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Experimental and computational EM analysis of MRI RF coils and of their interaction with implanted conductive objects
Full text linkRadio frequency (RF) coils are employed in Magnetic Resonance Imaging (MRI) to excite and detect the signal from nuclear spins at Larmor frequency. The intrinsic complexity of the phenomena involved inside an MR scanner leads to strong requirements on the magnetic field B1 generated by the RF coil which should be fulfilled to ensure reliable results. Nevertheless, the presence of an external conductive object, such as a metallic implant, strongly interacts with the B1 field and potentially affects the performance of an even ideal RF coil.
Whereas the B1 magnetic field prevalently impacts on the quality of the MRI result, the electric field generated by the RF coil has important safety consequences and should be accounted for as well.
This thesis is developed in the framework of the evaluation of the electromagnetic fields generated by MRI RF coils and of their interaction with conductive passive objects.
In this context, the third chapter of the thesis describes the realization of a dosimetric experimental set-up, able to generate and measure RF electromagnetic fields in a “tissues mimicking” phantom. The generation equipment consists of an RF synthesizer whose amplified signal is used to supply suitable antennas. The acquisition system is based on two power meters (connected to a directional coupler to measure the incident and reflected power), an electromagnetic field acquisition station and a tri-axial automatic system for specific field probes positioning.
The first characterization of the set-up is described and has been obtained employing a self-made shielded loop antenna. Its validation has been performed both in terms of magnetic and electric fields by comparing the experimental measurements with numerical simulation results. In such a context, an uncertainty budget has been studied and it has been associated to the relevant dosimetric set-up.
A second activity is presented in the same chapter and involved a double-tuned (23Na/1H) loop coil specifically designed for a 7 T MRI scanner and provided by the IMAGO7 foundation, Pisa. The experimental electromagnetic measurements have been compared to the numerical results carried out by the IMAGO7 research group by means of the same simulation codes they use for coil design routine evaluations.
The plan for the implementation of a realistic scaled-down 3 T MR body coil, conceived to increase the versatility of the dosimetric set-up, led to the design of a 16-leg high-pass birdcage coil. A detailed theoretical study of the coil structure represents the topic of the second chapter, whereas the coil design and fabrication become part of the chapter devoted to the experimental set-up.
The distortion of the RF coil sensitivities due to the presence of elongated passive implants, which leads to RF inhomogeneity artefacts, is evaluated through numerical simulations and represents the subject of the fourth chapter. The simulation set-up includes a birdcage volume resonator and a proper phantom inside which the metallic objects (i.e. a metallic cylinder and a realistic hip prosthesis stem model) are plunged. Special attention is given to the achievement of an easy but reliable description of the physical phenomenon. On the basis of these results, a solution is hence proposed to reduce the impact of RF artefacts in MRI exams. This proposal consists in covering the metallic objects with a suitable dielectric coating to make them invisible to the radiating antenna. The optimum coating parameters and the general effectiveness of the coating are studied using different commercial electromagnetic numerical codes both at 64 MHz and 128 MHz.
Finally, following a specific request from an MRI medical staff, the possible interactions between body-art tattoos and MRI electromagnetic fields are evaluated. The study is developed especially from a safety point of view. The tattoo inks and pigments electrical and magnetic properties experimental characterization is performed to assess electromagnetic and thermal simulations. The results are reported and discussed in the fifth chapter of the thesis
A Near-Field Cloaking Study to Reduce MRI RF-Artefacts in Presence of Elongated Prostheses
Full text linkThe objective of this paper is to analyze a near-field electromagnetic cloaking to reduce the radiofrequency (RF) magnetic field inhomogeneities (responsible for the RF-artefacts onset) in magnetic resonance imaging (MRI) in the presence of an elongated metallic hardware. A lumped circuit is considered to explain the role that a dielectric coat has on hiding ametallic cylinder to the RF antenna. The theoretical assumptions are proved by means of full-wave simulations that are also applied to a realistic hip prosthesis considering a frequency equal to 64 and 128 MHz. The numerical results confirm the theoretical assumptions. Both the theoretical analysis and the numerical simulations highlight the different role that the coat thickness and electric permittivity have in the definition of a proper dielectric coat. A particular cloaking approach leads to a dielectric coat whose constitutive electrical parameters may be simple enough to fit the considered application reducing the interaction between an elongated prosthesis and the RF antenna. Furthermore, results obtained at 64 MHz suggest the possibility to employ an existing biocompatible material to achieve the envisaged purposes
Polynomial chaos expansion of SAR and temperature increase variability in 3 T MRI due to stochastic input data
Full text linkObjective. Numerical simulations are largely adopted to estimate dosimetric quantities, e.g. specific absorption rate (SAR) and temperature increase, in tissues to assess the patient exposure to the radiofrequency (RF) field generated during magnetic resonance imaging (MRI). Simulations rely on reference anatomical human models and tabulated data of electromagnetic and thermal properties of biological tissues. However, concerns may arise about the applicability of the computed results to any phenotype, introducing a significant degree of freedom in the simulation input data. In addition, simulation input data can be affected by uncertainty in relative positioning of the anatomical model with respect to the RF coil. The objective of this work is the to estimate the variability of SAR and temperature increase at 3 T head MRI due to different sources of variability in input data, with the final aim to associate a global uncertainty to the dosimetric outcomes. Approach. A stochastic approach based on arbitrary Polynomial Chaos Expansion is used to evaluate the effects of several input variability's (anatomy, tissue properties, body position) on dosimetric outputs, referring to head imaging with a 3 T MRI scanner. Main results. It is found that head anatomy is the prevailing source of variability for the considered dosimetric quantities, rather than the variability due to tissue properties and head positioning. From knowledge of the variability of the dosimetric quantities, an uncertainty can be attributed to the results obtained using a generic anatomical head model when SAR and temperature increase values are compared with safety exposure limits. Significance. This work associates a global uncertainty to SAR and temperature increase predictions, to be considered when comparing the numerically evaluated dosimetric quantities with reference exposure limits. The adopted methodology can be extended to other exposure scenarios for MRI safety purposes
Efficient prediction of MRI gradient‐induced heating for guiding safety testing of conductive implants
Full text linkPurposeTo propose an efficient numerical method to predict the temperature increase of an implantable medical device induced by any linearly polarized homogeneous magnetic field, according to the ISO 10974 methodology for testing of gradient-induced device heating. Theory and MethodsThe concepts of device-specific power and temperature tensors are introduced to mathematically describe the electromagnetic and thermal anisotropic behavior of the device, from which the device heating for an arbitrary exposure direction can be predicted. The proposed method is compared to a brute-force approach based on simulations, and validated by applying it to four reference orthopedic implants with a commercial simulation software. ResultsThe proposed method requires about 5% of the time required by the brute-force approach, and 30% of the memory occupancy. The temperature increase predicted by the proposed method over a range of incident magnetic field exposures deviated from brute-force direct simulations by less than & PLUSMN;0.3%. ConclusionThe proposed method allows efficient prediction of the heating of an implantable medical device induced by any linearly polarized homogeneous magnetic field using a small fraction of the simulations required by the brute-force approach. The results can be used to predict the worst-case orientation of the gradient field, for subsequent experimental characterization according to the ISO 10974 standard
An ideal dielectric coat to avoid prosthesis RF-artefacts in Magnetic Resonance Imaging
Full text linkThe number of people submitted to total hip or knee arthroplasty increased in the last years and it is
likely to grow further. Hence, the importance of a proper investigation tool that allows to determine
and recognize the potential presence of perioperative and/or postoperative diseases becomes clear.
Although the Magnetic Resonance Imaging (MRI) technique demonstrated several advantages over
the other common tomography tools, it suffers from the arise of image artefacts if it is performed
in presence of metallic prostheses. In particular, the so-called RF-artefacts are caused by the
inhomogeneity in the radiofrequency magnetic field of MRI, due to the electric currents induced on
the metal surface by the field itself. In this work, a near-zero permittivity dielectric coat is simulated
to reduce those currents and, therefore, the RF-artefacts onset in the final image. Numerical results
confirm that the dielectric coat strongly reduces the magnetic field inhomogeneity, suggesting a
possible solution to a well-known problem in the MRI field
safety testing related to gradient‐induced heating of medical devices
Full text linkPURPOSE: To theoretically investigate the feasibility of a novel procedure for testing the MRI gradient‐induced heating of medical devices and translating the results into clinical practice. METHODS: The concept of index of stress is introduced by decoupling the time waveform characteristics of the gradient field signals from the field spatial distribution within an MRI scanner. This index is also extended to consider the anisotropy of complex bulky metallic implants. Merits and drawbacks of the proposed index of stress are investigated through virtual experiments. In particular, the values of the index of stress evaluated for realistic orthopedic implants placed within an ASTM phantom are compared with accurate heating simulations performed with 2 anatomic body models (a man and a woman) implanted through a virtual surgery procedure. RESULTS: The manipulation of the proposed index of stress allows to identify regions within the MRI bore where the implant could affect the safety of the examinations. Furthermore, the conducted analysis shows that the power dissipated into the implant by the induced eddy currents is a dosimetric quantity that estimates well the maximum temperature increase in the tissues surrounding the implant. CONCLUSION: The results support the adoption of an anisotropic index of stress to regulate the gradient‐induced heating of geometrically complex implants. They also pave the way for a laboratory characterization of the implants based on electrical measurements, rather than on thermal measurements. The next step will be to set up a standardized experimental procedure to evaluate the index of stress associated with an implant
Computational dosimetry in MRI in presence of hip, knee or shoulder implants: do we need accurate surgery models?
Full text linkObjective. To quantify the effects of different levels of realism in the description of the anatomy around hip, knee or shoulder implants when simulating, numerically, radiofrequency and gradient-induced heating in magnetic resonance imaging. This quantification is needed to define how precise the digital human model modified with the implant should be to get realistic dosimetric assessments. Approach. The analysis is based on a large number of numerical simulations where four 'levels of realism' have been adopted in modelling human bodies carrying orthopaedic implants. Main results. Results show that the quantification of the heating due to switched gradient fields does not strictly require a detailed local anatomical description when preparing the digital human model carrying an implant. In this case, a simple overlapping of the implant CAD with the body anatomy is sufficient to provide a quite good and conservative estimation of the heating. On the contrary, the evaluation of the electromagnetic field distribution and heating caused by the radiofrequency field requires an accurate description of the tissues around the prosthesis. Significance. The results of this paper provide hints for selecting the 'level of realism' in the definition of the anatomical models with embedded passive implants when performing simulations that should reproduce, as closely as possible, the in vivo scenarios of patients carrying orthopaedic implants
Magnetic Resonance-Based Electric Properties Tomography via Green’s Integral Identity
Full text linkA new approach to Magnetic Resonance-based Electric Properties Tomography (EPT) is presented. The method applies Green's integral identity to the equation that regulates the EPT problem. The resultant integral equations are used to impose the consistency of the measured values of the radiofrequency field. This is achieved by seeking dielectric properties values that allow satisfying the identity within suitable kernels of voxels. In each kernel, an overdetermined system of equations is written, and the corresponding problem is solved in the least squares sense, providing an index of trustworthiness of the solution. Both the complete formulation and its phase-based approximation are presented. The application of a filter, which post-processes the raw results based on the index of trustworthiness, is also discussed. The performance of the method is evaluated on synthetic data and experimental measurements acquired on a heterogeneous brain phantom and on four human volunteers. The reconstructions are compared to those produced through a Helmholtz-EPT with adaptive kernel. The new EPT method performs well in all tests
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