Introduction

Recently, substantial effort has been made to bring fMRI to the pediatric population and it has already given a great insight into the developmental changes that take place after birth across multiple fields of inquiry [1-3]. Applications of fMRI with infants and toddlers became a fast-paced expanding field of research. However, the use of MRI during infancy and toddlerhood remains a challenging undertaking due to practical, methodological, and analytical issues that arise when imaging this young and uncooperative population. The rapid changes in head sizes are a major issue for obtaining optimum signal reception in infant MR brain imaging applications during early life, where a close-fitting nature of the coil helmet design is critical to gaining maximum sensitivity from high-density coil arrays. Therefore, the concept of an adjustable coil array is explored for awake infant brain imaging.

Methods

A 32-channel pediatric head coil was developed to accommodate head sizes for infants of 1 to 18 months. We implemented the coil helmet design with three anatomical-shaped independent coil segments (Fig. 1). The posterior part was integrated into the coil base, so that the infant can be easily laid down without any other restrictive coil parts. Instead of lying down flat on the patient’s table, we designed the coil base as an inclined cradle seat to improve the subject’s overall MRI scanning compliance (Fig. 2). A critical design component was the implementation of dedicated earmuff compartments in both right and left anterior coil housing sections. For further head immobilization, we placed inflatable cushions to the outer side of the anterior coil parts.
We subdivided the total channel count into the posterior part with 12 elements, while the two anterior head parts houses 10 elements each. 30 loop elements were measured to be approximately 65 mm. Two larger loop elements which surround the earmuff compartments comprise elliptical loop diameters of $$$d_1 = 85$$$ mm and $$$d_2 = 91$$$ mm. Bench measurements verified the element’s $$$Q_U / Q_L$$$-ratio, tuning, active detuning, nearest-neighbor coupling and preamplifier decoupling [6] for each coil element. Pairs of coils were attached to a twin preamplifier.
Initial imaging was carried out on a 3T Scanner (Prisma, Siemens Healthineers, Erlangen, Germany). SNR [7], noise correlation, and G-factor calculations [8,9] were calculated from PD weighted raw-data images and compared to a commercially available 32ch head coil. Initial fMRI scans were conducted on 19 awake infants (3-9 months old) while watching videos of faces, bodies, objects, and scenes. Critically, infants had adequate hearing protection and listened to lullabies on infant-specific, MR-compatible headphones. EPI data were collected with 44 near-axial slices (TR/TE/α=3s/30ms/90°, FOV=160mm, matrix=80x80, SL=2mm).

Results

The coil elements showed a $$$Q_U / Q_L$$$-ratio of 4.3 (ear loops $$$ Q_U / Q_L = 8.6 $$$). The average geometrical decoupling of nearest neighbors was measured to be -(15 ± 3) dB. The coupling between non-adjacent pairs ranged from -11 dB to -26 dB. All decoupling values were further reduced by (17 ± 2) dB via preamplifier decoupling. Active detuning provided >42 dB isolation. The noise correlation between the channels ranged from 0.2% to 38.5%, when the coil housing is adjusted for the smallest head size. The SNR gain (Fig. 3) of the constructed 32ch infant coil in the cortical region was measured to be 3-fold, when compared to the 32ch adult head coil. A 25% SNR improvement was still observed in the center. Figure 4 shows the inverse G-factor for Simultaneous MultiSlice techniques. The SMS G-factor maps show substantial improvements using the infant array coil when compared to the adult head coil. Figure 5 shows data from a single awake infant (6 months old) with 13.5 minutes of low-motion data on two canonical contrasts: faces>scenes and faces>objects.

Discussion

Several design considerations have to be addressed for an array coil when imaging this young population. The three-segment design provides a compromise between a high-degree of geometrical adaptability, and it greatly simplifies the handling of the coil setup on the scanner’s patient table. Maintaining a tightly fit of the 32-channels around the subject’s head is critical in gaining SNR. However, it potentially limits the usage of proper ear protection. Therefore, we implemented dedicated compartments in the lateral housing parts. The coil was designed as an inclined seated cradle. According to our infant MRI scan experiences obtained from initial studies [10], the infant favors the seated position over the regular supine position, which is typically used during adult MRI brain examinations.

Conclusion

By optimizing the shape and functionality of an infant brain array coil to allow head size adaptability, high-level sound protection, and head motion restriction, we improved infant MRI brain imaging in terms of sensitivity, spatial resolution, and accelerated encoding capabilities. We could demonstrate SNR gains, fast image encoding power, and improved scan completion rates by capitalizing on technical advances for both coil array technology and mechanical features, especially tailored for infant population. The coil is well-suited for awake infant neuro MRI studies.

Acknowledgements

MINT Fondation, MIT

References

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