Passive tracking exploiting local signal conservation: The white marker phenomenon

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 In endovascular interventional MRI, consistent and reliable tracking of the inserted devices is one of the major requirements for the success of an MR-guided endovascular intervention. In the past, several methods have been suggested and shown valuable. In the active approach a combination of small catheter mounted receiver coils and readout-gradients along the coordinate axes can be used to determine the actual position of the coil (1). This method is very time-efficient because only three readouts are needed for coil localization. However, a significant drawback of this active approach is the yet unsolved problem of unacceptable potential heating of long connecting signal cables (2).Passive tracking is not subject to heating problems. In this approach, small paramagnetic rings are mounted as markers on catheters and guidewires (3). These paramagnetic rings produce local field distortions, which show up as areas of signal loss in gradient-echo (GE) imaging. A disadvantage of passive tracking is that it is image-based, resulting in a relatively time-consuming tracking scheme. Furthermore, this tracking method is often hampered by the need for subtraction due to weak negative contrast of the passive markers to their background, especially if thick imaging slices are used. This subtraction leads to an undesired increased sensitivity to motion and flow artifacts.The passive tracking approach would significantly improve if the described disadvantages could be overcome. In this article, we present a novel approach to passive tracking using positive contrast of the markers to their background, so-called "white marker tracking." The positive contrast results from dephasing the background signal with a slice gradient, while near the marker the signal is conserved because the dipole field induced by the marker compensates the dephasing gradient. The idea of local gradient compensation is well known in the literature (4 -6) and has mostly been used in functional MRI experiments to recover signal loss in brain regions that suffer from global field inhomogeneities due to air cavities. In this article, the idea of gradient compensation is exploited for the depiction and tracking of p...

Morphing steady‐state free precession

“…[5]), but peak signal intensities indicate properly adjusted frequencies; i.e., frequency matching (Eq. [6]). As a result, morphing SSFP is not prone to off-resonance-related image degradations that may hamper proper localization of marker materials.…”

“…It has found application in the passive tracking of interventional devices by the enhancement of susceptibility artifacts from markers over the background (5,6). An extrinsic gradient field dephases background signals, whereas marker-related local gradients may compensate for the dephasing to restore the signal to its original amplitude.…”

MR‐guided intravascular interventions: Techniques and applications

“…To visualize instruments equipped with susceptibility markers (eg, dysprosium oxide rings) projection imaging can be combined with an incomplete refocusing of the transverse magnetization. Here, use is made of the fact that the markers create strong static gradients that are selectively compensated by the imaging gradients, so that signal is only acquired from the vicinity of the markers (white marker phenomenon) (35,36).…”

Adiabatic pulse preparation for imaging iron oxide nanoparticles

“…Methods including the white marker method that refocuses the macroscopic signal using unbalanced gradient (8) and spectrally selective radio frequency pulses that excite and refocus the off resonance signal (9) produce signal enhancement extending beyond the volume producing the contrast. Although susceptibility mapping is also being developed, it requires multiple orientations and may not be readily amenable for biomedical applications (10).…”