Determination Of Specific Absorption Rate Research Articles

Real-time assessment of potential peak local specific absorption rate value without phase monitoring: Trigonometric maximization method for worst-case local specific absorption rate determination.

PurposeMulti‐transmit MRI systems are typically equipped with dedicated hardware to sample the reflected/lost power in the transmit channels. After extensive calibration, the amplitude and phase of the signal at the feed of each array element can be accurately determined. However, determining the phase is more difficult and monitoring errors can lead to a hazardous peak local specific absorption rate (pSAR10g) underestimation. For this purpose, methods were published for online maximum potential pSAR10g estimation without relying on phase monitoring, but these methods produce considerable overestimation. We present a trigonometric maximization method to determine the actual worst‐case pSAR10g without any overestimation.Theory and MethodThe proposed method takes advantage of the sinusoidal relation between the SAR10g in each voxel and the phases of input signals, to return the maximum achievable SAR10g in a few iterations. The method is applied to determine the worst‐case pSAR10g for three multi‐transmit array configurations at 7T: (1) body array with eight fractionated dipoles; (2) head array with eight fractionated dipoles; (3) head array with eight rectangular loops. The obtained worst‐case pSAR10g values are compared with the pSAR10g values determined with a commonly used method and with a more efficient method based on reference‐phases.ResultsFor each voxel, the maximum achievable SAR10g is determined in less than 0.1 ms. Compared to the reference‐phases‐based method, the proposed method reduces the mean overestimation of the actual pSAR10g up to 52%, while never underestimating the true pSAR10g.ConclusionThe proposed method can widely improve the performance of parallel transmission MRI systems without phase monitoring.

Simple and fast evaluation of relaxation parameters of magnetic nanoparticles

The efficacy of magnetic hyperthermia treatment depends on the optimal available magnetic nanoparticles (MNPs) that are excited in a given alternating magnetic field and viscosity of the region of interest. In this regard, assessing the relevant relaxation parameters is of upmost importance and could improve the speed of development of efficient applications. Here, we demonstrate how to deduce all relevant magnetic parameters from fast, independent, and simple experimental measurements such as dynamic light scattering, vibrating sample magnetometer, and lock-in thermography. We study the thermal behaviour of two MNPs with different forms, i.e. spherical and cubical, synthesized in-house by thermal decomposition and coated with 4 different surface agents. By determination of specific absorption rate (SAR) values, hydrodynamic diameters and M−H curves it is possible to compute the magnetic particle volume, dominant relaxation time and magnetocrystalline anisotropy constant. The calculated SAR values derived from these parameters, show good agreement with the experimental determined SAR data, demonstrating the applicability of the reported procedure. Additionally, our results indicate that surface coatings can have minor impacts on the thermal dissipation of Néel relaxation dominated MNPs.

Specific absorption rate determination of magnetic nanoparticles through hyperthermia measurements in non-adiabatic conditions

An experimental setup for magnetic hyperthermia operating in non-adiabatic conditions is described. A thermodynamic model that takes into account the heat exchanged by the sample with the surrounding environment is developed. A suitable calibration procedure is proposed that allows the experimental validation of the model. Specific absorption rate can then be accurately determined just from the measurement of the sample temperature at the equilibrium steady state. The setup and the measurement procedure represent a simplification with respect to other systems requiring calorimeters or crucial corrections for heat flow. Two families of magnetic nanoparticles, one superparamagnetic and one characterised by larger sizes and static hysteresis, have been characterised as a function of field intensity, and specific absorption rate and intrinsic loss power have been obtained.

Feasibility of electric property tomography of pelvic tumors at 3T.

Investigation of the validity of the "transceive phase assumption" for Electric Property Tomography of pelvic tumors at 3T. The acquired electric conductivities of pelvic tumors are beneficial for improved specific absorption rate determination in hyperthermia treatment planning. Electromagnetic simulations and magnetic resonance imaging measurements of a pelvic-sized phantom and the human pelvis of a volunteer and a cervix cancer patient. The reconstructed conductivity values of the phantom tumor model are in good quantitative agreement (mean deviation: 1-10%) with the probe measurements. Furthermore, the average reconstructed conductivity of a pelvic tumor model was in close agreement with the input conductivity (0.86 S/m vs. 0.90 S/m). The reconstructed tumor conductivity of the presented patient (cervical carcinoma, Stage: IVA) was 1.16 ± 0.40 S/m. This study demonstrates the feasibility of electric property tomography to measure quantitatively the conductivity of centrally located tumors in a pelvic-sized phantom and human pelvis with a standard magnetic resonance (MR) system and MR sequences. A good quantitative agreement was found between the reconstructed σ-values and probe measurements for a wide range of σ-values and for off-axis located spherical compartment. As most pelvic tumors are located in the central region of the pelvis, these results can be exploited in hyperthermia treatment planning systems.

Adiabatic vs. non-adiabatic determination of specific absorption rate of ferrofluids

The measurement of temperature variations in adiabatic conditions allows the determination of the specific absorption rate of magnetic nanoparticles and ferrofluids from the correct incremental expression, SAR=(1/ m MNP ) C(Δ T/Δ t). However, when measurements take place in non-adiabatic conditions, one must approximate this expression by SAR≈ Cβ/ m MNP , where β is the initial slope of the temperature vs. time curve during alternating field application. The errors arising from the use of this approximation were estimated through several experiments with different isolating conditions, temperature sensors and sample–sensor contacts. It is concluded that small to appreciable errors can appear, which are difficult to infer or control.

FDTD chiral brain tissue model for specific absorption rate determination under radiation from mobile phones at 900 and 1800 MHz

A new electrodynamics model formed by chiral bioplasma, which represents the human head inner structure and makes it possible to analyse its behaviour when it is irradiated by a microwave electromagnetic field from cellular phones, is presented. The finite-difference time-domain (FDTD) numeric technique is used, which allows simulation of the electromagnetic fields, deduced with Maxwell's equations, and allows us to simulate the specific absorption rate (SAR). The results show the SAR behaviour as a function of the input power and the chirality factor. In considering the chiral brain tissue in the proposed human head model, the two more important conclusions of our work are the following: (a) the absorption of the electromagnetic fields from cellular phones is stronger, so the SAR coefficient is higher than that using the classical model, when values of the chiral factor are of order of 1; (b) ‘inverse skin effect’ shows up at 1800 MHz, with respect to a 900 MHz source.

Simplified model and measurement of specific absorption rate distribution in a culture flask within a transverse electromagnetic mode exposure system.

In vitro experiments in bioelectromagnetics frequently require the determination of specific absorption rate (SAR) within a layer of cells on the bottom of a culture flask when the SAR has rapid spatial variation both horizontally within the cell layer and vertically in the medium bathing the cells. This problem has only recently been treated in the literature; and it is here approached differently for another irradiation system. It is shown that a simple two-dimensional frequency-domain guided-wave treatment yields results qualitatively comparable to those of more computationally intensive three-dimensional time-domain free-field scattering treatments. The problem of inferring local SARs from temperature-vs.-time curves is shown to be seriously confounded by thermal diffusion; and specific analytic and numerical results are presented to aid in understanding this effect. A novel experimental technique is introduced for measuring millikelvin temperature offsets with subsecond resolution, and illustrative experimental data are presented. Finally, present experimental and theoretical uncertainties are considered; and it is pessimistically asserted that, in a culture flask where spatial SAR variation is rapid, point SAR measurements by thermal methods may be in error by as much as +/- 3 dB. More reliable thermal determinations will require extreme care, challenging technological innovations, or both.