Deep learning-based fully automatic segmentation of wrist cartilage in MR images

The study objective was to investigate the performance of a dedicated convolutional neural network (CNN) optimized for wrist cartilage segmentation from 2D MR images. CNN utilized a planar architecture and patch-based (PB) training approach that ensured optimal performance in the presence of a limited amount of training data. The CNN was trained and validated in twenty multi-slice MRI datasets acquired with two different coils in eleven subjects (healthy volunteers and patients). The validation included a comparison with the alternative state-of-the-art CNN methods for the segmentation of joints from MR images and the ground-truth manual segmentation. When trained on the limited training data, the CNN outperformed significantly image-based and patch-based U-Net networks. Our PB-CNN also demonstrated a good agreement with manual segmentation (Sorensen-Dice similarity coefficient (DSC) = 0.81) in the representative (central coronal) slices with large amount of cartilage tissue. Reduced performance of the network for slices with a very limited amount of cartilage tissue suggests the need for fully 3D convolutional networks to provide uniform performance across the joint. The study also assessed inter- and intra-observer variability of the manual wrist cartilage segmentation (DSC=0.78-0.88 and 0.9, respectively). The proposed deep-learning-based segmentation of the wrist cartilage from MRI could facilitate research of novel imaging markers of wrist osteoarthritis to characterize its progression and response to therapy

Tunable hybrid metasurfaces for image quality enhancement

Metasurfaces created a new paradigm in the research of artificial electromagnetic structures owing to their potential to overcome many challenges typically associated with bulk metamaterials. However, a majority of demonstrated metasurface structures possess fixed properties, e.g. fixed operational bandwidth or functionality. An active control of metasurface functionalities and bandwidth is highly desirable for engineering an advanced electromagnetic and photonic devices. Here, we suggest and demonstrate experimentally a novel type of metasurface capable of dramatic enhancing the image quality.This work was supported by Russian Science Foundation (Project No. 15-19-20054)

Tunable hybrid metasurfaces for MRI applications

One of many exciting application of metasurfaces is in the magnetic resonance imaging (MRI). Here we demonstrate theoretically and experimentally how to improve substantially the MRI sensitivity by employing the concept of hybrid metasurfaces. We design a novel hybrid metasurface as an array of nonmagnetic metallic wires with high-permittivity dielectric blocks at the edges. We demonstrate that tunability of this metasurface can be achieved via a change of the effective permittivity of blocks near the edges. Moreover, altering the coupling strength between the dielectric and metallic elements allows to obtain nearly homogeneous shapes of the near-field magnetic modes.This work was supported by Ministry of Education and Science of the Russian Federation (Zadanie No. 3.2465.2017/4.6) and by the Grant of the Government of the Russian Federation (No. 074-U01). AGW acknowledges support by European Research Council Advanced Grant 670629 NOMA MRI and NWO Topsubside

Locally Enhanced Image Quality with Tunable Hybrid Metasurfaces

Metasurfaces represent a new paradigm in artificial subwavelength structures due to their potential to overcome many challenges typically associated with bulk metamaterials. The ability to make very thin structures and change their properties dynamically makes metasurfaces an exceptional meta-optics platform for engineering advanced electromagnetic and photonic metadevices. Here, we suggest and demonstrate experimentally a tunable metasurface capable of enhancing significantly the local image quality in magnetic resonance imaging. We present a design of the hybrid metasurface based on electromagnetically coupled dielectric and metallic elements. We demonstrate how to tailor the spectral characteristics of the metasurface eigenmodes by changing dynamically the effective permittivity of the structure. By maximizing a coupling between metasurface eigenmodes and transmitted and received fields in the magnetic resonance imaging (MRI) system, we enhance the device sensitivity that results in a substantial improvement of the image quality.Experimental studies are supported by the Ministry of Education and Science of the Russian Federation (Zadanie No. 3.2465.2017/4.6). A. P. S. acknowledges support by the IEEE MTT-S and Photonics Graduate Fellowships. A. G. W. acknowledges support by European Research Council Advanced Grant No. 670629 NOMA MRI and NWO Topsubside

Metasurface-based wireless coils for magnetic resonance imaging

Magnetic resonance imaging (MRI) is the cornerstone diagnostics technique for medicine, biology, and neuroscience. This imaging method is innovative, noninvasive and its impact continues to grow. During the past several decades the quality of MRI scans has been substantially improved through the design of very sensitive multichannel receivers and compatible receive radio-frequency (RF) coil arrays for parallel imaging. However, conventional designs of coils have already reached their “saturation point” in terms of provided signal-to-noise ratio. In this contribution unique properties of metasurfaces are used in designing new surface and volumetric wireless coils improving imaging efficiency of high-field MR systems. We employ metasurfaces organized as arrays of parallel metal wires placed close to a scanned subject inside an MRI bore to produce a wireless coil, which is driven by an external body coil via inductive coupling. The wireless coil in this approach enhances both the transmit power efficiency and the receive sensitivity of the body coil with respect to the region of interest. By full-wave numerical simulations the two metasurface-based wireless coils were compared: the surface coil using a single array of wires and the volumetric coil using two separate arrays of wires.This work was supported by the Ministry of Education and Science of the Russian Federation (project No. 14.587.21.0041 with the unique identifier RFMEFI58717X0041)

Enhancement of magnetic resonance imaging with metasurfaces

\u3cp\u3eIt is revealed that the unique properties of ultrathin metasurface resonators can improve magnetic resonance imaging dramatically. A metasurface formed when an array of metallic wires is placed inside a scanner under the studied object and a substantial enhancement of the radio-frequency magnetic field is achieved by means of subwavelength manipulation with the metasurface, also allowing improved image resolution.\u3c/p\u3

Applications of dielectric pads, novel materials and resonators in 1.5T and 3T MRI

\u3cp\u3eIn order to boost the performance of magnetic resonance imaging without increasing the static magnetic field, it is necessary to increase its intrinsic sensitivity. This allows a reduction in the scanning time, increased spatial resolution, and can enable low-field strength systems (which are much cheaper and can be used to scan patients with metallic implants) to have a higher signal-to-noise ratio (SNR) so that they are comparable to more expensive higher field strength systems. In this contribution, we demonstrate radiofrequency field enhancing and shaping devices based on novel materials, such as high permittivity dielectric structures and metamaterials. These materials can substantially enhance SNR, thus potentially increasing image resolution or allowing faster examinations.\u3c/p\u3

Element decoupling of 7T dipole body arrays by EBG metasurface structures : Experimental verification

Metasurfaces are artificial electromagnetic boundaries or interfaces usually implemented as two-dimensional periodic structures with subwavelength periodicity and engineered properties of constituent unit cells. The electromagnetic bandgap (EBG) effect in metasurfaces prevents all surface modes from propagating in a certain frequency band. While metasurfaces provide a number of important applications in microwave antennas and antenna arrays, their features are also highly suitable for MRI applications. In this work we perform a proof-of-principle experiment to study finite structures based on mushroom-type EBG metasurfaces and employ them for suppression of inter-element coupling in dipole transceive array coils for body imaging at 7T. We firstly show experimentally that employment of mushroom structures leads to reduction of coupling between adjacent closely-spaced dipole antenna elements of a 7T transceive body array, which reduces scattering losses in neighboring channels. The studied setup consists of two active fractionated dipole antennas previously designed by the authors for body imaging at 7T. These are placed on top of a body-mimicking phantom and equipped with the manufactured finite-size periodic structure tuned to have EBG properties at the Larmor frequency of 298MHz. To improve the detection range of the B1+ field distribution of the top elements, four additional elements were positioned along the bottom side of the phantom. Bench measurements of a scattering matrix showed that coupling between the two top elements can be considerably reduced depending on the distance to the EBG structure. On the other hand, the measurements performed on a 7T MRI machine indicated redistribution of the B1+ field due to interaction between the dipoles with the structure. When the structure is located just over two closely spaced dipoles, one can reach a very high isolation improvement of -14dB accompanied by a strong field redistribution. In contrast, when put at a certain height over the antennas the structure provides a moderate isolation improvement together with a slight increase of B1+ level. This study provides a tool for the decoupling of dipole antennas in ultrahigh field transceive arrays, possibly resulting in denser element placement and/or larger subject-element spacing