Principles of Radiographic Interpretation
Radiographic interpretation is an art form. Effective evaluation requires selecting appropriate views for the radiographic study (Chapter 10), use of the proper viewing tools, a consistent means for viewing digital images on a monitor or placing films on a view box, understanding basic concepts of image formation, and using a systematic approach for image assessment (Chapter 11). This chapter focuses on the fundamentals of radiographic image formation and evaluation.
X-RAY FILM–VIEWING TOOLS
The minimum requirements for an x-ray film–viewing room include a view box, magnifying glass, and spotlight.
A view box should be used whose size is consistent with the size of film being viewed. For example, if 10- × 12-in films are used, then the view box size should be 10 × 12 in or multiples thereof. If the lit viewing area is larger than the film, extraneous light escapes around the sides of the film and the user perceives less useful visual information from the radiograph. The radiograph appears more dense (blacker) if extraneous light surrounds the film.1 To illustrate, place a high-contrast dorsoplantar foot radiograph on a fully lit view box that accommodates two or more films. Notice how parts of the image that are relatively darker, such as the toes, are barely visible. Then turn off or cover all remaining view box lights except that being used to view the radiograph. Finally, take the x-ray folder and cover up the entire foot except the toes. Notice how the toes become more visible with each step. Therefore, not only should the size of the view box correspond to the film size but also each single film–viewing area should have its own on/off power switch and there should be reflectors that divide and separate each 10- × 12-in viewing area so that there is no crossover illumination.
Overhead lighting, lamps, and sunlight can also impair the viewer’s ability to interpret radiographs by producing glare and surface reflections. Therefore, all other light sources should be turned off or subdued when viewing films.
Cool white or daylight fluorescent light bulbs are used as the view box light source. Cool white bulbs are preferable; daylight bulbs are “bluer” and do not appear to provide as much light through the illuminator surface and film. If the view box uses more than one light bulb, be sure to use the same type of bulb in all fixtures. Colored (green) fluorescent bulbs are also available; one manufacturer claims they are less straining to the eyes. This is fine if you do not mind looking at a green radiograph! (Personally, I favor the black and white/shades of gray variety.)
A white, transparent illuminator surface diffuses light of uniform brightness for viewing the radiograph. This surface must be kept clean and free of scratches. Dirt or other foreign matter on the illuminator surface will appear as artifact or may mimic pathology in the radiograph.
Every viewing area should have an accessible magnifying glass. Glasses come in many shapes, sizes, and powers. Invest in a precision-cut piece of glass. Inexpensive, bargain-brand magnifiers tend to distort the image because they are improperly manufactured. A small handheld magnifying glass suffices.
A spotlight generally uses an incandescent bulb for its light source. The bulb wattage is typically 60 or 75 W. The device can be purchased with or without a pedal or dial for variable adjustment of light intensity.
When viewing a film over a spotlight, be extremely careful not to damage the film. The proper method for viewing the film with this light source is to slowly but constantly move and/or rotate the film. A film will warp if it is held over the high-intensity light source too long. (Consider the following analogy: If your hand is held still over a lit match, the skin will burn. However, continually moving your hand in a circular motion over the flame will prevent the skin from burning.) The emulsion may eventually crack in the damaged area, ruining the image. Another way of knowing if the film is burning is to continually hold a finger against the film near the area being highlighted. If your finger feels intense heat, then you know the film is being damaged. Burning x-ray film also emits an unpleasant smell. This damage, by the way, is permanent and cannot be reversed.
DIGITAL IMAGE–VIEWING TOOLS
Digital images, whether viewed at a PACS (picture archiving and communications system) workstation or on a personal computer, are best displayed on a high-resolution monitor. Two types of monitors, the cathode ray tube (CRT) and the liquid crystal display (LCD), are used. Images of the foot and ankle are best viewed on a portrait monitor with a screen resolution of at least 1600 × 1200 (2k). The LCD flat panel monitor should be viewed straight on, as is recommended for most LCD television monitors.
Ambient light, including overhead lighting, lamps, and sunlight, can produce glare and surface reflections, especially on CRT monitors, that will impair the viewer’s ability to interpret radiographs. Therefore, all other light sources should be turned off or subdued when viewing images.
A CD/DVD drive is necessary for viewing studies performed outside your practice setting that may be brought in by the patient. Additional DICOM software is necessary to view and manipulate the images on a personal computer. CDs provided by outside imaging centers typically include software to “play” or view the images, but do not allow the end user to “record” or copy the images. The provided software is a limited version of the full software, but allows the end user to manipulate the image’s brightness (window level), contrast (window width), extraneous light (“shuttering”), orientation, and size; in addition, the image can be magnified and measurements made.2
How should radiographic images be viewed? The conventional method entails positioning the image so that the patient is facing the viewer. For example, the patient’s left ankle is seen on the viewer’s right. And, by this method, the anteroposterior/dorsoplantar foot view would be placed so that the toes are at the bottom, rearfoot at the top. Most people do not view foot images in this manner, however. The pedal image is commonly positioned (and reproduced in textbooks or journals) so that the patient’s right foot is on the viewer’s right and the toes are at the top. Merrill’s atlas mentions the exception as to how foot images are viewed, with the toes pointed up.3