Spatial pulse length SPL is defined as n × λ. If the pulse contains 6 cycles and the wavelength is 0.3 mm, what is the SPL?

• A. 1.8 mm
• B. 0.9 mm
• C. 3.6 mm
• D. 0.6 mm

Answer: (A)

Spatial pulse length is the physical length of the ultrasound pulse in tissue and is found by multiplying the number of cycles in the pulse by the wavelength. With 6 cycles and a wavelength of 0.3 mm, SPL = 6 × 0.3 mm = 1.8 mm. So the SPL is 1.8 mm. This shows that increasing either the number of cycles or the wavelength lengthens the pulse, while shorter wavelengths or fewer cycles shorten it. Note that wavelength itself depends on tissue speed of sound and frequency (λ = c/f).