The present study was undertaken to measure normal aortic blood velocity profiles by means of continuous wave Doppler echocardiography from apical and suprasternal positions in 40 healthy adults. The profiles in the ascending aorta were measured from both positions, whereas the profiles in the descending aorta were measured only from the suprasternal position. The highest values for the maximal flow velocities were found in the ascending aorta from the suprasternal position (1.28 +/- 0.18 m/s), the lowest values for the maximal flow velocities were found in the descending aorta (1.17 +/- 0.22 m/s). This trend was also found for the mean velocity values. As to the time parameters, a longer acceleration time for the descending aorta was remarkable (106 +/- 24 ms), whereby the mean values for the ejection time for the descending aorta (308 +/- 25 ms, suprasternal: 320 +/- 30 ms). The highest values for the acceleration were found in the ascending aorta (suprasternal: 1526 +/- 531 cm/s2, apical: 1422 +/- 330 cm/s2) and the lowest values in the descending aorta (1208 +/- 378 cm/s2). Regarding the maximal velocity index there was no significant difference for the maximal values between the different transducer positions. The results of the regression analysis showed only a weak correlation of the maximal velocity values in the different transducer positions. This means that for follow-up examinations only those values should be used that were obtained from the same transducer position.

Determination of normal values of the aortic blood flow profile using continuous Doppler echocardiography from apical and suprasternal echo position

En Medecine Nucleaire, l'imagerie par ouverture codee a ete developpee comme une alternative aux collimateurs a canaux paralleles pour atteindre une grande sensibilite de detection ainsi qu'une haute resolution spatiale. Les principales limites sont la formation d'artefacts et l'effet de collimation pour l'imagerie 2D, et une degradation de la resolution spatiale longitudinale en imagerie 3D. Au cours de cette these, nous avons developpe un algorithme de reconstruction ML-EM (Maximum Likelihood - Expectation Maximization) base sur une matrice systeme estimee par des simulations Monte Carlo, et incluant la geometrie du masque code ainsi que les effets physiques entrant en compte dans l'imagerie par ouverture codee. Nous avons valide la construction du masque code dans la plate-forme de simulation GATE en comparant les donnees simulees avec les resultats experimentaux. Le masque code est base sur un ensemble cyclique de differences de Singer et est monte sur une gamma-camera clinique. Nous avons trouve un bon accord entre les spectres en energie mesures et simules avec et sans milieu diffusant et entre les fonctions de dispersions ponctuelles 3D mesuree et simulee. Nous avons calcule les coefficients de probabilite de la matrice systeme avec des simulations du masque code en utilisant GATE. Les resultats de la reconstruction d'une etude sur un fantome simule montrent que les artefacts en champ proche sont totalement supprimes en utilisant l'algorithme ML-EM a la place de la methode de decodage classique par correlation. Des d'etudes experimentales sur fantome et sur patient montrent la faisabilite de cette methode de reconstruction pour la scintigraphie thyroidienne 3D.

Optimisation d'un collimateur à ouverture codée avec la plate-forme de simulation Monte Carlo GATE

En medecine nucleaire, les images planaires et tomographiques sont principalement limitees en resolution (inferieure au centimetre) et en sensibilite par la necessite d'utiliser des collimateurs. La tomographie par ouverture codee est une methode permettant d'ameliorer ces caracteristiques en remplacant le collimateur a canaux paralleles par un masque muni de trous stenopeiques dont les avantages sont lies au systeme multi - pinhole : augmentation de la sensibilite, amelioration de la resolution spatiale, possibilite d'acquisitions tomographiques dynamiques et multi spectrales en energie. Le masque a ouverture codee de notre systeme a ete realise a partir d'un algorithme mathematique de singer. Selon cette sequence pseudo - aleatoire, cent quatre-vingt-seize trous coniques de 3,1 mm de diametre ont ete perces dans une plaque de plomb de 3,0 mm d'epaisseur. Cette plaque est fixee a une distance focale de 336,0 mm du cristal de la gamma - camera. La reconstruction d'un objet a partir d'une image (projection planaire de l'objet a travers le masque) permet d'obtenir des coupes jointives paralleles au detecteur. Cette reconstruction utilise une technique iterative selon un algorithme faisant appel au maximum de vraisemblance maximum likelihood - expectation maximization : ml - em qui permet de prendre en compte cette geometrie particuliere d'acquisition. La sensibilite d'acquisition du systeme a ete mesuree dix fois superieure a celle obtenue grace a un collimateur a canaux paralleles de basse energie - ultra haute resolution (low energy ultra high resolution : leuhr). Ce systeme a permis de reconstruire un volume de 133,30 176,70 130,33 mm 3, i l h, echantillonne en 29 coupes de 29 39 pixels correspondant a des voxels isotropes de 4,5 mm de cote. Des reconstructions de simulations numeriques et physiques d'objets ont ete realisees. L'acquisition d'une seconde image de projection orthogonale codee a permis d'ameliorer la resolution en profondeur (effet tomographique) et d'obtenir une resolution homogene dans les trois directions de l'espace mesuree a 5,4 mm. Des corrections de la diffusion compton et de l'attenuation des photons peuvent etre integrees dans l'algorithme iteratif de reconstruction afin d'obtenir des valeurs quantitatives.

Tomographie par projections orthogonales et ouverture codee

This work deals with the estimation of the concentration of molecules in arterial blood which are labelled with positron-emitting radioelements. This concentration is called “ B+ arterial input function”. This concentration has to be estimated for a large number of pharmacokinetic analyses. Nowadays it is measured through series of arterial sampling, which is an accurate method but requiring a stringent protocol. Complications might occur during arterial blood sampling because this method is invasive (hematomes, nosocomial infections).The objective of this work is to overcome this risk through a non-invasive estimation of B+ input function with an external detector and a collimator. This allows the reconstruction of blood vessels and thus the discrimination of arterial signal from signals in other tissues.Collimators in medical imaging are not adapted to estimate B+ input function because their sensitivity is very low. During this work, they are replaced by coded-aperture collimators, originally developed for astronomy.New methods where coded apertures are used with statistical reconstruction algorithms are presented. Techniques for analytical ray-tracing and for the acceleration of reconstructions are proposed. A new method which decomposes reconstructions on temporal sets and on spatial sets is also developped to efficiently estimate arterial input function from series of temporal acquisitions.This work demonstrates that the trade-off between sensitivity and spatial resolution in PET can be improved thanks to coded aperture collimators and statistical reconstruction algorithm; it also provides new tools to implement such improvements.

https://tel.archives-ouvertes.fr/tel-00536849/document

Feasibility study of the non-invasive estimation of the b+ arterial input function for human PET imaging

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Feasibility study of the non-invasive estimation of the b+ arterial input function for human PET imaging