Addressing Patient-Related Artifacts: Implants & Motion
Ives R Levesque1
1McGill University, Canada

Synopsis

This lecture will review the manifestations, fundamental physics, and mitigation strategies of patient-related artifacts, namely those that arise from motion and from implants. In the first portion, artifacts arising from implants will be addressed, focusing on susceptibility induced signal dropout, image distortion, and signal displacement. In the second portion, we will turn to the broad category of motion-related artifacts to explore how various sources of motion corrupt MR images. Possible “confound” artifacts will be included. Approaches for mitigation of these artifacts will be discussed, emphasizing the practical tradeoffs and the latest solutions proposed in research.

Learning Objectives

At the end of this lecture, audience members will be able to:
  1. Recognize manifestations of patient-related artifacts such as implants and motion.
  2. List approaches for mitigation of these artifacts.

Introduction

Artifacts in MRI arise from a variety of sources and may have deleterious effects on image quality that negatively impacts the diagnostic potential of the images [1,2]. Artifacts can have a measurable impact on throughput and overall performance of an imaging clinic [3]. Among these sources of artifacts [4] are hardware (such as RF interference), sampling (such as wrap-around), field inhomogeneity effects [5], magnetic susceptibility (e.g. [6]), and finally, motion [7]. Each artifact typically has an explanation rooted in MR physics, from mitigation strategies have been derived. Certain artifacts, such as phase artifacts due to flow or other motion, can exploited to derive new imaging approaches [8].

Patient related artifacts

Image artifacts may arise from the patient, namely those related to implants and from patient motion. Understanding the fundamental physics of these artifacts can help us more easily understand their manifestations and aspire to correctly identify them. Artifacts can be specific to anatomical regions under investigation (e.g. [9,10,11]) or critical for select populations [12]. There are a few examples of “confound” artifacts—i.e. those that may resemble hardware artifacts such as RF zipper artifacts—and distinguishing them can be important to identifying the source and taking corrective action. Other artifacts may mimic abnormalities [13].

Artifacts due to implants

Artifacts arising from implants are most commonly characterized by signal dropout, image distortion, and signal displacement. The source of many of these artifacts, magnetic susceptibility variations, are common and can be native to the body (such as the presence of air or bone) or the result of the presence of foreign objects or implants on the body. Artifacts due to such implants can be understood as arising from perturbations in any of the magnetic fields used for imaging, namely the static, gradient, and/or radio-frequency field. These perturbations corrupt the anticipated MR signal and result in loss or misplacement of information in the image. Implants containing various materials (plastics, metals and alloys, and silicone) will be discussed, along with the role of size and shape. Building on an understanding of these basic principles, imaging approaches exist to reduce the impact of susceptibility variations (e.g. [14,15]). Safety of imaging in the presence of implants is an important related concern [16].

Artifacts due to motion

In the broad category of motion-related artifacts, various forms of motion corrupt MR images. Motion artifacts arise from deviations from the expected data in k-space, which upon reconstruction propagate to unanticipated errors in the image [7]. The main causes of motion artifacts include involuntary or voluntary bulk movement of parts of the anatomy. This also includes other involuntary movements of portions of the anatomy such as swallowing and peristalsis, and the periodic cardiac and respiratory motions. A final source of motion giving rise to artifacts is that of fluid flow throughout the body, such as blood, cerebrospinal fluid, and urine.
As summarized in the literature [7], motion artifacts mostly show up as "ghosts" or unwanted signal features often repeated across images. Sequential k-space trajectories are relatively insensitive to slow continuous motion, and tracking translation as a function of k-space location is predictive of artifact appearances. Quick motion is generally more important in terms of artifacts, especially when motion occurs during the collection of data around the centre of k-space. Periodic motion, and its repeated deviations in k-space data, generates artifacts with periodic features, while random motion results in aperiodic artifacts. The special case of flow artifacts, which are related to shifts in the voxel magnetization and phase angle accumulation [17].
Mitigation strategies include those aimed at reduction of motion (patient preparation and cooperation, immobilization, pharmaceutical interventions), and technical methods aimed at mitigating effects on image acquisition and/or providing information for correction of motion (pulse sequence and trajectory design, gating, triggering, navigators). The tradeoffs and pitfalls of these approaches can include longer scan times (e.g. when triggering), compromised patient comfort (e.g. breath-hold), or compromises in image appearance, such as limited contrast control flexibility.

Acknowledgements

Acknowledgments to Jonathan Kalinowski for portions of the research into motion artifacts, and to Laurian Rohoman, Véronique Fortier, and Evan McNabb for useful discussion.

References

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Proc. Intl. Soc. Mag. Reson. Med. 30 (2022)