Accurate Specific Absorption Rate Research Articles

Individualized and accurate SAR characterization method based on equivalent circuit model for MRI system.

To protect patients from RF heating in MRI scan, this work proposes an accurate and patient-specific whole-body specific absorption rate (SAR) characterization method based on an equivalent circuit model. Compared to the standard pulse energy method defined in NEMA MS 8-2016, this method avoids the complexity of integrating flux loops and has the potential to be easily implemented in MRI scanners. In this study, we use an equivalent parallel circuit to model the power distribution on the transmit coil and subject. The coil and subject equivalent resistances are fitted by the frequency response functions of reflection coefficient and are thereafter used to calculate the power ratio between them. To assess the accuracy of this method, we measured the subject absorbed power of 2 phantoms and 5 volunteers and compared it with the standard pulse energy method with flux loops. The resistances, resonant frequencies, and quality factors of the transmit coil are fitted with the equivalent circuit model in both unloaded and loaded conditions. Whole-body SAR of 5 volunteers is measured at 2 different landmarks. In addition, the relationship between SAR and the working frequencies of the transmit coil is measured and analyzed. The subject absorbed power measured by the proposed method demonstrates good accuracy (RMS error and maximum error of 3.77% and 9.47%, respectively) relative to the flux loop method. The equivalent circuit model-based method enables individualized, accurate, and simplified SAR characterization for clinical applications and research with moderate implementation complexity.

Average SAR prediction, validation, and evaluation for a compact MR scanner head-sized RF coil

A recently developed compact 3 T (C3T) MRI scanner with high performance gradients [1, 2] has a dedicated radiofrequency (RF) transmit coil that exposes only the head, neck and a small portion of the upper body region during head-first scanning. Due to the unique coil geometry and patient positioning, the established SAR model used for a conventional whole-body scanner cannot be directly translated to the C3T. Here a specific absorption rate (SAR) estimation and validation framework was developed and used to implement a dedicated and accurate SAR prediction model for the C3T. Two different SAR prediction models for the C3T were defined and evaluated: one based on an anatomically derived exposed mass, and one using a fixed anatomical position located caudally to the RF coil to determine the exposed mass. After coil modeling and virtual human body simulation, the designed SAR prediction model was implemented on the C3T and verified with calorimetry and in vivo scan power monitoring. The fixed-demarcation exposed mass model was selected as appropriate exposed mass region to accurately estimate the SAR deposition in the patient on the C3T.

An investigation into the minimum number of tissue groups required for 7T in‐silico parallel transmit electromagnetic safety simulations in the human head

Safety limits for the permitted specific absorption rate (SAR) place restrictions on pulse sequence design, especially at ultrahigh fields (≥ 7 tesla). Due to intersubject variability, the SAR is usually conservatively estimated based on standard human models that include an applied safety margin to ensure safe operation. One approach to reducing the restrictions is to create more accurate subject-specific models from their segmented MR images. This study uses electromagnetic simulations to investigate the minimum number of tissue groups required to accurately determine SAR in the human head. Tissue types from a fully characterized electromagnetic human model with 47 tissue types in the head and neck region were grouped into different tissue clusters based on the conductivities, permittivities, and mass densities of the tissues. Electromagnetic simulations of the head model inside a parallel transmit head coil at 7 tesla were used to determine the minimum number of required tissue clusters to accurately determine the subject-specific SAR. The identified tissue clusters were then evaluated using 2 additional well-characterized electromagnetic human models. A minimum of 4-clusters-plus-air was found to be required for accurate SAR estimation. These tissue clusters are centered around gray matter, fat, cortical bone, and cerebrospinal fluid. For all 3 simulated models, the parallel transmit maximum 10g SAR was consistently determined to within an error of <12% relative to the full 47-tissue model. A minimum of 4-clusters-plus-air are required to produce accurate personalized SAR simulations of the human head when using parallel transmit at 7 tesla.

Global and peak local specific absorption rate control on parallel transmit systems using k-means SAR compression model.

To improve the specific absorption rate (SAR) compression model capability in parallel transmission (pTx) MRI systems. A k-means clustering method is proposed to group voxels with similar SAR behaviors in the scanned object, providing a controlled upper-bounded estimation of peak local SARs. This k-means compression model and the conventional virtual observation point (VOP) model were tested in a pTx MRI framework. The pTx pulse design with different SAR controlling schemes was simulated using a numerical human head model and an eight-channel 7T coil array. Multiple criteria (including RF power, global and peak local SARs, and excitation accuracy) were compared for the performance testing. The k-means compression model generated a narrower overestimation bound, leading to a more accurate local SAR estimation. Among different pTx pulse design approaches, the k-means compression model showed the best trade-off between the SAR and excitation accuracy. The developed SAR compression model is advantageous for pTx framework given the narrower overestimation bound and control over the compression ratio. Results also illustrate that a moderate increase of maximum RF power can be useful for reducing the maximum local SAR deposition.

Ultrathin surface coated water-soluble cobalt ferrite nanoparticles with high magnetic heating efficiency and rapid in vivo clearance

To obtain magnetic nanoparticles with high magnetic heating efficiency and rapid in vivo clearance, this study utilized an improved linear response theory model to theoretically simulate the specific absorption rate (SAR) value versus the particle size of cobalt ferrite nanoparticles (CFNPs). An accurate SAR curve consistent with experimental results was obtained using cubes instead of spheres as the shape of CFNPs, given that cube was closer to the actual shape of prepared CFNPs. Under the guidance of simulation, we predicted and prepared water-soluble cubic CFNPs of 10–13 nm in size, with an ultrathin surface coating less than 1 nm in thickness. These CFNPs were experimentally verified to have high magnetic heating efficiency and rapid in vivo clearance rate. Our CFNPs of 11.8 nm in size had a high intrinsic loss power of 12.11 nHm2/kg. Most of the cells were killed within 30 min under magnetic heating with CFNPs. In an in vivo study, these CFNPs can heat a tumor area to 45 °C (ΔT > 9 °C) within 120 s under a weak alternating magnetic field (27 kA/m, 115 kHz). Notably, these CFNPs had significant tumor inhibition rate in vivo and can be cleared from the body by more than 64% within 2 weeks, demonstrating excellent rapid in vivo clearance. This result was close to the clearance level of the magnetic resonance imaging contrast agent Feridex. Therefore, our CFNPs had high magnetic heating efficiency and rapid in vivo clearance rate, indicating their great potential for future clinical applications.

Uncertainties in the estimation of specific absorption rate during radiofrequency alternating magnetic field induced non-adiabatic heating of ferrofluids

Magnetic fluid hyperthermia (MFH) is becoming a viable cancer treatment methodology where the alternating magnetic field induced heating of magnetic fluid is utilized for ablating the cancerous cells or making them more susceptible to the conventional treatments. The heating efficiency in MFH is quantified in terms of specific absorption rate (SAR), which is defined as the heating power generated per unit mass. In majority of the experimental studies, SAR is evaluated from the temperature rise curves, obtained under non-adiabatic experimental conditions, which is prone to various thermodynamic uncertainties. A proper understanding of the experimental uncertainties and its remedies is a prerequisite for obtaining accurate and reproducible SAR. Here, we study the thermodynamic uncertainties associated with peripheral heating, delayed heating, heat loss from the sample and spatial variation in the temperature profile within the sample. Using first order approximations, an adiabatic reconstruction protocol for the measured temperature rise curves is developed for SAR estimation, which is found to be in good agreement with those obtained from the computationally intense slope corrected method. Our experimental findings clearly show that the peripheral and delayed heating are due to radiation heat transfer from the heating coils and slower response time of the sensor, respectively. Our results suggest that the peripheral heating is linearly proportional to the sample area to volume ratio and coil temperature. It is also observed that peripheral heating decreases in presence of a non-magnetic insulating shielding. The delayed heating is found to contribute up to ~25% uncertainties in SAR values. As the SAR values are very sensitive to the initial slope determination method, explicit mention of the range of linear regression analysis is appropriate to reproduce the results. The effect of sample volume to area ratio on linear heat loss rate is systematically studied and the results are compared using a lumped system thermal model. The various uncertainties involved in SAR estimation are categorized as material uncertainties, thermodynamic uncertainties and parametric uncertainties. The adiabatic reconstruction is found to decrease the uncertainties in SAR measurement by approximately three times. Additionally, a set of experimental guidelines for accurate SAR estimation using adiabatic reconstruction protocol is also recommended. These results warrant a universal experimental and data analysis protocol for SAR measurements during field induced heating of magnetic fluids under non-adiabatic conditions.

MRI scanner-independent specific absorption rate measurements using diffusion coefficients.

ObjectiveThe purpose of this study was to measure specific absorption rate (SAR) during MRI scanning using a human torso phantom through quantification of diffusion coefficients independently of those reported by the scanner software for five 1.5 and 3 T clinical MRI systems from different vendors.MethodsA quadrature body coil transmitted the RF power and a body array coil received the signals. With diffusion tensor imaging, SAR values for three MRI sequences were measured on the five scanners and compared to the nominal values calculated by the scanners.ResultsFor the GE 1.5 T MRI system, the MRI scanner‐reported SAR value was 1.58 W kg‐1 and the measured SAR value was 1.38 W kg‐1. For the Philips 1.5 T MRI scanner, the MRI system‐reported SAR value was 1.48 W kg‐1 and the measured value was 1.39 W kg‐1. For the Siemens 3 T MRI system, the reported SAR value was 2.5 W kg‐1 and the measured SAR value was 1.96 W kg‐1. For two Philips 3 T MRI scanners, the reported SAR values were 1.5 W kg‐1 and the measured values were 1.94 and 1.96 W kg‐1. The percentage differences between the measured and reported SAR values on the GE 1.5 T, Philips 1.5 T, Siemens 3 T, and Philips 3 T were 13.5, 6.3, 24.2, 25.6, and 26.6% respectively.ConclusionThe scanner‐independent SAR measurements using diffusion coefficients described in this study can play a significant role in estimating accurate SAR values as a standardized method.

In vivo electric conductivity of cervical cancer patients based on maps at 3T MRI

The in vivo electric conductivity (σ) values of tissue are essential for accurate electromagnetic simulations and specific absorption rate (SAR) assessment for applications such as thermal dose computations in hyperthermia. Currently used σ-values are mostly based on ex vivo measurements. In this study the conductivity of human muscle, bladder content and cervical tumors is acquired non-invasively in vivo using MRI.The conductivity of 20 cervical cancer patients was measured with the MR-based electric properties tomography method on a standard 3T MRI system.The average in vivo σ-value of muscle is 14% higher than currently used in human simulation models. The σ-value of bladder content is an order of magnitude higher than the value for bladder wall tissue that is used for the complete bladder in many models. Our findings are confirmed by various in vivo animal studies from the literature. In cervical tumors, the observed average conductivity was 13% higher than the literature value reported for cervical tissue.Considerable deviations were found for the electrical conductivity observed in this study and the commonly used values for SAR assessment, emphasizing the importance of acquiring in vivo conductivity for more accurate SAR assessment in various applications.

Evaluation of radiofrequency safety by high temperature resolution MR thermometry using a paramagnetic lanthanide complex.

The current practice of calculating the specific absorption rate (SAR) relies on local temperature measurements made using temperature probes. For an accurate SAR measurement, a temperature imaging method that provides high temperature sensitivity is desirable, because acceptable levels of SAR produce small temperature changes. MR thermometry using paramagnetic lanthanide complexes can be used to obtain absolute temperature measurements with sub-degree temperature and sub-millimeter spatial resolution. The aim of this study was to develop and evaluate a high temperature resolution MR technique to determine SAR. MR thermometry using a paramagnetic lanthanide complex thulium 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrakis (methylene phosphonate) (TmDOTP(5-)), which has an almost 10(2) times stronger chemical shift temperature dependence than water, was used to develop a novel method for SAR measurement. Three-dimensional temperature and SAR images were calculated using MR images acquired with a conventional gradient recalled echo sequence and SAR-intensive T1ρ sequence. Effects of the presence of conducting wire and increasing T1ρ spin-lock pulse duration were also examined. SAR distribution could be visualized clearly and surges associated with conducting wires and increasing pulse duration were identified clearly in the computed high spatial resolution SAR images. A novel method with high temperature sensitivity is proposed as a tool to evaluate radiofrequency safety in MRI.

Patient‐individual local SAR determination: In vivo measurements and numerical validation

Tissue heating during magnetic resonance measurements is a potential hazard at high-field MRI, and particularly, in the framework of parallel radiofrequency transmission. The heating is directly related to the radiofrequency energy absorbed during an magnetic resonance examination, that is, the specific absorption rate (SAR). SAR is a pivotal parameter in MRI safety regulations, requiring reliable estimation methods. Currently used methods are usually based on models which are neither patient-specific nor taken into account patient position and posture, which typically leads to the need for large safety margins. In this work, a novel approach is presented, which measures local SAR in a patient-specific manner. Using a specific formulation of Maxwell's equations, the local SAR is estimated via postprocessing of the complex transmit sensitivity of the radiofrequency antenna involved. The approximations involved in the proposed method are investigated. The presented approach yields a sufficiently accurate and patient-specific local SAR measurement of the brain within a scan time of less than 5 min.

Analysis of the effect of mobile phone base station antenna loading on localized SAR and its consequences for measurements

In this work, the effect of antenna element loading on the localized specific absorption rate (SAR) has been analyzed for base station antennas. The analysis was conducted in order to determine whether localized SAR measurements of large multi-element base station antennas can be conducted using standardized procedures and commercially available equipment. More specifically, it was investigated if the antenna shifting measurement procedure, specified in the European base station exposure assessment standard EN 50383, will produce accurate localized SAR results for base station antennas larger than the specified measurement phantom. The obtained results show that SAR accuracy is affected by the presence of lossy material within distances of one wavelength from the tested antennas as a consequence of coupling and redistribution of transmitted power among the antenna elements. It was also found that the existing standardized phantom is not optimal for SAR measurements of large base station antennas. A new methodology is instead proposed based on a larger, box-shaped, whole-body phantom.

Specific Absorption Rates and Magnetic Properties of Ferrofluids with Interaction Effects at Low Concentrations

The specific absorption rate (SAR) of a maghemite-based ferrofluid, measured at 315 K, 3 kA/m, and 109 kHz, was found to double as the ferrofluid concentration was decreased by a factor of 4. The ferrofluid contained nonagglomerated, highly crystalline, and monodisperse nanoparticles with an average size of 11.6 nm and an initial concentration of 8.14 mg/mL. The magnetic characterization of three different concentrations of this ferrofluid revealed several effects typical of the presence of magnetic interactions, such as the decrease of initial susceptibility values (liquid ferrofluid) and Neel relaxation times, τN (frozen ferrofluid), with increasing concentration. The accurate SAR determination in adiabatic conditions allowed estimating the τN values of the liquid ferrofluid, which displayed the same trend against concentration as those obtained in the frozen state. Such a trend allowed explaining qualitatively the degradation of the heating performance of the ferrofluid upon increasing concentration. E...

Specific Absorption Rate Measurement of Birdcage Coil for 3.0-T Magnetic Resonance Imaging System Employing Thermographic Method

Magnetic resonance imaging (MRI) is a noninvasive imaging techniques used to visualize the inside of the human body based on nuclear magnetic resonance. Currently, RF coils that transmit high frequency and high power of electromagnetic (EM) wave pulses are being investigated to obtain high-quality images and to shorten the imaging time. Due to these approaches, the specific absorption rate (SAR) rise inside the human body has to be concerned during the imaging. Thus far, the SAR has been evaluated by calculating the EM fields inside the human body, and the safety of the human body has been reported. Meanwhile, according to the IEC standards, there are two methods to measure the total absorption energy of the human body model, although these methods cannot be used to evaluate detailed SAR distribution inside the body. It is important to comprehend detailed SAR distribution by measurement in order to consider reducing the SAR. In this paper, by use of the thermographic method, detailed SAR distribution inside the human head model was intended to measure employing birdcage coil for a 3.0-T MRI system. In the thermographic method, short time radiation and a sufficient temperature rise are required to obtain accurate SAR distribution. However, the effect of temperature diffusion inside the phantom caused by long time radiation depending on circumstances in the experiment must also be considered. In order to determine the temperature diffusion, temperature transition inside the phantom during the experiment was also measured by using a fluoroptic thermosensor. Then, multiplying the correction coefficients, which were calculated from the temperature transition by the measured SAR values, it was found that accurate SAR values could be obtained despite the long radiation time, and in addition, the measurement and calculation of SAR distribution were in good agreement. From these results, it has been verified that the proposed measurement method has a possibility of measuring detailed SAR distribution inside the phantom when any conventional RF coil is used.

DOSIMETRIC COMPARISON BETWEEN DIFFERENT QUANTITIES FOR LIMITING EXPOSURE IN THE RF BAND: RATIONALE AND IMPLICATIONS FOR GUIDELINES

While source type and its distance from the subject are influential parameters, the fundamental exposure metrics are the physically measurable quantities of currents, external electric field and magnetic field strengths, and incident power density (when appropriate), which induce electric and magnetic fields that bring about the interaction of radiofrequency (RF) electromagnetic energy with biological systems. Induced fields are the primary cause for biological effect of RF exposure, regardless of the mechanism. Thus, in order to achieve a quantitative understanding of biological response, the induced electric field and the derived dosimetric quantities of specific absorption rate (SAR) and current density must be quantified and correlated with the observed phenomenon. In the established exposure guidelines, reference levels expressed in terms of physical quantities are introduced for practical exposure assessment purposes since the basic restrictions are often specified as dosimetric quantities that may be impractical to measure. The dosimetric quantity SAR, current density, and their determinations are tissue-type dependent and require a region of specific tissue mass for averaging. Thus, a smaller averaging region is scientifically more relevant and precise. It is emphasized that the sensitivity and resolution of present-day computational resources and experimental measurement techniques can provide accurate SAR values with a spatial resolution on the order of a 1 mm, in dimensions. Furthermore, most exposure guidelines are promulgated on a 4 W kg SAR to prevent any whole-body exposure from raising the body temperature to 1 degrees C above the norm at 37 degrees C. Special guidance may be needed for higher ambient temperature and humidity variations.

High spatial resolution measurements of specific absorption rate around ICD leads

Introduction: It has been shown that strong electric shocks can cause local refractoriness in the heart. This is of particular concern if the region of refractoriness is the area sensed by an implant to determine cardiac rhythm, as is the case with many Implantable Cardioverter Defibrillator (ICD) leads which use the same electrodes for shocking and sensing. Failure to sense the true cardiac rhythm can cause application of unnecessary shocks and potential induction of arrhythmias. We developed a system to accurately map the areas where local refractoriness is most probable. We measured the Specific Absorption Rate (SAR) around typical ICD leads. Current density ( J), a parameter that determines defibrillation effectiveness, is proportional to the square root of SAR. Methods and results: SAR measurements were performed in a homogeneous saline media using a variety of ICD leads. Gated 60 Hz shocks were used to produce heating, which was measured by thermistor probes. The temperature-rate-of-change is directly proportional to the SAR. Measurement techniques were developed that produced accurate SAR results at high spatial resolutions. Multiple polarities and configurations of ICD leads were tested. Conclusions: We confirmed the spatial distribution of the SAR and corresponding current density possessed sharp peaks and were highly localized around the leads' electrodes. Scans with a resolution of 1 mm or less are required in the area of peak SAR in order to capture the peak's value.

Analytic SAR computation in a multilayer elliptic cylinder for bioelectromagnetic applications

The specific absorption rate (SAR) is usually considered as the basic quantity to derive the reference levels for the exposure of workers and general population. In this paper, we propose an analytical procedure for the SAR computation inside a biological elliptic cylinder model made up of layers of different biological tissues. The procedure makes it possible to obtain accurate SAR values in simplified models of biological subsystems, and it is also helpful to test numerical techniques to be used for more realistic models and to generate synthetic input data for diagnostic methodologies. For the assumed model, the calculation of the analytical solution has been obtained by generalizing a known procedure that deals only with lossless materials, and the model makes possible the calculation of the SAR for realistic human tissues. Various calculations prove the reliability of the technique.

Analytic SAR computation in a multilayer elliptic cylinder for bioelectromagnetic applications

The specific absorption rate (SAR) is usually considered as the basic quantity to derive the reference levels for the exposure of workers and general population. In this paper, we propose an analytical procedure for the SAR computation inside a biological elliptic cylinder model made up of layers of different biological tissues. The procedure makes it possible to obtain accurate SAR values in simplified models of biological subsystems, and it is also helpful to test numerical techniques to be used for more realistic models and to generate synthetic input data for diagnostic methodologies. For the assumed model, the calculation of the analytical solution has been obtained by generalizing a known procedure that deals only with lossless materials, and the model makes possible the calculation of the SAR for realistic human tissues. Various calculations prove the reliability of the technique. Bioelectromagnetics 20:365–371, 1999. © 1999 Wiley-Liss, Inc.