Wireless amplified NMR detector for improved visibility of image contrast in heterogeneous lesions

Evaluation of condylar osseous changes using a wireless detector with proton density-weighted imaging sequences

Background

Cone-beam computed tomography (CBCT) is the gold standard for evaluating condylar osseous changes. However, the radiation risk and low soft-tissue resolution of CBCT make it unsuitable for evaluating soft tissue such as the articular disc and lateral pterygoid muscle. This study aimed to qualitatively and quantitatively evaluate the feasibility and advantages of using wireless detectors (WD) with proton density-weighted imaging (PDWI) sequences to image condyle changes in patients with temporomandibular disorders (TMD).

Methods

This study involved 20 patients (male =8, female =12; mean age 31.65 years, SD 12.68 years) with TMD. All participants underwent a closed oblique sagittal PDWI scan with head/neck coupling coiling (HNCC) and wireless detector-HNCC (WD-HNCC) on a 3.0 T magnetic resonance imaging (MRI) scanner. Subsequently, the changes in the condyle bones in the scanned images for the 2 image types were scored subjectively and compared qualitatively. The contrast-to-noise ratio (CNR) of the 2 types of scanned images was compared quantitatively. The comparison of CNR differences between the 2 types of images was performed using the paired t-test. The kappa statistic was used to test the consistency of quantitative analyses of MRI images between observers. The subjective scores of condylar osseous changes in the 2 types of images were compared by paired rank-sum test. A P value <0.05 was considered statistically significant.

Results

A total of 40 condyles from 20 patients were scanned. Among them, 8 condyles showed no bone changes, and the other 32 condyles demonstrated condylar osseous changes of varying degrees and nature. These 32 condyles were used in the subsequent analysis. As compared to images acquired by HNCC in the PDWI sequence, the WD-HNCC images more clearly showed mandibular osteophyte, bone cortical erosion, subcortical cystic focus, and bone cortical hyperplasia and thickening. In addition, the WD-HNCC was demonstrated to improve image CNR by 158.9% compared to HNCC (28.17±16.01 vs. 10.88±6.53; t=8.63; P=0.001).

Conclusions

WD-HNCC in PDWI sequences is suitable for imaging the condylar bone changes of patients with TMD and significantly improves the image quality.

Parallel magnetic resonance image reconstruction from a single-element parametric amplifier

In magnetic resonance imaging (MRI), acquisition speed is always an important issue. In this paper, we propose a promising technique to achieve parallel MRI (pMRI) on a single-channel spectrometer, using a novel Wireless Amplified Nuclear MR Detector (WAND) for spatial encoding in image reconstruction. For this, a planar structure double frequency WAND is designed and fabricated, where two of its frequencies - 'signal', ω1 and 'idler', ω2 are effectively utilized as two separate "channels" for accelerated acquisition. We provided a thorough background needed for the method and subsequently parallel imaging algorithms. Sum-of-Squares (SoS) reconstruction and GeneRalized Autocalibrating Partially Parallel Acquisition (GRAPPA) reconstruction are used to reconstruct as well as to analyze the SNR in the resulting images and validate our hypothesis. Experimental results using phantom datasets demonstrate that the proposed method of parallel imaging yield a better sensitivity for the combined images ('idler' + 'signal') than the sensitivity acquired for each individual image and thus significantly improving the reconstruction quality with optimal signal-to-noise ratio. We also demonstrated the achievable acceleration factor of this approach.

High-resolution MRI of kidney microstructures at 7.05 T with an endo-colonic Wireless Amplified NMR detector

To map the hemodynamic responses of kidney microstructures at 7.05 T with improved sensitivity, a Wireless Amplified NMR Detector (WAND) with cylindrical symmetry was fabricated as an endoluminal detector that can convert externally provided wireless signal at 600.71 MHz into amplified MR signals at 300.33 MHz. When this detector was inserted inside colonic lumens to sensitively observe adjacent kidneys, it could clearly identify kidney microstructures in the renal cortex and renal medullary. Owing to the higher achievable spatial resolution, differential hemodynamic responses of kidney microstructures under different breathing conditions could be individually quantified to estimate the underlying correlation between oxygen bearing capability and local levels of oxygen unsaturation. The WAND's ability to map Blood Oxygen Level Dependent (BOLD) signal responses in heterogeneous microstructures will pave way for early-stage diagnosis of kidney diseases, without the use of contrast agents for reduced tissue retention and toxicity.

Signal Sensitivity Enhancement of High-Spatial-Resolution MR Imaging with a Concatenated Cylindrical Parametric RF-Resonator

Phased array MRI coils can increase sensitivity of superficial tissues owing to their proximity to the detection region. Deep-lying tissues, on the other hand, do not benefit to the same degree. Here we investigate the use of a localized cylindrically symmetric quadruple frequency resonator concatenated with a double frequency resonator to increase the longitudinal field-of-view (FOV) without compromising the spatial-resolution and detection sensitivity. These concatenated array coils work on the principle of a parametric amplification to provide wireless amplification of the locally detected NMR signal prior to inductively coupling the coil to an external pick-up loop with connection to the system receiver. When both the detectors are activated together, the effective range of both overlay to create a larger FOV enabling better identification of detectable regions. Furthermore, the in-vivo test of the concatenated detector provides a worst-case 5-fold SNR gain in regions separated from the cylindrical surface larger than its own diameter. This proposed approach of concatenated detector realization can be individually activated and manipulated to enlarge the sensitivity-enhanced region without sacrificing their individual performance. Compared to double frequency detectors, quadruple frequency detectors offer more flexibility in the choice of detector dimension, enabling multi-element concatenation over an extended FOV.

A Novel Expandable Catheter Wireless Amplified NMR Detector for MR Sensitivity Accessing the Kidney in Rodent Model

This paper demonstrates the enlarged effective range for MRI sensitivity enhancement with a deformable catheter MRI coils integrated with a wirelessly powered amplifier. The expandable balloon wireless amplified nuclear magnetic resonance detector (WAND) is constructed on a copper-clad polyimide film to resonate at the first and second harmonics of the proton Larmor frequency at 7 Tesla. The WAND is then mounted on a balloon catheter system for easy delivery inside confined orifice. Upon reaching the region of interest, it is unfolded out of the sheath tube to increase its effective size. Magnetic resonance (MR) imaging experiments with and without the WAND are performed both in a water phantom and in a live rat to evaluate the WAND's sensitivity advantage. Expanded from a 3 mm diameter in its folded state, this deformable WAND can change its width by >100% in its inflated state to at least 6 mm, leading to a sensitive detection region extending to up to 20 mm in the transverse direction. When the deformable WAND is placed in an artery in the region of the kidney of a live rat, it could achieve at least a 10-fold SNR gain over images acquired by a standard external detector of 22 mm diameter, even though the region of interest is separated from the WAND's surface by a distance larger than the WAND's own width. The proposed expandable catheter WAND could significantly enlarge the effective range for MR sensitivity enhancement in-vivo, enabling versatile applications in interventional MRI.