Metabolic evolution of cortex before, during and
Metabolic evolution of cortex before, during and immediately after global ischemia: an ultra-short echo time
1H
MRS study
Mario G. Lepore and Hongxia Lei1,2 , 1AIT,
Center for Biomedical Imaging (CIBM), Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland, 2 Laboratory of functional and metabolic imaging LIFMET, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland, 3 Faculty of Medicine, University of Geneva, Geneva, Switzerland
INTRODUCTION
RESULT I: Quality MR angiography images revealed the complete 4VO in rats
RESULT III: Metabolic evolution in cortex before, during and after a 15min-4VO
Studying metabolic evolution of brain right before, during and immediately after stroke insults in experimental models might improve diagnosis but remains challenging. A 4-vessel occlusion (4VO) delivers bilateral hemispheric ischemia in rats (1) and has been shown to be remotely performed (2). AIM: Evaluate whether both MR angiography and localized 1H MRS could be applied for diagnosis purpose on rat after global ischemia induced by 4VO at 14.1T. The second aim of this study was to study metabolic evolution of cortex during and immediately after stroke
Figure 3. Localized short-echo time 1H MRS of one rat cortex before, during and immediately after global ischemia revealed changes in Cr, PCr and Lac upon a 15min 4VO. The bottom axis indicated one minute 1H MRS acquisition (nt=16) on rat cortex. A red arrow indicated the 4VO and a blue arrow pointed at the end of the 15-min 4VO.
METHODS ANIMALS oAll studies were approved by the local animal care and use committee. We applied one-stage anterior approach (1) to achieve 4VO in adult male Wistar rats (300-350g). oIn brief, rats were anesthetized for surgical preparation under 2.5% isoflurane mixed with air and oxygen (1:1). oBoth vertebral arteries (VAs) were carefully occluded between cervical diapophysis C2 and C3 using a cauterizer. Common carotid arteries (CCAs) were isolated and encircled loosely with one pair of 2mm inflatable vascular occluders, both of which were saline-filled and extended to two syringes at the entrance of the magnet. oOnce CCAs were ligated at the entrance of the magnet, 4VO can be achieved. MR STUDIES oImmediately, animals were carefully secured, placed in prone position and transferred in a horizontal 14.1T magnet. oA quadrature coil (two 16-mm-inner-diameter loops) was placed on the top of cortex and used for MR angiography and localized 1H MRS. oOnce the field inhomogeneity was adjusted, 3D GRE images were acquired to cover the entire head and some of the neck for MR angiography (maximum intensity projection (MIP), 3&4). o Localized 1H MR spectra were acquired using SPECIAL (5). Bilateral cortical tissue (1×8×4.2mm3) was defined based on anatomical images (FSE/GRE). o1H MR spectra were quantified using LCMODEL referencing the endogenous water signal from the identical voxel (80% water contents in cortex). REFERENCES 1) Yamaguchi M et al. Stroke. 2005; 36:2212-2214; 2) Schmidt-Kastner R et al. Stroke 1989; 20:938-946;3) Beckmann N et al. JMR 1999; 140:442-450; 4) Hilger T et al. JCBFM 2002 ; 22 :652-662 ; 5) Berthet C et al. 2014 JCBFM 2014 ; 34:1848-1855
Figure 1. Representative MIPs in sagittal (at the midline of brain), coronal (the blue dotted line) and axial (the orange dashed line) planes of one control rat (top row), one after 4VO (2nd row) and one after 3VO (bottom row). BA, Basilar artery; VA, vertebral artery; CCA, common carotid artery; STV, superficial temporal vein. Orientation: A, anterior; P, posterior; R, animal right; L, animal left. The circle of willies (BA towards to the anterior of the head) can be visualized in both control and after 3VO. The bright spot at the animal left side was a reference tube containing 10mM MnCl2, which was attached to the side of the surface coil and with a final position at the animal left side. 3D GRE parameters: flip angle=90° , TE/TR=1.5/6ms, 45×35×35mm3, 160×96×96, SW=100kHz, nt=8.
RESULT II:
1H
MRS showed different spectral evolution
DISCUSSION MRA images illustrated that the vascular signal intensities globally decreased significantly during both 4VO and 3VO, but a complete reduction was reached in 4VO only. Thus, the incomplete occlusion, i.e. 3VO, would be sufficient in maintaining normal cortical metabolic profiles and possibly attenuate ischemic attack in cortex. (Figure 1)
1H
MRS data suggested modification in cortical tissue occurs only in case of permanent global ischemia (Figure 2). Similar results were obtained previously in ipsilateral striatum (5).
We also noticed that some metabolites changes were slightly different from murine studies (5), e.g. myo-inositol (Ins) and taurine (Tau), which might due to different brain regions and species, and remained to be studied. With such model, we can observe the striking increase of Cr and the decrease of PCr were accompanied by elevated lactate in rat cortex after during a 15-min 4VO. Figure 2 Typical localized 1H MRS (TE/TR=2.8/4000ms) of rat cortex (white squares in FSE images, A) from one control (B), during 4VO (C) and during an incomplete 4VO, namely 3VO (D) at 14.1T. Arrows indicated visually observed spectrum changes when comparing to that was acquired in control (B). ). In E, summary of selected metabolites quantified from localized 1H MRS of rat cortex from control (white, B), 4VO (black, C) and 3VO (gray). Abbreviations: GABA, g-amino butyrate; tCr, total creatine; Glu, glutamate; NAA, N-acetyl-aspartate; Lac, lactate, NAAG, N-acetyl-aspartate-glutamate; Gly, glycine.
CONCLUSION A complete 4VO can be visualized by MRA and metabolic changes in cortex can be successfully assessed by 1H MRS during and immediately after stroke.
1H
MRS study
Mario G. Lepore and Hongxia Lei1,2 , 1AIT,
Center for Biomedical Imaging (CIBM), Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland, 2 Laboratory of functional and metabolic imaging LIFMET, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland, 3 Faculty of Medicine, University of Geneva, Geneva, Switzerland
INTRODUCTION
RESULT I: Quality MR angiography images revealed the complete 4VO in rats
RESULT III: Metabolic evolution in cortex before, during and after a 15min-4VO
Studying metabolic evolution of brain right before, during and immediately after stroke insults in experimental models might improve diagnosis but remains challenging. A 4-vessel occlusion (4VO) delivers bilateral hemispheric ischemia in rats (1) and has been shown to be remotely performed (2). AIM: Evaluate whether both MR angiography and localized 1H MRS could be applied for diagnosis purpose on rat after global ischemia induced by 4VO at 14.1T. The second aim of this study was to study metabolic evolution of cortex during and immediately after stroke
Figure 3. Localized short-echo time 1H MRS of one rat cortex before, during and immediately after global ischemia revealed changes in Cr, PCr and Lac upon a 15min 4VO. The bottom axis indicated one minute 1H MRS acquisition (nt=16) on rat cortex. A red arrow indicated the 4VO and a blue arrow pointed at the end of the 15-min 4VO.
METHODS ANIMALS oAll studies were approved by the local animal care and use committee. We applied one-stage anterior approach (1) to achieve 4VO in adult male Wistar rats (300-350g). oIn brief, rats were anesthetized for surgical preparation under 2.5% isoflurane mixed with air and oxygen (1:1). oBoth vertebral arteries (VAs) were carefully occluded between cervical diapophysis C2 and C3 using a cauterizer. Common carotid arteries (CCAs) were isolated and encircled loosely with one pair of 2mm inflatable vascular occluders, both of which were saline-filled and extended to two syringes at the entrance of the magnet. oOnce CCAs were ligated at the entrance of the magnet, 4VO can be achieved. MR STUDIES oImmediately, animals were carefully secured, placed in prone position and transferred in a horizontal 14.1T magnet. oA quadrature coil (two 16-mm-inner-diameter loops) was placed on the top of cortex and used for MR angiography and localized 1H MRS. oOnce the field inhomogeneity was adjusted, 3D GRE images were acquired to cover the entire head and some of the neck for MR angiography (maximum intensity projection (MIP), 3&4). o Localized 1H MR spectra were acquired using SPECIAL (5). Bilateral cortical tissue (1×8×4.2mm3) was defined based on anatomical images (FSE/GRE). o1H MR spectra were quantified using LCMODEL referencing the endogenous water signal from the identical voxel (80% water contents in cortex). REFERENCES 1) Yamaguchi M et al. Stroke. 2005; 36:2212-2214; 2) Schmidt-Kastner R et al. Stroke 1989; 20:938-946;3) Beckmann N et al. JMR 1999; 140:442-450; 4) Hilger T et al. JCBFM 2002 ; 22 :652-662 ; 5) Berthet C et al. 2014 JCBFM 2014 ; 34:1848-1855
Figure 1. Representative MIPs in sagittal (at the midline of brain), coronal (the blue dotted line) and axial (the orange dashed line) planes of one control rat (top row), one after 4VO (2nd row) and one after 3VO (bottom row). BA, Basilar artery; VA, vertebral artery; CCA, common carotid artery; STV, superficial temporal vein. Orientation: A, anterior; P, posterior; R, animal right; L, animal left. The circle of willies (BA towards to the anterior of the head) can be visualized in both control and after 3VO. The bright spot at the animal left side was a reference tube containing 10mM MnCl2, which was attached to the side of the surface coil and with a final position at the animal left side. 3D GRE parameters: flip angle=90° , TE/TR=1.5/6ms, 45×35×35mm3, 160×96×96, SW=100kHz, nt=8.
RESULT II:
1H
MRS showed different spectral evolution
DISCUSSION MRA images illustrated that the vascular signal intensities globally decreased significantly during both 4VO and 3VO, but a complete reduction was reached in 4VO only. Thus, the incomplete occlusion, i.e. 3VO, would be sufficient in maintaining normal cortical metabolic profiles and possibly attenuate ischemic attack in cortex. (Figure 1)
1H
MRS data suggested modification in cortical tissue occurs only in case of permanent global ischemia (Figure 2). Similar results were obtained previously in ipsilateral striatum (5).
We also noticed that some metabolites changes were slightly different from murine studies (5), e.g. myo-inositol (Ins) and taurine (Tau), which might due to different brain regions and species, and remained to be studied. With such model, we can observe the striking increase of Cr and the decrease of PCr were accompanied by elevated lactate in rat cortex after during a 15-min 4VO. Figure 2 Typical localized 1H MRS (TE/TR=2.8/4000ms) of rat cortex (white squares in FSE images, A) from one control (B), during 4VO (C) and during an incomplete 4VO, namely 3VO (D) at 14.1T. Arrows indicated visually observed spectrum changes when comparing to that was acquired in control (B). ). In E, summary of selected metabolites quantified from localized 1H MRS of rat cortex from control (white, B), 4VO (black, C) and 3VO (gray). Abbreviations: GABA, g-amino butyrate; tCr, total creatine; Glu, glutamate; NAA, N-acetyl-aspartate; Lac, lactate, NAAG, N-acetyl-aspartate-glutamate; Gly, glycine.
CONCLUSION A complete 4VO can be visualized by MRA and metabolic changes in cortex can be successfully assessed by 1H MRS during and immediately after stroke.
