Feasibility of low contrast volume and low injection flow rate in CT pulmonary angiography

Background Computed Tomography Pulmonary Angiography (CTPA) is currently the gold standard for diagnosing Pulmonary Embolism (PE), with a high flowrate (>4.5ml/s) for contrast media (CM) administration recommended for sufficient pulmonary artery opacification. However, this may not be achievable for patients with challenging IV access. Aim To determine if a low volume CM, low flowrate (LVLF) CTPA protocol produces images of similar image quality compared to a standard protocol in two aspects, in terms of peak arterial enhancement through the quantitative measurement of Hounsfield unit (HU) and based on subjective overall image quality. Methods Retrospective collection of 151 patients who underwent CTPA via 320 slice multi-detector CT due to clinical suspicion of PE. 80 patients underwent the standard protocol, with a fixed flowrate of 4.5ml/s and 50ml of CM, while 71 patients underwent the LVLF protocol with up to a 37% and 30% reduction in flowrate and CM administered, respectively. Two independent radiographers measured the attenuation of multiple pulmonary arteries in HU, with ≥200HU being considered diagnostic. Overall image quality was also reviewed using a 5-point close-ended questionnaire by two independent radiologists. Results There was no significant difference in terms of attenuation measured in HU for the seven regions of interest (main pulmonary trunk, right and left pulmonary arteries, right and left lobar arteries, and right and left subsegmental arteries (RSA and LSA)) between the LVLF and standard CTPA protocol. Similarly, there were no significant differences in the overall image quality score obtained from standard and LVLF protocols reported by both radiologists. Conclusion The LVLF protocol can achieve similar enhancement and subjective image quality as the standard CTPA protocol, potentially allowing for further optimisation in the CM dosage.