Improved Image Quality of Low-Dose CT Pulmonary Angiograms

The use of CT pulmonary angiography (CTPA) to evaluate for pulmonary embolism has been increasing, and carries a significant radiation dose. We evaluate image quality of lower-dose images, taking into account patient size as well as the effects of image postprocessing.

A total of 250 CTPAs were retrospectively reviewed. The following parameters were obtained: kVp, mA, dose length product, Hounsfield units (HU) with standard deviation in the main pulmonary artery, transverse scout measurement, and subjective image quality.

Radiation dose decreased 55% by reducing kVp from 120 to 100, and 60% from 100 to 80 kVp. Radiation dose decreased 82% from 120 to 80 kVp. Noise increased 38% from 120 kVp to 100 kVp, and increased 23% from 100 kVp to 80 kVp. Adding an overlapped reconstructed image decreased noise by 16% to 21%. Despite the increase in image noise, diagnostic quality was significantly improved at 80 and 100 kVp, compared with 120 kVp, with an average subjective quality rating of 3.8, 4.0, and 3.2, respectively, and an average pulmonary artery density of 536, 423, and 278 HU. Even in larger patients, qualitative image quality was better at 100 kVp compared with 120 kVp, with an average quality rating of 3.6 versus 2.9, respectively.

Radiation dose exposure can be easily reduced on CTPA by lowering kVp, which at the same time improves image quality. Studies using a lower kVp were of significantly higher diagnostic quality. This held true even in larger patients.