Diagnostic ultrasound is non-invasive, inexpensive, widely available and uses no ionizing radiation. A transducer is used to convert electrical pulses to ultrasound and vice versa. Pulses of ultrasound are directed into the body and allowed to interact with the body structures. A fraction of the ultrasound energy is scattered back towards the transducers. In B-mode imaging a greyscale image is constructed from the amplitude envelope of the backscatter signals.
Image contrast arises from the variability of echo strength.
Boundaries between tissues of different acoustic impedance act as specular reflectors of ultrasound and produce strong echoes. These signals eventually appear as a bright streak on the image. Within the bulk of a tissue, ultrasound is scattered by many smaller structures which combine coherently to produce a speckle pattern on the image. Red blood cells have extremely small scattering cross- sections. Therefore, the lumina of vessels are echo-poor.