The majority of our evaluations will be done using this window setting. Sinus spaces are also nicely delineated on bone windows because of the contrast between air (black) and all other tissues (gray shades).īrain windows are useful for evaluation of brain hemorrhage, fluid-filled structures including blood vessels and ventricles, and air-filled spaces. Air remains black on bone windows and can be readily identified-for example, intracranial air can easily be seen on bone window settings. At the same time, they sacrifice detailed evaluation of structures less dense than bone (brain, cerebrospinal fluid, and blood vessels). By shifting the gray scale to center on the range of densities typical of bone bone windows allow detection of abnormalities such as subtle fracture lines. In general, head CT images are viewed on brain or bone windows to allow most emergency pathology to be assessed (see Figure 1-2).īone windows are useful for evaluation in the setting of trauma. The same image data can be displayed in different window settings to allow evaluation of injury to different tissues. Instead, the tissues of interest are highlighted by devoting the visible gray shades to a narrow portion of the full density range, a process called “windowing” ( Figures 1-1 and 1-2). Because the human eye can perceive only a limited number of gray shades, the full range of density values is typically not displayed for a given image. By convention, low-density tissues are assigned darker (blacker) colors and high-density structures are assigned brighter (whiter) colors. The density of a tissue is represented using the Hounsfield scale, with water having a value of zero Hounsfield units (HU), tissues denser than water having positive values, and tissues less dense than water having negative values ( Figure 1-1). Joshua Broder MD, FACEP, Robert Preston MD, in Diagnostic Imaging for the Emergency Physician, 2011 Hounsfield Units and Windows
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