Pre-distorted amplitude modulated (PDAM) chirps for transducer compensation in second and sub-harmonic imaging

A pre-distortion by amplitude modulate (PDAM) method is evaluated for use in linear frequency modulated (LFM) coded harmonic imaging. LFM coded signals are known to boost the SNR of the low amplitude harmonics, but due to the nature of harmonic imaging, are located at inefficient regions of the transducer’s bandwidth. As a consequence, the LFM signal is subjected to transducer-induced tapering which is particularly severe at the start or end of the signal. The motivation for this work is to investigate whether compensating for this tapering effect results in improved compression of the harmonic signal. PDAM signals are designed using an inverse filter method. Hydrophone measurements show successful transducer compensation of the fundamental signal. Fundamental and second harmonic images of a tissue mimicking phantom indicate minimal improvement when compared with non pre-distorted LFM signals (N-PDAM). A discussion is provided as to why this result can be attributed to the use of a sub-optimal compression filter.