Radiation Dose Reduction at Low Tube Voltage CCTA Based on the CNR Index.

RATIONALE AND OBJECTIVES: We compared the radiation dose and diagnostic accuracy on 120- and 100-kVp coronary computed tomography angiography (CCTA) scans whose contrast-to-noise ratio (CNR) was the same. MATERIALS AND METHODS: We studied 1311 coronary artery segments from 100 patients. For 120-kVp scans, the targeted image level was set at 25 Hounsfield units (HU). For 100-kVp scans, the targeted noise level was set at 30 HU to obtain the same CNR as at 120 kVp. We compared the CNR and the radiation dose on scans acquired at 120 and 100 kVp. Invasive coronary angiography (ICA) images were evaluated by an interventional coronary angiography specialist, and CCTA images were evaluated by a radiologist. Coronary artery disease was defined as a luminal narrowing ≧50% for ICA and CCTA. With ICA considered the gold standard, the diagnostic accuracy (sensitivity, specificity, positive predictive value, and negative predictive value) was analyzed on both 120- and 100-kVp CCTA images. We also compared the diagnostic accuracy for area under the receiver operating characteristic curve of the ICA and CCTA performed at 120 and 100 kVp. Two blinded observers visually evaluated the septal branch. RESULTS: The mean dose-length product was 48% lower at 100 kVp than at 120 kVp (P < .01). Under the 120-kVp CCTA protocol, the area under the curve, 95% confidence interval, sensitivity, specificity, positive predictive value, and negative predictive value were 0.94%, 0.91%-0.96%, 94.0%, 93.0%, 82.3%, and 98.1%, respectively; at 100 kVp these values were 0.94%, 0.92%-0.97%, 96.1%, 92.0%, 85.2%, and 98.0%, respectively. Area under the receiver operating characteristic curve analysis revealed no significant difference in diagnostic accuracy between the two protocols (P = .87). CONCLUSIONS: At the same CNR, the 100-kVp CCTA protocol may help to reduce the radiation dose by approximately 50% compared to the 120-kVp protocol without degradation of diagnostic accuracy.

[Optimization of the chest computed tomography scan by varying the position of the arms].

Computed tomography automatic exposure control (CT-AEC) technique is calculated from a localizer radiograph. When we perform neck and chest CT examination, at first, we acquire localizer radiograph and neck images by placing the arm in a lowered position. Next, the arm is raised for the chest scan. Therefore, the localizer radiograph and subject information are different in the chest scan. In this situation, the chest scan with the use of the CT-AEC causes radiation over-dose. The purpose of this study is to optimize the CT-AEC by controlling noise index (NI), and make a chest CT scan condition considering the position of the arms. We measured the image noise (SD) in the phantom by using CT-AEC. In addition, dose length product (DLP) was recorded. Moreover, we examined the correlation with the clinical images. The results of our experiments show that radiation dose can be reduced with the image quality kept by controlling NI.