Improved Decoupling for Low Frequency MRI Arrays Using Non-Conventional Preamplifier Impedance

Abstract

 Abstract-Objective: In this study, we describe a method to improve preamplifier decoupling in low frequency MRI receive coil arrays, where sample loading is low and coils exhibit a high Qfactor. Methods: The method relies on the higher decoupling obtained when coils are matched to an impedance higher than 50 Ω. Preamplifiers with inductive (and low resistive) input impedance, increase even further the effectiveness of the method. Results: We show that for poorly sample loaded coils, coupling to other elements in an array is a major source of SNR degradation due to a reduction of the coil Q-factor. An 8-channel 13 C array at 32 MHz for imaging of the human head has been designed following this strategy. The improved decoupling even allowed constructing the array without overlapping of neighboring coils. Parallel imaging performance is also evaluated demonstrating a better spatial encoding of the array due to its non-overlapped geometry. Conclusion: The proposed design strategy for coil arrays is beneficial for low frequency coils where the coil thermal noise is dominant. The method has been demonstrated on an 8channel array for the human head for 13 C MRI at 3 T (32 MHz), with almost 2-fold SNR enhancement when compared to a traditional array of similar size and number of elements. Significance: The proposed method is of relevance for low frequency arrays, where sample loading is low, and noise correlation is high due to insufficient coil decoupling.

Multi-site benchmarking of clinical 13C RF coils at 3T

“…The coils are placed around a cylinder with 250 mm diameter; a suitable size for human head imaging. Each loop is critically overlapped with its neighbors, while decoupling to the next-neighboring coils is provided by the preamplifiers as described in [18] , [19] .…”

A flexible MRI coil based on a cable conductor and applied to knee imaging

“…6 c). One explanation for the apparent discrepancy is that decoupling and tuning have a complex relationship with SNR because of interplay among additional properties such as coil loss, noise correlation, matching circuitry, and preamplifier impedance mismatch 22 – 30 . From our data on two-channel arrays, we selected cable coils for the six-channel knee array because the plastic hinges (Fig.…”

RF coil design for accurate parallel imaging on 13C MRSI using 23Na sensitivity profiles

“…In order to achieve the required decoupling, a high mismatch between the coil and preamplifier is created, as described in Ref. [16], where the coils are matched to a higher impedance (200 Ω, for the sample‐loaded coils) than the optimum for the preamplifier (50 Ω). In this way, a controlled noise penalty is accepted by not matching the coils to the optimal noise impedance of the LNA (WMA32C, WanTCom, USA).…”

“…The design proposed here relies on creating a high level of preamplifier decoupling, which can be obtained by matching the low noise amplifier (LNA) to a higher impedance than the noise optimal, in a controlled way. 16 Then, its frequency response can be tuned to the intermediate frequency between 13 C and 23 Na, such that the coupling levels will still be low (and equal) at both frequencies. As a proof of principle, a flexible eight-channel array implementing this method is designed and evaluated.…”