Figure 35

A retrospectively-gated, cine spoiled gradient echo pulse sequence (top) is shown in detail for four cardiac phases. In this example the standard imaging gradients are modified in the slice-selection direction to encode flow velocity components perpendicular to the image plane. For each cardiac phase the gradient echo pulse sequence is acquired twice, with two different flow sensitivities, indicated here by the red and green gradient pulses. The heart phase interval or ‘effective TR’ of the velocity encoded sequence is double the actual TR of the gradient echo pulse sequence. The M_x and M_y components of the MR signal are used to calculate both the signal magnitude, M, and the signal phase, ϕ, for each pixel. The phase maps for the two different flow sensitivities are subtracted to remove background phase shifts, creating a velocity map, which contains only velocity-related phase shifts due to the difference in flow velocity sensitivity between the two acquisitions. The flow sensitivities are chosen such that a subtracted phase difference of 180° corresponds to a predefined maximum velocity or VENC. The sign of the subtracted phase indicates the direction of flow along the encoded direction. The value of relative phase shift is mapped onto the pixel intensity scale such that a zero subtracted phase (stationary tissue) maps on to the centre of the pixel intensity scale (mid-grey). Positive and negative subtracted phases (corresponding to opposite directions of flow) are mapped onto higher and lower pixel intensities, respectively.