@article {Sobczyk, author = {O. Sobczyk and A.P. Crawley and J. Poublanc and K. Sam and D.M. Mandell and D.J. Mikulis and J. Duffin and J.A. Fisher}, title = {Identifying Significant Changes in Cerebrovascular Reactivity to Carbon Dioxide}, year = {2016}, doi = {10.3174/ajnr.A4679}, publisher = {American Journal of Neuroradiology}, abstract = {BACKGROUND AND PURPOSE: Changes in cerebrovascular reactivity can be used to assess disease progression and response to therapy but require discrimination of pathology from normal test-to-test variability. Such variability is due to variations in methodology, technology, and physiology with time. With uniform test conditions, our aim was to determine the test-to-test variability of cerebrovascular reactivity in healthy subjects and in patients with known cerebrovascular disease.MATERIALS AND METHODS: Cerebrovascular reactivity was the ratio of the blood oxygen level{\textendash}dependent MR imaging response divided by the change in carbon dioxide stimulus. Two standardized cerebrovascular reactivity tests were conducted at 3T in 15 healthy men (36.7 {\textpm} 16.1 years of age) within a 4-month period and were coregistered into standard space to yield voxelwise mean cerebrovascular reactivity interval difference measures, composing a reference interval difference atlas. Cerebrovascular reactivity interval difference maps were prepared for 11 male patients. For each patient, the test-retest difference of each voxel was scored statistically as z-values of the corresponding voxel mean difference in the reference atlas and then color-coded and superimposed on the anatomic images to create cerebrovascular reactivity interval difference z-maps.RESULTS: There were no significant test-to-test differences in cerebrovascular reactivity in either gray or white matter (mean gray matter, P = .431; mean white matter, P = .857; paired t test) in the healthy cohort. The patient cerebrovascular reactivity interval difference z-maps indicated regions where cerebrovascular reactivity increased or decreased and the probability that the changes were significant.CONCLUSIONS: Accounting for normal test-to-test differences in cerebrovascular reactivity enables the assessment of significant changes in disease status (stability, progression, or regression) in patients with time.AbbreviationsBOLDblood oxygen level{\textendash}dependentCO2carbon dioxideCVRcerebrovascular reactivityEC-ICextracranial to intracranialIDinterval differencePaCO2arterial partial pressure of carbon dioxidePetCO2end-tidal partial pressure of carbon dioxide}, issn = {0195-6108}, URL = {https://www.ajnr.org/content/early/2016/02/04/ajnr.A4679}, eprint = {https://www.ajnr.org/content/early/2016/02/04/ajnr.A4679.full.pdf}, journal = {American Journal of Neuroradiology} }