Question

relate the EI for this system and patient dose: what range or level of exposure index should be utilized in order to keep patient dose to a minimum with this system? (Radiology question)

Answer 1

Radiation does delivered to a patient is a necessary consequence of acquiring the x-ray images used to define anatomical and patho-physiological  process and make a diagnosis. The radiographic study should be optimized in terms of achieving necessary message quality at the lowest possible radiation does, in order to maximize the benefit-to-risk ratio. As long as the examination is appropriate, the benfi to an individual patient (to confirm or eliminate disease or trauma) will far outweigh the associated risk.

patient radiation does and the exposure index for digital radiographic detectors :

The does to the patient is determine by the x-ray technique factor (kv,mas,grid,sid, filtration, beam collimation), x-ray beam penetrability and quality, the amount of energy imparted to the body, and the size and area of the body irradiated. The exposure incident on the detector is determined by the remnant radiation ( primary radiation transmitted through the patient and scattered radiation transmitted from the patient) that is absorbed, converted to electronics signals and formatted into a digital radiographic image with a given detective quantum efficiency (DQE). The exposure index is a measure of the signal level produced by a digital detector for a given incident exposure transmitted through the patient is proportional to the signal-to-noise ratio squared (SNR2) and is related to image quality.

SO if a large patient is imaged on a digital detector through a thick body part, the x-ray technique must be adjusted upward to compensate for x-ray attenuation and to ensure a sufficient number of transmitted x-ray incident on the detector in order to achieve a desired exposure index value. This represents a relatively high does to the patient. Now if a thin patient is imaged on the same detector, the x-ray technique must be adjusted downward to achieve the same number of transmitted x-ray to the detector (for the same SNR2), with the begin the same exposure index. Thus with out knowledge of the x-ray acquisition techniques and patient habitus, it is not possible to directly calculate the patient does from the exposure index value. However digital radiography devices the ability to correct for under and overexposure conditions, the exposure index can give the technologist an idea of whether a proper radiography technique was used. this is extremely important in the effort to optimize radiographic studies and acquisition techniques, especially for children, in whom radiation sensitivity is relatively high.