Passive tracking exploiting local signal conservation: The white marker phenomenon

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Abstract
This article presents a novel approach to passive tracking of paramagnetic markers during endovascular interventions, exploiting positive contrast of the markers to their background, so-called “white marker tracking.” The positive contrast results from dephasing of the background signal with a slice gradient, while near the marker the signal is conserved because a dipole field induced by the marker compensates the dephasing gradient. Theoretical investigation shows that a local gradient induced by the local dipole field will nearly always cancel the dephasing gradient somewhere, regardless of marker composition, gradient strength, orientation, and acquisition parameters. The actual appearance of the white marker is determined by the marker strength, echo-time, slice thickness, and gradient strength, as shown both theoretically and experimentally. The novel concept is demonstrated by tracking experiments in a flow phantom and in pig models and is shown to allow reliable and robust depiction of paramagnetic markers with positive contrast and significant suppression of the background signal. Magn Reson Med 50:784–790, 2003.