Evaluation of regional cerebral blood flow using single photon emission tomography for the selection of patients for local fibrinolytic therapy of acute cerebral embolism

Diagnostic and prognostic benefit of arterial spin labeling in subacute stroke

BACKGROUND AND PURPOSE

Brain perfusion measurement in the subacute phase of stroke may support therapeutic decisions. We evaluated whether arterial spin labeling (ASL), a noninvasive perfusion imaging technique based on magnetic resonance imaging (MRI), adds diagnostic and prognostic benefit to diffusion-weighted imaging (DWI) in subacute stroke.

METHODS

In a single-center imaging study, patients with DWI lesion(s) in the middle cerebral artery (MCA) territory were included. Onset to imaging time was ≤7 days and imaging included ASL and DWI sequences. Qualitative (standardized visual analysis) and quantitative perfusion analyses (region of interest analysis) were performed. Dichotomized early outcome (modified Rankin Scale [mRS] 0-2 vs. 3-6) was analyzed in two logistic regression models. Model 1 included DWI lesion volume, age, vascular pathology, admission NIHSS, and acute stroke treatment as covariates. Model 2 added the ASL-based perfusion pattern to Model 1. Receiver-operating-characteristic (ROC) and area-under-the-curve (AUC) were calculated for both models to assess their predictive power. The likelihood-ratio-test compared both models.

RESULTS

Thirty-eight patients were included (median age 70 years, admission NIHSS 4, onset to imaging time 67 hr, discharge mRS 2). Qualitative perfusion analysis yielded additional diagnostic information in 84% of the patients. In the quantitative analysis, AUC for outcome prediction was 0.88 (95% CI 0.77-0.99) for Model 1 and 0.97 (95% CI 0.91-1.00) for Model 2. Inclusion of perfusion data significantly improved performance and outcome prediction (p = 0.002) of stroke imaging.

CONCLUSIONS

In patients with subacute stroke, our study showed that adding perfusion imaging to structural imaging and clinical data significantly improved outcome prediction. This highlights the usefulness of ASL and noninvasive perfusion biomarkers in stroke diagnosis and management.

PET measurements of CBF, OEF, and CMRO2 without arterial sampling in hyperacute ischemic stroke: Method and error analysis

A method for relative measurement of cerebral blood flow (CBF), oxygen extraction fraction (OEF), and metabolic rate of oxygen (CMRO2) using positron emission tomography (PET) without arterial sampling in patients with hyperacute ischemic stroke was presented.

METHODS

The method requires two PET scans, one for H2(15)O injection and one for 15O2 inhalation, and calculates regional CBF, CMRO2, and OEF relative to those at the reference brain region by means of table-lookup method. In this study, we calculated "relative lookup-tables" which relate relative CBF to relative H2(15)O count, relative CMRO2 to relative 15O2 count, and relative OEF to relative 15O2/H2(15)O count. Two assumptions were applied to the lookup-table calculation: 1) In the reference region. CBF and OEF were assumed to be 50.0 ml/min/100 ml and 0.40, respectively, 2) Cerebral blood volume (CBV) was assumed to be constant at 4.0 ml/100 ml over the whole brain. Simulation studies were done to estimate the error of the present method derived from the assumptions.

RESULTS

For relative CBF measurements, 20% variation in reference CBF gave about +/- 10% error for measured relative CBF at maximum. Changes in CBV caused relatively large errors in measured OEF and CMRO2 when relative CBF and OEF decreased. Errors for measured relative OEF caused by 50% variation in CBV were within +/- 8% at 0.8 of relative CBF and +/- 12% at 0.4 of relative CBF when relative OEF was greater than 1.0.

CONCLUSION

CBV effects caused larger errors in estimated OEF and CMRO2 in the region of the ischemic core with decreasing relative CBF and/or OEF but only slight errors in the region of "misery perfusion" with relative OEF values greater than 1.0. The present method makes PET measurements simpler than with the conventional method and increases understanding of the cerebral circulation and oxygen metabolism in patients with hyperacute stroke of several hours after onset.

Computed tomography perfusion imaging in acute stroke

The development of thrombolytic and neuroprotective agents for the treatment of acute stroke has created an imperative for improved imaging techniques in the assessment of acute stroke. Five cases are presented to illustrate the value of perfusion CT in the evaluation of suspected acute stroke. To obtain the perfusion data, a rapid series of images was acquired without table movement following a bolus of contrast medium. Cerebral blood flow, cerebral blood volume and mean transit time were determined by mathematically modelling the temporal changes in contrast enhancement in the brain and vascular system. Pixel-by-pixel analysis allowed generation of perfusion maps. In two cases, CT-perfusion imaging usefully excluded acute stroke, including one patient in whom a low-density area on conventional CT was subsequently proven to be tumour. Cerebral ischaemia was confirmed in three cases, one with an old and a new infarction, one with a large conventional CT abnormality but only a small perfusion defect, and one demonstrating infarct and penumbra. Perfusion CT offers the ability to positively identify patients with non-haemorrhagic stroke in the presence of a normal conventional CT, to select those cases where thrombolysis is appropriate, and to provide an indication for prognosis.

Comparison of admission perfusion computed tomography and qualitative diffusion- and perfusion-weighted magnetic resonance imaging in acute stroke patients

BACKGROUND AND PURPOSE

Besides classic criteria, cerebral perfusion imaging could improve patient selection for thrombolytic therapy. The purpose of this study was to compare quantitative perfusion CT imaging and qualitative diffusion- and perfusion-weighted MRI (DWI and PWI) in acute stroke patients at the time of their emergency evaluation.

METHODS

Thirteen acute stroke patients underwent perfusion CT and DWI or PWI on admission. The size of infarct and ischemic lesion (infarct plus penumbra) on the admission perfusion CT was compared with that of the MR abnormalities as shown on the DWI trace and on the relative cerebral blood volume, cerebral blood flow, time to peak, and mean transit time maps calculated from PWI studies.

RESULTS

The most significant correlation was found between infarct size on the admission perfusion CT and abnormality size on the admission DWI map (r=0.968, P<0.001). A significant correlation was also observed between the size of the ischemic lesion (infarct plus penumbra) on the admission perfusion CT and the abnormality size on the mean transit time map calculated from admission PWI (r=0.946, P<0.001). Information about cerebral infarct and total ischemia (infarct plus penumbra) carried by both imaging techniques was similar, with slopes of 0.913 and 0.905, respectively.

CONCLUSIONS

An imaging technique may be helpful in the identification of cerebral penumbra in acute stroke patients and thus in the selection of patients for thrombolytic therapy. Perfusion CT and DWI/PWI are equivalent in this task.

Comparative evaluation of cerebral blood volume and cerebral blood flow in acute ischemic stroke by using perfusion-weighted MR imaging and SPECT

PURPOSE

To investigate the relationship between relative cerebral blood volume (CBV) measured with perfusion-weighted (PW) MR imaging and relative cerebral blood flow (CBF) measured with SPECT in acute ischemic stroke.

MATERIAL AND METHODS

Fifteen patients who had acute unilateral middle cerebral artery occlusion underwent both PW MR imaging and 99mTc-HMPAO SPECT with an interval less than 20 min between the two examinations within 6 h after stroke onset. Lesion-to-contralateral relative CBV and CBF ratios measured in multiple regions of interest were compared to evaluate the relationship of the two parameters.

RESULTS

An overall linear relationship was found between relative CBV and relative CBF ratios (R2 = 0.54, p < 0.0001). The two parameters correlated linearly to each other in regions with evolving infarction (R2 = 0.43, p<0.0001), but not in regions without evolving infarction (R2 = 0.001, p>0.05). Regions with evolving infarction had more severe hypoperfusion (mean relative CBF ratio, 0.38 +/- 0.22) than regions without (mean relative CBF ratio, 0.70+/-0.13) (p<0.0001).

CONCLUSION

A significant linear relationship existed between relative CBV and relative CBF in acute ischemic stroke, although relative CBV did not change linearly to relative CBF in mild hypoperfusion. Relative CBV can be used as an alternative to relative CBF within 6 h after stroke onset, particularly in regions with severe hypoperfusion proceeding to infarction.

Prognostic accuracy of cerebral blood flow measurement by perfusion computed tomography, at the time of emergency room admission, in acute stroke patients

The purpose of this study was to determine the prognostic accuracy of perfusion computed tomography (CT), performed at the time of emergency room admission, in acute stroke patients. Accuracy was determined by comparison of perfusion CT with delayed magnetic resonance (MR) and by monitoring the evolution of each patient's clinical condition. Twenty-two acute stroke patients underwent perfusion CT covering four contiguous 10mm slices on admission, as well as delayed MR, performed after a median interval of 3 days after emergency room admission. Eight were treated with thrombolytic agents. Infarct size on the admission perfusion CT was compared with that on the delayed diffusion-weighted (DWI)-MR, chosen as the gold standard. Delayed magnetic resonance angiography and perfusion-weighted MR were used to detect recanalization. A potential recuperation ratio, defined as PRR = penumbra size/(penumbra size + infarct size) on the admission perfusion CT, was compared with the evolution in each patient's clinical condition, defined by the National Institutes of Health Stroke Scale (NIHSS). In the 8 cases with arterial recanalization, the size of the cerebral infarct on the delayed DWI-MR was larger than or equal to that of the infarct on the admission perfusion CT, but smaller than or equal to that of the ischemic lesion on the admission perfusion CT; and the observed improvement in the NIHSS correlated with the PRR (correlation coefficient = 0.833). In the 14 cases with persistent arterial occlusion, infarct size on the delayed DWI-MR correlated with ischemic lesion size on the admission perfusion CT (r = 0.958). In all 22 patients, the admission NIHSS correlated with the size of the ischemic area on the admission perfusion CT (r = 0.627). Based on these findings, we conclude that perfusion CT allows the accurate prediction of the final infarct size and the evaluation of clinical prognosis for acute stroke patients at the time of emergency evaluation. It may also provide information about the extent of the penumbra. Perfusion CT could therefore be a valuable tool in the early management of acute stroke patients.

Estimation of Regional Cerebral Blood Flow Levels in Ischemia Using [15O]Water or [11C]Flumazenil PET Without Arterial Input Function

Determination of residual flow within the ischemic brain may be important in patients eligible for thrombolytic therapy. Conventional cerebral blood flow (CBF) quantification often cannot be achieved in these cases because arterial blood sampling required for the input function is prohibited. Quantitative CBF was measured in 20 patients with ischemic stroke using [(15)O]water and [(11)C]flumazenil PET with arterial blood sampling and compared with normalized nonquantified data of the same patients. For both comparisons, percentiles were derived, allowing the determination of a normalized blood flow level corresponding to an absolute CBF value below which 95% of all pixels are located. These normograms can be used to derive absolute CBF levels in milliliters per 100 g per minute from normalized data without the need for measuring an arterial input function.

Quantitative assessment of the ischemic brain by means of perfusion-related parameters derived from perfusion CT

BACKGROUND AND PURPOSE

Besides the delineation of hypoperfused brain tissue, the characterization of ischemia with respect to severity is of major clinical relevance, because the degree of hypoperfusion is the most critical factor in determining whether an ischemic lesion becomes an infarct or represents viable brain tissue. CT perfusion imaging yields a set of perfusion related parameters which might be useful to describe the hemodynamic status of the ischemic brain. Our objective was to determine whether measurements of the relative cerebral blood flow (rCBF), relative cerebral blood volume (rCBV), and relative time to peak (rTP) can be used to differentiate areas undergoing infarction from reversible ischemic tissue.

METHODS

In 34 patients with acute hemispheric ischemic stroke <6 hours after onset, perfusion CT was used to calculate rCBF, rCBV, and rTP values from areas of ischemic cortical and subcortical gray matter. Results were obtained separately from areas of infarction and noninfarction, according to the findings on follow-up imaging studies. The efficiency of each parameter to predict tissue outcome was tested.

RESULTS

There was a significant difference between infarct and peri-infarct tissue for both rCBF and rCBV but not for rTP. Threshold values of 0.48 and 0.60 for rCBF and rCBV, respectively, were found to discriminate best between areas of infarction and noninfarction, with the efficiency of the rCBV being slightly superior to that of rCBF. The prediction of tissue outcome could not be increased by using a combination of various perfusion parameters.

CONCLUSIONS

The assessment of cerebral ischemia by means of perfusion parameters derived from perfusion CT provides valuable information to predict tissue outcome. Quantitative analyses of the severity of ischemic lesions should be implemented into the diagnostic management of stroke patients.

Prediction of acute embolic stroke outcome after local intraarterial thrombolysis: value of pretreatment and posttreatment 99mTc-ethyl cysteinate dimer single photon emission computed tomography

The aim of this study was to investigate the efficacy of pre- and posttreatment 99mTc-ethyl cysteinate dimer (99mTc-ECD) single photon emission computed tomography (SPECT) for predicting the ischemic outcome of embolic middle cerebral artery occlusion after treatment with local intraarterial thrombolysis. The authors examined 28 patients with a moderately ischemic area (ratio of affected regional activity to cerebellar activity (A/C ratio) of 0.4 to 0.7) determined using pretreatment SPECT, and with complete recanalization within 6 hours. Posttreatment dynamic and static SPECT studies were performed immediately after thrombolysis. The extent of the affected area outlined on pretreatment SPECT was used for the posttreatment SPECT images, and A/C ratios were calculated. The relative retention ratio of 99mTc-ECD in the affected area was also analyzed using posttreatment dynamic SPECT. Fourteen patients either without infarction or with small subcortical and basal ganglial infarction, 11 patients with medium or large cortical infarction, and 3 patients with hemorrhage were identified by follow-up computed tomography. Ischemic outcome correlated with the relative retention ratio of 99mTc-ECD more closely than either the pre- or posttreatment A/C ratios. In particular, a threshold value for the development of hemorrhage was distinct only in the relative retention ratio of 99mTc-ECD. Pretreatment 99mTc-ECD SPECT did not always predict the occurrence of hemorrhagic transformation, whereas dynamic 99mTc-ECD SPECT performed immediately after thrombolysis allowed clear identification of patients at risk for hemorrhagic transformation.

Ischemic penumbra: evidence from functional imaging in man

The ischemic penumbra is defined as tissue with flow within the thresholds for maintenance of function and of morphologic integrity. Penumbra tissue has the potential for recovery and therefore is the target for interventional therapy in acute ischemic stroke. The identification of the penumbra necessitates measuring flow reduced less than the functional threshold and differentiating between morphologic integrity and damage. This can be achieved by multitracer positron emission tomography (PET) and perfusion-weighted (PW) and diffusion-weighted magnetic resonance imaging (DW-MRI) in experimental models, in which the recovery of critically perfused tissue or its conversion to infarction was documented in repeat studies. Neuroimaging modalities applied in patients with acute ischemic stroke--multitracer PET, PW- and DW-MRI, single photon emission computed tomography (SPECT), perfusion, and Xe-enhanced computed tomography (CT)-- often cannot reliably identify penumbra tissue: multitracer studies for the assessment of flow and irreversible metabolic damage usually cannot be performed in the clinical setting; CT and MRI do not reliably detect irreversible damage in the first hours after stroke, and even DW-MRI may be misleading in some cases: determinations of perfusion alone yield a poor estimate of the state of the tissue as long as the time course of changes is not known in individual cases. Therefore, the range of flow values in ischemic tissue found later, either within or outside the infarct, was rather broad. New tracers--for example, receptor ligands or hypoxia markers--might improve the identification of penumbra tissue in the future. Despite these methodologic limitations, the validity of the concept of the penumbra was proven in several therapeutic studies in which thrombolytic treatment reversed critical ischemia and decreased the volume of final infarcts. Such neuroimaging findings might serve as surrogate targets in the selection of other therapeutic strategies for large clinical trials.

Outcome in acute stroke with successful intra-arterial thrombolysis and predictive value of initial single-photon emission-computed tomography

This study investigates retrospectively, in selected patients, the ischemic outcome (reversible ischemia, infarction, and hemorrhage) and neurologic outcome of acute stroke treated with intra-arterial thrombolysis and the predictive value of pretreatment single-photon emission-computed tomography (SPECT). Thirty patients with complete recanalization within 12 hours were analyzed. The extent of ischemia was outlined on SPECT, and two CBF parameters were calculated: the ratio of ischemic regional activity to CBF in the cerebellum and the asymmetry index. Reversible ischemia, infarction, and hemorrhage were identified by comparing SPECT and follow-up computed tomography. Nine patients (30%) had no or small infarction, 14 (47%) had medium or large infarction, and seven (23%) had hemorrhage. Forty-two lesions were identified (22 reversible ischemia, 13 infarction, and 7 hemorrhage). Duration of ischemia, urokinase dose, disease type, and occlusion site were nonsignificant factors, whereas neurologic outcome and CBF parameters were significant among the three patient groups and three types of ischemic lesions. Ischemic tissue with CBF greater than 55% of cerebellar flow still may be salvageable, even with treatment initiated 6 hours after onset of symptoms. Ischemic tissue with CBF greater than 35% of cerebellar flow still may be salvageable with early treatment (less than 5 hours). Ischemic tissue with with CBF less than 35% of cerebellar flow may be at risk for hemorrhage within the critical time window. Pretreatment SPECT can provide useful parameters to increase the efficacy of thrombolysis by reducing hemorrhagic complications and improving neurologic outcome.

Tissue at risk of infarction rescued by early reperfusion: a positron emission tomography study in systemic recombinant tissue plasminogen activator thrombolysis of acute stroke

Thrombolytic therapy of acute ischemic stroke can be successful only as long as there is penumbral tissue perfused at rates between the thresholds of normal function and irreversible structural damage, respectively. To determine the proportion of tissue at risk of infarction, cerebral perfusion was studied in 12 patients with acute ischemic stroke who underwent treatment with systemic recombinant tissue plasminogen activator (0.9 mg/kg body weight according to National Institute of Neurological Disorders and Stroke protocol) within 3 hours of onset of symptoms, using [15O]-H2O positron emission tomography (PET) before or during, and repeatedly after thrombolysis. The size of the regions of critically hypoperfused gray matter were identified on the initial PET scans, and changes of perfusion in those areas were related to the clinical course (followed by the National Institutes of Health stroke scale) and to the volume of infarcted gray matter demarcated on magnetic resonance imaging 3 weeks after the stroke. Whereas the initial clinical score was unrelated to the size of the ischemic area, after 3 weeks there was a strong correlation between clinical deficit and volume size of infarcted gray matter (Spearman's rho, 0.96; P < 0.001). All patients with a severely hypoperfused (< 12 mL/100 g/min) gray matter region measuring less than 15 mL on first PET showed full morphologic and clinical recovery (n = 5), whereas those with ischemic areas larger than 20 mL developed infarction and experienced persistent neurologic deficits of varying degree. Infarct sizes, however, were smaller than expected from previous correlative PET and morphologic studies of patients with acute stroke: only 22.7% of the gray matter initially perfused at rates below the conventional threshold of critical ischemia became necrotic. Actually, the percentage of initially ischemic voxels that became reperfused at almost normal levels clearly predicted the degree of clinical improvement achieved within 3 weeks. These sequential blood flow PET studies demonstrate that critically hypoperfused tissue can be preserved by early reperfusion, perhaps related to thrombolytic therapy. The results correspond with experimental findings demonstrating the prevention of large infarcts by early reperfusion to misery perfused but viable tissue.

PTA for Internal Carotid Artery Stenosis

SUMMARY

Thirty-one lesions of internal carotid artery (ICA) stenosis were treated by percutaneous transluminal angioplasty (PTA). All the patients were males except two. There were 18 extracranial ICA stenosis and 13 intracranial ICA stenosis. Four patients were treated in acute stage and 27 were in chronic stage. In 27 patients in the chronic stage, clinical presentation was transient ischemic attack or reversible ischemic neurological deficits or minor completed stroke. Dilatation of any degree was observed in all patients except one in the chronic group. Good dilatation (stenosis ratio after PTA is less than 50%) was observed in 11 out of 13 intracranial lesions and in 10 out of 14 extracranial lesions. Restenosis was two out of 11 intracranial good dilated lesions and 6 out of 10 good dilated extracraniallesions. No patients but one have experienced recurrent symptoms. Symptomatic complications occurred in only one patient who encountered arterial dissection during PTA resulting in severe hemiparesis. In four patients in the acute stage, dilatation of any degree was observed in two and good dilatation was seen in two. In conclusion, PTA for chronic ICA stenosis is an effective and safe technique. However, extracranial lesions tend to develop restenosis. Self-expandable stent will be one of the solutions for extracranial lesions. Major complications of PTA are distal embolism and arterial dissection. PTA for acute ICA stenosis is a challenging technique.

Permanent cortical damage detected by flumazenil positron emission tomography in acute stroke

BACKGROUND AND PURPOSE

Therapy of acute ischemic stroke can only be effective as long as neurons are viable and tissue is not infarcted. Since gamma-aminobutyric acid receptors are abundant in the cortex and sensitive to ischemic damage, specific radioligands to their subunits, the central benzodiazepine receptors (BZR), may be useful as indicators of neuronal integrity and as markers of irreversible damage. To test this hypothesis we studied the binding of the BZR ligand [11C]flumazenil (FMZ) early after ischemic stroke in comparison to the extent of final infarcts and hypometabolic cortical areas.

METHODS

In 10 patients cerebral blood flow, cerebral metabolic rate for oxygen (CMRO2), oxygen extraction fraction (OEF), and FMZ binding were studied by positron emission tomography 3.5 to 16 hours after onset of their first hemispheric stroke. Early changes in flow, oxygen metabolism, and FMZ binding were compared with permanent disturbances in glucose metabolism, and the size of the final infarcts was determined on MRI or CT 12 to 22 days after the stroke.

RESULTS

In all patients except one cerebral blood flow was disturbed, with marked decreases in eight and a hyperperfusion in one patient corresponding to the location of neurological deficits. In these areas CMRO2 was also reduced but to a variable degree, inducing highly variable OEF. Areas with markedly decreased CMRO2 (<60 micromol/100 g per minute) corresponded to regions with decreased FMZ binding (<4.0 times the mean value in the white matter). In all patients the final cortical infarcts were visible on the early FMZ images. Infarcts could be discriminated from noninfarcted cortex by decreased FMZ binding despite a wide range of OEF. In finally hypometabolic cortex FMZ binding was initially decreased or normal, with OEF covering a wide range; this suggested neuronal loss and/or deactivation as the cause of metabolic disturbance. Additionally, a highly significant correlation was found between FMZ distribution within the first 2 minutes after injection and regional cerebral blood flow.

CONCLUSIONS

These results demonstrate that permanently and irreversibly damaged cortex can be detected by reduced FMZ binding early after stroke. Since FMZ distribution additionally images regional cerebral perfusion, BZR radioligands have a potential as clinically useful tracers in patients with acute ischemic stroke. The evidence of tissue damage furnished by these tracers might be of relevance for the selection of individual therapeutic strategies.