Christian Michel

Effect of familiarity on the processing of human faces

Most brain imaging studies on face perception have investigated the processing of unknown faces and addressed mainly the question of specific face processing in the human brain. The goal of this study was to highlight the effects of familiarity on the visual processing of faces. Using [15O]water 3D Positron Emission Tomography, regional cerebral blood flow distribution was measured in 11 human subjects performing an identical task (gender categorization) on both unknown and known faces. Subjects also performed two control tasks (a face recognition task and a visual pattern discrimination task). They were scanned after a training phase using videotapes during which they had been familiarized with and learned to recognize a set of faces. Two major results were obtained. On the one hand, we found bilateral activations of the fusiform gyri in the three face conditions, including the so-called fusiform-face area, a region in the right fusiform gyrus specifically devoted to face processing. This common activation suggests that different cognitive tasks performed on known and unknown faces require the involvement of this fusiform region. On the other hand, specific regional cerebral blood flow changes were related to the processing of known and unknown faces. The left amygdala, a structure involved in implicit learning of visual representations, was activated by the categorization task on unknown faces. The same task on known faces induced a relative decrease of activity in early visual areas. These differences between the two categorization tasks reveal that the human brain processes known and unknown faces differently.

Neuronal Mechanisms of Perceptual Learning: Changes in Human Brain Activity with Training in Orientation Discrimination

Using 15O-water 3D positron emission tomography, regional cerebral blood flow was measured twice in six human subjects: before and after extensive training in orientation discrimination. In each session subjects performed two orientation discrimination tasks, during which they discriminated the orientation of a grating at either the trained or untrained reference orientation, and a control task, during which they detected a randomly textured pattern. By comparing the discrimination to the detection tasks, we observed a main effect of task bilaterally in the posterior occipital cortex, extending into the left posterior fusiform gyrus and the right inferior occipital gyrus, bilaterally in the intraparietal sulcus, as well as in the cerebellum, thalamus, and brainstem. When we compared the activation pattern before and after the training period, all the changes observed were activity decreases. The nonspecific changes, which were not related to the orientation used during the training, were situated in the cerebellum and bilaterally in the extrastriate visual cortex. The orientation-specific changes, on the other hand, were restricted to the striate and extrastriate visual cortex, more precisely the right calcarine sulcus, the left lingual gyrus, the left middle occipital, and the right inferior occipital gyrus. These findings confirm our hypothesis concerning the existence of learning related changes at early levels of visual processing in human adults and suggest that mechanisms resulting in neuronal activity decreases might be involved in the present kind of learning.

Attenuation correction in whole-body FDG oncological studies: the role of statistical reconstruction

Abstract. Whole-body fluorine-18 fluoro-2-d-deoxyglucose positron emission tomography (FDG-PET) is widely used in clinical centres for diagnosis, staging and therapy monitoring in oncology. Images are usually not corrected for attenuation since filtered backprojection (FBP) reconstruction methods require a 10 to 15-min transmission scan per bed position on most current PET devices equipped with germanium-68 rod transmission sources. Such an acquisition protocol would increase the total scanning time beyond acceptable limits. The aim of this work is to validate the use of iterative reconstruction methods, on both transmission and emission scans, in order to obtain a fully corrected whole-body study within a reasonable scanning time of 60xa0min. Fivexa0minute emission and 3-min transmission scans are acquired at each of the seven bed positions. The transmission data are reconstructed with OSEM (ordered subsets expectation maximization) and the last iteration is reprojected to obtain consistent attenuation correction factors (ACFs). The emission image is then also reconstructed with OSEM, using the emission scan corrected for normalization, scatter and decay together with the set of consistent ACFs as inputs. The total processing time is about 35xa0min, which is acceptable in a clinical environment. The image quality, readability and accuracy of uptake quantification were assessed in 38 patients scanned for various malignancies. The sensitivity for tumour detection was the same for the non-attenuation-corrected (NAC-FBP) and the attenuation-corrected (AC-OSEM) images. The AC-OSEM images were less noisy and easier to interpret. The interobserver reproducibility was significantly increased when compared with non-corrected images (96.1% vs 81.1%, P<0.01). Standardized uptake values (SUVs) measured on images reconstructed with OSEM (AC-OSEM) and filtered backprojection (AC-FBP) were similar in all body regions except in the pelvic area, where SUVs were higher on AC-FBP images (mean increase 7.74%, P<0.01). Our results show that, when statistical reconstruction is applied to both transmission and emission data, high quality quantitative whole-body images are obtained within a reasonable scanning (60xa0min) and processing time, making it applicable in clinical practice.

Absolute CBF and CBV measurements by MRI bolus tracking before and after acetazolamide challenge: repeatabilily and comparison with PET in humans.

Bolus tracking magnetic resonance imaging (MRI) is a powerful technique for assessing cerebral perfusion, but its capability to measure absolute cerebral blood flow (CBF) and volume (CBV) values is still debated. To validate the MRI technique, absolute CBF and CBV values in healthy humans obtained by echo planar gradient echo MRI were compared to H(2)(15)O and (11)CO positron emission tomography (PET) before and after acetazolamide (ACZ) (n = 8) or saline (n = 4) administration. The repeatability of CBF and CBV measurements was moderate with both methods, and slightly lower with MRI than with PET. At rest, the mean CBF values were similar with both techniques except in the cortex where they were moderately higher with MRI. CBV was higher with MRI than with PET in all areas, which may reflect an underestimation of the arterial input function (AIF). After ACZ, a significant CBF increase was observed in gray matter with both MRI and PET, suggesting that MRI might be used to assess the cerebrovascular reserve. In individual subjects, the correlation between MRI and PET measurements was good for both CBF and CBV (R(2) between 0.70 and 0.84). However, when all results were considered as a group, R(2) was lower (0.40 to 0.65), and the limits of agreement between the two methods (SD of the difference) were large. Our data suggest that physiologic CBF values and systematically overestimated CBV values may be obtained with MRI in healthy humans, but that an individual scale factor should be applied to MRI measurements to improve the agreement with PET.

PET study of the human foveal fixation system

Positron emission tomography (PET) was used to investigate the functional anatomy of the foveal fixation system in 10 subjects scanned under three different conditions: at rest (REST), during the fixation of a central point (FIX), and while fixating the same foveal target during the presentation of peripheral visual distractors (DIS). Compared with the REST condition, both FIX and DIS tasks activated a common set of cortical areas. First, in addition to the involvement of the occipital visual cortex, both the frontal eye field (FEF) and the intraparietal sulcus (IPS) were bilaterally activated. Right frontal activation was also found in the dorsolateral prefrontal cortex, the inferior part of the precentral gyrus, and the inferior frontal gyrus. These results suggest that both FEF and IPS may constitute the main cortical regions subserving bilaterally the foveal fixation system in humans. The remaining right frontal activations may be considered as part of the anterior attentional network, supporting a role for the right frontal lobe in the allocation of the attentional mechanisms. Compared with the FIX condition, the DIS task also revealed the perceptual and cognitive processes related to the presence of peripheral visual distractors during foveal fixation. In addition to a bilateral activation of the V5/MT motion‐sensitive area, a right FEF‐IPS network was activated which may correspond to the engagement of the visuospatial attention. Finally, normalized regional cerebral blood flow (NrCBF) decreases were also observed during both DIS and FIX condition performance. Such NrCBF decreases were centered in the superior and middle temporal gyri, the prefrontal cortex, and the precuneus and the posterior retrosplenial part of the cingulate gyrus. Hum. Brain Mapping 8:28–43, 1999.

Brain glucose metabolism in children with the autistic syndrome: positron tomography analysis.

Brain glucose metabolism was measured in 18 autistic children, using high resolution positron emission tomography. Global brain glucose utilization in the autistic population was slightly more elevated than in young adult volunteers but did not differ significantly from that of control children. Regional metabolic maps were also normal, although there was evidence for heterogeneities, particularly at the level of prefrontal and parieto-temporo-occipital association areas: 6 children showed a relative hyperfrontality whilst hypofrontality was found in 2 cases; these heterogeneities were not correlated with clinical symptoms. These data suggest that both the rate and the regional distribution of brain glucose metabolism are normal in autistic children. Variations in terms of relative metabolic rates in association cortex remains to be investigated further.

Advances in scatter correction for 3D PET/CT

We report on several significant improvements to the implementation of image-based scatter correction for 3D PET and PET/CT. Among these advances are: a new algorithm to scale the estimated scatter sinogram to the measured data, thereby largely compensating for external scatter; the ability to handle CT image truncation during this scaling; the option to iterate the scatter calculation for improved accuracy; the use of ordered subset estimation maximization (OSEM) reconstruction for the estimated emission images from which the scatter contributions are simulated; reporting of data quality parameters such as scatter and randoms fractions, and noise equivalent count rate (NECR), for each patient bed position; and extensive quality control output. Scatter correction (2 iterations, OSEM) typically requires 15-45 sec per bed. Very good agreement between the estimated scatter and measured emission data for several typical clinical scans is reported for CPS Pico-3D and HiRez LSO PET/CT systems. The data characteristics extracted during scatter correction are useful for patient specific count rate modeling and scan optimization

Online brain attenuation correction in PET: towards a fully automated data handling in a clinical environment

We have improved the calculation of the brain attenuation correction in positron emission tomography (PET) and set up a procedure which allows the clinician to get a fully corrected image in a single reconstruction step, without human intervention. By using a general object description scheme based on polygonal contour trees we are able to calculate the attenuation correction for brain tissue, bone and head holder. The head contour is generated from the emission sinogram. On a set of 15 adult patients, the emission values obtained using this calculated attenuation compare favorably with those obtained with an attenuation resulting from a transmission measurement. Residual discrepancies are attributed to incomplete scatter compensation between emission and transmission. The robustness of the algorithm has been tested on more than 100 brain fluorodeoxyglucose (18FDG) studies in adult patients, including pathological cases. Its applicability for18FDG studies in children and for other tracer such as water (H215O) and fluoroethylspiperone (18FESP) is also presented.

Variance reduction on randoms from coincidence histograms for the HRRT

A new algorithm for variance reduction on random coincidences (VRR) has been validated for the HRRT. VRR is crucial to achieve quantitation for low statistics dynamic studies reconstructed with iterative methods based on ordinary Poisson model. On HRRT, VRR cannot be performed in projection space since individual LORs are mixed after histogramming in parallel projection space using nearest neighbor approximation and axial compression. The proposed algorithm uses the classical random rate equation on the 4.5 109LORs. However, crystal singles are registered at block level and have lower deadtime than coincidences. Variations in layer identification with countrate were reported biasing random estimation from block singles. Our method overcomes these problems by estimating the singles per crystal from delayed coincidences. A singles map is created histogramming every delayed event into 2 singles. Each element represents the number of coincidences between that crystal and the ones in the 5 opposite coincident heads. The algorithm finds iteratively the crystal singles rates compatible with the delayed coincidence events. The method has been validated on decaying phantoms. We compared estimated and measured block singles to identify deadtime difference between singles and coincidences

Automatic registration of 3D MR images with a computerized brain atlas.

We present an automatic and unsupervised method for non-rigid registration of 3D magnetic resonance (MR) images with the Stockholm Computerized Brain Atlas (CBA). This method can be used in the context of multimodal medical image registration, fusion and automatic brain segmentation. In these applications anatomical images (MR) are coregistered with low spatial resolution functional imaging modalities (PET and SPECT) and fused with the neurological database of the CBA. The proposed matching method is based on the minimization of a 3D Chamfer distance function between the surface of the brain extracted from the MR image and the CBA brain surface. The surface-to-surface distance function is efficiently calculated by using a precomputed point-to-surface Euclidean distance map. The non-rigid inter-patient transformation of the CBA is modeled by a generalized 3D second order transformation. This transformation is easily differentiable and, as a consequence, fast and efficient minimization methods can be used. First, a quasi-rigid, first order transformation is computed. Then, the matching is improved by introducing the second order coefficients into the transformation. After this global matching, a local adaptation of the CBA is performed by a morphing method. The combination of a second order global transformation with a 3D local morphing allows the user to obtain a registration accuracy of one pixel, i.e. a mean distance between the surface of the brain in the MR image and the CBA of one pixel, which is significantly better than what can be expected from a human operator.