0209

An Improved Intraoral Transverse Loop Coil Design for High Resolution Dental MRI

Ali Caglar Özen¹, Serhat Ilbey¹, Feng Jia¹, and Michael Bock¹
¹Division of Medical Physics, Department of Radiology, University Medical Center Freiburg, Freiburg, Germany

Synopsis

Keywords: Non-Array RF Coils, Antennas & Waveguides, New Devices, dental MRI

MRI can simultaneously image soft and hard tissues such as glands, teeth, gum, nerves and bone, thus could be valuable for diagnosis of dental pathologies and implant planning. Intraoral coils have been proved essential to dental MRI due to superior sensitivity compared to external coils. In this study, we introduce a modified transverse loop coil design, which provides improved sensitivity, homogeneity, comfort and safety. We also introduce a bio-compatible, artefact-free and MR-silent coating for intraoral coils. Phantom and in vivo dental MR images demonstrate the advantages of the new intraoral coil design.

Introduction

The value of the dental MRI (dMRI) is evident in diagnosis of various pathologies in endodontics [1]–[3], orthodontics [4], craniomaxillofacial surgery [5], and implantology [6]. In dMRI, submillimeter structures must be imaged such as the root canals. To achieve this high resolution in clinically acceptable measurement times, the imaging FOV must be restricted to the target volume. This can be realized by local RF excitation, B₁⁺, or limited receive sensitivity B₁^- which is tailored to the anatomy, or a combination of both. So far mainly extraoral surface coils and coil arrays have been used [7]–[9]. For an average-sized patient, the distance between an extraoral coil element and a molar tooth is about 30–50 mm, which reduces the sensitivity significantly. Therefore, intraoral coils are preferred [10]–[13].
A transmit/receive (Tx/Rx) intraoral loop coil with the coil plane orthogonal to 𝐵₀ was proposed [10], where the transverse 𝐵₁ fields are sensitive to MR signal. The transverse loop coil (tLoop) can image the whole dental arch including both the upper and the lower teeth. It’s sensitive volume in the occlusal position includes the teeth and supporting structures, and excludes a large portion of the cheeks and the tongue. However, there are several limitations of the tLoop design which we aim to overcome in a modified coil setup (mtLoop) (Fig.1D).
In this work, tLoop and mtLoop designs are compared in terms of sensitivity distributions, image SNR, and eddy currents using electromagnetic (EM) simulations and MRI measurements. Insulation thickness is optimized using EM simulations. Finally, an artefact-free, MR-silent coating was introduced. In vivo dMRI with 250-µm-isotropic resolution was demonstrated for T2-SPACE and UTE sequences.

Methods

The design of a tLoop and a mtLoop coil are shown in Fig.1. Compared to the tLoop, the mtLoop uses a longer bent coil conductor with a higher inductance (L=50nH and 63nH). Capacitive tuning and matching was applied, and coil conductors were laser-cut from 100µm-thick copper plates. At the mtLoop, a 100µm-thick dielectric tape was used to form a parallel-plate-capacitor at the posterior section.
FDTD simulations were performed (Sim4Life v7.0, ZMT, Zurich, Switzerland) to compare the sensitivity along the coil conductor, and the results were compared to measurements. Different thicknesses of the insulating layer (d = 0.2, 0.5, 1.0, 2.0mm) were evaluated. Eddy currents were computed to compare tLoop and mtLoop using a FEM solver (Comsol, The COMSOL AB, Stockholm). MRI measurements were performed at 3T (Prisma-Fit, Siemens). To study the different sensitivity profiles, both tLoop and mtLoop were tested in Rx-only mode. The tLoop was also assessed in Tx/Rx mode [10]. A phantom was 3D-printed from an optical intraoral scan using the insulation material of the intraoral coils; thus, the coils could be tested without additional insulation.
As ultrashort-TE sequences are used in dMRI due to the ultra-short T₂* of dentin [12]–[16], the MR-silence as well as image artefacts were tested of the potential insulation materials such as dental resin, bonding fluids, gutta-percha, 3D-printing materials, impression putties and dental cement using a modified ZTE/PETRA sequence [17], [18] (TE=20ms). After coating, coil safety was evaluated [19], and in vivo images were acquired of the whole dental arch at 0.25-mm-isotropic resolution.

Results

In Fig. 2 simulation results show the mtLoop improvement for overlapped feed-port and forming a parallel-plate capacitor at the posterior section. A 3.7-fold sensitivity increase along the incisors, and a 76% signal reduction at the back of the tongue is seen. An insulation-thickness of 1mm was optimal for the given coating material properties at 128MHz (Fig.3).
A UV-cured 3D-printing resin mixed with resin powder was MR-silent and did not show image artefacts, and was therefore used to coat the intraoral coils (Fig. 1C). Dental cement was also usable, but had a high conductivity of 0.014S/m (measured with DAKS12, ZMT).
Transverse and coronal phantom images showed a higher SNR in at the incisors (SNR=6721) for the Rx-mtLoop compared to the Rx-tLoop (602) and Tx/Rx-tLoop (72) (Fig. 4), and the SNR ratio of the incisors over the 3^rd molars are 1.04, 0.48, and 1.04, respectively. On the contrary, SNR ratio between the tongue and 3^rd molars are 0.56, 0.60, 0.84 for mtLoop and Rx-only and Tx/Rx tLoops, respectively.
Maximum temperature rise of 0.4°C at the hot spots was measured with a high-SAR protocol. In Fig. 5, in vivo panoramic images were reconstructed from T2-SPACE (300µm–in-plane-resolution, 1mm-slice thickness), and UTE (250µm-isotropic) sequences. In the transverse slices of mandible, a missing tooth and a metallic filling is observed. The artefact around the metallic filling is larger for T2-SPACE than UTE. In T2-space images, root canals are better visualized than UTE.

Discussion

With mtLoop, we introduced an intraoral transverse loop coil design which has an improved performance over the previously proposed tLoop. As intraoral coils are indispensable components of dMRI, the modifications might help to overcome sensitivity problems, anatomical constraints, patient comfort limitations and loading factor issues.

Acknowledgements

The first author gratefully acknowledges the fruitful discussions on the mechanical and material properties of intraoral coils with Djaudat Idiyatullin and Michael Garwood of Center for Magnetic Resonance Research and Beth Groenke, Laurence Gaalaas and Donald Nixdorf of School of Dentistry at the University of Minnesota, Minneapolis, MN. We also thank Frank Pfefferkorn of Dentsply DeTrey GmbH, Konstanz, Germany for sending dental material samples.

References

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Figures

Fig.1: 3D drawings (A) and circuit schematics (B) of tLoop and mtLoop designs. Modifications are numbered in A) according to D) as such: 1. Rx-only mode using a conventional tune/match and active detuning interface; 2. Adjacent conductors at the feed port are overlapped to improve the signal along the incisor teeth; 3. Bending the posterior part of the coil conductor releases the tongue; 5. Coil conductor forms a capacitor at the posterior section. C) A photo of the mtLoop. D) a summary of the advantages of the mtLoop in comparison to tLoop.

Fig.2: tLoop vs mtLoop simulations. 1.row: Overlapping improves the sensitivity along the incisors by 3.7-fold compared to adjacent conductors at the feed port. 2.row: Posterior section of the mtLoop is a parallel-plate capacitor, which reduces signal from the tongue 76%. The sensitivity distribution in the transverse plane selected 10mm above the coil conductor resembles the dental arch. 3.row: Eddy current simulations for Gy-axis show reduced surface current density for mtLoop. The direction of the current indicates, a mechanical force along y direction might be experienced.

Fig.3: Insulation thickness optimization. Intraoral coils need to be properly isolated to prevent direct tissue contact. A thicker insulation layer increases distance between the tissue and the coil, reducing efficiency. Thinner insulation cause increased parasitic impedance due to lossy tissue. For the EM properties of the MR-silent insulation material developed in this study, 1mm-thickness yielded higher sensitivity. The line plots represent the sensitivity profile along the black line along z direction. Reduced losses in the insulation improves coil performance.

Figure 4: Phantom results to demonstrate the improvements in the sensitivity profile. A) The intraoral phantom made of a 3D printed hollow mandible and maxilla pair filled with gel (10g/L Agar Agar, 1g/L CuSO4). B) transverse (1.row) and coronal (2.row) slices of mtLoop, tLoop and tLoop in Tx/Rx mode acquired using a GRE sequence with (TR/TE=20/4ms, FOV=192x192mm2, Voxel size=1mm3, a=10°). C) Line profile comparison for the transverse and coronal slices shown in (B). mtLoop shows improved signal along the incisor teeth, a better sensitivity along the transverse line across the roots.

Figure 5: In vivo images acquired with a water-excited UTE implemented using binomial pulses (left) and fat-saturated T2-SPACE (right). The panoramic reconstructions were generated along a vertical axis, and therefore doesn’t cover the whole teeth in the same slice plane. T2-space provides better visualization of root canals than UTE. However, UTE has reduced metal artefacts.

Proc. Intl. Soc. Mag. Reson. Med. 31 (2023)

0209

DOI: https://doi.org/10.58530/2023/0209