Geometrical and Monte Carlo projectors in 3D PET reconstruction.

PURPOSE: In the present work, the authors compare geometrical and Monte Carlo projectors in detail. The geometrical projectors considered were the conventional geometrical Siddon ray-tracer (S-RT) and the orthogonal distance-based ray-tracer (OD-RT), based on computing the orthogonal distance from the center of image voxel to the line-of-response. A comparison of these geometrical projectors was performed using different point spread function (PSF) models. The Monte Carlo-based method under consideration involves an extensive model of the system response matrix based on Monte Carlo simulations and is computed off-line and stored on disk. METHODS: Comparisons were performed using simulated and experimental data of the commercial small animal PET scanner rPET. RESULTS: The results demonstrate that the orthogonal distance-based ray-tracer and Siddon ray-tracer using PSF image-space convolutions yield better images in terms of contrast and spatial resolution than those obtained after using the conventional method and the multiray-based S-RT. Furthermore, the Monte Carlo-based method yields slight improvements in terms of contrast and spatial resolution with respect to these geometrical projectors. CONCLUSIONS: The orthogonal distance-based ray-tracer and Siddon ray-tracer using PSF image-space convolutions represent satisfactory alternatives to factorizing the system matrix or to the conventional on-the-fly ray-tracing methods for list-mode reconstruction, where an extensive modeling based on Monte Carlo simulations is unfeasible.

Accuracy of the initial endoscopic diagnosis in the discrimination of gastric ulcers: is endoscopic follow-up study always needed?

BACKGROUND: Endoscopic follow-up study of gastric ulcers has been recommended routinely because of the possibility that a gastric neoplasm will be missed in the initial endoscopy. Some authors, most of them reporting data from areas of low gastric carcinoma incidence, have questioned this policy because of the low numbers of curable cancers detected and the high cost of such a program. GOALS: To assess the accuracy of endoscopy diagnosis of gastric ulcers, and to evaluate the efficacy and cost of a gastric ulcer follow-up endoscopic program in an area with an intermediate incidence rate of gastric cancer. STUDY: A retrospective study was used to identify all the gastroscopies in which a gastric ulcer had been diagnosed during a 6-year period. The endoscopic impression was compared with the histologic diagnosis, sensitivity, specificity, positive and negative predictive values, and the likelihood ratio. Patients who completed a follow-up program also were reviewed. For each neoplasm discovered, the number of endoscopies and global cost were calculated. RESULTS: In the 741 gastroscopies performed, 547 gastric ulcers were diagnosed in 529 patients. Biopsies were taken in 330 patients, in whom 341 gastric ulcers were found. At the index endoscopy, 41 gastric neoplasms (12.4%) were diagnosed. The accuracy of endoscopic malignancy diagnosis was as follows: positive predictive value of 0.68, negative predictive value of 0.98, sensitivity of 0.82, and specificity of 0.95. The likelihood ratio was 16. A total of 117 patients completed the follow-up program. Three new cases of gastric cancer (2.6%) were identified. In these three cases, the initial opinion of the endoscopist was uncertain. In the authors' experience, the cost of each gastric cancer diagnosed has been $4.653 (U.S. dollars). CONCLUSIONS: The endoscopic impression correlates with the histologic diagnosis even in a area of intermediate gastric cancer incidence. Endoscopic follow-up study may be restricted to cases of uncertain or malignant endoscopic impression.