M. Sibomana

Tri-dimensional automatic segmentation of PET volumes based on measured source-to-background ratios: influence of reconstruction algorithms.

A method for automatic volume segmentation of functional imaging based on a relationship between source-to-background ratio and the iso-activity level to be used is described. Such method, which has been established with radioactive spheres in a phantom, is independent of the a priori knowledge of the lesion of interest and is valid for small (>2 ml) and/or poorly contrasted (S/B>1.5) lesions. Accuracy varies slightly with the image reconstruction algorithm used.

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.

Preserving Poisson characteristics of PET data with weighted OSEM reconstruction

To evaluate the importance of preserving the Poisson characteristics of PET data when using OSEM reconstruction, the authors have examined several data weighting schemes with increasing complexity which progressively account for attenuation, normalization, random coincidences and Compton scatter with both ordinary and shifted Poisson models. All schemes have first been tested on 2D dynamic data from a decaying phantom filled with H/sub 2//sup 15/O. The images produced by the various weighting schemes were compared to the ones obtained by filtered backprojection reconstruction. Small positive bias (1%) was detected for the FW and SP schemes and may be due to implementation. The impact of these reconstruction methods on physiological parameters is illustrated with clinical protocols measuring myocardial blood flow with /sup 13/NH/sub 3/ and Standard Uptake Values with /sup 18/FDG. Despite the high random rate in cardiac studies, no positive bias was detected in the images when using the AW and ANW schemes with scans precorrected for random and scatter. For whole-body studies, at low random rates, the ANW scheme produced the same quantitative results as the FW and SP ones. That scheme is likely the best choice for obtaining high quality emission images and unbiased ROIs in most situations on current 2D scanners.

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

Mediman: an object oriented programming approach for medical image analysis

Mediman is an image analysis package which has been developed to analyze quantitatively PET (positron emission tomography) data. It is object-oriented, written in C++, and based on InterViews on top of which new classes have been added. Mediman accesses data through NSF using external data representation or an import/export mechanism which avoids data duplication. Multimodality studies are organized in a database which includes images, headers, color tables, lists and objects of interest (OOIs), and history files. Stored color table parameters allow one to focus directly on the interesting portion of the dynamic range. Lists allow one to organize the image database according to modality, acquisition protocol, and time and spatial properties. OOIs are stored in absolute 3-D coordinates, allowing correlation with other coregistered imaging modalities such as magnetic resonance imaging or single photon emission computed tomography. OOIs have visualization properties and are organized into groups. Quantitative region-of-interest analysis of anatomic images consists of position, distance, and volume calculation on selected OOIs. >

Count-Rate Dependent Component-Based Normalization for the HRRT

Component-based normalization is an important technique for PET scanners with a high number of lines of response (LOR), e.g., 4.5 times 109 for the HRRT. It reduces the problem of measuring the sensitivity of each LOR to that of estimating the individual crystal efficiencies(epsiv), e.g., 119808 for the HRRT. We propose a component-based method to compute epsiv for the HRRT. In addition, the block design of the HRRT produces pulse pile-up which causes apparent changes in epsiv with count rate. These effects occur within the block and between the front (LSO) and back (LYSO) crystal layers. We use a rotating source to measure the values and a decaying uniform phantom to account for variations with count rate. The computation of efficiencies is achieved with ~1% statistical noise with an acquisition of ~1 h. Count rate dependency of epsiv is implemented as a linear model in terms of block singles rate. Four approaches to modify epsiv with count rate were compared. Among them, an independent parameter for each crystal produced the best results, both visually and quantitatively. Failure to account for the count rate dependency in epsiv leads to high resolution artifacts in the reconstructed images, most visible in the transverse plane, in the center of the field-of-view.

Interactive delineation of brain sulci and their merging into functional PET images

A set of software tools has been developed to assist the neuro(physio)logist in the analysis of a series of cerebral images from single subjects by fusing sulci manually delineated on MRI brain surface into PET functional data. The procedure requires coregistered datasets and involves 4 steps: (i) segmentation of anatomical MRI data in order to extract the brain surface, (ii) generate the brain surface views by parallel ray casting, (iii) manual delineation of the relevant sulci from the surface views and (iv) fusion of the landmarks into any coregistered dataset from the same subject. The brain surface is segmented automatically from 3D MRI data using a new Directional Watershed Transform algorithm. From the segmented brain surface, 8 orthogonal surface views are calculated as visual support for interactive stereo definition of the major brain sulci. Each sulcus is built as a 3D trace line using a few vertices which are manually defined on one or several surface views. This technique allows one to follow the brain surface curvature rather independently of the number and the position of the vertices. The sulci are saved in an individual file for further use. The brain surface viewer is linked (via the 3D cursor position) to an independent volume viewer containing a coregistered (anatomical or functional) volume. Sulcal landmarks are finally projected onto this volume allowing further volume of interest definition. The use of the tool set is illustrated by a single subject brain activation study after /sup 15/O water injection.

Increased FDG Uptake in the Ipsilesional Sensorimotor Cortex in Congenital Hemiplegia

The resting brain metabolism was estimated in six children suffering from a right congenital hemiplegia (CH) of subcortical origin. This estimate was based on the 18F-labeled 2-deoxy-2-fluoro-d-glucose (FDG) uptake measured by means of positron emission tomography and compared, using statistical parametric mapping (SPM99), with that of six control subjects. The contrast [CH children - Controls] showed that CH children had two loci of relatively higher FDG uptake. The larger voxel cluster was found in the ipsilesional hemisphere and comprised the primary motor and somatosensory cortices and left inferior parietal lobule. The other cluster was located in the contralesional hemisphere and encompassed the primary motor cortex, callosomarginal sulcus, and cingulate gyrus. The reverse contrast [Controls - CH children] showed that control subjects had a relatively higher FDG uptake bilaterally in the temporal and hippocampal gyri, the rostral part of the brain stem, the thalami, the putamen, and the superior frontal gyri. A crossed cerebellar diaschisis was not observed in CH children. This relatively higher FDG uptake in the ipsi- and contralesional motor areas of CH children stands out in contrast to the hypometabolism (diaschisis) frequently observed in adult stroke patients with a subcortical lesion. This increased FDG uptake in the disconnected ipsilesional motor areas may reflect a long-term adaptation leading, for example, to an increased synaptic density and/or activity or to a change in the density of glucose transporters.

Decreased benzodiazepine receptor density in the cerebellum of early blind human subjects.

As a first approach to study the effect of early visual deprivation in the GABA-ergic inhibitory system, the distribution of benzodiazepine receptors (BZR) was accurately estimated using [11C]flumazenil ([11C]FMZ). Measurements were carried out in five subjects who became blind early in life and in five sighted control subjects. The interactions between [11C]FMZ and BZR were described using a non-linear compartmental analysis which permitted to estimate the BZR synaptic density independently of other model parameters. The distribution of BZR in the visual areas and other cortical regions of blind subjects was qualitatively and quantitatively similar to that of controls. However, the BZR density in the cerebellum was significantly lower in blind than in control subjects (P<0.01). Our findings suggest that modifications of the cerebellar neural circuitry may be concomitant to the already observed compensatory reorganization in cerebral areas of blind subjects.

Données actuelles et perspectives de la tomographie par émission de positrons en oncologie-radiothérapie

Resume La tomographie par emission de positrons (TEP) est une methode dimagerie tres prometteuse dans le domaine de loncologie et de la radiotherapie. Lutilisation du fluorodeoxyglucose, analogue du glucose, a permis de developper les applications de la TEP grâce aux acquisitions ≪ corps entier ≫. Le diagnostic de malignite et la determination du stade sont possibles en un seul examen et ce, avec une grande sensibilite. Nous abordons ici trois situations cliniques dans lesquelles la TEP a demontre non seulement son efficacite diagnostique mais egalement son impact sur la prise en charge du patient: le bilan dextension du cancer bronchique, la recidive de cancer colorectal et levaluation du patient symptomatique apres traitement dune tumeur de la sphere cervicofaciale. Nous envisageons egalement dans quelle mesure la TEP peut entrer dans larsenal des methodes dimagerie actuellement utilisee pour la planification therapeutique en radiotherapie. Cette application constitue en effet une voie davenir, meme si de nombreux ecueils methodologiques doivent dabord etre resolus.