Local Magnetic Susceptibility Research Articles

Local Magnetic Susceptibility in Rare-Earth Compounds

The element specific magnetic susceptibilities of some rare-earth compounds are estimated by measuring magnetic circular dichroism at rare-earth M 4,5 absorption edges. The temperature dependences ...

Po-topic I-02: Bold MRI applied to renal medullary hypoxia

Renal medulla operates in a hypoxic environment (1) and results in hypoxic injury when oxygenation or blood flow is further impaired. Acute renal failure (ARF) involves damage to the medulla (1) probably initiated by a reduction in medullary blood flow or increase in oxygen demand. Hence, an understanding of mechanisms that influence medullary hypoxia is important in the study of the pathophysiology of ARF. The conventional measurements of intrarenal oxygen tension depend on implanted oxygen-sensitive microelectrodes into the renal parenchyma of animals, a delicate and tedious procedure and impractical for human studies. The Blood Oxygenation Level Dependent (BOLD) magnetic resonance imaging (MRI) depends on the principle that variations in the oxygen saturation of hemoglobin result in changes in local magnetic susceptibility and hence in the signal intensity of T2∗-weighted MR images. Because the ratio of oxyhemoglobin to deoxyhemoglobin is related to the pO2 of blood, and since the pO2 of capillary blood is thought to be in equilibrium with the surrounding tissue, changes in BOLD MRI signals can be interpreted as changes in tissue pO2(2). The sigmoid relationship of blood pO2 to the hemoglobin oxygen saturation makes the BOLD MRI technique especially suited for oxygenation measurements in the hypoxic renal medulla because it falls on the linear regime.

Local magnetic susceptibility of the positive muon in the quasi-one-dimensional S=1/2 antiferromagnet dichlorobis (pyridine) copper (II).

We report muon spin rotation measurements of the local magnetic susceptibility around a positive muon in the paramagnetic state of the quasi-one-dimensional spin 1/2 antiferromagnet dichlorobis (pyridine) copper (II). Signals from three distinct sites are resolved and have a temperature dependent frequency shift which is significantly different than the magnetic susceptibility. This difference is attributed to a muon induced perturbation of the spin 1/2 chain. The obtained frequency shifts are compared with transfer matrix density-matrix renormalization-group numerical simulations.

Improved spectral quality for 3D MR spectroscopic imaging using a high spatial resolution acquisition strategy

Spectral quality in 1H MR spectroscopic imaging (MRSI) of the brain is often significantly degraded in regions subject to local magnetic susceptibility variations, which results in broadened and distorted spectral lineshapes. In this report, a modified acquisition strategy for volumetric echo-planar spectroscopic imaging (3D EPSI) is presented that extends the region of the brain that can be observed. The data are sampled at higher spatial resolution, then corrected for local B 0 shifts and reconstructed such that the final spatial resolution matches that of 3D EPSI data acquired with the conventional lower spatial resolution. Comparison of in vivo data obtained at 1.5 T with these two acquisition schemes shows that the high spatial resolution acquisition provides considerable reduction of spectral linewidths in many problematic brain regions, though with a reduction in signal-to-noise ratio by a factor of approximately 1.4 to 1.6 for the matrix sizes used in this study. However, the effect of the increased noise was largely offset by the improved spectral quality, leading to an overall improvement of the metabolite image quality obtained using automated spectral analysis.

BOLD Magnetic Resonance Imaging of Skeletal Muscle

Blood-oxygen-level-dependent (BOLD) imaging was a concept introduced in 1990 for evaluating brain activation. The method relies on magnetic resonance imaging (MRI) contrast resulting from changes in the microvascular ratio of oxyhaemoglobin (oxyHb) to deoxyhaemoglobin (deoxyHb). OxyHb is diamagnetic, whereas deoxyHb is paramagnetic, which produces a local bulk magnetic susceptibility effect and subsequent MRI signal change. The changes are typically observed in T(2)*-weighted functional MRI scans. However, there has recently been interest in BOLD as a way to evaluate microcirculation of any normal or diseased tissue. This review focuses on the application of BOLD imaging in the understanding of normal and diseased skeletal muscle. In addition we present new findings showing the possible application of BOLD imaging with hyperoxia for evaluating skeletal muscle physiology.

Local magnetic susceptibility of the positive muon in the quasi-1D S=1/2 antiferromagnet KCuF 3

We report muon spin rotation measurements of the local magnetic susceptibility around a positive muon in the paramagnetic state of the quasi-one-dimensional spin 1/2 antiferromagnet KCuF 3. Signals from two distinct sites are resolved which have a temperature-dependent frequency shift which is different than the magnetic susceptibility. This difference is attributed to a muon-induced perturbation of the spin 1/2 chain.

Influence of the sample length and profile of the magnetoimpedance effect in FeCrSiBCuNb ultrasoft magnetic wires

The magnetoimpedance (MI) effect has been investigateded in Fe63.5Cr10Si13.5B9Cu1Nb3 alloy wire exhibiting an ultrasoft magnetic character after suitable thermal treatment at 525 °C. It has been measured as a function of the sample length and of the distance to the ends of the wire (MI profile). The observed dependence on both parameters is correlated with the local particularities of the magnetization process: complex domain structures emerging near the ends to decrease the magnetostatic energy modify the local magnetic susceptibility and consequently the MI response. The present study points out the importance of geometrical dimensions of samples and measuring conditions when reporting numerical values in MI studies. This wire, showing large magnetoimpedance together with optimized soft magnetic behavior and improved corrosion properties seems to be indicated as a sensing element in GMI based sensors.

Spectrally Inhomogeneous Effects of Contrast Agents in Breast Lesion Detected by High Spectral and Spatial Resolution MRI

Improvements in MR hardware and software allow acquisition of data at very high spectral and spatial resolution so that the details of the water and fat lineshapes in each small image voxel can be resolved (1). Data obtained with high spectral and spatial resolution (HiSS) MR imaging demonstrate that the water proton resonance in small voxels is complicated, and is frequently composed of multiple well-resolved components (2–5). This inhomogeneous broadening is generally caused by local magnetic susceptibility gradients that reflect local anatomy and physiology. Previous work in this laboratory showed that effects of contrast agents in very small image voxels are often spectrally inhomogeneous (2–5). This reflects effects of the contrast agent in different sub-voxelar, perhaps microscopic environments (2–4). For example, small areas with high angiogenic activity in tumors may be detectable with increased sensitivity in HiSS images, because the water signal from these regions may be slightly shifted relative to the water signal from the rest of the voxel. The purpose of these experiments was to image contrast media uptake in breast lesions using HiSS MRI. MATERIALS AND METHODS

Functional MRI in Epilepsy

*Department of Neurology, Medical College of Wisconsin, Milwaukee, Wisconsin, U.S.A.; †University Hospital Gent, Gent, Belgium; ‡Departments of Diagnostic Radiology and Neurosurgery, Yale University, New Haven, Connecticut; §Department of Neurology, University of Pennsylvania, Philadelphia, Pennsylvania; Clinical Epilepsy Section, National Institutes of Health, Bethesda, Maryland; ¶Department of Radiology, Mayo Clinic, Rochester, Minnesota; and **Department of Neurology, Medical College of Georgia, Augusta, Georgia, U.S.A.

Differentiation of nonmetastatic and metastatic rodent prostate tumors with high spectral and spatial resolution MRI.

MR images can be acquired with high spectral and spatial resolution to precisely measure lineshapes of the water and fat resonances in each image voxel. Previous work suggests that the high-resolution spectral information can be used to improve image contrast, SNR, sensitivity to contrast agents and to physiologic and biochemical processes that affect local magnetic susceptibility gradients. The potential advantages of high-resolution spectroscopic imaging (SI) suggest that it might be useful for early detection and characterization of tumors. The present experiments evaluate the use of high-resolution SI to discriminate between metastatic and nonmetastatic rodent Dunning prostate tumors. SI datasets were obtained at 4.7 Tesla with an in-plane resolution of 350-500 micron in a single 1.0-mm slice, and 6-8 Hz spectral resolution, before and after i.v. injection of an iron oxide contrast agent. Images of water signal peak height in nonmetastatic tumors were smoother in the tumor interior than images of metastatic tumors (P <.004 by t-test) before contrast media injection. This difference was stronger in contrast-enhanced images (P <.0004). In addition, the boundary between the tumor and muscle was more clearly demarcated in nonmetastatic than metastatic tumors. Combinations of image texture, tumor edge morphology, and changes in T2* following contrast media injection improved discrimination between metastatic and nonmetastatic tumors. The data presented here do not demonstrate that effective discrimination between metastatic and nonmetastatic tumors depends on the use of high-resolution SI. However, the results suggest that SI and/or other MR methods that provide similar contrast might be used clinically for early and accurate detection of metastatic disease.

Detection of central auditory compensation in unilateral deafness with functional magnetic resonance tomography

Functional magnetic resonance imaging (fMRI) is a noninvasive method to detect focal brain activity at high spatial resolution. Acoustic stimulation induces an increase of regional cerebral blood flow in the primary auditory cortex. This entails an increased concentration of diamagnetic oxyhemoglobin in the capillaries and the venous system. The resulting decrease of the local magnetic susceptibility was detected as a signal increase in T2*-weighted images. The central auditory pathways predominantly cross to the contralateral hemisphere in normally hearing subjects. The aim of the present study was to investigate the primary auditory cortex after acoustic stimulation in unilateral deaf patients using fMRI. Magnetic resonance images were acquired on a 1.5 T Siemens Vision scanner. For fMRI, a single shot gradient recalled, echo planar imaging (EPI) sequence with decreasing excitation order was used, allowing the aquisition of 9 slices within 1.8 s. The 9 slices covered a slab of 3.6 cm in cranio-caudal extension in the region of the temporal lobes. For statistical processing of the raw image data the SPM96 software package was used. A p-value of p < 0.01 was applied to differentiate between activated and non-activated. The resulting functional activation maps were superimposed onto the EPI scan. The number of activated pixels was used to quantitate the cortical response upon acoustic stimulation. Stimulation consisted of a 1000-Hz sine tone (100 dB SPL at the distal end of the head phone, pulsed at 6 Hz) to which the patients were asked to listen passively. A piezoelectric loudspeaker was mounted on the subject table and connected to a plastic tube system leading to a combination of bilateral ear- and headphones. Auditory paradigms require disentangling experimental excitation from the scanner noise that approximates 90 dB. Headphones suppress noise by approximately 30 dB. To decrease the acoustic background-to-stimulation ratio and to keep background noise constant during stimulation and resting, we employed short scanning (1.8 s) and long resting periods (10.2 s; TR = 12 s). This acquisition mode allows sufficient recovery during off-periods and sufficient excitation during on-periods. 14 unilateral deaf patients were examined. The mean duration of deafness was 22.5 years. Acoustic stimulation of the deaf ear revealed only weak cortical activation which could be explained by sound transmission via bone conduction to the other ear. A significant increase of BOLD (blood oxygen level dependent)-activation in the primary auditory cortex could be demonstrated in all patients after stimulation of the hearing ear. However, remarkable individual differences were noticed concerning the absolute number of activated pixels. The lateralization ratio was calculated by the number of activated pixels on the hearing side divided by the number of activated pixels on the deaf side. A mean lateralization ratio of 0.9 (Stdv +/- 0.6) was found. The mean lateralization ratio for patients with a right deaf ear (n = 8) and those with a left deaf ear (n = 5) was 1.1 (Stdv +/- 0.7) and 0.6 (Stdv +/- 0.3) respectively. However, the difference was not significant (Wilcoxon test: p = 0.08). Central-auditory compensation by bilateral cortical activation was demonstrated in unilateral deaf patients. Moreover, a tendency towards a dominance of the left primary auditory cortex was found, although the difference between both hemispheres was not significant. The lateralization ratio in unilateral deaf patients is similar to findings after binaural stimulation in normally hearing subjects.

A novel method for magnetic imaging: determination of local in-plane susceptibility at a surface

Using a combined read/write head, a new instrument has been developed to determine the local in-plane magnetic susceptibility in magnitude and orientation at a micron length scale. Applications of this instrument to image the susceptibility distribution over ferromagnetic, as well as superconducting surfaces are presented. As far as we know, such microscopic magnetic information cannot be measured by other techniques.

Unified theory of dynamical mean field and self-consistently renormalized spin-fluctuations

A unified theory of the dynamical mean-field and the self-consistently renormalized spin-fluctuations (SCR) for strongly correlated electron systems is proposed. Starting from the iterative perturbation theory, local dynamical magnetic susceptibility χ L( ω) is constructed with inclusion of the vertex corrections, and the effect of spin fluctuations is incorporated in the form similar to the SCR theory. Temperature dependence of specific heat and electrical resistivity are calculated to be proportional to T(1− const. T ) and T 3/2, respectively, at the quantum critical point and low temperatures.

Study of the positive muon Knight shift in : Evidence for a tetravalent -state and crystalline electric field splitting

We report on transverse field (TF) Muon Spin Rotation (μSR) measurements on a single crystal of the hexagonal heavy fermion superconductor UNi2Al3 between 5 K and 300 K. From the measured muon Knight shift (KS) in the easy ( a , b )-plane and along the c-axis we extracted the local magnetic susceptibility tensor [0pt] , which arises from the nearest U-neighbors. By comparison with the bulk susceptibility [0pt] it is found that [0pt] and [0pt] agree well above 150 K but deviate considerably in the basal plane below 150 K, due to the disturbance introduced by the . We succeed in reproducing both [0pt] and [0pt] on the basis of a crystalline electric field (CEF)-approach assuming U to be in the tetravalent state. The disturbance introduced by the affects the CEF-Hamiltonian in an expected manner, suggesting strongly that a CEF-picture implying a rather local 5 f-electron wave function is indeed valid. Reanalyzing older data on UPd2Al3 we arrive at the same conclusion. A necessary condition for extracting the local susceptibility was the knowledge of the -site, this information was derived from the analysis of the TF-relaxation rates. At low temperatures we found about 30% of the implanted at the d-site and none at this site above 200 K. The majority fraction was found to be in a tunneling state over six m (or k)-sites around the b-site. No long range diffusion was seen up to room temperature.

Ferromagnetic MRI artifact secondary to a previous mandibular modified condylotomy

The presence of artifact on diagnostic radiographs can sometimes lead to misinterpretation and misdiagnosis. Artifacts are often caused by improper handling or processing of radiographic film. Panoramic radiographs often exhibit shadows or ‘ghost’ images cast by the presence of objects, such as earrings, that were present on the patient while the film was being exposed. Computerized tomography (CT) scans may display a significant ‘scatter’ effect if metallic dental restorations, implants, or rigid fixation hardware are present. Magnetic resonance imaging (MRI) is also degraded by the presence of both ferromagnetic and non-ferromagnetic metal implants. 1,2 The use of steel surgical instruments, such as drills and saws, may result in a ferromagnetic artifact caused by the deposition of minute metallic particles into the surrounding bone and soft tissue. 3‐5 Ferromagnetic artifacts have been previously reported following orthopaedic and neurosurgical osteotomies, 2‐5 but not after facial bone osteotomies. Recently, at our institution, we encountered a case where a large ferromagnetic MRI artifact occurred in the area of a previous mandibular osteotomy. No rigid fixation or other metallic hardware was present in this area to explain the artifact. The patient was a 32-year-old white male who originally presented with internal derangement of his right temporomandibular joint. When conservative therapy for this complaint failed, a right modified condylotomy with a postoperative period of maxillomandibular fixation was performed. The osteotomy was performed using an electric oscillating saw with a stainless steel blade (Stryker Instruments, Kalamazoo, MI) and copious irrigation. The patient returned 5 months postoperatively with complaints of pain, swelling and trismus associated with the right side of the mandible. Examination showed a large, firm, tender, non-fluctuant swelling superficial to the right angle of the mandible with associated cervical lymphadenopathy. An ultrasound examination failed to show a fluid accumulation in the area. An MRI of the area was obtained in order to determine an explanation for the soft tissue swelling. Evaluation of the MRI showed a dramatic artifact associated with the mandibular ramus that made interpretation of the soft tissue in the area impossible. This artifact consisted of multiple halo-appearing areas joined centrally like the spokes on a wheel. These halos were not uniform in size, but all appeared to be teardrop shaped in appearance. The artifact was associated with the entire mandibular ramus, both medially and laterally, in the area of the previous osteotomy and was visible on both axial and coronal views (Figs 1 & 2). Upon consultation with the radiology service, it was determined that the abnormal appearance of the area was consistent with a ferromagnetic artifact. There was no macroscopic ferromagnetic or nonferromagnetic hardware located in this area to explain this phenomenon. Surgical exploration of the area failed to show any gross anatomic abnormality and no purulence was demonstrated. Several small hard and soft tissue specimens were obtained from the area. Histologic examination with both standard light microscopy and polarized light microscopy failed to reveal any foreign bodies in the tissue. The presence of microscopic metal particles could not be ruled out however, due to the relatively small size of the tissue specimens. Large MRI artifacts will result from the presence of even minute ferromagnetic particles present within the scanner’s radiofrequency coils. 3 Stainless steel saws and drills may deposit microscopic ferromagnetic debris in the tissue after their use. 3‐5 Even these tiny particles can lead to a significant change in the local magnetic susceptibility of the tissue which in turn leads to local distortions of the homogeneity of

Electronic and magnetic properties of intermetallic compound YCo 5

The electronic and magnetic properties of the intermetallic compound YCo 5 have been studied using density functional theory with the local spin density approximation. The calculated magnetic moments of Y, Co(2c) and Co(3g) are −0.61, 1.68 and 2.04 μ B, respectively, and the total magnetic moment is about 8.87 μ B per formula unit, which agrees well with the previous experimental results. The dependence of the magnetic moments of Y, Co(2c) and Co(3g) on the lattice spacing has been investigated. The local electronic structure of Y, Co(2c) and Co(3g) are discussed in detail. The local magnetic susceptibilities of Y, Co(2c) and Co(3g) are calculated. Based on our results, YCo 5 was found to have characteristic of a strong ferromagnet.

Ultrafast MR imaging of the abdomen: echo planar imaging and diffusion-weighted imaging.

Ultrafast MRI technique has become available with the introduction of new generation MR scanners for abdominal imaging. However, there is no consensus about the optimal imaging acquisition at the present time. Because single shot echo planar imaging (EPI) technique is based on high technology and had just applied in clinical imaging, further clinical investigation will be needed. Currently, the hypersensitivity to magnetic inhomogeneity and local magnetic susceptibility and the low spatial resolution may limit the widespread application of EPI technique. In addition to providing information for morphologic diagnosis, EPI will be more widely used for functional and qualitative diagnosis. Diffusion-weighted imaging can be used for differentiation of solid tumors according to their different cellular construction, evaluation of cystic lesions based on the different viscosity of their contents, and assessment of diffused pathologic changes in the parenchyma of solid organs. In addition to the previous parameters such as proton density and T1 and T2 values, diffusion factors may provide important information for the qualitative and dynamic evaluation of abdominal pathologic changes. Even though there are many difficulties that must be solved for diffusion-weighted imaging, a more wide application of this technique is expected through technologic improvement.

Magnetic resonance imaging of the Lake Agassiz–Lake Winnipeg transition

As part of the Geologic Survey of Canada (GSC) Lake Winnipeg Study, we have successfully imaged the Lake Agassiz to Lake Winnipeg transition in Section 4 of Core Namao 94-900-122a (i.e., from 313 cm to 465 cm), using a newly-developed magnetic resonance imaging (MRI) technique called SPRITE (Single-Point, Ramped Imaging with T1 Enhancement). Whole core, gamma-ray attenuation measurements have been used to calculate the bulk porosity of the sediment at 1 cm intervals for comparison with the SPRITE images. Image contrast and image intensities observed in the SPRITE images of Section 4 are related to local porosity and magnetic susceptibility variations. In general, regions of the core with low signal intensity contain high porosity and low magnetic susceptibility. The best contrast between sediment layers is observed from regions of the core with high magnetic susceptibility. High signal intensity is observed from regions with low porosity and/or high magnetic susceptibility.

TDPAC study of 140Ce local susceptibility in the Kondo insulator system CeNi 1 − xCo xSn

We have measured the local magnetic susceptibility ( χ loc) of 140Ce in the alloy system CeNi 1 − x Co x Sn for x = 0, 0.3 and 0.38 in the range 28–300 K using TDPAC method. The results show strongly reduced χ loc values in all the alloys. Using the Coqblin-Schrieffer model we obtain the Kondo temperatures T K to be 90, 112 and 170 K for x = 0.0, 0.38 and 0.30, respectively. In addition, for x = 0.38, the data show an abrupt drop in the χ loc value at 48 K indicating a first-order valence transition for Ce ions.

Interface formation and a structural phase transition for the spherical model of ferromagnetism

A detailed analysis is reported examining the local magnetic susceptibility χ(r), in relation to the correlation functionG(R) and correlation length ζ, of a spherical model ferromagnet confined to geometryΩ =Ld −d′ × ∞d′ (d′≤2,d>2) under a continuous set oftwisted boundary conditions. The “twist” parameter\(\underline \tau \) in this problem may be interpreted as a measure of the geometry-dependent doping level of interfacial impurities (or antiferromagnetic seams) in theextended system at various temperatures. For τj →0, ∀j∈d-d′, no seams are present except at infinity, whereas if τj = 1/2, impurity saturation occurs. For 0 L), defining the region between seams containing the origin, depends on temperature above a certain threshold (T>T0). Below that temperature (T>T0), seams are frozen at the same position (D≈L/2τ,d-d'=1), revealing a smoothly varying largescale structural phase transition.