Development of novel RF coils for $^{23}$Na imaging at ultra-high field MRI

Autor: Ha, Yonghyun
Přispěvatelé: Shah, Nadim Joni, Heberling, Dirk, Mayer, Joachim
Jazyk: angličtina
Rok vydání: 2019
Předmět:
Zdroj: Aachen 1 Online-Ressource (XXII, 122 Seiten) : Illustrationen, Diagramme (2019). doi:10.18154/RWTH-2019-10616 = Dissertation, RWTH Aachen University, 2019
DOI: 10.18154/RWTH-2019-10616
Popis: Dissertation, RWTH Aachen University, 2019; Aachen 1 Online-Ressource (XXII, 122 Seiten) : Illustrationen, Diagramme (2019). = Dissertation, RWTH Aachen University, 2019
Sodium (23Na) magnetic resonance imaging (MRI) provides valuable cellular and metabolic information. Since the Larmor frequency of the 23Na nucleus is different from that of protons (1H), a radiofrequency (RF) coil tuned to the 23Na Larmor frequency is required for sodium MR imaging. However, the MR signal of 23Na is much smaller in amplitude than that of 1H due to its low intrinsic MR sensitivity and lower natural abundance and 1H images are very often required in conjunction with the 23Na images. For this reason, 23Na/1H double-tuned RF coils are often used for 23Na studies. In this case the proton signal is used for localisation and B0 shimming. Double-tuned RF coils are often designed by including additional lossy components such as LC-traps or PIN-diodes. Thus, the sensitivity of the coil – which is proportional to the quality factor of the resonant circuit – is reduced at both resonance frequencies, when compared to that of a single-tuned coil. The aim of this thesis is to develop both surface and volume double-tuned RF coils for 23Na brain imaging with high efficiency. Of special interest is the quality factor at the 23Na Larmor frequency in order to maintain the best possible signal-to-noise ratio (SNR) for the lower MR sensitive X-nucleus. For a heterogeneous resonator, a double-tuned surface coil can be designed using PIN-diodes with series capacitors in parallel with the tuning capacitor. When the PIN-diode is forward biased, a capacitor is added in parallel to the tuning capacitor. The resonance frequency of the coil is then shifted to the lower frequency (for example to the 23Na Larmor frequency). While not simultaneously double-resonant, such networks allow rapid switching between two resonance frequencies under electronic control. However, the sensitivity at the lower frequency can be significantly decreased due to the residual RF resistance of the PIN-diode. A PIN-diode controlled, quadrature enhanced, double-tuned RF coil is developed for an animal scanner to enhance the SNR of the 23Na image. A single-tuned 23Na butterfly coil is added on the PIN-diode controlled coil to drive this coil in quadrature. The coil is connected to the scanner using a home-built transmit/receive (T/R) switch. Either forward current or reverse voltage is applied simultaneously to the individual PIN-diodes using a home-built PIN-diode control unit. In another approach, for a homogeneous resonator, the four-ring birdcage coil is comparable to a single-tuned birdcage coil terms of efficiency and homogeneity. However, it requires a physical addition in length for the two outer end-rings and this limits its use for brain studies due to the presence of shoulders forming a natural restriction of overall coil dimensions. In order to resolve this issue, a three-dimensional (3D) spatially folded four-ring birdcage coil is developed that mitigates space limitations. For the suggested surface coil, the SNR values of gradient echo (GRE) images were compared to that of single-tuned coils under the same condition. It was found that 23Na SNR is higher than that of single-tuned coil, for both the loop and the butterfly arrangements. The performance of the folded four-ring birdcage coil was also compared with a single-tuned and a larger four-ring birdcage coil on a phantom. It is shown that both B1 field homogeneity and image SNR of 23Na are comparable to that of the single-tuned birdcage coil. In vivo healthy rat brain images at 9.4 T using the novel folded four-ring birdcage coil are presented to validate the suitability of the novel design for brain imaging. It was demonstrated that using of the quadrature-enhanced surface coil and folded four-ring birdcage coil has a distinct benefit for 23Na imaging. It is also possible to combine both coils as a transmit-only/receive-only RF system to generate homogeneous B_1^+ field using the folded four-ring birdcage coil and for the higher SNR using the quadrature-enhanced surface coil.
Published by Aachen
Databáze: OpenAIRE
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