The impact of grating lobe clutter on plane wave DCE-US parametric imaging

Parametric imaging is a tool to visualize the parameters extracted from dynamic contrast enhanced ultrasound (DCE-US) and the corresponding time amplitude curves (TAC) to illustrate the heterogeneity of those parameters across a tumour. TAC generated using DCE-US can be influenced by imaging mode, plane wave (PWI) or focussed imaging (FI) and the parameter choices. This has been previously studied for a single vessel phantom and a micro vasculature phantom when laterally located clutter from grating lobes was observed for PWI. Here we study the effect of such clutter on parametric imaging for three TAC metrics: peak intensity (PI), time from injection to peak (TIP) and mean transit time (MTT), using a microvascular flow phantom. DCE-US images were obtained using a Vantage (Verasonics Inc.) and a pulse-inversion algorithm. 800 frames were recorded at 10 Hz for PWI and FI. All measurements were repeated 3 times, injecting 0.4 ml of contrast agent (Sonozoid). The concept of TAC coherence, the similarity of the TACs between any two locations in the field of view, was introduced and used as a measure of TAC information leakage, a blurring of the ability to see spatial variation in TAC shape. Such leakage may be due, for example, to a broad beam (high side lobes) or strong grating lobes. A greater signal leakage was observed for PWI which raises concerns over the use of PWI DCE- US when it is important to maintain sensitivity to vascular heterogeneity in tumours, especially when interested in response to treatment.