Several surface and volume rendering techniques are compared using nuclear medicine data including several new methods developed by the authors specifically for scintigraphic data. The techniques examined are summed projection, thresholded projection, threshold-based surface illumination, volumetric compositing, maximum-activity projection, sun-weighted maximum-activity projection, and variable attenuation. The advantages and disadvantages of each method are discussed in relation to the goals of three-dimensional display, which are defined herein. Selected images are shown to illustrate the usefulness of the methods.

The fetal radiation dose from 18 F-FDG was estimated in a series of pregnant women who underwent a PET scan during a clinical workup for malignancies. Methods: Six pregnant patients were injected with 18 F-FDG (activity range, 296-385 MBq). Three patients were scanned during the first trimester (1 with PET and 2 with PET/CT), 2 were scanned during the second trimester (with PET/MR imaging), and 1 was scanned during the third trimester (with PET). The time-integrated activity coefficients were derived from the fetal radioactivity concentrations measured on the images for all but 1 patient (in early pregnancy [5 wk]), in whom the activity in the uterus was used as a proxy. The coefficients of the mother's organs were derived from standard values (from the International Commission on Radiological Protection). Results: Fetal doses ranged from 6.29E-03 to 2.46E-02 mGy/MBq. An earlier bladder voiding reduced these doses by 25%-45%. The 2 patients who underwent PET/MR imaging-in whom fetal contours could be accurately delineated-displayed the lowest fetal absorbed dose, likely because of more accurate region drawing, with the inclusion of areas of both low and high fetal uptake. Moreover, PET/MR imaging did not necessitate additional radiation for attenuation correction. The placenta, delineated on a PET/MR imaging scan, concentrated 0.27% of the injected activity. Conclusion: Fetal radiation doses are higher in early pregnancy than in late pregnancy, and there can be considerable intersubject variability. However, the total absorbed dose is always well below the threshold for noncancer health effects throughout pregnancy. PET/MR imaging is the optimal PET procedure for imaging pregnant women because it is not associated with radiation for attenuation correction and allows more accurate dosimetric calculations.

Most adrenocortical carcinomas accumulate and retain FDG and thus can be visualized by PET. However, false-negative findings are possible, especially with very small lesions.

For implementations of iterative reconstruction algorithms that rotate the image matrix, the characteristics of the rotator may affect the reconstruction quality. Desirable qualities for the rotator include: 1) preservation of global and local image counts; 2) accurate count positioning; 3) a uniform and predictable amount of blurring due to the rotation. A new rotation method for iterative reconstruction is proposed which employs. Gaussian interpolation. This method was compared to standard rotation techniques and is shown to be superior to standard techniques when measured by these qualities. The computational cost was demonstrated to be only slightly more than bilinear interpolation.

This paper reports on experiments designed to identify and implement mechanisms for enhancing the explanation capabilities of reasoning programs for medical consultation. The goals of an explanation system are discussed, as is the additional knowledge needed to meet these goals in a medical domain. We have focussed on the generation of explanations that are appropriate for different types of system users. This task requires a knowledge of what is complex and what is important; it is further strengthened by a classification of the associations or causal mechanisms inherent in the inference rules. A causal representation can also be used to aid in refining a comprehensive knowledge base so that the reasoning and explanations are more adequate. We describe a prototype system which reasons from causal inference rules and generates explanations that are appropriate for the user.