Tomography

Vol. 3 No. 1 - Mar 2017

Tomography is a scientific journal for publication of articles in imaging research

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Low-Noise Active Decoupling Circuit and its Application to 13 C Cryogenic RF Coils at 3 T Juan Diego Sanchez-Heredia 1 , Esben Søvsø Szocska Hansen 2 , Christoffer Laustsen 2 , Vitaliy Zhurbenko 1 , and Jan Henrik Ardenkjær-Larsen 1 1 Department of Electrical Engineering, Technical University of Denmark, Kongens Lyngby, Copenhagen, Denmark and 2 MR Research Centre, Department of Clinical Medicine, Aarhus University, Aarhus, Denmark Corresponding Author: Jan Henrik Ardenkjær-Larsen, PhD Department of Electrical Engineering, Technical University of Denmark, Kongens Lyngby, Copenhagen, Denmark; E-mail: jhar@elektro.dtu.dk Key Words: RF coil, SNR, cryogenic, 13 C MRI Abbreviations: Signal-to-noise ratio (SNR), magnetic resonance (MR), magnetic resonance imaging (MRI), radiofrequency (RF), carbon-13 ( 13 C), hydrogen-1 ( 1 H), equivalent series resistance (ESR), vector network analyzer (VNA) We analyze the loss contributions in a small, 50-mm-diameter receive-only coil for carbon-13 ( 13 C) magnetic resonance imaging at 3 T for 3 different circuits, which, including active decoupling, are compared in terms of their Q-factors and signal-to-noise ratio (SNR). The results show that a circuit using unsegmented tuning and split matching capacitors can provide .20% SNR enhancement at room temperature compared with that using more traditional designs. The performance of the proposed circuit was also measured when cryo- genically cooled to 105 K, and an additional 1.6-fold SNR enhancement was achieved on a phantom. The enhanced circuit performance is based on the low capacitance needed to match to 50 V when coil losses are low, which significantly reduces the proportion of the current flowing through the matching network and therefore minimizes this loss contribution. This effect makes this circuit particularly suitable for receive-only cryogenic coils and/or small coils for low-gamma nuclei. INTRODUCTION The sensitivity of the magnetic resonance (MR) detection circuit is one of the most important aspects of a magnetic resonance imaging (MRI) experiment. Signal-to-noise ratio (SNR) im- provement allows higher image resolution and/or reduced ac- quisition time. In an MR experiment, the noise mainly comes from thermal noise of the coil (and electronics), and the sample noise is due to the interaction of radiofrequency (RF) fields with the lossy sample. For proton imaging in humans, often, the sample losses are dominant because of the relatively high Lar- mor frequency of protons and the subject's (patient) large size. However, increasing attention has been drawn toward imaging of other nuclei with lower Larmor frequencies, emphasizing the importance of coil losses. Carbon-13 ( 13 C) is of particular inter- est because it is used for hyperpolarized metabolic MR. This is an exciting new method with potential in early diagnosis of dis- ease, staging, and therapy monitoring (1-5). Following the trend already established for proton imaging, the SNR of 13 C imaging can, in principle, also be improved using smaller surface coils in phased arrays or for acceleration with parallel imaging. However, the assumption is that sample-dom- inated noise can be achieved. Examples of receive-only double- tuned (4-6) and phased-array (7) coils using small surface ele- ments for 13 C have already been reported, showing improved performance over detection using bigger-volume coils. How- ever, developing low-loss 13 C small surface coils that work in a sample-dominated noise regime to replicate the enhancements provided by surface coils already seen for proton imaging re- mains a challenge. At the lower Larmor frequency of 13 C (4 times lower than that of hydrogen-1 [ 1 H]), the noise contribution of the coil becomes more significant. One can approximate the SNR of a nuclear magnetic resonance experiment, in terms of coil and sample losses, as it is done in Styles et al.'s study (8) and calculated using the following equation: SNR < v · B 1 2 Ï R S · T S 1 R C · T C (1) where R S and R C are the equivalent resistances of sample and coil, respectively, T S and T C their temperatures, v the operating frequency, and B 1 2 the field per unit current. In this context, the SNR of the experiment can be further increased by cooling the coil. The resistivity of copper decreases ca. 8 times when cooled to liquid nitrogen temperature (77 K), allowing a potential ;3-fold SNR increase for the case of no sample losses. The SNR gain obtained from cooling then de- pends on the balance between coil and sample losses, making it difficult to directly compare coils with different geometries and resonance frequencies. The following are some examples found in the literature of Q factors for cryogenic coils similar in size to the ones we will study here: RESEARCH ARTICLE ABSTRACT © 2017 The Authors. Published by Grapho Publications, LLC This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). ISSN 2379-1381 http://dx.doi.org/10.18383/j.tom.2016.00280 60 TOMOGRAPHY.ORG | VOLUME 3 NUMBER 1 | MARCH 2017

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