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Misleading changes of the signal intensity on opposed-phase MRI after injection of contrast medium
S H Heywang-Köbrunner1, H D Wolf, M Deimling
1Department of Diagnostic Radiology, Martin Luther University of Halle-Wittenberg, Halle, Germany.
Objective:
The effect of opposed-phase imaging on the interpretation of MR contrast studies is highlighted.
Materials And Methods:
A model calculation is performed. It demonstrates the change of signal intensity of an average tumor before and after application of Gd-DTPA on an in-phase and an opposed-phase image, depending on the percentage of fat within the voxels. The effect is then demonstrated, using a small cotton stick soaked with water or a solution of contrast agent representing a tumor before and after i.v. application of Gd-DTPA.
Results:
If an average enhancing tumor, which is surrounded by fat, occupies less than 50-60% of the slice thickness, it becomes undetectable on opposed-phase images. The reason is that due to signal cancellation on the opposed image, no signal change or even signal decrease results, while signal increase is visible on the in-phase image.
Conclusion:
In those areas of the body where significant partial volume of a tumor with fat may occur (such as for breast tumors growing along ducts, which are surrounded by fat), severe errors can result. Therefore we explicitly warn from using opposed-image sequences for MR contrast studies.