Principles and applications of the balanced steady-state free precession sequence in small animal low-field MRI.

Magnetic resonance imaging (MRI) in small animal practice is largely based on classic two-dimensional spin-echo, inversion recovery and gradient-echo sequences which are largely limited by low spatial resolution, especially in low-field (LF)-MRI scanners. Nowadays, however, the availability of volumetric sequences can open new perspectives and enhance the diagnostic potential of this imaging modality. Balanced steady-state free precession (bSSFP) is a three-dimensional gradient-echo sequence in which image contrast is given by the ratio of T2 and T1, resulting in low soft-tissue signal, poor cerebral grey/white matter distinction and a bright signal from free fluid and fat. Such properties, along with a high signal-to-noise ratio and a very high spatial resolution deriving from acquisition of contiguous blocks of data, make this sequence perfectly suited for morphologic imaging, particularly for fluid-containing structures. Although bSSFP is widely adopted in human medical imaging, the use of this sequence in veterinary radiology is limited to anatomic studies of the inner ear and quadrigeminal cistern. This review aims to discuss the technical background of the bSSFP sequence and its possible advantageous applications in small animal LF-MRI for different specific disorders of the spine (arachnoid diverticula, small disc herniation, facet joint synovial cysts), brain (supracollicular fluid accumulation, traumatic injuries) and ligaments (complete and partial tears).