Basic principles of magnetic resonance imaging

Multimodal Magnetic Resonance and Photoacoustic Imaging of Tumor-Specific Enzyme-Responsive Hybrid Nanoparticles for Oxygen Modulation

Multimodal imaging contrast agents for cancer that can not only perform diagnostic functions but also serve as tumor microenvironment–responsive biomaterials are encouraging. In this study, we report the design and fabrication of a novel enzyme-responsive T₁ magnetic resonance imaging (MRI) contrast agent that can modulate oxygen in the tumor microenvironment via the catalytic conversion of H₂O₂ to O₂. The T₁ contrast agent is a core–shell nanoparticle that consists of manganese oxide and hyaluronic acid (HA)–conjugated mesoporous silica nanoparticle (HA-MnO@MSN). The salient features of the nanoparticle developed in this study are as follows: 1) HA serves as a targeting ligand for CD44-expressing cancer cells; 2) HA allows controlled access of water molecules to the MnO core via the digestion of enzyme hyaluronidase; 3) the generation of O₂ bubbles in the tumor by consuming H₂O₂; and 4) the capability to increase the oxygen tension in the tumor. The r₁ relaxivity of HA-MnO@MSN was measured to be 1.29 mM⁻¹s⁻¹ at a magnetic field strength of 9.4 T. In vitro results demonstrated the ability of continuous oxygen evolution by HA-MnO@MSN. After intratumoral administration of HA-MnO@MSN to an HCT116 xenograft mouse model, T₁ weighted MRI contrast was observed after 5 h postinjection and retained up to 48 h. In addition, in vivo photoacoustic imaging of HA-MnO@MSN demonstrated an increase in the tumor oxygen saturation over time after i. t. administration. Thus, the core–shell nanoparticles developed in this study could be helpful in tumor-targeted T₁ MR imaging and oxygen modulation.

Detecting and Monitoring Hydrogels with Medical Imaging

Hydrogels, water-swollen polymer networks, are being applied to numerous biomedical applications, such as drug delivery and tissue engineering, due to their potential tunable rheologic properties, injectability into tissues, and encapsulation and release of therapeutics. Despite their promise, it is challenging to assess their properties in vivo and crucial information such as hydrogel retention at the site of administration and in situ degradation kinetics are often lacking. To address this, technologies to evaluate and track hydrogels in vivo with various imaging techniques have been developed in recent years, including hydrogels functionalized with contrast generating material that can be imaged with methods such as X-ray computed tomography (CT), magnetic resonance imaging (MRI), optical imaging, and nuclear imaging systems. In this review, we will discuss emerging approaches to label hydrogels for imaging, review the advantages and limitations of these imaging techniques, and highlight examples where such techniques have been implemented in biomedical applications.

Estimating bone mineral density using MRI in medicine and dentistry: a literature review

OBJECTIVES

Studies performed in the medical area have shown that an indirect diagnosis of bone mineral density (BMD) is feasible by assessing the amount of bone marrow fat with non-ionizing magnetic resonance imaging (MRI). In dentistry, radiographic methods are still the most used for alveolar bone diagnosis. The present literature review aimed at addressing the role of MRI in assessing BMD in medicine and dentistry.

METHODS

MEDLINE and EMBASE databases were searched for articles published up to 2019.

RESULTS

A total of 428 potentially eligible papers were screened. Of these, 397 were excluded after title, abstract and keyword assessment, yielding 31 papers that potentially met the inclusion criteria. Eleven studies were then excluded because their full texts did not discuss the role of MRI in the indirect diagnosis of BMD. As a result, a total of 20 studies were finally identified as eligible for inclusion in this literature review. Most studies found satisfactory accuracy of MRI for indirectly assessing BMD by quantifying bone mineral fat (BMF). However, only one of these studies was on dentistry.

CONCLUSION

Within the limitations of this study, the present findings suggest that MRI is accurate to indirectly estimate bone density by assessing BMF, and could be clinically relevant during dental treatment planning.

Denoising high angular resolution diffusion imaging data by combining singular value decomposition and non-local means filter

BACKGROUND

High angular resolution diffusion imaging (HARDI) data is typically corrupted with Rician noise. Although larger b-values help to retrieve more accurate angular diffusivity information, they also lead to an increase in noise generation.

NEW METHOD

In order to sufficiently reduce noise in HARDI images and improve the construction of orientation distribution function (ODF) fields, a novel denoising method was developed in this study by combining the singular value decomposition (SVD) and non-local means (NLM) filter. Similar 3D patches were first recruited into a matrix from a search volume. HARDI signals in the matrix were then re-estimated using the SVD low rank approximation, and a NLM filter was employed to filter out any residual noise.

RESULTS

The performance of the proposed method was evaluated against the state-of-the-art denoising methods based on both synthetic and real HARDI datasets. Results demonstrated the superior performance of the developed SVD-NLM method in denoising HARDI data through preserving fine angular structural details and estimating diffusion orientations from improved ODF fields.

CONCLUSION

The proposed SVD-NLM method can improve HARDI quantitative computations, such as MRI brain tissue segmentation and diffusion profile estimation, that rely on the quality of imaging data.

Magnetic resonance imaging in cerebral ischemia: focus on neonates

Magnetic resonance imaging (MRI) has dramatically changed our ability to diagnose and treat stroke as well as follow its evolution and response to treatment. Early stroke and ischemia can be visualized using diffusion-weighted imaging (DWI), which utilizes proton diffusion within tissues as a reporter for evolving neuropathology that reflects cytotoxic edema, particularly during the first several days after injury. Historically, T2-weighted imaging (T2WI) has been used for evaluation of vasogenic edema and also is a reliable indicator of injured tissue late after injury. While visual analysis of MR data can provide information about the evolution of injury, quantitative analyses allow definitive and objective evaluations of injury size and location and the effectiveness of novel therapeutic strategies. We review the clinical basis of imaging for stroke and ischemia diagnosis and the methods for post-processing of MR data that could provide novel insights into the evolution and pathophysiology of stroke in the newborn.