Increase In Specific Absorption Rate Research Articles

Steering the synthesis of Fe3O4 nanoparticles under sonication by using a fractional factorial design

Superparamagnetic iron oxide nanoparticles (MNPs) have the potential to act as heat sources in magnetic hyperthermia. The key parameter for this application is the specific absorption rate (SAR), which must be as large as possible in order to optimize the hyperthermia treatment. We applied a Plackett-Burman fractional factorial design to investigate the effect of total iron concentration, ammonia concentration, reaction temperature, sonication time and percentage of ethanol in the aqueous media on the properties of iron oxide MNPs. Characterization techniques included total iron content, Fourier Transform Infrared Spectroscopy, X-Ray Diffraction, High Resolution Transmission Electron Microscopy, and Dynamic Magnetization. The reaction pathway in the coprecipitation reaction depended on the initial Fe concentration. Samples synthesized from 0.220 mol L−1 Fe yielded magnetite and metastable precipitates of iron oxyhydroxides. An initial solution made up of 0.110 mol L−1 total Fe and either 0.90 or 1.20 mol L−1 NH3(aq) led to the formation of magnetite nanoparticles. Sonication of the reaction media promoted a phase transformation of metastable oxyhydroxides to crystalline magnetite, the development of crystallinity, and the increase of specific absorption rate under dynamic magnetization.

The effect of metal objects on the SAR and temperature increase in the human head exposed to dipole antenna (numerical analysis)

Wearable metal objects with high electromagnetic reflection characteristics can cause interference with the incident waves during exposure to the electromagnetic (EM) radiation. Therefore, it is of interest to investigate the effect of metal objects capable of increasing the absorption of EM energy and temperature within the tissues when get exposed to EM radiation. A numerical analysis of increase in specific absorption rate (SAR) and temperature distribution in a human head model when metal objects are placed between the head and radiating source is performed. A realistic three-dimensional heterogeneous human head model, metal objects of different shapes and sizes, and spectacles with different lenses are used. A half-wavelength dipole antenna operating at 1800 MHz served as an EM radiation source. Results show that the presence of metal objects in proximity to the head alters SAR and temperature increase within the tissues. In most cases, metal objects redistribute the EM field incident upon them to a smaller region increasing power absorption, thereby increasing SAR and temperature in that region. The power absorption in head layers is found to be sensitive to metal object's size and shape, and distance of the antenna from the objects.

Simultaneous proton density, T1 , T2 , and flip-angle mapping of the brain at 7 T using multiparametric 3D SSFP imaging and parallel-transmission universal pulses.

Performing simultaneous quantitative MRI at ultrahigh field is challenging, as B0 and heterogeneities as well as specific absorption rate increase. Too large deviations of flip angle from the target can induce biases and impair SNR in the quantification process. In this work, we use calibration-free parallel transmission, a dedicated pulse-sequence parameter optimization and signal fitting to recover 3D proton density, flip angle, T1 , and T2 maps over the whole brain, in a clinically suitable time. Eleven optimized contrasts were acquired with an unbalanced SSFP sequence by varying flip-angle amplitude and RF phase-cycling increment, at a 1.0 × 1.0 × 3.0mm3 resolution. Acquisition time was 16minutes36seconds for the whole brain. Parallel transmission and universal pulses were used to mitigate heterogeneity, to improve the results' reliability over 6 healthy volunteers (3 females/3 males, age 22.6 ± 2.7 years old). Quantification of the physical parameters was performed by fitting the acquired contrasts to the simulated ones using the Bloch-Torrey equations with a realistic diffusion coefficient. Whole-brain 3D maps of effective flip angle, proton density, and relaxation times were estimated. Parallel transmission improved the robustness of the results at 7 T. Results were in accordance with literature and with measurements from standard methods. These preliminary results show robust proton density, flip angle, T1 , and T2 map retrieval. Other parameters, such as ADC, could be assessed. With further optimization in the acquisition, scan time could be reduced and spatial resolution increased to bring this multiparametric quantification method to clinical research routine at 7 T.

Risonanza Magnetica della Mammella con i moderni scanner 3T: principi fisici e vantaggi tecnici rispetto alle apparecchiature 1,5 T

In recent decades the Magnetic Resonance Imaging (MRI) world, for diagnostic uses, offered a very rapid and extremely dynamic and necessary technological evolution. Recently approved in Italy also for clinical use, in addition to the scientific one, the introduction of very high-field MRI, or 3 Tesla, provided considerable benefits. Therefore, the high magnetic field (3T) allows an increase in the signal-to-noise ratio (SNR) and in spatial and temporal resolution, and other several advantages. Certainly, there are some disadvantages, which can be found in the field of protectionism and safety, due to the increase in intensity of the static magnetic field and, specifically, to the increase in Specific Absorption Rate (SAR). Many applications of MRI improved significantly, among these, the brest MRI study, where the 3T magnetic field allows an increase of diagnostic accuracy in terms of specificity, providing a better visualization and characterization of breast lesions presenting post-contrastographic enhancement, so breast cancer and other lesions, showing a progressive better application field. Magnetic resonance is an emerging application with great potential, and the spread of very high-field scanners will allow 3 Tesla to become the excellence for many studies, especially in breast imaging.

Risonanza Magnetica della Mammella con i moderni scanner 3T: principi fisici e vantaggi tecnici rispetto alle apparecchiature 1,5 T

In recent decades the Magnetic Resonance Imaging (MRI) world, for diagnostic uses, offered a very rapid and extremely dynamic and necessary technological evolution. Recently approved in Italy also for clinical use, in addition tothe scientific one, the introduction of very high-field MRI, or 3 Tesla, provided considerable benefits. Therefore, thehigh magnetic field (3T) allows an increase in the signal-to-noise ratio (SNR) and in spatial and temporal resolution,and other several advantages. Certainly, there are some disadvantages, which can be found in the field of protectionism and safety, due to the increase in intensity of the static magnetic field and, specifically, to the increase in SpecificAbsorption Rate (SAR). Many applications of MRI improved significantly, among these, the brest MRI study, wherethe 3T magnetic field allows an increase of diagnostic accuracy in terms of specificity, providing a better visualizationand characterization of breast lesions presenting post-contrastographic enhancement, so breast cancer and otherlesions, showing a progressive better application field. Magnetic resonance is an emerging application with greatpotential, and the spread of very high-field scanners will allow 3 Tesla to become the excellence for many studies,especially in breast imaging.

Exchange-coupled Fe3O4/CoFe2O4 nanoparticles for advanced magnetic hyperthermia

We report a systematic study of the effects of core and shell size on the magnetic properties and heating efficiency of exchange-coupled Fe3O4/CoFe2O4 core/shell nanoparticles. The nanoparticles were synthesized using thermal decomposition of organometallic precursors. Transmission electron microscopy (TEM) confirmed the formation of spherical Fe3O4 and Fe3O4/CoFe2O4 nanoparticles. Magnetic measurements showed high saturation magnetization for the nanoparticles at room temperature. Increasing core diameter (6.4±0.7, 7.8±0.1, 9.6±1.2 nm) and/or shell thickness (∼1, 2, 4 nm) increased the coercive field (HC), while an optimal value of saturation magnetization (MS) was achieved for the Fe3O4 (7.8±0.1nm)/CoFe2O4 (2.1±0.1nm) nanoparticles. Magnetic hyperthermia measurements indicated a large increase in specific absorption rate (SAR) for 8.2±1.1 nm Fe3O4/CoFe2O4 compared to Fe3O4 nanoparticles of same size. The SAR of the Fe3O4/CoFe2O4 nanoparticles increased from 199 to 461 W/g for 800 Oe as the thickness of the CoFe2O4 shell was increased from 0.9±0.5 to 2.1±0.1 nm. The SAR enhancement is attributed to a combination of the large MS and the large HC. Therefore, these Fe3O4/CoFe2O4 core/shell nanoparticles can be a good candidate for advanced hyperthermia application.

Increase in Specific Absorption Rate (SAR) Of Mobile Phone - A Threat to Human

Drastic increase of mobile phone usage increases the concern about the effect of electromagnetic radiation on human brain. During the last decade, several researches were conducted in this area and reported the effect of radiation on brain. Unfortunately most of these findings cannot be reproduced. Findings of a number of studies ascertain that, there is no clear evidence to link the relationship between the adverse effects and mobile phone usage. The reason of the changes observed in human body due to radiation and the causes that leads to non reproducibility of these results has to be investigated. In [1] it is reported that, adverse effect of radiation occurs in cellular level if SAR is much higher. So we tried to study the change in SAR using FDTD simulation by increasing the power of excitation. Frequency range is selected as the specified range of frequency permissible for mobile phone communication. The value of SAR obtained for the above specified range of frequency is found within the permissible limit insisted by ICNIRP/IEEE. But on increasing the magnitude of power of excitation, the magnitude of SAR also increased. It can be concluded that, if the transmitting power is not within the permissible limit, there is a possibility to increase the values of SAR, which may cause health hazards.

Electrodynamics and radiofrequency antenna concepts for human magnetic resonance at 23.5T (1GHz) and beyond.

This work investigates electrodynamic constraints, explores RF antenna concepts and examines the transmission fields (B 1 (+) ) and RF power deposition of dipole antenna arrays for (1)H magnetic resonance of the human brain at 1GHz (23.5T). Electromagnetic field (EMF) simulations are performed in phantoms with average tissue simulants for dipole antennae using discrete frequencies [300MHz (7.0T) to 3GHz (70.0T)]. To advance to a human setup EMF simulations are conducted in anatomical human voxel models of the human head using a 20-element dipole array operating at 1GHz. Our results demonstrate that transmission fields suitable for (1)H MR of the human brain can be achieved at 1GHz. An increase in transmit channel density around the human head helps to enhance B 1 (+) in the center of the brain. The calculated relative increase in specific absorption rate at 23.5 versus 7.0T was below 1.4 (in-phase phase setting) and 2.7 (circular polarized phase setting) for the dipole antennae array. The benefits of multi-channel dipole antennae at higher frequencies render MR at 23.5T feasible from an electrodynamic standpoint. This very preliminary finding opens the door on further explorations that might be catalyzed into a 20-T class human MR system.

An efficient fat suppression technique for stimulated-echo based CMR

Background Stimulated-echo acquisition mode (STEAM) is one of the key pulse sequences in CMR imaging, as it alleviates rapid magnetization decay with T2 relaxation. Fat suppression is frequently implemented in cardiac imaging to improve visualization and tissue characterization, albeit at the cost of reduced temporal resolution and signal-to-noise ratio (SNR), as well as possible increase of specific absorption rate (SAR). The purpose of this work is to develop an efficient fat suppression method (Spectrally-Presaturated Modulation (SPM)) for STEAM sequences to enable imaging at high temporal-resolution, with high SNR, and no increase in scan time. Methods The developed method is based on saturating the fat magnetization prior to applying the STEAM modulation; therefore, only the water-content of the tissues is modulated by the sequence, resulting in fat-suppressed images without the need to run the fat suppression module during image acquisition (Figure 1). The potential significance of the proposed method has been tested in two STEAM-based cardiac MRI applications: complementary spatial-modulation of magnetization (CSPAMM) tagging, and black-blood cine imaging. In vivo experiments were conducted to evaluate the developed technique and compare it to the commonly implemented chemical-shift selective (CHESS)

Ultrahigh field single-refocused diffusion weighted imaging using a matched-phase adiabatic spin echo (MASE)

To improve ultrahigh field diffusion-weighted imaging (DWI) in the presence of inhomogeneous transmit B1 field by designing a novel semi-adiabatic single-refocused DWI technique. A 180° slice-selective, adiabatic radiofrequency (RF) pulse of 4 ms duration was designed using the adiabatic Shinnar-Le Roux algorithm. A matched-phase slice-selective 90° RF pulse of 8 ms duration was designed to compensate the nonlinear phase of the adiabatic 180° RF pulse. The resulting RF pulse combination, matched-phase adiabatic spin echo (MASE), was integrated into a single-shot echo planar DWI sequence. The performance of this sequence was compared with single-refocused Stejskal-Tanner (ST), twice-refocused spin echo (TRSE) and twice-refocused adiabatic spin echo (TRASE) in simulations, phantoms, and healthy volunteers at 7 Tesla (T). In regions with inhomogeneous B1 , MASE resulted in increased signal intensity compared with ST (up to 64%). Moderate increase in specific absorption rate (35-39%) was observed for adiabatic RF pulses. MASE resulted in higher signal homogeneity at 7T, leading to improved visualization of measures derived from diffusion-weighted images such as white matter tractography and track density images. Efficient adiabatic SLR pulses can be adapted to single-refocused DWI, leading to substantially improved signal uniformity when compared with conventional acquisitions.

Tilt optimized flip uniformity (TOFU) RF pulse for uniform image contrast at low specific absorption rate levels in combination with a surface breast coil at 7 Tesla.

Going to ultrahigh field MRI (e.g., 7 Tesla [T]), the nonuniformity of the B1+ field and the increased radiofrequency (RF) power deposition become challenging. While surface coils improve the power efficiency in B1+, its field remains nonuniform. In this work, an RF pulse was designed that uses the slab selection to compensate the inhomogeneous B1+ field of a surface coil without a substantial increase in specific absorption rate (SAR). A breast surface coil was used with a decaying B1+ field in the anterior-posterior direction of the human breast. Slab selective RF pulses were designed and compared with adiabatic and spokes RF pulses. Proof of principle was demonstrated with FFE and B1+ maps of the human breast. In vivo measurements obtained with the breast surface coil show that the tilt optimized flip uniformity (TOFU) RF pulses can improve the flip angle homogeneity by 31%, while the SAR will be lower compared with BIR-4 and spokes RF pulses. By applying TOFU RF pulses to the breast surface coil, we are able to compensate the inhomogeneous B1+ field, while keeping the SAR low. Therefore stronger T1 -weighting in FFE sequences can be obtained, while pulse durations can remain short, as shown in the human breast at 7T.

Structured superparamagnetic nanoparticles for high performance mediator of magnetic fluid hyperthermia: Synthesis, colloidal stability and biocompatibility evaluation

Core–shell structures with magnetic core and metal/polymer shell provide a new opportunity for constructing highly efficient mediator for magnetic fluid hyperthermia. Herein, a facile method is described for the synthesis of superparamagnetic LSMO@Pluronic F127 core–shell nanoparticles. Initially, the surface of the LSMO nanoparticles is functionalized with oleic acid and the polymeric shell formation is achieved through hydrophobic interactions with oleic acid. Each step is optimized to get good dispersion and less aggregation. This methodology results into core–shell formation, of average diameter less than 40nm, which was stable under physiological conditions. After making a core–shell formulation, a significant increase of specific absorption rate (up to 300%) has been achieved with variation of the magnetization (<20%). Furthermore, this high heating capacity can be maintained in various simulated physiological conditions. The observed specific absorption rate is almost higher than Fe3O4. MTT assay is used to evaluate the toxicity of bare and core–shell MNPs. The mechanism of cell death by necrosis and apoptosis is studied with sequential staining of acridine orange and ethidium bromide using fluorescence and confocal microscopy. The present work reports a facile method for the synthesis of core–shell structure which significantly improves SAR and biocompatibility of bare LSMO MNPs, indicating potential application for hyperthermia.

Numerical simulation of SAR induced around Co‐Cr‐Mo hip prostheses in situ exposed to RF fields associated with 1.5 and 3 T MRI body coils

When patients with metallic prosthetic implants undergo an MR procedure, the interaction between the RF field and the prosthetic device may lead to an increase in specific absorption rate (SAR) in tissues surrounding the prosthesis. In this work, the distribution of SAR(10g) around bilateral CoCrMo alloy hip prostheses in situ in anatomically realistic voxel models of an adult male and female due to RF fields from a generic birdcage coil driven at 64 or 128 MHz are predicted using a time-domain finite integration technique. Results indicate that the spatial distribution and maximum values of SAR(10g) are dependent on body model, frequency, and the position of the coil relative to the body. Enhancement of SAR(10g) close to the extremities of a prosthesis is predicted. Values of SAR(10g) close to the prostheses are compliant with recommended limits if the prostheses are located outside the coil. However, caution is required when the prostheses are within the coil since the predicted SAR(10g) close to an extremity of a prosthesis exceeds recommended limits when the whole body averaged SAR is 2 W kg(-1) . Compliance with recommended limits is likely to require a reduction in the time averaged input power.

Adapted RF pulse design for SAR reduction in parallel excitation with experimental verification at 9.4 T

Parallel excitation holds strong promises to mitigate the impact of large transmit B 1 ( B 1 + ) distortion at very high magnetic field. Accelerated RF pulses, however, inherently tend to require larger values in RF peak power which may result in substantial increase in Specific Absorption Rate (SAR) in tissues, which is a constant concern for patient safety at very high field. In this study, we demonstrate adapted rate RF pulse design allowing for SAR reduction while preserving excitation target accuracy. Compared with other proposed implementations of adapted rate RF pulses, our approach is compatible with any k-space trajectories, does not require an analytical expression of the gradient waveform and can be used for large flip angle excitation. We demonstrate our method with numerical simulations based on electromagnetic modeling and we include an experimental verification of transmit pattern accuracy on an 8 transmit channel 9.4 T system.

Spectrally selective B1‐insensitive T2 magnetization preparation sequence

A T(2) magnetization-preparation (T(2) Prep) sequence is proposed that is insensitive to B(1) field variations and simultaneously provides fat suppression without any further increase in specific absorption rate (SAR). Increased B(1) inhomogeneity at higher magnetic field strength (B(0) > or = 3T) necessitates a preparation sequence that is less sensitive to B(1) variations. For the proposed technique, T(2) weighting in the image is achieved using a segmented B(1)-insensitive rotation (BIR-4) adiabatic pulse by inserting two equally long delays, one after the initial reverse adiabatic half passage (AHP), and the other before the final AHP segment of a BIR-4 pulse. This sequence yields T(2) weighting with both B(1) and B(0) insensitivity. To simultaneously suppress fat signal (at the cost of B(0) insensitivity), the second delay is prolonged so that fat accumulates additional phase due to its chemical shift. Numerical simulations as well as phantom and in vivo image acquisitions were performed to show the efficacy of the proposed technique.

Magnetic Resonance Angiography of Chest and Abdomen at 3 T

During the past decade, contrast-enhanced magnetic resonance angiography (CE-MRA) has been proven to be a powerful tool to visualize the thoracoabdominal vasculature and, consequently, has become a widely accepted noninvasive imaging modality. With the more recent introduction of high-field whole-body magnetic resonance scanners, a further improvement of diagnostic accuracy can be expected. General considerations for performing high-resolution CE-MRA at higher field strength include the benefits of higher signal-to-noise ratio and an improved contrast between vascular and background tissues. Although there are many positive attributes for performing CE-MRA at 3 T, there are also some tradeoffs, such as static magnetic field inhomogeneity and increase in specific absorption rate. This review describes the main technical innovations of advanced CE-MRA techniques at 3 T, illustrated by characteristic cases.

Nitrogen absorption by roots as a cause of interspecific variations in leaf nitrogen concentration and photosynthetic capacity

Summary Optimal biomass allocation models predicted that an increase in specific absorption rate (SAR: nitrogen absorption rate per unit root dry weight) increases the optimal leaf N concentration (LNC) which maximizes whole‐plant growth rates. From this prediction, we hypothesized that inherent differences in the N absorption ability of roots, which is represented by differences in SAR, causes interspecific differences in LNC and photosynthetic capacity (Pmax). Four deciduous tree species and three herb species were grown under controlled conditions to test this hypothesis. Despite growing under the same soil N conditions, the SAR of these species differed more than sixfold, with the deciduous trees having a smaller SAR than the herbs. Consistent with the hypothesis, there were strong positive correlations between SAR, LNC and Pmax. Leaf dry mass per area and the LNC–Pmax relationship, factors correlated with leaf life span, differed little among the species, suggesting a small effect of differences in these properties on the variation in LNC. Simulations were performed using an optimal biomass allocation model and parameter values determined from measurements of each species. These demonstrated that the observed variation in LNC could be explained largely by differences in the N absorption ability of the species. Additionally, the causal relationship between N absorption ability and LNC, suggested by optimal biomass allocation models, was verified by manipulating the biomass allocation between roots and leaves of Morus bombycis using uniconazole, an inhibitor of gibberellin synthesis. These results indicate a close functional link between N absorption ability and LNC, which would account for large variations in LNC and Pmax across species.

Increase in specific absorption rate in human heads arising from implantations

The effect of metallic implantations on the absorbed power distribution within a human head is studied numerically using a lossy dielectric sphere containing conducting elements excited by the near field of a dipole antenna.

The biology of mycorrhiza in the Ericaceae

SUMMARYThe effect of mycorrhizal infection on the uptake and distribution of iron in Calluna vulgaris was examined when plants were grown in sand supplied with different concentrations of calcium as CaC12 or CaCO3. Infection consistently caused highly significant increases in Fe concentration of Calluna shoots in CaCl2 treatments. A similar effect was obtained with CaCO3 but only in the lowest Ca treatments. Iron concentrations in roots followed the same pattern, being particularly high in all CaCl2 and in the three lowest CaCOa treatments.Total iron contents of shoots were increased by 225% with infection in CaCl2 treatments. In the presence of CaCO3 the average increase in Fe content in shoots of infected plants was 77%, enhancement being restricted to the lowest Ca treatments (0‐50μg ml−1). In roots, the patterns were similar except that since the total Fe contents were higher than in shoots, the proportional differences between infected and uninfected plants were greater.Mycorrhizal infection caused a significant overall increase in specific absorption rate of iron in CaCl2 treatments. In CaCO3 the specific absorption rates of mycorrhizal plants were approximately double those of uninfected plants at the four lowest Ca concentrations but at higher concentrations of the salt the reverse situation was obtained.The nutritional implications of the results are explored, special emphasis being placed upon the possible role of mycorrhizal infection in soils of high calcium content and in situations where iron is present in sparingly soluble mineral combination with phosphates.