Synopsis

The high SNR provided by whole body 7T also provides enticing opportunities for ex-vivo imaging. However, such MRI systems are usually only equipped with a limited number of coils, rarely optimized for ex-vivo imaging. In this work, we leverage the new-found degrees of freedom provided by High Impedance Coil elements to create a versatile modular array-coil that can be re-configured in seconds to fit each sample optimally.

Introduction

Methods

Eight HIC elements resonant at 297MHz were built (Fig. 1). The coaxial structure (∅=3.5cm) was made out of micro-coax. Two pin diodes were used to detune each element during transmission. The interface board, including the matching circuit, was shielded using a 3D-printed housing covered with copper tape. The cable length was adjusted to achieve a “reversed” pre-amplifier decoupling condition⁵. Each element was stitched to a soft Velcro disk such that they can easily be re-arranged on a Velcro sheet (Fig 2) Five different HIC arrangements were tested on two different phantoms (Fig. 2); the larger of the two approximates a container used to hold a macaque brain (∅=6.5cm, length=14cm, σ=0.5S/m), the smaller one representative of a container for excised tissue samples such as a human spine segment (∅=2.5cm, length=12cm, σ=0.5S/m).

For transmission an 8-rung de-tunable birdcage resonant at 297 MHz was build (∅=15cm, length=15cm). To detune the transmit-coil during receive, PIN diodes were placed on the rungs. The two channels of the birdcage were driven in quadrature, each equipped with a transmit-receive switch and pre-amplifier interface.

Signal to Noise Ratio (SNR) measurements were performed using the Kellmann method⁶ and B₁⁺ normalized based on the gradient echo signal equation⁷. Two spoiled gradient echo images were acquired, one with RF on and one without (noise measurement). Transmit sensitivity maps were obtained using a Turbo FLASH based B₁⁺ mapping sequence8.

The coils elements were built at Kyoto University and tested in the 7T (Siemens MAGNETOM 7T, Erlangen, Germany) located at New York University.

Results & Discussion

The Velcro backing of the coil elements enables easy and quick re-arrangement of the array for use with samples of different size. Despite the use of non-critical overlap⁹, the noise correlation matrix remains relatively clean for all considered HIC arrangements (Fig. 3). However, some residual coupling is observed between elements 1 and 2 (7 and 8) in the “4x2L” (“8x1L”) arrangement. These may be attributed to a possible imperfection in the interface. In particular, we noticed that it is crucial to shield the inductors on the matching circuits as they may start to couple when placed in close proximity (on the small phantom the inductors are more orthogonal to each other).

In the center of the large phantom, the HIC array and birdcage provide a similar SNR. At the periphery, however, the SNR is considerably higher, as may be expected from a surface array⁹. In the center for the small phantom, a closefitting arrangement of 4 (or more) HIC elements can provide more than 10 times the SNR observed with the birdcage (Fig. 4). This boost in SNR can first and foremost be attributed to the close proximity of the elements, which, in this case, also happen to have a near optimal diameter to maximize the SNR in the center. It should therefore be noted, that a customized solenoid coil, for example, may be expected to provide a more comparable SNR^4,10. However, such a rigid coil cannot easily adapt to samples¹⁰.

The dual rows configurations expand the field of view at the expense of azimuthal coverage. Due to the relatively small size of the HIC elements at 7T, additional elements may be needed to coverage large samples. Nevertheless, these initial results already show great promise in terms of SNR and versatility (Fig. 5).

Conclusion

Acknowledgements

References

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