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Our studies of the venom from the Levantine viper Vipera lebetina have demonstrated the existence of both coagulants and anticoagulants in the same venom. We showed that V. lebetina venom contains: (1) proteases that degrade fibrinogen, but not fibrin; (2) fibrinolytic enzyme (lebetase); (3) factor X activator (VLFXA); (4) factor V activator (VLFVA). Fibrinolytic enzyme and VLFXA are metalloproteases; the other studied enzymes are serine proteases. α-Fibrinogenase has no homolog among known serine proteases. β-Fibrinogenase is a typical thermostable arginine esterase that hydrolyzes esters and amides of arginine and attacks the β-chain of fibrinogen. Lebetase is a direct-acting fibrinolytic zinc metalloendopeptidase related in amino acid sequence to reprolysins. We used the matrix-assisted laser desorption/ionization time-of-flight mass spectrometry technique for the recovery and identification of peptides released by protease hydrolysis and for the detection of human factor X cleavage products after VLFXA hydrolysis. VLFXA cleaves the Arg52-Ile53 bond in the heavy chain of human factor X and the Arg226-Val227 bond in human factor IX precursor; VLFVA cleaves Arg1545-Ser1546 in factor V.

A new transmission source-based attenuation correction system, c.clear, has been developed for the Siemens c.cam cardiac SPECT camera. The transmission sources are two arrays of 14 Gd-153 line sources placed in cassette wings affixed to the detectors. The source strengths decrease geometrically away from the array center. The absorbed radiation dose received from these sources is about 0.01% of that received from a conventional Tl-201 myocardial scan. The line sources are stationary relative to the detector during the SPECT scan. The emission and transmission radiation are acquired simultaneously and are inherently registered spatially and temporally, so avoiding misregistration artifacts. Transmission reconstruction is performed using an ordered subset maximum likelihood algorithm with gradient descent. It models the geometry of the sources and the radiation emanating from these sources as narrow fan beams. The parameters of this model are estimated from near and far blank calibration images. It includes correction for the Compton emission down-scatter into the transmission energy window that occurs during the simultaneous Tc-99m and Gd-153 imaging. The down-scatter is estimated from down-scatter measured for a short interval when the line sources are obstructed by shutters in the cassette wings. The down-scatter is estimated by linear filtering using the measured short-interval down-scatter and the emission data. The transverse FOV of the camera can truncate the transmission radiation, particularly for obese or improperly positioned patients. Compensation for such truncations is achieved by employing robust data extrapolation techniques and extending the model of the source geometry beyond the measured FOV. This attenuation correction method has been investigated using both phantom and clinical studies and has been shown to produce corrected emission reconstructions of excellent quality.

The Differential Attenuation Method (DAM) was developed to simultaneously estimate activity and attenuation distributions from multi-emission projection data alone. Previously, DAM was shown to improve the quality and quantitative accuracy compared with filtered-backprojection without attenuation compensation. Here, that work is extended by comparing the performance of DAM to an iterative penalized-weighted least-squares reconstruction with: (1) uniform attenuation, and (2) the true attenuation. A single-slice numerical torso phantom was used to simulate data from /sup 201/Tl cardiac studies. Noise-free projections and data with additive Poisson-distributed noise were simulated with and without a myocardial perfusion defect. Scatter was not simulated. Images reconstructed with DAM from data at realistic count densities were more accurate than those reconstructed with an assumed uniform attenuation distribution, and nearly as accurate as those obtained using the true attenuation distribution.

The Differential Attenuation Method (DAM) simultaneously estimates SPECT activity and attenuation distributions from emission data. In this work, we extend the investigation of DAM to measured data and compare the results obtained with FBP without compensating for attenuation, and Penalized Weighted Least Squares (PWLS) with uniform and no attenuation. An anthropomorphic torso phantom with a heart insert containing a myocardial defect filled with /sup 111/In is used to acquire data on a GE VG tomograph. An accurate scatter correction before reconstruction is crucial. Scatter is subtracted with an Energy Window Based Subtraction method (EWBS), with positions and widths of the energy windows selected using Monte Carlo simulated source distributions. The results show the potential benefits of DAM as the activity estimate has fewer artifacts and is more accurate than FBP without compensating for attenuation or PWLS with uniform attenuation. However, artifacts remain and additional constraints on the allowed attenuation values were required to produce acceptable images.