Assessment of MR-based R2* and quantitative susceptibility mapping for the quantification of liver iron concentration in a mouse model at 7T

Reactive Oxygen Species-Triggered Dissociation of a Polyrotaxane-Based Nanochelator for Enhanced Clearance of Systemic and Hepatic Iron

Chronic blood transfusions are used to alleviate anemic symptoms in thalassemia and sickle cell anemia patients but can eventually result in iron overload (IO) and subsequently lead to severe oxidative stress in cells and tissues. Deferoxamine (DFO) is clinically approved to treat transfusional IO, but the use of the iron chelator is hindered by nonspecific toxicity and poor pharmacokinetic (PK) properties in humans, resulting in the need to administer the drug via long-term infusion regimens that can often lead to poor patient compliance. Herein, a nanochelator system that uses the characteristic IO physiological environment to dissociate was prepared through the incorporation of DFO and reactive oxygen species (ROS)-sensitive thioketal groups into an α-cyclodextrin-based polyrotaxane platform (rPR-DFO). ROS-induced dissociation of this nanochelator (ca. 10 nm) into constructs averaging 2 nm in diameter significantly increased urine and fecal elimination of excess iron in vivo. In addition to significantly improved PK properties, rPR-DFO was well-tolerated in mice and no adverse side effects were noted in single high dose or multiple dose acute toxicity studies. The overall features of rPR-DFO as a promising system for iron chelation therapy can be attributed to a combination of the nanochelator's improved PK, favorable distribution to the liver, and ROS-induced dissociation properties into constructs <6 nm for faster renal elimination. This ROS-responsive nanochelator design may serve as a promising alternative for safely prolonging the circulation of DFO and more rapidly eliminating iron chelates from the body in iron chelation therapy regimens requiring repeated dosing of nanochelators.

R1ρ at high spin-lock frequency could be a complementary imaging biomarker for liver iron overload quantification

PURPOSE

To compare the correlations among the R1ρ, R2, and R2* relaxation rates with liver iron concentration (LIC) in the assessment of rat liver iron content and explore the application potential of R1ρ in assessing liver iron content.

METHODS

Iron dextran (dosage of 0, 25, 50, 100, and 200 mg/kg body weight) was injected into 35 male rats to increase the amount of iron storage in the liver. After one week, all rats were euthanized with isoflurane. A portion of the largest hepatic lobe was extracted to quantify the LIC by inductively coupled plasma, and the remaining liver tissue was stored in 4% buffered paraformaldehyde for 24 h before MRI. Spin-lock preparation with a RARE (rapid acquisition with relaxation enhancement) readout (9 different spin-lock times and 7 different spin-lock frequencies (FSLs)) and multi-echo UTE (ultrashort TE) pulses were developed to quantify R1ρ and R2 * on a Bruker 11.7 T MR system. For comparisons with R1ρ and R2*, R2 was acquired using the CPMG sequence.

RESULTS

Mean R1ρ values displayed dispersion, with decrease in R1ρ at higher FSLs. Spearman's correlation analysis (two-tailed) indicated that the R1ρ values were significantly associated with LIC at FSL = 2000, 2500, and 3000 Hz (r = 0.365 and P = 0.031, r = 0.608 and P < 0.001, and r = 0.764 and P < 0.001, respectively), and were not significantly associated with LIC at FSL = 500, 1000, 1250, and 1500 Hz (all P > 0.05). R2 and R2* showed significant linear correlations with LIC (r = 0.787 and P < 0.001, and r = 0.859 and P < 0.001, respectively). Correlation analysis across R1ρ, R2, and R* also suggested that the correlation strength between R1ρ and R2 and between R1ρ and R* showed an increasing trend with increase in FSL.

CONCLUSION

In this study, a strong association was observed between R1ρ and LIC at high FSLs further confirming previous findings. The results demonstrated that R1ρ at high FSL might serve as a complementary imaging biomarker for liver iron overload quantification.

Abdominal magnetic resonance imaging examination of Tibetan patients with abnormal iron metabolism and a preliminary study of correlations with blood cell analysis

OBJECTIVE

The objective was to investigate factors influencing the high incidence of abnormal iron metabolism in a Tibetan population.

METHODS

This was a retrospective observational study. Magnetic resonance imaging and blood analysis of 363 Tibetan patients were performed and patients divided into normal and abnormal groups based on the clinical diagnostic standard. The upper limit of normal liver iron content was 50 μmol/g. We analyzed the association between abnormal iron metabolism and blood cell indicators using the Spearman rank correlation test.

RESULTS

In male patients, differences in mean corpuscular hemoglobin (MCH) and MCH concentration in blood between the normal and abnormal groups were significant. Abnormal iron metabolism in male patients was positively correlated with MCH and MCH concentration. In female patients, differences in erythrocytes, hemoglobin, and hematocrit levels between the two groups were significant. Erythrocyte counts and hemoglobin and hematocrit levels of female patients were positively correlated.

CONCLUSION

Iron overload in male patients was correlated with an increase in MCH and MCH concentration, and that in female patients was correlated with levels of erythrocytes, hemoglobin, and hematocrit. The incidence rate of iron overload was higher in males than in females and was correlated with age in this Tibetan population.

Sensitivity of quantitative relaxometry and susceptibility mapping to microscopic iron distribution

PURPOSE

Determine the impact of the microscopic spatial distribution of iron on relaxometry and susceptibility-based estimates of iron concentration.

METHODS

Monte Carlo simulations and in vitro experiments of erythrocytes were used to create different microscopic distributions of iron. Measuring iron with intact erythrocyte cells created a heterogeneous distribution of iron, whereas lysing erythrocytes was used to create a homogeneous distribution of iron. Multi-echo spin echo and spoiled gradient echo acquisitions were then used to estimate relaxation parameters ( R 2 and R 2 * ) and susceptibility.

RESULTS

Simulations demonstrate that R 2 and R 2 * measurements depend on the spatial distribution of iron even for the same iron concentration and volume susceptibility. Similarly, in vitro experiments demonstrate that R 2 and R 2 * measurements depend on the microscopic spatial distribution of iron whereas the quantitative susceptibility mapping (QSM) susceptibility estimates reflect iron concentration without sensitivity to spatial distribution.

CONCLUSIONS

R 2 and R 2 * for iron quantification depend on the spatial distribution or iron. QSM-based estimation of iron concentration is insensitive to the microscopic spatial distribution of iron, potentially providing a distribution independent measure of iron concentration.

Quantitative ferumoxytol-enhanced MRI in pregnancy: A feasibility study in the nonhuman primate

OBJECTIVES

To assess the feasibility of ferumoxytol-enhanced MRI in pregnancy with a nonhuman primate model.

MATERIALS AND METHODS

In this prospective study, eleven pregnant rhesus macaques at day 98 ± 5 of gestation were divided into three groups, untreated control (UC) (n = 3), saline control (SC) (n = 4) and interleukin 1 beta (IL-1β) treated (IT) (n = 4), which were administered with either saline or IL-1β into the amniotic fluid. All animals were imaged at multiple time points before and after ferumoxytol administration (4 mg/kg). Longitudinal R2* and susceptibility of tissues were obtained using region-of-interest analysis and the longitudinal changes were assessed using linear mixed models and Student's t-test.

RESULTS

In fetuses, a slope of 0.3 s-1/day (P = 0.008), 0.00 ppm/day (P = 0.699) and - 0.2 s-1/day (P = 0.023) was observed in liver R2*, liver susceptibility, and lung R2*, respectively. In placentas, R2* and susceptibility increased immediately after ferumoxytol administration (P < 0.001) and decreased to baseline within two days. The mean change from baseline showed no significant difference between the SC group and the IT group at all scan time points. In maternal livers, R2* increased immediately after ferumoxytol administration, further increased at one-day, and then decreased but remained elevated (P < 0.001). The mean change from baseline showed no significant difference between the SC group and the IT group at all scan time points.

CONCLUSIONS

This work demonstrates the feasibility of quantitative ferumoxytol-enhanced MRI to measure dynamics of ferumoxytol delivery and washout in the placenta. Stable MRI measurements indicated no evidence of iron deposition in fetal tissues of nonhuman primates after maternal ferumoxytol exposure.

Susceptibility based multiparametric quantification of liver disease: Non-invasive evaluation of steatosis and iron overload

PURPOSE

To evaluate if single-voxel MR spectroscopy (MRS) of iron and fat correlates with biopsy results of hepatic steatosis and iron overload, and to compare MR-measurements with room-temperature susceptometer (RTS), ultrasound, controlled attenuation parameter (CAP) and serum ferritin.

MATERIAL AND METHODS

In this prospective study, a set of 42 patients out of 47 screened patients with several chronic liver diseases underwent MRI-examination at 1.5 T including R2-measurements by single-voxel high-speed T2-corrected multiecho spectroscopy, additional liver biopsy, abdominal ultrasound, CAP, and RTS. Routine blood and serum parameters were determined, including ferritin. Atomic absorption spectroscopy (AAS) and histologically confirmed extent of hepatic steatosis from liver biopsy were used as reference standard. For correlation of R2, RTS, CAP, ferritin, and ultrasound with results of AAS and histologically determined fat fraction of liver biopsy specimen, Spearman's and Pearson's correlation as well as receiver operating characteristics curve (ROC) analysis with cut-off values determined by maximizing Youden index was used.

RESULTS

MRS iron assessment correlated best with AAS, with a Pearson correlation coefficient of 0.715 (p < 0.001), followed by RTS 0.520 (p < 0.001), and serum ferritin 0.213 (p = 0.088, not significant). MRS fat quantification correlated best with the histological confirmed extent of steatosis hepatis with a Spearman correlation coefficient of 0.836 (p < 0.001), followed by CAP 0.604 (p < 0.001) and sonographically diagnosed steatosis 0.358 (p = 0.013).

CONCLUSION

MRS by T2-corrected multiecho single-voxel spectroscopy correlated best with histological results of hepatic fat and iron content compared to RTS, CAP, abdominal ultrasound, and ferritin. Non-invasive methods to assess hepatic fat and iron are of clinical interest for follow-up examinations of patients with chronic liver diseases, where repeated biopsy is not indicated.

Patents on Quantitative Susceptibility Mapping (QSM) of Tissue Magnetism

BACKGROUND

Quantitative susceptibility mapping (QSM) depicts biodistributions of tissue magnetic susceptibility sources, including endogenous iron and calcifications, as well as exogenous paramagnetic contrast agents and probes. When comparing QSM with simple susceptibility weighted MRI, QSM eliminates blooming artifacts and shows reproducible tissue susceptibility maps independent of field strength and scanner manufacturer over a broad range of image acquisition parameters. For patient care, QSM promises to inform diagnosis, guide surgery, gauge medication, and monitor drug delivery. The Bayesian framework using MRI phase data and structural prior knowledge has made QSM sufficiently robust and accurate for routine clinical practice.

OBJECTIVE

To address the lack of a summary of US patents that is valuable for QSM product development and dissemination into the MRI community.

METHOD

We searched the USPTO Full-Text and Image Database for patents relevant to QSM technology innovation. We analyzed the claims of each patent to characterize the main invented method and we investigated data on clinical utility.

RESULTS

We identified 17 QSM patents; 13 were implemented clinically, covering various aspects of QSM technology, including the Bayesian framework, background field removal, numerical optimization solver, zero filling, and zero-TE phase.

CONCLUSION

Our patent search identified patents that enable QSM technology for imaging the brain and other tissues. QSM can be applied to study a wide range of diseases including neurological diseases, liver iron disorders, tissue ischemia, and osteoporosis. MRI manufacturers can develop QSM products for more seamless integration into existing MRI scanners to improve medical care.

Fat spectral modeling on triglyceride composition quantification using chemical shift encoded magnetic resonance imaging

PURPOSE

To explore, at a high field strength of 7T, the performance of various fat spectral models on the quantification of triglyceride composition and proton density fat fraction (PDFF) using chemical-shift encoded MRI (CSE-MRI).

METHODS

MR data was acquired from CSE-MRI experiments for various fatty materials, including oil and butter samples and in vivo brown and white adipose mouse tissues. Triglyceride composition and PDFF were estimated using various a priori 6- or 9-peak fat spectral models. To serve as references, NMR spectroscopy experiments were conducted to obtain material specific fat spectral models and triglyceride composition estimates for the same fatty materials. Results obtained using the spectroscopy derived material specific models were compared to results obtained using various published fat spectral models.

RESULTS

Using a 6-peak fat spectral model to quantify triglyceride composition may lead to large biases at high field strengths. When using a 9-peak model, triglyceride composition estimations vary greatly depending on the relative amplitudes of the chosen a priori spectral model, while PDFF estimations show small variations across spectral models. Material specific spectroscopy derived spectral models produce estimations that better correlate with NMR spectroscopy estimations in comparison to those obtained using non-material specific models.

CONCLUSION

At a high field strength of 7T, a material specific 9-peak fat spectral model, opposed to a widely accepted or generic human liver model, is necessary to accurately quantify triglyceride composition when using CSE-MRI estimation methods that assume the spectral model to be known as a priori information. CSE-MRI allows for the quantification of the spatial distribution of triglyceride composition for certain in vivo applications. Additionally, PDFF quantification is shown to be independent of the chosen a priori spectral model, which agrees with previously reported results obtained at lower field strengths (e.g. 3T).