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Chennai Breast Centre, Chennai, India
Breast Ultrasound: Physics and Equipment Settings
Breast ultrasound is a very important tool in diagnostic breast imaging. Understanding the physics of ultrasonography helps in maximizing the outcome of breast ultrasound examination.
Ultrasound pulses of a particular frequency are transmitted into the tissues. The ultrasound energy interacts with different tissue interfaces that are being examined. The returning echoes are received to generate an image of the underlying tissue. Acoustic impedance is the opposition of a medium to the passage of sound waves. Impedance is proportional to both the density of the medium and the velocity of the sound within it. The body consists of a range of medium like air in the lungs, gas in bowels, bone, water etc. Acoustic impedance of different human tissues is mostly similar. Impedance mismatch can occur in air and tissue is large, and hence 90 % of the incident ultrasound is reflected.
The speed of propagation of ultrasound is the rate at which ultrasound energy travels through tissues. Speed is determined by density and stiffness of the tissues. The speed of sound in a fat is 1,470 m/s and in soft tissues is 1,540 m/s.
The returning echoes are processed to generate an image of the tissue. An image consists of a number of discrete lines, each one of which contains information of the tissue examined. The more lines, the better the image appearance (i.e., the higher the resolution, the better the tissue details displayed).
As it travels through the tissue, the ultrasound energy is either absorbed (attenuation) or scattered (reflected). Attenuation determines the maximum depth imaged in any given frequency. As the frequency increases, the attenuation increases. This limits the depth of tissues that can be imaged.
Ultrasound waves lose energy as they travel through tissues; therefore, the returning echoes from deeper structures are weaker than the superficial structures. Time gain compensation amplifies the returning echoes from the deeper tissues. All ultrasound units are equipped with side switches for manually adjusting the TGC slope
“B”-Mode Scan
B mode or the brightness modulation is used in breast examination. The echoes are displayed on the monitor as illuminated spots where the brightness is proportional to the amplitude of the echoes received.
The transducers provide the link between the ultrasound imaging system and the tissues. Transducers are available in a wide variety of shape and size each designed for a particular range of clinical application. The transmission and reception of ultrasound waves by a transducer are based on the piezoelectric effect.
When alternating voltage is applied to a piezoelectric crystal, the crystal will expand and contract at a similar frequency emitting a sound wave that has the same frequency as the applied voltage.
A linear array transducer consists of a large number of small crystals arranged in groups that function alternately as transmitter and receivers. The linear array produces a uniform resolution over the full depth of the image field.
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