Ultrasound
Ultrasound Imaging Ultrasound - Sound waves (longitudinal) with frequencies greater than 20KHz. Typical frequencies used in imaging 1 – 5 MHz The transducer produces short pulses and acts as a receiver in between pulses.
Piezoelectric Crystals When pressure is exerted on these crystals, a voltage is produced between opposite faces of the crystal. The charges within the crystal structure are forced out of balance. The process also works in reverse, so an applied voltage will result in pressure at the surface of the crystal. An alternating voltage generates a longitudinal oscillation (and therefore a sound wave). The frequency of the a.c. is chosen so that it matches one of the resonant frequencies of the crystal.
Ultrasound Pulses Typical Pulse Data f – 3 MHz v – 1500 m/s - 0.5mm Pulse length - 1s Scale of body 5-20cm Time until return of pulse ~ 70 s+
Building the Image A-scan. The peaks correspond to reflection from surfaces inside the body. Taller peak represent stronger reflections. B-Scan. The transducer is moved across the body and the return signals are stored electronically. The signal strength controls the brightness/colour of each part of the image displayed on a screen. Blood vessels inside the eye
Doppler Scans Resolution Diffraction will occur if the sound waves have a wavelength that is too long. As a general rule, the Ultrasound scanner will have a resolution approximately equal to its wavelength. Therefore resolution is of the order of millimetres (typical ~0.5mm). Doppler Scans Movement of the reflecting surface causes a shift in wavelength of the ultrasound pulse (The Doppler Effect). e.g. to assess blood vessels and the heart. Doppler ultrasound of a kidney