<unk> , or SEEP, is a contrast mechanism for functional magnetic resonance imaging (fMRI), which is an alternative to the more commonly employed BOLD (blood-oxygen-level dependent) contrast. This mechanism for image contrast changes corresponding to changes in neuronal activity was first proposed by Dr. Patrick Stroman in 2001. SEEP contrast is based on changes in tissue<unk>  content which arise from the increased production of extracellular fluid and swelling of neurons and glial cells at sites of neuronal activity. Because the dominant sources of MRI<unk>  in biological tissues are<unk>  and lipids, an increase in tissue<unk>  content is reflected by a local increase in MR<unk>  intensity. A correspondence between BOLD and SEEP<unk>  changes, and sites of activity, has been observed in the brain and appears to arise from the common dependence on changes in local blood flow to cause a change in blood oxygenation or to produce extracellular fluid. The advantage of SEEP contrast is that it can be detected with MR imaging methods which are relatively insensitive to magnetic susceptibility differences between air, tissues, blood, and bone. Such susceptibility differences can give rise to spatial image distortions and areas of low<unk> , and magnetic susceptibility changes in blood give rise to the BOLD contrast for fMRI. The primary application of SEEP to date has been fMRI of the spinal cord (spinal fMRI) because the bone/tissue interfaces around the spinal cord cause poor image quality with conventional fMRI methods. The disadvantages of SEEP compared to BOLD contrast are that it reveals more localized areas of activity, and in the brain the<unk>  intensity changes are typically lower, and it can therefore be more difficult to detect.
Signal enhancement by extravascular water protons
Transient hepatic attenuation differences
Hypervascularity
Very low cerebral blood volume
Massive perivillous fibrin deposition
Collagenous spherulosis
Ring-enhancing lesion
Twin reversed arterial perfusion