Our observations in these two cases suggest visual assessment of T1 hyperintensity on fat suppressed images may be misleading and falsely suggest blood-containing pathology. Such pseudohyperintensity is a potential pitfall in MRI that the interpreting radiologist must be aware of when examining fat suppressed studies. We believe the mechanism of this pseudohyperintensity has two components. First, the fact that the hyperintense T1 signal was seen on fat-suppressed images but not on unsuppressed images suggests the apparent high signal partially reflects the altered dynamic range when fat signal is subtracted. Essentially, the next brightest signal is assigned to a whiter shade on the display grayscale. However, this is likely only a partial explanation, because the T1 hyperintensity was much less apparent on post-gadolinium images. Review of the cases suggests this reflects altered visual appreciation of signal intensity as the ambient contrast is changed, the same principle that underlies many popular optical illusions. This is perhaps most famously illustrated by the checker-shadow illusion (). These optical illusions occur due to subconscious processing of retinal input [8
], demonstrating that our perception of the intensity of various shades of gray can be manipulated by their surroundings - a point with obvious parallels in MRI interpretation. Thus, it is especially important to be aware of this diagnostic limitation as it is inherent to how our visual system was designed to perceive light intensity rather than any technical flaw of image acquisition which may be correctable.
Yousem et al described paradoxically low signal intensity in structures after intravenous contrast administration that appeared of high signal intensity on precontrast imaging [10
], and suggested the effect was due to gadolinium-related changes in signal intensity of tissues due to altered relaxation times at varying TR and TE values [10
]. While this mechanism might contributes to changed contrast in solid tissue, such effects would probably not explain changes in signal intensity of a purely cystic structure ().
Some limitations of our study deserve mention. First, this was a single institution retrospective review. The individual cases were not identified systematically and with our small sample size we were unable to estimate the frequency of which pseudohyperintensity contributes to diagnostic errors. As no cases have been previously reported we can speculate that it is either very rare or under-recognized - further study with larger case series are needed to clarify. Also, each patient in our study obtained only one MR scan, not allowing us the opportunity to examine whether or not these findings were reproducible or had evolved significantly with disease progression.
In conclusion, we recommend that all suspicious hyperintense lesions on fat suppressed MRI studies be compared with the corresponding non fat-suppressed MR image or with supplemental CT or ultrasound studies. Otherwise pseudohyperintense signals, especially when resembling pathologic lesions, may lead to unnecessary patient workup, misdiagnosis or diagnostic delay of more serious pathology.