Clinical comparison of pulse and chirp excitation

Coded excitation (CE) using frequency modulated signals (chirps) combined with modified matched filtering has earlier been presented showing promising results in simulations and in-vitro. In this study an experimental ultrasound system is evaluated in a clinical setting, where image sequences are assessed by skilled medical doctors. The effect on penetration depth and image quality were measured. A modified clinical scanner with a 4 MHz single element mechanical transducer, and external transmitter and receiver boards (RASMUS system) were used. The system allowed rapid toggling between chirp and short pulse excitation to simultaneously produce identical image sequences using both techniques. Nine healthy male volunteers were scanned in abdominal locations. All sequences were evaluated by 3 skilled medical doctors, blinded to each other and to the technique used. They assessed the depth (1) in which image quality decreased and (2) in which the image would be insufficient for clinical diagnosis. Furthermore they compared image quality in matching pairs of conventional and CE images. The average increase in penetration depth were almost 2 cm. Side-by-side comparison showed that coded image quality was consistently rated better; significant (p≤0.05) when images were cropped at minimum the depth for good image quality and highly significant (p<0.001) when cropped at maximum depth sufficient for clinical diagnosis. We conclude that coded excitation with linear FM chirps improves penetration and image quality significantly in a clinical setting.