Radiology and Radiation Safety
Physics, protection, interpretation
X-rays are ionizing electromagnetic radiation produced when accelerated electrons strike a tungsten target. Know the units: absorbed dose is measured in gray (1 Gy = 100 rad); equivalent and effective dose in sievert (1 Sv = 100 rem). The occupational effective dose limit is 50 mSv per year, the general public limit is 1 mSv, and after a declared pregnancy the fetal limit is 0.5 mSv for the gestation. ALARA governs practice: rectangular collimation, F-speed film or digital receptors, lead apron with thyroid collar, accurate technique, and only justified exposures. Minimum filtration is 2.5 mm aluminum for units above 70 kVp, and a 16-inch position-indicating device is preferred.
Technique: paralleling with a long cone is the standard; bisecting-the-angle is reserved for selected situations. Beam alignment errors cause elongation or foreshortening, cone cuts, and overlapped contacts. Processing errors mimic exposure errors. Quality assurance includes processor monitoring, retake analysis, and viewbox checks.
Interpretation: bitewings survey interproximal caries and crestal bone; periapicals show apices; panoramic images provide a broad survey. Radiographs reveal demineralization only after roughly 30–40% mineral loss, so incipient caries is a clinical diagnosis. Use the SLOB rule for localization: an object moving in the same direction as the tube head is lingual; opposite direction, buccal.
Rule of thumb: 1 Gy = 100 rad and 1 Sv = 100 rem - never mix the two systems. Worked example: the tube head shifts mesially and the object moves mesially on the image, so the object lies lingual.