Characterizing Microbeads' Free Fluid Volume

To confirm that the free fluid volume surrounding the microbeads increases with increasing particles size, additional measurements were performed employing MR T₁ mapping and contrast-enhanced micro computed-tomography (µCT).
For T₁ mapping, all phantoms were prepared de novo and scanned jointly using spin-echo sequence (2D-RARE; Table 1B) with variable repetition times. For each phantom, dry microbeads were put for 24 hr directly into the phantom lumen filled with 0.001 mM/ml gadobutrol solution. Before MRI, any possible air bubbles were removed, and the phantom was sealed (as above). Microbeads of different size were expected to differently infiltrate surrounding water, and partially gadobutrol. Thus, the relative concentration of gadobutrol was expected to be altered compared to original solution, and shorter T₁ relaxations were expected to be observed for phantoms with decreasing microbeads size. For reference, T₁ mapping was performed in the phantom filled solely with gadobutrol solution. To correct T₁ maps for B₁-filed inhomogeneities using double-angle method (Insko and Bolinger, 1993 (link); Cunningham et al., 2006 (link)), all phantoms were scanned with the same spin-echo sequence with a maximal repetition time used for T₁ mapping, and using two different flip angles (Table 1B).
For free fluid space estimation using µCT, all phantoms previously scanned for DWI were unsealed and flushed with a distilled water solution via 5 ml syringe to remove gadobutrol. Subsequently, distilled water was replaced with a 1:1 dilute of non-ionic iodine Omnipaque 350 contrast agent (Iohexol, 350 mg iodine/ml; GE Healthcare AS) in normal saline via 3 ml syringe. Each phantom was scanned using Vector4uCT system (MILabs, Utrecht, Netherlands) using the scan parameters of 15 μm isomteric resolution, 50 kVp, 0.24 mA (75ms exposure), 360 degrees rotation, 0.2 degree rotation step, 2 frames for averaging, 0.5 mm thick beam aluminum filter, Hann filter with cone-beam reconstruction.
To verify the relation between MR diffusion values and the T₁ relaxation times as well as free fluid estimates using µCT, DWI was performed 4 times in each phantom (as above).