To access the performance afforded by OMAG, we used the OMAG system described in to image micro-vascular blood perfusion over the brain cortex in the normal mouse with the cranium left intact. shows one representative image (B scan) obtained from raw spectral interferograms captured by the detecting spectrometer system. is the flow map showing all moving components, such as the moving red blood cells in all blood vessels, including capillaries. is the structure map containing the normal OCT/OMAG cross-sectional image within which the anatomically important layers such as cranium and cortex can be clearly demonstrated, but it is almost impossible to identify the blood vessels. Because the flow and structure images are co-registered in OMAG, they can be combined into a single image. This combined image can be used to precisely localize the blood vessels within the brain tissue (), which can be compared with the corresponding histological section (H&E) (). Similar to conventional OCT, OMAG is able to resolve the cortical structures and blood perfusion at depths of ~ 1.5 mm through the skull, a penetration depth that cannot be achieved by confocal microscopy. The axial resolution was ~ 8 μm within the biological tissue, which is determined by the bandwidth of the light source used and is capable of resolving the capillaries with an average size of ~10μm. The lateral imaging resolution was approximately 16 μm as determined by the objective lens that focused the sample light onto tissue. Compared with confocal microcopy and laser speckle imaging, OMAG does not need to prepare “cranial window” for imaging, and thus it potentially avoids any complications involved during the procedure, such as the changes in temperature and cranial pressure that may affect the alterations in CBF.
Figure 2 A cross-section of a mouse brain with the skull intact was imaged with OMAG in vivo. (A) and (B) are the flow and structural images obtained by OMAG. (C) The fused OMAG image showing the blood vessel locations in the cortex and skull bone. (D) The corresponding (more ...)
By scanning the probe beam progressively in x-y direction and then processing the raw spectrograms slice by slice, we can reconstruct a 3D OMAG image, from which the detailed information regarding micro-vascular blood perfusion can be obtained. is an example obtained from a tissue volume of 2.5 × 2.5 × 1.7(x-y-z) mm3 of an adult mouse brain with the cranium intact. With this 3D volume data set, the orientation and location of blood vessels can be identified (). Using suitable software, the 3D view can be rotated, cut from any angle to examine the blood flows at different depths within tissue in detail. Additionally, with the application of volume segmentation algorithm, the blood perfusion in the skull bone and meninges are able to be separated from those within brain cortex, shown in , respectively. This transcranial blood perfusion with high imaging resolution and sufficient imaging depth would provide us a good opportunity to capture the perfusion information of specific areas where other imaging methods are difficult to reach. By stitching eight images together as shown in , shows a mosaicing OMAG image that provide us an ability to examine micro-vascular morphology in a whole brain tissue, compared with the photography taken with and without skull. Imaging acquisition time is another critical factor to investigate the response of cerebrovascular perfusion under a variety of pathological and therapeutic conditions. Our current preliminary system can provide the temporal resolutions of 0.1 s and 50 s to obtain 2D and 3D OMAG images, respectively. As we are aware, currently no other imaging tools can non-invasively delineate such detailed cerebral vasculature within one minute.
Figure 3 A volume of 2.2×2.2×1.7 (x-y-z) mm3 of an adult mouse brain with the skull intact was imaged with OMAG in vivo. (A) 3D volumetric rendering of the blood perfusion within the scanned tissue volume. (B) 2D x–y projection view of (more ...)
Figure 4 The entire cerebro-vascular flow over the cortex of an adult mouse with the skull intact was imaged with OMAG in vivo. (A) is the projection view of blood perfusion that reveals the detailed blood perfusion network over the cortex at capillary level resolution. (more ...)
In our preliminary experiments, OMAG was performed in a typical ischemic stroke model to show its potential on monitoring structural and functional cerebral blood perfusion in the study of cerebro-vascular disease in mice. is the OMAG flow image from baseline, i.e. the control, where it shows the capability of OMAG to delineate the cerebral perfusion over the cortex while the skull was left intact. The result at 30 minutes after the beginning of MCAO shows that OMAG is able to indicate the alterations in blood perfusion within cortex (). Compared to baseline, the ischemic status caused by progressive focal occlusion was apparent in the ipsilateral region at this time point. At the ipsilateral side where ischemia was induced, the vessel constriction can be seen, which is supposed to be the direct result of brain autoregulation that the animal has to prevent it from the further hypoxia and ischemia. After 30 minutes onset of reperfusion, the blood perfusion in ipsilateral side restored in some region, while the residual occlusion was still apparent (). This suggested that some occlusions were temporary, while others persisted even the filament was removed from MCA. Therefore, our OMAG results showed us the occlusive components of ischemic stroke model independent of whether the foreign body existed in MCA. These data paved a way for evaluating the potential use of this novel imaging technology in the aid of understanding the pathophysiology of cerebro-vascular diseases or brain disorder and the potential benefits of pharmacological interventions.
Figure 5 3D OMAG imaging of the cortex in ischemic stroke. Compared to baseline (A), progressive focal ischemia (B) developed during MCAO; (B) was taken at 30 min during MCAO. After 30 minutes onset of reperfusion (C), the blood perfusion in ipsilateral side restored (more ...)