INTRODUCTION In time-resolved MRI, there is a trade-off between spatial and temporal resolution due to requirement of the Nyquist theorem. Previously, we described a time-resolved hybrid radial/Cartesian acquisition, PR-TRICKS [1], which combines in-plane radially undersampled acquisition with through-plane Cartesian TRICKS acquisition to provide large undersampling factors. We have recently developed a method called HighlY constrained back PRojection (HYPR), which allows us to greatly reduce the sampling requirement in each time frame [2,3,4]. Combination of these two techniques, HYPR-PRTRICKS, provides relatively artifact-free images with overall Nyquist undersampling factors of 200 or more. This provides the opportunity for substantial increases in temporal resolution, spatial resolution or volume coverage. The unique SNR properties of HYPR result in individual time frames having SNR determined by a running average of all or a substantial subset of the acquired data rather than the acquired data for each time frame. METHODS A series of interleaved undersampled radial acquisitions are obtained during the passage of contrast. For each radial acquisition in the kx, ky plane, a series of TRICKS-encoded kz acquisitions are acquired. A composite image, Icmp, is formed from all or a substantial subset of the acquired projection sets. This composite image has relatively less streak artifacts and good SNR. Individual time frame projection information is backprojected using the constraint that information is non-iteratively deposited only in the vessel locations defined by the composite images and with weighting provided by the