Calculate pulse repetition frequency and the Nyquist limit from imaging depth for ARDMS SPI exam preparation.
Interactive Nyquist Limit & Aliasing Calculator
Enter imaging depth and measured Doppler shift frequency to determine whether the shift exceeds the Nyquist limit.
Formula: PRF = 77 ÷ depth (cm)
Compare the measured shift with PRF ÷ 2.
Estimated PRF
11.00 kHz
Nyquist limit
5.50 kHz
Status
NO ALIASING
How the formula works
For this exam-style calculation, pulse repetition frequency is estimated from imaging depth:
PRF = 77 ÷ depth (cm)
Nyquist limit = PRF ÷ 2
Increasing imaging depth lowers PRF because the system needs more time for echoes to return. A lower PRF also lowers the Nyquist limit, which can make aliasing more likely.
SPI exam takeaway
Aliasing can occur when a Doppler frequency shift exceeds half the PRF. Common controls include increasing the PRF or scale, lowering transmit frequency, and shifting the baseline.