Study: Inconsistent perfusion values pervasive in stroke imaging
MR and CT perfusion imaging for acute ischemic stroke is characterized by inconsistency among optimum perfusion values to identify tissue at risk of infarction in acute stroke, according to a review published online June 2 in Annals of Neurology.
“The target region of tissue for reperfusion therapy in ischemic stroke is that which is hypoperfused and at risk of infarction, but still viable–the 'ischemic penumbra' as salvage of this tissue may improve functional outcome. This tissue needs to be distinguished from definitely nonviable and from not-at-risk/normal tissue,” explained Krishna A. Dani, MBChB, MRCP, from the University of Glasgow Southern General Hospital in the U.K., and colleagues.
The researchers pointed out that both perfusion MRI and CT are widely available for evaluation of acute ischemic stroke. However, their clinical utility depends on the ability to differentiate benignly oligemic, at-risk and non-viable tissue.
Dani and colleagues reviewed 69 studies that analyzed MR or CT perfusion imaging with 24 hours of stroke onset.
The researchers found eight different definitions of non-viable tissue in 43 out of the 49 studies of MR imaging and four different definitions in 10 out of 20 CT studies. The most commonly used MRI definition used the acute diffusion weighted lesion to indicate non-viable tissue. For CT perfusion imaging, visible infarct on follow-up imaging performed between one and 90 days provided the most common definition of non-viable tissue.
Similarly, there were 10 and eight different definitions of at-risk tissue in MR and CT studies, respectively. Finally, there were six definitions for normal tissue in 20 out of 49 MR studies and two different definitions in 11 out of 20 CT studies. The majority of studies that provided a definition of normal tissue used the perfusion value obtained from a mirror image region in the contralateral hemisphere.
The authors wrote, “Despite the promising number of studies on perfusion imaging, and that rescuing the penumbra is a major stroke treatment target, we found very limited amounts of CT or MR data on perfusion thresholds for key tissue states. The data were further fragmented by the small study size, the variety of perfusion parameters reported and the considerable heterogeneity of definitions of tissue viability status and do not tell us if use of perfusion imaging improves clinical outcomes because there were no functional outcome data.”
Dani et al stressed the disparity between the limited evidence base and the increasing use of perfusion imaging, particularly CT imaging, while acknowledging the difficulty of establishing consistent tissue viability threshold values via perfusion imaging because of technical factors involved.
The authors concluded, “This review suggests that if MR or CT perfusion imaging is to be used to guide stroke treatment, then clinical decisions should not be based on threshold values at present.” They also proposed additional research, referring to “the large amount of original perfusion imaging data already collected,” which could be combined in a database for reanalysis using agreed standards, as proposed by the Stroke Imaging Repository (STIR) Collaboration.
“The target region of tissue for reperfusion therapy in ischemic stroke is that which is hypoperfused and at risk of infarction, but still viable–the 'ischemic penumbra' as salvage of this tissue may improve functional outcome. This tissue needs to be distinguished from definitely nonviable and from not-at-risk/normal tissue,” explained Krishna A. Dani, MBChB, MRCP, from the University of Glasgow Southern General Hospital in the U.K., and colleagues.
The researchers pointed out that both perfusion MRI and CT are widely available for evaluation of acute ischemic stroke. However, their clinical utility depends on the ability to differentiate benignly oligemic, at-risk and non-viable tissue.
Dani and colleagues reviewed 69 studies that analyzed MR or CT perfusion imaging with 24 hours of stroke onset.
The researchers found eight different definitions of non-viable tissue in 43 out of the 49 studies of MR imaging and four different definitions in 10 out of 20 CT studies. The most commonly used MRI definition used the acute diffusion weighted lesion to indicate non-viable tissue. For CT perfusion imaging, visible infarct on follow-up imaging performed between one and 90 days provided the most common definition of non-viable tissue.
Similarly, there were 10 and eight different definitions of at-risk tissue in MR and CT studies, respectively. Finally, there were six definitions for normal tissue in 20 out of 49 MR studies and two different definitions in 11 out of 20 CT studies. The majority of studies that provided a definition of normal tissue used the perfusion value obtained from a mirror image region in the contralateral hemisphere.
The authors wrote, “Despite the promising number of studies on perfusion imaging, and that rescuing the penumbra is a major stroke treatment target, we found very limited amounts of CT or MR data on perfusion thresholds for key tissue states. The data were further fragmented by the small study size, the variety of perfusion parameters reported and the considerable heterogeneity of definitions of tissue viability status and do not tell us if use of perfusion imaging improves clinical outcomes because there were no functional outcome data.”
Dani et al stressed the disparity between the limited evidence base and the increasing use of perfusion imaging, particularly CT imaging, while acknowledging the difficulty of establishing consistent tissue viability threshold values via perfusion imaging because of technical factors involved.
The authors concluded, “This review suggests that if MR or CT perfusion imaging is to be used to guide stroke treatment, then clinical decisions should not be based on threshold values at present.” They also proposed additional research, referring to “the large amount of original perfusion imaging data already collected,” which could be combined in a database for reanalysis using agreed standards, as proposed by the Stroke Imaging Repository (STIR) Collaboration.