Quantitative comparison of the bolus and steady-state methods for measurement of cerebral perfusion and oxygen metabolism: positron emission tomography study using 15O-gas and water

Enhanced parameter estimation in multiparametric arterial spin labeling using artificial neural networks

PURPOSE

Multiparametric arterial spin labeling (MP-ASL) can quantify cerebral blood flow (CBF) and arterial cerebral blood volume (CBVa). However, its accuracy is compromised owing to its intrinsically low SNR, necessitating complex and time-consuming parameter estimation. Deep neural networks (DNNs) offer a solution to these limitations. Therefore, we aimed to develop simulation-based DNNs for MP-ASL and compared the performance of a supervised DNN (DNNSup), physics-informed unsupervised DNN (DNNUns), and the conventional lookup table method (LUT) using simulation and in vivo data.

METHODS

MP-ASL was performed twice during resting state and once during the breath-holding task. First, the accuracy and noise immunity were evaluated in the first resting state. Second, CBF and CBVa values were statistically compared between the first resting state and the breath-holding task using the Wilcoxon signed-rank test and Cliff's delta. Finally, reproducibility of the two resting states was assessed.

RESULTS

Simulation and first resting-state analyses demonstrated that DNNSup had higher accuracy, noise immunity, and a six-fold faster computation time than LUT. Furthermore, all methods detected task-induced CBF and CBVa elevations, with the effect size being larger with the DNNSup (CBF, p = 0.055, Δ = 0.286; CBVa, p = 0.008, Δ = 0.964) and DNNUns (CBF, p = 0.039, Δ = 0.286; CBVa, p = 0.008, Δ = 1.000) than that with LUT (CBF, p = 0.109, Δ = 0.214; CBVa, p = 0.008, Δ = 0.929). Moreover, all the methods exhibited comparable and satisfactory reproducibility.

CONCLUSION

DNNSup outperforms DNNUns and LUT with respect to estimation performance and computation time.

Effects of the Training Data Condition on Arterial Spin Labeling Parameter Estimation Using a Simulation-Based Supervised Deep Neural Network

OBJECTIVE

A simulation-based supervised deep neural network (DNN) can accurately estimate cerebral blood flow (CBF) and arterial transit time (ATT) from multidelay arterial spin labeling signals. However, the performance of deep learning depends on the characteristics of the training data set. We aimed to investigate the effects of the ground truth (GT) ranges of CBF and ATT on the performance of the DNN when training data were prepared using arterial spin labeling signal simulation.

METHODS

Deep neural networks were individually trained using 36 patterns of the training data sets. Simulation test data (1,000,000 points), 17 healthy volunteers, and 1 patient with moyamoya disease were included. The simulation test data were used to evaluate accuracy, precision, and noise immunity of the DNN. The best-performing DNN was determined by the normalized mean absolute error (NMAE), normalized root mean squared error (NRMSE), and normalized coefficient of variation over repeated training (CV Net ). Cerebral blood flow and ATT values and their histograms were compared between the GT and predicted values. For the in vivo data, the dependency of the predicted values on the GT ranges was visually evaluated by comparing CBF and ATT maps between the best-performing DNN and the other DNNs. Moreover, using the synthesized noisy images, noise immunity was compared between the best-performing DNN based on the simulation study and a conventional method.

RESULTS

The simulation study showed that a network trained by the GT of CBF and ATT in the ranges of 0 to 120 mL/100 g/min and 0 to 4500 milliseconds, respectively, had the highest performance (NMAE CBF , 0.150; NRMSE CBF , 0.231; CV NET CBF , 0.028; NMAE ATT , 0.158; NRMSE ATT , 0.257; and CV NET ATT , 0.028). Although the predicted CBF and ATT varied with the GT range of the training data sets, the appropriate settings preserved the accuracy, precision, and noise immunity of the DNN. In addition, the same results were observed in in vivo studies.

CONCLUSIONS

The GT ranges to prepare the training data affected the performance of the simulation-based supervised DNNs. The predicted CBF and ATT values depended on the GT range; inappropriate settings degraded the accuracy, whereas appropriate settings of the GT range provided accurate and precise estimates.

Diagnostic value of PET imaging in clinically unresponsive patients

Rapid advancements in the critical care management of acute brain injuries have facilitated the survival of numerous patients who may have otherwise succumbed to their injuries. The probability of conscious recovery hinges on the extent of structural brain damage and the level of metabolic and functional cerebral impairment, which remain challenging to assess via laboratory, clinical, or functional tests. Current research settings and guidelines highlight the potential value of fluorodeoxyglucose-PET (FDG-PET) for diagnostic and prognostic purposes, emphasizing its capacity to consistently illustrate a metabolic reduction in cerebral glucose uptake across various disorders of consciousness. Crucially, FDG-PET might be a pivotal tool for differentiating between patients in the minimally conscious state and those in the unresponsive wakefulness syndrome, a persistent clinical challenge. In patients with disorders of consciousness, PET offers utility in evaluating the degree and spread of functional disruption, as well as identifying irreversible neural damage. Further, studies that capture responses to external stimuli can shed light on residual or revived brain functioning. Nevertheless, the validity of these findings in predicting clinical outcomes calls for additional long-term studies with larger patient cohorts suffering from consciousness impairment. Misdiagnosis of conscious illnesses during bedside clinical assessments remains a significant concern. Based on the clinical research settings, current clinical guidelines recommend PET for diagnostic and/or prognostic purposes. This review article discusses the clinical categories of conscious disorders and the diagnostic and prognostic value of PET imaging in clinically unresponsive patients, considering the known limitations of PET imaging in such contexts.

Improved reliability of perfusion estimation in dynamic susceptibility contrast MRI by using the arterial input function from dynamic contrast enhanced MRI

The arterial input function (AIF) plays a crucial role in estimating quantitative perfusion properties from dynamic susceptibility contrast (DSC) MRI. An important issue, however, is that measuring the AIF in absolute contrast-agent concentrations is challenging, due to uncertainty in relation to the measuredR 2 ∗ -weighted signal, signal depletion at high concentration, and partial-volume effects. A potential solution could be to derive the AIF from separately acquired dynamic contrast enhanced (DCE) MRI data. We aim to compare the AIF determined from DCE MRI with the AIF from DSC MRI, and estimated perfusion coefficients derived from DSC data using a DCE-driven AIF with perfusion coefficients determined using a DSC-based AIF. AIFs were manually selected in branches of the middle cerebral artery (MCA) in both DCE and DSC data in each patient. In addition, a semi-automatic AIF-selection algorithm was applied to the DSC data. The amplitude and full width at half-maximum of the AIFs were compared statistically using the Wilcoxon rank-sum test, applying a 0.05 significance level. Cerebral blood flow (CBF) was derived with different AIF approaches and compared further. The results showed that the AIFs extracted from DSC scans yielded highly variable peaks across arteries within the same patient. The semi-automatic DSC-AIF had significantly narrower width compared with the manual AIFs, and a significantly larger peak than the manual DSC-AIF. Additionally, the DCE-based AIF provided a more stable measurement of relative CBF and absolute CBF values estimated with DCE-AIFs that were compatible with previously reported values. In conclusion, DCE-based AIFs were reproduced significantly better across vessels, showed more realistic profiles, and delivered more stable and reasonable CBF measurements. The DCE-AIF can, therefore, be considered as an alternative AIF source for quantitative perfusion estimations in DSC MRI.

Neuronal Alterations in Secondary Thalamic Degeneration Due to Cerebral Infarction: A 11 C-Flumazenil Positron Emission Tomography Study

Studies using animal experiments have shown secondary neuronal degeneration in the thalamus after cerebral infarction. Neuroimaging studies in humans have revealed changes in imaging parameters in the thalamus, remote to the infarction. However, few studies have directly demonstrated neuronal changes in the thalamus in vivo. The purpose of this study was to determine whether secondary thalamic neuronal damage may manifest as a decrease in central benzodiazepine receptors in patients with cerebral infarction and internal carotid artery or middle cerebral artery disease.

Deep-learning-based attenuation correction in dynamic [15O]H2O studies using PET/MRI in healthy volunteers

Quantitative [¹⁵O]H₂O positron emission tomography (PET) is the accepted reference method for regional cerebral blood flow (rCBF) quantification. To perform reliable quantitative [¹⁵O]H₂O-PET studies in PET/MRI scanners, MRI-based attenuation-correction (MRAC) is required. Our aim was to compare two MRAC methods (RESOLUTE and DeepUTE) based on ultrashort echo-time with computed tomography-based reference standard AC (CTAC) in dynamic and static [¹⁵O]H₂O-PET. We compared rCBF from quantitative perfusion maps and activity concentration distribution from static images between AC methods in 25 resting [¹⁵O]H₂O-PET scans from 14 healthy men at whole-brain, regions of interest and voxel-wise levels. Average whole-brain CBF was 39.9 ± 6.0, 39.0 ± 5.8 and 40.0 ± 5.6 ml/100 g/min for CTAC, RESOLUTE and DeepUTE corrected studies respectively. RESOLUTE underestimated whole-brain CBF by 2.1 ± 1.50% and rCBF in all regions of interest (range -2.4%- -1%) compared to CTAC. DeepUTE showed significant rCBF overestimation only in the occipital lobe (0.6 ± 1.1%). Both MRAC methods showed excellent correlation on rCBF and activity concentration with CTAC, with slopes of linear regression lines between 0.97 and 1.01 and R² over 0.99. In conclusion, RESOLUTE and DeepUTE provide AC information comparable to CTAC in dynamic [¹⁵O]H₂O-PET but RESOLUTE is associated with a small but systematic underestimation.

Visit-to-Visit Blood PressureVariations and Hemodynamic Deterioration in Atherosclerotic Major Cerebral ArteryDisease

OBJECTIVE

Visit-to-visit variations in blood pressure (BP) in patients with atherosclerotic major cerebral artery disease could impair the function of cerebral collaterals, leading to hemodynamic deterioration at follow-up. However, few studies have investigated the relationship between visit-to-visit BP variability and changes in hemodynamic parameters at follow-up.

MATERIALS AND METHODS

We evaluated 35 medically treated patients with atherosclerotic internal carotid artery or middle cerebral artery disease with no ischemic episodes during follow-up (mean: 35 ± 20 months); these patients had a three-time visit for positron emission tomography examinations with ¹⁵O-gas. Differences in the mean hemispheric values of hemodynamic parameters in the cortical territory of the diseased artery between the first and third examinations (changes at follow-up) were correlated with the coefficient of variation (CoV) in three systolic BP (SBP) values at the three examinations (BP variability during follow-up).

RESULTS

CoV values were negatively correlated with changes in oxygen metabolism or cerebral blood flow/cerebral blood volume (CBF/CBV) ratio. In 17 patients with higher CoV values (> group median, 0.072), decreases in CBF, cerebral metabolic rate of oxygen, and CBF/CBV ratio were observed at follow-up; CBV decreased in 18 patients without elevated CoV. A higher CoV was associated with a lack of statin use.

CONCLUSION

In patients with atherosclerotic major cerebral artery disease, high visit-to-visit SBP variations during follow-up may be associated with deterioration in cerebral hemodynamics and metabolism.

Separating spin compartments in arterial spin labeling using delays alternating with nutation for tailored excitation (DANTE) pulse: A validation study using T2 -relaxometry and application to arterial cerebral blood volume imaging

PURPOSE

To clarify the type of spin compartment in arterial spin labeling (ASL) that is eliminated by delays alternating with nutation for tailored excitation (DANTE) pulse using T₂ -relaxometry, and to demonstrate the feasibility of arterial cerebral blood volume (CBV_a ) imaging using DANTE-ASL in combination with a simplified two-compartment model.

METHOD

The DANTE and T₂ -preparation modules were combined into a single ASL sequence. T₂ values under the application of DANTE were determined to evaluate changes in T₂ , along with the post-labeling delay (PLD) and the relationship between transit time without DANTE (TT_noVS ) and T₂ . The reference tissue T₂ (T_{2_ref} ) was also obtained. Subsequently, the DANTE module was embedded into the Hadamard-encoded ASL. Cerebral blood flow (CBF) and CBV_a were computed using two Hadamard-encoding datasets (with and without DANTE) in a rest and breath-holding (BH) task.

RESULTS

While T₂ without DANTE (T_{2_noVS} ) decreased as the PLD increased, T₂ with DANTE (T_{2_DANTE} ) was equivalent to T_{2_ref} and did not change with the PLD. Although there was a significant positive correlation between TT_noVS and T_{2_noVS} with short PLD, T_{2_DANTE} was not correlated with TT_noVS nor PLD. Baseline CBV_a values obtained at rest were 0.64 ± 0.12, 0.64 ± 0.11, and 0.58 ± 0.15 mL/100 g for anterior, middle, and posterior cerebral arteries, respectively. Significant CBF and CBV_a elevations were observed in the BH task.

CONCLUSION

Microvascular compartment signals were eliminated from the total ASL signals by DANTE. CBV_a can be measured using Hadamard-encoded DANTE-ASL in combination with a simplified two-compartment model.

Differences in Hemodynamic Alteration between Atherosclerotic Occlusive Lesions and Moyamoya Disease: A Quantitative 15O-PET Study

To clarify the differences in hemodynamic status between atherosclerotic steno-occlusive lesions (SOL) and moyamoaya disease (MMD), hemodynamic parameters were compared using ¹⁵O-PET. Twenty-four patients with unilateral SOL (67 ± 11 y) and eighteen with MMD (33 ± 16 y) were assigned to this study. MMD patients were divided into twelve unilateral and six bilateral lesions. All patients underwent ¹⁵O-PET to measure cerebral blood flow (CBF), blood volume (CBV), oxygen extraction fraction (OEF), and metabolic rate (CMRO₂). Acetazolamide was administered after the baseline scan and the second ¹⁵O-water PET was performed to evaluate cerebrovascular reactivity (CVR). For the CBF calculation in ¹⁵O-water PET, the three-weighted integral method was applied based on a one-tissue compartment model with pixel-by-pixel delay correction to measure precise CBF and arterial-to-capillary blood volume (V₀). Baseline hemodynamic parameters showed significantly lower CBF, V₀, and CMRO₂, but greater CBV, OEF, and delay (p < 0.01) in the affected hemispheres than in the unaffected hemispheres. After ACZ administration, both hemispheres showed a significant increase in CBF (p < 0.0001), but not in V₀. CVR differed significantly between the hemispheres. The arterial perfusion pressure of the functioning arterial part tended to be reduced after acetazolamide administration in patients with past neurologic events caused by hemodynamic impairment. MMD patients showed greater inactive vascular and venous volumes compared with common atherosclerotic SOL patients. The hemodynamic status of cerebral circulation may vary according to the chronic process of steno-occlusive change and the development of collateral circulation. In order to evaluate physiologic differences between the two diseases, ¹⁵O-PET with an acetazolamide challenge test is useful.

Arterial spin labeling magnetic resonance imaging at short post-labeling delay reflects cerebral perfusion pressure verified by oxygen-15-positron emission tomography in cerebrovascular steno-occlusive disease

BACKGROUND

Arterial transit time correction by data acquisition with multiple post-labeling delays (PLDs) or relatively long PLDs is expected to obtain more accurate imaging in cases of the cerebrovascular steno-occlusive disease. However, there have so far been no reports describing the significance of arterial spin labeling (ASL) images at short PLDs regarding the evaluation of cerebral circulation in ischemic cerebrovascular disease.

PURPOSE

To clarify the role of short-PLD ASL in cerebrovascular steno-occlusive disease.

MATERIAL AND METHODS

Fifty-three patients with cerebrovascular steno-occlusive disease were included in this study. All patients underwent ASL magnetic resonance imaging and ¹⁵O-PET within two days of each modality. To compare the ASL findings with each parameter of PET, the right-to-left (R/L) ratio, defined as the right middle cerebral artery (MCA) value/left MCA value, was calculated.

RESULTS

There is a significant correlation between the ASL images at a short PLD and the ratio of cerebral blood flow and cerebral blood volume by ¹⁵O-PET, which may accurately reflect the cerebral perfusion pressure. A receiver operating characteristic curve analysis indicated that ASL images at PLD 1000 and 1500 ms were more accurate than at PLD 2000-3000 ms for the detection of a ≥10% change in the PET cerebral blood flow.

CONCLUSION

ASL images at shorter PLDs may be useful at least as a screening modality to detect the changes in the cerebral circulation in cerebrovascular steno-occlusive disease. We must evaluate ASL images at multiple PLDs while considering the arterial transit time of each case at present.

Quantification of brain oxygen extraction and metabolism with [15O]-gas PET: A technical review in the era of PET/MRI

Oxygen extraction fraction (OEF) and the cerebral metabolic rate of oxygen (CMRO₂) are key cerebral physiological parameters to identify at-risk cerebrovascular patients and understand brain health and function. PET imaging with [¹⁵O]-oxygen tracers, either through continuous or bolus inhalation, provides non-invasive assessment of OEF and CMRO₂. Numerous tracer delivery, PET acquisition, and kinetic modeling approaches have been adopted to map brain oxygenation. The purpose of this technical review is to critically evaluate different methods for [¹⁵O]-gas PET and its impact on the accuracy and reproducibility of OEF and CMRO₂ measurements. We perform a meta-analysis of brain oxygenation PET studies in healthy volunteers and compare between continuous and bolus inhalation techniques. We also describe OEF metrics that have been used to detect hemodynamic impairment in cerebrovascular disease. For these patients, advanced techniques to accelerate the PET scans and potential synthesis with MRI to avoid arterial blood sampling would facilitate broader use of [¹⁵O]-oxygen PET for brain physiological assessment.

Selective neuronal damage and blood pressure in atherosclerotic major cerebral artery disease

OBJECTIVE

In patients with atherosclerotic major cerebral artery disease, low blood pressure might impair cerebral perfusion, thereby exacerbate the risk of selective neuronal damage. The purpose of this retrospective study was to determine whether low blood pressure at follow-up is associated with increased selective neuronal damage.

METHODS

We retrospectively analysed data from 76 medically treated patients with atherosclerotic internal carotid artery or middle cerebral artery disease with no ischaemic episodes on a follow-up of 6 months or more. All patients had measurements of the distribution of central benzodiazepine receptors twice using positron emission tomography and ¹¹C-flumazenil. Using three-dimensional stereotactic surface projections, we quantified abnormal decreases in the benzodiazepine receptors of the cerebral cortex within the middle cerebral artery distribution and correlated these changes in the benzodiazepine receptors index with blood pressure values at follow-up examinations.

RESULTS

The changes in the benzodiazepine receptor index during follow-up (mean 27±21 months) were negatively correlated with systolic blood pressure at follow-up. The relationship between changes in benzodiazepine receptor index and systolic blood pressure was different among patients with and without decreased cerebral blood flow at baseline (interaction, p<0.005). Larger increases in benzodiazepine receptor index (neuronal damage) were observed at lower systolic blood pressure levels in patients with decreased cerebral blood flow than in patients without such decreases.

CONCLUSION

In patients without ischaemic stroke episodes at follow-up but with decreased cerebral blood flow due to arterial disease, low systolic blood pressure at follow-up may be associated with increased selective neuronal damage.

Misery perfusion and amyloid deposition in atherosclerotic major cerebral artery disease

Although experimental studies have shown that global cerebral hypoperfusion leads to amyloid deposition in the hemisphere with carotid artery occlusion in rodents, the results of such occurrence are controversial in humans. Hence, we aim to determine whether global cerebral hypoperfusion leading to decreased blood flow relative to metabolic demand [increased oxygen extraction fraction (OEF), misery perfusion] is associated with increases in amyloid deposition in the hemisphere with atherosclerotic major cerebral artery disease in patients. We evaluated the distribution of β-amyloid plaques using positron emission tomography and a [18F]-pyridylbenzofuran derivative (¹⁸F-FPYBF-2) in 13 patients with unilateral atherosclerotic disease of the internal carotid artery (ICA) or middle cerebral artery (MCA) disease and no cortical infarction. The distribution volume ratio (DVR) of ¹⁸F- FPYBF-2 was calculated using dynamic data and Logan graphical analysis with reference tissue and was correlated with the cerebral blood flow (CBF), cerebral metabolic rate of oxygen (CMRO₂), and OEF, obtained from ¹⁵O-gas PET. The mean cortical value was calculated as the mean value within the frontal, posterior cingulate, precuneus, parietal, and lateral temporal cortical regions. Significant reductions in CBF and CMRO₂ and increases in OEF were found in the hemisphere ipsilateral to the arterial lesion compared with the contralateral hemisphere. There was no significant difference for ¹⁸F-FPYBF-2 DVR between hemispheres. The ipsilateral to contralateral ratio of the ¹⁸F- FPYBF-2 DVR was increased in 3 patients, while the ipsilateral to contralateral OEF ratio was increased in 4 patients. The incidence of an increased hemispheric DVR ratio was significantly higher in patients with an increased hemispheric OEF ratio (3/4) than in patients without (0/9) (p < 0.02). Although the ¹⁸F- FPYBF-2 DVR in the ipsilateral hemisphere was positively correlated with OEF after adjustment for the ¹⁸F- FPYBF-2 DVR in the contralateral hemisphere using multiple regression analysis (p < 0.05), the contribution rate of OEF was small (R² = 5.5%). Only one of the 4 patients with an increased hemispheric OEF ratio showed amyloid positivity based on the DVR value. In atherosclerotic major cerebral artery disease, misery perfusion accompanied only small increases of amyloid deposition at best. Misery perfusion was not associated with amyloid positivity.

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Misery perfusion accompanied only small increases of amyloid deposition at best.

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Relative oxygen extraction fraction correlated with relative amyloid deposition.

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Misery perfusion was not associated with amyloid positivity.

Long-term hemodynamic changes and blood pressure in atherosclerotic major cerebral artery disease

In patients with major cerebral artery disease, lower blood pressure might reduce blood flow in the collateral pathways, thereby impairing the growth of cerebral collaterals, inhibiting hemodynamic improvement. We evaluated the hemodynamic status twice using positron emission tomography and ¹⁵O-gas, over time, in 89 medically treated patients with atherosclerotic internal carotid artery or middle cerebral artery disease that had no ischemic episodes during follow-up (mean, 28 ± 23 months). Changes in the mean hemispheric values of hemodynamic parameters in the territory of the diseased artery at follow-up were correlated with the mean blood pressure values at the baseline and follow-up examinations. There was a positive linear relationship between the degree of hemodynamic improvement and systolic blood pressure. Patients with low systolic blood pressure (<130 mmHg) ( n = 18) showed hemodynamic deterioration as indicated by significant decreases in cerebral blood flow, cerebral blood flow/cerebral blood volume ratio, and increases in oxygen extraction fraction during follow-up. In contrast, there were no significant changes in patients without low systolic blood pressure. In patients with atherosclerotic internal carotid artery or middle cerebral artery disease and no ischemic episodes of stroke during follow-up, lower systolic blood pressure was associated with lesser hemodynamic improvement.

Progressive Cortical Neuronal Damage and Extracranial-Intracranial Bypass Surgery in Patients with Misery Perfusion

BACKGROUND AND PURPOSE

Misery perfusion may cause selective neuronal damage in atherosclerotic ICA or MCA disease. Bypass surgery can improve misery perfusion and may prevent neuronal damage. On the other hand, surgery conveys a risk for neuronal damage. The purpose of this retrospective study was to determine whether progression of cortical neuronal damage in surgically treated patients with misery perfusion is larger than that in surgically treated patients without misery perfusion or medically treated patients with misery perfusion.

MATERIALS AND METHODS

We evaluated the distribution of benzodiazepine receptors twice by using PET and ¹¹C-labeled flumazenil in 18 surgically treated patients with atherosclerotic ICA or MCA disease (9 with misery perfusion and 9 without) and no perioperative stroke before and after bypass surgery; in 8 medically treated patients with misery perfusion and no intervening ischemic event; and in 7 healthy controls. We quantified abnormal decreases in the benzodiazepine receptors of the cerebral cortex within the MCA distribution and compared changes in the benzodiazepine receptor index among the 3 groups.

RESULTS

The change in the benzodiazepine receptor index in surgically treated patients with misery perfusion (27.5 ± 15.6) during 7 ± 5 months was significantly larger than that in surgically treated patients without misery perfusion (-5.2 ± 9.4) during 6 ± 4 months (P < .001) and in medically treated patients with misery perfusion (3.2 ± 15.4) during 16 ± 6 months (P < .01).

CONCLUSIONS

Progression of cortical neuronal damage in surgically treated patients with misery perfusion and no perioperative stroke may occur and may be larger than that in medically treated patients with misery perfusion and no intervening ischemic event.

Pixel-by-pixel precise delay correction for measurement of cerebral hemodynamic parameters in H215O PET study

OBJECTIVE

A new method of delay time estimation was proposed to measure precise cerebral blood flow (CBF) and arterial-to-capillary blood volume (V ₀) using ¹⁵O-water PET.

METHODS

Nineteen patients with unilateral arterial stenoocclusive lesions were studied to evaluate hemodynamic status before treatment. The delay time of each pixel was calculated using least squares fitting with an arterial blood input curve adjusted to the internal carotid artery counts at the skull base. Pixel-by-pixel delay estimation provided a delay map image that could be used for precise calculation of CBF and V ₀ using a one-tissue compartment model, and the values from this method were compared with those from the slice-by-slice correction method.

RESULTS

The affected side showed a longer delay time than the contralateral cerebral hemisphere. Although the mean cortical CBF values were not different between the two methods, the slice-by-slice delay correction overestimated CBF in the hypo perfused area. The scatter plot of V ₀ pixel values showed significant difference between the two correction methods where the slice-by-slice delay correction significantly overestimated V ₀ in the whole brain (P < 0.05).

CONCLUSION

Pixel-by-pixel delay correction provides delay images as well as better estimation of CBF and V ₀, thus offering useful and beneficial information for the treatment of cerebrovascular disease.

Progressive Cortical Neuronal Damage and Chronic Hemodynamic Impairment in Atherosclerotic Major Cerebral Artery Disease

Background and Purpose—

Cross-sectional studies suggest that chronic hemodynamic impairment may cause selective cortical neuronal damage in patients with atherosclerotic internal carotid artery or middle cerebral artery occlusive disease. The purpose of this longitudinal study was to determine whether the progression of cortical neuronal damage, evaluated as a decrease in central benzodiazepine receptors (BZRs), is associated with hemodynamic impairment at baseline or hemodynamic deterioration during follow-up.

Methods—

We evaluated the distribution of BZRs twice using positron emission tomography and 11 C-flumazenil over time in 80 medically treated patients with atherosclerotic internal carotid artery or middle cerebral artery occlusive disease that had no ischemic episodes during follow-up. Using 3D stereotactic surface projections, we quantified abnormal decreases in the BZRs in the cerebral cortex within the middle cerebral artery distribution and correlated changes in the BZR index with the mean hemispheric values of hemodynamic parameters obtained from 15 O gas positron emission tomography.

Results—

In the hemisphere affected by arterial disease, the BZR index in 40 patients (50%) was increased during follow-up (mean 26±20 months). In multivariable logistic regression analyses, increases in the BZR index were associated with the decreased cerebral blood flow at baseline and an increased oxygen extraction fraction during follow-up. Increases in the oxygen extraction fraction during follow-up were associated with a lack of statin use.

Conclusions—

In patients with atherosclerotic internal carotid artery or middle cerebral artery disease, the progression of cortical neuronal damage was associated with hemodynamic impairment at baseline and hemodynamic deterioration during follow-up. Statin use may be beneficial against hemodynamic deterioration and therefore neuroprotective.

Increase in [18F]-Fluoroacetate Uptake in Patients With Chronic Hemodynamic Cerebral Ischemia

Background and Purpose—

[18F]-fluoroacetate ( 18 F-FACE) can be used for evaluating glial cell metabolism. Experimental studies have shown an increase in 18 F-FACE uptake in rodent models of cerebral ischemia. The aim of this study was to determine whether 18 F-FACE uptake is increased in the noninfarcted cerebral cortex in patients with hemodynamic ischemia owing to atherosclerotic internal carotid artery or middle cerebral artery disease.

Methods—

We evaluated 9 symptomatic patients with unilateral atherosclerotic internal carotid artery or middle cerebral artery disease and no cortical infarction using positron emission tomography with 18 F-FACE and 15 O-gases. 18 F-FACE uptake during 40 to 60 minutes after injection was compared with the cerebral blood flow, cerebral metabolic rate of oxygen, oxygen extraction fraction, and cerebral blood volume in the middle cerebral artery distributions.

Results—

Significant decreases of cerebral blood flow and cerebral metabolic rate of oxygen and increases of oxygen extraction fraction and cerebral blood volume were found in the hemisphere ipsilateral to the arterial lesion, and 18 F-FACE uptake in this region was greater than that in the contralateral hemisphere. The relative 18 F-FACE uptake (ipsilateral/contralateral ratio) was negatively correlated with cerebral blood flow or cerebral metabolic rate of oxygen values and was positively correlated with oxygen extraction fraction values. Multivariate analysis showed that the ipsilateral/contralateral 18 F-FACE uptake ratio was independently correlated with the cerebral blood flow (or oxygen extraction fraction) and cerebral metabolic rate of oxygen values.

Conclusions—

In patients with atherosclerotic internal carotid artery or middle cerebral artery disease, 18 F-FACE uptake is increased in the noninfarcted cerebral cortex with chronic hemodynamic ischemia characterized by misery perfusion with decreased oxygen metabolism. Increased 18 F-FACE uptake may indicate the cortical regions that are at particular risk for ischemic damage.

Impaired perfusion modifies the relationship between blood pressure and stroke risk in major cerebral artery disease

OBJECTIVE

Blood pressure (BP) lowering may increase stroke risk in patients with symptomatic major cerebral artery disease and impaired perfusion. To investigate the relationships among BP, impaired perfusion and stroke risk.

METHODS

We retrospectively analysed data from 130 non-disabled, medically treated patients with either symptomatic extracranial carotid occlusion or intracranial stenosis or occlusion of the carotid artery or middle cerebral arteries. All patients had baseline haemodynamic measurements with (15)O-gas positron emission tomography and were followed for 2 years or until stroke recurrence or death.

RESULTS

There was a negative linear relationship between systolic BP (SBP) and risk of stroke in the territory of the diseased artery. The 2-year incidence of ischaemic stroke in the territory in patients with normal SBP (<130 mm Hg, 5/32 patients) was significantly higher than in patients with high SBP (2/98, p<0.005). Multivariate analysis revealed that normal SBP and impaired perfusion were independently associated with increased risk of stroke in the previously affected territory, while risk of stroke elsewhere was positively correlated with SBP. Overall, high total stroke risk was observed at lower BP in patients with impaired perfusion and at higher BPs in patients without (interaction, p<0.01). Overall, the relationship between SBP and total stroke recurrence was J-shaped.

CONCLUSIONS

Impaired perfusion modified the relationship between blood pressure and stroke risk, although this study had limitations including the retrospective analysis, the potentially biased sample, the small number of critical events and the fact that BP was measured only as a snapshot in clinic.

Chronic hemodynamic compromise and cerebral ischemic events in asymptomatic or remote symptomatic large-artery intracranial occlusive disease

BACKGROUND AND PURPOSE

In asymptomatic or remote symptomatic LAICOD, the risk of ischemic events is low in general, but there may be a subgroup of higher risk patients who require aggressive medical management. The purpose of this study was to determine whether chronic hemodynamic compromise is a predictor of ischemic events in asymptomatic or remote symptomatic LAICOD.

MATERIALS AND METHODS

We prospectively studied 51 asymptomatic, 19 coexistent asymptomatic, and 19 remote (>6 months) symptomatic patients with atherosclerotic intracranial internal carotid artery or middle cerebral artery disease by using (15)O-PET. MP was defined as decreased CBF, increased OEF, and a decreased CBF/CBV ratio. All patients were followed up for 2 years or until occurrence of stroke or TIA or death.

RESULTS

Bypass surgery was performed in 4 patients (2 with MP). Three cerebral ischemic events (1 TIA in an asymptomatic patient, 1 stroke, and 1 TIA in a remote symptomatic patient) occurred in the vascular territory ipsilateral to LAICOD. Kaplan-Meier analysis with censoring at the time of bypass surgery revealed that the incidence of ipsilateral ischemic events in patients with MP (2/5) was significantly higher than that in patients without MP (1/84) (log-rank test; P < .0001). The relative risk conferred by MP was 83.1 (95% confidence interval, 6.8-1017.4; P < .001). The incidence of ipsilateral ischemic events in patients with decreased CBF/CBV (2/9) was also significantly higher than that of patients without it (1/80) (P = .0001).

CONCLUSIONS

Chronic hemodynamic compromise may be a predictor of ischemic events in both asymptomatic and remote symptomatic LAICOD.

Cerebral oxygen metabolism of rats using injectable (15)O-oxygen with a steady-state method

To develop a less-stressful and simple method for measurement of the cerebral metabolic rate of oxygen (CMRO(2)) in small animals, the steady-state method was applied to injectable (15)O(2)-PET ((15)O(2)-positron emission tomography) using hemoglobin-containing vesicles ((15)O(2)-HbV). Ten normal rats and 10 with middle cerebral arterial occlusion (MCAO) were studied using a small animal PET scanner. A series of (15)O-PET scans with C(15)O-labeled HbV, H(2)(15)O, and (15)O(2)-HbV were performed with 10 to 15 minutes intervals to measure cerebral blood volume (CBV), cerebral blood flow (CBF), and CMRO(2). Positron emission tomography scans were started with a tracer injection using a multiprogramming syringe pump, which provides a slowly increasing injection volume to achieve steady-state radioactivity for H(2)(15)O and (15)O(2)-HbV scans. The radioactivity concentration of (15)O rapidly achieved equilibrium in the blood and whole brain at about 2 minutes after H(2)(15)O and (15)O(2)-HbV administration, which was stable during the scans. The whole brain mean values of CBF, CBV, and CMRO(2) were 54.3±2.0 mL per 100 g per minute, 4.9±0.4 mL/100 g, and 2.8±0.2 μmoL per g per minute (6.2±0.4 mL per 100 g per minute) in the normal rats, respectively. In the MCAO model rats, all hemodynamic parameters of the infarction area on the occlusion side significantly decreased. The steady-state method with (15)O-labeled HbV is simple and useful to analyze hemodynamic changes in studies with model animals.

Day-to-day test-retest variability of CBF, CMRO2, and OEF measurements using dynamic 15O PET studies

Purpose

We assessed test–retest variability of cerebral blood flow (CBF), cerebral blood volume (CBV), cerebral metabolic rate of oxygen (CMRO₂), and oxygen extraction fraction (OEF) measurements derived from dynamic ¹⁵O positron emission tomography (PET) scans.

Procedures

In seven healthy volunteers, complete test–retest ¹⁵O PET studies were obtained; test–retest variability and left-to-right ratios of CBF, CBV, OEF, and CMRO₂ in arterial flow territories were calculated.

Results

Whole-brain test–retest coefficients of variation for CBF, CBV, CMRO₂, and OEF were 8.8%, 13.8%, 5.3%, and 9.3%, respectively. Test–retest variability of CBV left-to-right ratios was <7.4% across all territories. Corresponding values for CBF, CMRO₂, and OEF were better, i.e., <4.5%, <4.0%, and <1.4%, respectively.

Conclusions

The test–retest variability of CMRO₂ measurements derived from dynamic ¹⁵O PET scans is comparable to within-session test–retest variability derived from steady-state ¹⁵O PET scans. Excellent regional test–retest variability was observed for CBF, CMRO₂, and OEF. Variability of absolute CBF and OEF measurements is probably affected by physiological day-to-day variability of CBF.

Silent cortical neuronal damage in atherosclerotic disease of the major cerebral arteries

In atherosclerotic internal carotid artery (ICA) or middle cerebral artery (MCA) disease, hemodynamic compromise may cause selective neuronal damage manifested as loss of central benzodiazepine receptors (BZRs) in the normal-appearing cerebral cortex, without overt episode of stroke. To investigate the association of decreases in cortical BZRs with hemodynamic compromise and the effect of angiotensin receptor blockers (ARBs) on these receptors in patients whose atherosclerotic ICA or MCA disease is asymptomatic, we measured BZRs using positron emission tomography and (11)C-flumazenil in 79 patients with asymptomatic atherosclerotic ICA or MCA disease and no cortical infarction. Three-dimensional stereotactic surface projections were used to calculate the BZR index, a measure of abnormally decreased BZRs in the cerebral cortex within the MCA distribution. Multiple regression analysis showed this index to be positively correlated with the value of oxygen extraction fraction, with the presence of silent subcortical infarcts, and with the presence of ischemic heart disease, whereas it was negatively correlated with the treatment of hypertension with ARBs. In asymptomatic atherosclerotic ICA or MCA disease, hemodynamic compromise is associated with selective neuronal damage manifested as decreases in cortical BZRs in the noninfarcted cerebral cortex, whereas ARBs are associated with preservation of cortical BZRs.

Selective neuronal damage and Wisconsin Card Sorting Test performance in atherosclerotic occlusive disease of the major cerebral artery

BACKGROUND

In atherosclerotic internal carotid artery (ICA) or middle cerebral artery (MCA) disease, selective neuronal damage can be detected as a decrease in central benzodiazepine receptors (BZRs) in the normal-appearing cerebral cortex. This study aimed to determine whether a decrease in the BZRs in the non-infarcted cerebral cortex is associated with poor performance on the Wisconsin Card Sorting Test (WCST), which assesses executive functions.

METHODS

The authors measured the BZRs using positron emission tomography and (11)C-flumazenil in 60 non-disabled patients with unilateral atherosclerotic ICA or MCA disease and no cortical infarction. Using three-dimensional stereotactic surface projections, the abnormally decreased BZR index (extent (%) of pixels with Z score >2 compared with controls × average Z score in those pixels) in the cerebral cortex of the anterior cerebral artery (ACA) or MCA territory was calculated and found to be correlated with the patient's score on the WCST.

RESULTS

On the basis of the WCST results, 39 patients were considered abnormal (low categories achieved) for their age. The BZR index of the ACA territory in the hemisphere affected by arterial disease was significantly higher in abnormal patients than in normal patients. The BZR index of the MCA territory differed significantly between the two groups when patients with left arterial disease (n=28) were analysed separately.

CONCLUSIONS

In atherosclerotic ICA or MCA disease, selective neuronal damage that is manifested as a decrease in BZRs in the non-infarcted cerebral cortex may contribute to the development of executive dysfunction.

Omission of [(15)O]CO scan for PET CMRO(2) examination using (15)O-labelled compounds

OBJECTIVE

CBF, OEF and CMRO(2) provide us important clinical indices and are used for assessing ischemic degree in cerebrovascular disorders. These quantitative images can be measured by PET using (15)O-labelled tracers such as C(15)O, C(15)O(2) and (15)O(2). To reduce the time of scan, one possibility is to omit the use of CBV data. The present study investigated the influence of fixing the CBV to OEF and CMRO(2) values on subjects with and without cerebrovascular disorders.

METHODS

The study consisted of three groups, namely, GROUP-0 (n = 10), GROUP-1 (n = 9), and GROUP-2 (n = 10), corresponding to--without significant disorder, with elevated CBV, and with reduced CBF and elevated OEF, respectively. All subjects received PET examination and using the PET data OEF and CMRO(2) images were computed by fixing CBV and with CBV data. The computed OEF and CMRO(2) values were compared between the methods.

RESULTS

The OEF and CMRO(2) values obtained by fixing the CBV were around 10% underestimation against that with CBV data. The regression analysis showed that these values were comparable (r = 0.93-0.98, P < 0.001). The simulation showed that fixing of the CBV would not derive significant error in either OEF or CMRO(2) values, when changed from 0 to 0.08 ml/g.

CONCLUSION

This study shows the feasibility of fixing the CBV value for computing OEF and CMRO(2) values in the PET examination, suggesting the CO scan could be eliminated.

Hemodynamic Compromise as a Cause of Internal Border-Zone Infarction and Cortical Neuronal Damage in Atherosclerotic Middle Cerebral Artery Disease

Background and Purpose— Hemodynamic compromise due to atherosclerotic middle cerebral artery (MCA) disease may induce internal border-zone infarction and cortical neuronal damage. This study aimed to determine whether internal border-zone infarction is associated with increased oxygen extraction fraction (OEF) and a decrease in central benzodiazepine receptors (BZRs) in the overlying cerebral cortex in atherosclerotic MCA disease.

Methods— We measured the OEF by using positron emission tomography and 15 O gas in 100 nondisabled patients with atherosclerotic MCA disease in the chronic stage. On MRI, the infarcts were categorized as territorial, border-zone (external or internal), deep perforator, and superior perforator infarcts. In 62 patients, BZRs were measured using 11 C-flumazenil. By using 3-dimensional stereotactic surface projections, the abnormally decreased BZR index (“BZR index”) [(the extent of the pixels with Z score more than 2 compared with controls)×(average Z score in those pixels)] was calculated. In the hemisphere affected by MCA disease, the type of infarcts was correlated with the value of OEF or BZR index in the cerebral cortex of the MCA distribution.

Results— Compared with patients without internal border-zone infarcts, those with these infarcts (n=18) had significantly increased OEF and significantly high BZR index. Multivariate analysis revealed that internal border-zone infarction was independently associated with increased OEF and high BZR index.

Conclusions— In atherosclerotic MCA disease, internal border-zone infarction is associated with increased OEF and a decrease in BZRs in the overlying cerebral cortex, suggesting that hemodynamic compromise may induce internal border-zone infarction and cortical neuronal damage.

Automatic labeling method for injectable 15O-oxygen using hemoglobin-containing liposome vesicles and its application for measurement of brain oxygen consumption by PET

INTRODUCTION

The aim of this study was to develop an injectable (15)O-O(2) system using hemoglobin-containing vesicles (HbV), a type of artificial red blood cell, and to investigate the feasibility of (15)O(2)-labeled HbV ((15)O(2)-HbV) to measure cerebral metabolic rate of oxygen (CMRO(2)) in rats.

METHODS

The direct bubbling method was combined with vortexing to enhance labeling efficiency of HbV with (15)O-O(2) gas. L-Cysteine was added as a reductant to protect hemoglobin molecules in HbV from oxidation at different concentrations, and labeling efficiencies were also compared. Measurement of cerebral blood flow (CBF) and CMRO(2) in five normal rats was performed using a small animal PET scanner after the injection of H(2)(15)O and (15)O(2)-HbV to evaluate the precision of hemodynamic parameters quantitatively.

RESULTS

The labeling efficiency of HbV was significantly increased when vortexing and bubbling were combined compared with the simple bubbling method (P<.05). The most efficient method for labeling was bubbling of (15)O-O(2) combined with vortexing and the addition of 2.8 mM L-cysteine in HbV solution. The mean radioactivity of 214.4+/-7.8 MBq/mL HbV was obtained using this method. PET scans using (15)O(2)-HbV and H(2)(15)O yielded a mean CMRO(2) value of 6.8+/-1.4 (mL/min per 100 g) in rats with normal CBF of 51.4+/-7.9 (mL/min per 100 g).

CONCLUSION

Addition of l-cysteine to HbV and simple direct bubbling of (15)O-O(2) gas combined with vortexing was the most efficient method for preparation of (15)O(2)-HbV. The present injectable system using (15)O(2)-HbV was successfully utilized to measure CMRO(2) in rats, indicating that this new method could be useful for animal models to measure oxygen metabolism in the brain.

Influence of residual oxygen-15-labeled carbon monoxide radioactivity on cerebral blood flow and oxygen extraction fraction in a dual-tracer autoradiographic method

OBJECTIVE

Cerebral blood flow (CBF), cerebral metabolic rate of oxygen (CMRO(2)), oxygen extraction fraction (OEF), and cerebral blood volume (CBV) are quantitatively measured with PET with (15)O gases. Kudomi et al. developed a dual tracer autoradiographic (DARG) protocol that enables the duration of a PET study to be shortened by sequentially administrating (15)O(2) and C(15)O(2) gases. In this protocol, before the sequential PET scan with (15)O(2) and C(15)O(2) gases ((15)O(2)-C(15)O(2) PET scan), a PET scan with C(15)O should be preceded to obtain CBV image. C(15)O has a high affinity for red blood cells and a very slow washout rate, and residual radioactivity from C(15)O might exist during a (15)O(2)-C(15)O(2) PET scan. As the current DARG method assumes no residual C(15)O radioactivity before scanning, we performed computer simulations to evaluate the influence of the residual C(15)O radioactivity on the accuracy of measured CBF and OEF values with DARG method and also proposed a subtraction technique to minimize the error due to the residual C(15)O radioactivity.

METHODS

In the simulation, normal and ischemic conditions were considered. The (15)O(2) and C(15)O(2) PET count curves with the residual C(15)O PET counts were generated by the arterial input function with the residual C(15)O radioactivity. The amounts of residual C(15)O radioactivity were varied by changing the interval between the C(15)O PET scan and (15)O(2)-C(15)O(2) PET scan, and the absolute inhaled radioactivity of the C(15)O gas. Using the simulated input functions and the PET counts, the CBF and OEF were computed by the DARG method. Furthermore, we evaluated a subtraction method that subtracts the influence of the C(15)O gas in the input function and PET counts.

RESULTS

Our simulations revealed that the CBF and OEF values were underestimated by the residual C(15)O radioactivity. The magnitude of this underestimation depended on the amount of C(15)O radioactivity and the physiological conditions. This underestimation was corrected by the subtraction method.

CONCLUSIONS

This study showed the influence of C(15)O radioactivity in DARG protocol, and the magnitude of the influence was affected by several factors, such as the radioactivity of C(15)O, and the physiological condition.

A physiologic model for recirculation water correction in CMRO2 assessment with 15O2 inhalation PET

Cerebral metabolic rate of oxygen (CMRO(2)) can be assessed quantitatively using (15)O(2) and positron emission tomography. Determining the arterial input function is considered critical with regards to the separation of the metabolic product of (15)O(2) (RW) from a measured whole blood. A mathematical formula based on physiologic model has been proposed to predict RW. This study was intended to verify the adequacy of that model and a simplified procedure applying that model for wide range of species and physiologic conditions. The formula consists of four parameters, including of a production rate of RW (k) corresponding to the total body oxidative metabolism (BMRO(2)). Experiments were performed on 6 monkeys, 3 pigs, 12 rats, and 231 clinical patients, among which the monkeys were studied at varied physiologic conditions. The formula reproduced the observed RW. Greater k values were observed in smaller animals, whereas other parameters did not differ amongst species. The simulation showed CMRO(2) sensitive only to k, but not to others, suggesting that validity of determination of only k from a single blood sample. Also, k was correlated with BMRO(2), suggesting that k can be determined from BMRO(2). The present model and simplified procedure can be used to assess CMRO(2) for a wide range of conditions and species.

Separation of input function for rapid measurement of quantitative CMRO2and CBF in a single PET scan with a dual tracer administration method

Cerebral metabolic rate of oxygen (CMRO(2)), oxygen extraction fraction (OEF) and cerebral blood flow (CBF) images can be quantified using positron emission tomography (PET) by administrating (15)O-labelled water (H(15)(2)O) and oxygen ((15)O(2)). Conventionally, those images are measured with separate scans for three tracers C(15)O for CBV, H(15)(2)O for CBF and (15)O(2) for CMRO(2), and there are additional waiting times between the scans in order to minimize the influence of the radioactivity from the previous tracers, which results in a relatively long study period. We have proposed a dual tracer autoradiographic (DARG) approach (Kudomi et al 2005), which enabled us to measure CBF, OEF and CMRO(2) rapidly by sequentially administrating H(15)(2)O and (15)O(2) within a short time. Because quantitative CBF and CMRO(2) values are sensitive to arterial input function, it is necessary to obtain accurate input function and a drawback of this approach is to require separation of the measured arterial blood time-activity curve (TAC) into pure water and oxygen input functions under the existence of residual radioactivity from the first injected tracer. For this separation, frequent manual sampling was required. The present paper describes two calculation methods: namely a linear and a model-based method, to separate the measured arterial TAC into its water and oxygen components. In order to validate these methods, we first generated a blood TAC for the DARG approach by combining the water and oxygen input functions obtained in a series of PET studies on normal human subjects. The combined data were then separated into water and oxygen components by the present methods. CBF and CMRO(2) were calculated using those separated input functions and tissue TAC. The quantitative accuracy in the CBF and CMRO(2) values by the DARG approach did not exceed the acceptable range, i.e., errors in those values were within 5%, when the area under the curve in the input function of the second tracer was larger than half of the first one. Bias and deviation in those values were also compatible to that of the conventional method, when noise was imposed on the arterial TAC. We concluded that the present calculation based methods could be of use for quantitatively calculating CBF and CMRO(2) with the DARG approach.

Selective neuronal damage and chronic hemodynamic cerebral ischemia

OBJECTIVE

In atherothrombotic internal carotid artery or middle cerebral artery (MCA) occlusive disease, chronic hemodynamic compromise may increase the risk for cerebral ischemic damage. To determine whether selective neuronal damage demonstrated as a decrease in central benzodiazepine receptor (BZR) in the normal-appearing cerebral cortex is associated with increased oxygen extraction fraction (OEF) (misery perfusion).

METHODS

We measured BZR and OEF using positron emission tomography in 105 nondisabled patients with atherothrombotic internal carotid artery or MCA occlusive disease and no cortical infarction. By using three-dimensional stereotactic surface projections and the stereotactic extraction estimation method, without correction for partial volume effects, the abnormally decreased BZR index [(the extent of the pixels with Z-score more than 2 compared with controls) x (average Z-score in those pixels)] in the cerebral cortex of the MCA distribution with arterial disease was calculated, and it was found to be correlated with the mean hemispheric value of OEF and several clinical variables.

RESULTS

All patients had pixels with abnormally decreased BZR, with the extent varying from 0.04 to 60.91%. Multivariate analysis showed that the abnormally decreased BZR index was positively correlated with the value of OEF and the history of stroke, whereas it was negatively correlated with the presence of hypercholesterolemia with statin treatment. Follow-up examinations of 17 patients without ischemic episode showed that a decrease of BZR was associated with an increase of OEF.

INTERPRETATION

In atherothrombotic internal carotid artery or MCA occlusive disease, misery perfusion may cause selective neuronal damage, and statins might have beneficial effects against neuronal damage.

Altered patterns of blood flow response during visual stimulation in carotid artery occlusive disease

To correctly interpret the results of functional neuroimaging studies in stroke, it is essential to understand how cerebrovascular disease influences hemodynamic response during neural activation. To investigate the effect of internal carotid artery (ICA) occlusive disease on the pattern of cerebral blood flow (CBF) response during visual stimulation, we used positron emission tomography to study 13 patients with ICA steno-occlusive lesions. We measured the changes of CBF during visual stimulation in the primary visual cortex and in the surrounding region, including the higher-order visual cortex, and examined their correlation with the baseline value of oxygen extraction fraction, a measure of hemodynamic impairment, in the ICA distribution. With visual stimulation, CBF in the primary visual cortex significantly increased in all patients, while in the surrounding region, CBF showed variable changes, including decreases in some patients. In 9 patients with unilateral ICA lesions, the CBF change in the surrounding region ipsilateral to the ICA lesion was significantly decreased compared with the value in the contralateral hemisphere, while the CBF change in the primary visual cortex showed no hemispheric difference. The hemispheric values of oxygen extraction fraction in the ICA distribution and the amount of CBF increase in the visual cortex were independently and negatively correlated with the CBF change in the surrounding region. We conclude that the pattern of CBF response during visual stimulation may change in ICA occlusive disease. We suggest that the redistribution of CBF during visual stimulation may be a contributing mechanism.

Multislice continuous arterial spin-labeled perfusion MRI in patients with chronic occlusive cerebrovascular disease: a correlative study with CO2 PET validation

PURPOSE

To compare the use of multislice continuous arterial spin labeling (CASL) and CO(2) positron emission tomography (PET) to assess CBF in patients with chronic occlusive cerebrovascular disease for the validation of quantitative CASL perfusion in an altered hemodynamic state.

MATERIALS AND METHODS

Eleven patients with occlusive carotid artery disease were studied with CASL and conventional MRI. Cerebral blood flow (CBF) was also measured with O(15)-labeled CO(2) gas using a whole-body PET scanner. The average values within region of interests (ROIs) drawn on coregistered CASL and PET images were used for the linear regression analysis and to assess the effect of transit time on the quantification using CASL.

RESULT

In all patients there was a significant correlation between the CBF values from CASL and PET (r = 0.71 +/- 0.07, P < 0.001). The slope of regression lines varied widely among patients (0.54-1.77). Longer transit times were estimated in four of 11 patients in the hypoperfusion region.

CONCLUSION

Quantification of CBF using CASL is feasible and reasonable, even when employed in a routine clinical setting. However, the long transit time may lead to underestimation of the affected side in occlusive cerebrovascular disease.

Rapid quantitative measurement of CMRO(2) and CBF by dual administration of (15)O-labeled oxygen and water during a single PET scan-a validation study and error analysis in anesthetized monkeys

Cerebral blood flow (CBF) and rate of oxygen metabolism (CMRO(2)) may be quantified using positron emission tomography (PET) with (15)O-tracers, but the conventional three-step technique requires a relatively long study period, attributed to the need for separate acquisition for each of (15)O(2), H(2)(15)O, and C(15)O tracers, which makes the multiple measurements at different physiologic conditions difficult. In this study, we present a novel, faster technique that provides a pixel-by-pixel calculation of CBF and CMRO(2) from a single PET acquisition with a sequential administration of (15)O(2) and H(2)(15)O. Experiments were performed on six anesthetized monkeys to validate this technique. The global CBF, oxygen extraction fraction (OEF), and CMRO(2) obtained by the present technique at rest were not significantly different from those obtained with three-step method. The global OEF (gOEF) also agreed with that determined by simultaneous arterio-sinus blood sampling (gOEF(A-V)) for a physiologically wide range when changing the arterial PaCO(2) (gOEF=1.03gOEF(A-V)+0.01, P<0.001). The regional values, as well as the image quality were identical between the present technique and three-step method for CBF, OEF, and CMRO(2). In addition, a simulation study showed that error sensitivity of the present technique to delay or dispersion of the input function, and the error in the partition coefficient was equivalent to that observed for three-step method. Error sensitivity to cerebral blood volume (CBV) was also identical to that in the three-step and reasonably small, suggesting that a single CBV assessment is sufficient for repeated measures of CBF/CMRO(2). These results show that this fast technique has an ability for accurate assessment of CBF/CMRO(2) and also allows multiple assessment at different physiologic conditions.

Pattern of collaterals, type of infarcts, and haemodynamic impairment in carotid artery occlusion

BACKGROUND

In internal carotid artery (ICA) occlusion, increased oxygen extraction fraction (OEF) indicates inadequate collateral blood flow distal to the occlusion, which may be caused by poor function of collateral pathways. In ICA occlusion, the circle of Willis may be the major collateral pathway, while the collaterals through the ophthalmic artery and leptomeningeal vessels may be recruited when collateral flow through the circle of Willis is inadequate. Conversely, ischaemic lesions may affect the adequacy of collateral blood flow by reducing the metabolic demand of the brain.

OBJECTIVE

To determine whether the pattern of collateral pathways and the type of infarcts are independent predictors of OEF in ICA occlusion.

METHODS

We studied 42 patients with symptomatic ICA occlusion. The presence of Willisian, ophthalmic, or leptomeningeal collaterals was evaluated by conventional four vessel angiography. The infarcts on magnetic resonance imaging were categorised as territorial, border zone (external or internal), striatocapsular, lacunar, and other white matter infarcts. The value of OEF in the affected hemisphere was measured with positron emission tomography as an index of haemodynamic impairment.

RESULTS

Using multivariate analysis, the presence of any ophthalmic or leptomeningeal collaterals and the absence of striatocapsular infarcts were significant and independent predictors of increased OEF.

CONCLUSIONS

In patients with symptomatic ICA occlusion, the supply of collateral flow, which is affected by the pattern of collateral pathways, and the metabolic demand of the brain, which is affected by the type of infarct, may be important factors determining the severity of haemodynamic impairment.

Positron emission tomography imaging of brain tumors

A wide variety of metabolic features of brain tumors can be imaged using PET, including glucose metabolism, blood flow, oxygen consumption, amino acid metabolism, and lipid synthesis. Currently, FDG is the most widely available PET tracer for body imaging and brain imaging. Malignant brain tumors, like many other soft tissue tumors, show increased glucose metabolism, which is reflected on FDG-PET imaging. FDG-PET imaging of brain tumors provides information on tumor grade and prognosis. Compared with other organ systems, FDG-PET imaging of the brain presents unique challenges because of the high background glucose metabolism of normal gray matter structures. Coregistration of the MRI (or CT) and FDG-PET images is essential for accurate evaluation of brain tumors and is performed routinely at the authors' institution. The heterogeneous nature of gliomas can result in significant sampling errors when patients are biopsied for primary tumor diagnosis or recurrent disease. FDG-PET can be used to define the most metabolically active targets for stereotactic biopsy. This in turn can improve diagnostic accuracy and reduce the number of biopsy samples required. FDG-PET is also useful for evaluating residual or recurrent tumor following therapy, and can be used to survey patients with low-grade brain tumors for evidence of degeneration into high-grade malignancy. In the case of suspected tumor recurrence or progression, PET can aid in defining appropriate targets for biopsy. One limitation of FDG-PET is the occasional inability to distinguish radiation necrosis from recurrent high-grade tumor. A second limitation is that FDG-PET is less sensitive than contrast-enhanced MRI for detecting intracranial metastases, and it is the authors' experience that brain studies should not be included as part of routine whole-body PET studies. Other tracers, such as 11C-methionine and FCH, also avidly accumulate in brain tumors and have the advantage of low background cortical activity. The relationship between degree of uptake of these agents and tumor grade is not established. These tracers may be useful in specific clinical situations, however, such as tumor localization for treatment planning or evaluation of low-grade tumors.

Evaluation of a commercial PET tomograph-based system for the quantitative assessment of rCBF, rOEF and rCMRO2 by using sequential administration of 15O-labeled compounds

The purpose of this study was to develop a reliable and practical strategy that generates quantitative CBF and OEF maps accurately from PET data sets obtained with 15O-tracers. Sequential sinogram data sets were acquired after the administration of 15O-tracers, and combined single-frame images were obtained. The delay time between sampled input function and the brain was estimated from the H2(15)O study with the whole brain and the arterial time-activity curves (TACs). The whole-brain TACs were obtained from the reconstructed images (image-base method) and the sinogram data (sinogram-base method). Six methods were also evaluated for the dead-time and decay correction procedures in the process of generating a single-frame image from the dynamic sinogram. The estimated delay values were similar with both the sinogram-based and image-based methods. A lumped correction factor to a previously added single-frame sinogram caused an underestimation of CBF, OEF and CMRO2 by 16% at maximum, as compared with the correction procedure for a short sinogram. This suggested the need for a dynamic acquisition of a sinogram with a short interval. The proposed strategy provided an accurate quantification of CBF and OEF by PET with 15O-tracers.

Changes in blood flow and oxygen metabolism during visual stimulation in carotid artery disease: effect of baseline perfusion and oxygen metabolism

BACKGROUND AND PURPOSE

Severe atherosclerotic disease of the carotid arteries may affect perfusion in the posterior circulation according to the degree of collateral supply. The purpose of this study was to determine whether the responses of regional cerebral blood flow (rCBF) and regional cerebral metabolic rate of oxygen (rCMRO2) during neural stimulation are affected by the baseline perfusion or oxygen metabolism.

METHODS

We used PET to measure rCBF, rCMRO2, and regional oxygen extraction fraction (rOEF) in 13 patients with carotid artery steno-occlusive lesions at baseline and during visual stimulation. We examined whether the changes in CBF, CMRO2, and OEF during visual stimulation were correlated with the baseline values of these parameters in the visual cortex.

RESULTS

With visual stimulation, rCBF increased in all patients, whereas rCMRO2 showed variable changes. The baseline rCMRO2 value showed a positive relationship with the degree of rCBF increase and a negative relationship to the degree of rCMRO2 increase. rCMRO2 decreased in patients with relatively high baseline rCMRO2 values, resulting in dissociation of the rCMRO2 response from the rCBF response. The rCBF increase was large in the region with an increased baseline rOEF value. These variable changes in rCBF and rCMRO2 resulted in a significant correlation between the values of rCBF and rCMRO2 during visual stimulation in contrast to the lack of such a correlation at baseline.

CONCLUSIONS

In patients with cerebrovascular disease, neural stimulation may induce variable changes in rCBF and rCMRO2 according to the baseline perfusion and oxygen metabolism.

Effects of acetazolamide on cerebral blood flow, blood volume, and oxygen metabolism: a positron emission tomography study with healthy volunteers

To evaluate changes in cerebral hemodynamics and metabolism induced by acetazolamide in healthy subjects, positron emission tomography studies for measurement of cerebral perfusion and oxygen consumption were performed. Sixteen healthy volunteers underwent positron emission tomography studies with 15O-gas and water before and after intravenous administration of acetazolamide. Dynamic positron emission tomography data were acquired after bolus injection of H2[15O] and bolus inhalation of 15O2. Cerebral blood flow, metabolic rate of oxygen, and arterial-to-capillary blood volume images were calculated using the three-weighted integral method. The images of cerebral blood volume were calculated using the bolus inhalation technique of C[15O]. The scans for cerebral blood flow and volume and metabolic rate of oxygen after acetazolamide challenge were performed at 10, 20, and 30 minutes after drug injection. The parametric images obtained under the two conditions at baseline and after acetazolamide administration were compared. The global and regional values for cerebral blood flow and volume and arterial-to-capillary blood volume increased significantly after acetazolamide administration compared with the baseline condition, whereas no difference in metabolic rate of oxygen was observed. Acetazolamide-induced increases in both blood flow and volume in the normal brain occurred as a vasodilatory reaction of functioning vessels. The increase in arterial-to-capillary blood volume made the major contribution to the cerebral blood volume increase, indicating that the raise in cerebral blood flow during the acetazolamide challenge is closely related to arterial-to-capillary vasomotor responsiveness.