Cross-excitation artifact (MRI)

Case contributed by Elmira Hassanzadeh


History of metastatic colon cancer. New liver lesion found on ultrasound, presents for further MR workup.

Patient Data

Age: 70 years
Gender: Female

On the coronal scout, the lumen of the gallbladder is sharply divided into low signal medially and high signal laterally with a "level" appearance suggesting layering sludge. However, the axial T2W shows no sludge in the gallbladder. This finding is attributed to cross-excitation artifact. This was an incidental finding.

The main area of concern was in segment 6 metastasectomy bed which shows nonspecific hyperintensity on axial T2 fat sat images.

Annotated coronal scout shows that the field of view (FOV) of the sagittal scout includes the medial half of the gallbladder, exactly matching the area of hypointensity.

Case Discussion

There are a few explanations for this finding:

  1. If a patient with sludge has been in the left decubitus position before the scan, the sludge may still be layering medially in the initial sequences, and slowly layers posteriorly during the rest of the exam. However, the lack of sludge in the subsequent T2 images in this patient excludes this possibility.  
  2. The other possibility, which is presumed to be the case here, is the cross-excitation artifact. This artifact happens when a tissue, which is currently being imaged, can not produce a signal because it is still under the influence of a prior excitation RF pulse from a different acquisition. In this case, the scouts were acquired in the following order: axial, sagittal and finally coronal. The sagittal FOV involved the medial gallbladder. Therefore, the immediate subsequent RF pulse of the coronal acquisition will result in a low signal in the medial gallbladder and can mimic sludge.  

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