PS12-16. Influence of freezing and thawing on MRI parameters of mid-gestation pig fetuses.

Abstract Magnetic resonance imaging (MRI) is a powerful diagnostic tool that can be used to evaluate physiological differences in development and variation in body composition. A challenge with MRI is the time required to scan each animal or tissue with high resolution to delineate different tissues from one another. However, if able to freeze and thaw samples while maintaining sample integrity similar to fresh tissue, MRI could be more commonly used to assess differences in body composition and development changes of fetal pigs throughout gestation. The objective of this study was to evaluate MRI parameters, T1 and T2, of multiple tissues from fresh and previously frozen fetal piglets from mid gestation. Sows (n = 2) were euthanized on 58 and 59 days of gestation. The gravid uterus was removed from the sow, all fetuses were removed, and morphometric analyses were conducted on each fetus. Three male fetuses from each litter were selected for MRI analysis (n = 6). Fetuses were transported on wet ice to the Huck Institute MRI facility to undergo imaging. Imaging was conducted using a 7T Bruker 70/30 BioSpec. Structural images were acquired using a 3D TurboRARE sequence. T1 and T2 were measured in different slices using the RAREVTR acquisition methods. After the first round of imaging, fetuses were frozen at -80 ºC until subsequent imaging. Fetuses were thawed for 24 – 48 h at 4 ºC prior to reimaging using the same parameters as the fresh scans. Regions of interest in the midbrain, eye, longissimus thoracis, and longissimus lumborum were identified for quantification of T1 and T2. Data were analyzed using a generalized mixed linear model with fresh or frozen as the fixed effect. Sow was initially included in all analyses but was not significant. Significance was declared at P ≤ 0.05 for all analyses. There was no difference (P > 0.05) in the T1 of the eye, longissimus thoracis, or longissimus lumborum regions of interest. There was a 16.7% reduction (P < 0.05) in T1 the brain was observed after freezing and thawing of the piglets. Similarly, there was a 51.2% reduction (P < 0.05) in T2 of the brain after freezing and thawing. There was a 51.8% decrease (P < 0.05) of T2 in the eye following freezing and thawing. The T2 of the longissimus thoracis decreased (P < 0.05) between 24.2% after a single freeze-thaw cycle and T2 of the longissimus lumborum decreased (P < 0.05) 21.6% after freeze-thaw. As water freezes and ice crystals are formed there is likely microstructure tissue damage leading to a reduction in the T2 values. Thus, the freezing and thawing process alters T2values, limiting the ability to freeze and thaw porcine fetuses to assess variation in body composition.