In Vivo and Post-mortem Comparisons of IVIM/Time-dependent Diffusion MR Imaging Parameters in Melanoma and Breast Cancer Xenograft Models

Purpose: We aimed to investigate the changes in intravoxel incoherent motion (IVIM) and diffusion parameters between in vivo and post-mortem conditions and the time dependency of these parameters using two different mouse tumor models with different vessel lumen sizes. Methods: Six B16 and six MDA-MB-231 xenograft mice were scanned using 7 Tesla MRI under both in vivo / post-mortem conditions. Diffusion weighted imaging with 17 b-values (0 - 3000 s/mm(2)) were obtained at two diffusion times (9 and 27.6 ms). The shifted apparent diffusion coefficient (sADC) using 2 b-values (200 and 1500 s/mm(2)), non-Gaussian diffusion and IVIM parameters (ADC(0) , K, f IVIM ) were estimated at each of the diffusion times. The results were evaluated by repeated measures two-way analysis of variance and post hoc Bonferroni test. Results: In B16 tumors, f IVIM significantly decreased with post-mortem conditions (from 12.6 +/- 6.5% to 5.2 +/- 1.9%, P < 0.05 at long diffusion time; from 11.0 +/- 2.4% to 4.6 +/- 2.7%, P < 0.05 at short diffusion time). In MDA-MB-231 tumors, f IVIM also significantly decreased (from 8.8 +/- 3.8% to 2.6 +/- 1.1%, P < 0.05 at long; from 7.9 +/- 5.4% to 2.9 +/- 1.1%, P < 0.05 at short). No diffusion time dependency was observed ( P = 0.59 in B16 and P = 0.77 in MDA-MB-231). The sADC and ADC 0 values tended to decrease and the K value tended to increase after sacrificing and when increasing the diffusion time. Conclusion: The f IVIM values dropped after sacrificing, confirming that IVIM MRI is a promising quantitative parameter to evaluate blood microcirculation. The presence of residual post-mortem f IVIM values suggested that the influence of water molecule diffusion in the blood lumen may contribute to the IVIM effect. Diffusion MRI parameter ' s time dependency and those changes after sacrificing could possibly provide additional insights into diffusion hindrance mechanisms.