Positron brain imaging--normal patterns and asymmetries

Positron emission tomography imaging of regional cerebral glucose metabolism

The (F-18) fluorodeoxyglucose (FDG) technique to measure local cerebral metabolic rate for glucose (LCMRglu) is well accepted and widely used by many institutions around the world. A large number of studies has been carried out in normal volunteers and patients with a variety of CNS disorders. Several investigators have noted that no significant age-related changes in cerebral glucose use occur with normal aging. Some important and interesting findings have been revealed following sensory, motor, visual, and auditory stimulations. Functional imaging with FDG in certain neurologic disorders has dramatically improved our understanding of their underlying pathophysiologic phenomena. Some abnormalities detected on the positron emission tomography (PET) images have no corresponding changes on either x-ray computed tomograms (XCT) or magnetic resonance images (MRI). In patients with Alzheimer's disease, primary sensorimotor, visual, and cerebellar metabolic activity appears relatively preserved. In contrast, parietal, temporal, and to some degree, frontal glucose metabolism is significantly diminished even in the early stages of the disease. Patients with Huntington's disease and those at risk of developing this disorder have a typical pattern of diminished CMRglu in the caudate nuclei and putamen. In patients with stroke, PET images with FDG have demonstrated abnormal findings earlier than either XCT or MRI and with a wider topographic distribution. FDG scans have revealed interictal zones of decreased LCMRglu in approximately 70% of patients with partial epilepsy. The location of the area of hypometabolism corresponds to the site of the epileptic focus as determined by electroencephalography and microscopic examination of the resected tissue. Ictal scans during partial seizures demonstrate areas of hypermetabolism corresponding to the sites of seizure onset and spread. Several investigators have reported relative hypofrontal CMRglu in patients with schizophrenia. In our center, FDG scans from patients with schizophrenia were successfully differentiated from those obtained in normal controls. Finally, our preliminary data (using PET, XCT, and MRI) in patients with CNS disorders indicate that MRI provides excellent delineation of the structural abnormalities. It may prove to be superior to XCT in the evaluation of certain diseases such as cerebral ischemia and infarcts, head injury, tumors, and white matter lesions. Metabolic imaging with FDG provides functional information not obtainable with either MRI or NMR spectroscopy. Therefore, PET studies will play a complementary role to the anatomic imaging in the management of patients with CNS disorders.

Hypotheses on mnemonic information processing by the brain

From a neuropsychological point of view, hypotheses are offered on the possible action of the brain in the processing of mnemonic information for long-term storage (or for retrieval of long-term stored information). It is argued that strict relations between damage of circumscribed brain structures and amnesia, as they have been suggested in recent case reports, are questionable for several reasons: Firstly, the involved regions differ between cases; secondly, there is some counter-evidence from other cases in which similar neuronal damage failed to result in lasting amnesic disturbances; thirdly, it is hypothesized that even from circumscribed brain damage it is not justifiable to conclude that the lesioned structure is solely or principally responsible for the observed mnemonic changes, as the brain acts in an integrative way, that is, on the basis of a wide-spread network of neuronal information processing. On the basis of these and related arguments, hypotheses and models on mnemonic information processing in the intact and in the damaged brain are derived. With these hypotheses even the frequent observation of interindividual differences in mnemonic information processing finds a possible explanation which is in conformity to known anatomical circuits and connections and to principles of neuronal coding.

Acquired perinatal leukoencephalopathy

Between 36 and 44 weeks after conception, telencephalic white matter in the newborn appears to be particularly vulnerable to insults that result in morphological disturbances. Available evidence indicates that this disorder (or group of disorders), named acquired perinatal leukoencephalopathy, reflects a decrease in blood flow through the distal vessels supplying paraventricular white matter and/or a metabolic disturbance produced by, or in response to, an infectious agent. Major advances in our understanding of the causes, mechanisms, and sequelae of acquired perinatal leukoencephalopathy may be made in the next decade because of improvement in brain imaging techniques, improved survival of high-risk babies, and relatively routine follow-up of these high-risk babies.