Knowledge of the spatial distribution of transmission field B(1)(+) and reception sensitivity maps is important in high-field (≥3 T) human magnetic resonance (MR) imaging for several reasons: these include post-acquisition correction of intensity inhomogeneities, which may affect the quality of images; modeling and design of radiofrequency (RF) coils and pulses; validating theoretical models for electromagnetic field calculations; testing the compatibility with MR environment of biomedical implants. Moreover, inhomogeneities in the RF field are an essential source of error for quantitative MR spectroscopy. Recent studies have also shown that B(1)(+) and reception sensitivity maps can be used for direct calculation of tissue electrical parameters and for estimating the local specific absorption rate (SAR) in vivo. Several B(1)(+) mapping techniques have been introduced in the past few years based on actual flip angle (FA) mapping, but, to date, none has emerged as a standard. For reception sensitivity calculation, the signal intensity equation can be used where the nominal FA distribution must be replaced with the actual FA distribution calculated by one of the B(1)(+) mapping techniques. This study introduces a quantitative comparison between two known methods for B(1)(+)/actual FA and reception sensitivity mapping: the double-angle method (DAM) and the fitting (FIT) method. Experimental data obtained using DAM and FIT methods are also compared with numerical simulation results.
"The FDTD validation results (Fig. 5A–C) show that the FA estimated with the FDTD simulations was within 10% of the FA measured with the actual flip angle imaging (AFI) method. Further numerical corrections to the AFI can improve even further the accuracy of the FA measurements with an even lower systematic error than the FDTD simulations . "
[Show abstract][Hide abstract] ABSTRACT: Intracranial electrocortical recording and stimulation can provide unique knowledge about functional brain anatomy in patients undergoing brain surgery. This approach is commonly used in the treatment of medically refractory epilepsy. However, it can be very difficult to integrate the results of cortical recordings with other brain mapping modalities, particularly functional magnetic resonance imaging (fMRI). The ability to integrate imaging and electrophysiological information with simultaneous subdural electrocortical recording/stimulation and fMRI could offer significant insight for cognitive and systems neuroscience as well as for clinical neurology, particularly for patients with epilepsy or functional disorders. However, standard subdural electrodes cause significant artifact in MRI images, and concerns about risks such as cortical heating have generally precluded obtaining MRI in patients with implanted electrodes. We propose an electrode set based on polymer thick film organic substrate (PTFOS), an organic absorbable, flexible and stretchable electrode grid for intracranial use. These new types of MRI transparent intracranial electrodes are based on nano-particle ink technology that builds on our earlier development of an EEG/fMRI electrode set for scalp recording. The development of MRI-compatible recording/stimulation electrodes with a very thin profile could allow functional mapping at the individual subject level of the underlying feedback and feed forward networks. The thin flexible substrate would allow the electrodes to optimally contact the convoluted brain surface. Performance properties of the PTFOS were assessed by MRI measurements, finite difference time domain (FDTD) simulations, micro-volt recording, and injecting currents using standard electrocortical stimulation in phantoms. In contrast to the large artifacts exhibited with standard electrode sets, the PTFOS exhibited no artifact due to the reduced amount of metal and conductivity of the electrode/trace ink and had similar electrical properties to a standard subdural electrode set. The enhanced image quality could enable routine MRI exams of patients with intracranial electrode implantation and could also lead to chronic implantation solutions.
PLoS ONE 09/2012; 7(9):e41187. DOI:10.1371/journal.pone.0041187 · 3.23 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Interest in techniques yielding quantitative information about brain tissue proton densities is increasing. In general, all parameters influencing the signal amplitude are mapped in several acquisitions and then eliminated from the image data to obtain pure proton density weighting. Particularly, the measurement of the receiver coil sensitivity profile is problematic. Several methods published so far are based on the reciprocity theorem, assuming that receive and transmit sensitivities are identical. Goals of this study were (1) to determine quantitative proton density maps using an optimized variable flip angle method for T(1) mapping at 3 T, (2) to investigate if systematic errors can arise from insufficient spoiling of transverse magnetization, and (3) to compare two methods for mapping the receiver coil sensitivity, based on either the reciprocity theorem or bias field correction. Results show that insufficient spoiling yields systematic errors in absolute proton density of about 3-4 pu. A correction algorithm is proposed. It is shown that receiver coil sensitivity mapping based on the reciprocity theorem yields erroneous proton density values, whereas reliable data are obtained with bias field correction. Absolute proton density values in different brain areas, evaluated on six healthy subjects, are in excellent agreement with recent literature results.
Magnetic Resonance in Medicine 07/2012; 68(1):74-85. DOI:10.1002/mrm.23206 · 3.57 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Numerical methods based on solutions of Maxwell's equations are usually adopted for the electromagnetic characterization of Magnetic Resonance (MR) Radiofrequency (RF) coils. In this context, many different numerical methods can be employed, including time domain methods, e.g., the Finite-Difference Time-Domain (FDTD), and frequency domain methods, e.g., the Finite Element Methods (FEM) and the Method of Moments (MoM). We provide a quantitative comparison of performances and a detailed evaluation of advantages and limitations of the aforementioned methods in the context of RF coil design for MR applications. Specifically, we analyzed three RF coils which are representative of current geometries for clinical applications: a 1.5 T proton surface coil; a 7T dual tuned surface coil; a 7T proton volume coil. The numerical simulation results have been compared with measurements, with excellent agreement in almost every case. However, the three methods differ in terms of required computing resources (memory and simulation time) as well as their ability to handle a realistic phantom model. For this reason, this work could provide "a guide to select the most suitable method for each specific research and clinical applications at low and high field".
Progress In Electromagnetics Research M 01/2013; 29:121-136. DOI:10.2528/PIERM12122113
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