Highly constrained backprojection for improving dynamic 3He MR ventilation imaging in rats

The highly constrained backprojection algorithm (HYPR) has recently been shown to allow accelerated acquisition in various fields of MRI, including angiography, perfusion and diffusion imaging as well as hyperpolarized gas imaging. Increase in temporal resolution is of particular interest in the case of small animal ventilation imaging due to the high respiration rate. In the present study, the two-dimensional HYPR technique and its iterative version (I-HYPR) were applied to (3)He ventilation imaging in rats. Two imaging protocols were used for two separate groups of animals. A single inspiration protocol consisted of (3)He imaging of the lungs during gas inflow and a following apnea. A multiple inspiration protocol involved spontaneous breathing of (3)He contained in a gas reservoir. Series of HYPR frames with four-fold increase in the temporal resolution were obtained in the case of the single inspiration experiment. For the multiple inspiration protocol, series of HYPR images corresponding to four different echo times were obtained and were used to reconstruct T(2)(*) maps at the inspiration and the expiration phases of the breathing cycle. The feasibility of using the two-dimensional HYPR technique for different (3)He ventilation protocols in small animals is demonstrated. Image quality and signal kinetics representations are compared for two variants of the HYPR algorithm.