Hurricane sign | Radiology Signs

Answer:

The hurricane sign is a swirling, curved distortion of the myocardial wall on reconstructed myocardial perfusion SPECT images, produced by patient motion during acquisition rather than by disease. Motion is one of the commonest sources of artefact in cardiac SPECT and can create apparent fixed or reversible perfusion defects, most often inferior or apical, that lead to false-positive interpretation. Detection is systematic and should be part of every read: the raw projection cine must be reviewed before the reconstructed images, looking for vertical or lateral shifts, upward creep of the heart as the lungs empty after exercise, and abrupt jumps, and the sinogram and linogram should be inspected for discontinuities. Motion correction software can compensate for small in-plane shifts, but substantial motion requires reacquisition. The other artefacts that must be separated from true disease are soft tissue attenuation, breast attenuation producing anterior defects and diaphragmatic attenuation producing inferior defects, which are recognised by preserved wall motion and thickening on gated images and can be corrected with CT-based attenuation correction or resolved with prone imaging; subdiaphragmatic tracer producing ramp filter artefacts; and reconstruction and misregistration errors when CT attenuation maps are misaligned. Gated analysis is central, since a genuine perfusion defect with preserved regional function raises the suspicion of artefact.

Why is it called so?

The reconstructed myocardium is smeared into a curved, swirling shape resembling a hurricane on a weather map.

Pathophysiology

SPECT reconstruction assumes that the object is stationary through the entire arc of projections. When the patient moves, the projections are inconsistent with a single object position, and filtered back projection or iterative reconstruction distributes the mismatched counts along incorrect lines, blurring and displacing the myocardial wall into a curved smear. Upward creep is a particular problem after exercise stress: the diaphragm gradually descends as respiration normalises, so the heart drifts downward through the acquisition, producing a progressive rather than abrupt shift that is easily missed on casual review. Because the resulting count deficit mimics a regional perfusion defect, and because attenuation and motion artefacts have preferred territories, correlation with gated wall motion and with the raw data is what protects against a false diagnosis.

Alternative names: Motion artefact; swirl artefact of cardiac SPECT

Other associated named signs: Breast and diaphragmatic attenuation artefacts, upward creep and the ramp filter artefact from subdiaphragmatic activity

References

1. Nichols KJ, Van Tosh A. Rotating and stationary SPECT system patient motion myocardial perfusion artifacts. J Nucl Cardiol. 2019;26(4):1323-1326. PMID: 29542014.