Search

Positron emission tomography (PET) is one of the most promising diagnostic procedures in oncology. Using the glucose analogue fluorodeoxyglucose, PET produces whole-body images and is highly sensitive for tumor diagnosis and staging. We review three particular clinical situations in which PET-FDG has proven not only its diagnostic accuracy, but also its impact on patient management, i.e., the staging of non-small cell lung cancer, diagnosis and staging of colo-rectal cancer and head and neck cancer recurrence. Image registration yields anatomo-metabolic images that could be used as additional information for the determination of radiation fields. Tracer and technical issues remain to be solved before PET can be routinely used for that purpose.

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.

The regional cerebral blood flow (rCBF) and metabolic rate for glucose (rCMRGlc) are associated with functional activity of the neural cells. The present work reports a comparison study between rCBF and rCMRGlc in a normal population as a function of age. 10 young (25.9+/-5.6 years) and 10 old (65.4+/-6.1 years) volunteers were similarly studied at rest. In each subject, rCBF and rCMRGlc were measured in sequence, during the same session. Both rCBF and rCMRGlc values were found to decrease from young (mean rCBF=43.7 ml/100 g per min; mean rCMRGlc=40.6 micromol/100 g per min) to old age (mean rCBF=37.3 ml/100 g per min; mean rCMRGlc=35.2 micromol/100 g per min), resulting in a drop over 40 years of 14.8% (0.37%/year) and 13.3% (0.34%/year), respectively. On a regional basis, the frontal and the visual cortices were observed to have, respectively, the highest and the lowest reduction in rCBF, while, for rCMRGlc, these extremes were observed in striatum and cerebellum. Despite these differences, the ratio of rCBF to rCMRGlc was found to have a similar behavior in all brain regions for young and old subjects as shown by a correlation coefficient of 88%. This comparative study indicates a decline in rCBF and rCMRGlc values and a coupling between CBF and CMRGlc as a function of age.

In order to better preserve the Poisson characteristics of PET data when using 3D OSEM iterative reconstruction, the authors have compared images reconstructed by various data weighting schemes: unweighted, attenuation weighted, attenuation and normalization weighted OSEM 3D, FORE+unweighted and attenuation weighted OSEM 2D using 3DRP and FORE+FBP images as reference. Due to better noise modeling, 3D weighted OSEM schemes are expected to produce better images (i.e. noise-bias trade-off) especially at low statistics, in short frames of dynamic studies and when bed overlap is suboptimal, as in 3D wholebody studies. These reconstruction methods were implemented on the ECAT Exact HR and quantification was validated on clinical data. Their influence on physiological parameter measurements was examined on a dynamic brain receptor study with /sup 11/C-Flumazenil and a whole-body study with /sup 18/FDG. The 3D weighted OSEM schemes provide unbiased results in volume of interests. The resulting noise reduction may lead to more accurate registration, acquisition of shorter frames and generation of parametric images., Anglais

During the first years of life, the human brain undergoes repetitive modifications in its anatomical, functional, and synaptic construction to reach the complex functional organization of the adult central nervous system. As an attempt to gain further insight in those maturation processes, the evolution of cerebral metabolic activity was investigated as a function of age in epileptic infants, children and adolescents. The regional cerebral metabolic rates for glucose (rCMRGlc) were measured with positron emission tomography (PET) in 60 patients aged from 6 weeks to 19 years, who were affected by complex partial epilepsy. They were scanned at rest, without premedication, in similar conditions to 20 epileptic adults and in 49 adult controls. The distribution of brain metabolic activity successively extended from sensorimotor areas and thalamus in epileptic newborns to temporo-parietal and frontal cortices and reached the adult pattern after 1 year of age. The measured rCMRGlc in the cerebral cortex, excluding the epileptic lesions, increased from low values in infants to a maximum between 4 and 12 years, before it declined to stabilize at the end of the second decade of life. Similar age-related changes in glucose metabolic rates were not observed in the adult groups. Despite the use of medications, the observed variations of rCMRGlc with age in young epileptic humans confirm those previously described in pediatric subjects. These metabolic changes are in full agreement with the current knowledge of the synaptic density evolution in the human brain.

This paper presents two new rebinning algorithms for the reconstruction of three-dimensional (3-D) positron emission tomography (PET) data, A rebinning algorithm is one that first sorts the 3-D data into an ordinary two-dimensional (2-D) data set containing one sinogram for each transaxial slice to be reconstructed; the 3-D image is then recovered by applying to each slice a 2-D reconstruction method such as filtered-backprojection, This approach allows a significant speedup of 3-D reconstruction, which is particularly useful for applications involving dynamic acquisitions or whole-body imaging, The first new algorithm is obtained by discretizing an exact analytical inversion formula, The second algorithm, called the Fourier rebinning algorithm (FORE), is approximate but allows an efficient implementation based on taking 2-D Fourier transforms of the data, This second algorithm was implemented and applied to data acquired with the new generation of PET systems and also to simulated data for a scanner with an 18 degrees axial aperture, The reconstructed images were compared to those obtained with the 3-D reprojection algorithm (3DRP) which is the standard ''exact'' 3-D filtered-backprojection method, Results demonstrate that FORE provides a reliable alternative to 3DRP, while at the same time achieving an order of magnitude reduction in processing time.

Quantitative measurement of brain glucose metabolism with positron emission tomography (PET) and fluorodeoxyglucose (FDG) involves arterial blood sampling to estimate the delivery of radioactivity to the brain. Usually, for an intra-venous bolus injection of 30 sec duration, an optimal manual sampling requires more than 25 blood samples since a frequency of 1 sample every 5 sec or less is necessary to determine the peak activity in arterial plasma during the first 2 minutes after injection. In the present work, 13 standardized sampling times were shown to be sufficient to accurately define the input curve. This standardized input curve was subsequently fitted by a polynomial function for its rising part and by spectral analysis for its decreasing part. Using the measured, the standardized as well as the fitted input curves, the regional cerebral metabolic rate for glucose (rCMRGlc) was estimated in 32 cerebral regions of interest (ROIs) in 20 normal volunteers. Metabolic values were obtained by both the autoradiographic method (ARG) and the kinetic analysis of dynamic data (DYN). Comparison of rCMRGlc values obtained with the measured and the fitted input curves showed that both procedures gave consistent results, with a maximal relative error in mean rCMRGlc of about 1% in ARG and 2% in DYN studies. This input curve fitting technique which was not dependent on the peak time occurrence, allowed an accurate determination of the input curve shape from reduced sampling schemes., Anglais

A new adaptive scheme is proposed for the lossless compression of positron emission tomography (PET) sinogram data. The algorithm uses an adaptive differential pulse code modulator (ADPCM) followed by a universal variable length coder (UVLC). Contrasting with Lempel-Ziv (LZ), which operates on a whole sinogram, UVLC operates very efficiently on short data blocks. This is a major advantage for real-time implementation. The algorithm is adaptive and codes data after some on-line estimations of the statistics inside each block. Its efficiency is tested when coding dynamic and static scans from two PET scanners and reaches asymptotically the entropy limit for long frames. For very short 3D frames, the new algorithm is twice more efficient than LZ. Since an ASIC implementing a similar UVLC scheme is available today, a similar one should be able to sustain PET data lossless compression and decompression at a rate of 27 MBytes/sec. This algorithms is consequently a good candidate for the next generation of lossless compression engine.

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., Anglais