion kurtosis imaging is an advanced magnetic resonance imaging technique that reveals additional information on the microstructure and micro-dynamics of different brain masses without the need for contrast agents. The aim of this study was to provide a comprehensive analysis of the role of MRI diffusion kurtosis and to compare it with magnetic resonance spectroscopy (MRS) and dynamic susceptibility contrast perfusion (DSC) in characterizing different brain masses, including gliomas, recurrent tumors, radiation necrosis, abscesses, and infarctions. Sixty-six patients with intracranial brain masses were enrolled in this prospective study. All patients were examined by conventional MRI sequences, DSC perfusion, MRS, and diffusion kurtosis imaging, with implemented b values which were 200, 500, 1000, 1500 and 2000s/mm2.ResultsMean kurtosis (MK) was higher (P < 0.001) in recurrent brain tumors than in radiation-induced necrosis; the optimal MK cutoff value for differentiation between them was 642 with 91.3% sensitivity and 85.7% specificity. Mean kurtosis was also higher (P < 0.001) in high-grade gliomas than in low-grade gliomas; the optimal MK cutoff value for differentiation between them was 639 with 91.6% sensitivity and 85.71% specificity. There was a good level of agreement between ADC and MD within the studied cases, with a correlation coefficient r = 0.815. MK had more sensitivity and specificity in differentiation between high- and low-grade gliomas, as well as RIN and tumoral recurrence, than MRS and DSC.ConclusionsDiffusion kurtosis imaging stands as an integral, noninvasive, and noncontrast tool for the characterization of various brain masses. It augments the capabilities of traditional and advanced MRI techniques, providing a deeper understanding of the microstructural changes in brain tissues.

Comparison of computed tomography perfusion and magnetic resonance dynamic susceptibility contrast perfusion-weighted imaging in canine brain.

Brain perfusion allows for the evaluation of cerebral hemodynamics, particularly in brain infarcts and tumors. Computed tomography (CT) perfusion (CTP) provides reliable data; however, it has a limited scan field of view and radiation exposure. Magnetic resonance (MR) perfusion provides detailed imaging of small structures and a wide scan field of view. However, no study has compared CTP and MR perfusion and assessed the correlation between the perfusion parameters measured using CTP and MR perfusion. The aim of the present study was to assess the correlation and agreement of the cerebral perfusion derived from dynamic susceptibility contrast (DSC)-MRI and CTP in dogs. In this crossover design study, the cerebral blood volume (CBV), cerebral blood flow (CBF), mean transit time, and time to peak were measured in the temporal cerebral cortex, caudate nucleus, thalamus, piriform lobe, and hippocampus using CTP and DSC-MRI in six healthy beagle dogs and a dog with a pituitary tumor. On the color map of healthy beagles, blood vessels and the perivascular brain parenchyma appeared as red-green, indicating high perfusion, and the areas distant from the vessels appeared as green-blue, indicating low perfusion levels in CTP and DSC-MRI. CTP parameters were highest in the piriform lobe (CBF = 121.11 ± 12.78 mL/100 g/min and CBV = 8.70 ± 2.04 mL/100 g) and lowest in the thalamus (CBF = 63.75 ± 25.24 mL/100 g/min and CBV = 4.02 ± 0.55 mL/100 g). DSC-MRI parameters were also highest in the piriform lobe (CBF = 102.31 ± 14.73 mL/100 g/min and CBV = 3.17 ± 1.23 mL/100 g) and lowest in the thalamus (CBF = 37.73 ± 25.11 mL/100 g/min and CBV = 0.81 ± 0.44 mL/100 g) although there was no statistical correlation in the quantitative perfusion parameters between CTP and DSC-MRI. In a dog with a pituitary tumor, the color map of the tumor appeared as a red scale, indicating high perfusion and higher CBF and CBV on CTP (149 mL/100 g and 20 mL/100 g/min) and on DSC-MRI (116.3 mL/100 g and 15.32 mL/100 g/min) compared to those measured in healthy dogs. These findings indicate that DSC-MRI and CTP maps exhibit comparability and interchangeability in the assessment of canine brain perfusion.

A potential NMR-based wettability index using free induction decay for rocks

The wettability of reservoir rocks saturated with oil and water is one of the most important factors influencing petrophysics and oil recovery. Minerals with different wettability constitute the overall heterogeneous wettability in rocks. Variations in sample composition can be detected by nuclear magnetic resonance (NMR) measurements. In this paper, the method of using the magnetic susceptibility contrast between rock skeleton and saturated fluid to estimate wettability is proposed. The theoretical feasibility was firstly analyzed, and then the internal gradients caused by magnetic susceptibility contrasts were employed to interpret wettability alteration before and after ageing process in rocks. It was discovered that water and oil in the same pores experienced different internal gradients after ageing, which were associated with the differences in magnetic susceptibility contrasts. After that, the free induction decay measurement was performed to acquire magnetic susceptibility contrasts of artificial sandstone samples with the intermediate-wet condition. A refined NMR wettability index was presented and correlated with the Amott wettability tests. The experimental results demonstrate that the new method for determining wettability is feasible.

Assessment of brain tumors by magnetic resonance dynamic susceptibility contrast perfusion-weighted imaging and computed tomography perfusion: a comparison study.

To investigate the association and agreement between magnetic resonance dynamic susceptibility contrast perfusion-weighted imaging (DSC-PWI) and computed tomography perfusion (CTP) in determining vascularity and permeability of primary and secondary brain tumors. DSC-PWI and CTP studies from 97 patients with high-grade glioma, low-grade glioma and solitary brain metastasis were retrospectively reviewed. Normalized cerebral blood flow (nCBF), cerebral blood volume (nCBV), capillary transfer constant (nK2) and permeability surface area product (nPS) values were obtained. Variables among groups were compared, and correlation and agreement between DSC-PWI and CTP were tested. All DSC-PWI and CTP parameters were higher in high-grade than in low-grade gliomas (p < 0.01 and p < 0.001). Metastases had greater DSC-PWI nCBV (p < 0.05), nCTP-CBF (p < 0.05), nCTP-CBV (p < 0.01) and nCTP-PS (p < 0.0001) than low-grade gliomas and more elevated nCTP-PS (p < 0.01) than high-grade gliomas. The correlation was strong between DSC-PWI nCBF and CTP nCBF (r = 0.79; p < 0.00001) and between DSC-PWI nCBV and CTP nCBV (r = 0.83; p < 0.00001), weaker between DSC-PWI nK2 and CTP nPS (r = 0.29; p < 0.01). Bland-Altman plots indicated that the agreement was strong between DSC-PWI nCBF and CTP nCBF, good between DSC-PWI nCBV and CTP nCBV and poorer between DSC-PWI nK2 and CTP nPS. DSC-PWI and CTP CBF and CBV maps were comparable and interchangeable in the assessment of tumor vascularity, unlike DSC-PWI K2 and CTP PS maps that were more discordant in the analysis of tumor permeability. CTP could be an alternative method to quantify tumor neoangiogenesis when MRI is not available or when the patient does not tolerate it.

Curie-Point Depths, Geothermal Gradients and Sub-Surface Heat Flow Estimation from Spectral Analysis of High-Resolution Aeromagnetic Data over Gongola Basin and Its Environs, Northeastern Nigeria

Geothermal energy resources have been established globally to be among the sustainable and environmentally harmless means of energy generation. Curie-point depths (CPD), temperature gradients, and heat flow data over the study area were computed using a spectral analysis method in order to have a preliminary view of the geothermal implications (prospect) of the current area. Aeromagnetic data acquired by the Nigerian Geological Survey Agency (NGSA) in the year 2009 was used for the research. The results showed the minimum, maximum, and the average CPD values as 8.18 km, 31.48 km, and 13.0 km, respectively. The minimum, maximum and average thermal gradients obtained were 18.42 °C/km, 70.91 °C/km, and 50.2 °C/km, respectively. The heat flow data obtained ranged from 33.15 mW/m2 to 177.28 mW/m2, with an average value of 110.65 mW/m2. Locations depicting shallow CPDs anomalies (Alkaleri, Darazo, Dukku, Misau, Wuyo, Deba, and Tula), also showed conformity with high heat flow areas. As such, they are regarded as areas of promising geothermal prospects and are recommended for further detailed investigation. Locations depicting a high magnetic susceptibility contrast from a generated analytic signal map, as well as high temperature gradients, high heat flow, and shallow CPDs are attributed to crustal thinning along the sedimentary basin and magmatic intrusions along basement areas, respectively. The magnetic depth to the basement calculated for the study area using the source parameter imaging (SPI) method ranges from 0.610 km to 3.055 km. The present study has provided an insight on preliminary information, regarding new areas of possible geothermal prospects for further detailed investigation.

Model Synthetic Samples for Validation of NMR Signal Simulations

Simulations of nuclear magnetic resonance (NMR) signal from fluids contained in porous media (such as rock cores) need to account for both enhanced surface relaxation and the presence of internal magnetic field gradients due to magnetic susceptibility contrast between the rock matrix and the contained fluid phase. Such simulations are typically focussed on the extraction of the NMR T2 relaxation distribution which can be related to pore size and indirectly to system permeability. Discrepancies between such NMR signal simulations on digital rock cores and associated experimental measurements are however frequently reported; these are generally attributed to spatial variations in rock matric composition resulting in heterogeneously distributed NMR surface relaxivities (ρ) and internal magnetic field gradients. To this end, a range of synthetic sediments composed of variable mixtures of quartz and garnet sands were studied. These two constituents were selected for the following reasons: they have different densities allowing for ready phase differentiation in 3D μCT images of samples to use as simulation lattices and they have distinctly different ρ and magnetic susceptibility values which allow for a rigorous test of NMR simulations. Here these 3D simulations were used to calculate the distribution of internal magnetic field gradients in the range of samples, these data were then compared against corresponding NMR experimental measurements. Agreement was reasonably good with the largest discrepancy being the simulation predicting weak internal gradients (in the vicinity of the quartz sand for mixed samples) which were not detected experimentally. The suite of 3D μCT images and associated experimental NMR measurements are all publicly available for the development and validation of NMR simulation efforts.

Magnetic resonance spectroscopy outperforms perfusion in distinguishing between pseudoprogression and disease progression in patients with glioblastoma.

BackgroundThere is a need to establish biomarkers that distinguish between pseudoprogression (PsP) and true tumor progression in patients with glioblastoma (GBM) treated with chemoradiation.MethodsWe analyzed magnetic resonance spectroscopic imaging (MRSI) and dynamic susceptibility contrast (DSC) MR perfusion data in patients with GBM with PsP or disease progression after chemoradiation. MRSI metabolites of interest included intratumoral choline (Cho), myo-inositol (mI), glutamate + glutamine (Glx), lactate (Lac), and creatine on the contralateral hemisphere (c-Cr). Student T-tests and area under the ROC curve analyses were used to detect group differences in metabolic ratios and their ability to predict clinical status, respectively.Results28 subjects (63 ± 9 years, 19 men) were evaluated. Subjects with true progression (n = 20) had decreased enhancing region mI/c-Cr (P = .011), a marker for more aggressive tumors, compared to those with PsP, which predicted tumor progression (AUC: 0.84 [0.76, 0.92]). Those with true progression had elevated Lac/Glx (P = .0009), a proxy of the Warburg effect, compared to those with PsP which predicted tumor progression (AUC: 0.84 [0.75, 0.92]). Cho/c-Cr did not distinguish between PsP and true tumor progression. Despite rCBV (AUC: 0.70 [0.60, 0.80]) and rCBF (AUC: 0.75 [0.65, 0.84]) being individually predictive of tumor response, they added no additional predictive value when combined with MRSI metabolic markers.ConclusionsIncorporating enhancing lesion MRSI measures of mI/c-Cr and Lac/Glx into brain tumor imaging protocols can distinguish between PsP and true progression and inform patient management decisions.

Applications of Hilbert transform and Newton's root finding algorithm in delineating buried fracture from magnetic anomaly data

ABSTRACTI propose a simple and sufficiently robust data‐based semi‐automated interpretation of a total magnetic intensity profile to delineate an isolated fracture or a fractured zone embedded within a homogeneous host medium. The interpretation technique, which is based on the premise of induced magnetization, uses a model of a thin, two‐dimensional dipping sheet embedded in a homogeneous half‐space. The proposed quantitative interpretation uses a set of closed form formulae in delineating model parameters; in those the Hilbert transform, the first‐ and second‐order derivatives of the local amplitude of the zero‐order analytical signal are the essential components. The locations of zero crossings of the first‐ and second‐order derivatives of analytical signal and the Hilbert transform remain the essential keys in estimating structural parameters of the model. The magnetic susceptibility contrast is delineated from the peak amplitude of the analytical signal. An appropriately designed Savitzky–Golay derivative filter and Hilbert–Noda transformation matrix are used as robust and efficient data‐processing tools. An additional tool, Newton's root finding algorithm, is proposed for locating the zero crossings. The reconstructed data fit is validated using a statistical appraisal, and the estimated model parameters are defined with an uncertainty measure using the confidence limit. The proposed technique has been applied to synthetic data generated from a model depicting a realistic example, and also on field data from two examples, one of which closely resembles the example of synthetic data and the other as a benchmark example. Numerical experiments validate the applicability of the method.

Interpretation of Aeromagnetic Data of Part of Gwagwalada Abuja Nigeria for Potential Mineral Targets

This study analyzes aeromagnetic data over a section of Gwagwalada in Abuja. The data were obtained from the Nigerian Geological Survey Agency acquired at 100 m terrain clearance. The study area spans longitudes 7.0875 E to 7.1458 E and latitude 8.9625 N to 9.0 N (about 27 km2). The dataset was reduced to the equator (RTE) and downward continued by 50 m. Analytic signal filter was applied on TMI-RTE grid to detect the edges of the magnetic bodies present. The structure was observed to trend NE-SW. The CET lineament map reveals intersections such as junctions and corners on the map. This revealed structure liable for potential mineralization zone. Euler deconvolution technique applied over the transformed dataset ascertain the location and depth of the structure,having a maximum depth of about 421 m and a minimum of about 59 m.Variation in magnetic depth and susceptibility contrast is specified by the gridded SPI depth map.

Amide proton transfer-weighted MRI in the clinical setting – correlation with dynamic susceptibility contrast perfusion in the post-treatment imaging of adult glioma patients at 3T

IntroductionWe investigated the correlation between amide proton transfer-weighted magnetic resonance imaging (APTw MRI) and dynamic susceptibility contrast (DSC) perfusion in order to assess the potential of APTw MRI as an alternative to DSC in adult brain tumor (glioma) imaging. MethodsAfter Ethical Committee approval, forty adult patients, treated for histopathologically confirmed glioma (World Health Organization (WHO) grade II-IV), were prospectively imaged at 3 Tesla (3 T) with DSC perfusion and a commercially available three-dimensional (3D) APTw sequence. Two consultant neuroradiologists independently performed region of interest (ROI) measurements on relative cerebral blood volume (rCBV) and APTw maps, co-registered with anatomical images. The correlation APTw MRI-DSC perfusion was assessed using Spearman's rank-order test. Inter-observer agreement was evaluated by the intraclass correlation coefficient (ICC) and Bland-Altman (BA) plots. ResultsA statistically significant moderately strong positive correlation was observed between maximum rCBV (rCBVmax) and maximum APTw (APTwmax) values (observer 1: r = 0.73; p < 0.01; observer 2: r = 0.62; p < 0.01). We found good inter-observer agreement for APTwmax (ICC = 0.82; 95% confidence interval (CI) 0.66–0.90), with somewhat broad outer 95% CI for the BA Limits of Agreement (LoA) (−1.6 to 1.9). ICC for APTwmax was higher than ICC for rCBVmax (ICC = 0.74; 95%; CI 0.50–0.86), but the difference was not statistically significant. ConclusionAPTwmax values correlate positively with rCBVmax in patients treated for brain glioma. APTw imaging is a reproducible technique, with some observer dependence. Results need to be confirmed by a larger population analysis. Implications for practiceAPTw MRI can be a useful addition to glioma follow-up imaging and a potential alternative to DSC perfusion, especially in patients where contrast agent is contraindicated.

Predicting 1p/19q codeletion status using diffusion-, susceptibility-, perfusion-weighted, and conventional MRI in IDH-mutant lower-grade gliomas.

Isocitrate dehydrogenase (IDH)-mutant lower-grade gliomas (LGGs) are further classified into two classes: with and without 1p/19q codeletion. IDH-mutant and 1p/19q codeleted LGGs have better prognosis compared with IDH-mutant and 1p/19q non-codeleted LGGs. To evaluate conventional magnetic resonance imaging (cMRI), diffusion-weighted imaging (DWI), susceptibility-weighted imaging (SWI), and dynamic susceptibility contrast perfusion-weighted imaging (DSC-PWI) for predicting 1p/19q codeletion status of IDH-mutant LGGs. We retrospectively reviewed cMRI, DWI, SWI, and DSC-PWI in 142 cases of IDH mutant LGGs with known 1p/19q codeletion status. Features of cMRI, relative ADC (rADC), intratumoral susceptibility signals (ITSSs), and the value of relative cerebral blood volume (rCBV) were compared between IDH-mutant LGGs with and without 1p/19q codeletion. Receiver operating characteristic curve and logistic regression were used to determine diagnostic performances. IDH-mutant and 1p/19q non-codeleted LGGs tended to present with the T2/FLAIR mismatch sign and distinct borders (P < 0.001 and P = 0.038, respectively). Parameters of rADC, ITSSs, and rCBVmax were significantly different between the 1p/19q codeleted and 1p/19q non-codeleted groups (P < 0.001, P = 0.017, and P < 0.001, respectively). A combination of cMRI, SWI, DWI, and DSC-PWI for predicting 1p/19q codeletion status in IDH-mutant LGGs resulted in a sensitivity, specificity, positive predictive value, negative predictive value, and an AUC of 80.36%, 78.57%, 83.30%, 75.00%, and 0.88, respectively. 1p/19q codeletion status of IDH-mutant LGGs can be stratified using cMRI and advanced MRI techniques, including DWI, SWI, and DSC-PWI. A combination of cMRI, rADC, ITSSs, and rCBVmax may improve the diagnostic performance for predicting 1p/19q codeletion status.

Magnetic susceptibility imaging of human habenula at 3\xa0T

The habenula plays an important role in brain reward circuitry and psychiatric conditions. While much work has been done on the function and structure of the habenula in animal models, in vivo imaging studies of the human habenula have been relatively scarce due to its small size, deep brain location, and lack of clear biomarkers for its heterogeneous substructure. In this paper, we report high-resolution (0.5 × 0.5 × 0.8 mm3) MRI of the human habenula with quantitative susceptibility mapping (QSM) at 3 T. By analyzing 48 scan datasets collected from 21 healthy subjects, we found that magnetic susceptibility contrast is highly non-uniform within the habenula and across the subjects. In particular, we observed high prevalence of elevated susceptibility in the posterior subregion of the habenula. Correlation analysis between the susceptibility and the effective transverse relaxation rate (R2*) indicated that localized susceptibility enhancement in the habenula is more associated with increased paramagnetic (such as iron) rather than decreased diamagnetic (such as myelin) sources. Our results suggest that high-resolution QSM could make a potentially useful tool for substructure-resolved in vivo habenula imaging, and provide a groundwork for the future development of magnetic susceptibility as a quantitative biomarker for human habenula studies.

A focused and constrained 2D inversion of potential field geophysical data through Delaunay triangulation, a case study for iron-bearing targeting at the Shavaz deposit in Iran

This work illustrates the performance of a focused algorithm for 2D inversion of potential field geophysics (gravity and magnetic) in regions with rugged topography. A non-structured model discretization mesh is performed through Delaunay triangular cells to accurately represent the rugged observation surface and recover complex-shaped sources. Several synthetic single and multi-source targets are simulated to evaluate the applicability of the focused algorithm for simultaneously preserving the edges, correct depth estimation and body distribution of the inverted models. The algorithm is written in such a way that it is able to improve the modeling result per iteration by imposing physical properties constraining (magnetic susceptibility or density contrast), depth weighting and focused term in the central system of linear equation. Whereby a preconditioned conjugate gradient scheme is used iteratively to solve efficiently the L2-norm cost function comprising of a data misfit norm and a model stabilizer. Potential field data pertaining to a real case study for iron-bearing targeting are also inverted along two profiles to investigate the geometry of the sought targets at the Shavaz deposit in Iran. Iron-oxide ore occurrence was approved by exploratory drilling indicating a shallow hematite target with minor compounds of magnetite. Of note is that the significant point of this study lies in accurate recovering of causative sources responsible for potential field anomaly when a severe undulating topography occurs.

Two Dimensional Modeling of Basaltic Rocks Intrusion Based on The Local Magnetic Anomalies Data in Jatilawang District Banyumas Regency

Two dimensional modeling to basaltic rocks intrusion in Pekuncen and Karanglewas Villages Jatilawang District, Banyumas Regency, Central Java based on the local magnetic anomalies data has been carried out in March – June 2020. The amount of magnetic data obtained from the acquisition in the field was 239 data stretching in position of 109.107222° – 109.134944°E and 7.561361° – 7.577306°S, with the local magnetic anomalies values ranging of -2,961.11 – 1,516.31 nT. To model anomalous sources in the subsurface in two dimensions, then the local magnetic anomalies data is transformed into pseudogravity anomalies data, so that anomalous value can be obtained as -27.815 – 41.087 mGal. Based on the pseudogravity anomalous map, the basaltic rock intrusion is interpreted to be located in the eastern part of the research area, so modeling of anomalous sources is conducted in this area. The results of 2D-modeling to local magnetic anomalies data indicate the presence of anomalous object interpreted as basaltic rock intrusion with magnetic susceptibility contrast value of 0.0223 cgs, located at depth of 52.61 – 505.97 m and a lateral length of 1777.94 m. This rock intrudes sediment rock from the Halang Formation and is connected to other basaltic rock near the surface with magnetic susceptibility contrast value of 0.0165 cgs, located at depth of 1.94 – 80.90 m and lateral length of 751.83 m. The results of lithological interpretation are in accordance with the geological information of the research area.

Deciphering channel networks from aeromagnetic potential field data: the case of the North Sea Quaternary tunnel valleys

Summary High-resolution magnetic data and potential field methods have been used to perform a detailed analysis of networks of late Quaternary subglacially-cut tunnel valleys (central Viking Graben, Norwegian sector of the North Sea). High-frequency, ribbon-like, sinuous, paired magnetic anomalies interpreted to be the signature of tunnel valleys are identified. Such magnetic anomalies have 1 to 8 nT amplitudes and reflect a magnetic susceptibility contrast between valley infills and the host sediments. Fractional vertical derivative and horizontal gradient transforms provide the best control on the accurately delineation of tunnel valleys by plotting automatically the extrema. The 2D forward modelling is a very effective approach to determinating the geometric parameters and magnetic susceptibility of the modeled valleys. It allows to determine the finite-width flat horizontal thin geometry as the most appropriate simple geometry to simulate the magnetic anomaly linked to a channel structure. The application of Euler deconvolution using complex algebra allows to substantiate the structural index (n = 1.5) for simple palaeovalley geometries and to determine fair valley depth estimates.

Magnetic and gravity signatures of the Kifl oil field in Iraq

Potential field methods are capable to provide satisfactory results on the tectonics and geology, especially when dealing with the hydrocarbon reservoirs and ore-bearing mineralization. In this study, magnetic and gravity survey were conducted at the west of Kifl area in central Iraq to image the subsurface geological structure and consequently the characteristics of the oil and gas reservoirs. Ground measurements were conducted on an area of 16 by 24 km with a regular grid spacing of 250 m. The results clarified the tectonic setting of the study area, where the fault system in the Kifl oil field was recognized by resembling both magnetic and gravity anomalies with distinct trends along NS and NW. A 3D inversion algorithm based on a weighted minimum length solution was used for reconstruction of the subsurface physical properties of magnetic susceptibility and density contrast. Weighting matrix is composed of multiplication of compactness and depth weighting functions, and truncated singular value decomposition (TSVD) was used for regularization. 3D inversion was first applied to a synthetic model to simulate the geological reality of the study area, while satisfactory results were obtained. Implementing the algorithm on the real potential field data, the physical properties of the hydrocarbon reservoir were determined. Four cross sections have been extracted from the inversion result to show geometric characteristics (depth and shape) of the main reservoir in the Kefl oil field. The results indicated the presence of a fault system in the middle area within a column of sedimentary sequences represented as a graben-horst structure. This phenomenon in the area could be attributed to the tectonic deformation and tension over the area and the Eurasian and Arabian plates collided back to the Permian - Paleocene periods.

Quantitative Susceptibility Mapping of Brain Function During Auditory Stimulation

Functional quantitative susceptibility mapping (fQSM) is a recently established method that, based on the same acquisition technique as conventional functional magnetic resonance imaging, has two very appealing features: 1) it is quantitative and 2) it is considerably less affected by nonlocal effects than the blood oxygenation level-dependent (BOLD) signal. Here, the response of the auditory cortex to the presentation of short acoustic stimuli has been studied with fQSM. fQSM responses to stimuli were obtained by using a data-driven approach based on signal deconvolution to avoid assumptions regarding the response shape. fQSM and BOLD responses showed similar shapes, however, while the majority (82%) of fQSM responses described transient decreases in magnetic susceptibility (explainable by the same mechanisms behind the typical, positive BOLD responses), some voxels exhibited transient increments in magnetic susceptibility, which might reflect the interplay among changes in fractional oxygen saturation, cerebral blood flow, and volume. Statistically significant fQSM responses were observed not only in veins (which constitute the largest sources of magnetic susceptibility contrast) but also in gray matter tissue, suggesting that this technique is sensitive to subtle, stimulus-dependent changes also in the parenchyma, and therefore it might be useful to localize and interpret the sources of activation.

Magnetization exchange in a single pore due to diffusion in internal fields: Simulation and experiment

Nuclear magnetic resonance measurements of spin relaxation are used in studies of liquid-saturated porous media where multidimensional relaxation correlations probe interpore diffusion and other transport processes. However, the magnetic susceptibility contrast between the solid and liquid results in pore-scale field inhomogeneities that influence all measurements of transverse ${T}_{2}$ relaxation time. In a previous publication we conjectured that experimentally observed exchange phenomena can correspond to intrapore diffusion between localized volumes of coherent magnetization generated by these internal gradients, rather than interpore diffusion. Here, we use a finite-element method to explore the decay of magnetization in a single pore (interstice between spheres) for the Carr-Purcell-Meiboom-Gill and two-dimensional ${T}_{2}\text{\ensuremath{-}}{T}_{2}$ exchange experiments. These simulations permit direct visualization of the time-dependent distribution of magnetization in the pore. The chosen grain size and susceptibility contrast are matched to values for Bentheimer sandstone, allowing for a comparison of simulated and experimental results. Despite the simplicity of the model pore geometry, the salient features of the magnetization decay observed experimentally are reproduced in simulation. We demonstrate that intrapore magnetization transfer can explain observations of diffusive exchange in porous materials characterized by a monomodal pore size distribution and large internal gradients.

Detecting the propped fracture by injection of magnetic proppant during fracturing

Hydraulic fracturing is key to enhancing the productivity of shale and tight hydrocarbon reservoirs, as well as geothermal resources from hot dry rock. Assessing the range and efficiency of hydraulic fracturing is very important for onsite fracturing operations and decision making on subsequent fracturing plans and well-pattern deployment. The idea of injecting fluid mixed with magnetic material into a formation to detect subterranean structure has been considered for a long time. The detectability of the characteristics of propped fractures using magnetic field measurement is discussed and evaluated with an analytical model. The magnetic proppants synthesized with superparamagnetic nanoparticles should be “silent” during injection and could be activated by an external current. By measuring the magnetic field before and after the fracture was created and the magnetic proppants were magnetized, the magnetic anomalies can be obtained. The result indicates that the magnetic field caused by a fracture filled with magnetic proppants is very weak, so that magnetometers with higher precision and resolution should be used to capture such subtle signal. The higher susceptibility contrast of the magnetic nanoparticles is, the stronger external magnetic induction field could be detected. It is suggested that the superparamagnetic nanomaterial with effective magnetic susceptibility contrast of greater than 0.4–0.5 should be used for fracture monitoring in deep wells. To infer the fracture characteristics with the magnetic anomaly, the ground and adjacent wells’ monitoring practices are recommended.

Interpreting regional 3D fault networks from integrated geological and geophysical data sets: An example from the Guichon Creek batholith, British Columbia

The effects of fault processes on rocks commonly include a lowered magnetic susceptibility and an increased predisposition to erosion. We present a workflow that uses these characteristics of faults and geological observations to interpret a 3D fault network. The location of faults on the topographic surface is first interpreted by a multi-layer lineament mapping method that includes geological and geophysical data sets. We then demonstrate how magnetic intensity data can be used to estimate the dip of fault-related magnetic anomalies by performing 2D inverse modeling along profiles extracted from a magnetic intensity grid. The accuracy of the method is assessed by modeling the dip of low magnetic anomalies of known geometry created in a synthetic 3D magnetic susceptibility model. Modeled dips are consistently accurate to better than 5° for input dips >60°. A requirement of the method, however, is that the magnetic susceptibility contrast between the background field and the magnetic anomaly must be >75% for the anomaly to be accurately modeled. Comparison between fault orientations modeled from magnetic data and measured in the field in the Guichon Creek batholith confirms that the methodology can be successfully applied to brittle faults in real, albeit relatively simple geological environments.