Assessing Complementary Information on Stroke Effects
PET and MRI have been used to study stroke in small animal models, examining changes in water diffusion and glucose metabolism. Ischemic events produce a cascade of rapidly evolving physiological changes1 , including a reduction in cerebral blood flow (CBF) and cerebral blood volume (CBV), that can be highly individualized in certain animal models, creating data interpretation challenges when animals are imaged sequentially. With instruments capable of simultaneous PET/MRI acquisition researchers can assess complimentary information about the evolution of injury and the effect of interventions such as neuroprotective compounds.
MRI has been used extensively to monitor water diffusion changes in stroke patients, quantified by the apparent diffusion coefficient (ADC) using diffusion weighted imaging (DWI). PET provides complimentary information, and has been used to evaluate changes in glucose metabolism in rat models.
![(A) Coronal and transverse slice of an animal showing [18F]FDG uptake. The PET image is overlaid and fused to an anatomical MRI image. The PET data is summed across all frames. (B) In the same animal, [18F]FDG time activity curve for the contralateral hemisphere (blue) and the ipsilateral hemisphere (red).](https://www.cubresa.com/wp-content/uploads/2019/04/Neuro_Fig_05.jpg "Neuro_Fig_05")
Benefits of Simultaneous PET/MRI to Observe Heterogenous Changes During Brain Injury
Recently a group of researchers from the University of California, Davis and Genentech demonstrated successful multi-modal evaluation of the time course of a hypoxic challenge in mouse models of cerebral hypoxia-ischemia2. The extent of brain injury is variable in these models3, and along with the speed of ischemic changes, make a strong argument for simultaneous PET/MRI acquisition in small-animal stroke research.
In most animals, [18F]FDG PET images fused with anatomical MRI images showed reduced uptake in the occluded side relative to the non-occluded side, although this was not true in all cases likely due to animal variability. Executing sequential imaging studies and relying on software co-registration techniques would require the assumption that animal physiology has not significantly changed between imaging sessions; a potential pitfall given the rapid progression of ischemic brain injury.4 Simultaneous PET/MRI instrumentation can arm researchers with a new, powerful tool for multi-modal investigation into physiological changes during stroke and the effectiveness of novel interventional strategies.
![(A) Time activity curves of contralateral (solid) and ipsilateral (dotted) hemisphere [18F]FDG uptake – shown on the same axis are examples of an unexpected [18F]FDG time activity curve (blue) and animal death at the end of H2 (at 45 min, green). (B) Ghosting artifacts.](https://www.cubresa.com/wp-content/uploads/2019/04/Neuro_Fig_06.jpg "Neuro_Fig_06")
Recent Research Highlights
Evaluations of oxidative phosphorylation and glycolysis metabolism in tumors using PET/MRI
Sanhita Sinharay, Ph.D., Instructor, Department of Cancer Systems Imaging, UT MD Anderson Cancer Center.
Tumor cells often show high uptake of glucose, which is used to feed the metabolic pathways of oxidative phosphorylation and glycolysis. Although glycolysis is often upregulated in tumor cells relative to normal tissues (known as the Warburg effect), oxidative phosphorylation may be upregulated or downregulated in tumors relative to normal tissues. Furthermore, the relative rates…
Combining [F18]FAZA PET and DCE MRI to evaluate anti-cancer drug treatments
Seth Gammon, Ph.D., Assistant Professor, Department of Cancer Systems Imaging, UT MD Anderson Cancer Center.
Historically we have sequentially utilized PET/CT and then MRI in both our existing projects and emerging projects. First we are utilizing PET/CT then MRI to validate new PET reporters of inflammation. While these experiments are possible and one can reasonably compare volume of interest based analysis, precise co-registration is difficult and does not leverage the…