The mismatch model between MRI perfusion-weighted imaging (PWI) and diffusion-weighted imaging (DWI) has been suggested to provide an estimation of real ischemic penumbra (IP) and is used as referring criteria to select patients who are within the first 6 hours after stroke onset and might benefit from thrombolysis [
The deoxyhemoglobin (deoxy-Hb) in cerebral capillaries and veins is an indicator of OEF, which can be visualized by T2-and T2*-based blood-oxygen-level-dependant (BOLD) MRI [
The purposes of this study were to investigate: (1) the value of R2′ in detecting IP in a monkey model of reversible MCAO; (2) time evolution of relative R2′ (rR2′) in infarcted core, IP, and oligemia.
This study was approved by the experimental animal ethical committee of our university. Six male monkeys (
The MCAO model was established by an interventional microcatheter method as described by De Grespigny et al. [
The criterions of a successful animal model included (1) the selected branch of MAC was not visualized on angiograms; (2) MR scan showed high signal intensity on DWI in the territory of selected branch of MCA after one-hour ischemia; (3) Postmortem specimen showed nonstained infarcted area on 2,3,5-triphenyltetrazolium chloride (TTC) staining.
MRI scan was performed on a 1.5 T superconductive scanner (GE Twin Speed Infinity with Excite I) with birdcage head 8-channel phase array coil. All animals were scanned at 0 hour (1 hour after MCAO) and 1, 3, 6, 12, 24, and 48 hours after reperfusion. Slice thickness was 5 mm and FOV was 15 cm × 15 cm. Scanning protocol included, (1) T2WI (fast recovery FSE, TR 9002 ms, TE 124 ms, NEX 2, matrix 256 × 256). (2) DWI (SE-EPI, b values of 0 and 1000 s/mm2, TR 3000 ms, TE 112.6 ms, NEX 2, matrix 128 × 128) (3) Quantitative T2-map[qT2] (4-echo SE, TR 1200 ms, TE 25, 50, 75, 100 ms; flip angle 150°, matrix 256 × 256) and Quantitative T2*-map[qT2*] (8-echo GRE, TR 150 ms, TE 3.6, 9.4, 15.3, 21.2, 27.1, 33.0, 37.5, 43.2 ms, flip angle 24°, matrix 256 × 256) (4) PWI (single-shot gradient recall echo planner imaging T2* WI, TR 2000 ms, TE 80 ms, flip of angle 90°, matrix 128 × 128, NEX 1, Gd-DTPA 0.1 mmol/kg, inject rate 2 mL/s).
qT2-map and qT2*-map were calculated according to the following formula:
At 48 hours after reperfusion, all animals were killed by an overdose of pentobarbital just after MRI scan. The brains were put into a −20°C refrigerator for an hour to make the specimens hardened. The brains were sliced into 5-mm axial slabs by using a home-made instrument and stained with 2,3,5- triphenyltetrazolium chloride (TTC) in 37°C water bath for 30 minutes. The stained sections were recorded with a digital camera.
Lesion volumes were calculated on DWI at 0 hour (0-DWI), T2WI at 48 hours after reperfusion (48-T2WI)- and TTC-stained sections by integrating products of lesion area and slice thickness on each lesion-containing slice. Lesion volume percentage (LVP) was calculated according to formula: LVP = lesion volumes/bilateral hemispheres volumes ×100%.
Ischemic brain tissues were classified into 3 subtypes: infarcted core (hyperintensive areas on both 0-DWI and 48-T2WI), IP (hyperintensive areas on 0-DWI but iso on 48-T2WI), oligemia (delayed mean transit time areas on PWI at 0 hour [0-MTT] but iso on both 0-DWI and 48-T2WI) (Figures
0 h after MCAO DWI shows high-signal ischemic lesion.
0 h after MCAO PWI (MTT) shows delayed MTT lesion.
48 h reperfusion T2WI shows high-signal infarction smaller than lesion on DWI.
Overlapped imaging shows infarcted core (white), IP (yellow), and oligemia (red).
Imaging registration was performed on an off-line workstation. The platform was Matlab 7.0, Windows XP SP2 with 3.0 GHz Intel Pentium 4 CPU and 2 G RAM. 0-DWI, 0-MTT, and 48-T2WI were coregistered to R2′ images of each time point using a free form deformation method [
5 region-of-interests (ROIs) were put into each subtype brain tissue, respectively. Each ROI was 10 mm2 in size. Relative R2′ (rR2′) was calculated according to the ratio of lesion and its minor area on contralateral healthy hemisphere.
Statistic analysis was performed with SPSS15.0 software. Paired
MCAO model was successful in all 6 monkeys. 2 monkeys were dead at 2 and 14 hours after reperfusion because of large area of hemorrhagic infarction, respectively, and their data were not included in this study.
0 h DWI demonstrated high-signal ischemic lesion and partial lesion can recover after reperfusion (Figures
0 h DWI shows high-signal ischemic lesion in basal ganglion (black arrow) and temporal lobe (white arrow).
1 h DWI shows increased signal in basal ganglion (black arrow) but decreased signal in temporal lobe (white arrow).
48 h DWI shows high-signal in basal ganglion (black arrow) and signal recovery in temporal lobe with scattered high signal (white arrow).
Same animal with Figures
LVP on 0-DWI, 48-T2WI- and TTC-stained sections were summarized in Table
LVP on DWI at 0 hour, T2WI at 48 hours after reperfusion and TTC.
Animal number | DWI (%) | T2WI (%) | TTC (%) |
---|---|---|---|
1 | 10.77 | 8.08 | 8.19 |
2 | 12.78 | 8.31 | 8.45 |
3 | 12.26 | 8.77 | 8.68 |
5 | 9.68 | 7.48 | 7.27 |
On 0 hour R2′ imaging, IP and oligemia showed hyperintensity compared to infarcted core. After reperfusion, the signal intensity on R2′ imaging was increased in IP and oligemia but decreased in infracted core with time evolution (Figures
0 h R2′ map shows that the signal of IP (black arrow) is higher than that of infarcted core (white arrow).
1 h R2′ map shows low signal of irreversible infarcted core (white arrow), slightly high signal of reversible IP (black arrow).
48 h R2′ map shows significant low signal of infarcted core (white arrow), high signal of IP (black arrow) (Figures
rR2′ values of infracted core, IP, and oligemia at each time point after reperfusion were summarized in Table
Comparison of rR2′ (
Subtype | 0 h | 1 h | 3 h | 6 h | 12 h | 24 h | 48 h |
---|---|---|---|---|---|---|---|
Infarcted core | |||||||
IP | |||||||
Oligemia | |||||||
36.19 | 134.09 | 256.30 | 803.25 | 743.74 | 1236.26 | 557.02 | |
Respective time evolution of rR2′ values in infarcted core, IP, and oligemia was summarized in Table
Time evolution of rR2′ in infarcted core, IP, and oligemia after reperfusion (compared by ANOVA).
Infracted core | IP | Oligemia | ||||||
( | ( | ( | ||||||
Reperfusion time | Compared time | Reperfusion time | Compared time | Reperfusion time | Compared time | |||
0 h | 1 h | * | 0 h | 1 h | — | 0 h | 1 h | — |
3 h | * | 3 h | — | 3 h | — | |||
6 h | * | 6 h | * | 6 h | * | |||
12 h | * | 12 h | * | 12 h | * | |||
24 h | * | 24 h | * | 24 h | * | |||
48 h | * | 48 h | * | 48 h | * | |||
1 h | 3 h | — | 1 h | 3 h | — | 1 h | 3 h | — |
6 h | * | 6 h | * | 6 h | * | |||
12 h | * | 12 h | * | 12 h | * | |||
24 h | * | 24 h | * | 24 h | * | |||
48 h | * | 48 h | * | 48 h | * | |||
3 h | 6 h | * | 3 h | 6 h | * | 3 h | 6 h | * |
12 h | — | 12 h | * | 12 h | * | |||
24 h | * | 24 h | * | 24 h | * | |||
48 h | * | 48 h | * | 48 h | * | |||
6 h | 12 h | — | 6 h | 12 h | — | 6 h | 12 h | — |
24 h | — | 24 h | — | 24 h | — | |||
48 h | — | 48 h | * | 48 h | * | |||
12 h | 24 h | * | 12 h | 24 h | — | 12 h | 24 h | — |
48 h | — | 48 h | * | 48 h | * | |||
24 h | 48 h | — | 24 h | 48 h | * | 24 h | 48 h | * |
*
Reperfusion time evolution of rR2′ shows decreased trend in infarcted core but increased trend in IP and oligemia.
As described by
Although many neuroprotective drugs have proven successful in animal (rodent) models, all have failed during clinical trials. A common reason for this is the lack of demonstrated efficacy of these drugs in larger animals such as cats or monkeys. The subhuman monkeys have similarity in cerebrovascular anatomy, neurochemistry, and immunology, which make them suitable for testing new drug therapies and endovascular instruments [
The delineation of the “penumbra” is of particular interest in acute stroke imaging. Since the “mismatch concept” applying PWI and DWI appears to be an oversimplification of the underlying electrophysiological tissue status, an additional parameter reflecting the metabolic state of brain tissue will improve the ability to describe the penumbra [
Our study demonstrated that the values of rR2′ were increased in IP and oligemia and decreased in infarcted core with time evolution after reperfusion. Thus, the hyperintensity in R2′ images may represent increased deoxy-Hb as an indicator of increased OEF, which can increase in IP and oligemia with ongoing CBF impairment. The infarcted core is unsalvable tissue which has lost the compensative ability of increased OEF. Thus, the infarcted core demonstrated consecutive hypointensity in R2′ images even after reperfusion. PET study has showed 50% increasement of OEF in IP [
An interesting finding demonstrated by Geisler et al. [
Although both IP and oligemia showed similar increased trend in this study, values of rR2′ in oligemia were higher than that in IP at every time point after reperfusion. There are two reasons for this. Firstly, CBF in oligemia is less impaired than those in IP [
Both PET and BOLD MRI studies had showed increased OEF in acute ischemic stroke [
For the infarcted core, the sharply decreased rR2′ within the first 6 hours may be due to the fact that no generation of deoxy-Hb is found even after early reperfusion. With the quick recovery of CBF, the previously produced deoxy-Hb is removed and rR2′ maintains as low level as 0 after the first 6 hours.
There are some methodological limitations for T2′ or R2′ images. Firstly, T2* is sensitive to large variations of the static magnetic field (ΔB0). This may affect the accuracy of measured T2′ or R2′. This problem can be solved by using side-to-side comparison which reveals more valuable relative results. Secondly, the concentration of deoxy-Hb in a single voxel is determined mainly by 2 factors: changes in the local generation of deoxy-Hb inside the voxel and changes in the transport of deoxy-Hb into or out of the voxel, which is closely correlated with CBF [
Although BOLD-based MR quantitative OEF measurement has been published and demonstrated good correlation with the existing OEF values [
BOLD-based R2′ MRI can be used to describe changes of cerebral oxygen extract in acute ischemic stroke, and it can provide additional information in detecting IP. The time evolution rR2′ in infarcted core, IP, and oligemia is in accordance with the underlying pathophysiolgy. With improvement in the accuracy, this technique would add more valuable information for patients selection for thrombolytic therapy.
This study was partially supported by Tianjin Municipal Nature Science Foundation (no. 09JCYBJC11500), National Basic Research Program of China (973 program, no. 2010CB732506), National Natural Science Foundation of China (NSFC, no. 30730036), and Doctorial Foundation of Ministry of Education of China (no. 20091202110006).