Implementation of a reduced field‐of‐view method for dynamic MR imaging using navigator echoes

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Abstract
A new technique was designed and implemented that increases imaging speed in dynamic imaging in which change is restricted to a fraction of the full field of view (FOV). The technique is an enhancement of a reduced FOV method first reported by Hu and Parrish. This enhancement extends the use of the Hu and Parrish method to cases in which there is cyclic motion throughout the entire FOV that normally would be aliased into the reduced FOV. This method requires the initial acquisition of a number of baseline k‐space data sets to characterize the background physiological motion during imaging. Projection navigator echoes along both the phase‐ and the frequency‐encoded directions are acquired and used to correct for motion outside the reduced FOV. Automatic placement or repositioning of the updated fraction of the FOV using navigators also is investigated. With this method, when using a 32‐echo rapid acquisition with relaxation enhancement (RARE) sequence, single‐shot updates of T2‐weighted, 128 × 128 pixel images are obtained, yielding a fourfold increase in temporal resolution compared to full k‐space update methods.