Paradoxical reduction of cerebral blood flow after acetazolamide loading: a hemodynamic and metabolic study with (15)O PET

Capillary transit time heterogeneity inhibits cerebral oxygen metabolism in patients with reduced cerebrovascular reserve capacity from steno-occlusive disease

The healthy cerebral perfusion demonstrates a homogenous distribution of capillary transit times. A disruption of this homogeneity may inhibit the extraction of oxygen. A high degree of capillary transit time heterogeneity (CTH) describes that some capillaries have very low blood flows, while others have excessively high blood flows and consequently short transit times. Very short transit times could hinder the oxygen extraction due to insufficient time for diffusion of oxygen into the tissue. CTH could be a consequence of cerebral vessel disease. We examined whether patients with cerebral steno-occlusive vessel disease demonstrate high CTH and if elevation of cerebral blood flow (CBF) by administration of acetazolamide (ACZ) increases the cerebral metabolic rate of oxygen (CMRO₂), or if some patients demonstrate reduced CMRO₂ related to detrimental CTH. Thirty-four patients and thirty-one healthy controls participated. Global CBF and CMRO₂ were acquired using phase-contrast MRI. Regional brain maps of CTH were acquired using dynamic contrast-enhanced MRI. Patients with impaired cerebrovascular reserve capacity demonstrated elevated CTH and a significant reduction of CMRO₂ after administration of ACZ, which could be related to high CTH. Impaired oxygen extraction from CTH could be a contributing part of the declining brain health observed in patients with cerebral vessel disease.

Estimation of simultaneous BOLD and dynamic FDG metabolic brain activations using a multimodality concatenated ICA (mcICA) method

Simultaneous magnetic resonance and positron emission tomography provides an opportunity to measure brain haemodynamics and metabolism in a single scan session, and to identify brain activations from multimodal measurements in response to external stimulation. However, there are few analysis methods available for jointly analysing the simultaneously acquired blood-oxygen-level dependant functional MRI (fMRI) and 18-F-fluorodeoxyglucose functional PET (fPET) datasets. In this work, we propose a new multimodality concatenated ICA (mcICA) method to identify joint fMRI-fPET brain activations in response to a visual stimulation task. The mcICA method produces a fused map from the multimodal datasets with equal contributions of information from both modalities, measured by entropy. We validated the method in silico, and applied it to an in vivo visual stimulation experiment. The mcICA method estimated the activated brain regions in the visual cortex modulated by both BOLD and FDG signals. The mcICA provides a fully data-driven analysis approach to analyse cerebral haemodynamic response and glucose uptake signals arising from exogenously induced neuronal activity.

Evaluation of Functional Connectivity in the Brain Using Positron Emission Tomography: A Mini-Review

Resting-state networks (RSNs) exhibit spontaneous functional connectivity in the resting state. Previous studies have evaluated RSNs mainly based on spontaneous fluctuations in blood oxygenation level-dependent (BOLD) signals during functional magnetic resonance imaging (fMRI). However, separation between regional increases in cerebral blood flow (CBF) and oxygen consumption is theoretically difficult using BOLD-fMRI. Such separation can be achieved using quantitative ¹⁵O-gas and water positron emission tomography (PET). In addition, ¹⁸F-FDG PET can be used to investigate functional connectivity based on changes in glucose metabolism, which reflects local brain activity. Previous studies have highlighted the feasibility and clinical usefulness of ¹⁸F-FDG-PET for the analysis of RSNs, and recent studies have utilized simultaneous PET/fMRI for such analyses. While PET provides seed information regarding the focus of the abnormalities (e.g., hypometabolism and reduced target binding), fMRI is used for the analysis of functional connectivity. Thus, as PET and fMRI provide different types of information, integrating these modalities may aid in elucidating the pathological mechanisms underlying certain diseases, and in characterizing individual patients.

Applications of PET/CT and PET/MR Imaging in Primary Bone Malignancies

Primary bone malignancies are characterized with anatomic imaging. However, in recent years, there has been an increased interest in PET/computed tomography scanning and PET/MRI with fludeoxyglucose F 18 for evaluating and staging musculoskeletal neoplasms. These hybrid imaging modalities have shown promise largely owing to their high sensitivity, ability to perform more thorough staging, and ability to monitor treatment response. This article reviews the current role of PET/computed tomography scanning and PET/MRI in primary malignancies of bone, with an emphasis on imaging characteristics, clinical usefulness, and current limitations.

Cerebrovascular Steal Phenomenon and Electroconvulsive Therapy: A Case Report and Review of the Literature

Electroconvulsive therapy (ECT) is a safe and effective treatment for major depressive disorder, but cerebrovascular and cardiovascular complications, although rare, remain the most concerning. This is particularly notable in those with preexisting cerebrovascular disease, which impacts dynamic cerebral autoregulation. In these patients, the increased blood flow to the seizing portions of the brain induced by ECT potentially can reduce cerebral blood flow to ischemic areas, possibly causing adverse neurological events. The authors describe a patient with chronic cerebral ischemic disease, chronic anemia, and major depressive disorder undergoing ECT to achieve remission. The patient developed recurrent focal neurological deficits after each ECT procedure, with neurological recovery within 48 hours post-ECT. Clinical guidelines may need to be updated for the management of ECT patients with cerebrovascular disease who may be at an increased risk of intraictal and possibly postictal regional ischemia, especially in areas already compromised by a prior stroke and/or by reduced cerebral oxygenation caused by symptomatic anemia at risk of ischemia. Research is needed to assess changes in regional cerebral blood flow during and after ECT in patients with cerebrovascular disease, including small-vessel cerebral ischemia, and to evaluate these changes in relation to the location, intensity, and duration of induced seizure.

X-ray angiography perfusion imaging with an intra-arterial injection: comparative study with 15O-gas/water positron emission tomography

Background

X-ray angiography perfusion (XAP) is a perfusion imaging technique based on conventional DSA.

Objective

In this study, we aimed to validate parameters derived from XAP by comparing them with 15O-gas/water positron emission tomography (PET), using data from patients with chronic ischemic cerebrovascular disease.

Methods

18 consecutive patients were included. XAP was performed with intra-arterial infusion of contrast media, and a time–density curve was constructed for each cerebral hemisphere. From the curves, the relative values of mean transit time (rMTT) and wash-in rate (rWiR) were obtained by dividing the values of the right hemisphere by those of the left hemisphere. These were then compared with the relative values of cerebral blood flow (rCBF) and rMTT calculated from the PET data.

Results

XAP rWiR correlated strongly with PET rCBF (r=0.86, P<0.0001). rMTT measurements from the two modalities were also strongly correlated (r=0.85, P<0.0001). Bland–Altman analysis revealed a bias of 0.14±0.18 (95% limits of agreement −0.22 to 0.51) for PET rCBF versus XAP rWiR, and 0.016±0.093 (95% limits of agreement −0.17 to 0.20) for rMTT between the two modalities.

Conclusions

The relative values obtained from XAP were validated across a population of patients with chronic ischemic cerebrovascular disease.

Sequential PET estimation of cerebral oxygen metabolism with spontaneous respiration of 15O-gas in mice with bilateral common carotid artery stenosis

Positron emission tomography with ¹⁵O-labeled gases (¹⁵O-PET) is important for in vivo measurement of cerebral oxygen metabolism both in clinical and basic settings. However, there are currently no reports concerning ¹⁵O-PET in mice. Here, we developed an ¹⁵O-PET method applicable to mice with spontaneous respiration of ¹⁵O-gas without a tracheotomy catheter. Sequential ¹⁵O-PET was also performed in a mouse model of chronic cerebral hypoperfusion with bilateral common carotid artery stenosis (BCAS) induced by placement of microcoils. ¹⁵O-gas with isoflurane was supplied to the nose of mouse with evacuation of excess ¹⁵O-gas surrounding the body. ¹⁵O-PET was performed on days 3, 7, 14, 21, and 28 after surgery. Cerebral blood flow (CBF), cerebral blood volume, oxygen extraction fraction (OEF), and cerebral metabolic rate of oxygen (CMRO₂) were calculated in whole brains. A significant decrease in CBF and compensatory increase in OEF in the BCAS group produced CMRO₂ values comparable to that of the sham group at three days post-operation. Although CBF and OEF in the BCAS group gradually recovered over the first 28 days, the CMRO₂ showed a gradual decrease to 68% of sham values at 28 days post-operation. In conclusion, we successfully developed a noninvasive ¹⁵O-PET method for mice.