Abstract
Phased array (PA) receive coils are built such that coil elements approximate independent antenna behavior. One method of achieving this goal is to use an available decoupling method to decouple adjacent coil elements. The purpose of this work was to compare the relative performance of two decoupling methods as a function of variation in sample load. Two PA receive coils with 5 channels (5-ch) each, equal outer dimensions, and formed on 12 cm diameter cylindrical phantoms of conductivities 0.3, 0.6, and 0.9 S/m were evaluated for relative signal-to-noise ratio (SNR) and parallel imaging performance. They were only tuned and matched to the 0.6 S/m phantom. Simulated and measured axial, sagittal, and coronal 5-ch PA coil SNR ratios were compared by dividing the overlap by the capacitive decoupled coil SNR results. Issues related to the selection of capacitor values for the two decoupling methods were evaluated by taking the ratio of the match and tune capacitors for large and small 2 channel (2-ch) PA coils. The SNR ratios showed that the SNR of the two decoupling methods were very similar. The inverse geometry-factor maps showed similar but better overall parallel imaging performance for the capacitive decoupled method. The quotients for the 2-ch PA coils’ maximum and minimum capacitor value ratios are 3.28 and 1.38 for the large and 3.28 and 2.22 for the small PA. The results of this paper demonstrate that as the sample load varies, the capacitive and overlap decoupling methods are very similar in relative SNR and this similarity continues for parallel imaging performance. Although, for the 5-ch coils studied, the capacitive decoupling method has a slight SNR and parallel imaging advantage and it was noted that the capacitive decoupled coil is more likely to encounter unbuildable PA coil configurations.
Highlights
Simultaneous and independent imaging with multiple surface coils, known as phased array (PA) coils, brings with it many benefits [1]. e benefits, cited by Roemer et al, are increased surface coil array field of view (FOV) while maintaining the local high signal-to-noise ratio (SNR) of each individual coil element [2]
PA coils can still be used even when strong coupling exists between coil elements if the magnitude and phase of all the mutual coupling between coil elements are known. e coupling information can be used to remove any signal and noise that was transferred from one coil element to another through the mutual coupling [3]. e resulting signals will appear as if they were received from coil elements with independent antenna behavior
An alternative magnetic decoupling technique is implemented by joining surface coils together with a common rung and using a capacitor, on that common rung, to eliminate the mutual inductance or the capacitive decoupled (CD) method [4,5,6,7]. ese types of PAs are commonly referred to as ladder coils [6, 8,9,10]
Summary
Simultaneous and independent imaging with multiple surface coils, known as phased array (PA) coils, brings with it many benefits [1]. e benefits, cited by Roemer et al, are increased surface coil array field of view (FOV) while maintaining the local high signal-to-noise ratio (SNR) of each individual coil element [2]. E benefits, cited by Roemer et al, are increased surface coil array field of view (FOV) while maintaining the local high signal-to-noise ratio (SNR) of each individual coil element [2]. Roemer proposed the first method of combining individual images on a pixel by pixel basis to form a composite image using optimal weights and phases; these were derived assuming independent antenna behavior (zero mutual coupling between coil elements). One of the most common techniques to minimize mutual inductance between adjacent coil elements is implemented by partially overlapping them or the overlap decoupled (OD) method [2]. There have been papers that present the different decoupling methods, a direct comparison of the different methods has not been published It remains to be shown whether one method has an advantage in SNR, is robust to variations in load, or has issues related to construction. The buildability of PA coils utilizing either of these decoupling methods was explored using small and large 2channel (2-ch) PA coils
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