Investigation of marine bivalve morphology by in vivo MR imaging: First anatomical results of a promising technique

Classically, investigation of the internal morphology and composition of molluscs and especially bivalves relies on destructive method (biometry, biochemistry and histology). These techniques have given essential information, but in contrast are time consuming and lead to the irreversible loss of the animal while they don't allow integration of the various levels of molecular-to-organism functioning. The aim of this study is to analyze for the first time the potential of NMR (nuclear magnetic resonance) imaging (MRI) to depict, with sufficient resolution and satisfactory contrast, the anatomy of a bivalve model, the Pacific oyster, Crassostrea gigas, without opening it. MRI experiments were carried out at 19 °C in several non-anaesthetized adult Pacific oysters, analyzed individually in a standard General Electric Signa 1.5T (whole body) instrument with actively shielded gradient coils (23 mT/m). To enhance signal detection, the oyster was centered in the middle of a 12-cm diameter Helmholtz-like radio-frequency coil (medical wrist coil). After several trials, the best MRI acquisition sequence retained was a T1-weighted procedure (spoiled gradient echo sequence) through two orthogonal directions (transversal and sagittal sections). According to direction, MR parameters were as follows: TR = 200–400 ms, TE = 4–5 ms, FOV = 120 × 90 mm, matrix = 512 × 256 units, 6 signal averages per echo, spatial resolution = 230 μm, total scan time = 3–6 min. The MR images obtained have satisfactory contrast-to-noise levels, and depict with a sufficient resolution all the main organs in the soft tissues of the oyster. Comparison with histology-based anatomical information shows that the MR images faithfully represent some detailed anatomical structures of Pacific oysters. Potential applications in shellfish aquaculture are reported, and perspectives are given which constitute starting point from furthers studies.