Tight collimation substantially increases air kerma rate and decreases image contrast during selected fluoroscopy...

International fluoroscopy guidance recommends tight collimation to optimise dose and image quality. This study found that, in system- and protocol-dependent conditions, clinically realistic collimation can substantially increase air kerma rate (AKR) and reduce image contrast. The underlying mechanism is partial coverage of the automatic dose rate control (ADRC) measuring region by collimation shutters.

Collimation was tightened on Philips Azurion and Allura angiography platforms using adult and paediatric phantoms. Collimated field size ranged from open 28 × 36 cm field to 1 cm in one axis. Clinically used field sizes were extracted from dose-management records, and the measured AKR response within these ranges was used to assess clinical relevance. Contrast was measured with a copper phantom (0.00-2.30 mm Cu, 3.0-17.5 cm PMMA). Staff dose rate was measured with dosimeters around the patient table.

At the mean clinically used field dimensions, AKR increased by 70% (adult) and 160% (paediatric) versus open field. In the smallest clinical fields AKR increased up to 300% and 850% respectively. Contrast decreased to one-third when 75% of the ADRC measuring region was covered, and 0.30 mm Cu became indistinguishable from 0.00 mm. Collimation reduced staff dose rate, but less than expected because of the AKR rise.

Without ADRC measuring region visualisation or coverage warnings, operators may not realise when ADRC feedback is unreliable. Adherence to collimation recommendations can inadvertently prevent adequate optimisation of radiation exposure and image quality. Collimation remains beneficial when the ADRC measuring region is fully inside the field.