Liver CT Perfusion: which is the relevant delay that reduces radiation dose and maintains diagnostic accuracy?

Objectives: High radiation dose during CT perfusion (CTp) studies contributes to prevent CTp application in daily clinical practice. This work evaluates the consequences of scan delay on perfusion parameters and provides guidelines to help reducing the radiation dose by choosing the most appropriate delay. Methods: 59 patients (34 men, 25 women; mean age 68±12) with colorectal cancer, without underlying liver disease, underwent liver CTp, with the acquisition starting simultaneously with iodinated contrast agent injection. Blood flow (BF) and hepatic perfusion index (HPI) were computed on the acquired examinations, and compared with those of the same examinations when a variable scan delay (τ) is introduced. Dose length product, CT dose index, and effective dose were also computed on original and delayed examinations. Results: Altogether, three groups of delays (τ≤4s, 5s≤τ≤9s, τ≥10s) were identified, yielding increasing radiation dose saving (RDS) (RDS≤9.5%, 11.9%≤RDS≤21.4%, RDS≥23.8%) and decreasing perfusion accuracy (high (τ≤4s), medium (5s≤τ≤9s), low (τ≥10s)). In particular, single-input and arterial BF, and HPI were more insensitive to delay as regards the absolute variations (only 1ml/min/100g and 1%, respectively, for τ≤9s), than portal and total BF. Conclusion: Using delays lower than 4s does not change perfusion accuracy and convey unnecessary dose to patients. Conversely, starting the acquisition 9s after contrast agent injection yields a RDS of about 21%, with no significant losses in perfusion accuracy.