Comfort pads and incubator design impact radiation dose of neonates
It is common practice to take a chest x-ray of a neonate within an incubator if the incubator has a built-in tray or slot where the image detector can be inserted. Not needing to move a fragile infant reduces the chance for accidental displacement of catheters, endotracheal tubes, and/or other support devices. However, this safety precaution has a worrisome tradeoff: an increased radiation dose. An article published in the January issue of Pediatric Radiology is an alert that imaging a neonate inside an incubator should be reconsidered.
Researchers from the Department of Radiology at University of Chicago Medicine conducted a study to assess the effect of comfort pads and support trays on detector entrance exposure (DEE) and image quality for neonatal radiography and to assess the implications of their findings with respect to radiation dose. They discovered that comfort pads and tray support devices can have a considerable effect on DEE and image quality, and that there are large variations among different incubator designs.
Three incubator and warming systems from the same manufacturer were evaluated. These varied in design, thickness, and the type of material used. A 5 cm thick uniform acrylic phantom represented a typical neonatal patient, and thin aluminum inserts were used to investigate the change in image quality.
Attenuation of the primary beam
A narrow beam geometry was used to minimize the effect of scatter to measure the attenuation of the primary beam by the comfort pads. The researchers measured exposure first with the phantom alone, then with a support tray and comfort pad inserted into the beam’s path. They used the ratio of the two exposures to calculate the attenuation. Attenuation ranged from 6% to 15% at 60 kVp depending upon the comfort pad used. Two support trays made of 3-mm acrylic plastic resulted in a 10% attenuation at 60 kVp, whereas a thicker and denser support tray resulted in a 45% attenuation. Because the amount of attenuation did not correspond well with the thickness or the area density of the pads, the research team believes that the padding materials had varying radiolucency.
Impact on detector entrance exposure
A broad beam geometry that included the effect of scatter was used to measure the change in DEE due to the presence of the comfort pads and support trays. Three different geometric configurations representing different clinical imaging options for neonatal chest radiography were investigated. Removing the comfort pad alone in each of the incubators increased DEE by 17% to 22% over the kVp range investigated. When both the comfort pad and the support tray were removed, DEE increased by 28% to 36% for two of the incubators, and more than doubled with the incubator that had a dense support tray.
Changes in image quality
Contrast-to-noise ratio (CNR) was used to quantify changes in image quality. When comfort pads were removed, the CNR increased by 16%, 15%, and 12% and with the additional removal of the support trays, by 23%, 85% (the incubator with the dense support tray), and 21% for the three systems respectively.
Lead author Xia Jiang, a medical physicist, and colleagues estimated that 22% to 31% patient dose reduction could potentially be achieved by removing comfort pads and support trays based on the CNR results. They pointed out that comfort pads and support trays not only attenuate the primary beam but also cause increased scatter that further lowers the image quality. They also noted that the use of comfort pads may create image artifacts from nonuniformity in the padding, and that the use of a tray makes it more difficult to align the beam and the patient with the image detector, increasing the chance of anatomy cutoff.
The authors recommend that if a neonate can be moved and an image detector can be placed directly under the infant at minimal clinical risk, this option should be considered. They caution that this decision should be made for each patient based on the neonate’s clinical condition. Pediatric neonatologists and pediatric radiologists are developing a decision tree that takes into consideration the various clinical and physical conditions of the patient that will help technologists and nurses to make this assessment.
They also have a recommendation for incubator manufacturers: to design products that have lower attenuation material, more uniform padding, and external guidance for beam centering to create a dedicated imaging plate slot supportive of lower radiation dose exposure to the occupants of incubators.
REFERENCE
- Jiang X, Baad M, Reiser I, et al. Effect of comfort pads and incubator design on neonatal radiography. 2016 Pediatr Radiol 46;1: 112-118