|ECR 2019 / C-2411||
|An investigation on the appropriateness of Exposure Index (EI) calibration and Exposure Index target (EIT) values as a method of ensuring diagnostic image quality and optimum patient exposures in Digital Radiography.|
The initial dose audit revealed exposures for pelvis and lateral lumbar spines were above the DRL in the hybrid systems. Image optimisation was implemented whereby the “10kVp rule” was employed. Pre-programmed automatic exposure control (AEC) exposures for pelvis and lateral lumbar spines on the hybrid units were altered by raising the kVp by 10 kVp and halving the mAs to achieve the required density, hence reducing the exposure to the patient.
The second dose audit performed after recalibration of EI values revealed exposures for pelvis examinations were above the DRL in Manufacturer 2. After manually reviewing the images, it was ascertained that the examinations were performed using the AEC. To account for these higher doses than expected it is possible that the patients were not fully positioned over the AEC, in addition the x-ray field sizes lacked sufficient collimation which may have impacted on the dose received. Further audit of exposures is ongoing in these x-ray rooms to investigate use of collimation and DAP’s received.
Lateral lumbar spine exposure in the hybrid systems still exceeded the DRL, triggering an additional paper audit of lumbar spine exposures which is ongoing.
As a result of this investigation, all 8 DR rooms have EI now correctly calibrated and all EITs are inputted into the organ programs, so in theory, radiographers should be able to use DI as an indicator of correct exposure.
Herrmann et al, believe errors during exposure field recognition can cause inaccurate DI readings, which vary among manufacturers.
Radiographers must be aware that EI is not a measure of radiation dose to the patient but a method of tracking a pre-determined satisfactory exposure to the detector .This would suggest that DI alone cannot be used by the radiographer to assess image quality but in conjunction with DAP in a correctly calibrated DR unit.
The current authors argue that it is important for radiographers to have a visual indicator that patients are correctly exposed, by checking the DAP against the DRL, in conjunction with the DI, this will avoid dose creep .
There are limitations to the IEC 62494-1 standard as the EI is calibrated in the factory at only one kVp value which is not reflective of clinical work.
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