Abstract
Purpose: Many of today’s MR coils are still somehow rigid and inflexible in their size and shape as they are intentionally designed to image a specific anatomical region and to fit a wide range of patients. Adaptive coils on the other hand, are intended to follow an one-size-fits-all approach, by fitting different shapes and sizes. Such coils improve the SNR for a wide range of subjects by an optimal fit to the anatomical region of interest, and in addition allow an increased handling and patient comfort as one MRI receive-coil is maintained instead of multiple. Material and Methods: To overcome the SNR losses by non-fitting and thus poorly loaded RF coils, we propose a stretchable antenna design. Each loop has the ability to reversibly stretch up to 100% of its original size, to be anatomically adaptive to different shapes and sizes, and therefore make the coil usable for a wide patient population. Besides the mechanical challenge to find a robust but flexible conductive material, various other problems like frequency and matching shifts affect the SNR. Through bench measurements and MR Imaging at 1.5T, we investigated different stretchable conductor materials, that fit the defined requirements. Finally, a rigid reference coil and an adaptive 6-channel array for knee imaging at 1.5 Tesla were developed to investigate the potential improvement in SNR. Results: The material tests identified two potentially useful materials: Highly ductile copper and a silver-plated stranded copper wire. Although, the adaptivity causes a frequency shift of the resonance frequency, which entails in variations of the impedance that each coil presents to its connected pre-amplifier, there are strategies to mitigate these effects. The adaptive array made of partly-stretchable loops, showed an improved SNR of up to 100% in 20 mm depth from the phantom surface, and therefore demonstrates the effectiveness of adaptive coils.
Highlights
The past two decades of Magnetic Resonance Imaging (MRI) have seen immense advances in various fields, with a focus toward improved sensitivity, multi-modal imaging and reduced scan-time in clinical and research examinations
The goal of the material tests was to identify a suitable material fulfilling the needs of an adaptive coil array
Bench measurement results for the rigid double-loop array and the three stretchable double-loop arrays are summarized in Figure 5 and show the dependence of the following parameters on the amount of stretching: Qloaded (Figure 5A), Qunloaded (Figure 5B), Qratio (Figure 5C), coupling coefficient k (Figure 5D), shift in resonance frequency (Figure 5E), inductive decoupling (Figure 5F), pre-amplifier decoupling (Figure 5G), matching (Figure 5H), coil+sample resistance (Figure 5I), and unloaded coil resistance (Figure 5J)
Summary
The past two decades of Magnetic Resonance Imaging (MRI) have seen immense advances in various fields, with a focus toward improved sensitivity, multi-modal imaging and reduced scan-time in clinical and research examinations. MRI data acquisition is limited in spatial and temporal resolution due to the lack of signal-to-noise ratio (SNR). While the advancement of gradient coils in strength and slew-rate [1] ensured a speed up in image acquisition, the improvement of sensitivity with higher field strengths or wellcrafted detector geometries of MRI probes, have always been critical [3]. Back in 1980, Ackerman et al demonstrated that an improved SNR could be obtained by placing a small coil on the surface of the sample, close to the region of interest [4]. The use of small surface coils in the regime of sample dominated noise enables large sensitivity improvements, because it provides both, stronger magnetic coupling with the sample and noise reduction due to the smaller volume of tissue being visible for the coil [5]
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