Compact disordered magnetic resonators designed by simulated annealing algorithm

In the last few years there has been rapid progress in the application of nuclear magnetic resonance clinical imaging as a diagnostic aid (Partain et al, 1983; Steiner, 1983; Worthington, 1983). Records kept by the operators of NMR imaging equipment indicate that about 1800 individuals have now been examined without incident within the United Kingdom. In view of this progress, an advisory group (Appendix) appointed by the National Radiological Protection Board recommends some modifications to the original advice given in 1981 (NRPB, 1981). The principal modifications are that: (1) It is no longer considered necessary to exclude volunteers with a history of epilepsy or cardiac disease. (2) It is not obligatory that a registered medical practitioner trained in resuscitation techniques be present during an exposure. (3) Higher limits are recommended for rates of change of magnetic flux density when the period of change is less than 10 ms. (4) The radiofrequency limit of 15 MHz has been removed. Theoretical considerations indicate that the interaction of an applied magnetic field with the intrinsic magnetic moments of protons and electrons within a molecule could alter its rate of chemical reaction (McLauchlan, 1981). Chemical reactions involving radical pair intermediates are expected to be sensitive to magnetic fields for this reason and a number of enzyme reactions that might be expected to be affected have been identified (A. Cass, personal communication). However, there is no experimental confirmation of these theoretical possibilities at present.

HomeRadiologyVol. 231, No. 3 PreviousNext EditorialsMR Imaging: Its Development and the Recent Nobel PrizeStephen J. RiedererStephen J. RiedererAuthor Affiliations1From the MR Laboratory, Mayo Clinic, 200 First Street SW, Rochester, MN 55905. Received January 19, 2004; accepted January 21. Address correspondence to the author (e-mail: [email protected]).Stephen J. RiedererPublished Online:Jun 1 2004https://doi.org/10.1148/radiol.2313040114MoreSectionsFull textPDF ToolsImage ViewerAdd to favoritesCiteTrack CitationsPermissionsReprints ShareShare onFacebookTwitterLinked In References 1 Lauterbur PC. Image formation by induced local interactions: examples employing nuclear magnetic resonance. Nature 1973; 242:190-191. Crossref, Google Scholar2 USA MRI market Mountain View, Calif: Frost and Sullivan Market Research, 2002. Google Scholar3 Mansfield P, Grannell PK. NMR “diffraction” in solids? J Phys C Solid State Phys 1973; 6:L422. Crossref, Google Scholar4 Mansfield P. 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Computerized fluoroscopy in real time for noninvasive visualization of the cardiovascular system: preliminary studies. Radiology 1979; 130:49-57. Link, Google ScholarArticle HistoryPublished in print: June 2004 FiguresReferencesRelatedDetailsCited ByMedicine, Vol. 93, No. 28Skeletal Radiology, Vol. 40, No. 9Medicine - Programa de Formaci?n M?dica Continuada Acreditado, Vol. 9, No. 92Magnetic Resonance Imaging, Vol. 24, No. 4Applied Magnetic Resonance, Vol. 29, No. 1Pacing and Clinical Electrophysiology, Vol. 28, No. 10Magnetic Resonance Imaging, Vol. 22, No. 10Recommended Articles RSNA Education Exhibits RSNA Case Collection Vol. 231, No. 3 Metrics Altmetric Score PDF download

A dielectric sample resonator design for enhanced sensitivity of EPR spectroscopy

Recent developments in magnetic resonance (MR) imaging have opened up new avenues in the investigation of cardiovascular physiology. Inherent signal intensity of any tissue on MR images depends largely on proton concentration as well as longitudinal (T1) and transverse (T2) relaxation times. Myocardial contrast can be manipulated by using specific MR pulse sequences which are selectively sensitive to differences in any one of these parameters. Paramagnetic metal complexes are used as contrast media in MR imaging to enhance the inherent contrast. Contrast media in MR imaging are not directly visible but change the magnetic properties of other nuclei in close proximity, such as those of the water hydrogen. The signal of water can be altered by the contrast medium in different ways, either by changing the relaxation times or through bulb susceptibility effects, or both. The role of MR contrast media for quantitative characterization of ischemic heart disease has advanced considerably in the past 10 years. Conventional MR imaging techniques following the administration of contrast media are useful for identifying and sizing myocardial infarctions and for distinguishing between occlusive and reperfused myocardial infarctions as well as reversible (stunned) and irreversible injuries. Recent results suggest that contrast-enhanced MR imaging can also be used to identify dead cells in reperfused ischemically injured myocardium. The recently developed fast MR imaging techniques, with the aid of MR contrast media as a perfusion indicator, may be useful in estimating regional myocardial perfusion and blood volume. The assessment of capillary circulation or myocardial perfusion may be used for evaluating the extent of hypoperfusion and treatment efficacy. Experimental and clinical perfusion studies indicate that perfusion-sensitive MR imaging detects compromised myocardium (area at risk). Combining myocardial perfusion imaging with the anatomic and functional information provided by other MR imaging sequences could make MR imaging a comprehensive noninvasive technique for the evaluation of ischemic heart disease.

To prospectively evaluate magnetic resonance (MR) imaging and MR spectroscopy for depiction of local prostate cancer recurrence after external-beam radiation therapy, with step-section pathologic findings as the standard of reference. Study received institutional approval, and written informed consent was obtained. Study was compliant with Health Insurance Portability and Accountability Act. Sextant biopsy, digital rectal examination, MR imaging, MR spectroscopy, and salvage radical prostatectomy with step-section pathologic examination were performed in nine patients with increasing prostate-specific antigen levels after external-beam radiation therapy. MR imaging criterion for tumor was a focal nodular region of reduced signal intensity at T2-weighted imaging. MR spectroscopic criteria for tumor were voxels with choline (Cho) plus creatine (Cr) to citrate (Cit) ratio ([Cho + Cr]/Cit) of at least 0.5 or voxels with detectable Cho and no Cit in the peripheral zone. Sensitivity and specificity of sextant biopsy, digital rectal examination, MR imaging, and MR spectroscopy were determined by using a prostate sextant as the unit of analysis. For feature analysis, MR imaging and MR spectroscopic findings were correlated with step-section pathologic findings. MR imaging and MR spectroscopy showed estimated sensitivities of 68% and 77%, respectively, while sensitivities of biopsy and digital rectal examination were 48% and 16%, respectively. MR spectroscopy appears to be less specific (78%) than the other three tests, each of which had a specificity higher than 90%. MR spectroscopic feature analysis showed that a metabolically altered benign gland could be falsely identified as tumor by using MR spectroscopic criteria; further analysis of MR spectroscopic features did not lead to improved MR spectroscopic criteria for recurrent tumor. In summary, MR imaging and MR spectroscopy may be more sensitive than sextant biopsy and digital rectal examination for sextant localization of cancer recurrence after external-beam radiation therapy.

In recent years, magnetic resonance imaging (MRI)-guided radiotherapy (RT) has experienced a notable increase in utilization due to technological advancements that leverage MRI's superior soft-tissue contrast and its non-invasive, non-ionizing imaging mechanism. Integrating MRI scanners with linac systems comes with several technical challenges, including the complex interactions between the linac components and the magnetic field of the MRI scanner. This study presents a comprehensive in silico finite element method (FEM)-based model for a proximity-type MRI-guided RT system consisting of an MRI-on-rail and a C-arm linac. The methodology enables the precise characterization of the MRI magnet's fringe field both in free space and when interacting with the ferromagnetic structure of the linac system. A comprehensive in silico FEM-based model was developed to simulate the MRI magnet and linac configuration. The magnet coil configuration was generated using linear programming based on the manufacturer's specifications for the 5 G line. The linac structure was modeled from technical schematics and coupled with the simulated magnet to construct the simulation environment. Fringe field measurements were conducted in a controlled environment to validate the simulation results. The measurements were taken at various distances from the MRI isocenter and in different directions to assess the spatial distribution of the fringe field. Simulations showed good agreement with experimental measurements, with a maximum difference of 1 G observed between simulated and measured fringe fields within the 3 to 5 m range from the MR isocenter, consistent with the 1 G Hall probe measurement tolerance. The linac's ferromagnetic structure significantly perturbed the magnetic fringe field, locally increasing field values to 40 G from an initial range of 0-23 G, and inducing local field differences of up to 30 G at its closest proximity to the magnet. Conversely, a local decrease of up to 2 G was observed near the linac isocenter. Furthermore, the room environment influenced the fringe field's spatial distribution, evidenced by deviations of approximately 6 and 4.2 G from the Espree reference field at 2.5 and 3 m, respectively. Despite these environmental effects, the overall agreement between simulations and experimental values, including measurements at the linac head (maximum difference less than 2 G), was highly satisfactory, confirming axial field symmetry and minimal impact from room layout variations. This study developed and validated a comprehensive FEM-based simulation methodology to accurately characterize magnetic field interactions in a proximity-type MRI-guided RT system. The methodology mapped the MRI magnet's fringe field in both free space and as perturbed by the linac's ferromagnetic structure, with experimental data also supporting these findings. This robust framework offers a reliable tool for guiding engineering activities and defining safety bounds for system design.

In this paper, a band stop all-dielectric metamaterial frequency selective surface (FSS) is designed based on effective medium theory and dielectric resonator theory. The FSS is made of high-permittivity ceramics without using any metallic parts. The stop band FSS is composed of 2D arrays of Jerusalem cross-shaped ceramic resonators. Solid-state sintering method is adopted to prepare the high-permittivity ceramics, which is made of 0.7Ba <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.6</inf> Sr <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.4</inf> TiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> –0.3La(Mg <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</inf> Ti <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</inf> )O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> . The permittivity of this ceramics is about 110. The simulation results show that the FSS can achieve a stop band at 6.91–9.25 GHz. The relative effective permittivity, permeability and the normalized impedance of the structure are retrieved, showing that the first resonant is a magnetic resonant and the second resonant is an electric resonant. Due to the magnetic and electric resonant, the impedance matching becomes worse, so the normalized impedance is close to 0 and the stop band forms. The electric fields and magnetic fields of the resonant points are observed to further analysis the FSS. At the first resonant point, the electric field loop is formed, equivalent to a magnetic dipole. At the second resonant point, the magnetic field loop is formed, equivalent to an electric dipole. The magnetic and electric fields are accordance with the retrieved parameters. Since such FSSs are made of high-permittivity ceramics, they have potential engineering application in high-power or high-temperature.

Introduction: In recent years, the use of magnetic resonance (MR) images in radiation treatment planning has drawn considerable attention. However, although the extent of a tumor can be determined in great detail on MR images, the geometric accuracy of these images is limited by distortions stemming from the inhomogeneity of the static background magnetic field, the nonlinearity of the applied gradient magnetic fields, the magnetic susceptibility of the imaged tissues, and chemical shift artifacts. Our goal is to design and construct a new Anthropomorphic Head Phantom for Assessment of Image Distortion in Treatment Planning Systems. Materials and Methods: In this study, CT scan images of head were transferred to the Mimic software (Mimics® Innovation Suite). Using this software, the skull texture was removed and a hollow layer formed between the bone tissue in which the bone tissue would be equivalent to the material. Then fabricated it with a 3D printer and used K2HPO4 (as bone). we designed a new 3D grid containing 13,824 reference features (control points) with AutoCAD software, fabricated it with a 3D printer, and filled it with gels that include nickel-doped agarose, sucrose, urea, and sodium chloride (as soft tissue) then placed this grid Inside the head phantom. The phantom is supplied with specially designed pads that allow fixation with any stereotactic frame or mounting for end-to-end testing. This phantom was tested on the Siemens 3 Tesla Prisma MRI model using a 64-channel head coil. We used imaging from a six-slice CT scan (Siemens) as a reference and matched the reference features in the MRI images with the CT-scan images. To achieve this, we used a three-dimensional reference feature model. Reproducibility on the phantom was investigated with three different imaging sessions per day for three different days. Results: The CT numbers and relative electron density of the gels closely resembled those of real tissue and T1 and T2 of the gels closely resembled those of real tissue. The geometric distortion in the 3D results was found to be due to field non-uniformity and nonlinearity of the gradients and its reproducibility. The mean Euclidean distance error for MRI volume was less than 0.9 mm. The maximum Euclidean error was 1.3 mm. Distortion in the whole volume is pronounced exclusively at the edges of the magnetic field. Conclusion: As with other investigations carried out in our work, the amount of distortion in the middle of the field was less than at its sides. This phantom can be used to check Image Distortion in Treatment Planning Systems.

Ultra-high field (UHF) magnetic resonance (MR) systems are advancing in preclinical imaging offering the potential to enhance radiation research. However, system-dependent factors, such as magnetic field inhomogeneities ( ) and gradient non-linearity (GNL), induce geometric distortions compromising the sub-millimeter accuracy required for radiationresearch. This study tackles system-dependent distortions in 15.2T MR images by prospective shimming strategies optimization and comparing two imaging methods for voxel displacement correction. The methods were evaluated on a 3D-printed grid phantom and validated on in vivo mouse brain MR images. Additionally, a phantom-based displacement map was tested for GNL correction in mouse brainimages. Phantom MR and CT images were acquired with 200 resolution. In vivo mouse brain MR and CT images had 140 and 200 resolutions, respectively. Three shimming strategies were established to assess displacements ( ) in phantom MR images. was calculated using the acquired static field maps in three volumes of interest (VOIs) via Python script. A one-step distortion correction (1SDC) method, which simultaneously corrects and GNL distortions via non-rigid registration with CT, and a two-step distortion correction (2SDC) method, which corrects separately in two consecutive steps and GNL displacements, were assessed on phantom and in vivo images. For in vivo 2SDC validation, a phantom displacement map generated by MR to CT non-rigid registration was applied to correct GNL on the mouse brain. Total displacements ( ) were quantified in phantom VOIs and the in vivo skull region by measuring landmarks'positions. The in the phantom increased with distance from the VOI center and magnet isocenter. Shimming scenario-2 showed the lowest maximum displacement (0.26mm) for the largest VOI but required a longer acquisition time. Distortion correction methods were necessary for large VOIs (13-25mm, along the z-axis) in the phantom where 0.2mm. The 2SDC method outperformed 1SDC by achieving a 0.2mm accuracy in 100%, 92.1%, and 59.3% of the landmarks from the smallest to the largest VOI. Phantom dice scores confirmed the improvement in geometric precision after each correction step. In vivo results showed that 1SDC correction overcorrected MR images, increasing voxel displacements. The 2SDC exceeded the 1SDC, reducing by 85%, in accordance with the dice score analysis (0.97 2SDC vs. 0.84 1SDC). At 15.2T, in vivo MR images of even small regions (e.g., mouse brain) require geometric distortion correction for radiation research. The 2SDC method outperformed the 1SDC, emphasizing the need for separate and GNL corrections. Moreover, a phantom-based displacement map shows promise for in vivo GNL correction.

Purpose : To evaluate the usefulness of in vivo magnetic resonance (MR) imaging for tracking intravenously injected superparamagnetic iron oxide (SPIO)-labeled human umbilical vein endothelial cells (HUVECs) in an acute renal failure (ARF) rat model. Materials and Methods: HUVECs were labeled with SPIO and poly-L-lysine (PLL) complex. Relaxation rates at 1.5-T MR, cell viability, and labeling stability were assessed. HUVECs were injected into the tail vein of ARF rats (labeled cells in 10 rats, unlabeled cells in 2 rats). Follow-up serial T2*-weighted gradient-echo MR imaging was performed at 1, 3, 5 and 7 days after injection, and the MR findings were compared with histologic findings. Results: There was an average of 98.4±2.4% Prussian blue stain-positive cells after labeling with SPIOPLL complex. Relaxation rates (R2*) of all cultured HUVECs at day 3 and 5 were not markedly decreased compared with that at day 1. The stability of SPIO in HUVECs was maintained during the proliferation of HUVECs in culture media. In the presence of left unilateral renal artery ischemia, T2*-weighted MR imaging performed 1 day after the intravenous injection of labeled HUVECs revealed a significant signal intensity (SI) loss exclusively in the left renal outer medulla regions, but not in the right kidney. The MR imaging findings at days 3, 5 and 7 after intravenous injection of HUVECs showed a SI loss in the outer medulla regions of the ischemically injured kidney, but the SI progressively recovered with time and the right kidney did not have a significant change in SI in the same period. Upon histologic analysis, the SI loss on MR images was correspondent to the presence of Prussian blue stained cells, primarily in the renal outer medulla. Conclusion: MR imaging appears to be useful for in vivo monitoring of intravenously injected SPIO-labeled HUVECs in an ischemically injured rat kidney. Index words : Magnetic resonance imaging (MRI)∙Superparamagnetic iron oxide (SPIO) Human umbilical vein endothelial cells (HUVECs)∙Cell tracking

Ever since the clinical application of magnetic resonance (MR) imaging became an essential diagnostic tool, the lung has been a challenging area. Although many physicists and radiologists tried to assess different lung diseases as well as mediastinal and cardiac diseases by means of MR imaging during the 1980s and early 1990s, they could neither produce convincing image quality within a reasonable examination time nor demonstrate that MR imaging could serve as a substitute for computed tomography (CT), pulmonary angiography, and/or nuclear medicine studies. However, improvement of the 1.5-tesla (T) MR system and sequences, clinical application of new techniques such as parallel imaging, and utilization of more effective contrast media have continued since the late 1990s. These developments have made it possible to apply lung MR imaging to not only oncologic but also other pulmonary diseases, as well as to not only morphological but also functional assessment of various pulmonary and cardiopulmonary diseases. Moreover, these improvements have been continuing, resulting in the gradual shifting to the use of a higher magnetic strength field (≥3 T) for lung MR imaging during the 2000s.

Cervical spondylotic myelopathy (CSM) represents the most commonly acquired cause of spinal cord dysfunction among individuals over 55 years old. The pathophysiology of the condition involves mechanical factors, which result to injury of the cervical spinal cord. In T-2 weighted magnetic resonance (MR) images of the spine the site of injury is depicted as a region of high intensity signal within the cervical spine cord. The present study aims to investigate whether texture analysis of MR signal in CSM could provide novel quantitative prognostic factors, rendering possible the prognostic estimation of the outcome of a therapeutic surgical intervention for CSM. The sample of the study comprised 12 MR images of the cervical spine, corresponding to 6 CSM patients, who had undergone surgical intervention with anterior cervical discectomy and spinal canal decompression. Following a specific MR imaging protocol a pair of T2-weighted sagittal images of the spine, corresponding to pre- and post-operative MR scans, were obtained for each of the patients. Employing custom developed software, the region of high intensity signal, associated to CSM, was automatically segmented from each MR image. Utilizing custom developed algorithms a number of textural features were extracted from the segmented ROIs and employed in the design of a classification system, based on the Quadratic classifier. The latter was used for the discrimination between pre-operative and post-operative MR images. Statistical analysis revealed the existence of statistically significant differences between textural features, corresponding to pre- and post-operative CSM MR signals. The quadratic classifier characterized correctly all the pre- and post-operative MR images (100% classification accuracy). The results of the present study indicate that textural features, generated from MR images of the spine, may serve as prognostic factors regarding the prediction of the post-operative outcome of CSM patients.

Purpose To measure regional specific ventilation with free-breathing hydrogen 1 (1H) magnetic resonance (MR) imaging without exogenous contrast material and to investigate correlations with hyperpolarized helium 3 (3He) MR imaging and pulmonary function test measurements in healthy volunteers and patients with asthma. Materials and Methods Subjects underwent free-breathing 1H and static breath-hold hyperpolarized 3He MR imaging as well as spirometry and plethysmography; participants were consecutively recruited between January and June 2017. Free-breathing 1H MR imaging was performed with an optimized balanced steady-state free-precession sequence; images were retrospectively grouped into tidal inspiration or tidal expiration volumes with exponentially weighted phase interpolation. MR imaging volumes were coregistered by using optical flow deformable registration to generate 1H MR imaging-derived specific ventilation maps. Hyperpolarized 3He MR imaging- and 1H MR imaging-derived specific ventilation maps were coregistered to quantify regional specific ventilation within hyperpolarized 3He MR imaging ventilation masks. Differences between groups were determined with the Mann-Whitney test and relationships were determined with Spearman (ρ) correlation coefficients. Statistical analyses were performed with software. Results Thirty subjects (median age: 50 years; interquartile range [IQR]: 30 years), including 23 with asthma and seven healthy volunteers, were evaluated. Both 1H MR imaging-derived specific ventilation and hyperpolarized 3He MR imaging-derived ventilation percentage were significantly greater in healthy volunteers than in patients with asthma (specific ventilation: 0.14 [IQR: 0.05] vs 0.08 [IQR: 0.06], respectively, P < .0001; ventilation percentage: 99% [IQR: 1%] vs 94% [IQR: 5%], P < .0001). For all subjects, 1H MR imaging-derived specific ventilation correlated with plethysmography-derived specific ventilation (ρ = 0.54, P = .002) and hyperpolarized 3He MR imaging-derived ventilation percentage (ρ = 0.67, P < .0001) as well as with forced expiratory volume in 1 second (FEV1) (ρ = 0.65, P = .0001), ratio of FEV1 to forced vital capacity (ρ = 0.75, P < .0001), ratio of residual volume to total lung capacity (ρ = -0.68, P < .0001), and airway resistance (ρ = -0.51, P = .004). 1H MR imaging-derived specific ventilation was significantly greater in the gravitational-dependent versus nondependent lung in healthy subjects (P = .02) but not in patients with asthma (P = .1). In patients with asthma, coregistered 1H MR imaging specific ventilation and hyperpolarized 3He MR imaging maps showed that specific ventilation was diminished in corresponding 3He MR imaging ventilation defects (0.05 ± 0.04) compared with well-ventilated regions (0.09 ± 0.05) (P < .0001). Conclusion 1H MR imaging-derived specific ventilation correlated with plethysmography-derived specific ventilation and ventilation defects seen by using hyperpolarized 3He MR imaging. © RSNA, 2018 Online supplemental material is available for this article.

The purpose of this study was to evaluate the use of gadolinium and barium-enhanced magnetic resonance (MR) imaging in detecting intestinal and extraintestinal Crohn's disease and compare MRI with contrast-enhanced helical computed tomography (CT). Twenty-six patients with Crohn's disease underwent imaging examinations, including gadolinium-enhanced, fat suppressed fast multiplanar spoiled gradient-recalled (FMPSPGR) MR imaging with oral 2% barium sulfate and rectal water and with helical CT using i.v. and positive (13) or negative (13) intestinal contrast material. MR images and CT scans were reviewed separately by two radiologists for bowel wall thickness and enhancement, presence of abscess, phlegmon, and fistula. MR images and CT scans were then compared side by side. Surgical, endoscopic, and histopathologic findings and results of barium studies were reviewed to determine the location and severity of involvement of intestinal Crohn's disease. Depiction of mural thickening and/or enhancement was superior on the MR images, which showed 55 (85%) and 52 (80%) of 65 abnormal bowel segments for the two observers, compared with helical CT, which showed 39 (60%) and 42 (65%; P < 0.001, P < 0.05) of bowel segments affected by Crohn's disease. Segments of bowel with moderate or marked mural thickening were depicted equally on MR imaging and helical CT. In mildly diseased segments of bowel, with only slight thickening and enhancement, MR imaging depicted 22 (79%) and 19 (68%) of 28 segments, compared with helical CT, which depicted 9 (32%; P < 0.01), and 13 (46%; P > 0.05) of 28 segments. In the side-by side comparison, MR imaging was preferred over helical CT for depicting normal bowel wall (MR 71%, CT 4%, equal 25%; P < 0.001), mural thickening (MR 41%, CT 11% equal 48%; P < 0.01), mural enhancement (MR 89%, equal 11%; P < 0.001), and overall GI tract evaluation (MR 52%, CT 10%, equal 38%; P < 0.001). Gadolinium-enhanced MR imaging with oral dilute barium sulfate and rectal water depicts intestinal and extraintestinal changes of Crohn's disease and shows promise as a clinically useful tool.

Design of hourglass nanoantenna for magnetic field enhancement