68Ga-DOTATOC PET/MR imaging and radiomic parameters in predicting histopathological prognostic factors in patients with pancreatic neuroendocrine well-differentiated tumours.

PURPOSE: To explore the role of fully hybrid 68Ga-DOTATOC PET/MR imaging and radiomic parameters in predicting histopathological prognostic factors in patients with pancreatic neuroendocrine tumours (PanNETs) undergoing surgery. METHODS: One hundred eighty-seven consecutive 68Ga-DOTATOC PET/MRI scans (March 2018-June 2020) performed for gastroenteropancreatic neuroendocrine tumour were retrospectively evaluated; 16/187 patients met the eligibility criteria (68Ga-DOTATOC PET/MRI for preoperative staging of PanNET and availability of histological data). PET/MR scans were qualitatively and quantitatively interpreted, and the following imaging parameters were derived: PET-derived SUVmax, SUVmean, somatostatin receptor density (SRD), total lesion somatostatin receptor density (TLSRD), and MRI-derived apparent diffusion coefficient (ADC), arterial and late enhancement, necrosis, cystic degeneration, and maximum diameter. Additionally, first-, second-, and higher-order radiomic parameters were extracted from both PET and MRI scans. Correlations with several PanNETs' histopathological prognostic factors were evaluated using Spearman's coefficient, while the area under the curve (AUC) of the receiver operating characteristic (ROC) curve was used to evaluate parameters' predictive performance. RESULTS: Primary tumour was detected in all 16 patients (15/16 by 68Ga-DOTATOC PET and 16/16 by MRI). SUVmax and SUVmean resulted good predictors of lymphnodal (LN) involvement (AUC of 0.850 and 0.783, respectively). Second-order radiomic parameters GrayLevelVariance and HighGrayLevelZoneEmphasis extracted from T2 MRI demonstrated significant correlations with LN involvement (adjusted p = 0.009), also showing good predictive performance (AUC = 0.992). CONCLUSION: This study demonstrates the role of the fully hybrid PET/MRI tool for the synergic function of imaging parameters extracted by the two modalities and highlights the potentiality of imaging and radiomic parameters in assessing histopathological features of PanNET aggressiveness.

Radiomics in pancreatic neuroendocrine tumors: methodological issues and clinical significance.

PURPOSE: To present the state-of-art of radiomics in the context of pancreatic neuroendocrine tumors (PanNETs), with a focus on the methodological and technical approaches used, to support the search of guidelines for optimal applications. Furthermore, an up-to-date overview of the current clinical applications of radiomics in the field of PanNETs is provided. METHODS: Original articles were searched on PubMed and Science Direct with specific keywords. Evaluations of the selected studies have been focused mainly on (i) the general radiomic workflow and the assessment of radiomic features robustness/reproducibility, as well as on the major clinical applications and investigations accomplished so far with radiomics in the field of PanNETs: (ii) grade prediction, (iii) differential diagnosis from other neoplasms, (iv) assessment of tumor behavior and aggressiveness, and (v) treatment response prediction. RESULTS: Thirty-one articles involving PanNETs radiomic-related objectives were selected. In regard to the grade differentiation task, yielded AUCs are currently in the range of 0.7-0.9. For differential diagnosis, the majority of studies are still focused on the preliminary identification of discriminative radiomic features. Limited information is known on the prediction of tumors aggressiveness and of treatment response. CONCLUSIONS: Radiomics is recently expanding in the setting of PanNETs. From the analysis of the published data, it is emerging how, prior to clinical application, further validations are necessary and methodological implementations require optimization. Nevertheless, this new discipline might have the potential in assisting the current urgent need of improving the management strategies in PanNETs patients.

Harmonisation of PET/CT contrast recovery performance for brain studies.

PURPOSE: In order to achieve comparability of image quality, harmonisation of PET system performance is imperative. In this study, prototype harmonisation criteria for PET brain studies were developed. METHODS: Twelve clinical PET/CT systems (4 GE, 4 Philips, 4 Siemens, including SiPM-based "digital" systems) were used to acquire 30-min PET scans of a Hoffman 3D Brain phantom filled with ~ 33 kBq·mL-1 [18F]FDG. Scan data were reconstructed using various reconstruction settings. The images were rigidly coregistered to a template (voxel size 1.17 × 1.17 × 2.00 mm3) onto which several volumes of interest (VOIs) were defined. Recovery coefficients (RC) and grey matter to white matter ratios (GMWMr) were derived for eroded (denoted in the text by subscript e) and non-eroded grey (GM) and white (WM) matter VOIs as well as a mid-phantom cold spot (VOIcold) and VOIs from the Hammers atlas. In addition, left-right hemisphere differences and voxel-by-voxel differences compared to a reference image were assessed. RESULTS: Systematic differences were observed for reconstructions with and without point-spread-function modelling (PSFON and PSFOFF, respectively). Normalising to image-derived activity, upper and lower limits ensuring image comparability were as follows: for PSFON, RCGMe = [0.97-1.01] and GMWMre = [3.51-3.91] for eroded VOI and RCGM = [0.78-0.83] and GMWMr = [1.77-2.06] for non-eroded VOI, and for PSFOFF, RCGMe = [0.92-0.99] and GMWMre = [3.14-3.68] for eroded VOI and RCGM = [0.75-0.81] and GMWMr = [1.72-1.95] for non-eroded VOI. CONCLUSIONS: To achieve inter-scanner comparability, we propose selecting reconstruction settings based on RCGMe and GMWMre as specified in "Results". These proposed standards should be tested prospectively to validate and/or refine the harmonisation criteria.

Simultaneous reconstruction of attenuation and activity in cardiac PET can remove CT misalignment artifacts.

BACKGROUND: Misalignment between positron emission tomography (PET) and computed tomography (CT) data is known to generate artifactual defects in cardiac PET images due to imprecise attenuation correction (AC). In this work, the use of a maximum likelihood attenuation and activity (MLAA) algorithm is proposed to avoid such artifacts in time-of-flight (TOF) PET. METHODS: MLAA was implemented and tested using a thorax/heart phantom and retrospectively on fourteen (13)N-ammonia PET/CT perfusion studies. Global and local misalignments between PET and CT data were generated by shifting matched CT images or using CT data representative of the end-inspiration phase. PET images were reconstructed with MLAA and a 3D-ordered-subsets-expectation-maximization (OSEM)-TOF algorithm. Images obtained with 3D-OSEM-TOF and matched CT were used as references. These images were compared (qualitatively and semi-quantitatively) with those reconstructed with 3D-OSEM-TOF and MLAA for which a misaligned CT was used, respectively, for AC and initialization. RESULTS: Phantom experiment proved the capability of MLAA to converge toward the correct emission and attenuation distributions using, as input, only PET emission data, but convergence was very slow. Initializing MLAA with phantom CT images markedly improved convergence speed. In patient studies, when shifted or end-inspiration CT images were used for AC, 3D-OSEM-TOF reconstructions showed artifacts of increasing severity, size, and frequency with increasing mismatch. Such artifacts were absent in the corresponding MLAA images. CONCLUSION: The proposed implementation of the MLAA algorithm is a feasible and robust technique to avoid AC mismatch artifacts in cardiac PET studies provided that a CT of the source is available, even if poorly aligned.

Evaluation of image reconstruction algorithms encompassing Time-Of-Flight and Point Spread Function modelling for quantitative cardiac PET: phantom studies.

BACKGROUND: To perform kinetic modelling quantification, PET dynamic data must be acquired in short frames, where different critical conditions are met. The accuracy of reconstructed images influences quantification. The added value of Time-Of-Flight (TOF) and Point Spread Function (PSF) in cardiac image reconstruction was assessed. METHODS: A static phantom was used to simulate two extreme conditions: (i) the bolus passage and (ii) the steady uptake. Various count statistics and independent noise realisations were considered. A moving phantom filled with two different radionuclides was used to simulate: (i) a great range of contrasts and (ii) the cardio/respiratory motion. Analytical and iterative reconstruction (IR) algorithms also encompassing TOF and PSF modelling were evaluated. RESULTS: Both analytic and IR algorithms provided good results in all the evaluated conditions. The amount of bias introduced by IR was found to be limited. TOF allowed faster convergence and lower noise levels. PSF achieved near full myocardial activity recovery in static conditions. Motion degraded performances, but the addition of both TOF and PSF maintained the best overall behaviour. CONCLUSIONS: IR accounting for TOF and PSF can be recommended for the quantification of dynamic cardiac PET studies as they improve the results compared to analytic and standard IR.

Development of a new toolbox for mouse PET-CT brain image analysis fully based on CT images and validation in a PD mouse model.

Automatic analysis toolboxes are popular in brain image analysis, both in clinical and in preclinical practices. In this regard, we proposed a new toolbox for mouse PET-CT brain image analysis including a new Statistical Parametric Mapping-based template and a pipeline for image registration of PET-CT images based on CT images. The new templates is compatible with the common coordinate framework (CCFv3) of the Allen Reference Atlas (ARA) while the CT based registration step allows to facilitate the analysis of mouse PET-CT brain images. From the ARA template, we identified 27 volumes of interest that are relevant for in vivo imaging studies and provided binary atlas to describe them. We acquired 20 C57BL/6 mice with [18F]FDG PET-CT, and 12 of them underwent 3D T2-weighted high-resolution MR scans. All images were elastically registered to the ARA atlas and then averaged. High-resolution MR images were used to validate a CT-based registration pipeline. The resulting method was applied to a mouse model of Parkinson's disease subjected to a test-retest study (n = 6) with the TSPO-specific radioligand [18F]VC701. The identification of regions of microglia/macrophage activation was performed in comparison to the Ma and Mirrione template. The new toolbox identified 11 (6 after false discovery rate adjustment, FDR) brain sub-areas of significant [18F]VC701 uptake increase versus the 4 (3 after FDR) macro-regions identified by the Ma and Mirrione template. Moreover, these 11 areas are functionally connected as found by applying the Mouse Connectivity tool of ARA. In conclusion, we developed a mouse brain atlas tool optimized for PET-CT imaging analysis that does not require MR. This tool conforms to the CCFv3 of ARA and could be applied to the analysis of mouse brain disease models.

Physical performance of the new hybrid PET∕CT Discovery-690.

PURPOSE: The aim of this work was the assessment of the physical performance of the new hybrid PET∕CT system: Discovery-690. METHODS: The Discovery-690 combines a lutetium-yttrium-orthosilicate (LYSO) block detector designed PET tomograph with a 64-slice CT scanner. The system is further characterized by a dedicated powerful computing platform implementing fully 3D-PET iterative reconstruction algorithms. These algorithms can account for time of flight (TOF) information and∕or a 3D model of the PET point spread function (PSF). PET physical performance was measured following NEMA NU-2-2007 procedures. Furthermore, specific tests were used: (i) to measure the energy and timing resolution of the PET system and (ii) to evaluate image quality, by using phantoms representing different clinical conditions (e.g., brain and whole body). Data processing and reconstructions were performed as required by standard procedures. Further reconstructions were carried out to evaluate the performance of the new reconstruction algorithms. In particular, four algorithms were considered for the reconstruction of the PET data: (i) HD = standard configuration, without TOF and PSF, (ii) TOF = HD + TOF, (iii) PSF = HD + PSF, and (iv) TOFPSF = HD + TOF + PSF. RESULTS: The transverse (axial) spatial resolution values were 4.70 (4.74) mm and 5.06 (5.55) mm at 1 cm and 10 cm off axis, respectively. Sensitivity (average between 0 and 10 cm) was 7.5 cps∕kBq. The noise equivalent count rate (NECR) peak was 139.1 kcps at 29.0 kBq∕ml. The scatter fraction at the NECR peak was 37%. The correction accuracy for the dead time losses and random event counts had a maximum absolute error below the NECR peak of 2.09%. The average energy and timing resolution were 12.4% and 544.3 ps, respectively. PET image quality was evaluated with the NEMA IEC Body phantom by using four reconstruction algorithms (HD, TOF, PSF, and TOFPSF), as previously described. The hot contrast (after 3 iterations and for a lesion∕background activity ratio of 4:1) for the spheres of 10, 13, 17, and 22 mm was (HD) 29.8, 45.4, 55.4, and 68.1%; (TOF) 39.9, 53.5, 62.7, and 72.2%; (PSF) 28.3, 47.3, 60.4, and 71.8%; (TOFPSF) 43.8, 62.9, 70.6, and 76.4%. The cold contrast for the spheres of 28 and 37 mm was (HD) 62.4 and 65.2%; (TOF) 77.1 and 81.4%; (PSF) 62.0 and 65.2%; (TOFPSF) 77.3 and 81.6%. Similar hot and cold contrast trends were found during the analyses of other phantoms representing different clinical conditions (brain and whole body). Nevertheless, the authors observed a predominant role of either TOF or PSF, depending on the specific characteristics and dimensions of the phantoms. CONCLUSIONS: Discovery-690 shows very good PET physical performance for all the standard NEMA NU-2-2007 measurements. Furthermore, the new reconstruction algorithms available for PET data (TOF and PSF) allow further improvements of the D-690 image quality performance both qualitatively and quantitatively.

Performance measurements for the PET/CT Discovery-600 using NEMA NU 2-2007 standards.

PURPOSE: The aim of this study was to assess the performance measurements of the new PET/CT system Discovery-600 (D-600, GEMS, Milwaukee, WI). METHODS: Performance measures were obtained with the National Electrical Manufacturers Association (NEMA) NU 2-2007 procedures. RESULTS: The transverse (axial) spatial resolution FWHMs were 4.9 (5.6) mm and 5.6 (6.4) mm at 1 and 10 cm off axis, respectively. The sensitivity (average at 0 and 10 cm) was 9.6 cps/kBq. The scatter fraction was 36.6% (low energy threshold: 425 keV). The NEC peak rate (k=1) was 75.2 kcps at 12.9 kBq/cc. The hot contrasts for 10, 13, 17, and 22 mm spheres were 41%, 51%, 62%, and 73% and the cold contrasts for 28 and 37 mm spheres were 68% and 72%. CONCLUSIONS: The Discovery-600 has good performance for the NEMA NU 2-2007 parameters, particularly in improved sensitivity compared to the scanners of the same Discovery family, D-ST and D-STE.

Number of partitions (gates) needed to obtain motion-free images in a respiratory gated 4D-PET/CT study as a function of the lesion size and motion displacement.

PURPOSE: In this study we evaluate the number of data partitions (gates) needed to sort 4D-PET and 4D-CT data to obtain motion-free images as a function of lesion size and motion displacement. METHODS: Plexiglas spheres of various diameters (8, 10, 15, 20, and 25 mm) were filled with a radioactive solution of water and 18F. A PET/CT study was acquired for each sphere in a rest condition to reconstruct a motion-free image as a reference in terms of radioactivity concentration and spatial distribution. Each sphere was then moved sinusoidally in the superior-inferior direction over different motion displacements (5, 10, 15, 20, and 25 mm) with a periodic motion of 4 s. During motion a 4D-CT scan followed by a 4D-PET scan were acquired. Each set of 4D-CT and 4D-PET data was retrospectively sorted to generate one, two, four, six, eight, ten, and 12 partitions (gates) over the whole cycle of motion. 4D-PET gates were reconstructed by using the corresponding 4D-CT gates for attenuation correction, while PET data acquired, with the sphere in the rest condition were corrected for attenuation by using the corresponding CT image set. For each series of PET images, data analysis was performed by measuring (1) the maximum value of the radioactivity concentration (RACmax) in a VOI encompassing the radioactivity distribution over the volume of motion and (2) the axial-profile of the radioactivity distribution (Ax-p). RESULTS: The results show that radioactivity concentration is strongly underestimated due to motion in most conditions considered in this study. In particular, the underestimation of RACmax for the smallest sphere varied from -10.6% to -66.3% with motion displacements ranging from 5 to 25 mm. For the largest sphere, errors ranged from -1.4% to -26.7%. The 4D-PET/CT methodology allows motion-free or nearly motion-free images to be obtained. It also permits both radioactivity concentration (RACmax) and Ax-p to be recovered with residual differences with respect to the rest condition, depending on the number of partitions used to process the data. Within the limitation of the regular sinusoidal motion, used to simulate a general breathing condition, a scheme describing the number of partitions needed to obtain nearly motion-free images with Ax-p differences of around 10% with respect to the rest data is presented as a function of the lesion size and motion displacement. Such a scheme is proposed to guide the setup of a 4D-PET/CT acquisition and processing protocol for clinical applications. CONCLUSIONS: By using the 4D-PET/CT acquisition technique, it is possible to compensate for the degradation effect of lesion motion on the reconstructed PET images.

PET textural features stability and pattern discrimination power for radiomics analysis: An "ad-hoc" phantoms study.

PURPOSE: The analysis of PET images by textural features, also known as radiomics, shows promising results in tumor characterization. However, radiomic metrics (RMs) analysis is currently not standardized and the impact of the whole processing chain still needs deep investigation. We characterized the impact on RM values of: i) two discretization methods, ii) acquisition statistics, and iii) reconstruction algorithm. The influence of tumor volume and standardized-uptake-value (SUV) on RM was also investigated. METHODS: The Chang-Gung-Image-Texture-Analysis (CGITA) software was used to calculate 39 RMs using phantom data. Thirty noise realizations were acquired to measure statistical effect size indicators for each RM. The parameter η2 (fraction of variance explained by the nuisance factor) was used to assess the effect of categorical variables, considering η2 < 20% and 20% < η2 < 40% as representative of a "negligible" and a "small" dependence respectively. The Cohen's d was used as discriminatory power to quantify the separation of two distributions. RESULTS: We found the discretization method based on fixed-bin-number (FBN) to outperform the one based on fixed-bin-size in units of SUV (FBS), as the latter shows a higher SUV dependence, with 30 RMs showing η2 > 20%. FBN was also less influenced by the acquisition and reconstruction setup:with FBN 37 RMs had η2 < 40%, only 20 with FBS. Most RMs showed a good discriminatory power among heterogeneous PET signals (for FBN: 29 out of 39 RMs with d > 3). CONCLUSIONS: For RMs analysis, FBN should be preferred. A group of 21 RMs was suggested for PET radiomics analysis.

PET guidance in prostate cancer radiotherapy: Quantitative imaging to predict response and guide treatment.

Positron emission tomography (PET) allows a monitoring and recording of the spatial and temporal distribution of molecular/cellular processes for diagnostic and therapeutic applications. The aim of this review is to describe the current applications and to explore the role of PET in prostate cancer management, mainly in the radiation therapy (RT) scenario. The state-of-the art of PET for prostate cancer will be presented together with the impact of new specific PET tracers and technological developments aiming at obtaining better imaging quality, increased tumor detectability and more accurate volume delineation. An increased number of studies have been focusing on PET quantification methods as predictive biomarkers capable of guiding individualized treatment and improving patient outcome; the sophisticated advanced intensity modulated and imaged guided radiation therapy techniques (IMRT/IGRT) are capable of boosting more radioresistant tumor (sub)volumes. The use of advanced feature analyses of PET images is an approach that holds great promise with regard to several oncological diseases, but needs further validation in managing prostate diseases.

Non-Gaussian smoothing of low-count transmission scans for PET whole-body studies.

A non-Gaussian smoothing (NGS) technique is developed for filtering low count transmission (TR) data to be used for attenuation correction (AC) of positron emission tomography (PET) studies. The method is based on a statistical technique known as the generalized linear mixed model that allows an inverse link function that avoids the inversion of the observed transmission data. The NGS technique has been implemented in the sinogram domain in one-dimensional mode as angle-by-angle computation. To make it adaptive as a function of the TR count statistics we also develop and validate an objective procedure to choose an optimal smoothing parameter. The technique is assessed using experimental phantoms, simulating PET whole-body studies, and applied to real patient data. Different experimental conditions, in terms of TR scan time (from 1 h to 1 min), covering a wide range of TR counting statistic are considered. The method is evaluated, in terms of mean squared error (MSE), by comparing pixel by pixel the distribution for high counts statistics TR scan (1 h) with the corresponding counts distribution for low count statistics TR scans (e.g., 1 min). The smoothing parameter selection is shown to have high efficiency, meaning that it tends to choose values close to the unknown best value. Furthermore, the counts distribution of emission (EM) images, reconstructed with AC generated using low count TR data (1 min), are within 5% of the corresponding EM images reconstructed with AC generated using the high count statistics TR data (1 h). An application to a real patient whole-body PET study shows the promise of the technique for routine use.

Metabolic impairment in human amnesia: a PET study of memory networks.

Human amnesia is a clinical syndrome exhibiting the failure to recall past events and to learn new information. Its "pure" form, characterized by a selective impairment of long-term memory without any disorder of general intelligence or other cognitive functions, has been associated with lesions localized within Papez's circuit and some connected areas. Thus, amnesia could be due to a functional disconnection between components of this or other neural structures involved in long-term learning and retention. To test this hypothesis, we measured regional cerebral metabolism with 2-[18F]fluoro-2-deoxy-D-glucose ([18F]FDG) and positron emission tomography (PET) in 11 patients with "pure" amnesia. A significant bilateral reduction in metabolism in a number of interconnected cerebral regions (hippocampal formation, thalamus, cingulate gyrus, and frontal basal cortex) was found in the amnesic patients in comparison with normal controls. The metabolic impairment did not correspond to alterations in structural anatomy as assessed by magnetic resonance imaging (MRI). These results are the first in vivo evidence for the role of a functional network as a basis of human memory.

Assessment and comparison of three scatter correction techniques in single photon emission computed tomography.

The detection of scattered radiation is recognized as one of the major sources of error in single photon emission computed tomography (SPECT). In this work three scatter correction techniques have been assessed and compared. Scatter coefficients and parameters characteristic of each technique have been calculated through Monte Carlo simulations and experimentally measured for various source geometries. Their dependence on the source/matter distribution and their spatial non-stationarity have been described. Each of the three scatter correction methods has then been tested on several SPECT phantom studies. The three methods provided comparable results. Following scatter compensation, both image quality and quantitative accuracy improved. In particular a slight improvement in spatial resolution and a statistically significant increase in cold lesion contrast, hot lesion recovery coefficient, and signal/noise ratio have been demonstrated with all methods.

A procedure for patient repositioning and compensation for misalignment between transmission and emission data in PET heart studies.

A procedure for patient repositioning and compensation for misalignment between transmission and emission data in positron emission tomography (PET) heart studies has been developed. Following the transmission scan (TR1), patients are moved from the scanner bed for the administration of the tracer, and repositioned when ready for the emission scan (EM1). A short postinjection transmission scan (TR2) is performed at the end of the EM1 study. TR1 and TR2 images are compared to recognize misalignment between transmission and emission studies. TR1 sinograms are compensated for misalignment to allow for a proper attenuation correction. The procedure has been tested on phantom and [18F]FDG PET heart studies. Misalignments down to 2.5 mm translation and 1 degree rotation in the transaxial plane and 4 mm in the axial direction can be recognized and compensated for. The procedure is suitable for clinical purposes, allowing reduction of patient time on the scanner bed, increased patient comfort and significant increase of patient throughput.

Different neural systems for the recognition of animals and man-made tools.

Using positron emission tomography, we mapped brain activity in normal volunteers during the recognition of visual stimuli representing living (animals) and non-living (artefacts) entities. The subjects had to decide whether pairs of visual stimuli were different representations of the same object, or different objects. Animal recognition was associated with activations in the inferior temporo-occipital areas, bilaterally, whereas artefact recognition engaged a predominantly left hemispheric network, involving the left dorsolateral frontal cortex. These findings, which concur with clinical observations in neurological patients, provide in vivo evidence for a fractionation of the neural substrates of semantic knowledge in man.

Activation of precentral and mesial motor areas during the execution of elementary proximal and distal arm movements: a PET study.

Regional cerebral blood flow was measured using positron emission tomography (PET) in normal subjects while performing simple aimless proximal and distal arm movements. The aim of the experiment was to compare the somatotopic organization of precentral and mesial (the so called supplementary motor area, SMA) motor cortices and to evaluate whether in man, as in the monkey, the rostral and caudal sectors of SMA are functionally different. The results showed that proximal and distal arm movements are to a large extent segregated in the precentral motor cortex, but not in the SMA. They also showed that the SMA is made of at least two functional sectors. Only the caudal one is activated during simple aimless movements.

Intrapleural liquid flow down a gravity-dependent hydraulic pressure gradient.

We studied the vertical movement of 2 mg technetium-labeled albumin injected intrapleurally in 0.5 ml saline (15% of pleural liquid volume) in eight spontaneously breathing anesthetized dogs subject to a sudden change in posture (prone to supine or vice versa). The albumin movements were evaluated through a large field gamma camera placed laterally to the animal and detecting total (AT) and regional activities from two superimposed equal areas (At and Ab, top and bottom, respectively). The At/Ab ratio decreased from 2.1 to 1.3 in four animals up to 20 min from the change in posture and from 0.9 to 0.5 in four more animals studied from 50 to 90 min from turning maneuver. The rate of change in At and Ab was similar in the two groups of animals and unaffected by the acquisition posture. AT decreased by 7.7 and 3.5% for the two groups, respectively, reflecting albumin clearance from the pleural space. The opposite time course of regional activities and the independence of their rate of change of the At/Ab ratio and of the animal posture suggest a top-to-bottom albumin transfer occurring through a bulk flow of liquid estimated at 0.006 ml.kg-1.h-1. The data are consistent with a measured vertical pleural liquid pressure gradient that does not reflect a hydrostatic condition.

Relationship between corpus callosum atrophy and cerebral metabolic asymmetries in multiple sclerosis.

Corpus callosum (CC) atrophy by magnetic resonance imaging (MRI) is a common finding in multiple sclerosis (MS). In order to examine the relationship between CC atrophy and cortical brain metabolism, we compared the cerebral metabolic rates for glucose (CMRglc), measured by positron emission tomography (PET), of 8 MS patients with evidence of CC atrophy on midsagittal MRI, 8 MS patients without CC atrophy and 10 healthy controls. Results showed no significant differences in supratentorial CMRglc absolute values between the three groups, although a slight metabolic reduction was observed in both MS groups compared with normal controls. By contrast, only patients with CC atrophy showed greater directional metabolic asymmetry than normals, the left frontal, temporal and parietal association cortices being significantly lower than the right. Predominant left hemispheric metabolic reductions were not accompanied by a corresponding left-sided predominance in the extent of MRI-detected demyelinating lesions. Therefore our data suggest that CC atrophy interfers more with left than with right metabolic function.

Evaluation of pre- and post-reconstruction count-dependent Metz filters for brain PET studies.

In this study we evaluated pre- (PR-R) and post-reconstruction (PO-R) count-dependent Metz filters for PET brain studies in order to increase signal-to-noise ratio. A set of studies using a 3D Hoffman brain phantom was performed at various count levels, and a reference image set was created from extremely high count images. Several combinations of PR-R and PO-R filtering were considered to find the optimal means of processing, including: Hann filter alone; PR-R Metz filter without or with a PO-R low pass filter; and PO-R Metz filter without or with a PR-R low pass filter. A formula was established to correlate the optimal PO-R Metz filter order with the net counts. Resolution [full width at half-maximum (FWHM) and fill width at tenth maximum (FWTM)], normalized residual mean square differences (NRMSD) between the ideal and the processed images, noise reduction and contrast were used as parameters for the evaluation of the different filter combinations. Resolution is decreased by all filter combinations that can effectively control noise; however, FWTM increases less than FWHM. NRMSD indicates that the use of Hann and (optimal) PO-R Metz filter is the most powerful combination from among those tested. A close correlation (r = 0.969) was found between the net counts and the optimal order of the PO-R Metz filter. At the count levels of clinical studies the PO-R Metz filter was found to control noise much more effectively and enhance the contrast when compared to the routinely used Hann filter alone, and produced images of better quality.