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Stroke. 1995;26:1859-1866

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(Stroke. 1995;26:1859-1866.)
© 1995 American Heart Association, Inc.

Articles

Amino Acid Uptake in Ischemically Compromised Brain Tissue

Andreas Jacobs, MD

From the Max-Planck-Institut für neurologische Forschung and Neurologische Universitätsklinik Köln, Köln, Germany.

Correspondence to Dr A. Jacobs, Max-Planck-Institut für neurologische Forschung and Neurologische Universitätsklinik Köln, Josef-Stelzmann-Strasse 9, D-50931 Köln, Germany.


*    Abstract
up arrowTop
 *Abstract
down arrowIntroduction
 down arrowSubjects and Methods
 down arrowResults
 down arrowDiscussion
 down arrowReferences
 
Background and Purpose Multitracer positron emission tomography (PET) was used to investigate local amino acid accumulation in brain tissue surrounding focal ischemia.

Methods PET using 15O-labeled oxygen and water for measuring cerebral metabolic rate of oxygen (CMRO2) and cerebral blood flow (CBF), C15O for determination of blood volume (CBV) and calculation of oxygen extraction fraction, and L-[11C]methylmethionine (11C-MET) for the assessment of amino acid accumulation was applied in 14 patients (mean age, 52±9.1 years) with acute ischemic hemispheric stroke. Two multitracer PET studies were completed, the first 8 to 24 hours after onset of neurological symptoms and the follow-up study 14±1 days after the ischemic attack. Functional changes were compared with morphological damage on cranial CT or MRI. Three-dimensional matching and volume of interest evaluation procedures were used to study 11C-MET accumulation in relation to various physiological variables in infarcted and noninfarcted tissue.

Results Compared with contralateral mirror regions, initially increased regional 11C-MET uptake (21.2±10.9%, P<.001) was found in patchy areas in the immediate vicinity of infarction as well as in distant areas within the same hemisphere. In those areas, regional CBF (-11.4±21.2%, P<.01) and oxygen extraction fraction (2.8±29.1%, P=NS) were highly variable, and regional CMRO2 was preserved or slightly reduced (-12.4±16.0%, P<.001). CBF data comprised severely ischemic as well as high values (14.6 to 64.2 mL/100 g per minute). Cranial CT and coregistered MRI in five patients demonstrated preserved morphology. In all peri-infarct areas (n=62), the 11C-MET uptake showed a positive correlation with {Delta}CMRO2 as the relative improvement of ipsilateral CMRO2 between the two PET studies (r=.378, P<.01). Particularly in areas with increased oxygen extraction fraction (n=42), the 11C-MET uptake showed a mild correlation with CMRO2 at follow-up measurement (r=.31, P<.05). In all peri-infarct areas, 11C-MET uptake showed a negative correlation with oxygen extraction fraction (r=-.672, P<.001) and a positive correlation with CBF (r=.4, P=.001). In all infarcted and peri-infarct areas, normalized initial 11C-MET uptake was positively correlated with CMRO2 at follow-up (r=.603, P<.001).

Conclusions Focal increases of 11C-MET uptake seen in this study were generally mild. They might be seen in the core of ischemia, indicating breakdown of the blood-brain barrier with poor tissue prognosis, but they also frequently occurred during or after ischemic compromise in surviving brain tissue surrounding focal cerebral infarction, perhaps representing alterations of amino acid transport or protein synthesis in brain tissue with a favorable prognosis.


Key Words: amino acids • cerebral ischemia, focal • tomography, emission-computed • protein synthesis


*    Introduction
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 up arrowAbstract
 *Introduction
down arrowSubjects and Methods
 down arrowResults
 down arrowDiscussion
 down arrowReferences
 
The intact protein metabolism in the brain is the prerequisite of growth and maintenance of nerve and glial cells and is involved in neuropeptide formation and enzyme activation. It also plays a role in neuronal plasticity, in the response of target nerve cells to hormones, and probably in the process of learning and memory.1 2 The rate of protein synthesis in the brain is highly sensitive to temperature changes and ischemic disturbances. During ischemia, for example, protein synthesis is inhibited at much higher flow thresholds than those necessary for maintenance of tissue energy state, neuronal function, and morphological integrity.3 Despite impaired protein synthesis, it has been shown that amino acid extraction is increased at certain periods of reperfusion after transient ischemia4 and that free amino acids are markedly accumulated in brain tissue adjacent to an ischemic focus.5 Since these results suggest differences in amino acid incorporation in necrotic, penumbral, and nonischemic tissue, local 11C-MET uptake was studied in conjunction with multitracer PET to investigate the role of amino acid uptake for the assessment of ischemically affected but viable tissue in focal infarction.


*    Subjects and Methods
up arrowTop
 up arrowAbstract
 up arrowIntroduction
 *Subjects and Methods
down arrowResults
 down arrowDiscussion
 down arrowReferences
 
Patients
PET of 15O-labeled oxygen and water for measuring CMRO2 and CBF, C15O for determination of CBV and calculation of OEF, and 11C-MET for assessment of amino acid accumulation was performed in 24 patients with acute ischemic hemispheric stroke. In 14 patients (age range, 34 to 65 years; mean, 52±9.1 years; 3 women and 11 men), two multitracer PET studies were completed. The first PET measurement was performed 8 to 24 hours after onset of neurological symptoms and the follow-up study 14±1 days after the ischemic attack. After diagnosis was made, cranial CT clinically excluded hemorrhagic or nonischemic lesions. A TCD examination was performed. Those patients with proximal complete occlusion of middle cerebral artery who received angiography and local fibrinolytic therapy were excluded from the study. Further exclusion criteria were comatose or unstable clinical states with hypertension (>200/120 mm Hg), manifest congestive heart failure or unstable cardiac arrhythmias, diabetes mellitus with unstable blood glucose, severe liver disease or renal insufficiency, and pretreatment with anticoagulants. Routine clinical, laboratory, and additional workup included general and physical examination, routine laboratory tests (blood glucose, liver/kidney/thyroid function tests, red and white blood cell and thrombocyte, antithrombin III, plasma viscosity, and fibrinogen), electrocardiogram, chest x-ray, electroencephalogram, TCD, duplex sonography, somatosensory evoked potentials, and cranial CT. In all patients a second cranial CT and/or MRI was performed within 14 days after the beginning of symptoms to demonstrate the extent of morphological damage.

Multitracer PET
Consent for the study was obtained from the patient whenever possible or from the next of kin. For all PET studies, a positron scanner with 24 detector rings (ECAT EXACT HR [8 patients] or ECAT EXACT [6 patients], Siemens CTI) was used, providing 47 contiguous transaxial image planes (slice thickness of 3.125 mm [ECAT EXACT HR] or 3.375 mm [ECAT EXACT] with a transaxial resolution [FWHM] of 3.6 to 5.8 mm).6 Initial and follow-up studies were performed with the same scanner for each patient. The contiguous slices constituting the whole brain facilitated the creation of sagittal and coronal images by data resampling. Measurements of CBF, CMRO2, OEF, and CBV were performed with patients in a resting state in accordance with the guidelines given by Baron et al.7 First, CBF was measured after bolus injection of about 2.2 GBq H215O and 3 minutes of data acquisition. Single-breath inhalation of about 1.9 GBq 15O2 with 5 minutes of data acquisition followed to obtain CMRO2. During both measurements, arterial blood activity was measured using an automated blood sampling system.8 Third, 1 minute of continuous inhalation of about 1.9 GBq C15O followed by 10 minutes of data acquisition was used to measure CBV. In addition to the primary parameters, the OEF was calculated. Details of the investigative and scanning procedure as well as image processing have been described previously.9 10 11 12 13 14 Finally, 11C-MET uptake was measured after intravenous bolus injection of approximately 20 mCi 11C-MET and 30 minutes of data sampling. Because kinetic modeling of 11C-MET uptake is complicated by its rapid metabolism and by endogenously produced amino acids,15 16 total activity was summed up for between 0 and 30 minutes of data collection. These summed images represented count rates rather than quantitative values for 11C-MET uptake. Consequently, further regional evaluation had to be based on percent side-to-side differences from mirror regions.

MR Images
In five patients, high-resolution MRI for the assessment of the extent of morphological damage was performed on a 1-T Magnetom (Siemens) in a three-dimensional fast low-angle shot mode (echo time, 15 milliseconds; repetition time, 40 milliseconds; flip angle 40°), providing 64 contiguous slices with a pixel size of 1 mm and a slice thickness of 2 mm.

Three-dimensional Coregistration
With the aid of a previously described coregistration procedure,17 18 exact three-dimensional alignment of the two PET studies and the MRI was performed to create PET and MRI brain slices with exact anatomic correspondence. First, the MRI was reoriented three-dimensionally with transaxial planes parallel to the intercommissural (anterior-posterior) line, exact anterior-posterior orientation of the frontal and occipital lobe, and craniocaudal orientation of the interhemispheric fissure and the brain stem. If no MRI had been performed, the first CBF image set was reoriented. Subsequently, the multitracer PET images of the first and second study were aligned with the MRI or first CBF image set (Fig 1Down).



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  		Figure 1. Multitracer PET on day 1 and day 14 after left-sided middle cerebral artery ischemia of a 35-year-old patient. In contrast to peri-infarct tissue, increased 11C-MET uptake in the infarct core is not an indicator for tissue outcome. METH indicates 11C-MET.

Volumes of Interest
Images were displayed on the screen of a graphic workstation as three orthogonal slices (transaxial, coronal, and sagittal) (Fig 2Down). VOIs were selected individually in each patient according to the location of the ischemic lesion. VOIs were either generated by a volume growing algorithm, with lower and upper thresholds for voxel values, or were outlined manually by drawing their projections on all three orthogonal slices (the VOI is defined as the largest volume that can be included in these projections). The VOIs were placed in acute-stage PET images to differentiate between three tissue compartments: (1) initially infarcted tissue and initially penumbral tissue that will either (2) turn into infarction or (3) will survive. First, the core of infarction was defined within the CMRO2 image by an upper threshold for viable tissue of 60 µmol/100 g per minute. Second, three different sets of VOIs were determined within the tissue surrounding the infarct core: (1) volumes with increased 11C-MET uptake (VOI 1) (defined by all contiguous voxels with values higher than the mean 11C-MET uptake plus one SD of the contralateral hemisphere of the same patient; mean and SD of contralateral 11C-MET uptake were determined by all contiguous voxels of one VOI covering the whole contralateral hemisphere sparing the ventricles); (2) volumes with visually increased OEF (VOI 2); and (3) volumes where 11C-MET uptake and OEF were increased (VOI 3) (intersections of VOI 1 and 2). All VOIs were mirrored to the contralateral hemisphere, thus providing reference VOIs for side-to-side comparison. Finally, all VOIs were stored, and their contents were determined for all matched multitracer PET images from both studies in the identical location.



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  		Figure 2. Three-dimensional VOI evaluation technique of multitracer PET. VOI 1 is defined by increased 11C-MET uptake in the immediate vicinity of an infarction (bottom row), which seems to be an indicator for good tissue prognosis, and transferred to the other image modalities (CBF [top row] and CMRO2 [middle row]). In the anterior part of the VOI, an overlapping area with increased OEF was found with preserved morphology on MRI (see Fig 4Up).

Statistics
Data are reported as mean±SD of absolute values and as percent differences between VOIs and corresponding mirror volumes in the contralateral hemisphere. Statistical analysis for nonparametric Wilcoxon tests and correlations were performed; calculations were done with a commercial software package (SPSS 6.0, SPSS Inc).


*    Results
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 up arrowAbstract
 up arrowIntroduction
 up arrowSubjects and Methods
 *Results
down arrowDiscussion
 down arrowReferences
 
Five of 14 patients presented with a cortical/subcortical infarction within the territory of the middle cerebral artery (4 left, 1 right) (Fig 1Up). The size of infarction ranged from 3.4 to 48.4 cm3. Four patients suffered from a large striato-pallido-capsular infarction (1 left, 3 right) (8.7 to 15.9 cm3). Three patients had small infarcts (0.5 to 1.2 cm3) in the internal capsule (1 left, 2 right), 1 patient within the left-sided thalamus. In 1 patient (clinically a prolonged reversible ischemic neurological deficit in the territory of the right middle cerebral artery), no infarct could be seen on follow-up cranial CT. In the latter two patients, no focal abnormality was seen with PET, and no VOIs could be defined. Table 1Down, therefore, summarizes the multiparametric PET values of only 12 patients in the infarct and the surrounding tissue. Sixty-two peri-infarct VOIs (VOI 1 through 3) could be defined, at least three in each patient (mean, 5.3±2.0; maximum, nine). The size of most VOIs (55 of 62) ranged between 1.1 and 12.7 cm3 (mean, 4.2±2.7 cm3). One large VOI measured 28.6 cm3. Six VOIs in 5 patients were less than 1 cm3 (0.5 to 0.98 cm3; mean, 0.73±0.17 cm3). The relative 11C-MET accumulation was calculated as percent difference from the contralateral mirror region. To decrease interindividual variability, all other parameters were also expressed as percent difference between regions and corresponding mirror regions.


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  		Table 1. Mean±SD Values of CBF, CMRO2, OEF, and Percent 11C-MET Difference During 8 to 24 Hours After Ischemic Stroke and CMRO22 2 Weeks Later in the Core of Infarction, Surrounding Tissue, and Corresponding Contralateral Regions

Compared with contralateral mirror regions, regional CBF1 (-40.1±45.2%, P<.05), CMRO21 (-62.0±16.1%, P<.01), and OEF1 (-35.2±36.5%, P<.05) were grossly depressed within the infarct core at the first measurement (indicated by "1"; second measurement, "2"), whereas 11C-MET accumulation was variable (-13.0±28.5%, P=.21) (Table 1Up). In 7 patients, it was reduced between -10.4% to -41.4%; in 3 patients, it was increased by 4.5% to 12.7% (Fig 1Up); and in 1 patient with postischemic luxury perfusion it was even more increased (47.4%) (Table 2Down).


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  		Table 2. Individual Data (in Percent Difference) of CBF, CMRO2, OEF, and 11C-MET During 8 to 24 Hours After Ischemic Stroke and CMRO22 2 Weeks Later for 12 Subjects With Respect to Percent Asymmetries Between VOIs and Corresponding Mirror Volumes for Infarct VOIs and Type 1 VOIs (11C-MET Increased)

In the surrounding tissue, heterogeneous changes were observed. In the immediate vicinity of the core of infarction, areas with increased OEF1 (68.7±41.7%, P<.001; VOIs 2) showed a reduction of CBF1 (-35.7±18.6%, P<.001) and variable changes in 11C-MET uptake (range, -11.5% to 13.1%; mean, 0.2±6.2%). Volumes with increased 11C-MET uptake (21.2±10.9%, P<.001; VOIs 1) were found in immediate peri-infarct tissue (Fig 2Up) as well as in more distant areas (Fig 3Down). In those areas, CBF1 was highly variable (minimum 14.6 and maximum 64.2 mL/100 g per minute; mean, -11.4±21.2%; P<.01), CMRO21 was preserved or slightly reduced (-12.4±16.0%, P<.001), and OEF1 was also variable (range, -47.5% to 93.7%; mean, 2.8±29.1%) (Table 2Up). Preserved morphology was demonstrated on matched MRI in 5 patients in volumes with increased 11C-MET uptake as well as in peri-infarct regions, where OEF1 and 11C-MET uptake were both elevated (Fig 4Down). In all peri-infarct regions, changes in oxygen metabolism between the first and second measurement ({Delta}CMRO2=percent difference between CMRO21 and CMRO22) were variable (range, -64.5% to 65.9%; mean, 1.0±23.9%). In areas with increased OEF1 (VOIs 2), CMRO2 initially seemed to be preserved (1.3±22.3%, P=NS) but deteriorated over the next 2 weeks ({Delta}CMRO2=-11.6±19.4%, P=.01). In contrast, in areas with increased 11C-MET uptake (VOIs 1), a tendency for improvement of the initially slightly decreased CMRO2 was found ({Delta}CMRO2=9.0±23.9%, P=.08).



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  		Figure 3. Multitracer PET on day 1 and day 14 after left-sided middle cerebral artery ischemia of a 35-year-old patient (same as in Fig 1Up) showing increased 11C-MET accumulation distant from the infarct core in frontoparietal cortex with apparently preserved morphology on cranial CT and matched MRI despite initially reduced CMRO2, which improved at follow-up (mean values in parietal cortex were 113.9 and 136.5 µmol/100 g per minute in the acute and chronic stage, respectively). METH indicates 11C-MET.




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  		Figure 4. Multitracer PET of type 3 VOI (intersection of VOIs with increased 11C-MET uptake and increased OEF) demonstrating an example with regional increased OEF in conjunction with increased 11C-MET uptake (top) and where morphology was preserved on matched MRI (bottom). METH indicates 11C-MET.

The Spearman's correlation coefficients were calculated between the ranks of regional side-to-side asymmetries for all VOIs (n=73) as well as for the infarct core (n=11) and the peri-infarct VOIs (n=62). Spearman's correlation coefficients are given in Table 3Down. In all peri-infarct VOIs, the 11C-MET uptake showed a negative correlation with CMRO21 in the first measurement (r=-.339, P=.01), a nonsignificant correlation with CMRO22 at follow-up (r=-.069), and a positive correlation with {Delta}CMRO2 as the relative improvement of ipsilateral CMRO2 at follow-up (r=.378, P<.01) (Table 3Down). In contrast, OEF1 showed a positive correlation with CMRO21 (r=.494, P<.001), no correlation with CMRO22 (r=-.129), and a negative correlation with {Delta}CMRO2 as the relative deterioration of ipsilateral CMRO2 at follow-up (r=-.28, P<.05). Particularly in peri-infarct VOIs with increased OEF (n=42), the 11C-MET uptake was significantly correlated with CMRO22 (r=.31, P<.05). A positive correlation between initial 11C-MET uptake and CMRO22 at follow-up was also found for all brain volumes (infarct core+peri-infarct tissue; n=73) when normalized regional values (regional value divided by the mean of the ipsilateral hemisphere) were used (r=.603, P<.001; Fig 5Down). In all peri-infarct VOIs, the 11C-MET uptake showed a negative correlation with OEF1 (r=-.672, P<.001) and a positive correlation with CBF1 (r=.4, P=.001). A positive correlation with CBF1 was also found in the infarct core (r=.746, P<.01). In peri-infarct VOIs, OEF1 was negatively correlated with CBF1 (r=-.757, P<.001), and CBF1 was positively correlated with CMRO22 (r=.482, P<.001).


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  		Table 3. Summarized Spearman's Rank Correlation Coefficients of Regional Side-to-Side Asymmetries of CBF, OEF, CMRO2, {Delta}CMRO2, and 11C-MET in the First and Second Measurement for All Volumes as Well as Infarct Core and Peri-Infarct Tissue Alone



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  		Figure 5. Plot shows correlation between initial regional 11C-MET uptake and regional oxygen consumption at follow-up.


*    Discussion
up arrowTop
 up arrowAbstract
 up arrowIntroduction
 up arrowSubjects and Methods
 up arrowResults
 *Discussion
down arrowReferences
 
Increased 11C-MET uptake was found (1) in some areas of the core of cerebral infarction (Fig 1Up), (2) in viable tissue immediately adjacent to the core of infarction (Fig 2Up), and (3) in remote areas of viable tissue within the infarcted hemisphere (Fig 3Up). Also, in the two patients who exhibited no infarct on either PET or structural imaging, no focal increased 11C-MET uptake was observed. Although in most cases 11C-MET uptake was decreased within the core of infarction, in some patients it was increased. This is thought to be due either to a disruption of the blood-brain barrier or to postischemic hyperperfusion. In the following discussion, however, I will focus on the mechanisms of increased 11C-MET uptake in viable, presumably ischemically compromised, tissue in the vicinities both immediate and distant to the core of infarction and its role as indicator for tissue prognosis.

Increased 11C-MET uptake outside the core of infarction seems to be related to a favorable tissue prognosis deduced from its correlation with the oxygen consumption at the second PET study (Fig 5Up). Preserved morphology was demonstrated on matched MRI in five patients in areas with increased 11C-MET uptake as well as in peri-infarct regions, where OEF and 11C-MET uptake were both elevated (Fig 4Up). Particularly in areas with increased OEF, the 11C-MET uptake was correlated with CMRO2 at follow-up. The 11C-MET accumulation took place in areas where CBF and OEF were variable and where CMRO2 was slightly decreased. As deduced from the reduction of the initial CMRO21 in areas with increased 11C-MET uptake, this tissue is thought to have been ischemically compromised for a period of unknown duration before the PET study. Also, some areas were in ongoing ischemic compromise during the PET study (OEF1 increased).

The ischemic penumbra was first defined as a region around focal ischemia, where decreased flow led to functional impairment (electrical failure) but was high enough to prevent morphological damage (membrane failure) and had the capacity to recover if perfusion improved.19 20 21 The most pertinent results of the complex pathophysiological changes within the penumbra during the early course after ischemic stroke have been obtained by multitracer PET. Previous studies showed that most penumbra zones with decreased CBF, increased OEF, and relatively preserved metabolism turn into infarction if low-flow values persist.22 23 24 25 However, hyperperfusion occurring early after ischemic attack and affecting tissue with little metabolic alteration was associated with a good prognosis.26 Our findings suggest that two different partly overlapping zones of penumbra might exist: first, potentially viable tissue with increased OEF as a sign of preserved metabolism in demand of blood supply in the immediate vicinity of focal infarction with a questionable prognosis; and second, previously or still ischemically compromised tissue in near and remote areas, where only short-lasting ischemia took place, leading to increased 11C-MET uptake as a sign of a more favorable prognosis. Yet, how can the patchy distribution and the different interindividual character of increased 11C-MET uptake in ischemically stressed tissue and its role for tissue prognosis be explained?

Before being incorporated into proteins in brain tissue, amino acids must cross the blood-brain barrier. This crossing is governed by a transport mechanism that requires the presence of a carrier system.16 After being transported into the cell, any amino acid is converted primarily into aminoacyl tRNA, the first step of protein synthesis. In addition to incorporation into proteins, amino acids also serve for complex nonprotein metabolism. This and recycling of endogenous amino acids make kinetic modeling difficult.1 16 27 28 As pointed out by Vaalburg et al,28 11C-MET does not fit several of their proposed criteria, which have to be met to make an amino acid suitable for quantitative determination of protein synthesis. Its main value should be seen as a marker of methionine transport in tissue.28 Therefore, we only measured local activity (count rate) of 11C-MET, which was shown to be regionally correlated with incorporation into proteins in normal brain,29 and we had to rely on percent interhemispheric differences for regional and interindividual comparison.

In pathological states, eg, ischemia or tumor, different aspects have to be considered as influencing local 11C-MET uptake, such as blood supply, function of the blood-brain barrier, tissue heterogeneity, breakdown of protein synthesis with energy depletion, and secondary induction of widespread expression of immediate early genes and HSPs. The correlation with CBF demonstrates that 11C-MET is a flow-dependent marker. During the first few minutes after tracer application, the flow dependency of local accumulation of 11C-MET is greatest. Ignoring this early interval for image construction by summing up 11C-MET data during the 5- to 30-minute or 10- to 30-minute period, some of the influence of CBF could have been avoided. However, because of the rapid decay of 11C-MET, summing up data over the 0- to 30-minute interval has the advantage of improving the signal-to-noise ratio significantly. In addition, despite a comparatively high CBF threshold of protein synthesis,3 25 increased 11C-MET uptake was found in low-flow areas, indicating that flow-independent mechanisms must contribute to increased 11C-MET uptake also.

In transient ischemia, a very early alteration of the blood-brain barrier with increased capillary permeability to small molecules like amino acids could be demonstrated.4 30 Yoshimine et al5 could not differentiate whether the early increased level of free amino acids might be due to increased capillary permeability of affected vessels or to an activated carrier-mediated transport. The complex course of ischemia-induced changes of blood-brain barrier (including alterations of endothelial cell reactivity, coagulation system and platelet activation, granulocyte–endothelial cell interactions, and free oxygen radical and nitric oxide generation) is currently under investigation.31 32 In the clinically most relevant stroke model of nonocclusive common carotid artery thrombosis,32 acute (15 minutes to 4 hours) alterations of blood-brain barrier function were spatially correlated with platelet emboli and led to multiple foci of protein leakage throughout the ipsilateral hemisphere. Platelet-borne substances such as oxygen free radicals and serotonin are thought to play a major role in altering vasoactivity and permeability of neighboring downstream vessels.32 This could be one explanation for the patchy distribution of increased 11C-MET uptake even in areas distant to focal infarction. While acute microvascular changes seem to be widespread and largely transient, in later stages (more than 24 hours) leaky sites of prolonged protein extravasation were found to be more restricted and commonly associated with microinfarction.32 Because the temporal and spatial pattern, as well as the extent of infarction in the patient population under investigation, is heterogeneous, one can only assume that blood-brain barrier changes play a role in at least some areas of increased 11C-MET uptake.

An exploding body of literature stresses the regional time-dependent expression of immediate early genes and HSPs, which were found to be widespread and remote from infarction.33 34 35 36 37 38 39 40 Despite global depression of most mRNAs and their proteins during cerebral ischemia, several groups of genes (eg, immediate early genes and HSP) are expressed rapidly after ischemia, and an increased 11C-MET uptake might be related (in part) to this gene expression, although experimental studies on regional relationships between amino acid uptake and the expression of immediate early genes and HSP are missing. Transient and permanent ischemia induce temporal- and spatial-dependent c-fos protein expression immediately adjacent and remote from the ischemic territory.33 34 The mechanism of this diffuse cortical induction of c-fos is thought to be caused by spreading depression.36 39 HSP 70 induction can be considered as an index of cell stress and is proposed to represent some degree of injury that might or might not be lethal.39 With prolonged ischemia, HSP 70 induction takes place in neurons of the penumbra but not in areas destined to infarct.38 I speculate that increased 11C-MET uptake in part of the vicinity of focal infarction might indicate ischemically affected reperfused tissue where excitatory amino acid activation, immediate early gene, and HSP expression take place. The patchy distribution of 11C-MET accumulation might reflect the temporal pattern of immediate early gene and HSP expression in tissue with different stages of ischemia.

The VOI methodology used in this study has certain advantages and disadvantages: the pathophysiological changes in and around acute focal ischemia follow a complex spatial and temporal pattern. In the clinical setting, only a few of the many factors contributing to the complex processes (CBF, CBV, CMRO2, OEF, and 11C-MET uptake) can be assessed by means of PET. As different stages of ischemia may be in areas of immediate vicinity at one time, the main interest was to identify areas (respective volumes) where the same pathophysiological changes take place. With the definition of individual volumes (defined by the pathophysiological variable of interest) rather than schematic regions, the attempt was made to search for and identify these individual changes around the infarction area. Because penumbral tissue is of special interest in the evolution of new therapeutic strategies, attention was focused on the importance of the patchy areas of increased 11C-MET uptake in the immediate vicinity of infarction for tissue prognosis in comparison to the neighboring areas with established ischemic penumbra in terms of increased OEF. VOIs with increased 11C-MET uptake had to be based on pixels with uptake greater than 1 SD only (rather than 2 SD) of the contralateral hemisphere because side-to-side-differences were generally mild and because the variance calculated across all pixels in the contralateral hemisphere was high due to relatively high gray/white matter differences. Therefore, not each of the mild but obvious regional 11C-MET increases might necessarily be statistically significant if taken individually. From all PET parameters, the OEF images are built up with the highest signal-to-noise ratio. Therefore, the attempt to get reproducible and convenient VOIs with increased OEF by the growing algorithm failed, and the VOIs had to be determined subjectively by visual assessment.

In conclusion, two mechanisms of increased focal 11C-MET uptake in viable brain tissue surrounding acute infarction should be considered: (1) widespread transient ischemia with alteration of the blood-brain barrier and early reperfusion with blood flow–dependent 11C-MET accumulation, and (2) activated carrier-mediated transport of 11C-MET as an index of cell stress reflecting induction of immediate early gene and HSP expression. In both cases, the increased 11C-MET uptake would be an indicator for previously ischemically compromised tissue with a favorable prognosis. In contrast, in the rare cases of increased 11C-MET uptake within the core of infarction, 11C-MET cannot be seen as a marker for tissue prognosis but indicates failure of the blood-brain barrier or postischemic hyperperfusion.


*    Selected Abbreviations and Acronyms
 
CBF = cerebral blood flow
CBV = cerebral blood volume
11C-MET = L-[11C]methylmethionine
CMRO2 = cerebral metabolic rate of oxygen
HSP = heat shock protein
OEF = oxygen extraction fraction
PET = positron emission tomography
TCD = transcranial Doppler ultrasound
VOI = volume of interest
1 = first measurement
2 = second measurement

Received March 3, 1995; revision received July 6, 1995; accepted July 6, 1995.


*    References
up arrowTop
 up arrowAbstract
 up arrowIntroduction
 up arrowSubjects and Methods
 up arrowResults
 up arrowDiscussion
 *References
 

  1. Lajtha A, Dunlop D, Banay-Schwartz M. Cerebral protein turnover: aspects and problems. In: Mazoyer BM, Heiss W-D, Comar D, eds. PET Studies on Amino Acid Metabolism and Protein Synthesis. Dordrecht-Boston-London: Kluwer Academic Publishers; 1993:1-17.
  2. Smith CB. Determination of regional rates of cerebral protein synthesis in vivo with L-[1-14C]leucine as the tracer amino acid. In: Mazoyer BM, Heiss W-D, Comar D, eds. PET Studies on Amino Acid Metabolism and Protein Synthesis. Dordrecht-Boston-London: Kluwer Academic Publishers; 1993:19-39.
  3. Mies G, Ishimaru S, Xie Y, Seo K, Hossmann K-A. Ischemic thresholds of cerebral protein synthesis and energy state following middle cerebral artery occlusion in rats. J Cereb Blood Flow Metab. 1991;11:753-761. [Medline] [Order article via Infotrieve]
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