Deriving target exposure indices for common radiography exams based on automatic exposures of flat-field phantoms

The IEC exposure index (EI), deviation index (DI), and target exposure index (EI_T), represent critical standardized metrics for the evaluation of exposure and quality in radiographic imaging.

This work develops and validates a systematic procedure to estimate the EI_T for eight of the most common radiography imaging protocols utilizing automatic exposure control (AEC) from measurements acquired under reference conditions.

A model was developed to define the relationship between a systems AEC logic, and an estimation of the EI under flat-field conditions (EIFFC). Separately, clinical data and acquisition protocol information for resultant EI during patient studies were also collected for the eight protocols studied: Chest posteroanterior (PA), Chest lateral, Abdomen anteroposterior (AP), Pelvis AP, L-Spine AP, C-Spine AP, T-Spine AP, and Ribs AP. Data were collected from 41 x-ray units spanning seven institutions. For each protocol on each unit the EIFFC was computed based on the acquisition protocol, as well as median EI from clinical exams to produce a scaling factor (SF). Kruskal-Wallis statistical tests were used to compare SF's between vendors and AEC cell configurations.

SFs for eight radiographic imaging protocols have been produced per vendor and per AEC cell selection. A workflow has been established for end-users to follow to apply these SFs to flat-field measurements taken at their own locations to establish local EI_T.

The study results show that in seven of the eight imaging protocols, the SFs for most units included in the study report SFs within 1 DI (± 25%) of their respective final vendor reported SF (218/228). The ribs protocol is the exception to this finding (n = 26). SFs have high utility for establishing EI_T values on individual x-ray units and normalizing EI value distributions for quality assurance purposes.