Measurements of cerebral blood volume using quantitative susceptibility mapping, R2 * relaxometry, and ferumoxytol-enhanced MRI

[Reproducibility analysis of quantitative susceptibility mapping of cerebral subcortical nuclei in healthy adults]

OBJECTIVE

To investigate the intra- and inter-scanner reproducibility of quantitative susceptibility mapping (QSM) of cerebral subcortical nuclei in healthy adults.

METHODS

QSM was performed in 21 healthy adults on two different 3.0T MR scanners, and the region of interest (ROI) method was used to measure the magnetic susceptibility value of the left subcortical nuclei (the head of the caudate, putamen, globus pallidus, thalamus, substantia nigra and red nucleus). The intraclass correlation coefficient (ICC) and Bland-Altman method were used to evaluate the inter-scanner and intra-scanner reliability.

RESULTS

The ICCs of the susceptibility value ranged from 0.90 to 0.99 for all the subcortical gray nuclei except for the head of the caudate nucleus measured on the same MR scanner by the same observer. Bland-Altman analysis revealed that the points with susceptibility differences for all the subcortical gray nuclei except for substantia nigra located in the 95% CI of limits of agreement for the same MR scanner. The ICCs of the susceptibility value for the inter-scanner was 0.49 (0.08-0.75) for the head of the caudate nuleus, 0.80 (0.57-0.91) for the putamen, 0.77 (0.51-0.90) for the globus pallidus, 0.78 (0.54-0.91) for the thalamus, 0.80 (0.56-0.91) for the substantia nigra and 0.93 (0.83-0.97) for the red nucleus. The points with susceptibility difference (95.2%, 20/21) located in the 95% CI of limits of agreement for the putamen and the thalamus measured on two different MR scanners.

CONCLUSIONS

The intra-scanner reproducibility of QSM of the subcortical gray nuclei is superior to the inter-scanner reproducibility in healthy adults.

Iron Oxide Labeling and Tracking of Extracellular Vesicles

Extracellular vesicles (EVs) are essential tools for conveying biological information and modulating functions of recipient cells. Implantation of isolated or modulated EVs can be innovative therapeutics for various diseases. Furthermore, EVs could be a biocompatible drug delivery vehicle to carry both endogenous and exogenous biologics. Tracking EVs should play essential roles in understanding the functions of EVs and advancing EV therapeutics. EVs have the characteristic structures consisting of the lipid bilayer and specific membrane proteins, through which they can be labeled efficiently. EVs can be labeled either directly using probes or indirectly by transfection of reporter genes. Optical imaging (fluorescent imaging and bioluminescent imaging), single-photon emission computed tomography (SPECT)/positron emission tomography (PET), and magnetic resonance imaging (MRI) are currently used for imaging EVs. Labeling EVs with superparamagnetic iron oxide (SPIO) nanoparticles for MRI tracking is a promising method that can be translated into clinic. SPIO can be internalized by most of the cell types and then released as SPIO containing EVs, which can be visualized on T2*-weighted imaging. However, this method has limitations in real-time imaging because of the life cycle of SPIO after EV degradation. Further studies will be needed to validate SPIO labeling by other imaging modalities in preclinical studies. The emerging technologies of labeling and imaging EVs with SPIO in comparison with other imaging modalities are reviewed in this paper.