The influence of patient-specific factors on the exposure index
In diagnostic digital radiology, the exposure index (EI) is a decisive parameter as it reflects the balance between the radiation emitted and the image quality achieved. The EI is not a direct measure of the patient dose, but helps to ensure an adequately high image quality with the lowest possible radiation dose. However, numerous factors influence the EI. These factors include patient-specific variables such as body mass index (BMI), age and gender as well as the presence of implants and prostheses.1
High body mass index: adjustments to the exposure parameters
In most cases, individuals with a higher BMI require a higher radiation dose. In particular, if the proportion of fatty tissue is very high, the attenuation of the X-ray beam can lead to increased noise and motion artifacts due to longer exposure times.2 As a rule, overweight or obese individuals therefore require higher kilovolt peak (kVp) and milliampere-second (mAs) settings, which increases the overall EI but also leads to higher radiation exposure for the patient.3 This is particularly true for areas of the body with higher tissue density, such as the abdomen or pelvis.
TThis is also confirmed by studies: A recent study of 805 patients (chest X-ray examination) clearly showed that the EI, defined as the amount of X-ray photons entering the image receptor, typically decreases with increasing BMI, as the rays are absorbed or scattered more strongly by the thicker tissue.4 To compensate for this, the corresponding exposure parameters must be adjusted.
Accordingly, further studies demonstrated that in digital X-ray examinations of the chest, abdomen, lumbar spine, kidneys, bladder, and pelvis, the radiation doses used to achieve optimal image quality were significantly higher in overweight and obese individuals than in individuals of normal weight.5 Another study from 2022 came to the same conclusions for X-ray examinations of the chest, lumbar spine, or pelvis, but not for knee and shoulder images.6 The influence of BMI on EI therefore requires an adjustment of radiographic parameters in order to achieve optimal image quality despite differences in body type and composition.
Age and gender: influence of body composition and tissue density
Age and gender also influence the EI, but presumably in a more indirect way. This is because an age-related decrease in bone density and other changes in tissue composition as well as gender-specific differences in the distribution of fat and muscle mass influence the absorption behavior of the body.1 For example, a study from 2023 found a trend towards a decrease in EI values for chest X-ray examinations with increasing age of the patients, although this negative correlation between age and EI was only weakly pronounced.7 However, the increased radiation sensitivity in old age must be taken into account here,8 so that the overall significance of age for determining suitable exposure parameters is still unclear.
The influence of gender-specific differences on the EI has also not yet been conclusively clarified. For example, one study found no significant gender-specific difference in EI for chest X-ray examinations,7 whereas other studies found significantly higher median EI values in women than in male patients for the same examination.4,9 Conversely, male patients showed significantly higher EI values than female patients for abdominal examinations.9 Certainly, such different findings may also be due to the underlying radiographic technique, but these EI variations emphasize the need for further research efforts to better understand the direct influence of different body compositions and tissue densities on EI.
Prostheses and implants: Reduction of the exposure index
The presence of prostheses or implants in the examination area can significantly reduce the EI value.9 Reduced EI values could result from an influence on the automatic EI processing, as the changed radiation absorption due to the high-density prosthesis or implant material can interfere with the automatic anatomical segmentation and lead to lower pixel values.9 Implants and prostheses are typically made of high-density materials such as titanium, cobalt-chromium alloys or stainless steel, which absorb X-rays much more strongly than soft tissue or bone. The strong absorption reduces the radiation reaching the detector, which directly affects the EI.
Conclusion
The patient-specific factors that influence EI in digital radiography are therefore relatively complex, but crucial for optimizing image quality while minimizing radiation exposure in accordance with the ALARA principle (as low as reasonably achievable). Although the wide dynamic range of digital X-ray systems offers a certain tolerance with regard to exposure variations, patient-specific adaptation of the examination parameters remains essential.
Sources
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2Uppot RN. Technical challenges of imaging & image-guided interventions in obese patients. Br J Radiol 2018,91, 20170931.
3Yanch JC, et al. Increased radiation dose to overweight and obese patients from radiographic examinations. Radiology 2009,252, 128-39.
4Soulis P, et al. "Exposing" the exposure index: Navigating between image quality and radiation dose. J Med Imaging Radiat Sci 2024,55, 101766.
5Metaxas VI, et al. Patient dose in diigital radiography utilising BMI classification. Radiat Prot Dosimetry 2019,184, 155-67.
6Dolenc L, et al. The impact of body mass index on patient radiation dose in general radiography. J Radiol Prot 2022,42, 041505.
7Poudel SS, et al. Evaluation of Exposure Indices on PA Chest X-Ray in Direct Digital Radiography. J Radiol Med Imaging 2023,6, 1092.
8Hernández L, et al. Aging and radiation: bad companions. Aging Cell 2015,14, 153-61.
9Mothiram U, et al. Retrospective evaluation of exposure index (EI) values from plain radiographs reveals important considerations for quality improvement. J Med Radiat Sci 2013,60, 115-22.
