Focal depth in cm can be calculated by which formula? (diameter in mm, frequency in MHz)

Focal depth in cm can be calculated by which formula? (diameter in mm, frequency in MHz)

• A. focal depth (cm) = diameter (mm)^2 x frequency (MHz) / 61.6
• B. focal depth (cm) = diameter (mm)^2 x frequency (MHz) * 61.6
• C. focal depth (cm) = diameter (mm)^2 / frequency (MHz) * 61.6
• D. focal depth (cm) = diameter (mm) x frequency (MHz) / 61.6

Answer: (A)

Focal depth is determined by how the transducer’s aperture (diameter) and the ultrasound wavelength (set by frequency) interact to focus energy. For a focused transducer, the focal length scales with the square of the aperture and with frequency: bigger apertures push the focus farther, and higher frequencies (shorter wavelengths) also push the focus farther in tissue. The exact form that gives the depth in centimeters when the diameter is in millimeters and the frequency is in megahertz is F = (diameter)^2 × frequency / 61.6. The 61.6 is a constant that comes from unit conversions and the speed of sound in tissue. So the correct expression multiplies the diameter squared by frequency and then divides by 61.6. Using frequency in the denominator would imply the focus moves closer with higher frequency, which contradicts the physics, and other forms that don’t square the diameter or that multiply by 61.6 rather than divide would not match the established unit-consistent relationship.

Focal depth is determined by how the transducer’s aperture (diameter) and the ultrasound wavelength (set by frequency) interact to focus energy. For a focused transducer, the focal length scales with the square of the aperture and with frequency: bigger apertures push the focus farther, and higher frequencies (shorter wavelengths) also push the focus farther in tissue. The exact form that gives the depth in centimeters when the diameter is in millimeters and the frequency is in megahertz is F = (diameter)^2 × frequency / 61.6. The 61.6 is a constant that comes from unit conversions and the speed of sound in tissue.

So the correct expression multiplies the diameter squared by frequency and then divides by 61.6. Using frequency in the denominator would imply the focus moves closer with higher frequency, which contradicts the physics, and other forms that don’t square the diameter or that multiply by 61.6 rather than divide would not match the established unit-consistent relationship.