Indirect MRI of ¹⁷ o-labeled water using steady-state sequences: Signal simulation and preclinical experiment

Few studies have been reported for T₂ -weighted indirect ¹⁷ O imaging.

To evaluate the feasibility of steady-state sequences for indirect ¹⁷ O brain imaging.

Signal simulation, phantom measurements, and prospective animal experiments were performed in accordance with the institutional guidelines for animal experiments.

Signal simulations of balanced steady-state free precession (bSSFP) were performed for concentrations of ¹⁷ O ranging from 0.037-1.600%. Phantom measurements with concentrations of ¹⁷ O water ranging from 0.037-1.566% were also conducted. Six healthy beagle dogs were scanned with intravenous administration of 20% ¹⁷ O-labeled water (1 mL/kg).

Dynamic 3D-bSSFP scans were performed at 3T MRI. ¹⁷ O-labeled water was injected 60 seconds after the scan start, and the total scan duration was 5 minutes.

Based on the result of signal simulation and phantom measurement, signal changes in the beagle dogs were measured and converted into ¹⁷ O concentrations.

The ¹⁷ O concentrations were averaged for every 15 seconds, and compared to the baseline (30-45 sec) with Dunnett's multiple comparison tests.

Signal simulation revealed that the relationships between ¹⁷ O concentration and the natural logarithm of relative signals were linear. The intraclass correlation coefficient between relative signals in phantom measurement and signal simulations was 0.974. In the animal experiments, significant increases in ¹⁷ O concentration (P < 0.05) were observed 60 seconds after the injection of ¹⁷ O. At the end of scanning, mean respective ¹⁷ O concentrations of 0.084 ± 0.026%, 0.117 ± 0.038, 0.082 ± 0.037%, and 0.049 ± 0.004% were noted for the cerebral cortex, cerebellar cortex, cerebral white matter, and ventricle.

Dynamic steady-state sequences were feasible for indirect ¹⁷ O imaging, and absolute quantification was possible. This method can be applied for the measurement of permeability and blood flow in the brain, and for kinetic analysis of cerebrospinal fluid.

2 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2018;47:1373-1379.