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Reduced-Dose CT: Effect on Reader Evaluation in Detection of Pulmonary Embolism

¹ Department of Radiology, Brigham and Women's Hospital, Boston, MA.
² Present address: Department of Radiology, Lucile Packard Children's Hospital, 725 Welch Rd., Stanford, CA 94305H5654.
³ Present address: Department Radiology, Baylor College of Medicine, Houston, TX.
⁴ Department of Biostatistics, Harvard School of Public Health, Boston, MA.

OBJECTIVE. The purpose of this study was to evaluate the effect of reduction in radiation dose on CT detection of pulmonary embolism.

SUBJECTS AND METHODS. Emergency department patients were evaluated for pulmonary embolism with standard and simulated reduced-dose CT angiography. Simulated lower-dose CT angiograms obtained at 90, 45, 22, and 10 mAs_eff were reconstructed by mathematical addition of noise to the standard dose (180 mAs_eff) data from the images of 18 patients with and 20 patients without pulmonary embolism. Four radiologists blinded to the study parameters separately interpreted each CT angiogram. Dose trends for subjective measures (diagnostic certainty, image quality, and perceived technical limitations) were evaluated, test characteristics for the detection of pulmonary embolism were computed, and clot burden was measured.

RESULTS. Readers indicated significant reductions in diagnostic certainty (p < 0.02) and image quality (p < 0.02) and an increase in perceived technical limitations (p < 0.01) as the simulated radiation dose was decreased. These subjective measures also showed significant adverse dose trends when the mAs_eff was reduced (p < 0.001). At reduced radiation doses, the sensitivity and positive predictive value for detection of pulmonary embolism diminished significantly. The sensitivity was 0.94 (lower bound of 0.95 CI, 0.92); specificity, 0.99 (lower bound of 0.95 CI, 0.98); positive predictive value, 0.95 (lower bound of 0.95 CI, 0.92); and negative predictive value, 0.99 (lower bound of 0.95 CI, 0.97). All patients had a low to moderate clot burden.

CONCLUSION. Reduction in dose for CT angiography in the detection of pulmonary embolism has a significant adverse effect on readers' subjective assessment of diagnostic confidence and image quality. Detection of pulmonary embolism also decreases as the tube current dose is reduced.

Keywords: CT • CT angiography • pulmonary artery • pulmonary embolism • radiation dosage

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