Rapid-motion-perception based cardiac navigators: using the high flow blood volume as a marker for the position of the heart.

Navigators have been developed as one of the many approaches to reducing motion artifacts due to respiration. A typical navigator approach applies a pencil-beam style profile crossing the diaphragm to track the superior-inferior (SI) motion of the diaphragm, and subsequently applying correlations to determine the heart location. This approach necessitates a priori knowledge of the correlation coefficients between heart and diaphragm motion, a variable parameter among patients. This paper presents an alternative navigator method based on Rapid Motion Perception (RaMP). This method acquires profiles of the ventricular blood volume based on its high flow velocity. The position of the blood volume is a direct representation of the position of the heart. This method allows cardiac navigation in two orthogonal directions simultaneously, eliminates the need to obtain correlations to the diaphragm motion, and increases tracking reliability for individual patients. A prospective version of RaMP navigators has been implemented on a clinical 1.5 T scanner, and preliminary tests on human volunteers show that this method can successfully track the heart position over the entire respiratory period. This navigation scheme is tested for predicting superior-inferior and anterior-posterior (AP) motion of the heart for breath-hold and free breathing conditions. Bland-Altman plots comparing the motion predicted by the navigators and that computed from single-shot images immediately following the navigators, show that the accuracy of this method is +/- 1.43 mm in the SI direction and +/- 0.84 mm in the AP direction. The RaMP navigator is suited for real-time tracking of the bulk translational motion of the heart.