The capability of small animal scanning using a clinically ultra-highfield whole body scanner at 3 Tesla

Purpose: MRI animal scanning has become an essential tool for experimental studies in molecular imaging. While ultra-high-field small-bore animal scanners with powerful gradients have been in use for the last decade, the recent availability of high-field clinical scanners warrants investigating of how well they can do the job in conjunction with small dedicated rf coils.

Methods: More than 200 animals (rats, mice) were examined for different experimental purposes with images of the head and abdomen taken. Three different small rf coils were employed including a 2-cm circularly polarized surface coil, a T/R volume resonator with 4cm, and another with10cm diameter (all Rapid Biomedical, Würzburg, Bavaria). The coil performance was validated at signal-to-noise ratio (SNR) measurements for both T1 and T2-weighted fast spin-echo images. Efficient fat suppression proved valuable for diminishing moving artifacts in body imaging as well as subcutaneous fat signal next to a surface coil.

Results: Generally, the animal scans were sufficiently resolved to depict the anatomical detail in question non-invasively. Experimentally inoculated brain tumors and small structures (eg., inner ear) became visualized in submillimeter dimensions in T1W and T2W contrast with and without contrast enhancement. While a high image resolution of 78µm in-plane and 500µm thru-plane as well as good CNR could be achieved using 2D spin-echo techniques in conjunction with the receiver surface coil, it was more time-consuming than 3D magnetization-prepared gradient-echo imaging. In abdominal scans, for example, gastrointestinal and liver tumors became sufficiently detectable at in-plane resolutions of typically 260µm (FOV 100mm, matrix 384×384) and slice thickness of 1–2mm. Examination time in the anesthetized animals could always be kept at less than 10min. Chemical shift-selective fat saturation provided a ready means to improve image quality and afforded a homogeneous image impression by diminishing motion artifacts at large.

The typical exponential signal drop associated with the surface coil resulted in an inhomogeneous image impression, mainly, because of the high subcutaneous fat signal. Since fat contrast was mostly not valuable, especially with repect to identifying edema or fluid in T2-weighting, efficient fat saturation aliviated this problem substantially, in addition to applying surface coil intensity correction filters. In contrast, imaging with the volume resonators showed a better image homogeneity per se. While the small one had a better SNR than the larger, it accommodated only mice and young rats, and did not show the versatility of the surface coil.

Discussion: Animal scanning in a clinical whole-body MR scanner at 3.0-T in combination with custom-sized animal rf coils can be a ready means for answering many questions in experimental research. Exploitation of the high SNR and increased frequency dispersion of fat and water at high field with modern imaging tools provides non-invasively for adequate imaging material to answer pertinent morphologic questions. Thus, expensive extra-investment in animal scanners often may not be warranted. And, on the other hand, clinical ultra-highfield scanners could be used with a more efficient workload, especially at night.