Physics of Ultrasound

I. Principles and Physical Properties of Ultrasound A. Sound WavesSound is created when an ultrasound transducer comes in contact with a physical medium and causes it to vibrate. The vibrations propagate through tissue and have areas of compression and rarefaction that resemble a sine wave. 1. PeriodTime from start of 1 cycle to the start of the next cycle; determined by sound source. 2. FrequencyNumber of cycles that occur in one second; determined by sound source; affects depth of penetration and image resolution (Hz). Ultrasound is a wave with frequency greater than 20,000 Hz; echocardiography uses frequencies of 2-10 MHz. 3. AmplitudeRepresents peak pressure of a sound wave; appreciated as loudness; determined by sound source but can be changed by sonographer; decreases as sound passes through the body (dB). 4. PowerRate that work is performed; determined by sound source but can be changed by sonographer; decreases as sound passes through the body; proportional to amplitude squared (Watts). 5. IntensityPower in a beam divided by its cross sectional area; determined by sound source but can be changed by sonographer; decreases as sound passes through the body; very high intensity signals can damage tissue. Intensity is proportional to power and to the amplitude squared (Watts/cm). I≈P≈A 6. WavelengthLength of a single cycle; determined by source and medium; inversely related to frequency (high frequency means short wavelength). In soft tissue, sound with frequency of 1 MHz has wavelength of 1.54 mm. Shorter wavelength yields better axial resolution. 7. Propagation VelocityRate that sound travels through a medium; determined by density and stiffness of the medium; average speed of sound in soft tissue is 1540 m/s.