If energy strikes a body part at an angle away from 90°, what happens to absorption?

If energy strikes a body part at an angle away from 90°, what happens to absorption?

• A. Absorption increases
• B. Absorption remains the same
• C. Absorption decreases
• D. Absorption becomes maximal

Answer: (C)

When energy hits a tissue at an angle away from perpendicular, less energy actually enters the tissue because part of it is reflected at the surface. Absorption depends on how much energy propagates into the tissue, and perpendicular incidence couples energy most efficiently. As the angle moves away from 90°, the transmitted energy decreases due to reflection and refraction effects, so the amount absorbed by the tissue is reduced. In practice, you aim the modality so the beam enters perpendicularly to maximize absorption and therapeutic heating.

When energy hits a tissue at an angle away from perpendicular, less energy actually enters the tissue because part of it is reflected at the surface. Absorption depends on how much energy propagates into the tissue, and perpendicular incidence couples energy most efficiently. As the angle moves away from 90°, the transmitted energy decreases due to reflection and refraction effects, so the amount absorbed by the tissue is reduced. In practice, you aim the modality so the beam enters perpendicularly to maximize absorption and therapeutic heating.