A post-beamforming nonlinear compounding algorithm for ultrasonic imaging is presented. Fundamental and harmonic image components from beamfromed radio frequency (RF) data are extracted, envelope detected and compounded using a spatial compounding functions (SCFs) derived from the transmit/receive beamforming topology used in obtaining the RF data. This is specially useful for applications where single-transmit focus (STF) imaging is used. In this paper, we present results from STF imaging experiments using a novel dual-mode phased array system for image-guided surgery. In particular, we address the enhancement of the echogenicity of thermal lesions formed in ex vivo tissue. It is shown that new nonlinear image compounding algorithm produces 25-30 dB enhancement in lesion echogenicity without loss in spatial resolution. This is to be compared with a typical enhancement of 5 dB achieved by standard echographic imaging and 20 dB achieved by second harmonic imaging alone. In addition, images resulting from the new algorithm are virtually free of beamforming artifacts that can severely degrade the performance of 2nd harmonic imaging.