Human whole blood 1 H2 O transverse relaxation with gadolinium-based contrast reagents: Magnetic susceptibility and transmembrane water exchange

PURPOSE

To characterize transverse relaxation in oxygenated whole blood with extracellular gadolinium-based contrast reagents by experiment and simulation.

METHODS

Experimental measurements of transverse ¹ H₂ O relaxation from oxygenated whole human blood and plasma were made at 1.5 and 3.0 Tesla. Spin-echo refocused and free-induction decays are reported for blood and plasma samples containing four different contrast reagents (gadobenate, gadoteridol, gadofosveset, and gadobutrol), each present at concentrations ranging from 1 to 18 mM (i.e., mmol (contrast reagent (CR))/L (blood)). Monte Carlo simulations were conducted to ascertain the molecular mechanisms underlying relaxation. These consisted of random walks of water molecules in a large ensemble of randomly oriented erythrocytes. Bulk magnetic susceptibility (BMS) differences between the extra- and intracellular compartments were taken into account. All key parameters for these simulations were taken from independent published measurements: they include no adjustable variables.

RESULTS

Transverse relaxation is much more rapid in whole blood than in plasma, and the large majority of this dephasing is reversible by spin echo. Agreement between the experimental data and simulated results is remarkably good.

CONCLUSION

Extracellular field inhomogeneities alone make very small contributions, whereas the orientation-dependent BMS intracellular resonance frequencies lead to the majority of transverse dephasing. Equilibrium exchange of water molecules between the intra- and extracellular compartments plays a significant role in transverse dephasing. Magn Reson Med 77:2015-2027, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Establishing an accurate gas phase reference frequency to quantify 129 Xe chemical shifts in vivo

PURPOSE

¹²⁹ Xe interacts with biological media to exhibit chemical shifts exceeding 200 ppm that report on physiology and pathology. Extracting this functional information requires shifts to be measured precisely. Historically, shifts have been reported relative to the gas-phase resonance originating from pulmonary airspaces. However, this frequency is not fixed-it is affected by bulk magnetic susceptibility, as well as Xe-N₂ , Xe-Xe, and Xe-O₂ interactions. In this study, we addressed this by introducing a robust method to determine the 0 ppm ¹²⁹ Xe reference from in vivo data.

METHODS

Respiratory-gated hyperpolarized ¹²⁹ Xe spectra from the gas- and dissolved-phases were acquired in four mice at 2T from multiple axial slices within the thoracic cavity. Complex spectra were then fitted in the time domain to identify peaks.

RESULTS

Gas-phase ¹²⁹ Xe exhibited two distinct resonances corresponding to ¹²⁹ Xe in conducting airways (varying from -0.6 ± 0.2 to 1.3 ± 0.3 ppm) and alveoli (relatively stable, at -2.2 ± 0.1 ppm). Dissolved-phase ¹²⁹ Xe exhibited five reproducible resonances in the thorax at 198.4 ± 0.4, 195.5 ± 0.4, 193.9 ± 0.2, 191.3 ± 0.2, and 190.7 ± 0.3 ppm.

CONCLUSION

The alveolar ¹²⁹ Xe resonance exhibits a stable frequency across all mice. Therefore, it can provide a reliable in vivo reference frequency by which to characterize other spectroscopic shifts. Magn Reson Med 77:1438-1445, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

The interaction of sterically stabilized magnetic nanoparticles with fresh human red blood cells

Sterically stabilized superparamagnetic iron oxide nanoparticles (SPIONs) were incubated with fresh human erythrocytes (red blood cells [RBCs]) to explore their potential application as magnetic resonance imaging contrast agents. The chemical shift and linewidth of (133)Cs(+) resonances from inside and outside the RBCs in (133)Cs nuclear magnetic resonance spectra were monitored as a function of time. Thus, we investigated whether SPIONs of two different core sizes and with three different types of polymeric stabilizers entered metabolically active RBCs, consuming glucose at 37°C. The SPIONs broadened the extracellular (133)Cs(+) nuclear magnetic resonance, and brought about a small change in its chemical shift to a higher frequency; while the intracellular resonance remained unchanged in both amplitude and chemical shift. This situation pertained over incubation times of up to 90 minutes. If the SPIONs had entered the RBCs, the intracellular resonance would have become broader and possibly even shifted. Therefore, we concluded that our SPIONs did not enter the RBCs. In addition, the T 2 relaxivity of the small and large particles was 368 and 953 mM(-1) s(-1), respectively (three and nine times that of the most effective commercially available samples). This suggests that these new SPIONs will provide a superior performance to any others reported thus far as magnetic resonance imaging contrast agents.

Frequency-encoded MRI-CEST agents based on paramagnetic liposomes/RBC aggregates

Paramagnetic liposomes containing Dy-HPDO3A in their inner water compartment and carrying a residual positive charge on their outer surface have been electrostatically bound to the membrane of red blood cells (RBCs). These aggregates yield two chemical exchange saturation transfer (CEST) pools represented by liposomal water protons (LipoCEST) and cytoplasmatic water protons (ErythroCEST), respectively. The absorption frequencies of the two pools fall at the negative and positive side of the solvent water resonance as expected from the dipolar (LipoCEST) and BMS (bulk magnetic susceptibility) (ErythroCEST) origin of the paramagnetic induced shift of their water protons resonances, respectively. In vivo magnetic resonance imaging (MRI) shows that the liposomes/RBC aggregates report about the vascular volume whereas the residual LipoCEST effect informs about the presence of released liposomes in the region of interest (ROI). Besides being an innovative blood cell labeling for MRI, the LipoCEST/RBC aggregates provide a route to improve the circulation lifetime of the liposomes and the CEST procedure allows assessing the deassembly of the aggregates and accumulation of the liposomes in the ROI.

Magnetic susceptibility to measure total protein concentration from NMR metabolite spectra: Demonstration on blood plasma

PURPOSE

Accurate metabolite and protein quantification in blood plasma and other body fluids from one single NMR measurement, allowing for improved quantitative metabolic profiling and better assessment of metabolite-protein interactions.

THEORY AND METHODS

The total protein concentration is derived from the common chemical-shift changes-caused by protein-induced bulk magnetic susceptibility (BMS)-measured on well-accessible and exchange-free metabolite resonances. These BMS shifts are simply obtained by external referencing with respect to 3-(trimethylsilyl)propionic-2,2,3,3-d4 acid, sodium salt in a coaxial insert.

RESULTS

Based on blood-plasma data from five volunteers, the estimated accuracy of the BMS method is ≤ 5% with respect and comparable to the 3.8% error of the standard colorimetric, Biuret, method. Valine, alanine, glucose, leucine, and lactate display no exchange-induced shift changes. Their well-accessible signals act as reliable probes for pure protein-induced BMS. The slopes and intercepts of their chemical-shift change versus protein concentration were derived from metabolite mixtures with (fatted) human and bovine albumin acting as blood-plasma mimics.

CONCLUSION

The BMS method, demonstrated on blood plasma, can also be used on other samples containing sufficient protein (> 10 g/L). Also, it allows measurement of the presence and sign of exchange-induced chemical-shift changes.

Sensing lanthanide metal content in biological tissues with magnetic resonance spectroscopy

The development and validation of MRI contrast agents consisting of a lanthanide chelate often requires a determination of the concentration of the agent in ex vivo tissue. We have developed a protocol that uses 70% nitric acid to completely digest tissue samples that contain Gd(III), Dy(III), Tm(III), Eu(III), or Yb(III) ions, or the MRI contrast agent gadodiamide. NMR spectroscopy of coaxial tubes containing a digested sample and a separate control solution of nitric acid was used to rapidly and easily measure the bulk magnetic susceptibility (BMS) shift caused by each lanthanide ion and gadodiamide. Each BMS shift was shown to be linearly correlated with the concentration of each lanthanide ion and gadodiamide in the 70% nitric acid solution and in digested rat kidney and liver tissues. These concentration measurements had outstanding precision, and also had good accuracy for concentrations ≥10 mM for Tm(III) Eu(III), and Yb(III), and ≥3 mM for Gd(III), gadodiamide, and Dy(III). Improved sample handling methods are needed to improve measurement accuracy for samples with lower concentrations.