Variable field of view for spatial resolution improvement in continuously moving table magnetic resonance imaging.

Interest in whole-body MRI has been recently growing (1–4). In particular, acquisition using continuously moving table (CMT) has been studied for imaging of the peripheral vasculature in 3D contrast-enhanced MR angiography (CE-MRA) (2,5,6). The motivation to use CMT for “runoff” CE-MRA studies naturally arises when one considers that the peripheral vasculature from the renal artery origins to the ankles spans a distance greater than 100 cm and extends beyond the homogeneous volume of all commercially available MRI scanners. For peripheral CE-MRA, a CMT technique is an alternative to a multistation approach in which a data set is acquired at each of several overlapping stationary table positions after contrast injection (7–14). Lateral (right/left) and slice (anterior/posterior) spatial resolution in coronal angiograms become critical as vessel diameters in the peripheral vasculature approach several millimeters or less. In CMT, spatial resolution is intrinsically limited by the actively sampled field of view (FOV), the repetition time TR, and the table velocity used (2). Investigators have recently studied this limitation with parallel imaging using stationary (15) and continuously moving (16) coils as well as matching the table velocity to the known variable velocity of the contrast bolus in the peripheral vasculature (17), where slower contrast velocities in the distal vessels allow reduced table speeds and more sampled views, thus improving spatial resolution (18). Multistation approaches offer the flexibility of tailoring FOV, slice thickness, and the imaging matrix size to the vasculature of interest at individual stations (12–14). Spatial resolution can be optimized on a station-by-station basis by selecting the appropriate values. On the other hand, CMT FOV values have thus far been held constant over the entire longitudinal FOV, with the lateral and slice FOV typically determined by the widest portion of the peripheral vasculature over the extended anatomy. This approach can be inefficient, as unnecessary scan time is spent in regions where the object extent in either the right/left or the anterior/posterior direction is considerably smaller than the prescribed volume. This work tests the hypothesis that, compared to a fixed FOVY, CMT techniques can benefit from improved lateral spatial resolution by tailoring the FOVY to the width of the anatomy along the extended longitudinal FOV. Specifically, we allow for the switching from one lateral FOV to another by introducing a dual-FOV CMT method. The FOVY transition occurs dynamically during table motion at an operator-specified location along the longitudinal axis. Improvement in lateral spatial resolution is achieved by maintaining the sampling matrix size while reducing FOVY at appropriate locations. Raw data from a dual-FOV CMT technique do not allow for simple Fourier reconstruction, because of the mixing of ΔkY intervals from two different FOVY values. We describe a reconstruction algorithm for such mixed data and examine the hypothesis using hybrid k-space analysis, phantom experiments, and volunteer studies.