Imaging 2 M2L1

• histogram: a graph of signal intensity values that corresponds to a spectrum of pixel values. Raw data is analyzed and put into a histogram
• Look Up Table: is an “ideal” histogram for every radiographic projection
• histogram: Graphical representation of signal intensity of pixels
• histogram X-axis = amount of exposure or pixel grey shade value
• histogram Y-axis = # of pixels for each grey value.
• histogram: Each pixel is assigned a number that represents the shade of grey. The number of pixels in each value is plotted on the histogram
• S min = brightest value
• s max = least bright value
• s avg is used to calculate exposure index number
• The peaks and valleys of the histogram graph represent subject contrast
• Each projection will have a different histogram shape. The histogram shape for each procedure should be consistent.
• LUT: provides standard for s min, s max and s avg that histogram is compared and rescaled to
• LUT: each anatomical part and project has its own LUT
• when adjustments to contrast and brightness are made the LUT assigns each pixel a new value
• CR system uses a data recognition program (segmentation) to determine the exposure field recognition.
• Segmentation (CR) will determine: Where the collimated borders are, the part orientation, # of projections/plate and Only information inside the collimated field is processed
• Exposure field recognition: This establishes the area of useful values for the VOI. Only the range of pixel values that are in the VOI will be analyzed and compared to the LUT
• S min , s max and s avg are established during exposure field recognition
• If exposure (histogram) is greater than or less than the ideal (LUT) amount, automatic rescaling occurs
• Automatic rescaling adjusts the provided pixel values so that they more closely match the LUT histogram values by applying a special algorithm. The values in the image histogram are always adjusted to match the values in the LUT
• If the image histogram is farther to the left then the LUT, then the rescaling will adjust the pixel values so that they move toward the right making each pixel darker
• If the image histogram is farther to the right then the LUT, then the rescaling will adjust the pixel values so that they move toward the left making the pixels brighter.
• moving to left will make image brighter
• moving to right will make image darker
• Rescaling for contrast means that the number of shades of grey in the image histogram needs to be adjusted to match the number of shades of grey in the LUT. values S min and S max are examined
• If the image histogram is wider than the LUT, then the algorithm will make the histogram narrower. A wide histogram means more shades of grey which results in low contrast
• Widening and narrowing the image histogram does not change the number of pixels in each value. Therefore, this process may result in a brighter image if inadequate kV is used. If too much scatter reaches the IR, then there will be more darker grey shades