Introduction

Magnetic resonance imaging (MRI) of the developing brain and heart could greatly benefit from the higher signal levels and improved tissue contrast at ^1-4. However, while adult safe positioning is ensured by the mechanical structure of the MR scanner, and first studies of neonatal brain have been published⁵, there are currently no dedicated RF coils and associated patient handling for young infants (<3-months). In this study, we describe design criteria and solutions to build the first mechanical frame and coil formers for imaging the brain and heart in infants up to 3-months of age in an integrated manner at 7T.

Methods and Results

The main design criteria for the setup were established after consultation with a multidisciplinary team experienced in the field of neonatal MRI.
1.Infants up to 3-months of age must fit – with size ranges to be determined from biometric data.
2.The infant should be able to be prepared away from the 7T system with all receiver coils (Rx-arrays) in-place, and then transported to the magnet with minimal further disturbance.
3.The infant needs to be accessible and covering structures should be fast to remove in emergencies.
4.The design of the patient handling and the Rx-arrays geometry must be such that Rx-arrays can be positioned in close proximity to the organs-of-interest, despite a wide range of infants body habitus.
5.The structures should allow for a 64-channel Rx-array to fit with the associated circuitry.
6.The full setup must fit into a dedicated transmit coil array since there are no integrated body coils at 7T.

The biometric data for infants sizes^6-9 determined the bed design (length=780mm,weight=2.5kg) to place the infant with its head ready to go inside a head Rx-array (Fig.1B). Helmet’s size (Fig.2B-C) included additional space to allow placement of ear protection and immobilization devices for the head. The apertures at the apex of the helmet were added for optimal head positioning⁹. The U-shape of the bed surface (Fig.2B) prevents lateral movement of the infant, while ensuring close proximity between Rx-arrays and the organs-of-interest. The general thickness of the bed (10mm) and the side-ears (Fig.2B,arrows #1&#2) ensure high robustness against flexion, while regions near the brain and posterior Rx-array (pRx) kept a thickness of 4-5mm. Such precautions are essential as the bed structure is carried with one hand on a dedicated handle and the other under the helmet (Fig.2D,blue-arrows). Therefore, the centre of gravity of the infant is only supported by the bed structure. Small Teflon pads (20x15mm2) ensure the sliding of the neonatal bed over the pRx-frame and alignment (Fig.2D,red-arrows).

The pRx-frame supports the bed. The large hollow area (Fig.1C) enables safe and quick positioning of the bed. Supported by the Teflon rails of the main support structure (Fig.1E,blue-pieces), it can slides with minimal friction inside the Tx-array holder between the brain/heart-isocentred positions without direct interaction with the infant placed on the bed. The final position is held using star knobs easy to release.

The anterior Rx-array (aRx) frame is independent of the rest of the structure, and can easily be placed or quickly removed. The designed dimensions provide a full coverage of the infant chest, while keeping an open-design to reduce parental anxiety and give radiographers a view of the infant once positioned. The aRx-frame can slide over 67mm along the longitudinal axis to enable flexible alignment with the heart position of different infants. The min/max distance between the helmet and aRx-frame was 40/107mm so the mouth and nose regions are not obstructed (Fig.3C,red-arrow). It can also move along the vertical axis so that the distance between the aRx-frame and the bed can vary from 130mm to 160mm. The helmet cover shown in Fig.3B-C supports the frame together with the pRx-frame (Fig.3B,red-arrows).

All the setup is fitting inside the Tx-array holder for the volume dipole-antenna array (Fig. 1A)¹⁰, opened at both-ends for free movement of the neonatal frame. The bed dimensions and available space inside the MR scanner bore (600mm-diameter) determined the inner/outer diameters (286/406mm) of the coil holder that was separated in two halves to provide an open view when placing infants.

The main support structure (Fig.1E) sits on the surface of the standard MR scanner bed. Through the sliding rails, it transfers the pressure of neonatal structures to the scanner ground for vertical stability. Four handles are placed along the part for transport.

All pieces were 3D-printed in polycarbonate (Deed3D,China), which guarantee MR transparency and mechanical robustness. When feasible, they were printed as one-piece to prevent the use of glue that could alter sturdiness (max.size:914x610x914mm). All geometrical features (angles,chamfers,screw-clearances,filets) were designed so that the outer surfaces could be easily sanitized. Any exposed surfaces were also painted with a lacquer meeting the international standards in terms of lack of toxicity.

Discussion and Conclusion

In this work, we describe the first mechanical frame for infants for brain and cardiac imaging at 7T. The different parts incorporate important design aspects including mechanical sturdiness/safety, toxicity, sanitization and ease of use. The Rx-array holders were designed to maximize signal in the brain and cardiac regions of infants up to 3-months of age and to fit a 64-channel Rx-array.