The main advantages of stable Xe/CT is that it noninvasively provides rapid access to high-resolution, quantitative, local CBF information coupled to CT anatomy. The information obtained is valid even in disease states because a partition coefficient is directly calculated for each voxel as small as 1 x 1 x 5 mm3. CBF studies can be repeated within 20 minutes, allowing the assessment of hemodynamic states. The technology can be incorporated into all existing CT technology at relatively little expense. The disadvantages are the radiation dose from the CT scanner, the pharmacologic effects of xenon gas, and the limitations of survey. The radiation dose is significant but it is focused to the least radiation-sensitive region of the body (scalp, skull, and brain) so that the effective cancer causation risk is lower than any other CBF study that delivers isotope indescriminatly to the body. The pharmacologic properties of xenon are associated with several potential problems. Xenon alters the sensorium of many individuals and may cause patient motion. This can be minimized with careful prestudy positioning and reassurance of the patient during the study. Most patients find the sensation enjoyable; however, occasionally, a patient cannot tolerate the perception of "losing control" and the study has to be stopped. One of the concerns regarding Xe/CT is that at the 80% level, xenon has anesthetic properties. At less than 33% xenon (with the difference being oxygen), the side effects have, in fact, been few and transient, with a mild hyperesthetic and disassociated feeling being the rule. Although xenon-induced apnea was reported as a consequence of xenon inhalation, the report involved the inhalation of 100% xenon, and at the levels recommended for imaging, holding one's breath for more than 20 seconds is rare. The other concern is the degree of flow activation that occurs with xenon inhalation and its effect on the accuracy of the measurements. In part, because of the "robust" nature of the Kety-Schmidt equations and because a significant xenon-induced flow activation is delayed for about 2 minutes, the effect of the flow activation is, in fact, minimal (less than 5% elevation of calculated flow values from true values). Although an earlier study by Obrist36 suggested that the error induced by flow activation was significant, a recent computer simulation study by the same author identified a maximal theoretical error of less than 5%. Previously, Xe/CT studies were limited to three brain levels, but with the new helical CT scanners, nine levels can now be studied. Most of the concerns over the technology have been resolved (Table 1). As a result, more clinicians around the world are beginning to use this technology when they need rapid access to quantitative CBF information about their patients.