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P Dupont

Publications and source records attributed to P Dupont.

At least 19 recordsLinked to original sources
PubMed1998

Laryngeal tumor volume measurements determined with CT: a study on intra- and interobserver variability.

PURPOSE: To investigate the intra- and interobserver variability of computed tomography-based volume measurements of laryngeal tumors. METHODS AND MATERIALS: The volume of 13 laryngeal tumors was repeatedly measured by five independent observers in four different sessions, using the summation-of-areas technique. Mean tumor volume and its standard deviation were calculated for each tumor. Statistical analysis was done with analysis of variance, Spearman rank correlation, and linear regression. RESULTS: Both the effect of the observers (p < 0.0001) and the effect of the session (p < 0.01) on tumor volume was statistically significant. Interobserver variability was the most important component of total variability (89.3%). A significant rank correlation was found between mean volume and standard deviation (p < 0.01); the relationship between mean tumor volume and standard deviation can be described using linear regression [standard deviation = 0.28 volume + 0.35 (R = 0.79)]. CONCLUSION: Total variability in the computed tomography-based measurement of laryngeal tumor volume can be reduced by having the measurements done by a single trained observer.

Analysis of Variance↗
PubMed1998

Human brain activity related to speed discrimination tasks.

The regional cerebral blood-flow (rCBF) pattern of the human brain was measured using positron emission tomography (PET) while subjects viewed, detected, judged the speed of a moving random dot pattern (RDP) or compared speeds of successive RDPs. In all four conditions, retinal input was identical. Two additional conditions, continuous presentation of a moving and a stationary RDP, were included to identify human MT/V5 (hMT/V5). Both speed discrimination tasks involved the right cuneus and right lingual gyrus and to a lesser degree the left lingual gyrus and a more anterior lingual region in the right hemisphere. There was, however, little or no differential activity over hMT/V5 during either speed discrimination. Direct comparison of the two speed discrimination tasks revealed higher activity in the right middle fusiform gyrus, a result reminiscent of that obtained in earlier studies using orientation and direction as the attribute to be discriminated. These results confirm that processing in the human visual cortex is task dependent and underscore the role of the middle fusiform gyrus in temporal comparison of simple attributes.

Adult↗
PubMed1998

Iterative reconstruction for helical CT: a simulation study.

Iterative reconstruction algorithms for helical CT are presented. The algorithms are derived from two-dimensional reconstruction algorithms, by adapting the projector/backprojector to the helical orbit of the source, and by constraining the axial frequencies with a Gaussian sieve. Simulations have been carried out and the performance of the iterative algorithms is compared to that of filtered backprojection of synthetic (interpolated) two-dimensional sinograms. The iterative algorithms produce superior bias-noise curves. Axial resolution is superior, but disturbing edge-artefacts are introduced.

Algorithms↗
PubMed1998

Human cerebral activity evoked by motion reversal and motion onset. A PET study.

In this PET study, we have investigated the human brain activity evoked by a visual motion paradigm commonly used to measure motion-related visual evoked potentials (VEPs). Because standard PET activation studies have been performed with motion along four axes, we first determined the pattern of brain activation when motion was restricted to a single axis. Motion back and forward along a single horizontal axis compared with a static condition revealed weak differential activations in the cuneus and the parietal cortex. Human area MT/V5 (middle temporal area) was hardly activated at all in this subtraction. Additional functional MRI experiments proved that MT/V5 activity is significantly higher for motion along four axes than for motion along a single axis. Secondly, we attempted to isolate the pattern of brain activity related to the reversal of motion direction and to the onset of motion, i.e. two transient motion components commonly used in measuring motion-related VEPs. To that end, we added a continuous linear contrast modulation, that reached maximum contrast at reversal or onset of motion, and compared both conditions with a contrast-modulated static or continuous motion condition. Subtraction of the static random dot pattern condition from the single-axis motion reversal condition, both contrast-modulated, revealed three significant activations: the anterior parieto-occipital sulcus, the lateral sulcus and the anterior claustrum. Additional analysis showed that these activations were not due to motion appearance or disappearance, but were due to the combination of motion reversal and contrast modulation. Hence, these activations do not reflect the motion reversal transient per se. In order to isolate a metabolic response to the reversal transient per se, we used a conjunction analysis, which suggests that activity in human MT/V5, the cuneus and a parietal insular region could underlie the motion reversal VEP in our experiments. Subtraction of the static random dot pattern condition from the single-axis motion onset condition, both contrast-modulated, revealed a single significant activation in the posterior cingulate cortex. Although the significance of this activation is unclear, it adds further evidence for the visual function of this region.

Adult↗
PubMed1998

Human brain regions involved in direction discrimination.

To obtain further evidence for the functional specialization and task-dependent processing in the human visual system, we used positron emission tomography to compare regional cerebral blood flow in two direction discrimination tasks and four control tasks. The stimulus configuration, which was identical in all tasks, included the motion of a random dot pattern, dimming of a fixation point, and a tone burst. The discrimination tasks comprised the identification of motion direction and successive direction discrimination. The control tasks were motion detection, dimming detection, tone detection, and passive viewing. There was little difference in the activation patterns evoked by the three detection tasks except for decreased activity in the parietal cortex during the detection of a tone. Thus attention to a nonvisual stimulus modulated different visual cortical regions nonuniformly. Comparison of successive discrimination with motion detection yielded significant activation in the right fusiform gyrus, right lingual gyrus, right frontal operculum, left inferior frontal gyrus, and right thalamus. The fusiform and opercular activation sites persisted even after subtracting direction identification from successive discrimination, indicating their involvement in temporal comparison. Functional magnetic resonance imaging (fMRI) experiments confirmed the weak nature of the activation of human MT/V5 by successive direction discrimination but also indicated the involvement of an inferior satellite of human MT/V5. The fMRI experiments moreover confirmed the involvement of human V3A, lingual, and parietal regions in successive discrimination. Our results provide further evidence for the functional specialization of the human visual system because the cortical regions involved in direction discrimination partially differ from those involved in orientation discrimination. They also support the principle of task-dependent visual processing and indicate that the right fusiform gyrus participates in temporal comparison, irrespective of the stimulus attribute.

Acoustic Stimulation↗
PubMed1997

Positron-emission tomography imaging of long-term shape recognition challenges.

Long-term visual memory performance was impaired by two types of challenges: a diazepam challenge on acquisition and a sensory challenge on recognition. Using positron-emission tomography regional cerebral blood flow imaging, we studied the effect of these challenges on regional brain activation during the delayed recognition of abstract visual shapes as compared with a baseline fixation task. Both challenges induced a significant decrease in differential activation in the left fusiform gyrus, suggesting that this region is involved in the automatic or volitional comparison of incoming and stored stimuli. In contrast, thalamic differential activation increased in response to memory challenges. This increase might reflect enhanced retrieval attempts as a compensatory mechanism for restoring recognition performance.

Adult↗
PubMed1997

Attention to one or two features in left or right visual field: a positron emission tomography study.

In human vision, two features of the same object can be identified concurrently without loss of accuracy. Performance declines, however, when the features belong to different objects in opposite visual fields. We hypothesized that different positron emission tomography activation patterns would reflect these behavioral results. We first delineated an attention network for single discriminations in left or right visual field and then compared this with the activation pattern when subjects divided attention over two features of a single object or over two objects in opposite hemifields. When subjects attended to a single feature, parietal, premotor, and anterior cingulate cortex were activated. These effects were strongest in the right hemisphere and were, remarkably, unaffected by the direction of attention. In contrast, direction of attention affected occipital and frontal activity: right occipital and left lateral frontal activity were higher with attention to the left, whereas right lateral frontal activity was higher with attention to the right. When subjects identified two features of the same object, parietal, premotor, and anterior cingulate activity was enhanced further, predominantly this time in the left hemisphere. Again, there was no direction sensitivity. Direction-sensitive activation of lateral frontal cortex also was increased. Finally, when subjects divided their attention over opposite hemifields, activity in the direction-sensitive occipital and frontal regions fell to a level midway between those seen during exclusively leftward or rightward attention. Thus, the behavioral efficiency with which we attend to multiple features of a single peripheral object is paralleled by enhanced activity in structures generally active during peripheral selective attention as well as in structures that depend on the specific direction of attention, most notably lateral frontal cortex. In addition, in the direction-sensitive regions, dividing attention over hemifields causes a compromise pattern between the extreme levels obtained during unilateral attention.

Adolescent↗
PubMed1997

[Shortening arthrodesis of three wrist bones].

In advanced cases of wrist osteoarthritis with lesions of the radio-scaphoid and mediocarpal joints, and when a proximal row carpectomy is not possible because of lesions of the head of the capitate, we suggest a new technique: The hamate-capitate-lunate shortening arthrodesis with a scaphoid-triquetral resection. The good results observed with proximal row carpectomies, and particularly their long-term reliability, have encouraged us on this new path. Effectively, this operation takes the concept of proximal row carpectomy one step further by reconstructing the head of the capitate with the lunate whose proximal articular surface is often not deteriorated even in very advanced cases of radio and mediocarpal osteoarthritis. The two theoretical concepts of this operation are the shortening of the carpus and respect of the physiological congruence of the radio-lunate joint, the goal being obtain similar results those with proximal row carpectomy, particularly concerning mobility. We present our first two cases with this technique. This new procedure is an alternative to the four bone arthrodesis, particularly in SLAC wrist sequellae when they have evolved to the stage of radio and mediocarpal osteoarthritis.

Aged↗
PubMed1997

Human brain activity related to orientation discrimination tasks.

In order to relate regional activity in the human brain to the different components of discrimination tasks, we compared regional cerebral blood flow, measured with positron emission tomography, under four conditions: successive orientation discrimination, orientation identification, detection and passive viewing. By adding successive discrimination and passive viewing at a second, lower rate we were able to investigate the main effects and interaction between task and presentation rate. Four occipital regions--the posterior calcarine region bilaterally, the right lingual gyrus and the right interior occipital cortex--displayed a main effect of presentation rate. Two regions--a right posterolateral occipital region and a right posterior fusiform region--displayed a significant main effect of task. The involvement of this posterior fusiform region in successive discrimination was also revealed by the subtraction of detection from successive discrimination, as was that of the right middle fusiform gyrus. Finally, a more anterior right middle fusiform region was differentially active in successive discrimination compared to identification, suggesting that activity in this region is related to the temporal comparison of orientation.

Adult↗
PubMed1997

Size quantification of liver metastases in patients undergoing cancer treatment: reproducibility of one-, two-, and three-dimensional measurements determined with spiral CT.

PURPOSE: To assess the reproducibility of one-, two-, and three-dimensional measurements of the size of liver metastases. MATERIALS AND METHODS: The sizes of 10 liver metastases were determined 200 times: Ten observers measured each lesion by using images obtained at two different CT studies and with five different measurement techniques (maximum diameter, product of diameters, area, volume, and product of three diameters). They repeated each measurement in a separate session. The influence of measurement technique and lesion type (size and morphologic appearance) on measurement reproducibility was assessed. RESULTS: Three-dimensional measurements proved to be as reproducible as one- and two-dimensional measurements. Measurement reproducibility was influenced mainly by the size (P < .0001) and morphologic appearance (P < .01) of the lesions. CONCLUSION: Three-dimensional size measurements obtained with spiral CT are reproducible and could therefore replace the two-dimensional measurements of tumor size currently obtained with follow-up CT studies.

Humans↗
PubMed1997

Positron emission tomography, magnetic resonance imaging and proton NMR spectroscopy of white matter in multiple sclerosis.

OBJECTIVE: To assess characteristics of MS lesions and normal appearing white matter (NAWM) with various imaging modalities. Glucose metabolism was investigated with FDG-PET, metabolite concentration with proton NMR spectroscopy, and lesion detection with routine brain MRI. METHODS: Thirteen patients were studied in a stable phase of their disease, and two during an acute episode. Nine healthy volunteers served as controls. RESULTS: Three patients had a normal brain MRI, 12 had typical lesions. MR images were registered to the PET planes. Lesions and contra-lateral control areas were analyzed, 10/15 lesions showed relative hyper-metabolism and 2 hypo-metabolism. NAA concentration was significantly decreased in both lesions and NAWM. CONCLUSION: In stable MS, most large lesions have a relatively increased glucose utilization and decreased NAA concentration. NAWM showed a significantly decreased NAA concentration compared to healthy subjects, but no difference in glucose metabolism. Active lesions in acute MS are also hyper-metabolic. This finding opens a new window on the classification of white matter lesions based on glucose utilization.

Acute Disease↗
PubMed1996

Visualisation of loss of 5-HT2A receptors with age in healthy volunteers using [18F]altanserin and positron emission tomographic imaging.

We used [18F]altanserin and positron emission tomography (PET) to image serotonin 5-HT2A receptors in humans. The highest [18F]altanserin uptake is found in the cerebral cortex, with specific-to-nonspecific binding ratios varying from 0.53 to 1.91 in humans between 24 and 48 years of age. In all neocortical regions studied, [18F]altanserin uptake correlates negatively with age. No correlations were found between age and uptake in the cerebellum, the regional cerebral blood flow, or the time course of metabolization of [18F]altanserin. The reduction in cerebral 5-HT2A receptor binding thus directly reflects the loss of specific 5-HT2A receptors with age.

Adult↗
PubMed1996

Task dependency of visual processing in the human visual system.

In this review we contrast passive, attribute driven processing in the visual system with an active, task-dependent view and summarize the evidence from our Positron Emission Tomography (PET) work supporting the task-dependent view. The PET studies involved comparison of regional Cerebral Blood Flow (rCBF) in closely related detection and discrimination tasks. The major finding reported is that the same retinal input or input containing only a single cue activates different extrastriate areas depending on the task.

Adult↗
PubMed1996

The influence of stimulus location on the brain activation pattern in detection and orientation discrimination. A PET study of visual attention.

This PET study concerns changes in regional cerebral blood flow (rCBF) associated with orientation discrimination tasks and with simple detection of the stimulus. The difference in rCBF between discrimination and detection represents the discrimination, or 'task', component of the response. We have examined how such differences are influenced by the position of the visual stimulus and also how they change when a peripheral distractor is added to a relevant central stimulus. We first determined the regions in which the task produced the main effect regardless of stimulus position or distraction with an additional stimulus. In these selected regions, we determined the interactions between task and stimulus position and the interactions between task and stimulus addition. Five regions were more active during orientation discrimination than during stimulus detection: the inferior occipital cortex, the right putamen, the superior parietal lobule, the anterior cingulate cortex and the left lower and the right upper premotor area. Stimulus addition interacted with task only in the lower premotor area. Interactions between the task and stimulus position occurred in the occipital and parietal cortex and in the putamen. When a central stimulus was presented instead of a peripheral one the difference between orientation discrimination and stimulus detection was significantly larger in the inferior occipital lobe and in the right putamen. Conversely, the difference between orientation discrimination and stimulus detection in the superior parietal lobule was significantly larger when a peripheral stimulus was presented instead of a central one.

Adult↗
PubMed1995

A motion area in human visual cortex.

We have localized an area in the human brain involved in the processing of contours defined by motion differences (kinetic contours) by comparing with positron emission tomography the regional cerebral blood flow in tasks performed with kinetic and luminance-defined gratings. These tasks included passive viewing, counting the total number of grating stimuli, and counting the number of gratings of a given orientation. Comparison between the counting tasks and passive viewing with a given type of contour revealed a set of active areas that were similar for both luminance-defined and kinetic contours. Comparisons between these two types of contours revealed a single focus in the right hemisphere that did not overlap with the many regions activated by uniform motion. In particular this "kinetic focus" was clearly separated from the area previously defined as the human homologue of V5/middle temporal. Activity in this kinetic focus was stronger when orientation had to be processed than in the other two tasks. These results and control experiments with uniformly moving random dot patterns suggest the existence of an area in the human visual system that is activated much more by kinetic contours than by luminance contours or uniformly moving random dots. Up to now, such an area has not been described in the monkey visual system.

Animals↗
PubMed1995

Blood flow in human anterior temporal cortex decreases with stimulus familiarity.

In this positron emission tomography investigation of human nonverbal visual memory, we determined the anatomical distribution of changes in regional cerebral blood flow (rCBF) reflecting familiarity of visual shapes. All stimuli consisted of relatively simple, abstract outlines generated by Fourier expansions. Subjects performed the same task twice, once using unfamiliar and once using familiar stimuli. The task was a modified version of delayed nonmatching-to-sample, consisting of one sample presentation prior to tracer injection and a sequence of test trials during which images were acquired. When this task was performed with familiar instead of unfamiliar stimuli, only decreases in rCBF were observed, which were localized to the left lateral anterior temporal neocortex, the left medial temporal pole, and the rostral anterior cingulate. The correlates of familiarity measured in this human brain mapping experiment may correspond with what has been measured in single neurons in monkeys. This correspondence holds both for the type and for the localization of changes.

Adolescent↗
PubMed1995

A study of the liver-heart artifact in emission tomography.

UNLABELLED: With the introduction of 99mTc-teboroxime, a previously undocumented artifact has shown up in cardiac SPECT imaging. In the images, the uptake values near the inferior wall are lower than expected. The artifact has been reported in the literature, but an adequate explanation has not yet been provided. The high uptake of 99mTc-teboroxime in the liver has been demonstrated to be the cause of this artifact. METHODS: With simulations we show that an artifact can be reproduced by applying filtered backprojection (without corrections for attenuation) of attenuated and blurred projections. The conclusions from the simulations are validated with SPECT and PET phantom measurements. Maximum likelihood expectation maximization (ML-EM) reconstruction is applied to evaluate the effect of accurate attenuation correction. The influence of the high liver uptake on the convergence of ML-EM was also evaluated. RESULTS: The artifact results mainly when the photon attenuation during reconstruction is ignored. This results in a distorted reconstruction of the liver. These distortions affect the neighboring inferior wall of the myocardium. While the use of opposite projections reduces the effect, accurate attenuation correction nearly eliminates it. A small additional deformation is caused by the position dependence of the spatial resolution of the gamma camera. It was also noted that the presence of the liver slows down the convergence of ML-EM in the heart region. CONCLUSION: The liver-heart artifact is an attenuation effect and is eliminated by attenuation correction. The local convergence of ML-EM is affected by the total image content.

Animals↗