Filtering of the radial self-gating signal. (a): The raw signal is disturbed by higher harmonic oscillations. (b): A Butterworth low-pass filter is applied to suppress high frequency interferences. (c): Low frequency oscillations caused by respiratory motion are reduced with a gaussian-shaped window function. (d): Magnitude response of the low-pass filter. Used pass band frequency: 10–13 Hz. Stop band frequency: 15-20 Hz. (e): Magnitude response of the Gaussian window function (w(f)=1-exp(-(f/σ)2)). The used window width was σ=4-8 Hz.