What is the effective radiating area (ERA) in therapeutic ultrasound?

• A. The total area of the soundhead
• B. The depth of tissue penetration
• C. The area of the faceplate (crystal size) which is smaller relative to the soundhead
• D. The time-averaged energy across the beam

Answer: (C)

ERA is the actual emitting surface of the transducer—the part of the faceplate/crystal that produces ultrasound. This area is typically smaller than the overall size of the soundhead because only the crystal-facing portion radiates energy while the rest of the head may be non-emitting. When you calculate the intensity delivered to tissue, you use this active area, not the full soundhead size. So the effective radiating area corresponds to the area of the faceplate (crystal) that emits energy, which is smaller relative to the overall soundhead.