Multi-Tuned Coils
1High-Field MR Center, Max Planck Institute for Biological Cybernetics, Tübingen, Germany, 2Institute of Physics, Ernst-Moritz-Arndt University of Greifswald, Greifswald, Germany
Synopsis
X-nuclei (13C, 31P, 19F etc) MRI and spectroscopy are of great interest since these methods provide a non-invasive technique to study in-vivo metabolite changes due to various diseases. To provide anatomical landmarks for interpretation of X-nuclei spectroscopic data, 1H anatomical images are required. To eliminate uncertainties associated with repositioning the patient, the RF coil must also resonate at the 1H frequency. This technique is called double-tuning (DT) of the RF coils. The choice of DT design is determined by the requirements of a specific application. Various methods of constructing DT RF surface coils, volume coils, and phased arrays are discussed.
Background
Target audience: RF engineers, scientists and students interested in development, construction and usage of double-tuned (DT) RF MRI coils for X-nuclei spectroscopic studies.
Purpose: To learn various techniques of DT RF coil design and construction
Highlights:
-Introduction
-Usefull RF circuits (traps) used for DT coil design
-DT surface coils
-DT Volume coils
-DT phased Arrays
Introduction
X-nuclei (13C, 31P, 19F, 23Na, 7Li etc) magnetic resonance imaging (MRI) and spectroscopy (MRS, MRSI) are of great interest to our scientific community since these methods provide a unique non-invasive technique to study in-vivo metabolite changes due to various diseases. To provide anatomical landmarks for interpretation of X-nuclei spectroscopic data, high-resolution 1H anatomical images are also required. To eliminate uncertainties associated with repositioning either the patient or the RF coil and to support 1H imaging, the RF coil must also resonate at the 1H frequency. This technique is often referred to as double-tuning of the MRI RF coils. B0 shimming and proton decoupling (e.g. 13C) also require double-tuning the X-nuclei coils. Various methods of constructing double-tuned (DT) RF coils have been previously described. The choice of DT design is always determined by the requirements of a specific application, e.g. whether both channels (1H, X) are used simultaneously, or the 1H-channel should have high sensitivity. Required field-of-views of both channels are also of great importance. X-nuclei MR signals are intrinsically lower due to a large difference in the gyromagnetic ratio. This implies that DT design is mostly optimized for X-nuclei performance. For the same reason, recent advances in ultra-high field (UHF, >7T) MRI, which provide for higher signal-to-noise ratio (SNR), increase interest in MRS studies.
The lecture starts with a discussion of useful basic RF circuits followed by an introduction of various basic methods of double-tuning surface RF coils [1-13], volume coils [14-30], and phased arrays [31-43]. Phased arrays are especially important at UHF.
Surface Coils
The most common way of constructing DT surface coils is by introducing a resonance LC-trap circuit in series with distributed capacitors. This is mathematically referred as pole insertion [1]. In this case, the same inductive loop of the surface coil, L, is used to generate the RF B1 field at both frequencies. A trap, formed by a parallel resonance circuit (LT, CT) is connected in series with a capacitor, C, and an inductor, L (Fig.1A). At low frequencies (ω2LTCT < 1), the trap’s impedance, ZT, is equivalent to an inductor, Llow, while at higher frequencies (ω2LTCT > 1) it acts as a capacitor, Chigh, thus, producing a double resonant circuit (Fig.1A). The performance of the surface coil at both frequencies can be estimated as E= [PL/(PL+PT)]1/2=[RL/(RL+RT)]1/2, where PL,T and RL,T are power losses and resistances associated with inductors L and LT, respectively. Efficiency at low, Elow, and high, Ehigh, frequencies can then be calculated using Eqs. (1) and (2) as obtained in [1].
Elow=(L/(L+LT))1/2 (1)
Ehigh=(LT/(L+LT))1/2 (2)
Using these equations, for LT = 0.2 L, we obtain an Elow of 0.91 and and Ehigh of 0.41. Thus, LC-trap design does not allow optimization of both channels in a DT coil simultaneously. While it provides a certain convenience in design, the coil always possesses substantially lower sensitivity at 1H-frequency. Various DT matching circuits are also discussed [13].
Another approach to construct a DT coil is by utilizing separate surface loops (or other types of RF coils) resonating at the X-nuclei and 1H-frequencies [2,4-8,10-12] as shown in Fig.1B. Despite a large difference in resonance frequencies, two coils can still strongly interact with one another. In order to cancel cross-talk, resonant traps are, therefore still inserted into each loop, i.e. an X-trap into the 1H -loop and vice versa. However, in this case, traps resonating at substantially different frequencies do not compromise the coil performance. Performance of both loops can thus be optimized simultaneously. Size and geometry of X- and 1H-coils are determined by the requirements of the specific experiment. Using a pair of geometrically decoupled coils also helps to eliminate a cross-talk between two channels of the DT coil [6,11]. Other options include using orthogonal modes of the same coil [12] or switching the resonance frequency using PIN diodes [9]. The last option, however, does not allow the use of both channels simultaneously.
Volume Coils
Optimal SNR for in vivo studies is typically obtained when a homogeneous volume coil is used for transmission and a sensitive local receive-only coil (phased array) is used for reception. In this setup, double-tuning of the transmit volume coil is often required. In this section examples of DT volume coils based on common multi-mode birdcage [1,2] and other designs are provided [14-30]. As with the double-tuning of surface coils, DT volume coils can be constructed using either the same coil resonating at X- and 1H-frequencies simultaneously [17,18,20-26,27,28] or two different nested volume coils [19,26]. Switching the frequency of a single-tuned birdcage coil using PIN diodes [30] may provide a solution when simultaneous use of both channels is not required.Phased Arrays
The improved signal-to-noise ratio (SNR) at UHF provides significant advantages for both 1H and lower gyromagnetic ratio X-nuclei. Double resonant volume coils based on both birdcage and TEM [21] designs have been previously used at lower magnetic fields. However, at UHF, phased arrays [31] provide significant advantages for transmit B1 homogeneity and efficiency over volume coils. Similarly, phased arrays for reception provide additional SNR gains for peripheral locations in studies of X nuclei. Therefore, DT transceiver arrays may provide substantial advantages over conventional DT volume coils. Nevertheless, DT transceiver arrays are substantially more complicated than single tuned arrays as all individual elements must be decoupled from each other at both resonance frequencies. In the last part of the presentation, several recent developments of DT phased arrays are discussed (32-42). This includes both transceiver DT arrays [33-37, 42], and Transmit-only/ Receive-only (ToRo) array combinations [32, 38-41].Acknowledgements
No acknowledgement found.References
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Surface Coils
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