CTPA protocol optimisation audit: challenges of dose reduction with maintained image quality

To assess computed tomography (CT) pulmonary angiography (CTPA) dose and image quality in a large teaching hospital, and subsequently, to optimise the protocol in order to reduce the dose without affecting image quality.

Dose-length product (DLP), patient size, and objective quality parameters (contrast-to-noise ratio and signal-to-noise ratio on standardised levels) were recorded from 31 patients undergoing CTPA, where also a subjective image quality evaluation was carried out independently by three specialist cardiothoracic consultant radiologists. An equivalent objective and subjective quality assessment was carried out on a cohort of the same size in a different tertiary healthcare centre. Moreover, experimental tests using anthropomorphic chest phantoms were performed, using different scan parameters. In light of the above analysis, two of the scanner settings for CTPA were modified, i.e., the SureExposure pre-set was changed to "Standard" noise level, quantified with standard deviation (SD) of 19, and the minimum amperage setting lowered from 80 to 40 mA. A second cohort of patients using this new protocol was audited, following the same methodology.

The average DLP of patients undergoing CTPA was initially found to be higher than both local and national dose reference levels (DRLs; 559 versus 300 mGy·cm and 400 mGy·cm, respectively). The new protocol led to a reduction in average DLP (359 mGy·cm) while the image quality, assessed by three cardiothoracic consultant radiologists, was preserved.

The CTPA protocol was implemented in the Royal Infirmary of Edinburgh resulting in significant dose reduction, and is now compliant with national and local DRLs. The image quality was maintained.