Postcontrast, fluid-attenuated inversion recovery (FLAIR) sequences are reported to be of variable value in veterinary and human neuroimaging. The source of hyperintensity in postcontrast-T2 FLAIR images is inconsistently reported and has implications for the significance of imaging findings. We hypothesized that the main source of increased signal intensity in postcontrast-T2 FLAIR images would be due to gadolinium leakage into adjacent fluid, and that the resulting gadolinium-induced T1 shortening causes reappearance of fluid hyperintensity, previously nulled on precontrast FLAIR images. A retrospective, descriptive study was carried out comparing T2 weighted, pre- and postcontrast T1 weighted and pre- and postcontrast weighted T2 FLAIR images in a variety of intracranial diseases in dogs and cats. A prospective, experimental, phantom, in vitro study was also done to compare the relative effects of gadolinium concentration on T2 weighted, T1 weighted, and FLAIR images. A majority of hyperintensities on postcontrast-T2 FLAIR images that were not present on precontrast FLAIR images were also present on precontrast T2 weighted images, and were consistent with normal or pathological fluid filled structures. Phantom imaging demonstrated increased sensitivity of FLAIR sequences to low concentrations of gadolinium compared to T1 weighted sequences. Apparent contrast enhancement on postcontrast-T2 FLAIR images often reflects leakage of gadolinium across normal or pathology specific barriers into fluid-filled structures, and hyperintensity may therefore represent normal fluid structures as well as pathological tissues. Findings indicated that postcontrast-T2 FLAIR images may provide insight into integrity of biological structures such as the ependymal and subarachnoid barriers that may be relevant to progression of disease.
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