Challenges towards MR imaging of the peripheral inflammatory response in the subacute and chronic stages of transient focal ischemia

Abstract

Intravenous administration of iron oxide nanoparticles after experimental stroke has been shown to produce focal signal intensity changes in the ischemic boundary on MRI images. These changes have been attributed to the influx of iron-laden blood-borne macrophages, although it has been suggested that this effect might not always be completely specific to inflammatory cells. The aim of the present study was to investigate this phenomenon in a subacute time frame that is more relevant to the peripheral inflammatory response. Imaging experiments (T(2) -, T(2)*- and T(1) -weighted sequences) were acquired in Wistar rats 6 days after transient middle cerebral artery occlusion (MCAO). Animals were intravenously infused with different doses of ultrasmall superparamagnetic iron oxide (USPIO) nanoparticles (300, 600 or 1000 µmol Fe/kg), or saline and gadolinium, and imaged again 24 h later. Tissue was immediately processed for immunohistochemistry with the macrophage marker ED-1, in combination with Prussian blue for iron. Ischemic tissue exhibited a large increase in T(2) values, and overall contrast enhancement was apparent in the brain and surrounding muscle. In contrast with previous reports, there were no regions of focal signal intensity changes in the ischemic territory in any of the images, although a region of interest analysis revealed a trend towards iron accumulation in the ischemic hemisphere, particularly in the cortex of T(2)* images. However, histological examination revealed that, despite extensive ED-1-positive macrophage accumulation in the entire ischemic territory, none of these cells were Prussian blue positive, except in the meninges of one animal that received a high dose of USPIO nanoparticles. These results imply that the observed trend is a result of the presence of contrast agent in the blood, or meninges, and not iron-containing inflammatory cells.

White Matter Reorganization and Functional Response after Focal Cerebral Ischemia in the Rat

“…The ischemic lesion was induced in 15 rats by transient (60 minutes) occlusion of the right middle cerebral artery (MCA), using the modified intraluminal filament technique [15], [16] under 2% isoflurane anesthesia in 30/70% O 2 /N 2 O. The body core temperature was measured with a rectal temperature probe and maintained at 36.7±1°C using an automated heating blanket with temperature feedback (Medres, Cologne, Germany).…”

Imaging Neuroinflammation after Stroke: Current Status of Cellular and Molecular MRI Strategies

“…In this case, the presence of MRI contrast and iron-positive macrophages would not derive from the infiltration of iron-loaded blood-borne monocytes. Third, contrast enhancement may also be caused by general blood pool effects induced by the prolonged presence of a long-circulating contrast agent, such as Sinerem/Combidex [14]. Last, in case of severe BBB damage, iron oxides may also directly leak into the brain parenchyma [51], where they may be engulfed by activated macrophages/microglia or diffuse into the cerebrospinal fluid (CSF).…”

MRI Assessment of Post-Ischemic Neuroinflammation in Stroke: Experimental and Clinical Studies

“…Macrophages (ED1) and focal iron deposition were detected in the lesion at these timepoints, in agreement with T2/T2* hypointense areas. However, these results obtained with SPIO injection failed to be reproduced with USPIO in a recent study : Farr and colleagues did not observed T1,T2, nor T2* signal changes, despite a three-dose assay and extensive ED-1-positive macrophage accumulation at the sub-acute stage (Farr et al, 2011). In line with this negative results, the single study performed with a mouse model of transient ischemia reported no detectable MRI changes in the first 72h following stroke onset (Denes et al, 2007).…”