Other Finding — Signal Void
Signal void is the absence of signal due to susceptibility artifact, typically from a metallic biopsy clip, wireless localizer, or other ferromagnetic/paramagnetic foreign body. It is classified under Other Findings — Typically Benign in the BI-RADS v2025 MRI lexicon.
Definition
A signal void appears as a focal area of complete signal loss caused by local magnetic field inhomogeneity. The susceptibility difference between metal and surrounding tissue distorts the local magnetic field, causing rapid dephasing of nearby protons and loss of signal.
Common Causes
| Source | Context | Typical Appearance |
|---|---|---|
| Biopsy marker clip | Post-biopsy marker placement | Small focal void at known biopsy site |
| Wireless localizer | Pre-operative localization | Void within or adjacent to target lesion |
| Surgical clips | Post-lumpectomy or post-excision | Multiple clustered voids at surgical bed |
| Metallic foreign body | Incidental (e.g., shrapnel, wire fragment) | Variable size/shape void |
Imaging Appearance
T1-Weighted (Pre- and Post-Contrast)
- Complete signal loss at the location of the metallic object
- Surrounding blooming artifact — the void appears larger than the actual object
- On fat-suppressed post-contrast T1W, the void is sharply demarcated against enhancing tissue
T2-Weighted
- Signal void persists; may appear even larger due to greater susceptibility effect on T2/T2*-weighted sequences
- Blooming artifact is sequence-dependent — gradient echo sequences (T2*) show the most pronounced artifact
DWI
- Signal void persists; can cause geometric distortion in adjacent tissue on EPI-based DWI sequences due to susceptibility
Board Pearl
Signal voids appear larger than the actual metallic object due to blooming artifact. The size of the void depends on the sequence — gradient echo (T2) sequences* exaggerate the void most, while spin echo sequences minimize it. This is because spin echo refocuses static field inhomogeneities.
Clinical Utility
- Biopsy clip correlation: Confirms the location of a prior biopsy site — essential for correlating MRI findings with prior mammographic/ultrasound-guided biopsies
- Surgical planning: Signal voids from wireless localizers confirm device position relative to the target lesion
- Post-surgical assessment: Multiple clustered signal voids at a lumpectomy site represent surgical clips and help identify the operative bed
- Concordance check: The signal void from a biopsy clip should be at or near the biopsied finding — if displaced, consider clip migration
Board Pearl
When a signal void from a biopsy clip is not at the expected location of a previously biopsied finding, consider clip migration. This is clinically significant because it may lead to failure to excise the correct lesion at surgery. Always correlate clip position on MRI with the original biopsy imaging.
Pitfalls
- Obscured adjacent enhancement: The blooming artifact from a signal void can mask or distort enhancement of adjacent tissue, potentially hiding residual or recurrent disease near a biopsy or surgical site
- Misidentification as calcification: Signal voids are not calcifications — calcifications are typically too small to cause visible susceptibility artifact on standard breast MRI
- Over-calling artifact as pathology: Susceptibility artifact can distort signal in adjacent tissue, creating apparent signal abnormalities that do not represent true findings
- Clip in wrong location: A signal void distant from the expected biopsy site should raise concern for clip migration, not be dismissed as incidental
Differential Diagnosis
| Entity | Key Distinguishing Feature |
|---|---|
| Signal void (metallic) | Complete signal loss, blooming artifact, known clip/device history |
| Flow void (vessel) | Tubular morphology, follows vascular anatomy |
| Air (post-biopsy) | Transient, resolves on follow-up, associated with recent procedure |
| Calcification | Usually not visible as signal void on standard breast MRI sequences |
Reporting
- Signal voids from biopsy clips or surgical clips should be reported when relevant to clinical correlation (e.g., confirming biopsy site, post-lumpectomy assessment)
- Per BI-RADS v2025, benign reported findings may include evidence of prior surgeries and biopsy markers — these help document procedural history
- Not every signal void needs detailed description — context determines reporting necessity
Board Pearl
In the post-neoadjuvant chemotherapy setting, the signal void from the biopsy clip placed before treatment is the key landmark for identifying the original tumor bed, especially when there is complete imaging response. Without the clip, the treatment site may be impossible to localize for surgery.
Related
- Other Findings Typically Benign
- Other Finding — Fat Necrosis
- Other Finding — Postoperative Collections
- Masses
- Non-Mass Enhancement
- Dynamic Contrast Enhanced MRI
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Page expanded from 29 lines to ~95 lines. Note: the source text you provided was about rectal cancer staging (Kaur et al.), which is unrelated to signal void — I used the actual BI-RADS v2025 source from `raw/sections/MRI_birads2025/` (page 140) plus domain knowledge to enrich the page.
Key additions: Definition with MR physics explanation, Common Causes table, full Imaging Appearance across sequences (T1, T2, DWI), Clinical Utility expanded with concordance/migration, Pitfalls section, DDx table, Reporting guidance, and 3 Board Pearls (blooming artifact physics, clip migration, neoadjuvant clip landmark).
Would you like me to retry the file write, or should I adjust the content?