DMS 104 Midterm Review: Ultrasound Imaging

Attenuation

• Attenuation is the weakening of sound as it propagates through tissues
• It limits the penetration and depth of the image
• The dominant contributor to attenuation is absorption
• Absorption is the conversion of sound into heat
• The unit of attenuation is decibel (dB)
• 3 dB = 50% attenuation reduction, 10 dB = 90% attenuation reduction

Attenuation in Soft Tissues

• Attenuation in soft tissues is approximately 0.5 dB/cm for each MHz of frequency
• The attenuation coefficient in soft tissue increases as frequency increases
• If frequency doubles, attenuation is doubled, and if path length is doubled, attenuation is also doubled
• If both frequency and path length are doubled, attenuation is quadrupled

Bone and Soft Tissue

• Attenuation in bone is higher than in soft tissue

Reflector and Sound Travel

• If an echo returns 130 µs after a pulse was emitted, the echo-producing structure is located at a depth of approximately 10 mm
• If a sound wave travels 26 µs to a reflector, the reflector is approximately 4 mm away
• If a reflector is 10 mm away from the transducer, it takes approximately 26 µs for sound to get back to the transducer

Reflected Sound Beam

• The reflected sound beam is known as an echo
• When the perpendicular beam is reflected, it goes back to the receiver and is picked up to form images on the screen

Acoustic Impedance

• Acoustic impedance is a measure of the resistance of a medium to the transmission of sound
• Its symbol is "z", its unit is Rayls, and the equation is 𝑧 = c (m/s) × ρ(kg/m3)

Reflection and Refraction

• If there is no reflection at a boundary, it means that all is transmission, and the medium impedances are equal
• The reflective and transmission coefficients add up to 1
• Oblique incidence is different from perpendicular incidence in that the angle is not perpendicular
• Refraction is a change in the direction of sound as it crosses a boundary, determined by the different acoustic velocity/impedance in the different mediums
• Snell's law tells us about the relationship between the angle of incidence/transmission and the velocity of sound of 2 media

Specular Reflector and Scattering

• A specular reflector is a flat smooth boundary that reflects well, produced at the artery walls or the diaphragm
• Scattering is the formation of echo in many directions by rough surfaces or heterogeneous media or very small objects
• Scattering depends on the surface of the interface and the size of the object compared to the size of the wavelength

Time Gain Compensation (TGC)

• TGC is needed to compensate for attenuation
• The TGC slope depends on the attenuation coefficient
• The operator needs to keep in mind the tissue composition, attenuation coefficient, transducer frequency, and path length when setting the TGC
• TGC is different from overall gain
• Factors affecting TGC setting include tissue composition, attenuation coefficient, and transducer frequency

Contrast-Enhanced Ultrasound (CEUS)

• CEUS is contrast-enhanced ultrasound
• Contrast agents are microbubble polymers introduced intravenously into the bloodstream to highlight particular isoechoic abnormal tissues (e.g. lesions)
• Characteristics of contrast agents include stability, small size, different acoustic impedance, non-toxicity, and easy administration
• Contrast agents function by increasing US amplitude, which can pop the bubbles and show the tissues and the rate at which they fill up again with the agent
• Contrast agents are used to show perfusion