Recovery of vision after ischemic lesions: positron emission tomography

We used [18F]fluoro-2-deoxyglucose positron emission tomography (PET) to study serial changes in the local cerebral metabolic rate of glucose in 5 patients with ischemic lesions of the posterior afferent visual system causing homonymous visual field defects. All 5 patients had striking impairment of glucose metabolism in the striate cortex shortly after ictus. In 2 patients, visual field defects abated, and repeat PET scans showed reduced size of the metabolic lesion and improvement of striate metabolism. Three patients did not recover vision, and repeat PET scans did not show improvement. Patients who recovered vision had ischemic lesions outside the occipital lobe, while those who did not experience improvement had primary damage to the occipital lobe itself.     

Temporoparietal cortex in aphasia. Evidence from positron emission tomography

Forty-four aphasic patients were examined with (F18)-fluorodeoxyglucose positron emission tomography in a resting state to determine whether consistent glucose metabolic abnormalities were present. Ninety-seven percent of subjects showed metabolic abnormalities in the angular gyrus, 89% in the supramarginal gyrus, and 87% in the lateral and transverse superior temporal gyrus. Pearson product moment correlations were calculated between regional metabolic measures and performance on the Western Aphasia Battery. No significant correlations were found between the Western Aphasia Battery scores and right hemisphere metabolic measures. Most left hemisphere regions correlated with more than one score from the Western Aphasia Battery. Temporal but not frontal regions had significant correlations to the comprehension score. The left temporoparietal region was consistently affected in these subjects, suggesting that common features in the aphasias were caused by left temporoparietal dysfunction, while behavioral differences resulted from (1) the extent of temporoparietal changes, and (2) dysfunction elsewhere in the brain, particularly the left frontal and subcortical areas.     

Glucose utilization in the temporal cortex of affectively ill patients: positron emission tomography

As temporal lobe dysfunction has been postulated in the affective disorders, the authors investigated glucose utilization in the temporal lobes of 13 affectively ill patients in comparison with 18 normal volunteer controls and 17 previously reported schizophrenic patients, following injections of fluorine 18-2-deoxy-D-glucose (FDG) during somatosensory stimulation to the right forearm. Using a boundary-finding algorithm to outline each temporal lobe, maximum glucose use relative to maximums elsewhere in the same positron emission tomography (PET) slice were calculated. In a small group of moderately to severely depressed patients, this relative measure was significantly reduced in the right (with a similar trend in the left) temporal lobe compared to normal volunteers and the other comparison groups. The lack of a significant increase in glucose utilization, measured either as a maximum or in relation to other areas in the PET scan slice, suggests that a temporal lobe activation or a seizure-like process is not generally occurring during active depressive phases of the illness.     

Insular cortex involvement in mesiotemporal lobe epilepsy: a positron emission tomography study

Somesthetic and emotional symptoms that are common in patients with mesial temporal lobe epilepsy are usually related to hippocampo-amygdalar complex involvement. Recent stereo-electroencephalographic studies have shown a relationship between such symptoms and epileptic insular discharges. To further investigate this problem, we carried out a positron emission tomography study using fluorodeoxyglucose (18F-FDG) and flumazenil (11C-FMZ) in mesial temporal lobe epilepsy patients. The aim of our study was to assess the existence of a cortical insular involvement in order to examine its clinical correlates and the relationship between the postoperative outcome and the insular involvement. Fluorodeoxyglucose and flumazenil-positron emission tomography studies were carried out in 18 patients with mesial temporal lobe epilepsy patients. A statistical parametric mapping (SPM96) was performed to analyze the data in comparison to 18 healthy volunteers. For each set of fluorodeoxyglucose and flumazenil images a group and an individual analysis were performed. In addition, a region of interest analysis was performed to validate the results. Focusing on the metabolic abnormalities, we also investigated the role of insular cortex in the symptoms experienced by the patients and the prognostic value of insular metabolic abnormalities. Highly significant hypometabolism and BZR binding decreases were detected in the insular cortex. Results were similar using the region of interest approach. Insular involvement (mainly ipsilateral to the seizure focus) was present in 60% of the patients. Emotional symptoms correlated with hypometabolism in the anterior part of the ipsilateral insular cortex, whereas somesthetic symptoms correlated with hypometabolism in the posterior part. No relationship between postoperative outcome and ipsilateral insular hypometabolism was found. Unilateral mesial temporal lobe epilepsy is associated with insular hypometabolism and benzodiazepine receptor loss. Our results also suggest that the anterior part of the insular cortex is involved in the emotional symptoms and the posterior insular cortex is involved in the somesthetic symptoms. Hypometabolism located in the insula did not influence postoperative outcome after anterior lobectomy.     

Relative sparing of the parietal cortex in cerebellar ataxia documented by positron emission tomography

With the intention to assess remote effects of cerebellar dysfunction, 23 patients with inherited or idiopathic cerebellar ataxia were studied with positron emission tomography (PET) and 2[18F]fluoro-2-deoxy-D-glucose (FDG). Eight patients (group 1) suffered from early onset cerebellar ataxia (EOCA, age of symptom onset <20 years), nine patients (group 2) from late onset cerebellar ataxia (LOCA, symptom onset between the ages of 20 and 50), and six patients (group 3) experienced symptom onset beyond the age of 50 years. The pattern of cerebral glucose metabolism in cerebellar ataxia was compared to the results in a control group of 16 healthy subjects. In all patients, a reduction in relative (EOCA, group 1) or absolute (LOCA, groups 2 and 3) values of regional cerebral glucose metabolism (rCMR(glu)) occurred in both cerebellar hemispheres as well as the vermis and both dentate nuclei. In patients from all groups presenting with a clinical syndrome of pure cerebellar ataxia, impairment of regional glucose metabolism also extended to the pontine and brainstem regions. In contrast to this infratentorial reduction of rCMR(glu) in all patients, in those with LOCA, a significant relative increase in rCMR(glu) was present in distinct supratentorial cortical regions, namely the cuneus, the pre-cuneus and the gyrus supramarginalis in the patients of group 2. In group 3, this significant relative increase in rCMR(glu) was restricted to the cuneus. Thus, FDG-PET in patients suffering from cerebellar ataxia shows distinct patterns of altered glucose metabolism which exceed pure cerebellar impairment. Most importantly, FDG-PET yields insight into the influence of cerebellar disease on supratentorial glucose metabolism and documents impairment of supratentorial neuronal function with relative sparing of the parietal cortex.     

Instrumentation in positron emission tomography

The past 40 years have seen PET scanning evolve from a tool that was used predominantly for research to a valued clinical, imaging modality. Current PET scanners must perform high quality, whole-body, as well as brain, PET. There are several levels of PET devices that range from the dedicated, high-end scanners down to the hybrid PET-SPECT systems that offer varying levels of performance. The incorporation of PET into single, hybrid, multi-modality units that can provide functional and anatomic information is becoming extremely popular. Several manufacturers now provide hybrid PET-CT scanners. There is also a growing interest in dedicated devices for specific applications, such as high-resolution scanners for imaging small animals.     

Neurologic applications of positron emission tomography

The impact of computerized neuroimaging in the neurologic sciences has been so dramatic that it has completely changed our approach to the individual patient. Further changes may be expected from the newborn positron emission tomography (PET) and nuclear magnetic resonance (NMR) in order to help the reader digest a large bulk of data and fully realize the present state of the art of PET, the authors have shaped this review mainly on results rather than on methods and on published reports rather than on future potential.     

The precision of positron emission tomography: theory and measurement

The limits of quantitation with positron emission tomography (PET) are examined with respect to the noise propagation resulting from radioactive decay and other sources of random error. Theoretical methods for evaluating the statistical error have been devised but seldom applied to experimental data obtained on human subjects. This paper extends the analysis in several ways: (1) A Monte Carlo method is described for tracking the propagation of statistical error through the analysis of in vivo measurements; (2) Experimental data, obtained in phantoms, validating the Monte Carlo method and other methods are presented; (3) A difference in activation paradigm, performed on regional CBF (rCBF) data from five human subjects, was analyzed on 1.6-cm diameter regions of interest to determine the mean fractional statistical error in PET tissue concentration and in rCBF before and after stereotactic transformation; and (4) A linear statistical model and calculations of the various statistical errors were used to estimate the magnitude of the subject-specific fluctuations under various conditions. In this specific example, the root mean squared (RMS) noise in flow measurements was about three times higher than the RMS noise in the concentration measurements. In addition, the total random error was almost equally partitioned between statistical error and random fluctuations due to all other sources.     

In vivo functional localization of the human visual cortex using positron emission tomography and magnetic resonance imaging

Positron emission tomography (PET) and magnetic resonance imaging (MRI) are two recently developed methods for imaging the human brain in vivo. One application of PET measures stimulus-evoked changes in cerebral blood flow while MRI provides a detailed anatomical map of the brain. Here we report the combined application of these two techniques in the same human subject. Subtracted PET scans of a brain receiving visual stimulation were superimposed upon MRI images of the same brain. The PET scans were converted into the MRI coordinate space before superposition, which allowed for a more precise correlation between MRI anatomical data and PET physiological data. Responses were localized in striate and extrastriate visual areas as well as in the posterior thalamus.     

Functional imaging of vegetative state applying single photon emission tomography and positron emission tomography

Nuclear medicine techniques, such as single photon emission tomography (SPECT) and positron emission tomography (PET) have been applied in patients in a vegetative state to investigate brain function in a non-invasive manner. Parameters investigated include glucose metabolism, perfusion at rest, variations of regional perfusion after stimulation, and benzodiazepine receptor density. Compared to controls, patients in a vegetative state show a substantial reduction of glucose metabolism and perfusion. While patients post-anoxia exhibit a rather homogenous cortical reduction of glucose metabolism, patients after head trauma often show severe cortical and sub-cortical reductions at the site of primary trauma. To distinguish reduced glucose metabolism due to neuronal inactivation from neuronal loss, flumazenil-PET, an indicator of benzodiazepine receptor density, could add valuable information on the extent of brain damage. Activation studies focus on the evaluation of residual brain network, looking for processing in secondary projection fields. So far the predictive strength concerning possible recovery for the individual patient is limited, and PET and SPECT are not routine procedures in the assessment of patients in a vegetative state.     

Utility of positron emission tomography in schwannomatosis

Schwannomatosis is characterized by multiple non-intradermal schwannomas with patients often presenting with a painful mass in their extremities. In this syndrome malignant transformation of schwannomas is rare in spite of their large size at presentation. Non-invasive measures of assessing the biological behavior of plexiform neurofibromas in neurofibromatosis type 1 such as positron emission tomography (PET), CT scanning and MRI are well characterized but little information has been published on the use of PET imaging in schwannomatosis. We report a unique clinical presentation portraying the use of PET imaging in schwannomatosis. A 27-year-old woman presented with multiple, rapidly growing, large and painful schwannomas confirmed to be related to a constitutional mutation in the SMARCB1 complex. Whole body PET/MRI revealed numerous PET-avid tumors suggestive of malignant peripheral nerve sheath tumors. Surgery was performed on multiple tumors and none of them had histologic evidence of malignant transformation. Overall, PET imaging may not be a reliable predictor of malignant transformation in schwannomatosis, tempering enthusiasm for surgical interventions for tumors not producing significant clinical signs or symptoms.     

Imaging the epileptic brain with positron emission tomography

Positron emission tomography (PET) has an established role in the noninvasive localization of epileptic foci during presurgical evaluation. [18F]fluorodeoxyglucose (FDG) PET is able to lateralize and regionalize potentially epileptogenic regions in patients who have normal MR imaging and is also useful in the evaluation of various childhood epilepsy syndromes, including cryptogenic infantile spasms and early Rasmussen's syndrome. Novel PET tracers that were developed to image neurotransmission related to gamma-aminobutyric acid (GABA) [with [11C]flumazenil] and serotonin-mediated [with alpha-[11C]methyl-L-tryptophan (AMT)] function provide increased specificity for epileptogenic cortex and are particularly useful when FDG PET shows large abnormalities of glucose metabolism. Detailed comparisons of PET abnormalities with intracranial electroencephalographic findings also improve our understanding of the pathophysiology of human epilepsy.