Radiation Health and Safety (Dental Radiography)
Dose, units and ALARA
Radiography questions hinge on ALARA - doses as low as reasonably achievable - and on units. Absorbed dose is measured in gray (Gy) or rad (1 Gy = 100 rad); equivalent dose in sievert (Sv) or rem (1 Sv = 100 rem). No one holds the film or sensor in a patient's mouth: use a positioning holder or, if absolutely required, a trained adult who is not occupationally exposed. Never stand in the primary beam; stand at least 2 m (6 ft) away at 90 to 135 degrees to the beam, behind a barrier when available.
What the factors change
kVp governs penetrating power and contrast - higher kVp produces a longer-scale, lower-contrast image and lowers patient dose. mA and exposure time govern quantity, which drives density. The inverse square law explains distance protection: double the distance and intensity falls to one quarter. Collimation to the receptor size and correct PID length reduce scatter and skin dose; a lead apron with thyroid collar shields the patient.
Technique errors
Elongation results when the receptor is not parallel to the long axis of the tooth - too little vertical angulation. Foreshortening comes from excessive vertical angulation. Overlapped contacts are horizontal angulation errors. A cone cut means the PID was not centered over the receptor. A pale image suggests underexposure or depleted processing chemistry. Digital sensors require an intact barrier that is not punctured or doubled over.
Rule of thumb: kVp = contrast and quality; mA × time = quantity and density - and angulation is the leading cause of retakes.