Abstract
Sodium is one of the most abundant physiological cations and is a key element in many cellular processes. It has been shown that several pathologies, including degenerative brain disorders, cancers, and brain traumas, express sodium deviations from normal. Therefore, sodium magnetic resonance imaging (MRI) can prove to be valuable for physicians. However, sodium MRI has its limitations, the most significant being a signal-to-noise ratio (SNR) thousands of times lower than a typical proton MRI. Radiofrequency coils are the components of the MRI system directly responsible for signal generation and acquisition. This paper explores the intrinsic properties of a Koch snowflake fractal radiofrequency surface coil compared to that of a standard circular surface coil to investigate a fractal geometry’s role in increasing SNR of sodium MRI scans. By first analyzing the network parameters of the two coils, it was found that the fractal coil had a better impedance match than the circular coil when loaded by various anatomical regions. Although this maximizes signal transfer between the coil and the system, this is at the expense of a lower Q, indicating greater signal loss between the tissue and coil. A second version of each coil was constructed to test the mutual inductance between the coils of the same geometry to see how they would behave as a phased array. It was found that the fractal coils were less sensitive to each other than the two circular coils, which would be beneficial when constructing and using phased array systems. The performance of each coil was then assessed for B1+ field homogeneity and signal. A sodium phantom was imaged using a B1+ mapping sequence, and a 3D radial acquisition was performed to determine SNR and image quality. The results indicated that the circular coil had a more homogeneous field and higher SNR. Overall while the circular coil proved to generate a higher signal-to-noise ratio than the fractal, the Koch coil showed higher versatility when in a multichannel network which could prove to be a benefit when designing, constructing, and using a phased array coil.
Highlights
Sodium magnetic resonance imaging (23Na-MRI) has the potential to become a valuable tool in the clinical setting, by assisting physicians through the diagnosis, prognosis, and monitoring of a variety of pathologies including cancers, degenerative brain disorders, concussion, and osteoarthritis [1, 2]
This leads to the hypothesis that a Koch snowflake fractal geometry surface coil can improve the quality of 23Na-MRI images by increasing the resultant signal-to-noise ratio (SNR) due to a more homogeneous B1+ field, a superior filling factor, and more robust impedance matching than a typical circular geometry surface coil
The Koch coil has a significantly better match than the circular coil when being loaded by the anatomical regions, which can be observed in the lower S11 values
Summary
Sodium magnetic resonance imaging (23Na-MRI) has the potential to become a valuable tool in the clinical setting, by assisting physicians through the diagnosis, prognosis, and monitoring of a variety of pathologies including cancers, degenerative brain disorders, concussion, and osteoarthritis [1, 2]. There exist thousands of fractal and fractal-like geometries and a smaller subset are the focus of most antenna research and applications, but here only one pattern was studied: the Koch snowflake (Figure 1) This geometry was one of the fractals that was explored in [11, 12] and showed a higher Q, better impedance match, and less mutual inductance when in arrays. Previous simulations of a Koch snowflake coil performed by Dona Lemus et al [15] and Nowikow et al [16] have shown that physical construction and implementation is warranted This leads to the hypothesis that a Koch snowflake fractal geometry surface coil can improve the quality of 23Na-MRI images by increasing the resultant SNR due to a more homogeneous B1+ (and B1−) field, a superior filling factor, and more robust impedance matching than a typical circular geometry surface coil. The lower mutual inductance for the Koch snowflake geometry will facilitate implementation in phased array coils
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