Michael Weiss MD, Presenter: Nothing to Disclose

Aaron D. Sodickson MD, PhD, Abstract Co-Author: Consultant, Siemens AG Consultant, Bayer AG

To evaluate image quality and radiation dose of CT pulmonary angiography (CTPA) performed at reduced tube voltage and reduced contrast dose as a function of patient size.

We reviewed consecutive CTPA performed at 120 and 100 kVp between 9/1/2010 and 10/31/2010. All scans were performed on a 128-slice Siemens AS+ scanner using automated tube current modulation (CareDose4D) with quality reference mAs of 200 and IV contrast concentration of 370 mg I/mL followed by a saline flush. After excluding 41 scans not performed according to protocol, this yielded 99 patients scanned at 120 kVp using 75 ml of contrast at 5 ml/sec and 53 patients weighing less than 175 lbs scanned at 100 kVp using 50 ml of contrast at 4 ml/sec. We recorded gender and age, and measured mean water-equivalent diameter at the level of the carina using a topogram analysis tool provided by Siemens. We measured signal and noise from axial images as the mean enhancement and standard deviation of a region of interest in the main pulmonary artery (MPA), and calculated the local CTDIvol from the kVp and mAs at the same level. Linear regression models were created for dependent variables CTDIvol, signal, noise, and signal to noise ratio (SNR) as a function of the independent variables mean water equivalent diameter, age, gender, and kVp.

CTDIvol increased exponentially with patient size and was significantly lower (p<.0001) at 100 kVp compared to 120 kVp, with relative dose savings of 31% at all comparable sizes. MPA enhancement decreased with increasing patient size (p=.002), with a 95 HU increase (p<.0001) at 100 kVp relative to 120 kVp after controlling for other variables. Image noise was higher in the 100 kVp group (p<.0001), but there was no significant difference in SNR (p=0.99).

Relative to 120 kVp, 100 kVp CTPA allows simultaneous reduction of radiation dose by 31% and IV contrast dose by 33%, while maintaining image quality. The maximum patient size for which low kVp imaging can be performed is determined solely by the maximum available mAs that can be delivered by the CT scanner. 100 kVp is thus recommended in all patients that do not exceed tube current limits.

In patients small enough for the scanner to produce the required x-ray tube current, 100 kVp CTPA allows a reduction in radiation dose and IV contrast dose with maintained image quality.

Weiss, M, Sodickson, A, Effects of Patient Size on Radiation Dose Reduction and Image Quality in Low kVp CT Pulmonary Angiography Performed with Reduced IV Contrast Dose  .  Radiological Society of North America 2011 Scientific Assembly and Annual Meeting, November 26 - December 2, 2011 ,Chicago IL. http://archive.rsna.org/2011/11008470.html