Angiogenesis detected after embolic stroke in rat brain using magnetic resonance T2*WI

Abstract

Background and purpose: This study uses T(2)* weighted imaging (T2*WI) to measure the temporal evolution of cerebral angiogenesis in rats subjected to embolic stroke up to 6 weeks after stroke onset with or without sildenafil treatment. Method- Male Wistar rats were subjected to embolic stroke and treated with saline (n=10) or with sildenafil (n=11), with treatment initiated at 24 hours and continued daily for 7 days after onset of ischemia. T2*WI measurements were performed at 24 hours after embolization and weekly up to 6 weeks using a 7-Tesla system. Histological measurements were obtained at 6 weeks after MRI scans.

Results: Using T2*WI, cerebral angiogenesis was detected starting from 4 weeks and from 2 weeks after onset of embolic stroke in saline and sildenafil treated rats, respectively. Significant differences in the temporal and spatial features of angiogenesis after embolic stroke up to 6 weeks after onset of stroke were found between saline and sildenafil treated rats and were identified with T2*WI. MRI permeability parameter, K(i), complementarily detected angiogenesis after ischemia in embolic stroke rats. Sildenafil treatment of stroke rats significantly enhanced the angiogenesis, as confirmed histologically.

Conclusions: T2*WI can quantitatively measure the temporal evolution of angiogenesis in rats subjected to embolic stroke. Compared to control rats, sildenafil treatment significantly increased angiogenesis in treated animals up to 6 weeks after stroke.

Figure 1

T₂* provides evidence of angiogenesis in a representative saline-treated control rat (C, the 1^st row) starting from 4 weeks (red arrow) after stroke, and in a representative sildenafil treated rat (T, the 2^nd row) starting at 2 weeks (red arrow) after stroke. The 3^rd row of T₂* maps exhibited hemorrhagic transformation (HT) after ischemia. The K_i maps are from the same rats as T₂* maps, respectively. Regional K_i increased from 1 week to 6 weeks after stroke from the representative control rat (C, the 1^st row), and from 1 week to 3 weeks after stroke for the representative sildenfil-treated rat (T, the 2^nd row). The typical K_i evolution pattern for BBB disruption is the 3^rd row of K_i maps (HT).

Figure 2

Area, identified by mean value minus 3 times the SD in T₂* maps as low intensity related to angiogenesis, rapidly increased and reached maximum at 2 weeks after stroke for treated rats, which was significantly different from control rats whose area did not achieve a maximum before 6 weeks after stroke (P<0.01 for 1 to 3 weeks, P<0.05 at 4 weeks after stroke). The T₂* value ratio (b) of angiogenic to contralateral areas demonstrated that these ratios of treated rats arrived at a minimum at 3 weeks after stroke, whereas the ratios of control rats continuously decreased to 6 weeks after stroke (P<0.01 for 1 to 3 weeks, P<0.05 at 4 weeks after stroke).

Figure 3

Angiogenesis after embolic stroke was detected by SWI. For a control rat treated with saline, SWI images exhibited evidence of angiogenesis (white arrow) started from 3 weeks in a transverse section (the 1^st row) and 5 weeks in a coronal section (the 3^rd row) after stroke. For a sildenafil-treated animal, angiogenesis identified by SWI (white arrow) occurred at 1 week after stroke in both axial (the 2^nd row) and coronal (the 4^th rows) sections. The transverse images were reconstructed from the original coronal images.

Figure 4

Histological measurements by using EBA staining showed angiogenesis for a treated rat (A) indicated by the red arrows in the enlarged picture under 10× microscopy (B). The processed image (C) of the real picture (A), warped to a T2WI image (F), matched the results detected by T₂* map (D) and SWI image (E) at 6 weeks after stroke. Blood vessels were enlarged and vascular densities were higher in angiogenesis areas for both control (G) and treated (H) animals measured under 40× microscope, compared to those in corresponding areas of the contralateral hemisphere (I). Quantitatively, the angiogenesis was significantly enhanced by the sildenafil treatment in embolic rats compared with control rats treated with saline (P<0.03) (J). An embolic stroke rat with hemorrhage (red arrow) at 48 hours after stroke confirmed by H&E stained slice (K) and 20× microscope enlarged picture (L) was used to verify T₂* map and SWI image for detecting HT after stroke. SWI image (M) and T₂* map (N) detected the HT (red arrows) at 24 and 48 hours after embolic stroke, respectively. Bars=100 μm.