What defines spatial pulse length?

What defines spatial pulse length?

• A. It equals the transducer's focal depth
• B. It equals the echo arrival time
• C. It equals the dynamic range setting
• D. It equals the number of cycles in the pulse times the wavelength

Answer: (D)

Spatial pulse length is the physical length of the transmitted ultrasound pulse in tissue. It’s determined by how many cycles are in the pulse and the distance each cycle travels—the wavelength. So SPL = (number of cycles) × (wavelength). Because wavelength in tissue equals the speed of sound divided by frequency, SPL = (number of cycles) × c / f. Increasing the number of cycles or using a longer wavelength (lower frequency) makes the pulse longer. A longer SPL worsens axial resolution, since the system cannot distinguish two closely spaced reflectors as well. The other concepts—focal depth, echo arrival time, and dynamic range—relate to imaging depth, when echoes return, and the range of echo amplitudes, not the length of the transmitted pulse.

Spatial pulse length is the physical length of the transmitted ultrasound pulse in tissue. It’s determined by how many cycles are in the pulse and the distance each cycle travels—the wavelength. So SPL = (number of cycles) × (wavelength). Because wavelength in tissue equals the speed of sound divided by frequency, SPL = (number of cycles) × c / f. Increasing the number of cycles or using a longer wavelength (lower frequency) makes the pulse longer. A longer SPL worsens axial resolution, since the system cannot distinguish two closely spaced reflectors as well. The other concepts—focal depth, echo arrival time, and dynamic range—relate to imaging depth, when echoes return, and the range of echo amplitudes, not the length of the transmitted pulse.