Tamer S. Ibrahim

Dielectric resonance phenomena in ultra high field MRI.

PURPOSE Dielectric resonances have previously been advanced as a significant cause of image degradation at higher fields. In this work, a study of dielectric resonances in ultra high field MRI is presented to explore the real importance of dielectric resonances in the human brain in this setting. METHOD Gradient-recalled echo images were acquired using a transverse electromagnetic resonator at 1.5, 4.7, and 8 T. Images were obtained from the human head and from phantoms filled with pure water, saline, and mineral oil. In addition, an exact theoretical analysis of dielectric resonances is presented for a spherical phantom and for a model of the human head. RESULTS Theoretical results demonstrate that distilled water can sustain dielectric resonances in head-sized spheres near 200 and 360 MHz, but the presence of significant conductivity suppresses these resonances. These findings are confirmed experimentally with proton images of water and saline (0.05 and 0.125 M NaCl). For lossy phantoms, coupling between the source and phantom overwhelms the dielectric resonance. Because of their low relative permittivity, mineral oil phantoms with 20 cm diameter do not exhibit dielectric resonances below approximately 900 MHz. Significant dielectric resonances were not observed in human head images obtained at 1.5, 4.7, and 8 T.

Electromagnetic perspective on the operation of RF coils at 1.5–11.7 Tesla

In this work experimental and numerical studies of the MR signal were performed at frequencies ranging from 64 MHz to 485 MHz, utilizing three different MRI coils: a single‐strut transverse electromagnetic (TEM)‐based coil, a TEM resonator, and a high‐pass birdcage coil. The experimental analyses were conducted using 1.5 and 8 Tesla whole‐body systems and volume RF head coils. The simulation data were obtained utilizing an in‐house‐developed finite difference time domain (FDTD) model. Pertinent data from the numerical and experimental setups were compared, and a remarkable agreement between the two methods was found that clearly demonstrates the effectiveness of the FDTD method when it is applied rigorously. The numerical and experimental studies demonstrate the complexity of the electromagnetic (EM) fields and their role in the MR signal. These studies also reveal unique similarities and differences between the transmit and receive field distributions at various field strengths. Finally, for ultra high‐field operations, it was demonstrated mathematically, numerically, and experimentally that highly asymmetric inhomogeneous images can be acquired even for linear excitation, symmetrical load geometries, and symmetrical load positioning within the coil. Magn Reson Med, 2005.

Insight into RF power requirements and B1 field homogeneity for human MRI via rigorous FDTD approach.

To study the dependence of radiofrequency (RF) power deposition on B0 field strength for different loads and excitation mechanisms.

Understanding and manipulating the RF fields at high field MRI.

This paper presents a complete overview of the electromagnetics (radiofrequency aspect) of MRI at low and high fields. Using analytical formulations, numerical modeling (computational electromagnetics), and ultrahigh field imaging experiments, the physics that impacts the electromagnetic quantities associated with MRI, namely (1) the transmit field, (2) receive field, and (3) total electromagnetic power absorption, is analyzed. The physical interpretation of the above‐mentioned quantities is investigated by electromagnetic theory, to understand ‘What happens, in terms of electromagnetics, when operating at different static field strengths?’ Using experimental studies and numerical simulations, this paper also examines the physical and technological feasibilities by which all or any of these specified electromagnetic quantities can be manipulated through techniques such as B1 shimming (phased array excitation) and signal combination using a receive array in order to advance MRI at high field strengths. Pertinent to this subject and with highly coupled coils operating at 7 T, this paper also presents the first phantom work on B1 shimming without B1 measurements. Copyright

A numerical analysis of radio-frequency power requirements in magnetic resonance imaging experiment

In this paper, a numerical analysis of the radio-frequency (RF) power requirements in magnetic resonance imaging (MRI) is presented at frequencies that span 200-362 MHz. This was performed utilizing an anatomically detailed human head model and a high-frequency RF coil utilized in high-field MRI systems. For axial slices through the brain region, it is demonstrated that the power required in order to obtain an average flip angle across the slice increases with frequency plateauing at a certain value, and then dropping as the frequency increases. The results demonstrate the significance of the electromagnetic interactions between the load and coil and their effects on the so important issue of power-frequency dependence in MRI.

Proposed radiofrequency phased-array excitation scheme for homogenous and localized 7-Tesla whole-body imaging based on full-wave numerical simulations

In this article, a radiofrequency (RF) excitation scheme for 7‐Tesla (T) whole‐body applications is derived and analyzed using the finite difference time domain (FDTD) method. Important features of the proposed excitation scheme and coil (a potential 7T whole‐body transverse electromagnetic [TEM] resonator design), from both operational and electromagnetic perspectives, are discussed. The choice of the coils operational mode is unconventional; instead of the typical “homogenous mode,” we use a mode that provides a null field in the center of the coil at low‐field applications. Using a 3D FDTD implementation of Maxwells equations, we demonstrate that the whole‐body 7T TEM coil (tuned to the aforementioned unconventional mode and excited in an optimized near‐field, phased‐array fashion) can potentially provide 1) homogenous whole‐slice (demonstrated in three axial, sagittal, and coronal slices) and 2) 3D localized (demonstrated in the heart) excitations. As RF power was not considered as a part of the optimization in several cases, the significant improvements achieved by whole‐slice RF excitation came at the cost of considerable increases in RF power requirements. Magn Reson Med 57:235–242, 2007.

Ultrahigh-field MRI whole-slice and localized RF field excitations using the same RF transmit array

In this paper, a multiport driving mechanism is numerically implemented at ultra high-field (UHF) magnetic resonance imaging (MRI) to provide 1) homogenous whole-slice (axial, sagittal, or coronal) and 2) highly localized radio frequency (RF) field excitation within the same slices, all with the same RF transmit array (here chosen to be a standard transverse electromagnetic (TEM) resonator/coil). The method is numerically tested using a full-wave model of a TEM coil loaded with a high-resolution/18-tissue/anatomically detailed human head mesh. The proposed approach is solely based on electromagnetic and phased array antenna theories. The results demonstrate that both homogenous whole-slice as well as localized RF excitation can be achieved within any slice of the head at 7 T (298 MHz for proton imaging)

Analytical approach to the MR signal

In this work, an analytical evaluation of the MR signal is presented using the principle of reciprocity for any transmit and receive probe(s) with general geometrical structure(s). It is shown that the excitation of the spins cannot be explained using the principle of reciprocity in its standard form due to the asymmetry in the permeability tensor of the transverse magnetization. In the reception problem, the standard form of the principle of reciprocity is then utilized to calculate the NMR signal. As expected, the analysis shows that the strength of the NMR signal is a function of the receive coils (fictitious) circularly polarized component of the field that poses an opposite sense of rotation to that corresponding with the excitation field, produced by the same coil, if it were to be used for excitation. Magn Reson Med, 2005.

High-resolution spiral imaging on a whole-body 7T scanner with minimized image blurring

High‐resolution (∼0.22 mm) images are preferably acquired on whole‐body 7T scanners to visualize minianatomic structures in human brain. They usually need long acquisition time (∼12 min) in three‐dimensional scans, even with both parallel imaging and partial Fourier samplings. The combined use of both fast imaging techniques, however, leads to occasionally visible undersampling artifacts. Spiral imaging has an advantage in acquisition efficiency over rectangular sampling, but its implementations are limited due to image blurring caused by a strong off‐resonance effect at 7T. This study proposes a solution for minimizing image blurring while keeping spiral efficient. Image blurring at 7T was, first, quantitatively investigated using computer simulations and point‐spread functions. A combined use of multishot spirals and ultrashort echo time acquisitions was then employed to minimize off‐resonance‐induced image blurring. Experiments on phantoms and healthy subjects were performed on a whole‐body 7T scanner to show the performance of the proposed method. The three‐dimensional brain images of human subjects were obtained at echo time = 1.18 ms, resolution = 0.22mm (field of view = 220mm, matrix size = 1024), and in‐plane spiral shots = 128, using a home‐developed ultrashort echo time sequence (acquisition‐weighted stack of spirals). The total acquisition time for 60 partitions at pulse repetition time = 100 ms was 12.8 min without use of parallel imaging and partial Fourier sampling. The blurring in these spiral images was minimized to a level comparable to that in gradient‐echo images with rectangular acquisitions, while the spiral acquisition efficiency was maintained at eight. These images showed that spiral imaging at 7T was feasible. Magn Reson Med, 2010.

Computational and experimental optimization of a double-tuned (1)H/(31)P four-ring birdcage head coil for MRS at 3T.

To optimize the homogeneity and efficiency of the B1 magnetic field of a four‐ring birdcage head coil that is double‐tuned at the Larmor frequencies of both 31P and 1H and optimized to acquire magnetic resonance spectroscopy (MRS) data at 3T for the study of infants.