Maria Alfonsetti

Versatile coil design and positioning of transverse-field RF surface coils for clinical 1.5-T MRI applications

Clinical MRI/MRS applications require radio frequency (RF) surface coils positioned at an arbitrary angle α with respect to B0. In these experimental conditions the standard circular loop (CL) coil, producing an axial RF field, shows a large signal loss in the central region of interest (ROI). We demonstrate that transverse-field figure-of-eight (FO8) RF surface coils design are not subject to the same amount of signal loss in the central ROI as loop coils when their orientations are changed. The 1.5-T CL and FO8 prototypes (diameter = 10 cm) were built on Plexiglas using copper strips (width = 4 mm, thickness = 100 μm). The two linear elements of the FO8 coil were 1 cm apart. Axial spoiled gradient echo (SPGR) images of a phantom containing doped water were acquired with the coil plane at α=0°, 45°, and 90°. As α increases, the CL images show, in the central ROI, a signal that decreases from a maximum value to zero. Whereas the FO8 images show, in the same ROI, a signal that varies little from the maximum value (20%). Optimized FO8 coils can be oriented with the coil plane positioned along any direction with respect to B0 without significant signal loss. Transverse RF coil design should be useful for clinical MRS studies and also for parallel imaging techniques where versatile RF coils disposed along arbitrary directions are required.

VALIDATION OF NUMERICAL APPROACHES FOR ELECTROMAGNETIC CHARACTERIZATION OF MAGNETIC RESONANCE RADIOFREQUENCY COILS

Numerical methods based on solutions of Maxwells equations are usually adopted for the electromagnetic characterization of Magnetic Resonance (MR) Radiofrequency (RF) coils. In this context, many difierent numerical methods can be employed, including time domain methods, e.g., the Finite-Difierence Time-Domain (FDTD), and frequency domain methods, e.g., the Finite Element Methods (FEM) and the Method of Moments (MoM). We provide

A composite resonator for simultaneous NMR and EPR imaging experiments

Electron paramagnetic resonance imaging (EPRI) and nuclear magnetic resonance imaging (NMRI) are currently used for in vivo spectroscopy and imaging. A multimodal apparatus based on the simultaneous observation of electronic and nuclear signals is very desirable because it combines the ability of NMRI to provide an accurate description of the internal structure of a sample with the ability of EPRI to detect the presence of free radicals and map their distribution. At a given value of the magnetic field B0, the electronic and nuclear transitions have very different resonance frequencies. This required the development of a multimodal spectrometer that combined the two spectroscopic modalities. In spite of the complexity of the electronic apparatus involved, the main requirement was for a resonator that allowed the simultaneous irradiation and observation of the signals at the two frequencies with good sensitivity. The EPR section of the composite resonator consists of a one-loop, two-gap resonator tuned to 1 GHz and the NMR section is a solenoid, coaxial to the EPR section, tuned to 1.52 MHz. Both sections have been designed to produce a homogeneous rf field in a cylindrical region of diameter 3 cm and length 4 cm. The rf magnetic fields B1e and B1n are directed along the axis of the magnet. The probe was tested on a phantom comprising two separate regions containing 0.73 g of lithium phthalocyanine (LiPtc) powder and 20 ml of an aqueous solution of CuSO4, respectively, and both EPR and NMR signals have been collected. Measurements of the sensitivity have also been made.

Optimization of multi-element transverse field radio frequency surface coils

In several MRI and MRS applications it is necessary to select a well-defined region of interest (ROI) with optimized signal-to-noise ratio and radio frequency (RF) field homogeneity. We have shown, theoretically and experimentally, that transverse field RF surface coils comprising a multitude of parallel linear elements are characterized by improved B1 spatial homogeneity and sensitivity in the central ROI, with respect to the standard square loop design. Finite-element modelling of RF coils tuned to 64.0 MHz was used to study the effect of the number of linear current elements and relative positioning with respect to B1 field homogeneity and sensitivity in the central ROI, both when empty and in the presence of a homogeneous tissue model. Workbench B1 field calibrations obtained with RF coil prototypes tuned to 64.0 MHz are in good agreement with our theoretical finite-element modelling. We have shown that the number of linear current elements and their relative positioning can be carefully selected to optimize either the B1 field homogeneity or sensitivity in the central ROI. We have shown that a number of multi-element RF coil geometries can be chosen to obtain an optimized RF field distribution and/or field amplitude. For example, we found that a six-element RF coil with 6 mm element separation is a good compromise for optimizing the B1 sensitivity (up to a factor 2) and the spatial homogeneity (about 30 mm with ΔB1/B1 ≤ 20%) in the central ROI, with respect to the standard square loop or the two-element FO8 design. MRI images obtained at 2.35 T (proton frequency 100.34 MHz) with standard GE pulse sequences showed that the six-element FO8 prototype significantly increases the RF field homogeneity and RF field amplitude, with respect to the SL coil design.

EPR imaging from projections: errors due to misalignment of projection centres and their rectification by a novel acquisition modality.

Continuous wave and pulsed wave electron paramagnetic resonance imaging (EPRI) makes use of classical methods of acquisition of projections. Acquisition/reconstruction techniques, such as spin-echo, gradient-echo, etc, cannot be applied to EPRI because they would require very short switching times for the gradient coils. Due to the use of the polar acquisition technique, it is necessary to define a centre of rotation about which the measured projections are rotated during the reconstruction process. This centre represents the point at which the field gradient coils must produce zero magnetic field. Due to the presence of a magnetic field control system that serves to compensate for field variations, principally due to heating, some interference can occur in the control system between the main magnetic field and the magnetic field produced by the gradient coils. The effect changes as the orientation changes. This results in a shift of the centres of the projections as a function of the variation of magnetic field produced by the gradient coils on the control Hall probe. If this condition is present, some artefacts can appear on the reconstructed image. This effect is irrelevant when EPR is used for imaging of paramagnetic probes whose linewidths are of the order of 10(-4) T, while it can be significant in the case of linewidths of the order of 10(-5) T or lower or when EPR is used in microimaging applications (i.e. for high values of magnetic field gradient). We describe the effects that misalignments of the projections have on the reconstructed images. We present a useful method for estimating the real position of the centre and correcting the measured projections before the application of the reconstruction algorithm. Moreover, we demonstrate the functioning of our technique by presenting some examples of EPR reconstruction collected by an X-band EPR imaging apparatus.

Improved 1.5 T Magnetic Resonance Spectroscopy in the Human Calf with a Spatially Selective Radio Frequency Surface Coil

We describe the use of a transverse field RF surface coil that improves 1.5 T proton MR spectroscopy in the human calf. A 2-element figure-of-eight (FO8) transverse field RF surface coil (diameter 2R=10 cm; separation between the two linear current elements 2s=1cm) and a circular loop (CL) coil of equal diameter where built and tested with proton PRESS spectra at 1.5 T. The 1 H PRESS spectra obtained in the resting calf muscle of healthy volunteers showed that the FO8 coil allows a higher PRESS SNR (up to a factor 4.5) within a region of about 20 mm centred at about 12 mm from the coil plane, as compared to a standard CL coil. We found also a faster PRESS SNR decrease in the muscle tissue for anterior/posterior distance >20 mm using the FO8 coil. The measured PRESS SNR in the fat tissues of the calf showed a signal mostly localised within 10 mm from the coils surface and with an improved SNR (up to 5.5 times) observed in the presence of the FO8 coil as compared to the CL coil of equal diameter. The FO8 coil design can be advantageous for MRS applications, since it allows higher SNR from a small VOI positioned centrally within a relatively narrow region at a given depth in the human calf. The reported spatial SNR features of the FO8 coil design should also be useful for 1 H and 31 P MRS metabolites quantification in the human brain.