Principles of Medical Diagnostic Ultrasound Physics 3

Intensity and Power

The region of tissue in front of the transducer subjected to the vibrations is referred to as the ultrasound field or beam.

Intensity is measured at the focus of the field or within 1 or 2 cm of the transducer face.

The power of an ultrasonic beam is the rate of flow of energy through the cross-sectional area of the beam.

Output intensity in decibels (dB) = 10log(I Io )

Cavitation is the violent response of bubbles when subjected to the pressure fluctuations of an ultrasound wave. Thermal (TI) and mechanical indices (MI) relate to these phenomena and are displayed onscreen.

Gain = Output volts Input volts

Diffraction and Interference

Diffraction describes the spreading out of a wave as it passes from its source through a medium.

Within a diffraction pattern there may be fluctuations in intensity, particularly close to the transducer.

The narrowness of a beam or the sharpness of a focus is ultimately determined by diffraction.

The higher the frequency, the narrower the beam can be made and hence the finer the image detail.

 It is the shape of the beam, i.e. the combination of transmission and reception, that plays an important part in determining the detail in an image.