4577
A High Performance Gradient and RF Insert for Dental MRI1Division of Medical Physics, Department of Radiology, University Medical Center Freiburg, Germany, Freiburg, Germany, 2Division of Medical Physics, Department of Radiology, University Medical Center Freiburg, Freiburg, Germany, Freiburg, Germany
Synopsis
Keywords: Gradients, New Devices, Dental MRI
Dental MRI requires high resolution imaging, e.g., for detection of root canals, as well as, high readout bandwidth for short T2* tissues, such as dentin and nerve fibers. These requirements can only be satisfied by high-performance gradients. In this study, we show with Bloch simulations the necessity for high performance gradients, and consequently introduce a gradient and RF insert, which is optimized for dental MRI. We also explore the concept a cylindrical encoding for curved slices for 4-fold acceleration compared to conventional encoding.Introduction
The value of the dental magnetic resonance imaging (dMRI) is evident in diagnosis of various pathologies in endodontics (1–3), orthodontics (4), craniomaxillofacial surgery (5), and implantology (6). dMRI requires high resolution imaging, e.g., for detection of root canals and, as well as, high readout bandwidth for short T2* tissues. To achieve this high resolution within clinically acceptable measurement times, high-performance gradients are essential (7). Head-only gradient systems offer higher efficiency and improves acquisition speed, resolution, and diffusion weighting while reducing distortions, e.g., in EPI, and increasing peripheral nerve stimulation (PNS) thresholds compared to whole-body systems (8–11). Use of nonlinear spatial encoding (12,13) for local resolution enhancement (14–16), to accelerate imaging speed with enhanced undersampling schemes (17), and for imaging multiple regions simultaneously (18,19) was also demonstrated with local gradients, including gradient coil arrays (20–22). Furthermore, it has been shown, that local gradient coils developed for a target anatomy can achieve significantly higher sensitivity(26).Methods
The imaging volume of the gradient and RF inset is defined for the upper and lower dental arch including the apices of the roots and the surrounding trabecular bone(Fig.1A) of a patient supine position. The gradient and RF insert are meant to be pushed into the scanner bore from the backside over the patient's head.The gradient coils were designed using the open-source, MATLAB-based software package COILGEN (30) using a stream function optimization method (32). For the current-carrying-surface (CCS), a half elliptical shape and a mask-like surface are proposed (Fig.1C-D). The central curve that defines the target field is taken from [27].
A standard linear x,y,z gradient field, and a curved (Phi,R,z) encoding field which follows the dental arch are defined (Fig.1,F). The idea of the later approach is to speed up the data acquisition by using curved slice profiles (28) and therefore reducing the number of slices (Fig.1F).
A Tx-array of five galvanically independent low-profile loop elements was designed similar to (29), to make efficient use of the space (Fig.3A).
For high signal-to-noise ratio (SNR) efficiency, an intraoral coil (IOC) was also included in the model as receiver (Rx) element [24,29]. Both Tx-array and the IOC were tuned for fLarmor corresponding to 0.55T, 1.5T, and 3T fields, respectively. B1+ and B1- distribution of the Tx-array and the IOC were calculated using an FDTD solver (Sim4Life-v7.0, ZMT, Zurich).
To highlight the effect of improved gradient strength, MATLAB based Bloch simulations for a numerical phantom were performed, comparing commercial MRI gradient strengths (12-80 mT/m) and the targeted strength of the proposed gradient insert of 180mT/m(See Fig.3). Metal artefact simulation was also performed for the 3T case
Results
Gradient coil layouts were generated for the elliptic and mask-like surface, both for linear and cylindrical target fields (Fig.2). Obtained sensitivities are 1mT/m/A and 0.1 to 0.3mT/m/A for linear and cylindrical encoding fields, respectively. The linear gradients outperform the cylindrical fields with lower mean relative errors of 1-2% compared to 10-15% (Fig.2C,D).The mask-like surface has a significantly better performance in terms of sensitivity (~25% improvement) and field accuracy (around 1%, See Fig.2, C, D).
In Fig.3, ||B1+|| fields are shown for a transverse slice and along a curved trajectory matching the target field used to design the gradient coils. Maximum deviation from the mean ||B1+|| was 7.1%, 13.2%, and 17.1% within the target region, for 23.7, 63.9, and 127.7MHz, respectively. ||B1-|| simulations of the IOC show that the transverse loop design is sensitive to the target region and the sensitivity profile is constrained within the target region.
Simulation results (Fig.4) show that imaging performance can be improved at field strengths of 0.55, 1.5 and 3T using high power gradients. Metal artefacts are reduced using high bandwidth offered by the gradient insert.
Discussion
The sensitivities for linear encoding for both dedicated surfaces, outperform a standard whole body system by more than an order of magnitude (Fig.2C).Unfortunately, the coil layouts for the cylindrical target fields yield lower sensitivity (0.2-0.3mT/m/A) and higher field error (>10%, Fig.2,D). This is most reasonably due to the higher power demands in generating curved gradient fields.
Despite the better performance of the mask-like surface, the semi-elliptical approach remains easier to manufacture.
We show that suitable Tx and Rx coils can be designed for the proposed gradient insert for optimal performance in dental MRI applications. A low profile Tx array makes efficient use of the available space. Since the array performance was not sensitive to up to 20% phase and amplitude deviations, it can be driven by a fixed power distribution circuit instead of parallel Tx unit. We have shown previously that intraoral coils provide up to 10-fold higher SNR than external coils. Constrained sensitivity of the IOC is also useful to avoid signal acquisition from unwanted regions. Therefore, the gradient and the RF coil system proposed in this work forms an optimal combination for dental MRI. Tx coils will be simulated together with the gradient coils for more accurate field distributions. SAR and peripheral nerve stimulation (PNS) simulations will also be performed using anatomical human models using the complete system model.
Acknowledgements
No acknowledgement found.References
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