The role of Tc-99m-HMPAO SPET perfusion neuroimaging and cerebrovascular reactivity to acetazolamide in patients with mild cognitive impairment

Abstract

Background In contrast to the physiological age related decline of cognitive functions, mild cognitive impairment (MCI) is considered an early stage of dementia. In the presence of vascular risk factors, it is difficult to differentiate early stages of Alzheimer's disease (AD) from those of vascular dementia (VD), solely on the basis of clinical and neuroimaging (CT, MRI) findings. Single photon emission tomography (SPET) is a technique to obtain functional tomographic images of the 3-dimensional distribution of a radiopharmaceutical. One of the most widely used radiopharmaceutical for regional Cerebral Blood Flow (CBF) SPET is Technetium-99m (Tc-99m) labelled compound with HMPAO (Ceretec; Nycomed Amersham). After intravenous injection the Tc-99m-HMPAO lipophilic compound cross the intact blood brain barrier, distribute in the brain proportional to local blood flow and is retained in the brain with a fixed regional distribution for a sufficient time period to permit image acquisition. The peak brain activity is reached within 2 min post-injection. Since there is no redistribution, the initial tracer uptake and distribution remains almost unchanged for several hours and are independent of rCBF variations occurring after the fixation time (frozen images, which represent the rCBF at the time of injection). Acetazolamide (Diamox) is known to increase Cerebral Blood Flow (CBF) without changing the cerebral metabolic rate of oxygen. Patients with compromised circulation and resultant reduced CBF due to reduced perfusion pressure does not respond to acetazolamide. However, little is known about the cerebrovascular reactivity (CVR) in patients with MCI due to degenerative or perfusion related memory disturbances. Materials and methods This study included 18 patients with demonstrated (clinic and neuropsychologic findings) MCI. There were ten men and eight women (age range 52–82 years, mean 69 years) with neurological clinical examination including Hachinski scale score. Computerized tomography (CT) scanning and Magnetic resonance (MR) imaging was performed in all patients. SPET brain perfusion imaging was performed after the intravenous injection of a typical dose of 740 MBq Tc-99m-HMPAO in two different days before and after the intravenous administration of 1 gr Diamox. All SPET scans were interpreted visually.