²³Na Triple‐quantum signal of in vitro human liver cells, liposomes, and nanoparticles: Cell viability assessment vs. separation of intra‐ and extracellular signal

Abstract

Background Triple‐quantum (TQ) filtered sequences have become more popular in sodium MR due to the increased usage of scanners with field strengths exceeding 3T. Disagreement as to whether TQ signal can provide separation of intra‐ and extracellular compartments persists. Purpose To provide insight into TQ signal behavior on a cellular level. Study Type Prospective. Phantom/Specimen Cell‐phantoms in the form of liposomes, encapsulated 0 mM, 145 mM, 154 mM Na+ in a double‐lipid membrane similar to cells. Poly(lactic‐co‐glycolic acid) nanoparticles encapsulated 154 mM Na+ within a single‐layer membrane structure. Two microcavity chips with each 6 × 106 human HEP G2 liver cells were measured in an MR‐compatible bioreactor. Field Strength/Sequence Spectroscopic TQ sequence with time proportional phase‐increments at 9.4T. Assessment The TQ signal of viable, dead cells, and cell‐phantoms was assessed by a fit in the time domain and by the amplitude in the frequency domain. Statistical Tests The noise variance (σ) was evaluated to express the deviation of the measured TQ signal amplitude from noise. Results TQ signal >20σ was found for liposomes encapsulating sodium ions. Liposomal encapsulation of 0 mM Na+ and 154 mM Na+ encapsulation in the nanoparticles resulted in <2σ TQ signal. Cells under normal perfusion resulted in >9σ TQ signal. Compared with TQ signal under normal perfusion, a 56% lower TQ signal of was observed (25σ) during perfusion stop. TQ signal returned to 92% of the initial signal after reperfusion. Data Conclusion Our measurements indicate that TQ signal in liposomes was observed due to the trapping of ions within the double‐lipid membrane rather than from the intraliposomal space. Transfer to the cell results suggests that TQ signal was observed from motion restriction equivalent to trapping. Level of Evidence: 1 Technical Efficacy: Stage 3 J. Magn. Reson. Imaging 2019;50:435–444.

X‐nuclei imaging: Current state, technical challenges, and future directions

“…This prominence can be partially attributed to the initial difficulty in distinguishing between intra‐ and extracellular signal origin. In addition, the unambiguous assignment of the physical signal to its biological origin has been the subject of ongoing debate . Technical advances in terms of magnetic field strength B 0 , gradient performance, and sophisticated pulse sequences were achieved progressively over time.…”

Protein conformational changes affect the sodium triple‐quantum MR signal

“…Such signals can be used to assess cell viability. 4,6,7,[12][13][14][15][16] However, the effect of pH or protein conformation, which affects the availability of negatively charged groups, can contribute to the TQ signal and needs to be evaluated. In this study, we investigated the dependence of the sodium TQ signal on pH and the protein folding state of the well-known protein BSA, which can resemble the in vivo situation.…”

“…[4][5][6][7][8][9][10][11] Thus, the TQ signal can potentially be a valuable biomarker of cell viability. 4,6,7,[12][13][14][15][16] Both the sodium TQ signal and the sodium SQ signal can be valuable parameters for cell viability. [17][18][19][20][21][22] Yet, in the case of the sodium SQ signal, the differentiation between increased intracellular sodium content and increased extracellular space cannot be accomplished.…”

“…In this study, the TQ time proportional phase increment (TQTPPI) pulse sequence 3,12,13 was selected for sensitive detection of sodium TQ signals. The sodium TQ signal was separated by frequency from other MR signals comprising SQ and DQ signals.…”

“…44 The TQ/SQ signal ratio has been shown to be capable of reflecting the level of sodium interactions with macromolecules. 3,12,13 The experiments in this study were conducted using BSA, a protein that has been shown to create a strong TQ signal despite a weak affinity for sodium ions. 36,[45][46][47] BSA was selected to be a suitable model system, as BSA has been widely studied, is available in high purity, and has a known protein structure with a relatively large amino acid chain length of 583.…”