General X-Ray Equipment

After learning a little about X-Ray equipment and X-Ray generation I always wondered if Superman’s X-Ray vision caused cancer if you were exposed to it too long. X-Ray’s were first generated in December 1895 in the middle of the night, by Professor Wilhelm Conrad Roentgen. He had been working on the machine for several years and in the middle of that December night asked his wife to put her hand on a photographic plate. He pushed a button and then took the plate to a dark room and developed it. The result was an image of the bones in her hand and the ring she wore on her finger. X-Ray technology took the world by storm after that.

X-Rays are generated by using extremely high voltages and low current. Voltages range up to 150 kilovolt Peak-to-Peak or 150kVp. Current is normally in the milliamp range. An X-Ray tube is made up of two filaments on the Cathode end and a spinning disc called an Anode on the other end. The filaments are on the negative side of the high voltage cable and the anode is on the positive side. At the X-Ray console the user selects the kVp, (voltage) and mA or mAs (milliamps or milliamps per second). The filament current is turned on and the filament glows brightly just like the filament in a light bulb.

When the exposure switch is pressed the high voltage causes the photons in the filament to flow across an open space and strike the face of the spinning anode. The anode is not flat but has an angle on the face of it. The photons are changed to electrons when they hit the anode and are deflected out of a small window in the tube and pass through a collimator through the patient and into the film that is developed later. When the electrons hit the face of the target anode is when they change and become X-Rays. This effect is called brehmsstrahlung effect or braking radiation because the electrons are slowed tremendously when they strike the anode.

The collimator is a device that is positioned in front of the tube. Its purpose is to limit the amount of radiation being generated by the tube. The collimator is lined with lead just as the tube is to prevent X-Rays from going everywhere. Inside the collimator are movable lead fingers that open and close to limit the size of the field. For example if a 9” x 9” film cassette is inserted into the bucky the sizing sensors in the bucky tell the collimator to adjust the fingers and only allow a 9”x9” area of X-Rays to come through. Manual override can further limit the size to pinpoint an area of the body. The bucky is nothing more than a tray that holds the film cassettes and slides in and out of the machine.

Because over exposure to radiation can cause burns, radiation poisoning and other serious problems, most X-Ray machines come with a feature called AEC. AEC stands for Automatic Exposure Control. AEC is programmed into various types of exposures. For example an exposure on a small body part like a hand doesn’t have to be as lengthy as an exposure on a thicker part of the body like a chest would. AEC combines the kVp selected by the user with the mA or mAs and determines what the maximum time of the exposure should be for the selected body part and study type. If the selections are outside the range of the pre-programmed AEC limits the X-Ray machine will not expose but will send up an alert message to the user telling them to make a different selection.

X-Rays can also be used to do fluoroscopy. Fluoroscopy is continuous X-Ray exposure but usually at a low kVp and mAs which limits the amount of exposure to X-Rays. Fluoroscopy allows a doctor to watch what is happening inside the body as it happens. It’s like watching a movie of what’s going on inside the patient. For example a Barium Swallow test is done with fluoroscopy. A patient is given some barium to swallow and while they are swallowing it the image is being displayed fluoroscopically on a monitor and the doctor is watching the swallowing process to determine abnormalities in the patient’s digestive tract. The barium works its way through the digestive system and when it reaches the area of concern the doctor can tell if blockages are present or if some other obstruction is casing the patient discomfort.

Some of the primary concerns with X-Ray quality are Density, Contrast, and Sharpness to name a few. Density is defined as the amount of blackening on an image. Density is directly related to the amount of kVp selected. As kVp increases Density increases because more x-rays are getting through the patient. Contrast is defined as the amount of difference in two adjoining areas. For example the amount of contrast between the colors black and white is very high. You can clearly see the edge where one begins and the other stops. Apply the same theory to two colors of gray and the edge becomes less clear which is considered low contrast. Contrast is also controlled by kVp and mA. Sharpness is simply how clear is the image and is there any distortion apparent.