Applications of SPECT in Dementia and Related Conditions-Queen Mary Hospital Experience

SUMMARY The regional cerebral blood flow (rCBF) pattern is a reflection of neural activity in different brain regions due to the tight coupling between regional cerebral blood flow and neuronal activity. With Single Photon Emission Computed Tomography (SPECT) using the flow tracer Tc-99m-HMPAO, high resolution rCBF images can be obtained. SPECT images of five patients with various degree of cognitive impairment and features of dementia are reported. Different rCBF patterns are noticed in Alzheimer's disease and multi-infarct dementia. The clinical applications of SPECT in the management of dementia are discussed. INTRODUCTION Single Photon Emission Computed Tomography (SPECT) is an imaging technique of brain function and is a measurement of regional cerebral blood flow (rCBF). It is based on the observation that there is a tight coupling between CBF and metabolic activity. It involves injection of radiopharmaceuticals into a subject and the detection of its distribution by a rotating gamma camera. The gamma camera consists of a collimator (a focusing device),a crystal (sodium iodide) which will emit photons on impact by gamma rays from radioisotopes and a circuitry that detects and analyses the photon emission pattern. Radiopharmaceuticals commonly used are Tc-99m-HMPAO and I-123-IMP. These lipophilic isotopes once taken up by the neurons will be transformed to a lipophobic ligand and will stay in the cells. In other words, the pattern of the distribution or radioactivity will be determined by the pattern of regional cerebral blood flow at the time of the injection of radiopharmaceuticals. The patient can be at a resting position during injection or he or she can be asked to perform a variety of tasks like verbal fluency activation as well as pharmacological activation and sensory stimulation (Devous, 1989; Friberg, 1989). The evolution of various techniques of cerebral blood flow measurement like Positron Emission Tomography (PET) and SPECT and their application in the studies of dementia was discussed by Bonte et al (1990). Brain SPECT is generally regarded as the culmination of a series of investigations of CBF pioneered by Kety and Schmidt (1948). The relative inexpensiveness and simplicity in operation as compared with PET has made SPECT widely applied in clinical practice. A number of SPECT studies have now been performed using Tc-99m-HMPAO to investigate dementia and has been reviewed by Geaney and Abou-Saleh (1990). These studies have shown that SPECT can reveal cerebral abnormalities when CT and MRI do not because the latter are measures of cerebral structure while the former are measures of cerebral function. Various rCBF patterns have been reported in different demented conditions like Alzheimer's disease (AD), multi-infarct dementia (MID), Pick's disease and Huntington's disease with different degree of specificity. The aim of this study is to summarize our experience at Queen Mary Hospital in the SPECT brain imaging of patients with various demented conditions and to discuss the relevant clinical applications in psychiatry. PATIENTS & METHOD Two patients with Alzheimer's disease, one patient with multi-infarct dementia, one patient with delirium and one patient with depression were studied. The diagnosis in these patients satisfied the DSM-111-R criteria. Mini-Mental State examination (Folstein et al, 1975) was also performed. SPECT were performed in the first hour following the intravenous injection of 550MBq of Tc-99m-HMPAO. Injection was given within 30 minutes of preparation of the compound. The patient was imaged on a tomographic table using a rotating gamma camera (Elscint Apex). A head support was used that enables the camera to rotate with the minimum possible radius around the patient's head. Images were acquired for 25 seconds each at 60 angles over 360 degrees and the total study time was approximately 30 minutes. A low energy high resolution collimator was used and images structured in a 64 x 64 matrix. …