Inhomogeneity Correction in High Field Magnetic Resonance Images

Projecte realitzat en col.laboració amb el centre Swiss Federal Institute of Technology (EPFL)Magnetic Resonance Imaging, MRI, is one of the most powerful and harmless ways to study human inner tissues. It gives the chance of having an accurate insight into the physiological condition of the human body, and specially, the brain. Following this aim, in the last decade MRI has moved to ever higher magnetic field strength that allow us to get advantage of a better signal-to-noise ratio. This improvement of the SNR, which increases almost linearly with the field strength, has several advantages: higher spatial resolution and/or faster imaging, greater spectral dispersion, as well as an enhanced sensitivity to magnetic susceptibility. However, at high magnetic resonance imaging, the interactions between the RF pulse and the high permittivity samples, which causes the so called Intensity Inhomogeneity or B1 inhomogeneity, can no longer be negligible. This inhomogeneity causes undesired efects that afects quantitatively image analysis and avoid the application classical intensity-based segmentation and other medical functions. In this Master thesis, a new method for Intensity Inhomogeneity correction at high ¯eld is presented. At high ¯eld is not possible to achieve the estimation and the correction directly from the corrupted data. Thus, this method attempt the correction by acquiring extra information during the image process, the RF map. The method estimates the inhomogeneity by the comparison of both acquisitions. The results are compared to other methods, the PABIC and the Low-Pass Filter which try to correct the inhomogeneity directly from the corrupted data

Design of Radio-Frequency Arrays for Ultra-High Field MRI

Magnetic Resonance Imaging (MRI) is an indispensable, non-invasive diagnostic tool for the assessment of disease and function. As an investigational device, MRI has found routine use in both basic science research and medicine for both human and non-human subjects. Due to the potential increase in spatial resolution, signal-to-noise ratio (SNR), and the ability to exploit novel tissue contrasts, the main magnetic field strength of human MRI scanners has steadily increased since inception. Beginning in the early 1980’s, 0.15 T human MRI scanners have steadily risen in main magnetic field strength with ultra-high field (UHF) 8 T MRI systems deemed to be insignificant risk by the FDA (as of 2016). However, at UHF the electromagnetic fields describing the collective behaviour of spin dynamics in human tissue assume ‘wave-like’ behaviour due to an increase in the processional frequency of nuclei at UHF. At these frequencies, the electromagnetic interactions transition from purely near-field interactions to a mixture of near- and far-field mechanisms. Due to this, the transmission field at UHF can produce areas of localized power deposition – leading to tissue heating – as well as tissue-independent contrast in the reconstructed images. Correcting for these difficulties is typically achieved via multi-channel radio-frequency (RF) arrays. This technology allows multiple transmitting elements to synthesize a more uniform field that can selectively minimize areas of local power deposition and remove transmission field weighting from the final reconstructed image. This thesis provides several advancements in the design and construction of these arrays. First, in Chapter 2 a general framework for modeling the electromagnetic interactions occurring inside an RF array is adopted from multiply-coupled waveguide filters and applied to a subset of decoupling problems encountered when constructing RF arrays. It is demonstrated that using classic filter synthesis, RF arrays of arbitrary size and geometry can be decoupled via coupling matrix synthesis. Secondly, in Chapters 3 and 4 this framework is extended for designing distributed filters for simple decoupling of RF arrays and removing the iterative tuning portion of utilizing decoupling circuits when constructing RF arrays. Lastly, in Chapter 5 the coupling matrix synthesis framework is applied to the construction of a conformal transmit/receive RF array that is shape optimized to minimize power deposition in the human head during any routine MRI examination

Image Encoding and Reconstruction for Portable Magnetic Resonance Imaging

Magnetic Resonance Imaging (MRI) is a successful imaging tool, but due to high cost, high weight and complexity of equipment, MRI is not currently as easily accessible clinically as desired. By making MRI cheaper, lighter, less complex and therefore potentially portable, it can become more widely accessible. A portable MRI system can be used in primary health care, operating and emergency rooms, car and air ambulances, sport and war facilities and remote regions (including outer space). The objective of this study was to show the feasibility of reconstructing MRI signals generated by two different portable MRI systems. The first portable MRI, known as radiofrequency (RF) phase encoded MRI, encodes spatial information through the use of a non-linear spatially varying RF transmit ($B_1$) phase. The second portable MRI, known as rotating field MRI, encodes information through non-uniform radially varying main magnet field ($B_0$). The fact that there is no need for gradient coils in both systems, leads to a smaller, lighter and more affordable MRI than most conventional systems. In RF phase encoded MRI, since the $B_1$ phase spatially varies non-linearly, using Fourier transform (FT) to reconstruct images results in distorted images. Therefore, regularized least squares inversion was used in place of the usual FT. The RF phase encoding coil generates an inhomogeneous $B_1$ field that leads to RF pulse imperfection in terms of flip angles produced versus flip angles intended. Composite pulses were therefore used to minimize the effect of RF transmit field inhomogeneity on tip angles. In rotating field MRI, a Halbach magnet was used to generate a non-uniform radially varying $B_0$ field to encode information in the radial direction. For encoding information in the angular direction two separate Saddle RF receiver coils were used. The main magnet and receiver coils are fixed relative to each other, but both rotate around the object. A regularized least squares inversion (LS) method followed by total variation (TV) techniques were used to reconstruct the images. MRI simulation signals encoded in RF transmit field with non-linearly varying spatial phase may be accurately reconstructed using regularized LS method thus pointing the way to the use of simple RF coil designs for RF encoded MRI. Also, my results from simulation and experimental data, indicated the feasibility of reconstructing images from rotating field MRI. I have made progress in the realization of a novel approach to different MRI systems that do not rely on active magnetic gradient fields. These two methods can be combined to encode information in 3 dimensions (3D) in the future, for example inhomogeneous $B_0$ field can be used for slice selection and RF phase encoding can be used to encode information in the plane

Analysis of a Flexible Dual-Channel Octagonal Coil System for UHF MRI

Nowadays, MRI is focused on using ultra-high static magnetic fields (> 7 T) to increase the signal-to-noise ratio. The use of high fields, on the other hand, requires novel technical solutions as well as more stringent design criteria for specific absorption rate levels, reducing radiative effect and coil resistance. In this paper, two flexible RF coils for 7 T human magnetic resonance, and 298 MHz ultra-high frequency operations were analyzed and characterized. Imaging of lower human limbs is regarded as a case study. The lumped element theory and subsequent numerical simulations were used to fine-tune the single-coil element and the dual-coil array design, respectively. Here, we demonstrate how the shape, size, configuration, and presence of the sample influence the coil performance. The penetration depth of the B 1 -field and the specific absorption rate values have been determined numerically using two numerical surface phantoms: saline and a multilayer human tissue. A preliminary study in the presence of a saline solution phantom has been carried out to develop and validate the dual-coil system. The frequency response of the dual-coil array was measured to assess its robustness when coupled to twelve human volunteers. We found that our design is robust to variations in the anatomical properties of the human thighs, and hence to coil bending. The presented approach can be useful for the implementation of flexible devices with high sensitivity levels and low specific absorption rat

Quantitative MR Imaging of the Electric Properties and Local SAR based on Improved RF Transmit Field Mapping

This work presents three new quantitative methods for magnetic resonance imaging. A method for simultaneous mapping of B1 and T1 (MTM) is developed and validated. Electric Properties Tomography (EPT), a method for quantitative imaging of dielectric properties of tissue, is presented. Based on EPT, separate (phase-based) conductivity and (amplitude-based) permittivity measurements are introduced. Finally, a B1-based method for patient-specific local SAR measurements is presented

Multiple resonant multiconductor transmission line resonator design using circulant block matrix algebra

The purpose of this dissertation is to provide a theoretical model to design RF coils using multiconductor transmission line (MTL) structures for MRI applications. In this research, an MTL structure is represented as a multiport network using its port admittance matrix. Resonant conditions and closed-form solutions for different port resonant modes are calculated by solving the eigenvalue problem of port admittance matrix using block matrix algebra. A mathematical proof to show that the solution of the characteristic equation of the port admittance matrix is equivalent to solving the source side input impedance is presented. The proof is derived by writing the transmission chain parameter matrix of an MTL structure, and mathematically manipulating the chain parameter matrix to produce a solution to the characteristic equation of the port admittance matrix. A port admittance matrix can be formulated to take one of the forms depending on the type of MTL structure: a circulant matrix, or a circulant block matrix (CB), or a block circulant circulant block matrix (BCCB). A circulant matrix can be diagonalized by a simple Fourier matrix, and a BCCB matrix can be diagonalized by using matrices formed from Kronecker products of Fourier matrices. For a CB matrix, instead of diagonalizing to compute the eigenvalues, a powerful technique called “reduced dimension method� can be used. In the reduced dimension method, the eigenvalues of a circulant block matrix are computed as a set of the eigenvalues of matrices of reduced dimension. The required reduced dimension matrices are created using a combination of the polynomial representor of a circulant matrix and a permutation matrix. A detailed mathematical formulation of the reduced dimension method is presented in this thesis. With the application of the reduced dimension method for a 2n+1 MTL structure, the computation of eigenvalues for a 4n X 4n port admittance matrix is simplified to the computation of eigenvalues of 2n matrices of size 2 X 2. In addition to reduced computations, the model also facilitates analytical formulations for coil resonant conditions. To demonstrate the effectiveness of the proposed methods (2n port model and reduced dimension method), a two-step approach was adopted. First, a standard published RF coil was analyzed using the proposed models. The obtained resonant conditions are then compared with the published values and are verified by full-wave numerical simulations. Second, two new dual tuned coils, a surface coil design using the 2n port model, and a volume coil design using the reduced dimensions method are proposed, constructed, and bench tested. Their validation was carried out by employing 3D EM simulations as well as undertaking MR imaging on clinical scanners. Imaging experiments were conducted on phantoms, and the investigations indicate that the RF coils achieve good performance characteristics and a high signal-to-noise ratio in the regions of interest

Quantitative MR Imaging of the Electric Properties and Local SAR based on Improved RF Transmit Field Mapping

This work presents three new quantitative methods for magnetic resonance imaging. A method for simultaneous mapping of B1 and T1 (MTM) is developed and validated. Electric Properties Tomography (EPT), a method for quantitative imaging of dielectric properties of tissue, is presented. Based on EPT, separate (phase-based) conductivity and (amplitude-based) permittivity measurements are introduced. Finally, a B1-based method for patient-specific local SAR measurements is presented

B(1) inhomogeneity correction of RARE MRI with transceive surface radiofrequency probes

PURPOSE: The use of surface radiofrequency (RF) coils is common practice to boost sensitivity in (pre)clinical MRI. The number of transceive surface RF coils is rapidly growing due to the surge in cryogenically cooled RF technology and ultrahigh‐field MRI. Consequently, there is an increasing need for effective correction of the excitation field (B(1)(+)) inhomogeneity inherent in these coils. Retrospective B(1) correction permits quantitative MRI, but this usually requires a pulse sequence‐specific analytical signal intensity (SI) equation. Such an equation is not available for fast spin‐echo (Rapid Acquisition with Relaxation Enhancement, RARE) MRI. Here we present, test, and validate retrospective B(1) correction methods for RARE. METHODS: We implemented the commonly used sensitivity correction and developed an empirical model‐based method and a hybrid combination of both. Tests and validations were performed with a cryogenically cooled RF probe and a single‐loop RF coil. Accuracy of SI quantification and T(1) contrast were evaluated after correction. RESULTS: The three described correction methods achieved dramatic improvements in B(1) homogeneity and significantly improved SI quantification and T(1) contrast, with mean SI errors reduced from >40% to >10% following correction in all cases. Upon correction, images of phantoms and mouse heads demonstrated homogeneity comparable to that of images acquired with a volume resonator. This was quantified by SI profile, SI ratio (error 80% in vivo and ex vivo compared to PIU > 87% with the reference RF coil). CONCLUSIONS: This work demonstrates the efficacy of three B(1) correction methods tailored for transceive surface RF probes and RARE MRI. The corrected images are suitable for quantification and show comparable results between the three methods, opening the way for T(1) measurements and X‐nuclei quantification using surface transceiver RF coils. This approach is applicable to other MR techniques for which no analytical SI exists

A 64-channel personal computer based image reconstruction system and applications in single echo acquisition magnetic resonance elastography and ultra-fast magnetic resonance imaging.

Emerging technologies in parallel magnetic resonance imaging (MRI) with massive receiver arrays have paved the way for ultra-fast imaging at increasingly high frame rates. With the increase in the number of receiver channels used to implement parallel imaging techniques, there is a corresponding increase in the amount of data that needs to be processed, slowing down the process of image reconstruction. To develop a complete reconstruction system which is easy to assemble in a single computer for a real-time rendition of images is a relevant challenge demanding dedicated resources for high speed digital data transfer and computation. We have enhanced a 64 channel parallel receiver system designed for single echo acquisition (SEA) MRI into a real-time imaging system by interfacing it with two commercially available digital signal processor (DSP) boards which are capable of transferring large amounts of digital data via a dedicated bus from two high performance digitizer boards. The resulting system has been used to demodulate raw image data in real-time data and store them at rates of 200 frames per second (fps) and subsequently display the processed data at rates of 26 fps. A further interest in realtime reconstruction techniques is to reduce the data handling issues. Novel ways to minimize the digitized data are presented using reduced sampling rate techniques. The proposed techniques reduce the amount of data generated by a factor of 5 without compromising the SNR and with no additional hardware. Finally, the usability of this tool is demonstrated by investigating fast imaging applications. Of particular interest among them are MR elastography applications. An exploratory study of SEA MRE was done to study the temperature dependency of shear stiffness in an agarose gel and the results correlate well with existing literature. With the ability to make MRE images in a single echo, the SEA MRE technique has an advantage over the conventional MRE techniques