Polystyrene microsphere-ferritin conjugates: a robust phantom for correlation of relaxivity and size distribution

Human mesenchymal stromal cells undergo apoptosis and fragmentation after intravenous application in immune-competent mice

BACKGROUND AIMS

The biodistribution of human MSCs after systemic delivery is incompletely understood. We investigated the changes in cell size and cell surface markers of human MSCs after intravenous (IV) injection in immune competent mice.

METHODS

Male human MSCs were labeled with fluorescent vital dye PKH67 and tracked after IV administration in C57/BL6 mice. MSCs were tracked in blood and different murine tissues by human SRY gene quantitative polymerase chain reaction (qPCR) analysis, flow cytometry and fluorescence microscopy. Calibrated microbeads were used to track the size of transplanted MSCs.

RESULTS

The majority of injected MSCs were detected by qPCR in the lungs 5 min after transplantation, whereas <0.1% were detected in other tissues over 24 h. Flow cytometric and fluorescence microscopic analysis indicated that MSCs continuously decreased in size after transplantation and underwent fragmentation. The majority of PKH⁺ MSCs and their fragments were found in lungs and liver. PKH⁺ MSCs rapidly became positive for annexin V, propidium iodide and calreticulin, indicating loss of cell integrity. In addition, PKH⁺ fragments co-stained with antibodies against C3b, F4/80 and/or GR-1 indicating opsonization. Preincubation of MSCs in hyperosmolaric hydroxyethyl starch (HyperHAES) decreased MSCs size before transplantation, delayed the loss of viability markers and increased the frequency of traceable MSCs up to 24 h after transplantation.

CONCLUSIONS

PKH67 labeled MSCs are fragmented after IV injection in mice, acquire apoptotic and phagocytic cell markers and accumulate in the lungs and liver.

A model for hepatic fibrosis: the competing effects of cell loss and iron on shortened modified Look-Locker inversion recovery T1 (shMOLLI-T1 ) in the liver

PURPOSE

To propose a simple multicompartment model of the liver and use Bloch-McConnell simulations to demonstrate the effects of iron and fibrosis on shortened-MOLLI (shMOLLI) T₁ measurements. Liver T₁ values have shown sensitivity to inflammation and fibrosis, but are also affected by hepatic iron content. Modified Look-Locker inversion recovery (MOLLI) T₁ measurements are biased by the lower T₂ associated with high iron.

MATERIALS AND METHODS

A tissue model was generated consisting of liver cells and extracellular fluid (ECF), with iron-dependent relaxation rates. Fibrosis was imitated by increasing the ECF proportion. Simulations of the shMOLLI sequence produced a look-up table (LUT) of shMOLLI-T₁ for a given ECF fraction and iron content. The LUT was used to calculate ECF(shMOLLI-T₁ ), assuming normal hepatic iron content (HIC), and ECF(shMOLLI- T1,T2*), accounting for HIC determined by T2*, for 77 patients and compared to fibrosis assessed by liver biopsy.

RESULTS

Simulations showed that increasing HIC decreases shMOLLI-T₁ , with an increase in HIC from 1.0 to 2.5 mg/g at normal ECF fraction decreasing shMOLLI-T₁ by 160 msec, while increasing ECF increased ShMOLLI-T₁ , with an increase of 20% ECF at normal iron increasing shMOLLI-T₁ by 200 msec. Calculated patient ECF(shMOLLI-T₁ ) showed a strong dependence on Ishak score (3.3 ± 0.8 %ECF/Ishak stage) and 1/T2* (-0.23 ± 0.04 %ECF/Hz). However, when iron was accounted for to produce ECF(shMOLLI- T1,T2*), it was independent of HIC but retained sensitivity to Ishak score.

CONCLUSION

Use of this multicompartment model of the liver with Bloch-McConnell simulation should enable compensation of iron effects when using shMOLLI-T₁ to assess fibrosis.

LEVEL OF EVIDENCE

1 J. Magn. Reson. Imaging 2017;45:450-462.

Measurement of liver iron concentration by MRI is reproducible

PURPOSE

The objectives were (i) construction of a phantom to reproduce the behavior of iron overload in the liver by MRI and (ii) assessment of the variability of a previously validated method to quantify liver iron concentration between different MRI devices using the phantom and patients.

MATERIALS AND METHODS

A phantom reproducing the liver/muscle ratios of two patients with intermediate and high iron overload. Nine patients with different levels of iron overload were studied in 4 multivendor devices and 8 of them were studied twice in the machine where the model was developed. The phantom was analysed in the same equipment and 14 times in the reference machine.

RESULTS

FeCl3 solutions containing 0.3, 0.5, 0.6, and 1.2 mg Fe/mL were chosen to generate the phantom. The average of the intramachine variability for patients was 10% and for the intermachines 8%. For the phantom the intramachine coefficient of variation was always below 0.1 and the average of intermachine variability was 10% for moderate and 5% for high iron overload.

CONCLUSION

The phantom reproduces the behavior of patients with moderate or high iron overload. The proposed method of calculating liver iron concentration is reproducible in several different 1.5 T systems.

Effect of magnetic field and iron content on NMR proton relaxation of liver, spleen and brain tissues

Iron accumulation is observed in liver and spleen during hemochromatosis and important neurodegenerative diseases involve iron overload in brain. Storage of iron is ensured by ferritin, which contains a magnetic core. It causes a darkening on T2 -weighted MR images. This work aims at improving the understanding of the NMR relaxation of iron-loaded human tissues, which is necessary to develop protocols of iron content measurements by MRI. Relaxation times measurements on brain, liver and spleen samples were realized at different magnetic fields. Iron content was determined by atomic emission spectroscopy. For all samples, the longitudinal relaxation rate (1/T1 ) of tissue protons decreases with the magnetic field up to 1 T, independently of iron content, while their transverse relaxation rate (1/T2 ) strongly increases with the field, either linearly or quadratically, or a combination thereof. The extent of the inter-echo time dependence of 1/T2 also varies according to the sample. A combination of theoretical models is necessary to describe the relaxation of iron-containing tissues. This can be due to the presence, inside tissues, of ferritin clusters of different sizes and densities. When considering all samples, a correlation (r(2)  = 0.6) between 1/T1 and iron concentration is observed at 7.0 T. In contrast the correlation between 1/T2 and iron content is poor, even at high field (r(2)  = 0.14 at 7.0 T). Our results show that MRI methods based on T1 or T2 measurements will easily detect an iron overloading at high magnetic field, but will not provide an accurate quantification of tissue iron content at low iron concentrations.