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

BACKGROUND

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

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

METHODS

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.

RESULTS

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 EIT.

CONCLUSIONS

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 EIT values on individual x-ray units and normalizing EI value distributions for quality assurance purposes.