PHYS1008 Lecture Notes - Lecture 7: Voltage Spike, Sound, Deep Structure And Surface Structure
Document Summary
Main bang, which is short voltage spike of 100 to 200v ((cid:883) (cid:884)), is applied to the crystal. Short pulse of ultrasound means better axial resolution. The crystal vibrates for about 2-4 cycles at its natural frequency due to the inverse piezoelectric effect. After then main bang, receiving circuits are opened to allow reception and processing of returning sound waves. When receiving circuit is first opened, the crystal is still being damped to stop its ringing. Ring down time refers to the time taken for this to occur. During this ti(cid:373)e, the u(cid:374)it (cid:272)a(cid:374)"t pro(cid:272)ess a(cid:374)d retur(cid:374)i(cid:374)g sou(cid:374)d thus first few (cid:373)(cid:373) (cid:272)a(cid:374)(cid:374)ot be visualised (dead zone). Once crystal has stopped ringing, the returning sound wave cause the crystal to vibrate, causing an electrical signal. Signal is processed and used to form the image. Different tissues reflect signals to different degrees. Ultrasound wave travels at a know speed thus depth can be calculated. Spatial information is only in one dimension.
