Physiological assessment of in vivo human knee articular cartilage using sodium MR imaging at 1.5T

A non-uniform quantization scheme for visualization of CT images

Medical science heavily depends on image acquisition and post-processing for accurate diagnosis and treatment planning. The introduction of noise degrades the visual quality of the medical images during the capturing process, which may result in false perception. Therefore, medical image enhancement is an essential topic of research for the improvement of image quality. In this paper, a clustering-based contrast enhancement technique is presented for computed tomography (CT) images. Our approach uses the recursive splitting of data into clusters targeting the maximum error reduction in each cluster. This leads to grouping similar pixels in every cluster, maximizing inter-cluster and minimizing intra-cluster similarities. A suitable number of clusters can be chosen to represent high precision data with the desired bit-depth. We use 256 clusters to convert 16-bit CT scans to 8-bit images suitable for visualization on standard low dynamic range displays. We compare our method with several existing contrast enhancement algorithms and show that the proposed technique provides better results in terms of execution efficiency and quality of enhanced images.

Morphologic and quantitative magnetic resonance imaging of knee articular cartilage for the assessment of post-traumatic osteoarthritis

Orthopedic trauma, such as anterior cruciate ligament (ACL) disruption, is a common source of osteoarthritis in the knee. Magnetic resonance imaging (MRI) is a non-invasive multi-planar imaging modality commonly used to evaluate hard and soft tissues of diarthrodial joints following traumatic injury. The contrast provided by generated images enables the evaluation of bone marrow lesions as well as delamination and degeneration of articular cartilage. We will provide background information about MRI signal generation and decay (T₁ and T₂ values), the utility of morphologic MRI, and the quantitative MRI techniques of T_1ρ , T₂ , and T₂ * mapping, to evaluate subjects with traumatic knee injuries, such as ACL rupture. Additionally, we will provide information regarding the dGEMRIC, sodium, and gagCEST imaging techniques. Finally, the description and utility of newer post hoc analysis techniques, such as texture analysis, will be given. Continued development and refinement of these advanced MRI techniques will facilitate their clinical translation. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:412-423, 2017.

Prestructural cartilage assessment using MRI

Cartilage loss is irreversible, and to date, no effective pharmacotherapies are available to protect or regenerate cartilage. Quantitative prestructural/compositional MR imaging techniques have been developed to characterize the cartilage matrix quality at a stage where abnormal findings are early and potentially reversible, allowing intervention to halt disease progression. The goal of this article is to critically review currently available technologies, present the basic concept behind these techniques, but also to investigate their suitability as imaging biomarkers including their validity, reproducibility, risk prediction and monitoring of therapy. Moreover, we highlighted important clinical applications. This review article focuses on the currently most relevant and clinically applicable technologies, such as T2 mapping, T2*, T1ρ, delayed gadolinium enhanced MRI of cartilage (dGEMRIC), sodium imaging and glycosaminoglycan chemical exchange saturation transfer (gagCEST). To date, most information is available for T2 and T1ρ mapping. dGEMRIC has also been used in multiple clinical studies, although it requires Gd contrast administration. Sodium imaging and gagCEST are promising technologies but are dependent on high field strength and sophisticated software and hardware.

LEVEL OF EVIDENCE

5 J. Magn. Reson. Imaging 2017;45:949-965.

High-resolution morphologic and ultrashort time-to-echo quantitative magnetic resonance imaging of the temporomandibular joint

OBJECTIVE

To implement high-resolution morphologic and quantitative magnetic resonance imaging (MRI) of the temporomandibular joint (TMJ) using ultrashort time-to-echo (UTE) techniques in cadavers and volunteers.

METHODS

This study was approved by the institutional review board. TMJs of cadavers and volunteers were imaged on a 3-T MR system. High-resolution morphologic and quantitative sequences using conventional and UTE techniques were performed in cadaveric TMJs. Morphologic and UTE quantitative sequences were performed in asymptomatic and symptomatic volunteers.

RESULTS

Morphologic evaluation demonstrated the TMJ structures in open- and closed-mouth position. UTE techniques facilitated the visualization of the disc and fibrocartilage. Quantitative UTE MRI was successfully performed ex vivo and in vivo, reflecting the degree of degeneration. There was a difference in the mean UTE T2* values between asymptomatic and symptomatic volunteers.

CONCLUSIONS

MRI evaluation of the TMJ using UTE techniques allows characterization of the internal structure and quantification of the MR properties of the disc. Quantitative UTE MRI can be performed in vivo with short scan times.

Optimization and comparison of two practical dual-tuned birdcage configurations for quantitative assessment of articular cartilage with sodium magnetic resonance imaging

BACKGROUND

In this study, two practical dual-tuned birdcage configurations for quantitative assessment of articular cartilage with sodium magnetic resonance imaging (MRI) were designed and compared.

METHODS

Two 1.5 T dual-tuned birdcages, a four-ring birdcage (FRB) and an alternating rungs birdcage (ARB), were built and then characterized by bench and MRI measurements. The relative uniformity (RU) and the efficiency of the coils were compared using (23)Na and (1)H B1 maps. In vivo images of a volunteer were acquired.

RESULTS

Bench measurements showed matching and decoupling coefficients of the quadrature channels lower than -20 dB. The RUs and 180° pulse amplitudes of the FRB/ARB were determined as: (1)H RU =94.4/74.4%, (23)Na RU =95.2/93.6%, (1)H 180° pulse amplitude =69.2/75.4 V and (23)Na 180° pulse amplitude =45.1/45.9 V. The in vivo (23)Na images acquired with the FRB show a signal-to-noise ratio (SNR) of 6 to 14 in the cartilage.

CONCLUSIONS

Due to its superior (1)H homogeneity and efficiency and its slightly better (23)Na homogeneity, the FRB is the overall preferred coil for the given requirements of this study. The achieved in vivo SNR is adequate for quantitative (23)Na and high resolution (1)H imaging.

High Field Sodium MRI Assessment of Stem Cell Chondrogenesis in a Tissue-Engineered Matrix

The development of non-invasive assessment techniques in vitro and in vivo is essential for monitoring and evaluating the growth of engineered cartilage tissues. Magnetic resonance imaging (MRI) is the leading non-invasive imaging modality used for assessing engineered cartilage. Typical MRI uses water proton relaxation times (T1 and T2) and apparent diffusion coefficient (ADC) to assess tissue growth. These techniques, while excellent in providing the first assurance of tissue growth, are unspecific to monitor the progress of engineered cartilage extracellular matrix components. In the current article, we present high field (11.7 T, (1)H freq. = 500 MHz) sodium MRI assessment of tissue-engineered cartilage at the early stage of tissue growth in vitro. We observed the chondrogenesis of human bone marrow derived stromal cells seeded in a gradient polymer-hydrogel matrix made out of poly(85 lactide-co-15 glycolide)--PuraMatrix™ for 4 weeks. We calculated the sodium concentration in the engineered constructs using a model of sodium MRI voxels that takes into account scaffold volume, cell density and amount of glycosaminoglycan (GAG). The sodium concentration was then converted to the fixed charge density (FCD) and compared with FCD derived from biochemical GAG analysis. Despite the small amount of GAG present in the engineered constructs, the sodium MRI derived FCD is found to be correlated (Pearson correlation coefficient R = 0.79) with the FCD derived from biochemical analysis. We conclude that sodium MRI could prove to be an invaluable tool in assessing engineered cartilage quantitatively during the repair or regeneration of cartilage defects.

Magnetic resonance compositional imaging of articular cartilage: What can we expect in veterinary medicine?

Since cartilage has limited ability to repair itself, it is useful to determine its biochemical composition early in clinical cases. It is also important to assess cartilage content in research animals in longitudinal studies in vivo. In recent years, compositional imaging techniques using magnetic resonance imaging (MRI) have been developed to assess the biochemical composition of cartilage. This article describes MR compositional imaging techniques, and discusses their use and interpretation. Technical concerns still limit the use of some techniques for research and clinical use, especially in veterinary medicine. Glycosaminoglycan chemical-exchange saturation transfer and sodium imaging are better used with high field magnets, which have limited availability. Long acquisition times are sometimes required, for instance in T1rho (ρ) and diffusion-weighted imaging, and necessitate general anaesthesia. Even in human medicine, some techniques such as ultra-short echo T2 are not fully validated, and nearly all techniques require validation for veterinary research and clinical practice. Delayed gadolinium-enhanced MRI of cartilage and T2 mapping appear to be the most applicable methods for compositional imaging of animal cartilage. Combining T2 mapping and T1ρ allows for the assessment of proteoglycans and the collagen network, respectively.

Multinuclear MR and multilevel data processing: an insight into morphologic assessment of in vivo knee articular cartilage

RATIONALE AND OBJECTIVES

Quantitative assessment of knee articular cartilage (AC) morphology using magnetic resonance (MR) imaging requires an accurate segmentation and 3D reconstruction. However, automatic AC segmentation and 3D reconstruction from hydrogen-based MR images alone is challenging because of inhomogeneous intensities, shape irregularity, and low contrast existing in the cartilage region. Thus, the objective of this research was to provide an insight into morphologic assessment of AC using multilevel data processing of multinuclear ((23)Na and (1)H) MR knee images.

MATERIALS AND METHODS

A dual-tuned ((23)Na and (1)H) radio-frequency coil with 1.5-T MR scanner is used to scan four human subjects using two separate MR pulse sequences for the respective sodium and proton imaging of the knee. Postprocessing is performed using customized routines written in MATLAB. MR data were fused to improve contrast of the cartilage region that is further used for automatic segmentation. Marching cubes algorithm is applied on the segmented AC slices for 3D volume rendering and volume is then calculated using the divergence theorem.

RESULTS

Fusion of multinuclear MR images results in an improved contrast (factor >3) in the cartilage region. Sensitivity (80.21%) and specificity (99.64%) analysis performed by comparing manually segmented AC shows a good performance of the automated AC segmentation. The average cartilage volume (23.19 ± 1.38 cm(3); coefficient of variation [COV] -0.059) measured from 3D AC models of four data sets shows a marked improvement over average cartilage volume (23.24 cm(3); COV -0.19) reported earlier.

CONCLUSIONS

This study confirms the use of multinuclear MR data for cartilage morphology (volume) assessment that can be used in clinical settings.

Sodium magnetic resonance imaging using ultra-short echo time sequences with anisotropic resolution and uniform k-space sampling

A method for uniform k-space sampling of 3D ultra-short echo time (UTE) techniques with anisotropic resolution in one direction is introduced to increase signal-to-noise ratio (SNR). State-of-the-art acquisition schemes for sodium MRI with radial (projection reconstruction) and twisting (twisted projection imaging (TPI)) trajectories are investigated regarding SNR efficiency, blurring behavior under T2(⁎) decay, and measurement time in case of anisotropic field-of-view and resolution. 3D radial and twisting trajectories are redistributed in k-space for UTE sodium MRI with homogeneous noise distribution and optimal SNR efficiency, if T2(⁎) decay can be neglected. Simulations based on Voronoi tessellations and phantom simulations/measurements were performed to calculate SNR efficiency. Point-spread functions were simulated to demonstrate the influence of T2(⁎) decay on SNR and resolution. Phantom simulations/measurements and in vivo measurements confirm the SNR gain obtained by simulations based on Voronoi cells. An increase in SNR of up to 21% at an anisotropy factor of 10 could be theoretically achieved by TPI with projection adaption compared to the same sequence but without redistribution of projections in k-space. Sodium MRI with anisotropic resolution and uniform k-space sampling is demonstrated by in vivo measurements of human intervertebral disks and heart at 3 T. The SNR gain can be invested in a measurement time reduction of up to 32%, which is important especially for sodium MRI.

Non-invasive and in vivo assessment of osteoarthritic articular cartilage: a review on MRI investigations

Early detection of knee osteoarthritis (OA) is of great interest to orthopaedic surgeons, rheumatologists, radiologists, and researchers because it would allow physicians to provide patients with treatments and advice to slow the onset or progression of the disease. Early detection can be achieved by identifying early changes in selected features of degenerative articular cartilage (AC) using non-invasive imaging modalities. Magnetic resonance imaging (MRI) is becoming the standard for assessment of OA. The aim of this paper was to review the influence of MRI on the selection, detection, and measurement of AC features associated with early OA. Our review of the literature indicates that the changes associated with early OA are in cartilage thickness, cartilage volume, cartilage water content, and proteoglycan content that can be accurately, consistently, and non-invasively measured using MRI. Choosing an MR pulse sequence that provides the capability to assess cartilage physiology and morphology in a single acquisition and advanced multi-nuclei MRI is desirable. The results of the review indicate that using an ultra-high magnetic strength, MR imager does not affect early OA detection. In conclusion, MRI is currently the most suitable modality for early detection of knee OA, and future research should focus on the quantitative evaluation of early OA features using advances in MR hardware, software, and data processing with sophisticated image/pattern recognition techniques.