Faculty Bibliography

OBJECTIVE: This study presents a reproducible phantom which mimics oxygen-consuming tissue and can be used for the validation of (19)F MRI oximetry. MATERIALS AND METHODS: The phantom consists of a haemodialysis filter of which the outer compartment is filled with a gelatin matrix containing viable yeast cells. Perfluorocarbon emulsions can be added to the gelatin matrix to simulate sequestered perfluorocarbons. A blood-substituting perfluorocarbon fluid is pumped through the lumen of the fibres in the filter. (19)F relaxometry MRI is performed with a fast 2D Look-Locker imaging sequence on a clinical 3T scanner. RESULTS: Acute and perfusion-related hypoxia were simulated and imaged spatially and temporally using the phantom. CONCLUSIONS: The presented experimental setup can be used to simulate oxygen consumption by somatic cells in vivo and for validating computational biophysical models of hypoxia, as measured with (19)F MRI oximetry.

The spin-spin relaxation rate R-2 (=1/T-2) in hydrogel foams measured by use of a multiple spin echo sequence was found to be dependent on the echo time spacing. (Baete and De Deene. Proc. Intl. Soc. Magn. Reson. Med. 15:37, 2007). This property, referred to as R-2-dispersion, originates to a large extent from molecular self-diffusion of water within internal field gradients that result from magnetic susceptibility differences between the gel and air phase. Another contribution to the R-2 relaxation rate is surface relaxation. Numerical simulations are performed to investigate the relation between the foam microstructure (the mean air bubble radius and standard deviation of the air bubble radius) at one hand and the R-2-dispersion at the other hand. The simulated R-2-dispersions of gel foam are in agreement with measured R-2-dispersions. By correlating the R-2-dispersion parameters and simulated microstructure properties a relationship is obtained that enables the mean air bubble size to be derived from measured R-2-dispersion curves. The R-2-derived mean air bubble size of a hydrogel foam is in agreement with the bubble size measured with X-ray micro-CT. This illustrates the feasibility of using H-1 R-2-dispersion measurements to determine the size of air bubbles in hydrogel foams and of alveoli in lung tissue.

The spin-spin relaxation rate R2 (=1/T2) in hydrogel foams measured by use of a multiple spin echo sequence is found to be dependent on the echo time spacing. This property, referred to as R2-dispersion, originates to a large extent from molecular self-diffusion of water within internal field gradients that result from magnetic susceptibility differences between the gel and air phase. Another contribution to the R2 relaxation rate is surface relaxation. Numerical simulations are performed to investigate the relation between the foam microstructure (the mean air bubble radius and standard deviation of the air bubble radius) and foam composition properties (such as magnetic susceptibilities, diffusion coefficient and surface relaxivity) at one hand and the R2-dispersion at the other hand. The simulated R2-dispersions of gel foam are in agreement with the measured R2-dispersions. By correlating the R2-dispersion parameters and simulated microstructure properties a semi-empirical relationship is obtained that enables the mean air bubble size to be derived from measured R2-dispersion curves. The R2-derived mean air bubble size of a hydrogel foam is in agreement with the bubble size measured with X-ray micro-CT. This illustrates the feasibility of using 1H R2-dispersion measurements to determine the size of air bubbles in hydrogel foams and of alveoli in lung tissue.