Riccardo Stara

Tissue Border Enhancement by inversion recovery MRI at 7.0 Tesla.

IntroductionThis contribution presents a magnetic resonance imaging (MRI) acquisition technique named Tissue Border Enhancement (TBE), whose purpose is to produce images with enhanced visualization of borders between two tissues of interest without any post-processing.MethodsThe technique is based on an inversion recovery sequence that employs an appropriate inversion time to produce images where the interface between two tissues of interest is hypo-intense; therefore, tissue borders are clearly represented by dark lines. This effect is achieved by setting imaging parameters such that two neighboring tissues of interest have magnetization with equal magnitude but opposite sign; therefore, the voxels containing a mixture of each tissue (that is, the tissue interface) possess minimal net signal. The technique was implemented on a 7.0 T MRI system.ResultsThis approach can assist the definition of tissue borders, such as that between cortical gray matter and white matter; therefore, it could facilitate segmentation procedures, which are often challenging on ultra-high-field systems due to inhomogeneous radiofrequency distribution. TBE allows delineating the contours of structural abnormalities, and its capabilities were demonstrated with patients with focal cortical dysplasia, gray matter heterotopia, and polymicrogyria.ConclusionThis technique provides a new type of image contrast and has several possible applications in basic neuroscience, neurogenetic research, and clinical practice, as it could improve the detection power of MRI in the characterization of cortical malformations, enhance the contour of small anatomical structures of interest, and facilitate cortical segmentation.

Investigation of maximum local specific absorption rate in 7 T magnetic resonance with respect to load size by use of electromagnetic simulations

Local specific absorption rate (SAR) evaluation in ultra high field (UHF) magnetic resonance (MR) systems is a major concern. In fact, at UHF, radiofrequency (RF) field inhomogeneity generates hot-spots that could cause localized tissue heating. Unfortunately, local SAR measurements are not available in present MR systems; thus, electromagnetic simulations must be performed for RF fields and SAR analysis. In this study, we used three-dimensional full-wave numerical electromagnetic simulations to investigate the dependence of local SAR at 7.0 T with respect to subject size in two different scenarios: surface coil loaded by adult and child calves and quadrature volume coil loaded by adult and child heads. In the surface coil scenario, maximum local SAR decreased with decreasing load size, provided that the RF magnetic fields for the different load sizes were scaled to achieve the same slice average value. On the contrary, in the volume coil scenario, maximum local SAR was up to 15% higher in children than in adults.

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

SAR prediction in adults and children by combining measured B1+ maps and simulations at 7.0 Tesla

To predict local and global specific absorption rate (SAR) in individual subjects.

Electromagnetic characterization of an MR volume coil with multilayered cylindrical load using a 2-D analytical approach

We present an analytical method for the analysis of Radio Frequency (RF) volume coils for Magnetic Resonance Imaging (MRI), using a 2-D full wave solution with loading by multilayered cylinders. This allows the characterization of radio-frequency E, H, B1, B1(+) fields. Comparisons are provided with experimental data obtained at 7.0 T. The procedure permits us to clearly separate the solution to single line source problem (which we call the primordial solution) and the composite solution (i.e. full coil, i.e. the summations of primordial solutions according to the resonator drive configuration). The capability of separating the primordial solution and the composite one is fundamental for a thorough analysis of the phenomena of dielectric resonance, and of standing wave and multi-source interference. We show that dielectric resonance can be identified only by looking at the electromagnetic field from a single line source.

RF coil design for low and high field MRI: Numerical methods and measurements

Several clinical magnetic resonance applications at 1.5 T and higher field strengths require a careful selection of the RadioFrequency (RF) coil design to optimize the RF spatial distribution and sensitivity. Specifically, two basic requirements must be fulfilled for obtaining high quality images: in the transmission mode, RF coils must be able to produce a uniform magnetic field in the volume of interest so that the nuclei can be properly excited; in the receiving mode, a high signal to noise ratio is needed, and the coil must be able to collect the signal emitted by the nuclei with better sensitivity throughout the volume of interest. A number of analytical and numerical methods are reported in the literature to simulate the RF field distributions of surface and volume coils. In this work, we compared the performances of two computational methods (Method of Moments and Finite Elements Method), considering the modelling of both surface (single loop, figure of eight coil - FO8, dual loop) and volume (TEM) coils. Low (1.5 T) and high (7 T) field operating regimes have been analysed. Measurements obtained on the workbench and with a 1.5 T scanner are used for validation.

Magnetic Resonance Imaging at 7 Tesla with Dedicated Radiofrequency Coils - Application to Cervical Cord and Knee

Magnetic Resonance (MR) Imaging is a valuable tool in the diagnosis and monitoring of various musculoskeletal pathologies. New Ultra-High Field (UHF) 7 T MRI systems, with their enhanced Signal-toNoise Ratio, may offer increased image quality in terms of spatial resolution and/or shorter scanning time compared to lower field systems. However, these benefits can be difficult to obtain because of increased radio-frequency (RF) inhomogeneity, increased Specific Absorption Rate (SAR) and the relative lack of specialized and commercially available RF coils compared to lower field systems. This study reports the feasibility of imaging in bones and cartilages at UHF with a 7 T MR scanner available at the IMAGO7 Foundation (Pisa, Italy). Dedicated radio-frequency coils for proton imaging have been designed, developed, optimized for different anatomical regions and validated in vivo, and are now ready for clinical research studies. The performance of the RF coil prototypes in targeting different anatomical regions are also demonstrated, obtaining images of the neck (the cervical cord) and of the knee (trabecular bone and

A 7T double-tuned ( 1 H/ 31 P) microstrip surface RF coil for the IMAGO7 MR scanner

Ultra-High Field (UHF) (4-9.4T) Magnetic Resonance Imaging (MRI) and Spectroscopy (MRS) are valuable tools in the diagnosis and monitoring of many diseases thanks to the enhanced Signal-to-Noise Ratio (SNR) and spectral/spatial resolution. However, such UHF MR applications require the development and optimization of specially designed Radio Frequency (RF) coils. In this study we report the design, construction, workbench and MR testing of a novel 7T double-tuned (¹H/³¹P) RF surface coil made with microstrip technology. The RF coil is suitable for MRI and MRS human studies of the lower limbs and also of the human head. The double-tuned RF coil was tested at the IMAGO7 Foundation in Pisa (Italy). Ex vivo MR ¹H images and ³¹P spectra obtained with a large size fresh calf veal sample showed an excellent SNR within about 50 mm from the RF surface coil. It is believed that the combined 7T MR ¹H/³¹P will provide benefits in performing clinical follow-up of muscular disorders in young and aging patients. The proposed design can be easily extended to the simultaneous detection of the ¹H and other low-gamma nuclei, such as ⁷Li, ¹¹B, ¹³C, ¹⁷O, ²³Na.

Non-invasive assessment of Neuromuscular Disorders by 7 tesla Magnetic Resonance Imaging and Spectroscopy: Dedicated radio-frequency coil development

Magnetic Resonance (MR) Imaging and Spectroscopy of the muscle is a valuable tool in the diagnosis and monitoring of Neuromuscular Disorders (NMD). New Ultra-High Field (UHF) 7 T MRI systems, with their enhanced Signal-to-Noise Ratio, may offer increased image quality in terms of spatial resolution and/or shorter scanning time compared to lower field systems. In the study of NMD the new features provided by UHF MR may allow the use of functional techniques to improve biochemical and physiological information of skeletal muscle correlated to the pathogenesis and progression of the muscle involvement. This study reports the recent achievements in muscle imaging and spectroscopy obtained at the first Italian 7 T MR scanner available at the IMAGO7 Foundation (Pisa, Italy). Dedicated radio-frequency coils for proton imaging and phosphorous spectroscopy have been designed, developed and validated in vivo, and are now ready for clinical research studies.

Evaluation of 3D radio-frequency electromagnetic fields for any matching and coupling conditions by the use of basis functions.

A procedure for evaluating radio-frequency electromagnetic fields in anatomical human models for any matching and coupling conditions is introduced. The procedure resorts to the extraction of basis functions: such basis functions, which represent the fields produced by each individual port without any residual coupling, are derived through an algebraic procedure which uses the S parameter matrix and the fields calculated in one (only) full-wave simulation. The basis functions are then used as building-blocks for calculating the fields for any other S parameter matrix. The proposed approach can be used both for volume coil driven in quadrature and for parallel transmission configuration.