Which factors determine the total amount of irradiated material?

• A. Patient age and gender
• B. Beam filtration and exposure time
• C. Body part thickness and area of exposure
• D. Detector type and processing speed

Answer: (C)

Think of the X-ray beam’s footprint on the patient and how deep it goes. The total amount of tissue that gets irradiated is determined by two things: how large the exposed field is (the area of exposure) and how thick the tissue is along the beam’s path (the body part’s thickness). If you widen the field, you irradiate more tissue; if you image a thicker part, the beam passes through more tissue and more material is irradiated.

Filtration and exposure time change the energy delivered per unit mass (the dose) but not the amount of tissue in the beam. Detector type and processing speed don’t affect the volume of tissue irradiated either. Age or gender don’t set the exposure geometry; they may influence anatomy, but the controlling factors for how much tissue is irradiated are the field size and the part’s thickness.