Radiation exposure and image quality in chest CT examinations.

OBJECTIVE: The purpose of this study was to determine how changes in radiographic tube current affect patient dose and image quality in unenhanced chest CT examinations. SUBJECTS AND METHODS: Ten sets of CT images were obtained from patients undergoing CT-guided chest biopsies. For each patient, six images of the same region were obtained at settings between 40 and 280 mAs. CT data were used to reconstruct tomographic sections with a field of view limited to the normal contralateral lung. Images were printed using lung and mediastinal image display settings. Image quality was determined by asking radiologists to assess the perceived level of mottle in CT images. Five chest radiologists ranked the relative image quality of six images. Patient effective doses were computed for chest CT examinations performed at each milliampere-second setting. Radiologists indicated whether any perceived improvement of image quality at the higher radiation exposures was worth the additional radiation dose. RESULTS: The differences in quality of chest CT images generated at greater than or equal to 160 mAs were negligible. Reducing the radiographic technique factor below 160 mAs resulted in a perceptible reduction in image quality. Differences in CT image quality for radiographic techniques between 120 and 280 mAs were deemed to be insufficient to justify any additional patient exposure. However, the use of 40 mAs results in an inferior image quality that would justify increased patient exposure. CONCLUSION: Radiographic techniques for unenhanced chest CT examinations can be reduced from 280 to 120 mAs without compromising image quality.

Radiation doses to infants and adults undergoing head CT examinations.

For routine noncontrast CT examinations of the head, we compared the radiation doses of infant patients aged no more than two years old, with those of "adults" defined as any patient whose weight was greater than 40 kg. Data were obtained for 23 infants, and an equal number of "adults," who underwent CT head examinations between May 1997 and March 1998. Patient CT data acquired included the x-ray tube potential (kVp), mAs, section thickness, and total number of sections. For radiation dosimetry purposes, the head was modeled as a uniform cylinder of water using patient size data obtained from a representative cross-sectional image. CT techniques and patient size data permitted the computation of the mean section doses in the head region, total energy imparted, and the corresponding effective doses. All CT scans were performed at 120 kVp, with an average current-exposure time product of 271 +/- 73 and 340 +/- 0 mAs for infants and "adults," respectively. The radius of the water cylinder used to model the patient head increased from 58 mm for 4 kg newborns to 70 mm for 8 kg infants. For "adults," there was little correlation between the weight of the patient and the mean water equivalent radius of 88 mm (r2 = 0.14). Mean section doses were 44.4 +/- 11.1 mGy for infants, and 44.2 +/- 1.5 mGy for "adults." The energy imparted to infants correlated with patient weight (r2= 0.35) much more than did that of "adults" (r2= 0.02). The average infant energy imparted (66.4 +/- 28.7 mJ) was approximately half the value obtained for "adults" (140 +/- 10 mJ). The average effective dose to the infants (7.6 +/- 3.1 mSv), however, was approximately six times higher than that for "adults" (1.3 +/- 0.1 mSv). There was no significant correlation between patient effective dose and patient mass for either the infant (r2 = 0.12) or the "adult" group of patients (r2= 0.02). Infant doses varied much more than "adult" doses, primarily because of a wider range of x-ray technique factors selected and secondarily due to the variation in infant head size. The observed variability in the computed radiation dose parameters indicates that it should be possible to reduce infant doses routinely in head CT examinations without any adverse effect on diagnostic imaging performance. For such routine head CT scans, the average dose reduction for infants weighing between 4 and 8 kg would be expected to range between 40% and 60%.

Heparin-induced thrombocytopenia and thrombosis syndrome.

Heparin-induced thrombocytopenia and thrombosis syndrome (HITTS) is an immune-mediated response to the administration of heparin that results in life-threatening thrombosis. The pathophysiology of HITTS remains controversial. The onset of clinical symptoms and laboratory changes is usually delayed 1-2 weeks after exposure to heparin. Thrombosis occurs in both the arterial and venous circulation with significant morbidity and mortality. Complications include deep venous thrombosis, pulmonary embolus, stroke, myocardial infarction, chronic venous insufficiency, extremity ischemia, gangrene, and death. Diagnostic criteria for HITTS include thrombocytopenia during heparin exposure, exclusion of other causes such as sepsis or medications, resolution of thrombocytopenia after withdrawal of heparin, demonstration of in vitro heparin-dependent platelet antibodies, and development of vascular thrombosis. Despite having several disadvantages, the carbon-14-serotonin release assay is the most sensitive and specific test for HITTS. Angiography as an adjunct to other imaging modalities can document the presence, location, and extent of thrombus. Optimal treatment has not yet been defined but should include immediate discontinuation of use of all heparin products and heparin-coated catheters. In addition, alternate methods of antithrombotic therapy should be considered. In severe cases, thrombolysis or thrombectomy may be warranted. Familiarity with the pathophysiology, clinical manifestations, complications, diagnostic criteria, and treatment options associated with HITTS will enable timely recognition and facilitate prompt and effective treatment.