Modular Coils with Low Hydrogen Content Especially for MRI of Dry Solids

Abstract

IntroductionRecent advances have enabled fast magnetic resonance imaging (MRI) of solid materials. This development has opened up new applications for MRI, but, at the same time, uncovered new challenges. Previously, MRI-invisible materials like the housing of MRI detection coils are now readily depicted and either cause artifacts or lead to a decreased image resolution. In this contribution, we present versatile, multi-nuclear single and dual-tune MRI coils that stand out by (1) a low hydrogen content for high-resolution MRI of dry solids without artifacts; (2) a modular approach with exchangeable inductors of variable volumes to optimally enclose the given object; (3) low cost and low manufacturing effort that is associated with the modular approach; (4) accurate sample placement in the coil outside of the bore, and (5) a wide, single- or dual-tune frequency range that covers several nuclei and enables multinuclear MRI without moving the sample.Materials and MethodsThe inductors of the coils were constructed from self-supporting copper sheets to avoid all plastic materials within or around the resonator. The components that were mounted at a distance from the inductor, including the circuit board, coaxial cable and holder were manufactured from polytetrafluoroethylene.Results and ConclusionResidual hydrogen signal was sufficiently well suppressed to allow 1H-MRI of dry solids with a minimum field of view that was smaller than the sensitive volume of the coil. The SNR was found to be comparable but somewhat lower with respect to commercial, proton-rich quadrature coils, and higher with respect to a linearly-polarized commercial coil. The potential of the setup presented was exemplified by 1H / 23Na high-resolution zero echo time (ZTE) MRI of a model solution and a dried human molar at 9.4 T. A full 3D image dataset of the tooth was obtained, rich in contrast and similar to the resolution of standard cone-beam computed tomography.

Imaging of root canal treatment using ultra high field 9.4T UTE-MRI – a preliminary study

“…22,23 First approaches towards practicable intraoral MR imaging using special teeth coils are available -hence, there is potential for in vivo application of UTE-MRI after coil and sequence optimization. [24][25][26][27][28] While air-tissue interface artifacts are avoided by using UTE-MRI, signal artifacts from surrounding water-rich tissue remain an issue and could be minimized by using spiral sequences or the pointwise encoding time reduction with radial acquisition (PETRA) sequence. 29,30 Other approaches described to this issue include the use of hydrogen-poor materials for the construction of coil, bed and support.…”

“…29,30 Other approaches described to this issue include the use of hydrogen-poor materials for the construction of coil, bed and support. 24 In addition, other promising approaches are available to obtain additional information by MRI examination of dense biological materials about discriminating bound from pore water based on their relaxation properties. 31 The extent to which this possible approach for human bone can be transferred to human teeth should be clarified by further studies.…”

A virtually 1H‐free birdcage coil for zero echo time MRI without background signal

“…Therefore, sequences have been developed to capture short-lived signals, including single-point imaging (2) and the particularly efficient techniques employing three-dimensional (3D) radial center-out encoding with ultrashort (3,4) or even zero echo time (TE) (5)(6)(7)(8)(9). However, on the downside of increased short-T2 sensitivity, with these sequences also signals from solid hardware parts are detected, particularly from the radiofrequency (RF) coils (10)(11)(12).…”

“…Hence, ideally, the undesired signal is avoided in the first place at the level of coil design and construction. So far, this challenge has been approached with loop‐gap resonators where the self‐supporting conductor facilitates avoiding additional materials. However, such coils are often not suitable for human in vivo MRI due to their orientation with respect to the main field and missing electrical isolation of the conductor toward the sample.…”

Dental MRI using wireless intraoral coils

“…Only recently, however, MRI sequences with very short echo time (TE) were reported that enabled fast soft- and hard-tissue imaging of hydrogen and other nuclei like sodium 1 2 3 4 5 6 7 8 . This development stimulated a rising interest in dental MRI 9 10 11 12 13 14 15 , and detailed images with soft- and hard-tissue contrast of extracted human teeth were already obtained ex vivo: Using a small-bore MRI system at high field in conjunction with ultra-short-TE (UTE), zero-TE (ZTE), sweep imaging with Fourier transformation (SWIFT) or single-point imaging (SPI), a voxel size of the order of 100 μm 3 was achieved.…”