Exposure index in digital radiography and its dependence on acquisition parameters, anatomy, and manufacturer

The exposure index (EI), the target exposure index (EI_T), and the deviation index (DI) have been defined in the IEC Standard 62494-1 Ed.1 2008-08. This study investigates the impact of certain acquisition parameters, the imaged anatomy, and the manufacturer's specificities on the EI of radiological images and how these may affect EI_T setting procedure.

Images were acquired using two digital radiography (DR) systems of two different manufacturers, using aluminum attenuators and an anthropomorphic phantom. Acquisition parameters like the tube potential (kVp), the tube loading (mAs), the exposure time, the automatic exposure control (AEC) system settings (sensor and dose level selection), the grid (with or without), the additional filtration, the field size, and the imaged anatomy were varied and their effect on the EI was quantified separately for each system.

EI is linearly related to the incident air kerma (IAK) on the detector as expected (by definition). For constant IAK, EI increases with increasing kVp. While EI in general is reduced in the presence of scatter, this may not always be the case. Under AEC operation, even the exposure time can make a difference. EI is strongly affected by the imaged anatomy in combination with the AEC sensor and field size selections, the examination protocol, and the manufacturer.

Many parameters affect the EI calculation apart from IAK. Among them, the most important are the imaged anatomy and the manufacturer. Since the EI calculation is a complex procedure, setting of the EI_T values should be done with caution on a per-examination and manufacturer basis, since the values that apply for one digital system are not always applicable to another. Furthermore, when EI is used as an image quality tool, a DI variation of at least ±2 should be allowed before a possibly meaningful red flag is activated.