•Whole-cerebrum distortion-free 3D pCASL imaging at 7T using TFL readout.•New set of labeling parameters, OPTIM background suppression pulse, and accelerated 3D TFL readout.•Improved test-retest ...repeatability and reduced susceptibility artifacts compared to 3D GRASE pCASL.•Increased SNR compared to the same sequence at 3T and 2D simultaneous multislice (SMS) TFL pCASL at 7T.
Fulfilling potentials of ultrahigh field for pseudo-Continuous Arterial Spin Labeling (pCASL) has been hampered by B1/B0 inhomogeneities that affect pCASL labeling, background suppression (BS), and the readout sequence. This study aimed to present a whole-cerebrum distortion-free three-dimensional (3D) pCASL sequence at 7T by optimizing pCASL labeling parameters, BS pulses, and an accelerated Turbo-FLASH (TFL) readout. A new set of pCASL labeling parameters (Gave = 0.4 mT/m, Gratio = 14.67) was proposed to avoid interferences in bottom slices while achieving robust labeling efficiency (LE). An OPTIM BS pulse was designed based on the range of B1/B0 inhomogeneities at 7T. A 3D TFL readout with 2D-CAIPIRINHA undersampling (R = 2 × 2) and centric ordering was developed, and the number of segments (Nseg) and flip angle (FA) were varied in simulation to achieve the optimal trade-off between SNR and spatial blurring. In-vivo experiments were performed on 19 subjects. The results showed that the new set of labeling parameters effectively achieved whole-cerebrum coverage by eliminating interferences in bottom slices while maintaining a high LE. The OPTIM BS pulse achieved 33.3% higher perfusion signal in gray matter (GM) than the original BS pulse with a cost of 4.8-fold SAR. Incorporating a moderate FA (8°) and Nseg (2), whole-cerebrum 3D TFL-pCASL imaging was achieved with a 2 × 2 × 4 mm3 resolution without distortion and susceptibility artifacts compared to 3D GRASE-pCASL. In addition, 3D TFL-pCASL showed a good to excellent test-retest repeatability and potential of higher resolution (2 mm isotropic). The proposed technique also significantly improved SNR when compared to the same sequence at 3T and simultaneous multislice TFL-pCASL at 7T. By combining a new set of labeling parameters, OPTIM BS pulse, and accelerated 3D TFL readout, we achieved high resolution pCASL at 7T with whole-cerebrum coverage, detailed perfusion and anatomical information without distortion, and sufficient SNR.
The purpose of this study is to investigate feasibility of estimating the specific absorption rate (SAR) in MRI in real time. To this goal, SAR maps are predicted from 3T- and 7T-simulated magnetic ...resonance (MR) images in 10 realistic human body models via a convolutional neural network. Two-dimensional (2-D) U-Net architectures with varying contraction layers and different convolutional filters were designed to estimate the SAR distribution in realistic body models. Sim4Life (ZMT, Switzerland) was used to create simulated anatomical images and SAR maps at 3T and 7T imaging frequencies for Duke, Ella, Charlie, and Pregnant Women (at 3, 7, and 9 month gestational stages) body models. Mean squared error (MSE) was used as the cost function and the structural similarity index (SSIM) was reported. A 2-D U-Net with 4 contracting (and 4 expanding) layers and 64 convolutional filters at the initial stage showed the best compromise to estimate SAR distributions. Adam optimizer outperformed stochastic gradient descent (SGD) for all cases with an average SSIM of <inline-formula> <tex-math notation="LaTeX">90.5 \mp 3.6 </tex-math></inline-formula> % and an average MSE of <inline-formula> <tex-math notation="LaTeX">0.7 \mp 0.6 </tex-math></inline-formula>% for head images at 7T, and an SSIM of ><inline-formula> <tex-math notation="LaTeX">85.1 \mp 6.2 </tex-math></inline-formula> % and an MSE of <inline-formula> <tex-math notation="LaTeX">0.4 \mp 0.4 </tex-math></inline-formula>% for 3T body imaging. Algorithms estimated the SAR maps for <inline-formula> <tex-math notation="LaTeX">224\times 224 </tex-math></inline-formula> slices under 30 ms. The proposed methodology shows promise to predict real-time SAR in clinical imaging settings without using extra mapping techniques or patient-specific calibrations.
This paper reports a wireless passive resonator architecture that is used as a fiducial electronic marker (e-marker) intended for internal marking purposes in magnetic resonance imaging (MRI). As a ...proof-of-concept demonstration, a class of double-layer, sub-cm helical resonators were microfabricated and tuned to the operating frequency of 123 MHz for a three T MRI system. Effects of various geometrical parameters on the resonance frequency of the e-marker were studied, and the resulting specific absorption rate (SAR) increase was analyzed using a full-wave microwave solver. The B 1 + field distribution was calculated, and experimental results were compared. As an exemplary application to locate subdural electrodes, these markers were paired with subdural electrodes. It was shown that such sub-cm self-resonant e-markers with biocompatible constituents can be designed and used for implant marking, with sub-mm positioning accuracy, in MRI. In this application, a free-space quality factor (Q-factor) of approximately 50 was achieved for the proposed resonator architecture. However, this structure caused an SAR increase in certain cases, which limits its usage for in vivo imaging practices. The findings indicate that these implantable resonators hold great promise for wireless fiducial e-marking in MRI as an alternative to multimodal imaging.
To predict subject-specific local specific absorption rate (SAR) distributions of the human head for parallel transmission (pTx) systems at 7 T.
Electromagnetic energy deposition in tissues is ...nonuniform at 7 T, and interference patterns due to individual channels of pTx systems may result in increased local SAR values, which can only be estimated with very high safety margins. We proposed, designed, and demonstrated a multichannel 3D convolutional neural network (CNN) architecture to predict local SAR maps as well as peak-spatial SAR (ps-SAR) levels. We hypothesized that utilizing a three-channel 3D CNN, in which each channel is fed by a
map, a phase-reversed
map, and an MR image, would improve prediction accuracies and decrease uncertainties in the predictions. We generated 10 new head-neck body models, along with 389 3D pTx MRI data having different RF shim settings, with their B
and local SAR maps to support efforts in this field.
The proposed three-channel 3D CNN predicted ps-SAR
levels with an average overestimation error of 20%, which was better than the virtual observation points-based estimation error (i.e., 152% average overestimation). The proposed method decreased prediction uncertainties over 20% (i.e., 22.5%-17.7%) compared to other methods. A safety factor of 1.20 would be enough to avoid underestimations for the dataset generated in this work.
Multichannel 3D CNN networks can be promising in predicting local SAR values and perform predictions within a second, making them clinically useful as an alternative to virtual observation points-based methods.
PurposeTo predict subject‐specific local specific absorption rate (SAR) distributions of the human head for parallel transmission (pTx) systems at 7 T.Theory and methodsElectromagnetic energy ...deposition in tissues is nonuniform at 7 T, and interference patterns due to individual channels of pTx systems may result in increased local SAR values, which can only be estimated with very high safety margins. We proposed, designed, and demonstrated a multichannel 3D convolutional neural network (CNN) architecture to predict local SAR maps as well as peak‐spatial SAR (ps‐SAR) levels. We hypothesized that utilizing a three‐channel 3D CNN, in which each channel is fed by a B1+$$ {B}_1^{+} $$ map, a phase‐reversed B1+$$ {B}_1^{+} $$ map, and an MR image, would improve prediction accuracies and decrease uncertainties in the predictions. We generated 10 new head–neck body models, along with 389 3D pTx MRI data having different RF shim settings, with their B1 and local SAR maps to support efforts in this field.ResultsThe proposed three‐channel 3D CNN predicted ps‐SAR10g levels with an average overestimation error of 20%, which was better than the virtual observation points–based estimation error (i.e., 152% average overestimation). The proposed method decreased prediction uncertainties over 20% (i.e., 22.5%–17.7%) compared to other methods. A safety factor of 1.20 would be enough to avoid underestimations for the dataset generated in this work.ConclusionMultichannel 3D CNN networks can be promising in predicting local SAR values and perform predictions within a second, making them clinically useful as an alternative to virtual observation points–based methods.
•Strain can be successfully extracted independent of the interrogation distance for the first time.•The proposed structure allows for strong inductive coupling and, thus, a higher wireless ...signal-to-noise ratio.•Using full electrical characteristics of the system, strain can be precisely extracted at any interrogation distance.
We proposed and developed a novel wireless passive RF resonator scheme that enables telemetric strain sensing avoiding the need for calibration at different interrogation distances. The specific architecture of the proposed structure allows for strong inductive coupling and, thus, a higher wireless signal-to-noise ratio. Here, in operation, the frequency scan of the sensor impedance was used to measure simultaneously both the impedance amplitude and resonance frequency. Using this wireless sensor, we further introduced a new telemetric monitoring modality that employs full electrical characteristics of the system to achieve correct strain extraction at any interrogation distance. In principle, any deformation of the sensor structure results in the resonance frequency shift to track strain. However, changing of the interrogation distance also varies the inductive coupling between the sensor and its pick-up antenna at the interrogation distance. Therefore, at varying interrogation distances, it is not possible to attribute an individual resonance frequency value solely to an individual strain level, consequently resulting in discrepancies in the strain extraction if the interrogation distance is not kept fixed or distance-specific calibration is not used. In this work, we showed that by using both the proposed passive sensor structure and wireless measurement technique, strain can be successfully extracted independent of the interrogation distance for the first time. The experimental results indicate high sensitivity and linearity for the proposed system. These findings may open up new possibilities in applications with varying interrogation distance for mobile wireless sensing.
In this work, dual‐modal (fluorescence and magnetic resonance) imaging capabilities of water‐soluble, low‐toxicity, monodisperse Mn‐doped ZnSe nanocrystals (NCs) with a size (6.5 nm) below the ...optimum kidney cutoff limit (10 nm) are reported. Synthesizing Mn‐doped ZnSe NCs with varying Mn2+ concentrations, a systematic investigation of the optical properties of these NCs by using photoluminescence (PL) and time resolved fluorescence are demonstrated. The elemental properties of these NCs using X‐ray photoelectron spectroscopy and inductive coupled plasma‐mass spectroscopy confirming Mn2+ doping is confined to the core of these NCs are also presented. It is observed that with increasing Mn2+ concentration the PL intensity first increases, reaching a maximum at Mn2+ concentration of 3.2 at% (achieving a PL quantum yield (QY) of 37%), after which it starts to decrease. Here, this high‐efficiency sample is demonstrated for applications in dual‐modal imaging. These NCs are further made water‐soluble by ligand exchange using 3‐mercaptopropionic acid, preserving their PL QY as high as 18%. At the same time, these NCs exhibit high relaxivity (≈2.95 mM−1 s−1) to obtain MR contrast at 25 °C, 3 T. Therefore, the Mn2+ doping in these water‐soluble Cd‐free NCs are sufficient to produce contrast for both fluorescence and magnetic resonance imaging techniques.
Small, low‐toxicity, water‐soluble monodisperse Mn‐doped ZnSe nanocrystals (NCs) synthesized using nucleation doping strategy are reported as potential dual‐modal (fluorescence and magnetic resonance) imaging contrast agents. These NCs exhibit high quantum efficiency (18% in water) and high relaxivity (2.95 mm−1 s−1). Therefore, these NCs offer a potential alternative to traditional toxic Gd‐based MRI contrast agents, with an additional fluorescence tool for accurate biomedical diagnosis.
•Wireless metamaterial architecture is proposed, modeled and demonstrated.•New geometry allows for high Q-factor and compactness, simultaneously.•Frequency range includes various wireless ...applications such as sub-mm resolution MRI.•Exhibiting high-Q, despite tissue loading is preserved.
A wireless deep-subwavelength metamaterial architecture is proposed, modeled and demonstrated for a high-resolution magnetic resonance imaging (HR-MRI) application that is miniaturized to be resonant at approximately λ0/1500 dimensions. The proposed structure has the adjustable resonance frequency from 65 MHz to 5.5 GHz for the sub-cm footprint area (8 mm × 8 mm for this study) and provides a quality factor (Q-factor) of approximately 80 in free space for 123 MHz of operation. This structure consists of a cross-via metallized partial-double-layer metamaterial, sandwiching a dielectric thin film; this structure strongly localizes the electric field in this film and has a highly capacitive metal overlay that allows for a wide range of frequency adjustment. Although the achieved resonance frequencies enable a large number of applications, as a proof-of-concept demonstration, we experimentally showed the operation of this wireless metastructure in HR-MRI to highlight its precise frequency adjustment and signal-to-noise-ratio (SNR) improvement capabilities. The proposed metamaterial was found to maintains high Q-factors despite loading with a body-mimicking lossy phantom. The experimental results indicated that the proposed metastructure can be used as an SNR-enhancing device offering 15-fold SNR enhancements that allows for imaging of objects as small as 200 μm in diameter in its vicinity, at an unprecedented level of resolution at the given DC field using standard head coils. As a result of its deep-subwavelength miniaturization accompanied by reasonable Q-factor with outstanding resonance frequency adjustment capability, this class of metastructure is proved to be an excellent candidate for in vivo medical applications.
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