Cerebral blood flow in patients with dementia of Alzheimer's type

Simulating the effect of cerebral blood flow changes on regional quantification of [18F]flutemetamol and [18F]florbetaben studies

Global and regional changes in cerebral blood flow (CBF) can result in biased quantitative estimates of amyloid load by PET imaging. Therefore, the current simulation study assessed effects of these changes on amyloid quantification using a reference tissue approach for [¹⁸F]flutemetamol and [¹⁸F]florbetaben. Previously validated pharmacokinetic rate constants were used to simulate time-activity curves (TACs) corresponding to full dynamic and dual-time-window acquisition protocols. CBF changes were simulated by varying the tracer delivery (K₁) from +25 to -25%. The standardized uptake value ratio (SUVr) was computed and TACs were fitted using reference Logan (RLogan) and the simplified reference tissue model (SRTM) to obtain the relative delivery rate (R₁) and volume of distribution ratio (DVR). RLogan was least affected by CBF changes (χ² = 583 p < 0.001, χ² = 81 p < 0.001, for [¹⁸F]flutemetamol and [¹⁸F]florbetaben, respectively) and the extent of CBF sensitivity generally increased for higher levels of amyloid. Further, SRTM-derived R₁ changes correlated well with simulated CBF changes (R² > 0.95) and SUVr's sensitivity to CBF changes improved for later uptake-times, with the exception of [¹⁸F]flutemetamol cortical changes. In conclusion, RLogan is the preferred method for amyloid quantification of [¹⁸F]flutemetamol and [¹⁸F]florbetaben studies and SRTM could be additionally used for obtaining a CBF proxy.

Relationship between Aortic Compliance and Impact of Cerebral Blood Flow Fluctuation to Dynamic Orthostatic Challenge in Endurance Athletes

Aorta effectively buffers cardiac pulsatile fluctuation generated from the left ventricular (LV) which could be a mechanical force to high blood flow and low-resistance end-organs such as the brain. A dynamic orthostatic challenge may evoke substantial cardiac pulsatile fluctuation via the transient increases in venous return and stroke volume (SV). Particularly, this response may be greater in endurance-trained athletes (ET) who exhibit LV eccentric remodeling. The aim of this study was to determine the contribution of aortic compliance to the response of cerebral blood flow fluctuation to dynamic orthostatic challenge in ET and age-matched sedentary (SED) young healthy men. ET (n = 10) and SED (n = 10) underwent lower body negative pressure (LBNP) (-30 mmHg for 4 min) stimulation and release the pressure that initiates a rapid regain of limited venous return and consequent increase in SV. The recovery responses of central and middle cerebral arterial (MCA) hemodynamics from the release of LBNP (~15 s) were evaluated. SV (via Modeflow method) and pulsatile and systolic MCA (via transcranial Doppler) normalized by mean MCA velocity (MCAv) significantly increased after the cessation of LBNP in both groups. ET exhibited the higher ratio of SV to aortic pulse pressure (SV/_AoPP), an index of aortic compliance, at the baseline compared with SED (P < 0.01). Following the LBNP release, SV was significantly increased in SED by 14 ± 7% (mean ± SD) and more in ET by 30 ± 15%; nevertheless, normalized pulsatile, systolic, and diastolic MCAv remained constant in both groups. These results might be attributed to the concomitant with the increase in aortic compliance assessed by SV/_AoPP. Importantly, the increase in SV/_AoPP following the LBNP release was greater in ET than in SED (P < 0.01), and significantly correlated with the baseline SV/_AoPP (r = 0.636, P < 0.01). These results suggest that the aortic compliance in the endurance athletes is able to accommodate the additional SV and buffer the potential increase in pulsatility at end-organs such as the brain.

CNS Assessment Using Functional Neuro-PET Imaging

Disorders of the brain manifest in a variety of manners ranging from feeling or thought abnormalities to total paralysis. Until recently, most imaging methods of the brain have been limited to anatomic considerations, with little information about actual function of the brain except that deduced from clinical examination and physical and cognitive assessment testing. With the advent of positron emission tomography (PET) and enhanced computer and scintigraphic image detection systems, there is keen interest in applying this imaging technique to better understand brain physiology and pathophysiology. Potential applications of PET in CNS assessment is expanding avenues for improved diagnosis and staging of disease, as well as monitoring therapeutic interventions. A general review of the radiopharmaceuticals used for neuro-PET imaging, as well as their application in situations such as cerebrovascular disease, brain activation studies, various movement disorders and dementias, depression, epilepsy, obsessive-compulsive disorder, schizophrenia, and neuropharmacology (including cerebral receptor studies) will be presented.

Alzheimer's disease and non-demented high pathology control nonagenarians: comparing and contrasting the biochemistry of cognitively successful aging

The amyloid cascade hypothesis provides an economical mechanistic explanation for Alzheimer's disease (AD) dementia and correlated neuropathology. However, some nonagenarian individuals (high pathology controls, HPC) remain cognitively intact while enduring high amyloid plaque loads for decades. If amyloid accumulation is the prime instigator of neurotoxicity and dementia, specific protective mechanisms must enable these HPC to evade cognitive decline. We evaluated the neuropathological and biochemical differences existing between non-demented (ND)-HPC and an age-matched cohort with AD dementia. The ND-HPC selected for our study were clinically assessed as ND and possessed high amyloid plaque burdens. ELISA and Western blot analyses were used to quantify a group of proteins related to APP/Aβ/tau metabolism and other neurotrophic and inflammation-related molecules that have been found to be altered in neurodegenerative disorders and are pivotal to brain homeostasis and mental health. The molecules assumed to be critical in AD dementia, such as soluble or insoluble Aβ40, Aβ42 and tau were quantified by ELISA. Interestingly, only Aβ42 demonstrated a significant increase in ND-HPC when compared to the AD group. The vascular amyloid load which was not used in the selection of cases, was on the average almost 2-fold greater in AD than the ND-HPC, suggesting that a higher degree of microvascular dysfunction and perfusion compromise was present in the demented cohort. Neurofibrillary tangles were less frequent in the frontal cortices of ND-HPC. Biochemical findings included elevated vascular endothelial growth factor, apolipoprotein E and the neuroprotective factor S100B in ND-HPC, while anti-angiogenic pigment epithelium derived factor levels were lower. The lack of clear Aβ-related pathological/biochemical demarcation between AD and ND-HPC suggests that in addition to amyloid plaques other factors, such as neurofibrillary tangle density and vascular integrity, must play important roles in cognitive failure.

Unidirectional Influx and Net Accumulation of PIB

The compound {N-methyl-[(11)C]}2-(4'-methylaminophenyl)-6-hydroxybenzothiazole, "PIB", measured by positron emission tomography, has been demonstrated to image brain beta-amyloid deposition in Alzheimer's disease (AD). In the present study the benefit of measuring the PIB accumulation rate together with the unidirectional influx of PIB into the brain was investigated in healthy control subjects and patients with AD. In a monkey changes in the influx rate constant K(1) of PIB closely followed changes in CBF, caused by alteration of Pa(CO2). In addition, K(1) was high both in the monkey and in humans, suggesting that this parameter reflects CBF. Most AD patients studied showed clearly higher accumulation rate for PIB than the controls in cortical brain areas, while a few patients showed as low accumulation as the controls. K(1) did not correlate with the accumulation rate, indicating that K(1) for PIB provides extra information besides the accumulation rate.

Strategies for molecular imaging dementia and neurodegenerative diseases

Dementia represents a heterogeneous term that has evolved to describe the behavioral syndromes associated with a variety of clinical and neuropathological changes during continuing degenerative disease of the brain. As such, there lacks a clear consensus regarding the neuropsychological and other constituent characteristics associated with various cerebrovascular changes in this disease process. But increasing this knowledge has given more insights into memory deterioration in patients suffering from Alzheimer's disease and other subtypes of dementia. The author reviews current knowledge of the physiological coupling between cerebral blood flow and metabolism in the light of state-of-the-art-imaging methods and its changes in dementia with special reference to Alzheimer's disease. Different imaging techniques are discussed with respect to their visualizing effect of biochemical, cellular, and/or structural changes in dementia. The pathophysiology of dementia in advanced age is becoming increasingly understood by revealing the underlying basis of neuropsychological changes with current imaging techniques, genetic and pathological features, which suggests that alterations of (neuro) vascular regulatory mechanisms may lead to brain dysfunction and disease. The current view is that cerebrovascular deregulation is seen as a contributor to cerebrovascular pathologies, such as stroke, but also to neurodegenerative conditions, such as Alzheimer's disease. The better understanding of these (patho) physiological mechanisms may open an approach to new interventional strategies in dementia to enhance neurovascular repair and to protect neurovascular coupling.

Magnetic resonance imaging and magnetic resonance spectroscopy in dementias

This article reviews recent studies of magnetic resonance imaging and magnetic resonance spectroscopy in dementia, including Alzheimer's disease, frontotemporal dementia, dementia with Lewy bodies, idiopathic Parkinson's disease, Huntington's disease, and vascular dementia. Magnetic resonance imaging and magnetic resonance spectroscopy can detect structural alteration and biochemical abnormalities in the brain of demented subjects and may help in the differential diagnosis and early detection of affected individuals, monitoring disease progression, and evaluation of therapeutic effect.

The probable involvement of soluble and deposited melanins, their intermediates and the reactive oxygen side-products in human diseases and aging

The plasma soluble melanins (PSM) form spontaneously in vitro and in vivo and their formation involves oxidative polymerization and copolymerization of dopa, catecholamines, homogentisic acid, 3-hydroxyanthranilic acid, p-aminophenol, p-phenylenediamine, and other end(ex)ogenous ortho and para polyhydroxy-, (poly)hydroxy(poly)amino- and polyamino-phenyl compounds. The build up of PSM is visible within 2-3 h after the start of incubation at 37 degrees C with 1 mg/ml of plasma. PSM also form similarly in blood and these processes cause hemolysis. The mean quantity of PSM in normal human plasma is 1.61+/-0.1 (S.D.) mg/ml (n = 20) and in normal human urine is 1.1+/-1.2 g/24 h collection (n = 8). They contribute to the yellow color of plasma and urine. Antioxidants delay the formation of PSM. The deposited melanins also form from these precursors. Reactive oxygen side products (ROSP) are generated during and after melanogenesis. Melanins in vivo are generally associated with proteins or with proteins and lipids. The PSM-protein-lipid complexes are called plasma soluble lipofuscins (PSL), because they have histochemical and fluorescence properties similar to those of solid lipofuscins. The soluble and deposited melanins (SDM) and their intermediates have similar toxic chemical reactivities. The oxidizing quinoid (they can produce partially and completely substituted conjugates) and the semiquinoid free radical intermediates are also moieties in most human melanin structures. Soluble melanins formed from dopa, or dopamine, or norepinephrine in weak alkaline solution have been shown to be toxic to human CD4+ lymphoblastic cells (MT-2) at higher than 10 microg/ml concentrations. Alkaptonuria with high levels of homogentisic acid in the plasma is a potentially fatal disease, exhibiting the toxic effects of the homogentisic acid melanin (soluble and deposited), its intermediates and the ROSP. Patients with alkaptonuria develop arthritis and often suffer from other diseases too, including cardiovascular disease (frequent cause of death) and kidney disease. Pheochromocytoma, with high levels of catecholamines in the plasma is another potentially fatal disease. The catecholamine PSM of pheochromocytoma have very light yellow or practically no colors, due to the concentrations and chemical structures. Pheochromocytomas can cause hypertension, cardiovascular disease (frequent cause of death), kidney disease, stroke, cancer, amyloid formation and can mimic many other diseases, including acute pancreatitis, carcinoid, neuroblastoma, psychiatric illness, hypercalcemia, retinal vascular lesions, and diabetes mellitus. Pheochromocytoma is potentially fatal even in patients without hypertension. Following trauma and surgery, heavily pigmented eyes are apt to experience greater inflammation than lightly pigmented eyes. In Parkinson's disease those neurons are lost first in the substantia nigra and locus ceruleus which contain the greatest amounts of neuromelanins. The antihypertensive alphamethyldopa causes Parkinson's syndrome. It forms PSM in a short time in vitro. The side effects of L-dopa (immobility episodes alternate with normal or involuntary movements; psychotic abnormalities) suggest that the SDM, their intermediates and the ROSP present naturally in vivo are involved in the cause of Parkinson's disease and Alzheimer's disease. There is a large overlap between these two diseases. (ABSTRACT TRUNCATED)

Association between medial temporal lobe atrophy on CT and parietotemporal uptake decrease on SPECT in Alzheimer's disease

OBJECTIVES

Alzheimer's disease is the most frequent cause of degenerative dementia. Despite the available diagnostic criteria, improvement of diagnosic accuracy is still required. The aim of this prospective study was to assess in a large population of patients referred to a memory clinic the diagnostic value of the combination of medial temporal lobe atrophy on temporal oriented CT and decreased temporoparietal uptake on HMPAO single photon emission tomography (SPECT).

METHODS

The study was conducted in 125 patients aged 51-93: 64 with probable Alzheimer's disease (Mean (SD) mini mental state examination (MMSE)=18.34 (6.93)), duration of disease=6.48 (2.93) years, 13 possible Alzheimer's disease (MMSE=21.58 (5.48), duration of disease=6.08 (2.56)), 48 patients with miscellaneous memory disorders (MMSE=21.98 (6.10), duration the disease = 6.85 (3.91)).

RESULTS

For the diagnosis of probable Alzheimer's disease, the sensitivity of this association was 0.56, the specificity 0.93, the positive predictive value 0.95, and the negative predictive value 0.45. The diagnosic accuracy was 0.68. Both medial temporal atrophy and parietotemporal decrease in uptake were present in four of 13 patients with possible Alzheimer's disease and 11 of 48 with miscellaneous memory disorders. The association was absent in 27 of 29 patients with frontotemporal dementia. In mild stages (MMSE>18; n = 32), the sensitivity of the association was 0.34, the specificity 0.93, the positive predictive value 0.85, and the negative predictive value 0.57. The diagnosic accuracy was 0.53.

CONCLUSION

This association, although not sensitive, helps to select patients with high probability of Alzheimer's disease at an early stage which can be of interest for clinical and research purposes.