Thanks to recent developments in pharmaceutical and interventional therapies, stroke is considered a treatable medical emergency. Therefore it is now indispensable to demonstrate viable tissue that might be at risk of further infarction if nothing is undertaken.Classically an area called the penumbra has been postulated to exist and which is not yet infarcted but at risk. The penumbra concept based on the diffusion perfusion mismatch allows this area to be visualized easily.Within this area of penumbra there is a reduced Apparent Diffusion Coefficient (ADC) and anisotropy with an area of hyperintensity on the raw diffusion images. Methods such as improved ADC mapping, perfusion imaging and diffusion tensor imaging should allow us to improve the visualization of this area of penumbra. The uses of Diffusion imaging have not been restricted to cerebrovascular diseases, and vascular imaging has also employed other techniques such as parenchymography. Our aim to demonstrate the presence of thresholds for cerebral damage due to ischemia has been greatly attained.
Results from current study:
Diffusion-perfusion imaging of the penumbra
The diffusion-perfusion mismatch based on MR imaging is usable in the acute phase of stroke . However in order to assess the severity of ischemia exactly, improvements in the calculation of the penumbra are needed. One way is to also include the calculated ADC maps into the equation, as we did in the paper of El-Koussy .
Age-related changes in the Apparent Diffusion Coefficient
In order to additionally assess correctly the ADC values, it is important to know that they do not remain stable over a life-time. After initial reports of ADC changes related to myelination in the newborn period, we have observed a tendency of the ADC to rise with increasing age, which corresponds to what is seen leucoaraiosis. These changes have to be taken into account when calculating ADCs for the definition of cerebral penumbral tissue.Also, these changes while they can be assessed by DW and DTI, a substrate is found in the vascular hypoperfusion seen by parenchymography. One of the earliest change to occur is probably the occurrence of anisotropy related to diminished diffusion.
To demonstrate the presence of areas of barely discernible variations in perfusion and ADC reductions, the use of new post-processing methods must be evaluated. One new such method would be a new fuzzy logic algorithm such as the one developed by Buerki et al..The rationale behind this is that waves of spreading depression-like changes are present in the cerebral cortex during ischemia, close to those seen in epilepsy. In epilepsy changes in diffusion and perfusion have been observed.
Diffusion imaging of hypoxic-ischemic lesions
Additionally to its use in imaging of acute embolic ischemia, we have studied the use of diffusion-weighted MRI in patients with hypoxic-ischemic lesions of the brain. This was done in a first report where we saw that lesion extent was more severe on DWI thank on conventional MRI and that ADC reductions correlated better with eventual clinical outcome; this data was then later expanded to a series of patients who all underwent MRI of the brain along with DWI to assess the extent of brain damage. Based on the imaging findings and the clinical data we could establish criteria for clinical outcome. Indeed we found that lesion extension to the white matter, cerebellum and/or brainstem was associated with more severe clinical outcome and a more bleak outcome.