Apparent Magnetic Susceptibility Research Articles

3D linear inversion of magnetic susceptibility data acquired by frequency domain EMI

Low induction number EMI instruments are able to simultaneously measure a soil's apparent magnetic susceptibility and electrical conductivity. This family of dual measurement instruments is highly useful for the analysis of soils and archeological sites. However, the electromagnetic properties of soils are found to vary over considerably different ranges: whereas their electrical conductivity varies from ≤0.1 to ≥100mS/m, their relative magnetic permeability remains within a very small range, between 1.0001 and 1.01 SI. Consequently, although apparent conductivity measurements need to be inverted using non-linear processes, the variations of the apparent magnetic susceptibility can be approximated through the use of linear processes, as in the case of the magnetic prospection technique.Our proposed 3D inversion algorithm starts from apparent susceptibility data sets, acquired using different instruments over a given area. A reference vertical profile is defined by considering the mode of the vertical distributions of both the electrical resistivity and of the magnetic susceptibility. At each point of the mapped area, the reference vertical profile response is subtracted to obtain the apparent susceptibility variation dataset. A 2D horizontal Fourier transform is applied to these variation datasets and to the dipole (impulse) response of each instrument, a (vertical) 1D inversion is performed at each point in the spectral domain, and finally the resulting dataset is inverse transformed to restore the apparent 3D susceptibility variations.It has been shown that when applied to synthetic results, this method is able to correct the apparent deformations of a buried object resulting from the geometry of the instrument, and to restore reliable quantitative susceptibility contrasts. It also allows the thin layer solution, similar to that used in magnetic prospection, to be implemented. When applied to field data it initially delivers a level of contrast comparable to that obtained with a non-linear 3D inversion. Over four different sites, this method is able to produce, following an acceptably short computation time, realistic values for the lateral and vertical variations in susceptibility, which are significantly different to those given by a point-by-point 1D inversion.

THE DIFFERENT MAGNETIC RESULTS OF ANEMI AND PPM MEASUREMENTS ON THE BURIED REMAINS OF A 13TH CENTURY FORTRESS

This study aimed to evaluate the different magnetic results of an electromagnetic induction with proton magnetometer measurements on an archaeological site. The electromagnetic induction allows measuring both the apparent magnetic susceptibility in part per thousand (ppt) and the apparent electrical conductivity in millisiemens (mS/m). A proton magnetometer measures the total magnetic intensity in nanotesla (nT), caused by the induced and remanent magnetisations. An archaeological site where historical documents indicated the presence of a 13th century fortress that built by Lamuri Sultanate was selected as a test area. The measurement were conducted by divided the study area into 10 profiles.Some standard data processing have been applied to the measured data. The result of the first survey with electromagnetic induction showed low magnetic anomalies in the buried remains of Lamuri fortress. The similar value are shown as well by low magnetic field intensity in magnetometer measurement.

Neuromelanin Imaging and Dopaminergic Loss in Parkinson's Disease.

Parkinson's disease (PD) is a progressive neurodegenerative disorder in which the major pathologic substrate is a loss of dopaminergic neurons from the substantia nigra. Our main objective was to determine the correspondence between changes in the substantia nigra, evident in neuromelanin and iron sensitive magnetic resonance imaging (MRI), and dopaminergic striatal innervation loss in patients with PD. Eighteen patients and 18 healthy control subjects were included in the study. Using neuromelanin-MRI, we measured the volume of the substantia nigra and the contrast-to-noise-ratio between substantia nigra and a background region. The apparent transverse relaxation rate and magnetic susceptibility of the substantia nigra were calculated from dual-echo MRI. Striatal dopaminergic innervation was measured as density of dopamine transporter (DAT) by means of single-photon emission computed tomography and [123I] N-ω-fluoropropyl-2b-carbomethoxy-3b-(4-iodophenyl) tropane. Patients showed a reduced volume of the substantia nigra and contrast-to-noise-ratio and both positively correlated with the corresponding striatal DAT density. The apparent transverse relaxation rate and magnetic susceptibility values of the substantia nigra did not differ between patients and healthy controls. The best predictor of DAT reduction was the volume of the substantia nigra. Clinical and imaging correlations were also investigated for the locus coeruleus. Our results suggest that neuromelanin-MRI can be used for quantifying substantia nigra pathology in PD where it closely correlates with dopaminergic striatal innervation loss. Longitudinal studies should further explore the role of Neuromelanin-MRI as an imaging biomarker of PD, especially for subjects at risk of developing the disease.

The Characterization of a Former World War I Battlefield by Integrating Multiple Signals from a Multireceiver EMI Soil Sensor

World War I (WWI) battlefields contain a large diversity of buried features that are situated just below the plough layer. A noninvasive interdisciplinary approach is preferred to characterize the buried heritage embedded within its soil landscape. In this study, the potential of integrating both electrical conductivity and magnetic susceptibility measurements from a multireceiver electromagnetic induction (EMI) instrument at a heavily shelled WWI battlefield was evaluated. Primarily, both measurements allowed the attribution of approximate depth interval to the buried trench remains. These remains proved dominantly present within the 0.4–0.8 m depth interval. Subsequently, both apparent electrical conductivity and magnetic susceptibility measurements were separately integrated to improve the contrast between buried features and their surrounding background. Trenches that were ditches filled with topsoil material could be distinguished from trenches that were continuously or partially filled with metal reinforcements or debris. To conclude, integrating multiple signals from a multireceiver EMI system allowed for a more straightforward discrimination of the buried WWI relicts.

Demonstrating the contribution of dielectric permittivity to the in‐phase EMI response of soils: example from an archaeological site in Bahrain

ABSTRACTElectromagnetic induction instruments (also called loop–loop, dipole–dipole, or Slingram) are now commonly used in archaeological prospecting. They are truly light instruments, which are able to measure both the apparent electrical conductivity and the apparent magnetic susceptibility of the ground. During a field test in Bahrain Island where the soil has a high clay content and a high salt content, surprisingly high values of in‐phase response were obtained at all inter‐coil spacings using CMD “Mini‐Explorer” (GF Instruments Ltd., Brno, Czech Republic) at 30 kHz, in both horizontal coplanar and vertical coplanar configurations, and the horizontal coplanar and vertical coplanar susceptibility variations were in total opposition. This apparent discrepancy is explained by considering the in‐phase responses to be dominated by the relative dielectric permittivity. Using the raw and in‐phase vertical coplanar and horizontal coplanar data, it is possible to determine and map the apparent permittivity and apparent magnetic susceptibility. For this case of slated soils with high clay content, the relative permittivity is strong but in agreement with both experimental data at lower frequencies and theoretical models reported in the literature.

Interpretation of shallow electromagnetic instruments resistivity and magnetic susceptibility measurements using rapid 1D/3D inversion

We have developed an inversion process of electromagnetic induction (EMI) data based on a two-step approach with 1D inversion of the entire studied surface and a fast 3D inversion applied over limited areas. This process is similar to that formerly used in resistivity prospection. For the study of soil (environmental, engineering, or archaeological explorations), low-frequency electromagnetic instruments (referred to as Slingram EMI) have highly useful specificities. They are light, are easy to move in the field, and can simultaneously measure the ground’s electric conductivity and magnetic susceptibility; they have thus been used to map these properties over large surface areas, within relatively short periods of time, and at reasonable expense. The possibility of combining several coil geometries has opened up the potential for multidepth techniques and systematic 1D inversion, which are found to be sufficiently revealing to allow larger portions of surveyed areas to be analyzed. In the “targeted areas” selected for 3D inversion, the geometries of the 3D features and the resistivity and/or susceptibility contrasts are determined. This step is based on the method of moments, where only 3D heterogeneities are meshed, and only a small number of major characteristics, such as contrast, thickness, width, etc., are sought. We first applied this process to synthetic data, then to data acquired at an experimental test site, and finally to field cases. The rapid 3D inversion complements the 1D inversion by solving a series of issues: correction for the apparent anisotropy generated by the instrument configuration, multiarched anomalies, precise location of lateral changes, and determination of the properties contrasts. Our inversion results highlighted the importance of the instrument geometry. We also have determined that apparent magnetic susceptibility data can be more appropriate for the determination of the volume of man-made features and can be highly complementary to conductivity data.

An airborne magnetometry study across Zagros collision zone along Ahvaz–Isfahan route in Iran

Convergence between the Eurasian and Arabian plates formed the Zagros orogenic belt between Late Cretaceous and Pliocene as a relatively young and active fold-thrust belt in Iran. The structural geology along Ahvaz to Isfahan route across Zagros is investigated employing magnetic data in order to determine the crustal structure in the collision zone of the two Palaeo-continents. Airborne magnetometry data with a line space of survey of 7.5km have been used to image the variations of the apparent magnetic susceptibility along this route. At first the airborne data were stably 500-m downward continued to the ground surface in order to enhance subtle changes of the Earth's magnetic field. Then 3D inverse modeling of magnetic data was implemented, while the cross section of the magnetic susceptibility variations along the route was mapped down to a depth of 100km. The acquired magnetic susceptibility model could appropriately predict the observed magnetic data as well. In addition, the analytic signal filter was applied to the reduced-to-pole magnetic data leading to the determination of active faults in Zagros fold-thrust belt (ZFTB) structural zone based upon the generated peaks. Some probable locations of fault events were also suggested in Sanandaj–Sirjan Zone (SSZ). The locations of faults correspond well to the magnetic susceptibility variations on the inverted section. Probable direction, slope and depth extension of these faults were also plotted on the magnetic susceptibility model, showing an intensively tectonized zone of the SSZ. The main difference between two domains is that the Eurasian plate seems to contain high magnetic susceptible materials compared to the Arabian plate. The recovered model of the apparent magnetic susceptibility values indicated that the average thickness of the non-magnetic sedimentary units is about 11km and the Curie depth locates approximately at depth of 24km for the whole studied area.

Geophysical models of the Montresor metasedimentary belt and its environs, central Nunavut, Canada

Recent identification of hydrothermally altered rocks and breccia in the underexplored Montresor belt of Paleoproterozoic metasedimentary rocks suggests the possible presence of undiscovered mineralization. This study examines potential field data from the region with the goal of identifying subsurface features that could be associated with or serve as vectors to mineralization (subsurface alteration zones, faulting and (or) igneous intrusions). Gravity data were used to model regional and local geological features using known geology and physical properties from the study area and environs as constraints, and documents dense intrusive bodies underlying the Paleoproterozoic sequences. Maps of transformed apparent magnetic susceptibility values outline corridors of weak magnetization that correspond to observed zones of non-magnetic breccia and epidote–hematite–quartz alteration. Imputing the apparent susceptibility and rock property information into a magnetic forward model defines the geometry of this alteration zone, which is best explained as a northerly dipping non-magnetic or demagnetized, metasomatized intrusive sheet. The presence of previously undocumented igneous intrusions, their association with demagnetized hydrothermal breccia, and the continuity of the demagnetized zone suggests additional prospective areas within the region. This geological–geophysical framework for the nature and geometry of the Montresor belt and its surrounds highlights the importance of integrated modelling for areas with limited data.

Combining multi‐receiver electromagnetic induction and stepped frequency ground penetrating radar for industrial site investigation

SummaryThe soil at industrial sites is frequently characterized by very heterogeneous properties, which are often related to physical disturbance and contamination. A conventional approach to characterize the soil, with only a limited number of invasive observations, fails to capture the full extent of soil heterogeneity. Proximal soil sensing provides efficient tools to record spatially dense soil information. Nevertheless, because the output of most sensors is affected by more than one soil property, the simultaneous characterization of different soil properties requires the use of multiple sensors. Here, we apply multi‐receiver electromagnetic induction (EMI) and stepped frequency ground penetrating radar (GPR) to survey a former gasworks site in a seaport area of Belgium. We used the EMI and GPR sensors in a motorized system to obtain densely sampled measurements of apparent electrical conductivity, apparent magnetic susceptibility and contrasts in relative dielectric permittivity. Our study shows that the sensors give detailed information on the variation in these electromagnetic soil properties. Interpretation of the variation in terms of the stratification of the soil was hampered by localized anthropogenic disturbances. However, the sensors provided complementary information that enabled the identification, discrimination and accurate location of several of these localized disturbances, including underground utility services such as electric cables, buried structures such as the remains of foundations and contamination by salts. Because these represent typical targets in industrial site investigation, we conclude that multi‐receiver EMI and stepped frequency GPR provide a useful set of tools to expedite the investigation of industrial sites.

Combining EMI and GPR for non‐invasive soil sensing at the Stonehenge World Heritage Site: the reconstruction of a WW1 practice trench

SummaryIncreasingly, conventional soil sampling procedures face restrictions because of their destructive character. Hence there is a growing interest in non‐invasive techniques, on which proximal soil sensors are based. There is great interest in applying proximal soil sensing to improve the characterization of the buried heritage embedded in the soil landscape at sites such as the Stonehenge World Heritage Site, UK. Because calibration and validation procedures based on invasive practices are unconventional, we turned to the investigation of a well‐documented practice trench dug during the First World War (WW1) close to the prehistoric Stonehenge monument. A methodology was tested that would simultaneously invert frequency‐domain ground‐penetrating radar (GPR) and multi‐receiver electromagnetic induction (EMI) data, with the aim of reconstructing the trench network. This trench network could not be distinguished on the EMI apparent electrical conductivity (σa) measurements, but appeared on the apparent magnetic susceptibility (κa) data. The GPR measurements showed the trench infilling as strong reflections contrasting with the surrounding soil. However, converting the two‐way travel times to absolute depths requires knowledge of the relative permittivity (εr). Because of the preference for non‐invasive observation in this protected landscape, we developed a procedure integrating the GPR measurements with κa measurements obtained with EMI. A fitting procedure, assuming a constant susceptibility and permittivity of the sub‐surface layers, allowed us to estimate both the susceptibility of the trench fill and the surrounding soil material, and the εr value of the material above and within the trench. This provided absolute depth values for the GPR reflection data, improving the lateral and vertical reconstruction of the trench system. Moreover, these results allowed depth slices to be determined from EMI κa data. So, integrating both GPR and EMI measurements enabled the detailed reconstruction of the buried trench network at Stonehenge, offering new perspectives on the investigation of features buried within the soil of protected sites.

Density, porosity and magnetic susceptibility of the Košice meteorite shower and homogeneity of its parent meteoroid

Bulk and grain densities, porosities, and magnetic susceptibilities of 67 individuals of Košice H chondrite fall were measured. The mean bulk and grain densities were determined to be 3.43g/cm3 with standard deviation (s.d.) of 0.11g/cm3 and 3.79g/cm3 with s.d. 0.07g/cm3, respectively. Porosity is in the range from 4.2% to 16.1%. The logarithm of the apparent magnetic susceptibility (in 10−9m3/kg) shows narrow distribution from 5.17 to 5.49 with mean value at 5.35 with s.d. 0.08.These results indicate that all studied Košice meteorites are of the same composition down to ~g scale without the presence of foreign (non-H) clasts and are similar to other H chondrites. Košice is thus a homogeneous meteorite fall derived from a homogeneous meteoroid.

Quantitative magnetic susceptibility of the developing mouse brain reveals microstructural changes in the white matter

Cerebral development involves a complex cascade of events which are difficult to visualize and quantify in vivo. In this study we combine information from Diffusion Tensor Imaging (DTI) and Quantitative Susceptibility Mapping (QSM) to analyze developing mouse brains at five stages up to 56days postnatal. Susceptibility maps were calculated using frequency shifts in gradient echo MR images acquired at 9.4T. The mean apparent magnetic susceptibility and magnetic susceptibility anisotropy of major white matter tracts were evaluated as a function of age. During the first two weeks, susceptibility of white matter appeared paramagnetic relative to surrounding gray matter; it then gradually became more diamagnetic. While diffusion anisotropy was already apparent and high at postnatal day 2, susceptibility anisotropy only became significant during the third week. This mismatch indicated different microstructural underpinnings for diffusion anisotropy and susceptibility anisotropy. Histological exams were also performed to evaluate myelin and iron content. It is confirmed that the main source of susceptibility contrast in WM is the myelin content. The ability to quantify the magnetic properties of white matter will provide valuable information on the architecture of the brain during development and potentially a more specific indicator for myelin degenerative diseases.

Micromagnetic technology for detection of carbon impurity in crystalline silicon

This paper proposes a method of lossless micromagnetic detection in the geomagnetic field for detecting traces of carbon impurity defects in crystalline silicon. The magnetization tests show that crystalline silicon is a diamagnetic substance with a stronger relative permeability than carbon. Micromagnetic decay theory is gained according to the energy decay. When the geomagnetic field penetrates through the materials, the apparent magnetic susceptibility can be calculated and subsequently used to project the images. The resulting image clearly showed the location of the defects. Test results are proved by the metallographic phase and spectral analysis. New method and ideas are provided for effective detection of trace carbon impurity defects in the crystalline silicon.

Comparing Apparent Magnetic Susceptibility Measurements of a Multi‐receiver EMI Sensor with Topsoil and Profile Magnetic Susceptibility Data over Weak Magnetic Anomalies

ABSTRACTToday, most surveys in archaeogeophysical prospection use magnetic properties to detect archaeological features. Such magnetic surveys are usually conducted with magnetometers and, to a lesser extent, with magnetic susceptibility meters and electromagnetic induction (EMI) sensors. Although the latter are the only instruments that allow mapping multiple physical soil properties simultaneously, EMI remains the odd‐one‐out in archaeogeophysical prospection. Nevertheless, by simultaneously recording the electric and magnetic soil variability, EMI survey can be beneficial in early archaeological evaluation stages, because detailed pedological and archaeological information is gathered at the same time. Furthermore, by using multi‐receiver EMI instruments vertical soil variation also can be integrated into the survey. However, although the potential of EMI for mapping electric soil variations is well known from advances in soil science, magnetic susceptibility measurements have been investigated less. Here we show the potential of a multi‐receiver EMI survey to detect weak magnetic anomalies by measuring the apparent magnetic susceptibility (κa) of multiple soil volumes at a test site. The shallowκadata were compared with topsoil susceptibility measurements using a magnetic susceptibility loop sensor survey, and with magnetic susceptibility profiling using a probe sensor for evaluating the deeperκa‐data. Further comparisons were made between these datasets and aerial photography and field walking data. We found that the multiple EMIκameasurements allowed for a straightforward discrimination of the natural and anthropogenic magnetic variations of shallow and deeper soil volumes, and allowed visualizing weak magnetic anomalies. Copyright © 2013 John Wiley & Sons, Ltd.

Application of 3D Apparent Physical Properties Inversion Method in an Iron Mine Area

A rapid method for 3D physical properties inversion based on separation and continuation of potential field are introduced in this paper. The inversion technique characterized by faster computing speed, it does not need to divide the underground field sources region into many rectangular blocks in one time and does not need to solve linear algebraic equations as well. So, this technique can overcome the requirement of large computer memory, unstable solution and computation time bottleneck which hinders the application of 3D inversion to practice. The method is used to apparent magnetic susceptibility inversion of magnetic anomaly in a iron mine area, the inversion results show that the method has good accuracy, and the advantages of strong stability and a high computation speed, and is suitable for 3D inversion of magnetic data in large area.

Integrating multi‐receiver electromagnetic induction measurements into the interpretation of the soil landscape around the school of gladiators atCarnuntum

SummaryRecently, the unique foundations of a school of gladiators were discovered in theRoman town ofCarnuntum (40 km southeast ofVienna,Austria) by applying a combination of non‐invasive archaeological prospection techniques such as magnetometry, ground penetrating radar, aerial photography, airborne laser scanning and airborne imaging spectroscopy. Although the well‐preserved remains of the building complex were revealed down to a depth of 1.8 m by high‐resolution near‐surface geophysics, some questions about the surrounding soil landscape remained unanswered. Therefore, a proximal soil sensing procedure based on a survey with a multi‐receiver electromagnetic induction (EMI) instrument was conducted to interpret the surroundings of the school, covering an area of 5.6 ha. We investigated the usefulness of integrating the complementary apparent electrical conductivity (ECa) and apparent magnetic susceptibility (MSa) measurements for the mapping and investigation of this soil landscape. The multipleECameasurements allowed the identification of zones with low‐conductive gravel outcrops, and zones where silty‐clayey soils were deposited on top of the underlying gravel. AnEC‐depth slicing procedure enhanced the contrast between small soil features, such as frost‐wedge pseudomorphs and drainage gullies, and their background, and provided indications about the depth extent of these features. TheMS‐depth slices showed the foundations of the school of gladiators, an aqueduct and grave monuments. After combining these results with the topography, an integrated visualization of the school in its soil landscape was obtained. This study demonstrated the potential of multi‐receiverEMIsoil surveys to map and interpret the soil landscape and to discern small natural as well as archaeological features.

Electromagnetic induction sounding and 3D laser imaging in support of a Mars methane analogue mission

The Mars Methane Analogue Mission simulates a micro-rover mission whose purpose is to detect, analyze, and determine the source of methane emissions on the planet's surface. As part of this project, both an electromagnetic induction sounder (EMIS) and a high-resolution triangulation-based 3D laser scanner were tested at the Jeffrey open-pit asbestos mine to identify and characterize geological environments favourable to the occurrence of methane. The presence of serpentinite in the form of chrysotile (asbestos), magnesium carbonate, and iron oxyhydroxides make the mine a likely location for methane production.The EMIS clearly delineated the contacts between the two geological units found at the mine, peridotite and slate, which are separated by a shear zone. Both the peridotite and slate units have low and uniform apparent electrical conductivity and magnetic susceptibility, while the shear zone has much higher conductivity and susceptibility, with greater variability. The EMIS data were inverted and the resulting model captured lateral conductivity variations through the different bedrock geological units buried beneath a gravel road.The 3D point cloud data acquired by the laser scanner were fitted with triangular meshes where steeply dipping triangles were plotted in dark grey to accentuate discontinuities. The resulting images were further processed using Sobel edge detection to highlight networks of fractures which are potential pathways for methane seepage.

An Interdisciplinary Non‐invasive Approach to Landscape Archaeology of the Great War

ABSTRACTThe prospection and evaluation of former battlefields of the Great War or the First World War (WW I) poses specific challenges. For several reasons, large‐scale excavation campaigns of this conflict landscape are problematic. The vastness of the former Western Front (one of the largest archaeological sites in the world), the large amounts of buried unexploded ordnance and the possible presence of human remains hinder invasive practices. As an alternative, an integrated approach combining a geophysical survey, contemporary aerial photographs and a topographic model is proposed. This approach was evaluated for a 3.2 ha WW I battlefield using a multireceiver electromagnetic induction (EMI) sensor. Integrating multiple apparent electrical conductivity (ECa) and apparent magnetic susceptibility (MSa) EMI measurements allowed evaluation of the present WW I remains in the subsoil, while comparison with WW I aerial photographs and a digital terrain model led to a comprehensive understanding of the WW I landscape. It is suggested that this approach may be of value for the investigation of battlefields in other locations and periods. Copyright © 2013 John Wiley & Sons, Ltd.

Exploring the potential of multi-receiver EMI survey for geoarchaeological prospection: A 90 ha dataset

The archaeological evaluation of landscapes in the framework of developer-led archaeology is often based on extensive trenching programmes complemented with auger surveys in wetland environments. During the archaeological evaluation of a 90ha polder site in the north-west of Belgium, a mobile multi-receiver electromagnetic induction (EMI) survey was used as a main prospecting technique. The use of a mobile survey allowed to map the entire study area at a very fine resolution (over 25 measurements per m2). Incorporating a multi-receiver EMI instrument enabled measuring the apparent electrical conductivity (ECa) and the apparent magnetic susceptibility (MSa) of four different soil volumes simultaneously at each location. The detailed maps provided insight into the archaeological and geomorphological features of the site. Among the detected structures were a large medieval farmstead, a palaeoriver system and a number of military remains from World War I (WWI). The vertical discrimination potential added insight into the vertical facies changes, which allowed modeling the palaeolandscape and helped determining the depth of detected medieval features. The different MSa measurements gave additional insight into the WWI structures. In this paper, we give an overview of the possibilities of combining multiple ECa measurements for interpreting vertical soil variability together with an example of the added information from simultaneously gathered MSa data. More generally, the diverse potential of multi-receiver EMI survey for geoarchaeological research is demonstrated.

Quantitative susceptibility mapping for investigating subtle susceptibility variations in the human brain

Quantitative susceptibility mapping (QSM) is a novel magnetic resonance-based technique that determines tissue magnetic susceptibility from measurements of the magnetic field perturbation. Due to the ill-posed nature of this problem, regularization strategies are generally required to reduce streaking artifacts on the computed maps. The present study introduces a new algorithm for calculating the susceptibility distribution utilizing a priori information on its regional homogeneity derived from gradient echo phase images and analyzes the impact of erroneous a priori information on susceptibility map fidelity. The algorithm, Homogeneity Enabled Incremental Dipole Inversion (HEIDI), was investigated with a special focus on the reconstruction of subtle susceptibility variations in a numerical model and in volunteer data and was compared with two recently published approaches, Thresholded K-space Division (TKD) and Morphology Enabled Dipole Inversion (MEDI). HEIDI resulted in susceptibility maps without streaking artifacts and excellent depiction of subtle susceptibility variations in most regions. By investigating HEIDI susceptibility maps acquired with the volunteers' heads in different orientations, it was demonstrated that the apparent magnetic susceptibility distribution of human brain tissue considerably depends on the direction of the main magnetic field.