Synopsis

A size-adaptable RF receiver coil prototype which can displace inter-element distance with negligible SNR degradation was investigated. We applied a simple method to enhance blocking impedance of the preamp decoupling circuit by using intentionally small matching capacitance. SNR of the size-adaptable prototype was evaluated for phantoms of 5 sizes ranging from knee size to abdomen size against commercial coils dedicated for each body parts. Despite of its broad size adaptability across various body parts and sizes, the prototype showed higher SNR than each commercial coil.

Introduction

Generally, RF receiver coils are designed for dedicated body parts with fixed coil size. Even commercial flexible coils are usually restricted to fixed coil length which leads to insufficient adaptability to patient size. Not only a “one-size-fits-all” concept is desirable, but also a “one-coil-fits-all” concept capable of various body parts and patient sizes with a single type of coil would be even more cost effective. Several approaches have been considered to realize size-adaptable multi-channel RF coils.^1-6 Challenges are to prevent SNR degradation due to inter-element coupling for a broad range of sizes with manageable manufacturing complication.

In this study, a size-adaptable RF receiver coil prototype which can displace inter-element distance with negligible SNR degradation was investigated. We applied a simple method to enhance blocking impedance of the preamp decoupling circuit⁷ by using intentionally small matching capacitance. Experimental SNR evaluation was conducted to check coil performance robustness when overlap of a neighbor coil was changed. For proof of concept evaluation, we made an 8-channel prototype to change overlap to fit various body parts or patient sizes and compared SNR of phantoms ranging from knee size to abdomen size against commercial coils dedicated for each body parts.

Methods

Decoupling method

Fig. 1a is a schematic circuit diagram of a typical RF receiver coil with preamp decoupling. Fig. 1b is an equivalent circuit of the preamp decoupling circuit. The blocking impedance of the preamp decoupling circuit can be expressed with variables described in Fig. 1b as,$$\frac{1}{Z_{block}} = j\omega C_m + \frac{1}{j\omega L_m + Z_{in}}. (Eq. 1)$$ For typical MRI parameters where $$$\omega^2 C_m^2 Z_{in}^2 \ll 1$$$, Zblock can be simplified as, $$Z_{block} \sim \frac{1}{\omega^2 C_m^2 Z_{in}}. (Eq. 2)$$ With lower input impedance preamps, Zblock increases which enables preamp decoupling.⁷ Here, we propose to intentionally choose a small matching capacitance to enhance Zblock since it has a larger effect with a power of 2 compared to Zin.

Proof of concept evaluation

First, a bench measurement of Zblock with a network analyzer was conducted for Cm = 12, 34, 59 pF and Zin = 0.6 and 1.2 Ω.
Second, SNR degradation due to neighbor element displacement was evaluated with a 2-channel prototype with Cm = 12 pF. Loop size were then chosen to be 90 mm by 180 mm placed at 7 mm distance from the surface of a head sized cylinder phantom (diameter 165 mm, height 320 mm), so that the coil was matched to 50 Ω at 3 T. The long end of the element was placed parallel to B0. Center SNR (ROI 30 mm diameter) of the fixed channel was evaluated while the other was displaced (Fig. 1c). For comparison, same measurement was made for Cm = 59 pF as conventional, with 46 mm gap from the phantom for 50 Ω matching.
Third, an 8-channel prototype was made to evaluate SNR for phantoms of 5 sizes (Fig. 2) with diameters of 121, 138, 165, 186 and 300 mm, respectively (phantoms A-E). The same elements of the prototype were used for all phantom sizes without adjusting any circuitry. 8-elements were used for phantoms B-E, and 6-elements were used for phantom A assuming perfect detuning of the unused 2-elements. For comparison, commercial coils (Hitachi Ltd., Japan) closest to each phantom sizes were chosen; 12-channel extremity coil for phantom A, 15-channel head coil for phantoms B, C and D, 12-channel torso coil for phantom E. SNR was calculated with optimum reconstruction accounting noise correlations.⁷ SNR of 75% area ROI and center ROI was compared. For all scanning measurements, 2D SE sequence was used on a 3 T scanner.

Results

Discussion

Conclusion

Acknowledgements

References

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