Effects of patient size on radiation dose reduction and image quality in low-kVp CT pulmonary angiography performed with reduced IV contrast dose

The purpose of the study is to evaluate image quality and radiation exposure as a function of patient size for CT pulmonary angiography (CTPA) performed at reduced tube voltage and reduced intravenous (IV) contrast dose. We reviewed consecutive CTPAs performed between 9/1/2010 and 10/31/2010 on a 128-slice Siemens AS+ scanner using automated tube current modulation with quality reference mAs 200 and IV contrast concentration 370 mg I/ml followed by a saline flush: 99 scans at 120 kVp using 75 ml of contrast at 5 ml/s and 53 scans on patients lighter than 175 lbs at 100 kVp using 50 ml of contrast at 4 ml/s. We measured patient size (mean water-equivalent diameter) using a topogram analysis tool, signal (mean CT density) and noise (standard deviation) in the main pulmonary artery (MPA) on axial images, and calculated local CTDI_vol from the kVp and mAs. Linear regression models were created for dependent variables ln(CTDI_vol), signal, noise, and signal to noise ratio (SNR) as a function of independent variables size, age, gender, and kVp. After controlling for other variables, scanning at 100 kVp yielded CTDI_vol reduction of 33 % (p < 0.0001), signal increase of 96 HU (p < 0.0001), and increased image noise (p < 0.0001), but without significant difference in SNR (p = 0.99). Relative to 120 kVp, 100-kVp CTPA allows simultaneous reduction of radiation exposure by 33 % and IV contrast dose by 33 % while maintaining image quality. Scanning at 100 kVp is recommended in all patients for whom the required mAs does not exceed maximum X-ray tube output.