{ Digital Image Processing By Professor Stelmark

Postprocessing Postprocessing functions are computer software operations available to the radiographer and radiologist that allow manual manipulation of the displayed image. These functions allow the operator to adjust manually many presentation features of the image to enhance the diagnostic value.

Background Removal or Shuttering Anytime a radiographic image is viewed, whether it is film/screen or digital, unexposed borders around the collimation edges allow excess light to enter the eye. Known as veil glare, this excess light causes oversensitization of a chemical within the eye called rhodopsin that results in temporary white light blindness. Although the eye recovers quickly enough so that the viewer recognizes only that the light is very bright, it is a great distraction that interferes with image reception by the eye. In film/screen radiography, black cardboard glare masks or special automatic collimation view boxes were sometimes used to lessen the effects of veil glare, but no technique has ever been entirely successful or convenient. In CR, automatic shuttering is used to blacken out the white collimation borders, effectively eliminating veil glare. Shuttering is a viewing technique only and should never be used to mask poor collimation practices.

Electronic collimation has no effect on overall image quality or patient exposure.

Image Orientation Image orientation refers to the way anatomy is oriented on the imaging plate. The image reader has to be informed of the location of the patient’s head versus feet and right side versus left side. The image reader scans and reads the image from the leading edge of the imaging plate to the opposite end. The image is displayed exactly as it was read unless the reader is informed differently. Vendors mark the cassettes in different ways to help technologists orient the cassette in such a way that the image will be processed to display as expected. Fuji uses a tape-type orientation marker on the top and right side of the cassette. Kodak uses a sticker reminiscent of the film/screen cassette identification blocker. Some examinations, however, require unusual orientation of the cassette. In these cases, the reader must be informed of the orientation of the anatomy with respect to the reader. In DR, for which no cassette is used, the position of the part should correspond with the marked top and sides of the imaging plate.

Image Stitching When anatomy or area of interest is too large to fit on one cassette, multiple images can be “stitched” together using specialized software programs. This process is called image stitching. In some cases special cassette holders are used and positioned vertically, corresponding to foot-to-hip or entire spine studies. Images are processed in computer programs that nearly seamlessly join the anatomy for display as one single image. This technique eliminates the need for large (36-inch) cassettes previously used in film/screen radiography.

Image Annotation Many times, information other than standard identification must be added to the image. In screen/film radiography, time and date stickers, grease pencils, or permanent markers were used to indicate technical factors, time sequences, technologist identification, or position. The image annotation function allows selection of preset terms and/or manual text input and can be particularly useful when such additional information is necessary. (Function availability depends on the manufacturer.) The annotations overlay the image as bitmap images. Depending on how each system is set up, annotations may not transfer to PACS. Again, input of annotation for identification of the patient’s left or right side should never be used as a substitute for technologist’s anatomy markers.

Another common measurement a radiologist may use is a region of interest (ROI). It will determine the pixel intensity of a certain area. Because each type of tissue or fluid has a little bit different intensity reading, the radiologist can make a determination whether something is solid or fluid.

Magnification Two basic types of magnification techniques come standard with digital systems. One technique functions as a magnifying glass in the sense that a box placed over a small segment of anatomy on the main image shows a magnified version of the underlying anatomy. Both the size of the magnified area and the amount of magnification can be made larger or smaller. The other technique is a “zoom” technique that allows magnification of the entire image. The image can be enlarged enough so that only parts of it are visible on the screen, but the parts not visible can be reached through mouse navigation

Subtraction is a technique that can remove superimposed structures so that the anatomic area of interest is more visible. Because the image is in a digital format, the computer can subtract selected brightness values to create an image without superimposed structures

Black/white reversal is a postprocessing technique that reverses the grayscale from the original radiograph.

Edge enhancement is a postprocessing technique that improves the visibility of small, high-contrast structures. Image noise may be slightly increased, however.

Smoothing is a postprocessing technique that suppresses image noise (quantum noise). Spatial resolution is degraded, however.

3D Angiography A three-dimensional (3D) image may be produced from the image data acquired during a rotational acquisition. These data are processed by a sophisticated computer system with the use of digital reconstruction techniques similar to those used in computed tomography.