Real‐time rigid body motion correction and shimming using cloverleaf navigators

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Abstract
Subject motion during scanning can greatly reduce MRI image quality and is a major reason for discarding data in both clinical and research scanning. The quality of the high‐resolution structural data used for morphometric analysis is especially compromised by subject movement because high‐resolution scans are of longer duration. A method is presented that measures and corrects rigid body motion and associated first‐order shim changes in real time, using a pulse sequence with embedded cloverleaf navigators and a feedback control mechanism. The procedure requires a 12‐s preliminary mapping scan. A single‐path, 4.2‐ms cloverleaf navigator is inserted every repetition time (TR) after the readout of a 3D fast low‐angle shot (FLASH) sequence, requiring no additional RF pulses and minimally impacting scan duration. Every TR, a rigid body motion estimate is made and a correction is fed back to adjust the gradients and shim offsets. Images are corrected and reconstructed on the scanner computer for immediate access. Correction for between‐scan motion can be accomplished by using the same reference map for each scan repetition. Human and phantom tests demonstrated a consistent improvement in image quality if motion occurred during the acquisition. Magn Reson Med, 2006.

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