MISLEADING CHANGES OF THE SIGNAL INTENSITY ON OPPOSED-PHASE MRI AFTERINJECTION OF CONTRAST-MEDIUM

Objective: The effect of opposed-phase imaging on the interpretation o f MR contrast studies is highlighted. Materials and Methods: A model c alculation is performed. It demonstrates the change of signal intensit y 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 cott on stick soaked with water or a solution of contrast agent representin g a tumor before and after i.v. application of Gd-DTPA. Results: If an average enhancing tumor, which is surrounded by fat, occupies less th an 50-60% of the slice thickness, it becomes undetectable on opposed-p hase images. The reason is that due to signal cancellation on the oppo sed image, no signal change or even signal decrease results, while sig nal increase is visible on the in-phase image. Conclusion: In those ar eas of the body where significant partial volume of a tumor with fat m ay occur (such as for breast tumors growing along ducts, which are sur rounded by fat), severe errors can result. Therefore we explicitly war n from using opposed-image sequences for MR contrast studies.