Myth Buster: “Dose Increases with CR & DR System Grid Alignment”

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David H. Foos, Clinical Applications Research, Carestream

In recent conversations with clinical colleagues, the comment was made that “our sickest patients get the worst imaging.” On further investigation I found that in portable chest imaging, anti-scatter grids are infrequently used, despite their ability to improve image quality. To understand this situation better, a team from Carestream Health and Mount Sinai Hospital collaborated on a line of research that became the subject of a joint scientific poster at the recent United Kingdom Radiological Congress (UKRC) 2012. The poster, “The Bucky stops here: redefining the Bucky factor for digital portable chest radiography,” summarized what our investigation found: quality of digital portable chest radiographs can be improved for a large fraction of intensive care (ICU) patients by using a grid, and without increasing patient dose.

To set the context, our research team has been systematically optimizing exposure and image processing techniques for CR and DR systems, with the objective of recommending optimal techniques for different exam types that deliver diagnostic quality imagery at the lowest possible dose to the patient. With this goal in mind, we wanted to understand whether grids could be used more generally for ICU chest imaging with digital radiography systems.

Through discussions with practitioners at a variety of hospitals, we found that there is ostensibly a general acceptance that portable chest radiographs will simply have lower quality. In addition, there are workflow complexities when using grids for portable chest imaging, perhaps with the key issue being the difficulty of grid alignment. We also found that from the perspective of dose, the Bucky factor, defined as the reciprocal of the total attenuation of X-rays by the grid, seemingly continues to be the general basis for making upward technique adjustments when using grids with digital systems. These technique adjustments can be as large as X2 to X5, depending on grid characteristics and patient thicknesses. We suspected that these patient dose increases might be unwarranted when using a digital system.

Because the technique increase associated with the Bucky factor is a consequence of a screen-film imaging paradigm, we decided to take a closer look at its relevance for digital systems. Historically, when a grid was used in combination with a screen film-system, the sensitometric response of film required an increase in technique corresponding to the total X-ray attenuation from the grid, in order to produce a film having diagnostic quality density and contrast. This requirement doesn’t exist to the same degree for digital systems, where image brightness and contrast are determined by image processing, and where image quality is limited by signal to noise ratio (SNR). Interestingly, it can be shown that because grids preferentially transmit primary versus scattered radiation, using a grid with a digital system actually has the potential to improve SNR in certain situations.

To determine the relevance of the Bucky factor in digital imaging, we explored image quality differences between digital portable chest X-ray images that were captured of ICU patients with and without a grid. The images captured with the grid were acquired using the same standard exposure techniques as were used for the corresponding images captured without the grid. We used two different approaches to quantify the image quality differences between the grid and non-grid images.

We found that image quality was improved for 80% of patients when their images were captured with the grid without increasing the patient dose.

The chest images shown in the example below qualitatively demonstrate the image quality improvement obtained by use of a grid. It should be kept in mind that the appropriate acquisition techniques, when using digital systems, depends on many factors including the image quality performance of the detector, grid performance, the scattering characteristics of the patient, imaging geometry, image processing, and the degree to which grid alignment can be assured. Appropriate choice of the relevant “digital” Bucky factor is necessary to obtain the best quality diagnostic image at the lowest possible patient dose.

It was an honor to learn last week that our work received validation from The Institute of Physics and Engineering in Medicine, which awarded the poster its President’s Prize at UKRC 2012. We believe the results from this work, coupled with the newly available grid alignment feature with the Carestream DRX Revolution System, will help promote increased grid usage in digital portable chest X-ray imaging.  This should result in an associated improvement in portable chest X-ray image quality without requiring the traditional increase in patient dose.

Are you using an anti-scatter grid for portable chest radiography? Why not?