Collimators and X-Ray Film

The collimation of the x-ray beam starts with the tube design, specifically the focal spot track, which focuses the majority of the x-rays generated in one direction. The tube housing further reduces stray radiation and continues the collimation process.

The x-ray beam exits the housing through the “top hat,” a lead cone with a rectangular hole in it. The size of the hole determines the maximum film size that can be exposed at a given distance (also called source-to-image distance [SID] or focal-film distance [FFD]). This opening also helps reduce stray radiation and shape the beam. The collimator-mounting block, made from aluminum, and the rotation ring secure the collimator to the tube housing. These items should be checked regularly to ensure that the hardware is secure. An old trick is to paint nail polish from the screw or bolt head to the housing surface, and if the polish is cracked, the hardware is starting to loosen.

On some collimators, there may be a slot in the housing, near the mounting block, that allows for the placement of filters in the x-ray beam. These filters are usually thin sheets of copper or aluminum (but other materials are sometimes used) that are inserted to remove radiation-intensity spikes generated by characteristic radiation and to smooth the remaining radiation so it is consistent. Filtering the beam may also be referred to as “hardening the beam” or removing soft radiation. These filters should only be added or removed as directed by the physicist doing the radiation certification on the unit. The certification may need to be redone after major repairs to the generator but otherwise is done as required by local and federal regulations. (The maximum length of certification is 3 years.)

It is not uncommon for the technologist to have to adjust his or her technique when filtration is added or removed. Therefore, an increase in “retake” films is not uncommon right after changes are made in the system. Work closely with radiology-department supervisors and technologists to ensure that the “retake” rate returns to the previous level or below.

Below the filters are the upper-level shutters. These are flat lead alloy strips of metal with beveled mating edges that, when closed, totally block x-ray passage. The upper-level shutters are mechanically linked to the lower-level shutters so they work together. On some units, a feature called automatic collimation allows the unit to sense the size of the film cassette installed and automatically adjust the shutters to that size. When a problem occurs with the automatic collimation, it is generally traceable to the sensors in the Bucky tray. With manual collimation, problems are generally based on the looseness of the adjustment knob or the linkage between the knob and the shutters.

In the space between the upper and lower shutters is a lamp and a radiolucent mirror. This lamp is called the aiming light, centering light, collimation light, or field light. The light beam is focused through the protective lens on the bottom of the collimator. That lens has crosshairs, which are used to properly align the tube with the area of the patient to be x-rayed. The alignment of the centering light to the film cassette needs to be checked on each preventive maintenance (PM) inspection to ensure proper alignment. The lamp, generally, is replaced from the side of the collimator, never by removing the lens with the crosshairs and reaching through the shutters. The light should only remain on for less than 1 minute.

As part of a PM, the shutters should be fully closed and a film should be exposed. When developed, the film should be black since no exposure of the film occurs if the collimator works properly. If the shutters do not fully close, the exposure will indicate which shutter is not closing or is damaged. While it is not a major safety issue, a damaged or malfunctioning shutter can affect the quality of patient films, possibly leading to costly retakes.

At the bottom of the collimator on some units are channel slides used to mount cones, cylinders, or other metal units to further focus the beam to an even smaller spot on the patient. These metal focusing units are mechanical devices and need only to be checked when purchased and for mounting
security at PMs.

As the x-ray beam passes through a body, striking cells or bones, some of it is deflected and some of it will trigger the release of chrematistic radiation from cells/bones—both of which can cause a lack of definition of the object being studied. Placing a grid between the patient and the film reduces deflected and Compton scatter radiation, providing a sharper image on the film. Two basic grids are used: parallel and focused. Focused grids are limited in that they are designed for a single SID or FFD. Compton scatter is
caused by x-ray beams reacting with body parts and creating secondary x-rays (similar to chrematistic radiation).   

Grids come in various ratios of height of the strip versus the space between strips. A common grid is 8:1, meaning the lead strip is 8 times as high as the space between it and the next strip. The common ratios in use are 5:1 for low-voltage work, 8:1 for midrange voltages, 12:1 for general-purpose work, and 16:1 for high-voltage work. The next item to consider is the spacing of the strips. The common choices are 30 strips per cm for general use, 45 strips per cm for skull studies, and 60 strips per cm for vascular studies.

If the films come out with noticeable grid lines, there is a problem with the Bucky tray. The movement of the grid is the “clunk” that you hear during an x-ray exposure. Normally, the grid is moved 2 to 3 cm in one or more directions, so the grid lines do not appear. Working independently, Dr Hollis Potter and Dr Gustav Bucky developed this system in the 1920s. The drive mechanism to move the grids can be springs, the motor, solenoids, or some other simple drive. The film cassette is placed in the Bucky tray, which sets the collimator to that film size. The grid is mounted over the tray and moves during the exposure.

To further confuse you with terms, the object-film distance (OFD) is the amplification factor of the imaging system. The greater the distance, the larger the object appears on the film. This amplification is rarely used, but it is sometimes useful for locating breaks in small bones.

Film and Film Cassettes

For most x-ray examinations, dual-emulsion films are used. With dual-emulsion, the image forms on both sides of the film. More durable than the single-emulsion films used for mammography studies, double-emulsion films tend to produce brighter images but give up some detail.

In the film cassette, two intensifying screens emit light when exposed to x-ray. This light, along with the x-rays, is what exposes the film, creating an image. The light from the screens allows for lower radiation dosages to be delivered to the patient but causes a small loss in clarity of the image on the film.

Sometimes, the screens become damaged and the resulting images are not high quality. It is not rare that problems with the film-cassette screens get reported as problems with the x-ray unit. An indication of a screen problem is that only one of a group of films has a problem. Finding the bad cassette can be very time-consuming.

Cassettes should also be checked by the users for light leaks. Light leaks usually cause the edges of the developed film to be clear. For digital (DR) and computer (CR) systems, sensors replace the film and cassette. Almost any x-ray system can be converted to CR at a reasonable cost. Changing an old system to DR may not be a good fiscal choice.

David Harrington, PhD, is director of staff development and training at Technology in Medicine (TiM), Holliston, Mass, and a member of 24x7’s editorial advisory board.Kevin Earl is a TiM biomed assigned to MetroWest Medical Center, Natick, Mass.