Incorrect exposure can be caused by many factors; the most common being improper exposure factor settings. Improper time selection is the most likely error, because most intraoral x-ray units have fixed or unchangeable milliamperage (mA) and kilovoltage (kVp) settings. Time and milliamperage control density or the overall darkness of an image while kilovoltage controls contrast or the differences in darkness. The exposure time settings should be based on the receptor speed, the area being exposed, and patient size and stature. Every treatment room should have an exposure factor chart to guide the operator in selecting the appropriate kVp, mA and time settings for each periapical and bitewing. Refer to the recommended manufacturer settings for specific intraoral views or projections. Generally speaking, the recommended time setting for each area is based on the size of an average adult patient. Therefore, time adjustments are necessary when the patient size deviates from the average adult.
Improper exposure time settings can also result from an inaccurate evaluation of patient size and stature. The operator should use longer times for larger-than-average patients and shorter times for smaller-than-average patients. Usually one step up for large patients and one step down for small patients will be adequate adjustment for each periapical and/or bitewing. Failing to adjust exposure time will result in images that are either too light or too dark. Generally speaking child exposure time settings are lower than adult exposure time settings because of the child’s smaller size.
Underexposed receptors result in images that are too light or low in density. Light images can also be caused by an increase in the source-object distance, or not placing the tubehead close enough to the patient's face during exposure. Exposure times are prescribed on the assumption that the tubehead is no more than 2 centimeters away from the face of the patient. Receptors can be underexposed if the exposure switch is not activated for the indicated or correct length of time. In other words, the clinician let go of the exposure button too soon. Overexposure results in a high-density or dark image. The causes include improper exposure factor settings or improper assessment of patient size and stature.
When using digital image receptors, the density can be adjusted by specific enhancement features of the computer software program. However, images that are too light or low in density make it difficult to visualize the differences in structures and typically cannot be improved by software adjustments.4 Digital images that are dark or high in density can usually be salvaged by software adjustments in density unless the image appears totally black.4 This adjustment in density should be accomplished prior to printing or archiving the image. Extreme under or overexposure by necessity will result in retakes, thus exposing the patient to unnecessary radiation.
Double exposure results when the receptor is exposed twice and two images appear superimposed onto each other. This error results in two errors; one receptor that was double-exposed and another that was not exposed. It is extremely important to avoid this error because it exposes the patient to radiation twice. To avoid this error, maintain an organized work space in which exposed receptors are placed in one area or turned over to separate from unexposed receptors.
Double exposure of rigid digital receptors is not possible but phosphor plate receptors can be double-exposed just like film.
Poor Image Definition/Sharpness
Poor image definition or resolution can be the result of patient movement, receptor movement, and/or vibration of the x-ray tubehead. Movement has been discussed previously. Receptor movement may occur when the receptor is not properly stabilized in the mouth or when the patient moves the receptor because it feels uncomfortable.
To help prevent tubehead vibration or drifting, place the arm against the wall with the tubehead pointing down after each use. When the arm is fully extended for long periods of time, the joints can become weakened and fatigued, resulting in drifting and vibration. Tubeheads and arms should be professionally serviced if drifting and movement persist.
When a removable prosthesis is left in the mouth during exposure, a superimposed image of the appliance will appear over the teeth. Prior to radiographic procedures, ask patients to take out all removable items from the mouth, such as retainers, partial dentures, complete dentures, etc.
Eyeglass frames or stems may appear in maxillary periapical radiographs of patients with large eyeglasses and/or when extreme vertical angulation is utilized. Glasses should be removed prior to radiographic procedures. Facial jewelry in the path of the x-ray beam should be removed as well to avoid unwanted artifacts and unnecessary retakes.
Thyroid collars or lead aprons can also be recorded on radiographic images. This is normally caused by the collar being placed on too loosely or too high on the neck or when the vertical angulation is so steep that the collar is in the path of the x-ray beam.
Another technical error that occurs occasionally is when the receptor yields no image. This error can be caused by mechanical problems such as electrical failure, faulty generator, timer inaccuracy or faulty exposure switch. Failure to turn on the machine, placement of the receptor between the cheek and teeth, failure to line-up the x-ray beam and backward placement of digital receptors can also produce a blank or white image representative of this error.