The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
Lawrence L Wald - One of the best experts on this subject based on the ideXlab platform.
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computer vision techniques for water fat separation in ultra high field mri local Specific Absorption Rate estimation
IEEE Transactions on Biomedical Engineering, 2019Co-Authors: Angel Torradocarvajal, Elfar Adalsteinsson, Lawrence L Wald, Yigitcan Eryaman, J L Herraiz, Esra Abaci Turk, Juan Antonio Hernandeztamames, Norberto MalpicaAbstract:Objective: The purpose of this paper is to prove that computer-vision techniques allow synthesizing water-fat separation maps for local Specific Absorption Rate (SAR) estimation, when patient-Specific water-fat images are not available. Methods: We obtained ground truth head models by using patient-Specific water-fat images. We obtained two different label-fusion water-fat models generating a water-fat multiatlas and applying the STAPLE and local-MAP-STAPLE label-fusion methods. We also obtained patch-based water-fat models applying a local group-wise weighted combination of the multiatlas. Electromagnetic (EM) simulations were performed, and B1+ magnitude and 10 g averaged SAR maps were geneRated. Results: We found local approaches provide a high DICE overlap (72.6 ± 10.2% fat and 91.6 ± 1.5% water in local-MAP-STAPLE, and 68.8 ± 8.2% fat and 91.1 ± 1.0% water in patch-based), low Hausdorff distances (18.6 ± 7.7 mm fat and 7.4 ± 11.2 mm water in local-MAP-STAPLE, and 16.4 ± 8.5 mm fat and 7.2 ± 11.8 mm water in patch-based) and a low error in volume estimation (15.6 ± 34.4% fat and 5.6 ± 4.1% water in the local-MAP-STAPLE, and 14.0 ± 17.7% fat and 4.7 ± 2.8% water in patch-based). The positions of the peak 10 g-averaged local SAR hotspots were the same for every model. Conclusion: We have created patient-Specific head models using three different computer-vision-based water-fat separation approaches and compared the predictions of B1+ field and SAR distributions geneRated by simulating these models. Our results prove that a computer-vision approach can be used for patient-Specific water-fat separation, and utilized for local SAR estimation in high-field MRI. Significance: Computer-vision approaches can be used for patient-Specific water-fat separation and for patient Specific local SAR estimation, when water-fat images of the patient are not available.
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robust time shifted spoke pulse design in the presence of large b0 variations with simultaneous reduction of through plane dephasing b1 effects and the Specific Absorption Rate using parallel transmission
Magnetic Resonance in Medicine, 2016Co-Authors: Bastien Guerin, Lawrence L Wald, Andrew V Stenger, Angel Torradocarvajal, Jason P Stockmann, Mehran BaboliAbstract:Purpose To design parallel transmission spokes pulses with time-shifted profiles for joint mitigation of intensity variations due to B1+ effects, signal loss due to through-plane dephasing, and the Specific Absorption Rate (SAR) at 7T. Methods We derived a slice-averaged small tip angle (SA-STA) approximation of the magnetization signal at echo time that depends on the B1+ transmit profiles, the through-slice B0 gradient and the amplitude and time-shifts of the spoke waveforms. We minimize a magnitude least-squares objective based on this signal equation using a fast interior-point approach with analytical expressions of the Jacobian and Hessian. Results Our algorithm runs in less than three minutes for the design of two-spoke pulses subject to hundreds of local SAR constraints. On a B0/B1+ head phantom, joint optimization of the channel-dependent time-shifts and spoke amplitudes allowed signal recovery in high-B0 regions at no increase of SAR. Although the method creates uniform magnetization profiles (ie, uniform intensity), the flip angle varies across the image, which makes it ill-suited to T1-weighted applications. Conclusions The SA-STA approach presented in this study is best suited to T2*-weighted applications with long echo times that require signal recovery around high B0 regions. Magn Reson Med, 2015. © 2015 Wiley Periodicals, Inc.
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parallel transmission pulse design with explicit control for the Specific Absorption Rate in the presence of radiofrequency errors
Magnetic Resonance in Medicine, 2016Co-Authors: Adrian Martin, Elfar Adalsteinsson, Lawrence L Wald, Emanuele Schiavi, Yigitcan Eryaman, J L Herraiz, Borjan Gagoski, Bastien GuerinAbstract:Purpose A new framework for the design of parallel transmit (pTx) pulses is presented introducing constraints for local and global Specific Absorption Rate (SAR) in the presence of errors in the radiofrequency (RF) transmit chain. Methods The first step is the design of a pTx RF pulse with explicit constraints for global and local SAR. Then, the worst possible SAR associated with that pulse due to RF transmission errors (“worst-case SAR”) is calculated. Finally, this information is used to re-calculate the pulse with lower SAR constraints, iterating this procedure until its worst-case SAR is within safety limits. Results Analysis of an actual pTx RF transmit chain revealed amplitude errors as high as 8% (20%) and phase errors above 3° (15°) for spokes (spiral) pulses. Simulations show that using the proposed framework, pulses can be designed with controlled “worst-case SAR” in the presence of errors of this magnitude at minor cost of the excitation profile quality. Conclusion Our worst-case SAR-constrained pTx design stRategy yields pulses with local and global SAR within the safety limits even in the presence of RF transmission errors. This stRategy is a natural way to incorpoRate SAR safety factors in the design of pTx pulses. Magn Reson Med, 2015. © 2015 Wiley Periodicals, Inc.
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design of parallel transmission pulses for simultaneous multislice with explicit control for peak power and local Specific Absorption Rate
Magnetic Resonance in Medicine, 2015Co-Authors: Bastien Guerin, Lawrence L Wald, Kawin Setsompop, Benedikt A Poser, Andrew V StengerAbstract:Purpose To design parallel transmit (pTx) simultaneous multislice (SMS) spokes pulses with explicit control for peak power and local and global Specific Absorption Rate (SAR). Methods We design SMS pTx least-squares and magnitude least squares spokes pulses while constraining local SAR using the virtual observation points (VOPs) compression of SAR matrices. We evaluate our approach in simulations of a head (7T) and a body (3T) coil with eight channels arranged in two z-rows. Results For many of our simulations, control of average power by Tikhonov regularization of the SMS pTx spokes pulse design yielded pulses that violated hardware and SAR safety limits. On the other hand, control of peak power alone yielded pulses that violated local SAR limits. Pulses optimized with control of both local SAR and peak power satisfied all constraints and therefore had the best excitation performance under limited power and SAR constraints. These results extend our previous results for single slice pTx excitations but are more pronounced because of the large power demands and SAR of SMS pulses. Conclusions Explicit control of local SAR and peak power is required to geneRate optimal SMS pTx excitations satisfying both the system's hardware limits and regulatory safety limits. Magn Reson Med 73:1946–1953, 2015. © 2014 Wiley Periodicals, Inc.
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local Specific Absorption Rate sar global sar transmitter power and excitation accuracy trade offs in low flip angle parallel transmit pulse design
Magnetic Resonance in Medicine, 2014Co-Authors: Bastien Guerin, Matthias Gebhardt, Elfar Adalsteinsson, Lawrence L Wald, Steven CauleyAbstract:Purpose We propose a constrained optimization approach for designing parallel transmit (pTx) pulses satisfying all regulatory and hardware limits. We study the trade-offs between excitation accuracy, local and global Specific Absorption Rate (SAR), and maximum and average power for small flip-angle pTx (eight channels) spokes pulses in the torso at 3 T and in the head at 7 T. Methods We compare the trade-offs between the above-mentioned quantities using the L-curve method. We use a primal-dual algorithm and a compressed set of local SAR matrices to design radio-frequency (RF) pulses satisfying all regulatory (including local SAR) and hardware constraints. Results Local SAR can be substantially reduced (factor of 2 or more) by explicitly constraining it in the pulse design process compared to constraining global SAR or pulse power alone. This often comes at the price of increased pulse power. Conclusion Simultaneous control of power and SAR is needed for the design of pTx pulses that are safe and can be played on the scanner. Constraining a single quantity can create large increase in the others, which can then rise above safety or hardware limits. Simultaneous constraint of local SAR and power is fast enough to be applicable in a clinical setting. Magn Reson Med 71:1446–1457, 2014. © 2013 Wiley Periodicals, Inc.
Giorgio Bonmassar - One of the best experts on this subject based on the ideXlab platform.
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changes in the Specific Absorption Rate sar of radiofrequency energy in patients with retained cardiac leads during mri at 1 5t and 3t
Magnetic Resonance in Medicine, 2019Co-Authors: Laleh Golestanirad, Amir Ali Rahsepar, John E Kirsch, Kenichiro Suwa, Jeremy C Collins, Leonardo M Angelone, Boris Keil, Rod S Passman, Giorgio BonmassarAbstract:PURPOSE To evaluate the local Specific Absorption Rate (SAR) and heating around retained cardiac leads during MRI at 64 MHz (1.5T) and 127 MHz (3T) as a function of RF coil type and imaging landmark. METHODS Numerical models of retained cardiac leads were built from CT and X-ray images of 6 patients with retained cardiac leads. Electromagnetic simulations and bio-heat modeling were performed with MRI RF body and head coils tuned to 64 MHz and 127 MHz and positioned at 9 different imaging landmarks covering an area from the head to the lower limbs. RESULTS For all patients and at both 1.5T and 3T, local transmit head coils produced negligible temperature rise ( Δ T 0.1 ° C ) for ‖ ‖ B 1 + ‖ ‖ ≤ 3 μ T . For body imaging with quadrature-driven coils at 1.5T, Δ T during a 10-min scan remained ‖ ‖ B 1 + ‖ ‖ ≤ 3 μ T and ‖ ‖ B 1 + ‖ ‖ ≤ 4 μ T . For body imaging at 3T, Δ T during a 10-min scan remained ‖ ‖ B 1 + ‖ ‖ ≤ 2 μ T . For shorter pulse sequences up to 2 min, Δ T remained ‖ ‖ B 1 + ‖ ‖ ≤ 3 μ T . CONCLUSION For the models based on 6 patients studied, simulations suggest that MRI could be performed safely using a local head coil at both 1.5T and 3T, and with a body coil at 1.5T with pulses that produced ‖ ‖ B 1 + ‖ ‖ ≤ 4 μ T . MRI at 3T could be performed safely in these patients using pulses with ‖ ‖ B 1 + ‖ ‖ ≤ 2 μ T .
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Specific Absorption Rate in a standard phantom containing a deep brain stimulation lead at 3 tesla mri
International IEEE EMBS Conference on Neural Engineering, 2013Co-Authors: Giorgio Bonmassar, Peter Serano, Leonardo M AngeloneAbstract:Finite elements methods (FEM) simulations are presented showing the Specific Absorption Rate (SAR) distribution in the case of a Deep Brain Stimulation (DBS) lead inside a standard phantom exposed to 128 MHz / 3 Tesla magnetic resonance imaging (MRI). The electromagnetic model is proposed as a tool to study the safety of patients with an implanted DBS lead undergoing clinical 3T MRI systems. The simulations were based on an electromagnetic and circuit co-simulation methodology and allowed for fine tuning of the MRI radiofrequency transmit body coil. Results show that the presence of the lead did not influence the behavior of the coil, namely the S-parameters at the coil feed were largely unaffected by the presence of a lead. Conversely, significant changes of electric and magnetic field, as well as local SAR were observed due to the characteristic antenna effect. The lead path strongly affected the level of SAR at the distal tip of the lead, with a 10x fold difference between the two configurations evaluated.
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analysis of the role of lead resistivity in Specific Absorption Rate for deep brain stimulator leads at 3t mri
IEEE Transactions on Medical Imaging, 2010Co-Authors: Leonardo M Angelone, John W Belliveau, Jyrki Ahveninen, Giorgio BonmassarAbstract:Magnetic resonance imaging (MRI) on patients with implanted deep brain stimulators (DBSs) can be hazardous because of the antenna-effect of leads exposed to the incident radio-frequency field. This study evaluated electromagnetic field and Specific Absorption Rate (SAR) changes as a function of lead resistivity on an anatomically precise head model in a 3T system. The anatomical accuracy of our head model allowed for detailed modeling of the path of DBS leads between epidermis and the outer table. Our electromagnetic finite difference time domain (FDTD) analysis showed significant changes of 1 g and 10 g averaged SAR for the range of lead resistivity modeled, including highly conductive leads up to highly resistive leads. Antenna performance and whole-head SAR were sensitive to the presence of the DBS leads only within 10%, while changes of over one order of magnitude were observed for the peak 10 g averaged SAR, suggesting that local SAR values should be considered in DBS guidelines. With ?lead = ?copper , and the MRI coil driven to produce a whole-head SAR without leads of 3.2 W/kg, the 1 g averaged SAR was 1080 W/kg and the 10 g averaged SAR 120 W/kg at the tip of the DBS lead. Conversely, in the control case without leads, the 1 g and 10 g averaged SAR were 0.5 W/kg and 0.6 W/kg, respectively, in the same location. The SAR at the tip of lead was similar with electrically homogeneous and electrically heterogeneous models. Our results show that computational models can support the development of novel lead technology, properly balancing the requirements of SAR deposition at the tip of the lead and power dissipation of the system battery.
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mri based anatomical model of the human head for Specific Absorption Rate mapping
Medical & Biological Engineering & Computing, 2008Co-Authors: Leonardo M Angelone, Nikos Makris, Seann Tulloch, Scott F Sorg, Jonathan Kaiser, David N Kennedy, Giorgio BonmassarAbstract:In this study, we present a magnetic resonance imaging (MRI)-based, high-resolution, numerical model of the head of a healthy human subject. In order to formulate the model, we performed quantitative volumetric segmentation on the human head, using T1-weighted MRI. The high spatial resolution used (1 × 1 × 1 mm3), allowed for the precise computation and visualization of a higher number of anatomical structures than provided by previous models. Furthermore, the high spatial resolution allowed us to study individual thin anatomical structures of clinical relevance not visible by the standard model currently adopted in computational bioelectromagnetics. When we computed the electromagnetic field and Specific Absorption Rate (SAR) at 7 Tesla MRI using this high-resolution model, we were able to obtain a detailed visualization of such fine anatomical structures as the epidermis/dermis, bone structures, bone-marrow, white matter and nasal and eye structures.
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Specific Absorption Rate studies of the parallel transmission of inner volume excitations at 7t
Journal of Magnetic Resonance Imaging, 2008Co-Authors: Adam C Zelinski, Leonardo M Angelone, Giorgio Bonmassar, Vivek K Goyal, Elfar Adalsteinsson, Lawrence L WaldAbstract:Purpose: To investigate the behavior of whole-head and local Specific Absorption Rate (SAR) as a function of trajectory acceleration factor and target excitation pattern due to the parallel transmission (pTX) of spatially tailored excitations at 7T. Materials and Methods: Finite-difference time domain (FDTD) simulations in a multitissue head model were used to obtain B1 and electric field maps of an eight-channel transmit head array. Local and average SAR produced by 2D-spiral-trajectory excitations were examined as a function of trajectory acceleration factor, R, and a variety of target excitation parameters when pTX pulses are designed for constant root-mean-square excitation pattern error. Results: Mean and local SAR grow quadratically with flip angle and more than quadratically with R, but the ratio of local to mean SAR is not monotonic with R. SAR varies greatly with target position, exhibiting different behaviors as a function of target shape and size for small and large R. For example, exciting large regions produces less SAR than exciting small ones for R 4, but the opposite trend occurs when R 4. Furthermore, smoother and symmetric patterns produce lower SAR. Conclusion: Mean and local SAR vary by orders of magnitude depending on acceleration factor and excitation pattern, often exhibiting complex, nonintuitive behavior. To ensure safety compliance, it seems that model-based validation of individual target patterns and corresponding pTX pulses is necessary.
Cornelis A T Van Den Berg - One of the best experts on this subject based on the ideXlab platform.
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Specific Absorption Rate intersubject variability in 7t parallel transmit mri of the head
Magnetic Resonance in Medicine, 2013Co-Authors: Martijn De Greef, Ozlem Ipek, Alexander J E Raaijmakers, J Crezee, Cornelis A T Van Den BergAbstract:Patient-Specific radiofrequency shimming in high-field MRI strengthens the need for online, patient-Specific Specific Absorption Rate (SAR) monitoring. Numerical simulation is currently most effective for this purpose but may require a patient-Specific dielectric model. To investigate whether a generic model may be combined with a safety factor to account for variation within the population, generic SAR behavior is studied for 7T MRI of the head. For six detailed head models, radiofrequency fields were simulated for an eight-channel parallel transmit array. SAR behavior is studied through comparison of the eigenvalues/eigenvectors of the local Q-matrices. Furthermore, numerical radiofrequency shimming experiments without and with SAR constraints were performed where SAR during optimization was evaluated on a generic model. In both cases, the ability of different generic models to predict actual SAR levels was evaluated. The largest eigenvalue distribution is comparable between models. Radiofrequency shimming without constraints improves the |B +1| homogeneity while the SAR increases substantially. Imposing constraints on SAR during optimization, estimating SAR on a generic model, was effective. A safety factor of 1.4 was found to be sufficient. Generic SAR behavior makes a generic head model a practical alternative to patient-Specific models and allows effective |B +1| shimming with SAR constraints.
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fast design of local n gram Specific Absorption Rate optimized radiofrequency pulses for parallel transmit systems
Magnetic Resonance in Medicine, 2012Co-Authors: Alessandro Sbrizzi, Hans Hoogduin, Jan J W Lagendijk, Peter R Luijten, Gerard L G Sleijpen, Cornelis A T Van Den BergAbstract:Designing multidimensional radiofrequency pulses for clinical application must take into account the local Specific Absorption Rate (SAR) as controlling the global SAR does not guarantee suppression of hot spots. The maximum peak SAR, averaged over an N grams cube (local NgSAR), must be kept under certain safety limits. Computing the SAR over a three-dimensional domain can require several minutes and implementing this computation in a radiofrequency pulse design algorithm could slow down prohibitively the numerical process. In this article, a fast optimization algorithm is designed acting on a limited number of control points, which are stRategically selected locations from the entire domain. The selection is performed by comparing the largest eigenvalues and the corresponding eigenvectors of the matrices which locally describe the tissue's amount of heating. The computation complexity is dramatically reduced. An additional critical step to acceleRate the computations is to apply a multi shift conjugate gradient algorithm. Two transmit array setups are studied: a two channel 3 T birdcage body coil and a 12-channel 7 T transverse electromagnetic (TEM) head coil. In comparison with minimum power radiofrequency pulses, it is shown that a reduction of 36.5% and 35%, respectively, in the local NgSAR can be achieved within short, clinically feasible, computation times. Magn Reson Med, 2012. (c) 2011 Wiley Periodicals, Inc.
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simultaneous b1 homogenization and Specific Absorption Rate hotspot suppression using a magnetic resonance phased array transmit coil
Magnetic Resonance in Medicine, 2007Co-Authors: Cornelis A T Van Den Berg, Ob Van Den Berge, Jeroe Van De Kame, W Raaymakers, H Kroeze, Lambertus W Artels, Ja J W LagendijkAbstract:In high-field MRI severe problems with respect to B uniformity and Specific Absorption Rate (SAR) deposition pose a great challenge to whole-body imaging. In this study the potential of a phased array transmit coil is investigated to simultaneously reduce B nonuniformity and SAR deposition. This was tested by performing electromagnetic simulations of a phased array TEM coil operating at 128 MHz loaded with two different homogeneous elliptical phantoms and four dielectric patient models. It was shown that the wave interference of a circularly polarized RF field with an ellipse and a pelvis produces largely identical B and electric field patterns. Especially for obese patients, this results in large B nonuniformity and global areas with elevated SAR deposition. It is demonstRated that a phased array transmit coil can reduce these phenomena. The technique was especially successful in suppressing SAR hotspots with a decrease up to 50%. The application of optimized settings for an ellipse to the patient models leads to comparable results as obtained with the patient-Specific optimizations. This suggests that generic phase/amplitude port settings are possible, requiring no preinformation about patient-Specific RF fields. Such a scheme would, due to its simultaneous B homogenization and extra SAR margin, have many benefits for whole-body imaging at 3 T. Magn Reson Med 57:577–586, 2007. © 2007 Wiley-Liss, Inc.
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simultaneous b1 homogenization and Specific Absorption Rate hotspot suppression using a magnetic resonance phased array transmit coil
Magnetic Resonance in Medicine, 2007Co-Authors: Cornelis A T Van Den Berg, H Kroeze, B W Raaymakers, Lambertus W Bartels, Bob Van Den Bergen, Jeroen B Van De Kamer, Jan J W LagendijkAbstract:In high-field MRI severe problems with respect to B(1) (+) uniformity and Specific Absorption Rate (SAR) deposition pose a great challenge to whole-body imaging. In this study the potential of a phased array transmit coil is investigated to simultaneously reduce B(1) (+) nonuniformity and SAR deposition. This was tested by performing electromagnetic simulations of a phased array TEM coil operating at 128 MHz loaded with two different homogeneous elliptical phantoms and four dielectric patient models. It was shown that the wave interference of a circularly polarized RF field with an ellipse and a pelvis produces largely identical B(1) (+) and electric field patterns. Especially for obese patients, this results in large B(1) (+) nonuniformity and global areas with elevated SAR deposition. It is demonstRated that a phased array transmit coil can reduce these phenomena. The technique was especially successful in suppressing SAR hotspots with a decrease up to 50%. The application of optimized settings for an ellipse to the patient models leads to comparable results as obtained with the patient-Specific optimizations. This suggests that generic phase/amplitude port settings are possible, requiring no preinformation about patient-Specific RF fields. Such a scheme would, due to its simultaneous B(1) (+) homogenization and extra SAR margin, have many benefits for whole-body imaging at 3 T.
Leonardo M Angelone - One of the best experts on this subject based on the ideXlab platform.
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changes in the Specific Absorption Rate sar of radiofrequency energy in patients with retained cardiac leads during mri at 1 5t and 3t
Magnetic Resonance in Medicine, 2019Co-Authors: Laleh Golestanirad, Amir Ali Rahsepar, John E Kirsch, Kenichiro Suwa, Jeremy C Collins, Leonardo M Angelone, Boris Keil, Rod S Passman, Giorgio BonmassarAbstract:PURPOSE To evaluate the local Specific Absorption Rate (SAR) and heating around retained cardiac leads during MRI at 64 MHz (1.5T) and 127 MHz (3T) as a function of RF coil type and imaging landmark. METHODS Numerical models of retained cardiac leads were built from CT and X-ray images of 6 patients with retained cardiac leads. Electromagnetic simulations and bio-heat modeling were performed with MRI RF body and head coils tuned to 64 MHz and 127 MHz and positioned at 9 different imaging landmarks covering an area from the head to the lower limbs. RESULTS For all patients and at both 1.5T and 3T, local transmit head coils produced negligible temperature rise ( Δ T 0.1 ° C ) for ‖ ‖ B 1 + ‖ ‖ ≤ 3 μ T . For body imaging with quadrature-driven coils at 1.5T, Δ T during a 10-min scan remained ‖ ‖ B 1 + ‖ ‖ ≤ 3 μ T and ‖ ‖ B 1 + ‖ ‖ ≤ 4 μ T . For body imaging at 3T, Δ T during a 10-min scan remained ‖ ‖ B 1 + ‖ ‖ ≤ 2 μ T . For shorter pulse sequences up to 2 min, Δ T remained ‖ ‖ B 1 + ‖ ‖ ≤ 3 μ T . CONCLUSION For the models based on 6 patients studied, simulations suggest that MRI could be performed safely using a local head coil at both 1.5T and 3T, and with a body coil at 1.5T with pulses that produced ‖ ‖ B 1 + ‖ ‖ ≤ 4 μ T . MRI at 3T could be performed safely in these patients using pulses with ‖ ‖ B 1 + ‖ ‖ ≤ 2 μ T .
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Specific Absorption Rate in a standard phantom containing a deep brain stimulation lead at 3 tesla mri
International IEEE EMBS Conference on Neural Engineering, 2013Co-Authors: Giorgio Bonmassar, Peter Serano, Leonardo M AngeloneAbstract:Finite elements methods (FEM) simulations are presented showing the Specific Absorption Rate (SAR) distribution in the case of a Deep Brain Stimulation (DBS) lead inside a standard phantom exposed to 128 MHz / 3 Tesla magnetic resonance imaging (MRI). The electromagnetic model is proposed as a tool to study the safety of patients with an implanted DBS lead undergoing clinical 3T MRI systems. The simulations were based on an electromagnetic and circuit co-simulation methodology and allowed for fine tuning of the MRI radiofrequency transmit body coil. Results show that the presence of the lead did not influence the behavior of the coil, namely the S-parameters at the coil feed were largely unaffected by the presence of a lead. Conversely, significant changes of electric and magnetic field, as well as local SAR were observed due to the characteristic antenna effect. The lead path strongly affected the level of SAR at the distal tip of the lead, with a 10x fold difference between the two configurations evaluated.
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analysis of the role of lead resistivity in Specific Absorption Rate for deep brain stimulator leads at 3t mri
IEEE Transactions on Medical Imaging, 2010Co-Authors: Leonardo M Angelone, John W Belliveau, Jyrki Ahveninen, Giorgio BonmassarAbstract:Magnetic resonance imaging (MRI) on patients with implanted deep brain stimulators (DBSs) can be hazardous because of the antenna-effect of leads exposed to the incident radio-frequency field. This study evaluated electromagnetic field and Specific Absorption Rate (SAR) changes as a function of lead resistivity on an anatomically precise head model in a 3T system. The anatomical accuracy of our head model allowed for detailed modeling of the path of DBS leads between epidermis and the outer table. Our electromagnetic finite difference time domain (FDTD) analysis showed significant changes of 1 g and 10 g averaged SAR for the range of lead resistivity modeled, including highly conductive leads up to highly resistive leads. Antenna performance and whole-head SAR were sensitive to the presence of the DBS leads only within 10%, while changes of over one order of magnitude were observed for the peak 10 g averaged SAR, suggesting that local SAR values should be considered in DBS guidelines. With ?lead = ?copper , and the MRI coil driven to produce a whole-head SAR without leads of 3.2 W/kg, the 1 g averaged SAR was 1080 W/kg and the 10 g averaged SAR 120 W/kg at the tip of the DBS lead. Conversely, in the control case without leads, the 1 g and 10 g averaged SAR were 0.5 W/kg and 0.6 W/kg, respectively, in the same location. The SAR at the tip of lead was similar with electrically homogeneous and electrically heterogeneous models. Our results show that computational models can support the development of novel lead technology, properly balancing the requirements of SAR deposition at the tip of the lead and power dissipation of the system battery.
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mri based anatomical model of the human head for Specific Absorption Rate mapping
Medical & Biological Engineering & Computing, 2008Co-Authors: Leonardo M Angelone, Nikos Makris, Seann Tulloch, Scott F Sorg, Jonathan Kaiser, David N Kennedy, Giorgio BonmassarAbstract:In this study, we present a magnetic resonance imaging (MRI)-based, high-resolution, numerical model of the head of a healthy human subject. In order to formulate the model, we performed quantitative volumetric segmentation on the human head, using T1-weighted MRI. The high spatial resolution used (1 × 1 × 1 mm3), allowed for the precise computation and visualization of a higher number of anatomical structures than provided by previous models. Furthermore, the high spatial resolution allowed us to study individual thin anatomical structures of clinical relevance not visible by the standard model currently adopted in computational bioelectromagnetics. When we computed the electromagnetic field and Specific Absorption Rate (SAR) at 7 Tesla MRI using this high-resolution model, we were able to obtain a detailed visualization of such fine anatomical structures as the epidermis/dermis, bone structures, bone-marrow, white matter and nasal and eye structures.
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Specific Absorption Rate studies of the parallel transmission of inner volume excitations at 7t
Journal of Magnetic Resonance Imaging, 2008Co-Authors: Adam C Zelinski, Leonardo M Angelone, Giorgio Bonmassar, Vivek K Goyal, Elfar Adalsteinsson, Lawrence L WaldAbstract:Purpose: To investigate the behavior of whole-head and local Specific Absorption Rate (SAR) as a function of trajectory acceleration factor and target excitation pattern due to the parallel transmission (pTX) of spatially tailored excitations at 7T. Materials and Methods: Finite-difference time domain (FDTD) simulations in a multitissue head model were used to obtain B1 and electric field maps of an eight-channel transmit head array. Local and average SAR produced by 2D-spiral-trajectory excitations were examined as a function of trajectory acceleration factor, R, and a variety of target excitation parameters when pTX pulses are designed for constant root-mean-square excitation pattern error. Results: Mean and local SAR grow quadratically with flip angle and more than quadratically with R, but the ratio of local to mean SAR is not monotonic with R. SAR varies greatly with target position, exhibiting different behaviors as a function of target shape and size for small and large R. For example, exciting large regions produces less SAR than exciting small ones for R 4, but the opposite trend occurs when R 4. Furthermore, smoother and symmetric patterns produce lower SAR. Conclusion: Mean and local SAR vary by orders of magnitude depending on acceleration factor and excitation pattern, often exhibiting complex, nonintuitive behavior. To ensure safety compliance, it seems that model-based validation of individual target patterns and corresponding pTX pulses is necessary.
Teresa Pellegrino - One of the best experts on this subject based on the ideXlab platform.
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one pot synthesis of monodisperse water soluble iron oxide nanocrystals with high values of the Specific Absorption Rate
Journal of Materials Chemistry B, 2014Co-Authors: Pablo Guardia, Liberato Manna, Andreas Riedinger, Simone Nitti, Giammarino Pugliese, Sergio Marras, Alessandro Genovese, Maria Elena Materia, Christophe Lefevre, Teresa PellegrinoAbstract:We report a highly reproducible route to synthesize iron oxide nanoparticles (IONPs) with control over size and shape and with size dispersions around 10%. By tuning the relative ratio of squalane to dibenzyl ether, which were used as solvents in the synthesis, the size of the particles could be varied from 14 to around 100 nm, while their shape evolved from cubic (for size ranges up to 35 nm) to truncated octahedra and octahedra (for sizes from 40 nm up to 100 nm). Fine tuning of the size within each of these ranges could be achieved by varying the heating ramp and the iron precursor to decanoic acid ratio. We also demonstRate direct water transfer of the as-synthesized IONPs via in situ ligand exchange with gallol polyethylene glycol molecules, the latter simply added to the crude nanocrystal mixture at 70 °C. The Specific Absorption Rate (SAR) values measured on the water transferred IONPs, at frequencies and applied magnetic fields that are considered safe for patients, confirmed their high heating performance. Finally, this method allows the transfer of 35 nm nanocubes as individually coated and stable particles to the water phase. For the first time, the heating performance of such large IONPs has been studied. This work uncovers the possibility of using large IONPs for magnetic hyperthermia in tumor therapy.
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water soluble iron oxide nanocubes with high values of Specific Absorption Rate for cancer cell hyperthermia treatment
ACS Nano, 2012Co-Authors: Pablo Guardia, Riccardo Di Corato, Lenaic Lartigue, Claire Wilhelm, Ana Espinosa, M Garciahernandez, Florence Gazeau, Liberato Manna, Teresa PellegrinoAbstract:Iron oxide nanocrystals (IONCs) are appealing heat mediator nanoprobes in magnetic-mediated hyperthermia for cancer treatment. Here, Specific Absorption Rate (SAR) values are reported for cube-shaped water-soluble IONCs prepared by a one-pot synthesis approach in a size range between 13 and 40 nm. The SAR values were determined as a function of frequency and magnetic field applied, also spanning technical conditions which are considered biomedically safe for patients. Among the different sizes tested, IONCs with an average diameter of 19 ± 3 nm had significant SAR values in clinical conditions and reached SAR values up to 2452 W/gFe at 520 kHz and 29 kAm–1, which is one of the highest values so far reported for IONCs. In vitro trials carried out on KB cancer cells treated with IONCs of 19 nm have shown efficient hyperthermia performance, with cell mortality of about 50% recorded when an equilibrium temperature of 43 °C was reached after 1 h of treatment.
