Comparison of PET and fMRI activation patterns during declarative memory processes

Aim: In this study neuronal correlates of encoding and retrieval in paired association learning were compared using two different neuroimaging methods: positron emission tomography (PET) and functional magnetic resonance imaging (fMRI). Methods: 6 right-handed normal male volunteers took part in the study. Each subject underwent six 0-15-butanol PET scans and an fMRI study comprising four single epochs on a different day. The subjects had to learn and retrieve 12 word pairs which were visually presented (highly imaginable words, not semantically related). Results: Mean recall accuracy was 93% in the PET as well as in the fMRI experiment. During encoding and retrieval we found anterior cingulate cortex activation, and bilateral prefrontal cortex activation in both imaging modalities. Furthermore, we demonstrate the importance of the precuneus in episodic memory. With PET the results demonstrate frontopolar activations whereas fMRI fails to show activations in this area probably due to susceptibility artifacts. In fMRI we found additionally parahippocampal activation and due to the whole-brain coverage cerebellar activation during encoding. The distance between the center-of-mass activations in both modalities was 7.2 ± 6.5 mm. Conclusion: There is a preponderance of commonalities in the activation patterns yielded with fMRI and PET. However, there are also important differences. The decision to choose one or the other neuroimaging modality should among other aspects depend on the study design (single subject vs. group study) and the task of interest.

Neurocognition and PET

Aim: In cognitive neuroscience regional cerebral blood flow (rCBF) imaging with positron-emission-tomography (PET) is a powerful tool to characterize different aspects of cognitive processes by using different data analysis approaches. By use of an n-back verbal working memory task (varied from 0- to 3-back) we present cognitive subtraction analysis as basic strategy as well as parametric and covariance analyses and discuss the results. Methods: Correlation analyses were performed using the individual performance rate as an external covariate, computing inter-regional correlations, and as network analysis applying structural equation modelling to evaluate the effective connectivity between the involved brain regions. Results: Subtraction analyses revealed a fronto-parietal neuronal network also including the anterior cingulate cortex and the cerebellum. With higher memory load the parametric analysis evidenced linear rCBF increases in prefrontal, pre-motor and inferior parietal areas including the precuneus as well as in the anterior cingulate cortex. The rCBF correlation with the individual performance as external covariate depicted negative correlations in bilateral prefrontal and inferior parietal regions, in the precuneus and the anterior cingulate cortex. The network analysis demonstrated mainly occipito-frontally directed interactions which were predominantly left-hemispheric. Additionally, strong linkages were found between extrastriate and parietal regions as well as within the parietal cortex. Conclusion: The data analysis approaches presented here contribute to an extended and more elaborated understanding of cognitive processes and their different sub-aspects.

Focus on GABAA receptor function

Impairment of GABAA receptor function is increasingly recognized to play a major role in the pathophysiology of neuropsychiatric diseases including anxiety disorder (AD), major depressive disorder (MDD) and schizophrenia (SZ). Patients, method: We conducted a PUBMED search, which provided a total of 23 in vivo investigations with PET and SPECT, in which GABAA receptor binding in patients with the primary diagnosis of AD (n = 14, 160 patients, 172 controls), MDD (n = 2, 24 patients, 28 controls) or SZ (n = 6, 77 patients, 90 controls) was compared to healthy individuals. Results: A retrospective analysis revealed that AD, MDD and SZ differed as to both site(s) and extent(s) of GABAergic impairment. Additionally, it may be stated that, while the decline of GABAA receptor binding AD involved the whole mesolimbocortical system, in SZ it was confined to the frontal and temporal cortex. Conclusion: As GABA is known to inhibit dopamine and serotonin, GABAergic dysfunction may be associated with the disturbances of dopaminergic and serotonergic neurotransmission in neuropsychiatric disorders.

Cerebral haemodynamics during hypo- and hypercapnia

Aim: Transcranial Doppler sonography (TCD) is increasingly used in cerebrovascular disease for monitoring brain perfusion. It allows estimation of cerebral blood flow (CBF) by the measurement of cerebral blood flow velocity (CBFV). The CBFV as well as CBF are intimately associated with the intravascular CO2-concentration. Thus, hyper- or hypocapnia can be used to induce a defined range of blood flows. The aim of our study was the comparison of vasomotor reactivity assessed with simultaneous TCD and quantitative regional CBFmeasurements (rCBF) by PET (serving as the reference method for in-vivo quantification of rCBF). Patients, methods: Six healthy young volunteers participated in this study. CBF was measured using 15O-butanol PET. A flow and dispersion- model was fitted to the measured time activity curves using arterial input curves. Each subject underwent five scans at five different end-tidal CO2 levels (EtCO2): 25, 32, 40, 48, and 55 mmHg. CBFV was assessed by continuous bilateral TCD of the middle cerebral artery (MCA). Volumes of interest for rCBF determination were placed in grey matter of the prefrontal cortex (PFC) as determined from individual MRIs. Comparisons between the rCBF, EtCO2 and CBFV were carried out with regression and correlation analysis and paired t-tests. Results: Strong positive linear correlations of rCBF and CBFV with the CO2-concentration and linear relationships between rCBF and CBFV were found in each individual. Normalised CO2-reactivities measured by TCD and PET were closely correlated. Conclusions: TCD-measurements of vascular reactivity in healthy volunteers show a high correlation to those acquired with PET that serves as the reference method of quantitative rCBF-measurement. The results of the MCA insonation are a close approximation of the rCBF changes induced by variations of EtCO2.

Cerebellar fMRI Activation Increases with Increasing Working Memory Demands

The aim of the present study was to explore cerebellar contributions to the central executive in n-back working memory tasks using 7-T functional magnetic imaging (fMRI). We hypothesized that cerebellar activation increased with increasing working memory demands. Activations of the cerebellar cortex and dentate nuclei were compared between 0-back (serving as a motor control task), 1-back, and 2-back working memory tasks for both verbal and abstract modalities. A block design was used. Data of 27 participants (mean age 26.6 ± 3.8 years, female/male 12:15) were included in group statistical analysis. We observed that cerebellar cortical activations increased with higher central executive demands in n-back tasks independent of task modality. As confirmed by subtraction analyses, additional bilateral activations following higher executive demands were found primarily in four distinct cerebellar areas: (i) the border region of lobule VI and crus I, (ii) inferior parts of the lateral cerebellum (lobules crus II, VIIb, VIII, IX), (iii) posterior parts of the paravermal cerebellar cortex (lobules VI, crus I, crus II), and (iv) the inferior vermis (lobules VI, VIIb, VIII, IX). Dentate activations were observed for both verbal and abstract modalities. Task-related increases were less robust and detected for the verbal n-back tasks only. These results provide further evidence that the cerebellum participates in an amodal bilateral neuronal network representing the central executive during working memory n-back tasks.

Clinical Evaluation of the First Automated Assay for the Detection of Stimulating TSH Receptor Autoantibodies

Until recently, stimulating TSH receptor autoantibodies (sTRAbs) could only be measured by bioassays. A new assay system, which directly detects sTRAb in sera by applying bridge technology, has been established and is now available as automated chemiluminescence (bridge) immunoassay. We evaluated the automated bridge assay in clinical routine and compared it with a conventional automated TRAb assay (competition assay). Altogether, 226 Graves' disease (GD), 57 autoimmune thyroiditis, 74 non-autoimmune nodular goiter and 49 thyroid cancer patients, as well as 41 healthy controls were retrospectively evaluated. ROC plot analysis based on sera of newly diagnosed GD patients revealed an area under curve of 0.99 (95% CI: 0.99-1.0) indicating a high assay sensitivity and specificity. The highest sensitivity (100%) and specificity (99%) were seen at a cut-off level of 0.55 IU/l. The calculated positive predictive value was 94%, whereas the negative was 100%. Applying a ROC plot-derived cut-off of≥0.30 IU/l, derived from sera of GD patients already receiving antithyroid drug therapy for≤6 months, the sensitivity was 99% whereas the specificity was 98%. Detailed comparison of both assay systems used revealed a slightly different distribution of sTRAb and TRAb. Measurement of sTRAb during follow-up revealed a steady decline over one year of follow-up. In summary, our results demonstrate that the new automated bridge assay system for detecting sTRAb has a high sensitivity and specificity for diagnosing GD and to discriminate from other thyroid diseases, respectively. Our study, however, does not provide full evidence that the bridge assay is specific for sTRAb only.

Pharmacological treatment with L-DOPA may reduce striatal dopamine transporter binding in in vivo imaging studies

Numerous neurologic and psychiatric conditions are treated with pharmacological compounds, which lead to an increase of synaptic dopamine (DA) levels. One example is the DA precursor L-3,4-dihydroxyphenylalanine (L-DOPA), which is converted to DA in the presynaptic terminal. If the increase of DA concentrations in the synaptic cleft leads to competition with exogenous radioligands for presynaptic binding sites, this may have implications for DA transporter (DAT) imaging studies in patients under DAergic medication. This paper gives an overview on those findings, which, so far, have been obtained on DAT binding in human Parkinson's disease after treatment with L-DOPA. Findings, moreover, are related to results obtained on rats, mice or non-human primates. Results indicate that DAT imaging may be reduced in the striata of healthy animals, in the unlesioned striata of animal models of unilateral Parkinson's disease and in less severly impaired striata of Parkinsonian patients, if animal or human subjects are under acute or subchronic treatment with L-DOPA. If also striatal DAT binding is susceptible to alterations of synaptic DA levels, this may allow to quantify DA reuptake in analogy to DA release by assessing the competition between endogenous DA and the administered exogenous DAT radioligand.

Interim PET response criteria in paediatric non-Hodgkin's lymphoma. Results from a retrospective multicenter reading

AIM

To evaluate the use and reliability of the PET-based response criteria for interim PET (iPET) in terms of interobserver variability in pediatric and adolescent patients suffering from non-Hodgkin´s lymphoma (NHL). Particular attention was given to the identification of visual cutoff to separate patients with a favourable outcome.

PATIENTS, METHODS

Retrospective analysis of PET-datasets of 18 children and adolescents suffering from NHL who underwent iPET after two cycles of chemotherapy for response assessment. Datasets were evaluated and rated in three independent review centers (RC) (blinded-read, intra-center consensus) using a visual 5-point response scale. Ratings were compared to clinical outcome. Pairwise interobserver agreement was analysed with Cohen's kappa-test (κ). Overall agreement (between attended RCs) was assessed with Fleiss' κ-test.

RESULTS

Four patients suffered relapse (early, n = 2; late, n = 2). Per region analyses on interobserver variability revealed a "substantial" agreement (Fleiss' κ = 0.618). Per patient analyses revealed concordant iPET-ratings in eight patients: iPET-negative (iPET-), n = 5; iPET-positive (iPET+), n = 2; iPET-inconclusive (iPET±), n = 1. Discordant ratings were found in the remaining patients. Patients with early relapse were concordantly identified using mediastinal blood pool structures (MBPS, score ≥ 3) as visual cutoff between iPET+ or iPET-, respectively. However, patients with late relapse were not concordantly identified taking the MBPS as visual cutoff.

CONCLUSION

The iPET interpretation using a dedicated PET-based response scale assured a low interobserver variability in per-region but not in per-patient analyses in a multicenter read. Using a sensitive read out (iPET+, score ≥ 3) a reliable identification of patients suffering relapse was limited to those with early relapse.

A comparison of rh-TSH and thyroid hormone withdrawal in patients with differentiated thyroid cancer: preliminary evidence for an influence of age on the subjective well-being in hypothyroidism

We aimed to investigate the subjective well-being in patients with differentiated thyroid cancer after hormone withdrawal. Since this might be confounded by psycho-oncological processes unrelated to hypothyroidism we intended to minimize such factors by only including patients with a history of uneventful follow-up examinations for several years. We investigated 67 patients applying the General Health Questionnaire (GHQ-12) at 3 time points t1, t2, and t3. The time point t2 represented an intensified follow-up examination 5 years after thyroidectomy, which was performed either on hormone withdrawal (49 patients) or using rh-TSH (18 patients). The time points t1 and t3 took place during follow-up examinations 6 months before and after t2 in a euthyroid state. Additionally, we assessed the impact of age, gender, family status, and education on the GHQ-12 score at all 3 time points. Within the hormone withdrawal group the analyses demonstrated a significant difference between t1 and t2 as well as t3 and t2. Additionally, there was a significant negative correlation of age with GHQ-12 sum scores at t2, but not at t1 or t3. Subgroup analyses at t2 indicated that the subjective well-being in younger patients was more impaired compared to elderly patients. The between-group analysis showed no significant differences. However, concerning the age effect there was a significant difference between the subgroup of young hypothyroid patients and the total rh-TSH group at t2. We demonstrated preliminary evidence for an influence of age on the subjective well-being in hypothyroidism suggesting that younger subjects are subjectively more impaired by hypothyroidism than elderly ones.

In vivo imaging neurotransmitter function. The rat 6-hydroxydopamine model and its relevance for human Parkinson's disease

This article gives an overview of those small animal imaging studies which have been conducted on neurotransmitter function in the rat 6-hydoxydopamine (6-OHDA) model of Parkinson's disease, and discusses findings with respect to the outcome of clinical studies on Parkinsonian patients.

Evaluation of interim PET response criteria in paediatric Hodgkin's lymphoma--results for dedicated assessment criteria in a blinded dual-centre read

BACKGROUND

The aim of this study was to evaluate the use and reliability of the new positron emission tomography (PET)-based response criteria for interim positron emission tomography (iPET) in patients with paediatric Hodgkin's lymphoma (pHL). Particular emphasis was put on interobserver variability and on identification of a visual cut-off defining patients with very low risk for relapse.

PATIENTS AND METHODS

The iPET scans of 39 pHL patients were evaluated in two independent centres by two PET-experienced specialists in nuclear medicine (blinded read, centre consensus) each. The iPET scans were interpreted using a 5-point scale and were compared with the outcome. Cohen's kappa-test (κ) was used to analyse the interobserver agreement.

RESULTS

Concordant ratings were assessed in 19 patients with iPET-negative findings, in 11 patients with iPET-positive findings and in 2 patients with inconclusive ratings. A 'substantial agreement' between attended centres was achieved (κ = 0.748). All patients suffering relapse were concordantly identified, taking mediastinal blood pool structures (MBPS) as visual cut-off between PET-positive and PET-negative findings, respectively. All pHL patients with uptake lower than or equal to MBPS remained in complete remission.

CONCLUSION(S)

The iPET interpretation assured low interobserver variability. High sensitivity for identification of pHL patients suffering relapse is achieved if [18F]-fluorodeoxyglucose uptake above the MBPS value is rated as a PET-positive finding.

Neural traffic as voxel-based measure of cerebral functional connectivity in fMRI

To access functional connectivity by in vivo brain imaging voxel-by-voxel, we developed a novel approach named neural traffic (NT). NT depicts the intensity of functional connectivity on a voxel-by-voxel basis in the whole brain. Functional magnetic resonance imaging (fMRI) experiments were carried out on eight individuals during either hearing or viewing words. The blood oxygen level dependant (BOLD) signal was taken as measure of neural activity. For each voxel, functional connectivity with all other brain voxels was determined by calculating Pearson correlation coefficients at two connectivity thresholds (r=0.35 and 0.65). Then, NT images were derived by counting the number of suprathreshold connections for each individual voxel. Calculations based on random networks indicate that statistically reliable NT images can be derived in individuals. With regard to group analysis, at r=0.35 NT images are similar though not identical with the first component of principal component analysis (PCA), displaying a widespread but not ubiquitous pattern of functionally connected cortical areas. At r=0.65, NT group images display functional connectivity confined to circumscribed cortical regions which reach beyond the corresponding primary sensory areas, their known associated areas and the default network. In conclusion, NT goes beyond the approach of correlating the BOLD signal with the external stimulus-presentation time course by computing linear functional connectivity between all brain voxels based on any BOLD time course. First results demonstrate that the NT approach is likely - on an individual base - to reveal novel cortical and subcortical connectivities involved in stimulus processing.

Comparison of M22-based ELISA and human-TSH-receptor-based luminescence assay for the measurement of thyrotropin receptor antibodies in patients with thyroid diseases

Previously, a new procedure for measuring serum TSH receptor autoantibodies (TRAb) was reported in which the autoantibodies inhibit binding of a human monoclonal thyroid stimulating antibody M22 to TSHR-coated ELISA plate wells (TRAb ELISA). The aim of the present study was to evaluate the clinical performance of this assay in comparison to the second generation TRAb assay (TRAb LIA) based on the recombinant human TSH-receptor and chemiluminescence technology (TRAb LIA). Among the 158 patients, 84 patients suffered from Graves' disease (GD), 34 patients had Hashimoto's thyroiditis (HT), and 40 patients had euthyroid nodular thyroid disease (NTD) without signs of autoimmunity. TRAb measurements were performed according to the manufacturer's instructions. Out of 84 GD patients, 80 (95.2%) were TRAb positive as detected by the TRAb LIA. One GD patient had TRAb values within the grey zone (1.0-1.5 IU/l). All patients with HT and NTD were negative except in 6 (8.1%) cases whose TRAb values were within the grey zone. On the basis of the recommended cutoff value (TRAb 1.0 IU/l), the TRAb ELISA found 78 of 84 (92.9%) GD patients to be TRAb positive. None of the patients with HT, but two cases (5.0%) with NTD were TRAb positive. The diagnostic sensitivity of the TRAb LIA and TRAb ELISA assays was 95.2 and 92.9%, while the specificity was 100% and 97.3%, respectively. There was a close correlation (r=0.968, p<0.0001) between both assays in 84 patients with GD. Additionally, the between-run imprecision close to the cutoff limit was assessed. The calculated between-run coefficient of variation (CV) of the TRAb ELISA was 28.2% at the recommended cutoff value of 1.0 IU/l. Due to the evaluated imprecision data we propose a higher cutoff value correlating with a between-run CV of 20% (functional assay sensitivity). Our results indicate that due to a worse imprecision the TRAb ELISA has a slightly lower sensitivity and specificity compared to the TRAb LIA assay. These findings suggest that the M22 monoclonal antibody-based TRAb ELISA is not as reliable as other second generation TRAb assays in the diagnosis of Graves' diseases.

Graves' disease occurring after surgery of a multinodular goiter: a case report

The transition of toxic or nodular goiter to Graves' disease is known as a rare side effect of (131)I therapy. Here, we report the case of a 46-year-old German female with posttherapeutical Graves' disease after surgery of a multinodular goiter. Although the major part of the thyroid was excised the patient suffered from manifest Graves' disease including typical clinical and laboratory findings. Prior to surgery, no TSH receptor antibodies were found, although low TPO antibody titres could already be detected. It may thus be assumed that the therapeutic manipulation elicited the key change towards a TSH receptor antibody production in a predisposed organ or alternatively deteriorated a mild unapparent pre-existing Graves' disease. It might be concluded that the possibility of posttherapeutical Graves' disease should be considered in the presence of TPO antibodies prior to the surgical intervention.

Cerebral haemodynamics during hypo- and hypercapnia: determination with simultaneous 15O-butanol-PET and transcranial Doppler sonography

AIM

Transcranial Doppler sonography (TCD) is increasingly used in cerebrovascular disease for monitoring brain perfusion. It allows estimation of cerebral blood flow (CBF) by the measurement of cerebral blood flow velocity (CBFV). The CBFV as well as CBF are intimately associated with the intravascular CO2-concentration. Thus, hyper- or hypocapnia can be used to induce a defined range of blood flows. The aim of our study was the comparison of vasomotor reactivity assessed with simultaneous TCD and quantitative regional CBF-measurements (rCBF) by PET (serving as the reference method for in-vivo quantification of rCBF).

PATIENTS, METHODS

Six healthy young volunteers participated in this study. CBF was measured using 15O-butanol PET. A flow and dispersion-model was fitted to the measured time activity curves using arterial input curves. Each subject underwent five scans at five different end-tidal CO2 levels (EtCO2): 25, 32, 40, 48, and 55 mmHg. CBFV was assessed by continuous bilateral TCD of the middle cerebral artery (MCA). Volumes of interest for rCBF determination were placed in grey matter of the prefrontal cortex (PFC) as determined from individual MRIs. Comparisons between the rCBF, EtCO2 and CBFV were carried out with regression and correlation analysis and paired t-tests.

RESULTS

Strong positive linear correlations of rCBF and CBFV with the CO2-concentration and linear relationships between rCBF and CBFV were found in each individual. Normalised CO2-reactivities measured by TCD and PET were closely correlated.

CONCLUSIONS

TCD-measurements of vascular reactivity in healthy volunteers show a high correlation to those acquired with PET that serves as the reference method of quantitative rCBF-measurement. The results of the MCA insonation are a close approximation of the rCBF changes induced by variations of EtCO2.

Depression and anxiety in different thyroid function states

Previous studies on hypothyroid subjects have indicated serious psychiatric symptoms affecting the patients' quality of life. The present prospective cross-sectional study's aim was to examine these symptoms in thyroid patients with different functional states. A total of 254 patients (age: 56 +/- 14 years [mean +/- standard deviation], 181 female, 73 male) referred to a hospital for radioiodine treatment of hyperthyroidism or for follow-up of differentiated thyroid cancer, respectively, were included. All patients underwent the twelve-item general health questionnaire, which is an instrument for detecting mood disturbances. Euthyroid and hyperthyroid patients did not differ significantly in their general health questionnaire score (11 +/- 5 vs. 11 +/- 7), nor did subclinical hyperthyroid (11 +/- 6) or subclinical hypothyroid subjects (12 +/- 5). In contrast, hypothyroid patients showed a significantly higher mean score (17 +/- 7, p < 0.001, ANOVA). Binary logistic regression revealed that hypothyroidism increases age and gender-adjusted risk for critical mood deterioration by seven-fold. Thus, hypothyroidism represents a widely underestimated functional condition that may severely affect mental health.

When Planning Fails: Individual Differences and Error-related Brain Activity in Problem Solving

The neuronal processes underlying correct and erroneous problem solving were studied in strong and weak problem-solvers using functional magnetic resonance imaging (fMRI). During planning, the right dorsolateral prefrontal cortex was activated, and showed a linear relationship with the participants' performance level. A similar pattern emerged in right inferior parietal regions for all trials, and in anterior cingulate cortex for erroneously solved trials only. In the performance phase, when the pre-planned moves had to be executed by means of an fMRI-compatible computer mouse, the right dorsolateral prefrontal cortex was again activated jointly with right parahippocampal cortex, and displayed a similar positive relationship with the participants' performance level. Incorrectly solved problems elicited stronger bilateral prefrontal and left inferior parietal activations than correctly solved trials. For both individual ability and trial-specific performance, our results thus demonstrate the crucial involvement of right prefrontal cortex in efficient visuospatial planning.

[Neurocognition and PET. Strategies for data analysis in activation studies on working memory]

AIM

In cognitive neuroscience regional cerebral blood flow (rCBF) imaging with positron-emission-tomography (PET) is a powerful tool to characterize different aspects of cognitive processes by using different data analysis approaches. By use of an n-back verbal working memory task (varied from 0- to 3-back) we present cognitive subtraction analysis as basic strategy as well as parametric and covariance analyses and discuss the results.

METHODS

Correlation analyses were performed using the individual performance rate as an external covariate, computing inter-regional correlations, and as network analysis applying structural equation modelling to evaluate the effective connectivity between the involved brain regions.

RESULTS

Subtraction analyses revealed a fronto-parietal neuronal network also including the anterior cingulate cortex and the cerebellum. With higher memory load the parametric analysis evidenced linear rCBF increases in prefrontal, pre-motor and inferior parietal areas including the precuneus as well as in the anterior cingulate cortex. The rCBF correlation with the individual performance as external covariate depicted negative correlations in bilateral prefrontal and inferior parietal regions, in the precuneus and the anterior cingulate cortex. The network analysis demonstrated mainly occipito-frontally directed interactions which were predominantly left-hemispheric. Additionally, strong linkages were found between extrastriate and parietal regions as well as within the parietal cortex.

CONCLUSION

The data analysis approaches presented here contribute to an extended and more elaborated understanding of cognitive processes and their different sub-aspects.

[The (11C) acetate positron emission tomography in prostatic carcinoma. New prospects in metabolic imaging]

The exact staging of prostate cancer is mandatory to allow selection of the appropriate primary therapy. In addition, if the PSA level rises again it is extremely important to find the site(s) of local recurrence or metastatic spread as soon as possible. However, with the morphological and metabolic imaging techniques currently available it is often not possible to answer these questions with adequate sensitivity and specificity, since small metastases < or = 1 cm in diameter are likely to remain undetected by them. In the last few years new radioactive labelled tracers have been introduced for use in positron emission tomography (PET), and it is hoped that the shortcomings in the diagnostic procedures used for prostate carcinoma might be compensated by their use. Besides 11C- or 18F-labelled choline, [11C]Acetate is also attracting attention as a promising PET tracer. In this paper we review the various PET tracers available and evaluate the advantages and the drawbacks of [11C]Acetate in three case studies by comparing [11C]Acetate-PET with histology and with other imaging techniques. The use of [11C]Acetate appears to be feasible and helpful in the diagnosis of prostate carcinoma. However, its final value relative to other imaging techniques needs further investigation, with special reference to initial lymph node involvement, early localisation of recurrence and possible noninvasive differentiation between prostate cancer, prostatis and benign hyperplasia of the prostate.

Topographic segregation and convergence of verbal, object, shape and spatial working memory in humans

This functional magnetic resonance imaging study investigates commonalties and differences in working memory (WM) processes employing different types of stimuli. We specifically sought to characterize topographic convergence and segregation with respect to prefrontal cortex involvement using verbal, spatial, real object and shape memory items in a two-back WM task. Both the dorsolateral and ventrolateral prefrontal cortices are conjointly activated across all stimulus types. No stimulus-specific differences in the activation patterns of the prefrontal cortex could be demonstrated giving support to the view of an amodal prefrontal involvement during WM processes. However, extra-frontal regions specialized on feature processing and involved in the preprocessing of the stimuli were selectively activated by these different subtypes of WM. These selectively activated regions are assigned to parts of the ventral and dorsal stream.

The motion aftereffect: more than area V5/MT? Evidence from 15O-butanol PET studies

The motion aftereffect is a perceptual phenomenon which has been extensively investigated both psychologically and physiologically. Neuroimaging techniques have recently demonstrated that area V5/MT is activated during the perception of this illusion. The aim of this study was to test the hypothesis if a more broadly distributed network of brain regions subserves the motion aftereffect. To identify the neuronal structures involved in the perception of the motion aftereffect, regional cerebral blood flow (rCBF) measurements with positron emission tomography were performed in six normal volunteers. Data were analysed using SPM96. The motion-sensitive visual areas including area V5/MT were activated in both hemispheres. Additionally, the lateral parietal cortex bilaterally, the right dorsolateral prefrontal cortex, the anterior cingulate cortex and the left cerebellum showed significant increases in rCBF values during the experience of the waterfall illusion. In a further reference condition with identical attentional demand but no perception of a motion aftereffect elevated rCBF were found in these regions as well. In conclusion, our findings support the notion that the perceptual illusion of motion arises exclusively in the motion-sensitive visual area V5/MT. In addition, a more widespread network of brain regions including the prefrontal and parietal cortex is activated during the waterfall illusion which represents a non-motion aftereffect-specific subset of brain areas but is involved in more basic attentional processing and cognition.

[Comparison of PET and fMRI activation patterns durnig declarative memory processes]

AIM

In this study neuronal correlates of encoding and retrieval in paired association learning were compared using two different neuroimaging methods: positron emission tomography (PET) and functional magnetic resonance imaging (fMRI).

METHODS

6 right-handed normal male volunteers took part in the study. Each subject underwent six 0-15-butanol PET scans and on fMRI study comprising four single epochs on a different day. The subjects had to learn and retrieve 12 word pairs which were visually presented (highly imaginable words, not semantically related).

RESULTS

Mean recall accuracy was 93% in the PET as well as in the fMRI experiment. During encoding and retrieval we found anterior cingulate cortex activation, and bilateral prefrontal cortex activation in both imaging modalities. Furthermore, we demonstrate the importance of the precuneus in episodic memory. With PET the results demonstrate frontopolar activations whereas fMRI fails to show activations in this area probably due to susceptibility artifacts. In fMRI we found additionally parahippocampal activation and due to the whole-brain coverage cerebellar activation during encoding. The distance between the center-of-mass activations in both modalities was 7.2 +/- 6.5 mm.

CONCLUSION

There is a preponderance of commonalities in the activation patterns yielded with fMRI and PET. However, there are also important differences. The decision to choose one or the other neuroimaging modality should among other aspects depend on the study design (single subject vs. group study) and the task of interest.

Imaging and neural modelling in episodic and working memory processes

Neuroimaging studies using positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) have revealed the involvement of distributed brain regions in memory processes mainly by the use of subtraction strategy based data analyses. Covariance analysis based data analysis strategies have been introduced more recently which allow functional interactions between brain regions of a neuronal network to be assessed. This contribution focuses on studies aiming to (1) establish the functional topography of episodic and working memory processes in young and old normal volunteers, (2) to assess functional interactions between modules of networks of brain regions by means of covariance based analyses and systems level modelling, (3) to characterise the temporal dynamics by the use of magnetoencephalography (MEG) and (4) to relate neuroimaging data to the underpinning neural networks. Male normal young and old volunteers without neurological or psychiatric illness participated in neuroimaging studies (PET, fMRI, MEG). Studies were approved by the ethical committee and federal authorities. Our results in young volunteers show distributed brain areas that are involved in memory processes (episodic and working memory) and show much of an overlap with respect to the network components. Systems level modelling analyses support the hypothesis of bihemispheric, asymmetric networks subserving memory processes and revealed both similarities in general and differences in the interactions between brain regions during episodic encoding and retrieval as well as working memory. Changes in memory function with ageing are evident from functional topographic studies in old volunteers activating more brain regions as compared to young volunteers. There are more and stronger influences of prefrontal regions in elderly volunteers comparing the functional models between old and young subjects. We discuss the way that the systems level models of the PET and fMRI results have implications for the underlying neural network functioning of the brain. This is done by developing simplifying assumptions, which lead from the equations describing the activities of the coupled neural modules to the systems level model equations. The resulting implications for the neural interactions are then discussed, in terms of a set of synaptically coupled neural modules. Finally, we consider how a similar analysis could be extended from the spatial to the temporal domain thus including the EEG and MEG results. The implication of preliminary MEG results presented here for the temporality arising in the interaction between the coupled neural modules in a working memory paradigm is discussed in terms of the previously developed neural network models arising from the PET and fMRI data.

Transport mechanisms of 3-[123I]iodo-alpha-methyl-L-tyrosine in a human glioma cell line: comparison with [3H]methyl]-L-methionine

The amino acid analog 3-[(123)I]iodo-alpha-methyl-L-tyrosine (IMT) is under clinical evaluation as a SPECT tracer of amino acid transport in brain tumors. This study investigated the carrier systems involved in IMT transport in human glioma cells in comparison with [3H-methyl]-L-methionine (3H-MET).

METHODS

Human glioma cells, type 86HG-39, were cultured and incubated for 1 min at 37 degrees C with IMT and 3H-MET in the lag phase (1.2 d after seeding), exponential growth phase (3 d after seeding), and plateau phase (8 d after seeding). Experiments were performed in the presence and absence of Na+, during inhibition of system L amino acid transport by 2-aminobicyclo[2.2.1 ]heptane-2-carboxylic acid (BCH), and during inhibition of system A amino acid transport by 2-(methylamino)-isobutyric acid (MeAIB).

RESULTS

IMT and 3H-MET uptake decreased by 55%-73% when the cells entered from the exponential growth phase into the plateau phase (P< 0.05; n = 3-11). Inhibition by BCH reduced uptake of IMT in the lag phase, exponential growth phase, and plateau phase by 90%-98% (P < 0.001; n = 3-6) and the uptake of 3H-MET by 73%-83% (P < 0.001; n = 3-11). In a Na+-free medium 3H-MET uptake was reduced by 23%-33% (P < 0.05; n = 3-11), whereas IMT uptake was not significantly different. MeAIB showed no significant effect on IMT or 3H-MET uptake in either phase.

CONCLUSION

Transport of both IMT and 3H-MET depends on the proliferation rate of human glioma cells in vitro and is dominated by BCH-sensitive transport. These data indicate that system L is induced in rapidly proliferating glioma cells and is the main contributor to the uptake of both tracers. 3H-MET transport showed a minor Na+ dependency that was not attributable to system A. The similarity of transport mechanisms of both tracers emphasizes the clinical equivalence of IMT SPECT and (11)C-MET PET for the diagnostic evaluation of gliomas.

Early changes in fluorine-18-FDG uptake during radiotherapy

The aim of this case study was to quantify metabolic changes in tumor tissue during and directly after external radiotherapy.

METHODS

We performed six FDG-PET scans of the neck in a patient with two lymph node metastases of a papillary thyroid carcinoma and an antigranulocyte antibody scintigraphy of the same region to assess the inflammatory reaction.

RESULTS

The FDG-PET scans show an initial increase of tracer uptake in both metastases after application of 6 Gy followed by a slow but constant decline with increasing radiation dose. In the deep metastasis the FDG uptake returned to the initial level after 30 Gy total dose whereas the metabolic activity in the superficial lymph node remained high, even after 60 Gy total dose. The antigranulocyte scan demonstrated an intense inflammation in the latter metastasis.

CONCLUSION

There is an initial enhancement in metabolism induced by irradiation which is measureable by FDG-PET. With increasing dose, the metabolic activity declines constantly. The additional inflammatory reaction might contribute to the glucose uptake in irradiated tumors.