Physics of MRI Artefacts

Version 1.0 — July 2026

1. Executive Summary

This page covers, in depth, the eighth of the eleven planned topic groups listed in Section 4.8 of the MRI Physics — Fundamentals and Principles master page: the physical mechanisms behind five artefacts that recur across essentially every anatomical region and protocol documented on MRIninja — susceptibility artefact, aliasing/wrap-around, truncation (Gibbs) ringing, RF-related artefacts, and the magic angle effect. Flow- and motion-related artefacts, already covered in depth in the companion Flow, Motion, and Diffusion Physics child page, are not repeated here. Every artefact on this page has a specific, well-characterised physical origin — none of them are unexplained scanner quirks — and understanding that origin is what allows an artefact to be correctly recognised as artefact, rather than mistaken for genuine pathology, across the anatomical protocol pages elsewhere on MRIninja.

2. Relationship to the Master Page

The master page introduces MRI artefact physics only as a listed topic heading (Section 4.8), without detail. This page assumes, as direct prerequisites, the companion Relaxation Phenomena child page’s treatment of T2* and field inhomogeneity, the companion Signal Localisation and Image Formation (k-Space) child page’s treatment of k-space and the Fourier transform, and the companion RF Pulses and Pulse Sequence Physics child page’s treatment of frequency-selective RF pulses. What this page adds: the specific physical chain of causation linking each of these five named artefacts to the underlying physics already established elsewhere in this cluster, distinguishing artefacts that share a superficial appearance but have genuinely different physical origins.

3. Susceptibility Artefact

3.1 Physical Origin — Magnetic Susceptibility Variation

Every material — including every tissue type in the human body, and every object placed near the scanner — has a magnetic susceptibility: a physical property describing how strongly it becomes magnetised in response to an external field. Schenck’s comprehensive 1996 review formalised the practical classification of MRI-relevant susceptibility behaviour, distinguishing materials whose susceptibility is close enough to that of tissue and water to produce negligible field distortion from those whose susceptibility differs enough to produce clinically significant local field distortion [1]. Because different tissue types (and, most dramatically, interfaces between tissue and air, tissue and bone, tissue and metal, or tissue and haemorrhage/calcification products) have measurably different magnetic susceptibility, the local magnetic field near such interfaces is distorted from its nominal, intended value — directly perturbing the local Larmor frequency, exactly as introduced in the companion Relaxation Phenomena child page’s discussion of T2′ and field inhomogeneity (Section 5.2 there), but now considered as a spatially localised, interface-specific phenomenon rather than a general contributor to T2* decay.

3.2 Consequences — Signal Loss and Geometric Distortion

Susceptibility-related field distortion produces two distinct, simultaneously occurring practical effects. First, the accelerated local dephasing described in Section 3.1 produces localised signal loss, most pronounced on T2*-weighted gradient-echo sequences (which, as established in the companion RF Pulses and Pulse Sequence Physics child page, have no refocusing pulse to actively reverse this field-inhomogeneity-driven dephasing) and correspondingly much less pronounced on spin-echo-family sequences. Second, because spatial position is assigned based on local Larmor frequency (as established in the companion Signal Localisation and Image Formation (k-Space) child page), the same local frequency shift that causes signal loss also causes genuine geometric mislocalisation — pixels near a strong susceptibility interface are displaced from their true anatomical position — a distinct but physically related consequence of the same underlying field distortion.

4. Aliasing / Wrap-Around Physics

4.1 The Sampling Theorem and the Nyquist Criterion

Any system that samples a continuous signal at discrete intervals — exactly the situation in MRI, where k-space is sampled at discrete points rather than continuously, as established in the companion Signal Localisation and Image Formation (k-Space) child page — is subject to a fundamental mathematical constraint first rigorously established in the general signal-processing literature by Harry Nyquist in 1928: a signal can be unambiguously reconstructed from its samples only if it is sampled at a rate at least twice its highest frequency component [2]. Applied to MRI’s specific spatial-frequency (k-space) sampling, this becomes a direct geometric statement: the k-space sampling interval must be no coarser than 1/FOV (field of view) along a given encoding direction, or spatial frequencies originating from anatomy actually located outside the nominal field of view become mathematically indistinguishable, after Fourier transformation, from spatial frequencies properly originating from within it.

4.2 Wrap-Around as a Direct Consequence

When the Nyquist criterion described in Section 4.1 is violated — anatomy extends beyond the encoded field of view along the phase-encode or frequency-encode direction — the reconstructed image does not simply crop or blank the excess anatomy; instead, the excess anatomy’s signal is mathematically indistinguishable from signal originating from within the field of view, and appears wrapped around to the opposite edge of the image, superimposed on genuinely intended anatomy. Because the phase-encode direction (introduced in the companion Signal Localisation and Image Formation (k-Space) child page) is sampled discretely, one line at a time, aliasing is generally more prominent along phase-encode than frequency-encode, where oversampling the continuously-sampled readout signal beyond the nominal Nyquist rate is comparatively cheap and is, in practice, done routinely — the direct physical reason frequency-encode-direction aliasing is rarely a clinical problem while phase-encode-direction aliasing remains a genuine, actively-managed concern, addressed practically (phase oversampling, appropriately sized field of view) in the MRI Parameters cluster.

5. Truncation (Gibbs) Ringing

5.1 Physical and Mathematical Origin

Because any real, finite acquisition samples only a finite region of k-space rather than the theoretically infinite extent that would be required to represent a perfectly sharp image edge exactly (per the Fourier-transform relationship between k-space and image space established in the companion Signal Localisation and Image Formation (k-Space) child page), the reconstructed image of any genuinely sharp intensity transition — most classically, at high-contrast tissue boundaries such as the spinal cord/CSF interface or the boundary of a syrinx — necessarily shows a characteristic pattern of parallel, alternating bright and dark bands adjacent to the true edge, a direct mathematical consequence of abruptly truncating an infinite Fourier series at a finite number of terms, a phenomenon mathematically identical (though applied here to two-dimensional spatial data rather than the one-dimensional mathematical series it was first described for) to the general Gibbs phenomenon in Fourier analysis.

5.2 Determinants of Severity

Truncation ringing’s prominence depends directly on two factors that follow logically from Section 5.1: it is more pronounced with higher intrinsic image contrast at the relevant boundary (a larger abrupt intensity transition requires more high-spatial-frequency k-space content to represent sharply, per the k-space centre/periphery relationship introduced in the companion Signal Localisation and Image Formation (k-Space) child page), and it is more pronounced with coarser spatial resolution (fewer acquired phase-encode lines/k-space extent means the Fourier series is truncated earlier, at a lower maximum spatial frequency) — the direct physical reason truncation artefact is a well-recognised, specifically-named pitfall in spinal cord imaging (where high cord/CSF contrast combines with comparatively modest in-plane resolution) more than in most other anatomical contexts.

6. RF-Related Artefacts and the Magic Angle Effect

6.1 RF-Related Artefacts — the Zipper Artefact

Because the receiver is tuned to detect signal specifically at and immediately around the expected Larmor frequency (introduced in the companion Fundamentals of Nuclear Magnetic Resonance child page), any stray, unintended RF energy at or near that frequency — from imperfect RF shielding of the scan room, electronic interference from nearby equipment, or an unintentional resonance within the scanner’s own RF chain — is detected by the receiver exactly as though it were genuine MR signal. Because such stray RF energy is typically present at a single, narrow frequency rather than distributed smoothly across the expected signal bandwidth, it appears in the reconstructed image as a characteristic thin line of alternating bright and dark pixels running along the phase-encode direction, at the specific spatial position corresponding to its interference frequency — the “zipper” artefact, a direct, diagnostic-quality indicator of an RF shielding or interference problem rather than a normal, expected imaging phenomenon (distinguishing it from the other artefacts on this page, all of which occur routinely even with fully functioning hardware).

6.2 The Magic Angle Effect

The magic angle effect has a genuinely distinct physical origin from every other artefact on this page: it arises from the anisotropic nature of dipolar coupling in highly organised, collagen-rich tissue (tendons, ligaments, and similar structures), where water molecules are tightly bound in a restricted geometric relationship to collagen fibrils. Erickson and colleagues’ original 1991 clinical description demonstrated that when such a structure is oriented at approximately 54.7° (the “magic angle,” the specific angle at which the standard dipolar coupling term (3cos²θ − 1) equals zero) relative to B0, this dipolar coupling’s normally dominant contribution to rapid T2 decay is eliminated, causing an artefactual T2 lengthening and correspondingly increased signal on short-TE sequences — signal that can closely mimic the appearance of genuine tendinopathy or partial tear [3]. Because this effect depends on a genuine, real physical interaction (dipolar coupling) rather than a hardware or sampling artefact, it is not eliminated by any hardware or acquisition parameter adjustment in the way aliasing or truncation ringing can be — it is instead recognised and distinguished from true pathology primarily by its characteristic dependence on both anatomical orientation and echo time (the artefact diminishes or disappears on longer-TE sequences), discussed practically in the relevant musculoskeletal anatomical protocol pages elsewhere on MRIninja.

7. MRI Technologist and Radiologist Pearls — Common Misconceptions

• “Susceptibility artefact and aliasing look similar, so they’re probably related phenomena.” They have entirely unrelated physical origins (Sections 3 and 4) — susceptibility artefact arises from genuine local field distortion at tissue interfaces, while aliasing arises from a k-space sampling-rate violation; their superficially different but sometimes confusable appearances should not be taken as evidence of a shared mechanism.
• “Truncation ringing means something is wrong with the scan.” It is a normal, expected, mathematically inevitable consequence of any finite k-space acquisition at a sharp, high-contrast boundary (Section 5) — not evidence of a technical fault, and it can be reduced (though never entirely eliminated) by increasing spatial resolution, but never by “fixing” the scanner.
• “A zipper artefact and other RF-chain-related patterns are just another normal imaging artefact to expect on every scan.” Unlike susceptibility, aliasing, truncation ringing, and the magic angle effect — all of which occur routinely even with fully functioning, well-maintained hardware — a genuine zipper artefact (Section 6.1) specifically indicates an RF shielding or interference problem that generally warrants site engineering investigation, not routine acceptance.
• “The magic angle effect can be eliminated by adjusting acquisition parameters like a typical artefact.” Because it arises from a genuine physical interaction (dipolar coupling) dependent on anatomical geometry relative to B0, it cannot be eliminated by parameter adjustment in the way aliasing (oversampling) or truncation ringing (higher resolution) can — it is managed through recognition (echo-time dependence, characteristic distribution) rather than elimination.

8. Practical and Clinical Relevance

Recognising each of these five artefacts by its specific physical signature — rather than treating “artefact” as a single undifferentiated category — is directly relevant to correct image interpretation throughout MRIninja’s anatomical protocol pages: susceptibility artefact recognition is essential near metal, air-tissue interfaces, and haemorrhage; aliasing recognition and prevention (adequate FOV, phase oversampling) is a routine protocol-design consideration documented in the MRI Parameters cluster; truncation ringing recognition is specifically emphasised in spinal cord imaging; zipper artefact recognition is a site-quality-assurance signal distinct from routine patient-related artefact; and magic angle effect recognition is a specifically emphasised pitfall throughout musculoskeletal tendon and ligament imaging.

10. Evidence Gaps and Ongoing Debate

• Optimal susceptibility-artefact correction methodology remains an active research area. As referenced in Section 9.1, numerous distinct susceptibility-correction and distortion-mitigation techniques exist, with ongoing development (particularly for high-field and near-metal imaging) rather than a single settled, universally adopted approach.
• Quantitative characterisation of the magic angle effect’s clinical significance across tissue types. While the magic angle effect (Section 6.2) is well established in tendon and ligament imaging, its degree of clinical significance and optimal management in less classically studied collagen-rich structures (some peripheral nerve and intervertebral disc applications, for instance) remains a less thoroughly characterised area than the classic tendon/ligament literature.
• Zipper and RF-interference artefact source identification remains institution-dependent troubleshooting. Unlike the other artefacts on this page, which have standardised, well-characterised physical mitigation strategies, tracking down a specific zipper artefact’s source (Section 6.1) typically requires site-specific engineering investigation rather than a standardised physics-based diagnostic algorithm.

11. Evidence-Based References

A. Guidelines / Consensus / Society Recommendations

No dedicated society guideline exists for foundational MRI artefact physics as such — this is technical/physics literature rather than a clinical practice area subject to society guidance. Category A is therefore not populated for this child page.

C. Important Prospective / Original Studies

D. Technical MRI Papers

End of document — Physics of MRI Artefacts — Child Page under the MRIninja MRI Physics — Fundamentals and Principles master page — v1.0 — July 2026 Parent page: MRI Physics — Fundamentals and Principles