<prompt> Which of the following is not echogenic on ultrasonography-BileGasBoneGall Stones  <response>: Echoes are not produced if there is no difference in a tissue or between tissues. Homogenous fluids like blood, bile, urine, contents of simple cysts, ascites and pleural effusion are seen as echo-free structures. Ref : Physical principles of ultrasound Dr Daniel J Bell and Ass. Pr. Mirjan M. Nadrljanski et al. 