The Line That Follows the Frequency Axis
A 1.5 T scanner in a ground-floor suite has developed a problem. For the past week, every brain and cervical spine study shows a faint but distinct bright line and a corresponding dark line, always at the same position along the frequency-encoding axis, present on T1, T2, FLAIR, and DWI. The line is unchanged when the patient is repositioned, is present with both the 20-channel head coil and the body coil, is unaffected by whether the room door is open or closed, and appears on a phantom scan. The head coil was replaced two weeks ago for an unrelated preamplifier fault. The site recently installed a new patient-monitoring system and a new injector, and a neighboring tenant has begun operating heavy equipment on the other side of a shared wall.
1. Name the artifact and explain how its behavior constrains the differential diagnosis.
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This is a zipper artifact - narrowband radiofrequency interference received at a frequency offset from the Larmor frequency, reconstructed as a stripe perpendicular to the frequency-encoding direction at a fixed location.
What the behavior tells you:
- Same position on every sequence → the offset frequency is constant, so the source is narrowband and not patient- or sequence-dependent.
- Present on different coils → not a coil element or preamplifier failure; anything shared downstream of the coil (receive chain, cable routing, penetration panel, or the room's shielding) remains in the differential.
- Present on a phantom → the signal is entering the receive path independent of the body; it is not a patient-related or physiologic effect.
- Unchanged by door position → argues against a purely radiated leak at the door gasket, which typically worsens when the door is open. Conducted interference (entering on mains wiring through filters), an emitter inside the exam room, or a shield penetration elsewhere becomes more likely.
So: RF interference, most likely conducted or from a source inside the room, not a coil fault.
2. Outline the tests you would run, in order, to localize the source before calling service.
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- Noise-only acquisition: scan with the RF transmitter disabled (receive only). If the stripe is still there, the interference is genuinely being received and the problem is not the transmit chain.
- Equipment sweep: with a phantom in the bore and a fixed sequence, power down one suspect at a time - room lights, HVAC/inverter, the new patient monitor, the new injector, the scanner's own uninterruptible power supply, and any chargers. Reacquire after each change. The new monitor and injector were installed recently and are prime suspects.
- Mains/panel check: confirm nothing new has been tied into the room's electrical panel, and inspect the penetration panel and filter plate for a bypassed or removed filter and for new cables passed through the shield.
- Shield integrity: an RF cage test (or a sniff survey with a loop antenna and spectrum analyzer across the frequency range) localizes a breach or an external emitter - including through the shared wall.
- Receiver path swap: move the coil to a different port and cable to exclude a damaged receive cable acting as an antenna.
Log each test with a timestamp and a saved image; service and the medical physicist will need the trail, and the shield test should be repeated after any new penetration is found.
3. The source is traced to conducted interference on the building wiring. What are the interim and definitive actions, and what do you tell the radiologists?
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Interim:
- Do not run diagnostic reads on degraded images; zipper stripes can mimic or mask small lesions, and the artifact survives post-processing.
- If any scanning must continue, run with the offending circuit isolated and verify on a phantom before each list. If the stripe returns, stop.
- Document with images and a service ticket; escalate to the medical physicist and to facilities.
Definitive:
- Install or repair the appropriate line filters and confirm the penetration panel is intact.
- Have the shielding integrity re-tested and the results recorded before clinical scanning resumes.
- If the emitter is the tenant's equipment, the resolution is a facilities-level one (filtering, relocation, or shielding), not a scanner adjustment; scanning on that unit remains conditional until verified.
To the radiologists: the artifact is narrowband RF interference, not a patient or protocol problem; images acquired during the interval should be treated as degraded, and studies with a borderline finding may need to be repeated once the room is cleared. That framing prevents a technology problem from being read as clinical change.