Positron imaging of cerebral blood flow during continuous inhalation of C15O2

Early venous drainage after successful endovascular recanalization in ischemic stroke -- a predictor for final infarct volume?

INTRODUCTION

Peri-ischemic early venous filling (PEVD) has been reported to occur at certain stages of brain infarction and has previously been termed as "luxury perfusion". We report on the significance of PEVD after a successful endovascular recanalization.

METHODS

We retrospectively evaluated all patients who underwent endovascular stroke treatment from February 2006 to April 2010 in two centers. PEVD was rated as present or absent. Infarction was evaluated on computed tomography (CT) ≥ 18 h post-treatment. Localization of the PEVD and the infarction was noted for the anterior and posterior circulation; for the anterior circulation, also deep and superficial veins/brain regions were defined.

RESULTS

A total of 151 of the 175 patients developed an infarct. Of these 151 patients, 118 had PEVD (sensitivity 78.1%); meanwhile, 20 of 24 patients without an infarction had no PEVD (specificity 83.3%). Consistent localization of the PEVD and the infarct was seen in 107/151 patients (70.9%); in 28 of these 107 cases, the territory of PEVD was smaller than the infarct (26.2%) and exceeded it in 7/107 patients (5.6%). Territorial congruency of the PEVD and the final infarct was 57.6-75% for deep/superficial brain regions of the anterior, but only 16.7% for the posterior circulation. Separate evaluation for the anterior circulation resulted in a 94.9% sensitivity and an 81.0% specificity.

CONCLUSION

PEVD is a potential angiographic predictor for irreversible regional tissue damage and subsequent infarction despite successful recanalization. This finding deserves further studies and may influence therapeutic decisions such as post-treatment anticoagulative medication. It may also be considered in potential refined classifications of angiographic reperfusion success in the future.

Pharmacokinetics of a triarylmethyl-type paramagnetic spin probe used in EPR oximetry

The paramagnetic spin probe Oxo63 is used in oximetric imaging studies based on electron paramagnetic resonance (EPR) methods by monitoring the oxygen-dependent linewidth while minimizing the contributions from self-broadening seen at high probe concentrations. Therefore, it is necessary to determine a suitable dose of Oxo63 for EPR-based oxygen mapping where the self-broadening effects are minimized while signal intensity adequate for imaging can be realized. A constant tissue concentration of spin probe would be useful to image a subject and assess changes in pO2 over time; accumulation or elimination of the compound in specific anatomical regions could translate to and be mistaken for changes in local pO2, especially in OMRI-based oximetry. The in vivo pharmacokinetics of the spin probe, Oxo63, after bolus and/or continuous intravenous infusion was investigated in mice using a novel approach with X-band EPR spectroscopy. The results show that the half-life in blood was 17-21 min and the clearance by excretion was 0.033-0.040 min(-1). Continuous infusion following a bolus injection of the probe was found to be effective to obtain stable plasma concentration as well as image intensity to permit reliable pO2 estimates.

Radiopharmaceuticals for brain imaging

Brain imaging is performed using radiopharmaceuticals by single photon emission computed tomography (SPECT) and positron emission tomography (PET). SPECT and PET radiopharmaceuticals are classified according to blood-brain-barrier permeability, cerebral perfusion and metabolism receptor-binding, and antigen-antibody binding. The blood-brain-barrier (BBB) SPECT agents, such as 99mTcO4-, [99mTc]DTPA, 201TI and [67Ga]citrate are excluded by normal brain cells, but enter into tumor cells because of altered BBB. These agents were used in the earlier period for the detection of brain tumors. SPECT perfusion agents such as [123I]IMP, [99mTc]HMPAO, [99mTc]ECD are lipophilic agents and therefore, diffuse into the normal brain. These tracers have been successfully used to detect various cerebrovascular diseases such as stroke, Parkinson disease, Huntington's disease, epilepsy, dementia, and psychiatric disorders. Xenon-133 and radiolabeled microspheres have been used for the measurement of cerebral blood flow (CBF). Important receptor-binding SPECT radiopharmaceuticals include [123I]QNE, [123I]IBZM, and [123I]iomazenil. These tracers bind to specific receptors in the brain, thus displaying their distribution in various receptor-related cerebral diseases. Radioiodinated monoclonal antibodies were used for the detection of brain tumors. PET radiopharmaceuticals for brain imaging are commonly labeled with positron-emitters such as 11C, 13N, 15O, and 18F, although other radionuclides such as 82Rb, 62Cu and 68Ga also were used. The brain uptake of [13N]glutamate, [68Ga]EDTA and [82Rb]RbCl depends on the BBB permeability, but these are rarely used for brain imaging. Several cerebral perfusion agents have been introduced, of which [15O]water, [13N]ammonia, and [15O]butanol have been used more frequently. Regional CBF has been quantitated by using these tracers in normal and different cerebral disease states. Other perfusion agents include [15O]O2, [11C]CO, [11C]CO2, [18F]fluoromethane, [15O]O2, [11C]butanol, and [62Cu]PTSM. Among the PET cerebral metabolic agents, [18F]fluorodeoxyglucose (FDG) is most commonly used to detect metabolic abnormalities in the brain. Various brain tumors have been graded by [18F]FDG PET. This technique was used to detect epileptic foci by showing increased uptake in the foci during the ictal period and decreased uptake in the interictal period. Differentiation between recurrent tumors and radiation necrosis and the detection of Alzheimer's disease have been made successfully by [18F]FDG PET. Other PET metabolic agents such as [11C]deoxyglucose, and [11C]methylmethionine have drawn attention in the detection of brain tumors. [18F]fluorodopa is a cerebral neurotransmitter agent, which has been found very useful in the detection of Parkinson disease that shows reduced uptake of the tracer in the striatum of the brain.(ABSTRACT TRUNCATED AT 400 WORDS)

(68Ga)-EDTA positron emission tomography in the diagnosis of brain tumors

Fifty-two patients with active brain tumors and 8 patients with brain lesions from surgical treatment and/or radio- and/or chemotherapy of their brain tumors were examined by positron emission tomography (PET) using (68Ga)-EDTA in addition to conventional X-ray computed tomography (XCT). All patients with active brain tumors showed abnormal uptake of radioactivity in the tumor region, while all treated patients had normal PET scans. Site and shape of abnormal radioactivity accumulation were in good agreement with the tumor as demonstrated by XCT. Small tumors had a tendency to appear larger in PET than in XCT, while tumors with a mean largest diameter of more than 50.7 mm in XCT usually appeared smaller in PET. Despite considerable overlap to tumor classes with respect to their degree of tracer uptake a highly significant decreasing order of tumor-sagittal sinus ratios of radioactivity (TSR) was found, malignant gliomas ranking highest (median TSR 0.634), followed by meningiomas (median TSR 0.522) and metastases (median TSR 0.391), benign gliomas showing the least uptake (median TSR 0.307). These findings suggest that PET with (68Ga)-EDTA has a high sensitivity supplementing XCT in the diagnosis of brain tumors, and may be helpful in early detection of recurrent tumor growth after therapy.

Positron brain imaging--normal patterns and asymmetries

Regional brain physiology was investigated in 11 normal resting right-handed subjects using positron emission tomography. Cerebral blood flow was studied in all subjects. Cerebral oxygen metabolism was studied in six subjects, and cerebral glucose metabolism was also studied in one subject. In five subjects, physiological activity was higher in left frontotemporal regions than right. These findings may be related to structural cerebral asymmetries or to activation of brain language centers.

Regional blood flow and capillary permeability in the ethylnitrosourea-induced rat glioma

Regional cerebral blood flow and capillary permeability of rat brains bearing ethylnitrosourea-induced gliomas of various size were investigated with 14C-antipyrine autoradiography and Evans blue staining. In the small tumors (less than 2 mm in diameter), blood flow was uniformly reduced when compared to the adjacent brain. Even in tiny tumors (0.3 to 0.4 mm in diameter), reduction in blood flow was evident. In the medium (2 to 4 mm in diameter) and large (greater than 4 mm in diameter) tumors, the blood flow increased or decreased depending on the part of the tumor examined. The necrotic center and peripheral edge had low blood flows, whereas the viable portion adjacent to the necrotic center had high blood flows. Blood flow in the brain tissue adjacent to medium and large tumors was lower than control brain tissue, probably due to local edema. Leakage of intravenous Evans blue in the tissue was only evident in the large tumors with central necrosis. The present findings suggest that neovascularization of the tumor may occur when the tumor reaches a certain size, and leaky new vessels may be the cause of brain edema associated with tumor.

Reversal of focal "misery-perfusion syndrome" by extra-intracranial arterial bypass in hemodynamic cerebral ischemia. A case study with 15O positron emission tomography

Tomographic images of cerebral blood flow (CBF) and oxygen extraction fraction (OEF) using the 15O continuous inhalation technique, and positron emission tomography, were obtained from a patient with cerebral ischemia distal to an occluded left internal carotid artery. There was a focal mismatch between CBF and oxygen metabolism in the brain supplied by the middle cerebral artery where CBF was decreased and OEF increased ("misery-perfusion syndrome" as opposed to "luxury-perfusion syndrome"). These abnormalities were most marked in the parieto-occipital watershed area. After left superficial temporal to middle cerebral artery anastomosis, the clinical attacks ceased and a repeat study did not demonstrate the previous CBF and OEF abnormalities. This suggests that this pattern of abnormalities indicates potential viable tissue. The concept of "misery-perfusion" may be of some importance in the pathophysiological mechanisms of hemodynamic cerebral ischemia and serve as a rational basis for revascularization procedures.