129 Figure 5.5. A. Time–space representation of a complex UP pattern. The direction of UP propagation changes from F2C to C2F during the acquisition (highlighted in the red area). B. K-space representation of the UP. The sum of the spectral energies estimated within the physiological bandwidth from the first and third quadrants represents the energy of C2F propagation; the sum of the spectral energies estimated within the physiological bandwidth from the second and fourth quadrants represents the energy of F2C propagation. Simple binary classification based on the sign of vUP is not suitable when a complex propagation pattern occurs. Therefore, we propose an energy ratio (ER) metrics, where the sum of spectral energies is estimated from the first quadrant (E1) and the second quadrant (E2), representing the energy of C2F and F2C propagation, respectively (see Figure 5.5B). This is given as: C< =2 × C1 C + 1 C2 −1 (5) The parameter ER is comprised between −1 and 1. In this study, we conventionally assumed ER > 0.1 to indicate C2F propagation and otherwise for ER <−0.1 . ER around zero indicates the presence of opposing or “standing” waves. To estimate the global propagation direction, E1 and E2 were estimated on the full recording rather than within the moving window adopted for the estimation of the UP velocity.
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