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Improving Image Contrast

¹ Department of Radiology, Children's Hospital, 300 Longwood Ave., Boston, MA 02115.

View larger version (89K): [in a new window]   Fig. 1A. —Coronal sonogram of male neonate with postinfectious hydrocephalus. Low-contrast gray-scale sonographic image is shown on right. "Levels" tool is selected by clicking "Image" menu bar, then "Adjust," and then "Levels," as shown in upper left corner. Histogram located in lower left of figure shows distribution of pixel densities in sonographic image. Highest (brightest) pixel value is depicted by light gray arrowhead to right of histogram, lowest (darkest) value is depicted by dark gray arrowhead to left of histogram, and middle gray is depicted by medium gray arrowhead near center of histogram. Outermost markers are located beyond the brightest and darkest pixels in image.

View larger version (79K): [in a new window]   Fig. 1B. —Coronal sonogram of male neonate with postinfectious hydrocephalus. After modification using "Levels" tool, image shows relocation of outermost arrowhead markers to edges of histogram and midpoint marker to left of center of histogram, toward denser pixels. Note improved image contrast. Arrows show reverberation artifact in third ventricle.

View larger version (76K): [in a new window]   Fig. 1C. —Coronal sonogram of male neonate with postinfectious hydrocephalus. After modification using "Auto Levels" tool, image shows relocation of outermost arrowhead markers to edges of histogram and midpoint marker placed in center of histogram. Figure C is similar to B except that it does not depict reverberation artifact in dilated third ventricle as well.

View larger version (91K): [in a new window]   Fig. 1D. —Coronal sonogram of male neonate with postinfectious hydrocephalus. Low-contrast gray-scale sonographic image (same as A) before contrast modification. Curves tool is selected by clicking the "Image" menu bar, then "Adjust," and then "Curves," as shown in upper left corner. Default "Curves" graphic interface shows straight-line relationship between range of density values present in image (y-axis) and how each of those values is displayed as output (x-axis). "Eye Dropper" tool was selected from tools palette (white arrow) and placed over a relatively dark area of sonographic image. Density of image pixel selected is represented by circle on upper third of curve (black arrow).

View larger version (75K): [in a new window]   Fig. 1E. —Coronal sonogram of male neonate with postinfectious hydrocephalus. After modification using "Curves" tool, image shows curve with sigmoid shape: darker pixels (upper curve) are shifted upward, and lighter pixels (lower curve) are shifted downward. Note that mid portion of curve (middle gray pixels) remains straight. Resulting image contrast is close to that of analog original.

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